Charactering and optimizing cathode electrolytes interface for advanced rechargeable batteries: Promises and challenges

Zhongyng Zhng,Xinrn Wng,,Ying Bi,Chun Wu,

a Beijing Key Laboratory of Environmental Science and Engineering,School of Materials Science＆Engineering,Beijing Institute of Technology,Beijing,China

b Yangtze Delta Region Academy of Beijing Institute of Technology,Jiaxing,314019,China

Abstract With the advancement of secondary batteries,interfacial properties of electrode materials have been recognized as essential factors to their electrochemical performance.However,the majority of investigations are devoted into advanced electrode materials synthesis,while there is insufficient attention paid to regulate their interfaces.In this regard,the solid electrolyte interphase(SEI)at anode part has been studied for 40 years,already achieving remarkable outcomes on improving the stability of anode candidates.Unfortunately,the study on the cathode electrolyte interfaces(CEI)remains in infancy,which constitutes a potential restriction to the capacity contribution,stability and safety of cathodes.In fact,the native CEI generally possesses unfavorable characteristics against structural and compositional stability that requires demanding optimization strategies.Meanwhile,an in-depth understanding of the CEI is of great significance to guide the optimization principles in terms of composition,structure,growth mechanism,and electrochemical properties.In this literature,recent progress and advances of the CEI characterization methods and optimization protocols are summarized,and meanwhile the mutually-reinforced mechanisms between detection and modification are explained.The criteria and the potential development of the CEI characterization are proposed with insights of novel optimization directions.

Keywords: Cathode electrolyte interface;Secondary battery;Characterization methods;In situ/operando;Synchrotron radiation

1.Introduction

With electrification and portability of devices such as electric vehicles,smart electric power grid,and intelligent electronic devices,there is an overwhelmingly increased demand of rechargeable batteries with high energy density,long cycle life,and operational safety.Using a positive electrode with high-voltage and high-capacity has become an effective approach to satisfy this demand.However,the recent commercial cathodes face challenges in terms of structural and phase transition instability,transition metals(TM)dissolution,oxygen gas (O2) releasing,and the continuous decomposition of the electrolyte at the interface.These critical shortcomings have caused insufficient cycle stability,fast capacity degradation,and undesired thermal runaway in practical,severely restricting the state-of-the-art performance and development of rechargeable batteries.

The electrode/electrolyte interfaces have provided passivating yet ion-permeable layers for secondary batteries.Due to its formation,the separation of electrodes from electrolytes is fulfilled,which prevents the continuous electrolyte decomposition while facilitating the desolvation and solid-state migration of Li+[1,2].Such ion transportation across the interface determines many important properties for secondary batteries,such as cycle stability,rate capability,redox reversibility,electrode kinetics,and safety.Since Peled defined the solid electrolyte interphase(SEI)as a decomposition layer from liquid electrolytes in 1979[3,4],the interface issues have introduced a fundamental challenge,and become essential to improve battery performance while promoting industrial breakthroughs.The general understanding of SEI suggests an initial decomposition layer about 3-10 nm in thickness with both organic and inorganic components.It may grow up to 100-120 nm and maintain after cycles.The formation of SEI consumes electrolytes and anode materials,which will lead to the loss of active materials and a low columbic efficiency.Therefore,thin and protective SEI layers are suggested in order to minimize electrolyte decomposition.In this regard,some of the pre-lithiation procedures have been intensively investigated to solve the problem of SEI instability with large volume expansion materials,such silicon-based anodes[4-6].Similarly,the cathode electrolyte interfaces (CEI) formed on positive electrodes has played an equally important role in the performance optimization of secondary batteries[7].However,due to the extremely thin layers (0.5-1 nm) and chemical instability,CEI is more difficult to be observed,leading to the fact that the current research of the CEI remains in infancy[8-10].Based on the database online (Web of Science),Fig.1a summarized the growth of researches on CEI and SEI according to the lithium ion batteries (LIBs),sodium ion batteries (SIBs),and other rechargeable battery systems,such as aluminum ion battery (AIBs),potassium ion battery (KIBs),magnesium ion battery (MIBs).The figure evidently remains us that the investigation number of SEI exceeds more than twice of that in CEI during the past decade.More importantly,regulating SEI has proved to be effective on improving the capacity utilization of anode significantly.In this case,silicon anode is one of the most prominent examples,the performance of which is tightly related to the interfacial stability.According to the Fig.1b,the capacity utilization of the elemental silicon (theoretically 4200 mAh g-1) is generally limited to～640 mAh g-1,whereas the protected silicon anode positively boosts the capacity up to～2660 mAh g-1[11,12].As desired,similar capacity improvement has been also revealed in cases of SiOxand graphite,all in virtue of the SEI optimization[6,12-14].Learning from the development of the SEI,there are many fundamental issues and optimizations to be clarified with respect to the CEI [15-19].For example,the LiNix-CoyMn1-x-yO2(NCM) available for most of secondary batteries can theoretically deliver a specific capacity of～280 mAh g-1.Unfortunately,the practical one of NCM is limited to only～200 mAh g-1,and such capacity is increased to～220 mAh g-1by CEI modification [19].Similarly,only acceptable outcomes have been achieved with other cathode materials,such as LiNixCoyAl1-x-yO2(NCA),LiCoO2(LCO),which results in the low overall capacity of full cells.By comparison of components (as shown in Fig.1b and c),lithium fluoride(LiF)constitutes the major part of CEI,which suggests very different formation mechanism.Till now,because of the complexity and thinner thickness,the lack of understanding and fundamental ambiguity on the CEI still exist,such as the growth mechanism,composition transition and distribution,structural transformation and the structureactivity relationship and so on [20].

These challenges on CEI require direct information and proof of what actually occurs on the formation of the CEI[2].Relying on the optical,electronic,mechanical,spectroscopies,and electrochemical analysis,the CEI changes could be qualitatively/quantitatively examined,including their mechanical properties,chemical composition,elemental distribution,surface morphology and so forth.Herein,we have not only summarized the progress of conventional characterization methods on the basis of spectroscopy,X-ray,electronics,electrochemistry,etc.,but also introduced more advanced characterization methods,such as synchrotron radiation,cryogenic electron microscopy (cryo-EM),time of flight secondary ion mass spectrometry (ToF-SIMS),and other regulatory techniques,guiding researches for the further development of CEI.

Fig.1.The comparison of the literature of the CEI and the SEI for commonly-applied ion batteries from 2011 to 2020,the comparison of the main components and contents of typical CEI and SEI,and the improvement of mainstream battery materials after SEI/CEI optimization:(a)Number of studies in related fields.(b)Research data for the SEI.(c)Research data for the CEI;The data in(a)is obtained by search query CEI or cathode electrolyte interfaces and SEI or solid electrolyte interphase in the field of search topic utilizing the website,and"Other"in(a)represents related articles in the field of secondary batteries except LIBs and SIBs.

2.Progress on the characterization of CEI

The CEI has defined an interface with distinctive features than SEI,such as(I)thin layer(II)complex composition(III)inhomogeneity (IV) instability,and (V) special electrochemical properties.These features make more demanding requirements for characterization.In order to characterize these target properties,as shown in Fig.2,it is necessary to introduce for damage-resistance,high resolution,sensitivity,throughput,in situ/operando characterization methods and so on [2,21-23].These characteristics can be more feasible to shorten examine period (capture steady-state property),improve accuracy (trace but important components),widen detection range(wider range of component distribution),and so forth.To date,some characterization developments and improvements for CEI analysis are highlighted according to the literature (Fig.2): (a) The use of cryo-EM can maximally reduce electron beam damage (＜ -170°C),in particular available for the thin and fragile CEI.Synchrotron radiation technology provides sufficiently high signal intensity in order to improve the diagnostic resolution and shorten the detection time.(b) Computer technology has been introduced to cope with the increasingly demanding requirements for data processing.(c) The extensive promotion of ToF-SIMS facilitates to the detection of light but critical elements,including lithium.(d) In situ/operando diagnoses offer more comprehensive data on the transformation of the CEI (morphology,composition,thickness,etc.),ensuring timely inspection in unstable conditions.(e)In addition,the modified electrochemical analysis can focus on CEI rather than a mixed signal from other interference,such as the three-electrode electrochemical system.It is worth noting that most of methods for CEI analysis cannot be solely utilized.Multiple and simultaneous diagnostic methods are generally required,comprehensively explaining the formation,growth,and decomposition of CEI.In this point,we summarize the representative research using typical characterization for CEI and survey the progress of these technologies accordingly.

Fig.2.The characteristic methods for CEI analysis,mainly including: (a)damage-free and high resolution provided by cryo-EM (Reprinted with permission from Ref.[24].Copyright (2017) Science) and synchrotron radiation technology;(b) high sensitivity and throughput which could obtain by large amounts of data processed by computers (Reprinted with permission from Ref.[25].Copyright(2017) Springer Nature);(c) larger detection range brought by technologies such as TOF-SIMS(Reprinted with permission from Ref.[26].Copyright (1996) from Royal Society of Chemistry),synchrotron radiation (Reprinted with permission from Ref.[27].Copyright (2014) Royal Society of Chemistry),etc.;(d) in situ/operando diagnose;(e) special electrochemical detection such as multi-electrode system (Reprinted with permission from Ref.[28].Copyright (2017) Elsevier).

2.1.Formation of the CEI

The formation of CEI describes the decomposition of organic solvents,salts,and electrode materials at the cathodes/electrolytes interface.More recent studies have revealed the relationship between SEI and CEI that the dissolution and migration of the SEI-decomposed components may take part in the formation of CEI [29-31].Unmodified CEI basically generates loose surface structures with inhomogeneous composition distribution during the initial cycle stage.The substances at the interface are readily decomposed upon charging,producing radical species and forming conducting polymers consequently.Such process is apt to produce undesirable structures due to the uneven distribution of composition and reactive sites[32].The initial hierarchy of CEI is greatly determined by the morphology of electrodes as well as cut-off potentials,while its composition is largely influenced by lithium salts and solvent components applied.The CEI formed in carbonate-based electrolytes generally contains more carbon-containing species,which tend to decompose and release CO2at high potentials [33].The use of sulfone-based or sulfonate-based electrolytes gives rise to more stable sulfides in CEI [34-36].Other parameters,such as types of cathode materials,electrolyte additives,and charging/discharging procedures,could also vary the structures and components during the formation of CEI [23,37].When it is at the early stage,the significant changes of its components can be easily distinguished by general characterization methods,such as the analysis of organic bonds for organic substances,and the valent state of metal elements for transition metal oxides and organometallic components.

2.1.1.Chemical bond ＆organic components

The decomposition of salts and electrolytes results in both organic and inorganic species within CEI.The Fouriertransform infrared (FTIR) has become a useful characterization,which is available to identify organic species by qualitatively measuring the vibration of organic bonds caused by dipole changes [38-41].It accurately reflects the midinfrared region covering the vibration wavelengths between 1 μm and 100 μm,which contains signals of most organic functional groups.It can infer surface reactions,especially to study the reduction reaction of solvents and impurities on the electrode surface [42,43].In the early researches on CEI,the migration of SEI was considered to be the only reason for CEI generation [44].However,in 2009,Wu et al.investigated the formation of CEI on Li-doped spinel Li1.05Mn1.96O4by FTIR and found that the CEI can be formed directly on cathode surface without the presence of SEI.The composition of CEI grown on Li1.05Mn1.96O4has been assigned in 1 mol L-1LiPF6-ethylene carbonate (EC)/diethyl carbonate (DEC),which is mainly R-CO3Li and lithium carbonates (Li2CO3),similar to the composition found in SEI [45].Different electrolyte systems will affect the compositions of CEI,especially containing a variety of additives.These changes can also be examined through FTIR,so as to infer the different CEI formation mechanism [18,46,47].FTIR has characterized the composition of the CEI on LiNi0.5Mn1.5O4(LNMO) in the presence of fluoroethylene carbonate (FEC) additives.FEC decomposes into -CF3and -CF2species at the wavelength of 1332,1084,782 cm-1,which benefits to the LiF-rich interface to inhibit the dissolution of Mn [48].

Infrared-inactive components in CEI can be analyzed by the Raman spectra[42,49],which determines the symmetrical vibrations of nonpolar groups.A combination of FTIR and Raman is commonly applied for a comprehensive understanding of CEI components [50-53].However,Raman spectroscopy is somehow limited because the CEI is only a few nanometers in thickness,and the response is not detectable[40].Signal amplification with Raman can be achieved by the use of gold,silver,and copper electrodes,forming the surface-enhanced Raman spectroscopy (SERS) technology to obtain better surface-sensitivity [54].The SERS method has engaged to reveal the dynamic change of CEI on NCM and its variation with the state of charge.In typical SERS spectrum,due to the improvement of sensitivity,the band representing CEI(Marked in Fig.3aII with blue circles)can be observed as well as the ether and ester bands (1804.6 cm-1,1315.0 cm-1,etc.) that are not observable by common Raman detection.Through the tracking of these specific bands,it is found that the band intensity of CEI species is higher at low voltage(3 V).Considering that it is impossible for carbonate ester to be reduced at such potential range,an inference is made based on CEI signals and their formation mechanism is deduced.It is shown that dimethyl carbonate(DMC)or EC could be reduced at Li anode while the as-formed ester function groups can be reductively decomposed to alkoxide ions.These alkoxide ions will initiate nucleophilic chain reactions,through which ester molecules are polymerized and finally migrate to contribute to CEI formation [55].An upgraded Raman-based technique introduces shell-isolated nanoparticles (SHINs),which overcomes the disadvantage that the metal substrates cannot selectively recognize target molecules in SERS.By changing the substrates into a core-shell structure,the enhanced Raman spectroscopy (SHINERS) technology can thus ensures better reproducibility of the characterization results [52,56].The SHINERS method has been used to study the evolution of lithium oxide(Li2O)on lithium-rich cathodes,illustrating the surface reaction mechanism on lithium-rich cathodes for oxygen activating members.A limited amount of Li2CO3is formed in the CEI initially from electrolyte decomposition.The release of O2upon charging will react with the electrolyte to generate CO,CO2,and protons(H+).The generated H+will react with Li2O to form Li+and H2O at the end of charging,which migrate to the anode side to form LiOH·H2O finally[57].

Many other improvements to the FTIR have been also involved according to the literature.Attenuated total reflection FTIR (ATR-FTIR) provides a non-destructive surface sampling method by the total internal reflection from special reflection elements(Refractive index ＞1.5).It doesn't need for traditional tablet processing,so it's more suitable for surface diagnosis.ATR-FTIR can detect chemical information of 0.5-2 μm by differential spectroscopy and its detection sensitivity can reach the order of 10-9g [53,58,59].Showed in Fig.3b,the ATR-FTIR spectrum of CEI on the LNMO electrode proves the disappearance of the intermediate products after the introduction of tris (trimethylsilyl) borate (TMSB) additive(the peak at 1646.9 cm-1cor111responding C=C group),thereby demonstrating the inhibition of EC decomposition due to TMSB[10].ATR-FTIR is also employed in distinguishing the effect of di-(2,2,2 trifluoroethyl)carbonate -vinylene carbonate (DFDEC-VC) blended additives on the CEI composition in EC/DEC through the trivial changes in peaks.The CEI components in additive-free systems introduce more Li2CO3species (the peak at 1650-1580 cm-1),a potential precursor for the generation of CO2gas.The introduction of DFDEC-VC effectively reduces Li2CO3with more ester compounds in CEI[18].Such low and even missing signals in conventional can be effectively detected by ATR-FTIR[18,59].Furthermore,IR nanospectroscopy (nano-IR) is developed,engaging to overcome the limitation of resolution and narrow the detection regain into 10 nm.Nano-IR merges the high resolution of the atomic force microscope (AFM)with the chemical sensitivity due to the lightning rod effect[60].Upon illumination,the electrically polarized probe tip generates a spatially extended evanescent near-field and causes the light to converge locally.The light containing the spot composition information is scattered toward the far-field detection system [61].Three cor111responding technologies have been developed,namely photoinduced force microscopy(PiFM) [62-64],photothermal induced resonance (PTIR)[65,66],and infrared apertureless nearfield scanning optical microscopy (IR-aNSOM) [61,67-69].The detection of PiFM is driven by the light-induced dipole-dipole force between the needle tip and the sample,while PTIR relies on the local temperature rise and sample expansion caused by the interaction between the incident light and the probe.The IRaNSOM uses the tip enhancement effect to enhance the light scattering signal and provide the required information[60,70,71].Kostecki et al.have studied the chemical imaging of internal phase distribution of the LiFePO4cathode(Fig.3c)[67],while others have also studied the distribution and changes of the SEI composition of the anode [61,69].Although spectroscopic techniques provide great help for species identification in CEI,samples need to be prepared before testing,which unavoidably introduces oxidation of CEI[72].More in situ testing is required to clearly identify low quantities of products or intermediates(such as most inorganic products and lithium oxygen species)in the background noise.

Fig.3.(a)Operando SERS spectral evolution acquired in consecutive cycles for the LNMC cathode.Reprinted with permission from Ref.[55].Copyright(2019)American Chemical Society.(b) Operando-ATR-FTIR diagnosis of electrode surface components with and without additives.Reprinted with permission from Ref.[10].Copyright (2019) American Chemical Society.(c) Nano-FTIR chart to prove the existence of intermediate products.Reprinted with permission from Ref.[67].Copyright (2015) American Chemical Society.

2.1.2.Chemical state ＆inorganic components

TM-based cathode materials are important to the current set-up of LIBs,and the variable valent states of these transition metals provide the redox reaction with the charge transfer[73].Besides,the components in CEI is active and prone to be oxidized with electrolytes during the electrochemical process[22,74,75].The formation mechanism and related reactions of CEI can be further analyzed by characterizing the composition and the cor111responding chemical valent in particular with some inorganic species,such as LiF,Li2CO3,Li2S and other dissolved metal-induced CEI components.Therefore,energy dispersive X-ray spectroscopy (EDS),electron energy loss spectroscopy (EELS),X-ray photoelectron spectroscopy(XPS),X-ray absorption spectroscopy(XAS),SIMS and other characterization methods are also widely used in the study of CEI.

As one of the major techniques of electron microscopy testing,EDS takes advantage of X-ray photons with different characteristic energies for component analysis [76,77].Through EDS test,Yan et al.pointed out that the surface reconstruction layer of the lithium-rich and manganese-based cathode is a TM-rich and lithium-lean phase.Such phenomenon is caused by the migration of Ni from the bulk electrode to the surface as well as Mn accumulation on the surface.The elements changes along the cross-section direction show a linear reduction of element content from the bulk phase to the surface.It contributes to explain the substitution of lithium/TM in the surface layer,which leads to poor rate capability and capacity fading[78].In addition,EDS can also be used to judge changes of non-metallic elements in CEI,such as HF by-products,which can be eliminated by introducing bis(2,2,2-trifluoroethyl)phosphite (BTFEP) additives [77].

EELS has a higher energy resolution(up to 0.1 eV)because of the inelastic scattering caused by the incident electron beam.It is particularly available to distinguish light elements in CEI,such as lithium,compensating for EDS[79,80].Shown in Fig.4a-d,Botton et al.used the EELS spectrum (Fig.4d)and scanning transmission electron microscopy (STEM) images to illustrate changes of CEI composition on Li-rich cathode at different depths.The different colors in STEM image represent different composition at each depth of electrodes (Fig.4c).As suggested,the valence of Mn is reduced from Mn4+to Mn2+,which proves the consumption of TM due to the CEI formation.At the same time,a large amount of electrolyte decomposition products (such as LiF,LixPFy,Li2CO3,etc.) appear by judging signals of C,Li,and F [81].More importantly,its high sensitivity to Li is more conducive to analyze the formation mechanism of CEI [82].The energy loss of oxygen on the NaNi1/3Fe1/3Mn1/3O2characterized by EELS proves that the lattice oxygen is not released in the form of O2but in the form of reducing TM and oxidizing electrolyte during the process of migrating to the surface.The side reaction causes the dissolution of TMs and thus corresponds to the CEI growth.Driven by the growth of the CEI,the continuous consumption of TM generates microcracks and eventually lead to structural damage [83].

XPS,XAS,and X-ray emission spectroscopy (XES) are used to provide more detailed information of components in CEI owing to their strong X-ray penetrating power.Among them,XPS uses X-ray to radiate samples by exciting the inner-shell electrons or valence electrons and detecting the energy of photoelectrons to determine the chemical state of elements on the very surface of 2-5 nm [40,80,84].It is particularly available to identify the elements qualitatively or semi-quantitatively in CEI by changes of valent states [42].The sensitivity of XPS is high enough to differentiate the complex components and clarify the possible mechanism[22,85,86].For example,the changes in the valence of Fe and the content of LiF are detected in CEI of LiFePO4,and the phase transition from LiFePO4to FePO4on the surface is analyzed (Fig.4e).With the help of the semi-quantitative characterization of XPS,the peak areas of different valence states of iron are calculated.So the ratio of the content of Fe2+and Fe3+is obtained and the interfacial reaction between LiFePO4and the poly (propylene carbonate) (PPC) -solid polymer electrolyte (SPE) is suggested (Fig.4f and g): (1)Fe2+-e-→Fe3+(2) Fe3++PPC→Fe2++PPC+.After that,with the help of Ar+ion etching by XPS these chemical reactions are confirmed only took place at the surface of the cathode rather than in the bulk [87].In addition to this example,the sensitivity to valence analysis and the possibility of quantitative analysis could bring great convenience to the research of the generation mechanism of the CEI.For instance,the dynamic evolution of the formation and decomposition of CEI on LiCoO2cathodes are particularly investigated by XPS.The effect of the SEI on the CEI has been investigated by replacing the cycled lithium anodes with fresh one during the cycle and compared with the contents of species in the CEI and the SEI from the XPS spectra.Different from the semiquantitative relationship between the components in the CEI and the components in the SEI under conventional cycle strategy.The amount of Li2CO3and LiF in the CEI will not increase dramatically and the spectra of lattice oxygen could be maintained after discharge process.Accordingly,a supplement to the evolution mechanism of the CEI is put forward:the physical migration of SEI fragments also provides evolutionary support for CEI [31].

Fig.4.(a-d) The thickness of the CEI and composition distribution obtained by analyzing the changes of valent states of elements at different depths (a-c) The STEM images after 100 cycles.(d)A series of EELS spectra collected from the CEI to the subject from the color-coded region of interest marked in(c).Reprinted with permission from Ref.[81].Copyright (2018) American Chemical Society.(e) Fe 2p XPS spectra of LiFePO4 cathodes in different valent state and (f-g)Schematic diagram of interface between LFP and PPC-SPE in different states (Where the LEP,CLEP,and OLEP respectively represent LiFePO4,charged LFP,overcharged LFP,and TLFP is CLFP pressed on the PPC-SPE).Reprinted with permission from Ref.[87].Copyright (2013) Royal Society of Chemistry.

Furthermore,when X-rays pass through the sample,the intensity of light will be attenuated due to the absorption of X-rays.The intensity of transmitted light is related to the atomic mass of the sample composition and the absorption coefficient curve of different elements will have abrupt changes (defined as the absorption edge) at specific wavelengths.XAS is expert at retrieving structural,oxidation state,and local symmetry of atoms.Generally,the XAS spectrum can be divided into two regions according to energy: X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS),which correspond to regions with energy above the absorption edge 50 eV and 50-1000 eV,respectively.Among them,XANES can characterize chemical valence and electronic structure,while the adjacent atomic structure can be investigated by EXAFS[40,88-90].Combined with XPS,the dynamic evolutionary process of CEI on Ni-rich cathode is revealed by analyzing the changes of composition under different charge and discharge states.When the cathode is charged above 4.5 V,the CEI components of LiF and Li2CO3begin to decompose.The CEI begins to re-form until the cathode discharging voltage reaches 3 V [91].Moreover,the interactions between different species in CEI can be identified,such as the redox couple and the mechanism of species evolution,etc.Based on the high sensitivity of XAS,the content changes of different lithium species from the CEI are investigated with the different state of the electrode,including the initial lithiated state,delithiated state at room temperature,and the delithiated state at 60°C.And the conclusion has been drawn that the decomposition of LiPF6is related to the operation temperature,proving that the LiPF6is not appropriate to be cycled at 60°C.The instability of electrolyte salt will lead to the decomposition of both SEI and CEI,which will promote the reduction of transition metal in the electrode and reduce the battery life [92].

Therefore,the compositions of the CEI is different from that of the SEI due to the different chemical or electrochemical reactions under different operating potentials[30].In the CEI,inorganic components,such as LiF,Li2CO3and LiOH,are dominating,which is distinguished from the species presented in the SEI,such as Li2O and lithium alkoxides (ROLi)[29,30,45].In addition,except for the common ingredients such as R-Co3Li and organometals,the other organic components in the CEI are mainly determined by the solvent of the electrolytes [2,48,93].When combining characterization techniques using photons,electrons,and even ions,the formation time of the CEI and the species composition generated under different conditions can be judged through the generation of chemical bonds representing the components and the diagnosis of the valence states of each element in the CEI,thereby paving the way for the construction of models of internal components migration in the battery [59].

2.2.Growth of the CEI

The growth and quality of CEI are contributing to the electrochemical performance of the positive electrode.Previous studies have found that the growth process of the CEI involves both chemical and electrochemical reactions.Regardless of the static placement or charge-discharge cycling,the CEI film will become denser and its composition and electrochemical performance will change accordingly[46].This unstable state and uneven growth of the CEI is influenced by comprehensive factors,such as the leakage of cathode materials and even the migration of the SEI species(Fig.5) [29,74].These factors will determine the cycling,safety,capacity,and many other fundamental properties of batteries.Therefore,it is essential to conduct accurate,in situ,non-destructive,and high-throughput characterization on the morphology,composition,and electrochemical properties during the CEI growth.

2.2.1.Morphological characterizations

The surface microstructure of electrode materials has a great impact on the electrochemical performance.A smooth and uniform surface is often ideal for the improvement of cycle life and capacity.However,the nature of the CEI is often in an inhomogeneous and separated state,which constitutes the major reason for the decline of battery performance[94,95].The microscopy methods provide direct observation to evaluate the quality of CEI and they are mainly including the scanning electron microscope (SEM),transmission electron microscope (TEM),scanning tunneling microscope(STM),AFM,STEM,etc.

In virtue of a focused scanning electron beam,SEM collects the secondary electron and back-scattering electron to examine the surface morphology of electrodes [2,39].And it can be easily installed with the EDS or wavelength-dispersive spectrometer (WDS) for the element analysis [2,96].The combination of FIB and SEM facilitates the cross-sectional observation,which benefits to determine the thickness of CEI.In this regard,the CEI of commercial 26,650 battery is detected by FIB/SEM tomography to deduce the electrode degradation mechanism.Under the extreme test conditions of 22,000 cycles,it is observed that the CEI has reached up to 1 μm thick (Fig.6a,the dark region in cathode/electrode interface).The extremely thick CEI contributes to the increased resistance,which will increase the polarization of the aged electrodes during charge/discharge cycling,thus accelerating the battery decay.Additionally,the thicker CEI is speculated to be the result of the accelerated degradation of the electrolyte caused by the higher temperature [97].However,the electron beam can charge the electrode due to the insulating characteristics of CEI,which may reduce the resolution of the SEM.It is necessary to combine other characterization in order to comprehensively determine the properties of the CEI [39,96].In contrast,the morphology of the CEI can be notably obtained from TEM images with a much higher spatial resolution for the thickness and uniformity estimation of the CEI[49,98,99].Because of the improvement of resolution,the effect of different conditions on the thickness of CEI can be further evaluated by TEM clearly.Through the TEM analysis(Fig.6b III,VI,IX)and SEM analysis,Liu et al.compared the CEI of NCM surface with FEC additives (Fig.6b IV,V) and without FEC additives (Fig.6b VII,VIII),and found that the FEC-induced CEI is uniform with reduced cracks during cycling,confirming the more protective properties of the FECinduced CEI [100].Also,the result of TEM proves that the introduction of ethyl 5-amino-4-cyano-3-(2-ethoxycarbonylmethyl) thiophene-2-carboxylate (TAEC) additive triggers a thinner CEI (from 15.2 nm to 6.2 nm) on LCO cathode and contributes to the reduced resistance[99].Unlike SEM and TEM,which use electron beams as probes,AFM provides a convenient,non-destructive way to explore the surface morphology,mechanical properties,and electronic behavior of the CEI through interatomic forces [101-103].Both the short-range repulsion and the long-range Van-der-Waals forces between the probes and the CEI surface can be utilized to detect the slight changes of the CEI thickness during electrochemical reactions [96,103,104].The growth of native CEI on LCO is directly observed by the in situ AFM shown in Fig.6c.The filamentous structure of CEI film is not dense enough,so that it fails to separate the surface from the further reaction between the electrolyte and LCO[105].Apart from the CEI structure,the crystal faces of the electrode also play a significant role in the growth of the CEI.Relying on the AFM analysis,a CEI with a thickness of 4-5 nm is observed on the (111) surface of the LNMO particles with a voltage of 4.78 V,while the growth of the CEI is not observed on the(100) surface during the first cycle [106].

Fig.5.Schematic diagram of the growth mechanism of the CEI: (a) Representing the decomposition of lithium salts in the electrolyte.I and II represent the dissolution of the transition metals in the cathode and their reprecipitation on the cathode surface.(b)Species migrated forms the CEI.Reprinted with permission from Ref.[29].Copyright (2018) Elsevier.

Fig.6.The monitoring images of the CEI morphology captured by SEM (Including a,bI,bII,bIV,bV,bVII,bVIII),TEM (Including bIII,bVI,bIX) and AFM(Including c).Reprinted with permission from Ref.[97].Copyright (2018) Elsevier,Ref.[100].Copyright (1996) Springer Nature,and Ref.[105].Copyright(2017) American Chemical Society respectively.

Through conventional microscopy,the microstructure of CEI at different growth stages will be revealed initially.However,these commonly used morphology characterization methods of SEM,TEM,and AFM have several limitations.The data obtained at different time points and sampling regions may fluctuate significantly,while different battery systems (for example,the introduction of TMSB can reduce the thickness of the CEI about 3 nm) and charge-discharge systems will also have a greater impact on the data [31,97,107].

In order to maintain the stability of the data,improve the resolution and prevent the damage from the electron beams to the CEI,more emerging microscopy methods have been introduced according to literature,such as X-ray photoemission electron microscopy (X-PEEM),Scanning electrical microscopy (SECM),Scanning transmission X-ray microscopy (STXM).X-PEEM is a technique to accept the secondary electrons and photoemission electrons emitted or reflected from the sample surface onto the detector through a series of lenses(electromagnetic lenses or electrostatic lenses)to complete imaging.Owing to the support of synchrotron radiation sources,as a technology that can use X-ray as the excitation light source,it can be improved in terms of brightness,collimation,and coherence [108].At the same time,since the emission intensity of electrons used by XPEEM for imaging is proportional to the light absorption intensity,the imaging contrast of different elements can be selectively obtained by adjusting the wavelength of excitation light according to the absorption edge of the elements to be measured [109].By combining the composition analysis and microscopic characterization,X-PEEM can provide the surface composition distribution with a spatial resolution of tens of nanometer.It has successfully explained the effect of the suberonitrile (SUN) and lithium bis(oxalate)borate (LiBOB)binary additives on the CEI of LCO cathode.The uniform distribution of CoF2(The green and red interlaced parts like region 2 in Fig.7a) is the reason for the improved stability of the CEI.The distribution of components is presented by colorcoded (The color of the elements in the upper right corner of each picture corresponds to the color of the distribution area in each picture) composite maps of different combinations of individual elements into a graph as shown in Fig.7a.And just like this example,with the help of X-PEEM,not only the micros-morphology of the sample,but also the microscopic distribution of elements and even of different valence states can be explored [110].SECM is another type of scanning probe microscope based on the principle of electrochemistry,which can measure the electrochemical current given by the oxidation or reduction of substances in the micro-region with a small ultramicro electrode.And reaction rate imaging,a unique SECM technique,can be used to provide more evidence for determining the generation mechanism of the CEI[111].The evolution of the CEI in aqueous LIBs is first studied by SECM and it is proved that a discontinuous CEI with conductive properties is formed on the surface of LiMn2O4.The changes between the feedback current and the average current in different selection areas in Fig.7b III is shown in Fig.7b IV.The CEI growth region(dark blue region)has high insulation,which reduces the feedback current and thus proves the formation of the CEI and its certain passivation effect [112].STXM,as a spectral microscope,uses a zone plate to focus the X-rays on a small spot,while the sample is scanned on the focal plane of the zone plate,and finally the intensity of transmitted X-ray is recorded as a function of the sample position[96].Owing to the sensitivity to light elements and low concentration,more and more researchers have suggested or have applied this technology to the research of the SEI [113,114] and the CEI [115].

The fluctuation of data will affect the academic research on the model theory of the growth mechanism of the CEI.However,as shown in Fig.8,with the improvement of the characterization technology,its model is becoming more and more vivid and accurate.The original model of the SEI was inferred purely based on electrochemical data.It was a 2D model proposed by Paled from the characteristics of the electrode kinetics and deposition-dissolution mechanism of lithium metal with nonaqueous systems.In the later study,the dilatometer was introduced to characterize the expansion/contraction process of the crystal in the electrochemical process of the electrode,and it was verified that the SEI penetrated into the block,from which the 3D model was extended.Moreover,the two models (the multi-layer model and the mosaic model) currently recognized by the academic community are proposed respectively due to the introduction of IR and EIS to detect that the fitting circuit of the whole battery requires 4-5 resistance-capacitance and due to the further analysis of EIS data to take the grain-boundary resistance into account.With the continuous improvement of detection technology and experimental methods,many novel perspectives for understanding the model have also been proposed.For example,through further composition analysis by FTIR,the “double-layer capacitance model” in which the positive charges fixed in the SEI will counter negatively charged graphite is explained specifically from the point of view of the SEI adhesion and compactness;or with the help of Cryo-EM,the model is further refined from the point of view of morphology,and it is considered that the model should present as a “plum pudding model” [2,116-119].The continuous improvement of cognition further illustrates the necessity of developing characterization methods for mechanism analysis.At present,the two mainstream viewpoints about the spatial distribution of the CEI are mosaic model and layered model[44,120].The mosaic model believes that the inorganic and organic components in the CEI are randomly distributed,while the layered model considers that the distribution of each component is relatively regular(the inorganic components are distributed near the inside of the electrode surface and the organic components are distributed on the outside).The latest research believes that its structure is constantly changing dynamically.Therefore,the in situ/operando characterizations introduced later are more suitable for the diagnosis of unstable CEI compared to the above-mentioned morphology detection methods.

Fig.7.(a) Mapping diagram of element composition drawn by X-PEEM.Reprinted with permission from Ref.[110].Copyright (1996) Royal Society of Chemistry.(b)Scanning diagram of SECM area with different cycle times and the feedback current density and roughness of the feedback current surface in this area.Reprinted with permission from Ref.[112].Copyright (2013) Royal Society of Chemistry.

Fig.8.Model diagram of the SEI and the CEI.Reprinted with permission from Ref.[2].Copyright (2018) Springer Nature.

2.2.2.Diagnosis of electrochemical properties

The CEI has a notable contribution to the electrochemical performance of the battery as the unique benefits of blocking electrons,conducting ions,protecting the electrodes,and inhibiting the decomposition of the electrolyte [121].However,it also consumes lithium salts and electrolytes and increases the overall impedance.Consequently,some of the electrochemical measurements are available for the detection of the CEI and the study of its optimization.

Fig.9.(a) Two-electrode and three-electrode models.(b) The Nyquist diagram of cathode and anode measured by the three-electrodes system.Reprinted with permission from Ref.[126].Copyright (1948) Electrical Society (c-d) Commercial and optimized Swagelok T-cell device diagram.(e) System scheme of impedance measurement mode of Swagelok T-cell,where scheme I denotes potential-controlled impedance spectroscopy,scheme II denotes current-controlled impedance spectroscopy,and scheme III denotes modified potential-controlled impedance spectroscopy.Reprinted with permission from Ref.[128].Copyright(2016)Electrical Society.(f)Impedance diagram of LNMO with 1.5%conductive carbon black after 50 cycles at 1C and 40°C.Reprinted with permission from Ref.[16].Copyright (2019) Electrical Society.

Obstructing electron transfer and increasing internal resistance are some of the relatively intuitive manifestations of the electrochemical impact of the CEI.Therefore,characterization methods for diffusion coefficient such as electrochemical impedance spectroscopy (EIS),galvanostatic intermittent titration technique (GITT),potentiostatic intermittent titration technique(PITT),etc.,can be used to characterize the growth and change of the CEI [122].EIS measurements record the frequency response of an electrochemical system to an applied stimulus and give the electrochemical information of the system by discussing the values of the real part (Z’),the imaginary part(Z”),the modulus(｜Z｜)and the phase angle(φ)of the impedance at different frequencies [86,123,124].The detection range of the response frequency can even be from the diffusion processes (mHz range) to electron transfer kinetics occurring in electrochemical processes (MHz range)[123].Regarding the EIS tests of the CEI,the traditional twoelectrode set-up cannot exclude the electrochemical interference from the SEI,and therefore it is necessary to separate the electrochemical contribution through a three-electrode characterization system [125].For example,the resistance of cathode and anode of the LiFePO4/graphite battery is distinguished by the three-electrode system illustrated in Fig.9a and the results shown in Fig.9b indicate that the CEI of the cathode surface has made the major contribution to the resistance of the whole battery as compared with the EIS of cathode and anode,respectively[126].Additionally,the threeelectrode system is also used to elucidate the effect of TMSB additives on the CEI protection by effectively inhibiting the decomposition of the EC-DMC electrolyte on the NCM cathode [127].In addition to the three-electrode system,the other improvements of the cell configuration can also provide the possibility of a more detailed differentiation of impedance caused by the CEI or other factors.For instance,the Swagelok T-cells device optimized as shown in Fig.9d(The systematic scheme is from Fig.9e II to Fig.9e III)is used to illustrate the main source of resistance increase of graphite/LNMO battery during long charge/discharge cycling at 40°C [128].Accordingly,the increased resistance is proved to be not only derived from the increase of the Rfilmthat has been previously considered but also the increase of the contact resistance between the aluminum current collector and the cathode electrode.This is because it has the characteristics of contact resistance,such as low interfacial capacitance,very weak temperature dependence (Fig.9f I),and strong external compression dependence (Fig.9f II).Distinguishing the different parts of the battery impedance is not only conducive to the study of the impact of the CEI on the impedance,but also conducive to the improvement of the electrochemical intercalation reaction model [16,128].

Fig.10.(a)Rfilm values and(b)Rct values of different voltages and different cycle times separated by the three-electrodes system.Reprinted with permission from Ref.[132].Copyright (2018) John Wiley and Sons.

In the analysis of the EIS spectrum,researchers generally accept the surface layer model developed by Aurbach et al.[129-131].The model describes the EIS spectrum of Li+intercalation and de-intercalation with an equivalent circuit,mainly including four parts: The high-frequency region(Rfilm),the intermediate-frequency region (Rct),the lowfrequency region (Zw) and extremely low-frequency region(Rb) are related to film resistance,charge transfer,solid-state diffusion,and phase transformation,respectively [43,130].Because these processes will all affect the internal resistance of the battery when researching the mechanism of the CEI,it is often necessary to discuss the different influence of the formation of the CEI film from the charge transfer process on the electrochemical performance separately.Regarding the investigation of the CEI of LiMn2O4in ethyl methyl carbonate(EMC) and EC (1:1 in volume) by a three-electrode system,the changes of Rfilmand Rctare shown in Fig.10a-b according to different states of charge and cycle numbers.The Rctvalue is identified to be greater than the Rfilm,and the change of Rctis much more prominent than Rfilmat the same cut-off voltage,which proves that the change of Rctis the main reason to the increase of overall resistance of the cell [132].

The impact of the CEI on the stability of the battery performance can be also obtained through other common electrochemical methods,such as differential capacity plots (dQ/dV),linear sweep voltammetry(LSV)and cyclic voltammetry(CV): The active lithium ions will be consumed due to the growth process of the CEI,which will attenuate the voltage,but the formation of the CEI also protects electrodes and electrolytes from continuous consumption,thus improving cycle performance.

Although the growth of the CEI/SEI is often considered to be one of the most important reasons for battery degradation,there is a lack of direct proof linking the changes of the CEI with the interface characteristics and battery performance.Therefore,in the case that it is difficult to give a strong proof by electrochemical measurements alone,the combined characterization of electrochemical analysis(CV,charge-discharge cycling,EIS,etc.) with other measurements (mass spectrometry (MS) [133,134],electrochemical quartz crystal microbalance (EQCM) [135],digital image correlation (DIC)[136,137],and bending cantilever techniques [136,138],etc.)offers a more direct and in-depth understanding on the CEI.The relationship between the electrochemical properties and the surface mechanical properties of LiFePO4cathode in the electrolyte containing LiPF6or LiClO4is studied by in situ stress and strain measurement.From the data Fig.11a,LiPF6-induced CEI generates additional stress and strain than that of the LiClO4-induced CEI(Shown in Fig.11b).This is because the LiPF6-induced CEI is thicker and has high resistance,causing the dynamics of Li+transfer through the CEI to be blocked.And these additional stresses and strains will shorten the cycle life of the battery under high pressure [136].The in situ EQCM enables the simultaneous detection of electrochemical properties and the accurate mass changes in nanograms on the surface[28,139].The EQCM has been utilized to explain the activation process of Li2MnO3in Li-rich materials(xLi2MnO3·(1-x)LiNi0.3Co0.3Mn0.4O2(x=0,0.5,1)),because electrochemical decomposition leads to Li+deintercalation from electrode,whereas no Li+deintercalation occurs during chemical decomposition.The results show that this process is affected by electrochemical decomposition(oxygen redox) rather than chemical decomposition (that is Li2O precipitation)like pure Li2MnO3,which is proven by the mass accumulated per mole of electron transferred increased with the decomposition of the CEI displayed as a red line in Fig.11c.Importantly,is produced instead of Li2O through electrochemical decomposition,which provides a basis for studying the evolution of lattice oxygen from the bulk phase to the interface of Li-rich materials [135].

Fig.11.(a-b) The relationship between in situ pressure,pressure,and voltage,which illustrates the impact of the CEI growth on electrochemical performance.Reprinted with permission from Ref.[136].Copyright (1948) Electrical Society.(c) CV curve and simultaneously EQCM response curve for the first cycle.Reprinted with permission from Ref.[135].Copyright (2019) American Chemical Society.(d) Decomposition of the CEI at different potentials detected by electrochemical methods coupled with MS.Reprinted with permission from Ref.[134].Copyright(2016)Elsevier.(e)In situ diagnosis of surface gas evolution by CV and MS.Reprinted with permission from Ref.[133].Copyright (2016) Springer Nature.

When we try to establish the link between the quality of the CEI and electrochemical properties,gassing phenomenon is another important and convenient aspect to be utilized[140,141],because the process of gassing will cause irreversible consumption and directly affect the stability and safety of the battery while high-quality CEI can effectively inhibit the continuous gassing [33,133,142].As shown in Fig.11 d-e,the electrochemical tests coupled with MS are used to investigate the CEI decomposition and gas production during the electrochemical process.Analyzed by the online electrochemical MS,Fig.11d shows the change in the release of O2and CO2at different potentials through the electrolyte (1 mol L-1LiPF6in EC+EMC) decomposition and the degradation of NCM cathode.It is inferred that the growth of the CEI causes additional diffusion barrier and electron transfer,thereby deactivating the reaction on the electrode surface[134].Fig.11e introduces the application of operando differential electrochemical MS,which evaluates the gas released during the first cycle of the charge-discharge process.The investigation proves that the gassing phenomenon is mainly related to the activity of oxygen on the particle surface,which will cause excessive electrolyte decomposition and growth of the CEI.It results in increased electrochemical resistance eventually [133].These experiments with gassing detection also suggest that more side reaction products can be applied as a bridge to characterize the CEI and battery electrochemistry.And to the best of our understanding,a more convincing explanation of the effect of the CEI on the cell performance can be concluded unless the successful linking between electrochemical properties and the physicochemical changes of the CEI,which requires more combination of advanced characterization.

2.3.Advanced techniques for the CEI characterization

2.3.1.Synchrotron radiation

Synchrotron radiation provides higher resolution,improved detection accuracy,and a more detailed exploration of the extremely thin microstructure of the CEI due to its significantly improved brightness,collimation,coherency,and extremely short light pulse time to picoseconds [143,144].These characteristics provide good temporal and spatial resolution for the characterization technology,which are helpful to reveal the fast CEI changes and more details of the composition evolution in the CEI[144].Synchrotron radiation has been adapted to some traditional characterization methods,such as X-ray diffraction (XRD) [145-147],XAS [90,148],FTIR [149] with improved resolution and advantage.Moreover,some brand-new technologies have also been derived,such as Coherent diffraction imaging respectively (CDI),Synchrotron radiation infrared (SRIR),full-field transmission X-ray microscopy (TXM),Micro-X-ray fluorescence (XFM)and so on.Moreover,the technical achievements in this area are summarized as shown in Table 1.

Table 1 Typical characterization methods combined with synchrotron radiation technology [114,146,147,149].

Table 2 Comparison of time length,resolution,advantages,and disadvantages between the technologies using synchronous radiation and the original characterization methods (*Represents the advantages and disadvantages of characterization methods,·Represents resolution parameter).

Among the achievements obtained,the synchrotron radiation technique has greatly contributed to promote the research of the surface and interface of electrodes and the CEI.Although XAS detection has been used in material research before the development of synchrotron radiation technology,it was limited by the low brightness of the light source,which led to long data acquisition time and poor signal-to-noise ratio.It is not until the synchrotron radiation technology increased the brightness that XAS is widely applied to material characterization.With the high detection efficiency brought by synchrotron radiation,the total electron yield model (TEY) of XAS adopting photon-in-electron-out mode become a powerful diagnosis method in the field of surface characterization.Using the high-throughput synchrotron soft XAS coupled with atomic-level STEM-EELS,an efficient surface element information acquisition protocol is established,which helps explain the surface reconstruction of NCM cathode and detecting the changes of surface element distribution [148,150].As shown in Fig.12 a-c,the element distribution in the thickness of 5-10 nm on the surface of NCM cathodes is successfully obtained [148].By combining the tunable depth sensitivity,the synchrotron radiation based hard X-ray photoelectron spectroscopy (HAXPES) and soft X-ray photoelectron spectroscopy (SOXPES) can detect the elemental information with the “sputtering free” depth.The thickness and composition of the CEI and the SEI have been investigated in this study by detecting the components on the LiFePO4cathode and the graphite anode,and the schematic diagram (Fig.12e-f) of the composition that varies with the depth gradient is given.From the comparison of the composition of the SEI (≈20 nm) and the CEI (＞7 nm) shown in Fig.12d,it can be seen that the Li2O and ROLi components are lack in the CEI,which is inferred to be due to the different driving force for the various electrochemical reactions of the anode and the cathode [30].Furthermore,the synchrotron radiation technology provides higher sensitivity that can facilitate the detection of the trace amount of components.The HAXPES is used to detect the protection of Sm2O3coating on Li1.23Mn0.46Fe0.15Ni0.15O2cathode.The peak representing Li2O2species or oxygen atoms (530.5 eV) can be only detected in virtue of hard X-ray photoelectron spectroscopy,which is a solid proof to determine the formation of oxygen vacancies at the interface.And the latter is a direct hazard to the electrolyte decomposition and interface instability.The Sm2O3coating is found to deactivate the formation of oxygen vacancies and thus stabilize the Li1.23Mn0.46Fe0.15Ni0.15O2cathode [151].

Fig.12.(a-b)XAS spectra of Mn L-edge and Co L-edge before and after cycles,and(c)surface element distribution calculated with TXM data.Reprinted with permission from Ref.[148].Copyright(2016)Springer Nature.(d)The relative intensities of different components of the SEI and the CEI obtained by fitting the HAXPES spectral curve and schematic pictures of(e)the SEI and(f)the CEI drawing based on the PES data.Reprinted with permission from Ref.[30].Copyright(2013) Elsevier.

The synchrotron radiation techniques not only improve the diagnosis resolution,element sensitivity,interface sensitivity but also have advantages on shortening the detection time (as shown in Table 2),which are more conducive to the instantaneous tracking of the growth and change of the CEI with the unstable state [152].

Although synchrotron radiation has all these advantages,which is helpful to obtain high-resolution images,analyze trace components,characterize internal structure,and even characterize three-dimensional structure of the CEI,but highintensity X-rays will cause damage to the inherently unstable CEI.Thereby,the current application of synchrotron radiation technology to the characterization of the CEI is still in its infancy,and how to maintain the high intensity of incident Xrays without destroying the CEI is an urgent problem to be solved (such as the combination of freezing technology and synchrotron radiation in the detection of biomimetic biological crystals [153,154]).

2.3.2.In situ/operando characterization

In situ experiments and operando characterization have been widely applied to better clarify the dynamic changes of the CEI throughout the operational process and conditions.The concept of “in situ” emphasizes the original use and the natural scene,so the in situ characterizations sample and analyze the interior of the batteries without disassembling the structure of them [155].In contrast,“operando” emphasizes more realistic behavior,it is not like “in situ” to pause regularly for testing,but under ideal conditions to monitor the battery in real-time to ensure that the experiment is performed while carrying out the expected function [155-158].In other words,operando experiments are usually the development of in situ experiments,which enhance the time-resolving ability through external forces,so as to ensure that the data is obtained in the actually continuous experiment rather than the pseudo-persistence composed of the multiple transient states obtained by pauses as in the in situ experiment [28,157].Despite there are some differences between these two concepts after careful comparison,in the actual research,it is not necessary to make a complete distinction between the two concepts.Both in situ and operando measurements are both designed to optimize the existing characterization methods and devices to get more mechanisms of the CEI changes[158].Most of the in situ/operando devices are homemade with a variety of set-ups,as shown in Fig.13 [33,159].Such as the DIC technique (Fig.13a) is introduced to monitor the surface strain of the MWCNTs/V2O5cathode during the electrochemical process[159],the sputtering mode of XPS(Fig.13b)is changed to study the process of the SEI growth between solid electrolyte lithium phosphorus sulfuric oxide nitride(LiPSON) and metal lithium [160],and the gas analysis system (Fig.13c) is set up to in situ detect the gas production of different electrolytes (γ-butyrolactone (GBL),EC,DMC,EMC,and DEC) [33].

Fig.13.(a)Device diagram of the in situ stress-strain electrochemical monitoring system.Reprinted with permission from Ref.[159].Copyright(2019)Elsevier.(b) Scheme of the XPS setup for the in situ measurements.Reprinted with permission from Ref.[160].Copyright (2020) John Wiley and Sons.(c) Schematic diagram of gas in situ analysis system for LIBs.Reprinted with permission from Ref.[33].Copyright (2015) American Chemical Society.

Fig.14.Data of the CEI obtained by the in situ diagnosis methods: Image obtained by (a) in situ XRD,(b) in situ EIS analysis,and (c) in situ Raman.(a) XRD measured while heating electrodes after cycling in different voltage ranges.Reprinted with permission from Ref.[161].Copyright (2013) Royal Society of Chemistry (b-c) Raman spectra,EIS analysis and cor111responding voltage distribution recorded during the initial constant current cycle.Reproduced from Ref.[162].Copyright (2020) Elsevier.

The in situ/operando measurements offer great opportunities to an in-depth understanding of the dynamic evolution of the CEI.The failure mechanism of LCO cathode under high-voltage cycling is analyzed by in situ XRD (Fig.14a).Compared with the different cut-off voltage(2.7-4.2 V for the left and 2.7-4.7 V for the right),the evolutions of the (003)peaks of LiCoO2are different after 50 cycles.Combined with other measurements(EELS,STEM,and XPS),it is proved that the voltage decay is mainly due to the formation of the CEI.In addition,the capacity loss and voltage attenuation jointly cause the decline in electrode life[161].The formation and the evolution of the CEI on the lithium-rich and manganesebased cathode are studied using in situ EIS and in situ Raman (as shown in Fig.14b-c).The extraordinary decrease of RCEIobtained in the charging voltage window of 4.35-4.75 V and the enhancement of the Raman peak (805 cm-1,representing the peroxo O-O bond stretching) at the charging voltage above 4.6 V are interpreted to be all related to the evolution of oxygen in the cathode.With the Lideintercalation from the cathode,O2-will be oxidized toand the formation of the CEI obtain Li-source,so the RCEIand 805 cm-1peak will enhance.However,when the voltage window comes to 4.35-4.75 V,the CEI layer will suffer an oxidative decomposition,which led to the decrease of RCEI[162].

2.3.3.Cryo–EM

The cryo-EM,which is first developed to explore electronbeam-sensitive biomolecules [163-165],provides a solution to resist the beam damage and make significant progress on the beam-based analysis [166,167].It uses liquid nitrogen to freeze the sample,which avoids the damage of air and electron beams while retaining their inherent structure.It is particularly available for the fragile and unstable segments or beamsensitive regions in a very small scale during sample transfer and characterization [168].

The cryo-EM has been applied to the study of the SEI with many successful examples.As shown in Fig.15a-b,Cui et al.observed the nanostructures of the SEI and metallic lithium through cryo-EM in the presence of FEC additives.It shows that the FEC-induced SEI is more orderly and has a multilayer structure rather than a mosaic structure in elemental electrolytes (Fig.15b) [24].Cryo-EM can even indicate the SEI compositions by detecting the crystalline orientation.The Li in the (110) crystal orientation detected by cryo-TEM proves the existence of crystalline Li2O in the SEI.Therefore,it infers that there is a reaction between Li2O in the SEI and trace water in the carbonate solvent or electrolyte,resulting in LiOH in the results of cryo-TEM images [169].

Fig.15.(a-b)Cryo-EM image of the SEI with different electrolytes.Reprinted with permission from Ref.[24].Copyright(2017)Science(c-d)Cryo-EM graphics of the CEI on LiMn1.5Ni0.5O4 after 50 cycles with different electrolytes.Reprinted with permission from Ref.[169].Copyright(2019)Royal Society of Chemistry.

The thickness of the CEI on LNMO cathodes are compared after 50 cycles in different electrolytes.Fig.15d displays the image of the CEI in the sulfolane with lithium bis(-fluorosulfonyl)imide electrolyte,which provides a more uniform structure than that in the original electrolyte (Fig.15c)[34].In Li-S batteries,the cryo-TEM also revealed a crystalline CEI of the sulfurized polyacrylonitrile(SPAN)cathode,which contains LiF and LiNO2in a high-concentration etherbased electrolyte with LiTFSI and LiNO3.It explains the stable cycling of SPAN in ether-based electrolytes due to the protection of crystalline CEI[170].From the current research,it can be seen that,compared with the SEI,there is a stronger requirement to introduce the cryo-EM to guarantee the integrity of the thinner CEI(Even less than 1 nm)and provide assistance for analysis and diagnosis [171].Because the overall efficiency of the battery is not limited only by the short plate effect,the interaction between the various parts will also have a significant impact on it[163,167].Only after analyzing the overall complex system including the cathode,the anode,the electrolyte and the separator,the interaction between the various parts of the battery can be further explored.

2.3.4.ToF–SIMS

The ToF-SIMS is of high-resolution diagnosis to determine the ion mass according to the different flight time of the secondary ion to the detector.The specific structure of surface compounds and organic substances can be therefore determined even with isotopes of elements in the level of ppm or ppb.At the same time,it has an extremely high spatial resolution of 2 nm on the top layer,making it the most superficial mass spectroscopy on the sample surface[84,172].Some light but important elements,such as lithium,can be detected,making it suitable for the characterization of the CEI[173,174].

Fig.16.The works of ToF-SIMS in the characterization of the CEI and element of the electrode surface layer:(a)Elemental distribution maps obtained with the help of FIB-SEM/TOF-SIMS.Reprinted with permission from Ref.[175].Copyright (2015) Elsevier.(b) The stacked setup of the CEI showed by SIMS depth profile of cathode.Reprinted with permission from Ref.[176].Copyright (2014) Elsevier.(c) Depth profiles of different ion fragments obtained by TOF-SIMS.Reprinted with permission from Ref.[177].Copyright(2015)Elsevier.(d)3D rendering with TOF-SIMS data which shows the distribution of components in the CEI.Reprinted with permission from Ref.[178].Copyright (2020) Elsevier.

In the research of the CEI,many achievements have been made in collecting the information of its compositions and their distribution by ToF-SIMS.It is used to draw element distribution maps in combination with EM.Fig.16a shows the element distribution map on the surface of lithium-rich cathode in the fully charged/discharged state.The uneven element distribution shown in Fig.16a is related to the electrochemical state of the battery electrode,which affects the formation and growth of the CEI with precise lithium element detection[175].Fig.16b shows the changes of different species at different depths in the CEI.A stacking structure of the CEI is proposed,which consequently contains the decomposition product of LiPF6on the CEI surface.It detects the presence of the organic phase,lithium carbonate,and inorganic substances,such as phosphorus oxide,from the CEI surface to the bottom of the CEI[176].As shown in Fig.16c,the ToF-SIMS can be also used to plot the depth distribution of cationic fragments on the surface of cycled LNMO cathode before and after Al2O3coating,which proves that the CEI on the Al2O3coated LNMO is much thinner than that of bare LNMO[177].Combined with TEM,XPS,and other characterization methods,the LiF depositing onto the surface of LNMO by atomic layer deposition (ALD) technology is analyzed.The distribution of LiF and carbon-fluoride is shown in Fig.16d.The two sources of fluoride from TiF4and Hexafluoroacetylacetone (Hfac) give rise to different uniformity of LiF coating.Hfac brings a more uniform LiF distribution and contains more lithium,which provides an explanation for its better coulomb efficiency [178].

3.Strategies to tailor an ideal CEI

The original CEI demonstrates typical intrinsic disadvantages,such as the reduced migration of Li+,increased electrode polarization,unstable electrochemical properties,uneven element distribution,and so on.These problems will cause the capacity degradation [179],poor cycling,insufficient safety,additional resistance [180,181],increasing consumption of active materials [179,182-185],uneven stress distribution[92,136],and even changes in the composition or state of the SEI on the anode side[186].In practical applications,an ideal CEI should have characteristics,such as uniformity,compactness,chemical/electrochemical stability,strong ionic conductivity,and good mechanical flexibility.Considerable of the state-of-the-art results of battery performances are mostly related to the effective optimization of the CEI,such as elements doping,surface coating,electrolyte optimization,and other methods.However,the above modification results require the characterization aforementioned to verify the feasibility,so the combination of characterization and modification is a complete ring that complements each other.

3.1.Electrolyte optimization

The regulation of the electrolyte composition is a direct optimization route to control the formation,composition,and function of the CEI since the CEI is the product of the electrolyte decomposition [187].Electrolytes and additives for batteries are constantly innovated and improved,and a large part of the additives are designed to form a passivation layer on the surface of the cathode before the decomposition of the main electrolyte materials [20].Table 3 shows the effects of additives on the electrochemical properties of the CEI as well as the cor111responding characterization.

In addition to types of additives,changing the concentration of the electrolytes will also have a great impact on the quality of the CEI.Properly increasing the concentration is conducive to the formation of the CEI,whereas high concentration may lead to reduced lithium mobility and unfavorable CEI properties [192].After using the concentrated EC/DMC/Lithium Oxalyldifluoro Borate (LiODFB) electrolyte,the formation of a compact LiF-rich CEI layer is observed on the surface of the LCO cathode by EM and XPS,which effectively inhibits the decomposition of the electrolyte solvent and the dissolution of Co2+[193].In addition,since most of the commonly used electrolytes are non-aqueous,when exposed to air,the composition will change,which is not accurate to judge the impact of electrolyte.Therefore,it is necessary to adopt the methods mentioned above to avoid the possible oxidation and analysis bias,such as in situ diagnoses and synchrotron radiation.

3.2.Surface modifications

Fig.17.(a)Analysis of the effect of Zr coating on the CEI by STEM,EIS,and electrochemical performance.Reprinted with permission from Ref.[198].Copyright(2018) American Chemical Society.(b) XPS spectra of the surface of the LNMO cathode before and after Co3O4 coating.Reprinted with permission from Ref.[201].Copyright (2019) Elsevier.

Modifying the surface of the electrode materials can directly and effectively regulate the CEI as well.An effect surface modification can be achieved by coating (simple substance coatings,oxide coatings,and organic coating) to serve as a protective barrier,so as to enhance the inhibition of transition metal dissolution and consumption of electrolyte [187,194,195].Because the type of coating is often determined at the beginning of studies,the results of modification can be easily linked to the electrochemical changes through some relatively conventional diagnostic methods [95,196,197].And among the various coatings,the detection of the simple substance coatings is the easiest[179].For example,the use of Zr coating can improve the cycle stability and rate performance of Li+battery cathodes,but its specific mechanism of action has not been determined yet.By the combination of STEM and EIS(Fig.17a),it is found that the modification by Zr can constitute a Zr-based rock-salt structure layer on the surface,which can inhibit the rock-salt phase transition of cathodes.The EIS results show that it doesn't inhibit the growth of the CEI or affect the impedance,which explains why the Zr coating improves the performance of the battery[198].In addition to simple substance coatings,many oxides are also used to modify the surface of cathodes in order to pursue better surface properties [151,177,199-201].After coating LNMO with Co3O4,the excessive growth of the CEI is suppressed and the cycle life of the battery is improved.The XPS results shown in Fig.17b illustrate the component changes of the CEI after coating that the content of PEO,R2CO3,Li2CO3,LixPFyOz,and LiF are decreasing.These results provide pieces of evidence that the growth of the CEI is suppressed and the electrochemical performance can be improved due to the coating blocks the CEI from excessive raw materials and electrolyte [201].According to the literature,LiF is proven to be passivating for better preventing electrolyte decomposition.Therefore,it is often introduced into the CEI to modify the electrode surface.Wang et al.investigated vanadium oxy-acetylacetonate (VO(acac)2) as the surface protective layer of V2O5-based cathode.And the results of XPS and EDS show that VO(acac)2can form more inorganic components in the CEI,such as LiF and Li2CO3,and thence provide sufficient protection for the cathode interface during the cycle [195].

On the other hand,peeling off the CEI after surface modifying caused by the electro-strain becomes one of the biggest obstacles to this kind of optimization method[202].Therefore,surface sulfurization and nitridation,which improve the coherence of modification layers,have been highlighted in the cases of the CEI optimization.Through the electrostatic interaction between ions,the sulfidized LNMO surface is successfully synthesized.The 3D porous structure of the sulfidized layer not only plays the protection and isolation role but also adjust the interfacial strain to generate the more stable CEI [203].Compared with the regulation of electrolytes which may introduce more potentially uncontrollable factors,the method of surface modification is more targeted for the CEI,which is easier to track the related modification results.However,the loss of energy density and the barriers to charge transport caused by it are the problems that have to be considered [204].

Fig.18.Characterization results that prove the existence of artificial CEI:(a)SEM images and FTIR spectra for NCM811(I,II,and black)and modified NCM811(III,IV,and blue).Reprinted with permission from Ref.[207].Copyright(2017)Elsevier.(b)Surface and analysis of NCM811 and modified NCM811 by TEM and XPS.Reprinted with permission from Ref.[209].Copyright (2019) Elsevier.

3.3.Artificial CEI

Nevertheless,the optimization of electrolyte and surface modification are all aimed at improving the properties of the CEI to optimize the electrochemical performance of batteries,these approaches are difficult to meet expectations because they do not directly affect the CEI [205].Therefore,the artificial interphases have been directly engineered onto electrode surfaces to stabilize the electrode/electrolyte interface[194,206].And its original purpose,like surface modification,is to protect the electrode material through passivation.However,more recently,researchers have begun to focus on the sacrificial role of artificial CEI during the electrochemical process.Compared with traditional surface modification,it also participates in the electrochemical film formation process on the electrode surface,thereby saving more active substances.A variety of materials including metals,metal salts,organics,etc.have been explored for the construction of artificial CEI.In this regard,we mainly introduce the relatively easy-to-implement artificial CEI based on organics.

It is reported that the sulfonate-based artificial CEI layer(N,N-dimethylpyrrolidinium methyl sulfonate as the CEI precursor) with a 5% coating amount on NCM can make the optimized battery performance,including the better capacity retention rate from 86.5% to 97.4% and the higher coulomb efficiency (99.8%).The conclusion is proven by SEM and FTIR results,as shown in Fig.18a.Comparing the morphology of the electrode surface before and after cycles,the sample after 50 cycles can maintain a good state with the help of artificial CEI.Different from the modification method of simple surface coating,the artificially induced CEI can participate in the formation of primary CEI(the sulfur-oxygen bond in the FTIR spectrum provides the reaction site),thus reducing the loss of original active substances[207].Observed with a high-resolution TEM(HR-TEM)and FTIR,the CEI is inferred to be formed in situ on the Li-rich cathode through the oxidative decomposition of ionic liquid(RTIL)electrolyte doped with a sacrificial fluorinated salt additive(N-methyl-Npropylpyrrolidinium/bis (fluorosulfonyl)imide),which is then proved to improve the long-term capacity and energy retention of the lithium-rich and manganese-based cathode [208].Moreover,a conductive CEI film is formed in situ by electrochemical polymerization of N-methylpyrrole(MPL)on the surface of the LiNi0·5Mn1·5O4electrode,which could prevent the cathode from contacting with the electrolyte directly,thereby effectively reducing the decomposition of electrolyte and improving the cycling performance.And the formation of this artificial CEI is proved by the characterization of SEM,XPS,and ATR-FTIR [32].Another method of constructing artificial CEI on the NCM cathode is to use lithium tetra(trimethylsilyl)borate as the functional precursor,whose information is proven by TEM images and XPS spectrum in Fig.18b.Through TEM,artificial CEI of 5-8 nm can be observed on the electrode surface after induction and before cycling,and the reaction between LTB and NCM can be confirmed by XPS.It proved tetra (trimethylsilyl)borate can slow down the decomposition of electrolyte and remove harmful fluoride,thus bringing good cycling performance for the battery at a high temperature [209].

3.4.The CEI optimization beyond LIBs

At present,these strategies for the CEI optimization has also proved their availability on other batteries beyond LIBs[20,122].For example,Hu et al.put forward the strategy of using unconventional or artificial methods for sodium metal to form the SEI protection layer through additional chemical or physical treatment,which will protect the electrode and inhibit electrolyte consumption [210].With the AlPO4coating,the coulomb efficiency of FeF3·0.33H2O cathodes is improved from 84.26% to 91.39%.Through the microscopic techniques and the analysis of the element distribution,as shown in Fig.19,the improvement of the electrochemical performance is attributed to the effective reduction of the side reaction between the electrolyte and the electrode and the formation of high-quality CEI by the AlPO4coating [211].The surface of the Na0.80Ni0.22Zn0.06Mn0.66O2cathode coated with carbonized polydopamine (C-PDA) shows a higher discharge capacity,better rate capacity,and better cycle stability due to the generated thinner CEI,as suggested by XPS,XRD,and other electrochemical methods [212].Moreover,the pre-sodiation strategies like pre-lithiation techniques are also used to improve the quality of the CEI for SIBs.SEM and XPS measurements have proved that a simple and economical method through direct contact can be used to pre-deposit a thicker CEI (≈30 nm) on the Na0.67Fe0.5Mn0.5O2cathode,which helps to improve the cycling ability and reversible capacity of the battery[213].In this regard,ALD technology is a method that can deposit highly pure homogenous films with well-controlled film thickness and chemical contents.It offers an opportunity to tune the chemical reaction at the interface and to deposit ingredients that are poor effective by other methods [214].In virtue of this technology,a variety of metal oxides have been deposited on the cathode of SIBs,and their effect on the CEI can be demonstrated using EDS and some common electrochemical diagnosis methods.In the cases of the deposition of Fe2O3on NCM [215] or the deposition of Al2O3on Na0.67MnO2[216],these surface coatings all play a role similar to artificial CEI,which ultimately suppress the phase transition during cycles and the excessive consumption of electrolytes.Moreover,the CEI obtained after ALD will be more uniform and denser,which is beneficial to the performance of the battery.

From these examples,the CEI modification in SIBs is more difficult than the optimization of the CEI of LIBs due to difficulties such as the coating is difficult to be sodiation or doping is not suitable [217].However,since the characterization and modification of the CEI can significantly improve the performance of SIBs,more and more researches on SIBs have begun to be in this field [20].

The optimization of the CEI has also been adapted in other emerging kinds of batteries,such as AIBs [218-220],MIBs[221,222],and KIBs [223,224].In MIBs,the lithiummagnesium hybrid electrolyte introduces a much smaller charge-transfer resistance and a faster ion-diffusion rate.A stable CEI is formed on the surface of a sulfur-rich amorphous molybdenum polysulfide (a-MoS5.7)cathode,and it is proved to help to improve the electrochemical performance and stability (retention of 90% over 500 cycles) of the battery by SEM and EIS tests as shown in Fig.20a[222].In KIBs,a solid artificial CEI layer and an inactive K-poor spinel interlayer(as shown in Fig.20b)are constructed on the P2-K0.67MnO2(P2-KMO) cathode in the electrolyte of 6M of potassium bis(-fluorosulfonyl)amide in diglyme (KFSI/G2).Through the decomposition of the aggregated FSI-anion and G2 molecules under high potential,thus realizing the stable electrochemical performance of the potassium battery (retention of 90.5% over 300 cycles) [223].

So far,various methods of regulating the electrode surface and interface beyond LIBs can be introduced into the research of LIBs.The mechanisms and experience observed through the process of characterizing and regulating the CEI of LIBs can also be extended to other ion batteries.For example,drawing lessons from the electrolyte optimization for the CEI of LIBs,the 0.8 mol L-1NaPF6-Propylene carbonate (PC)/EMC system was selected to successfully optimize the surface of NIBs and improve their cycle stability [225].The mutual reference of the CEI research between different kinds of batteries will surely promote the entire development and breakthrough of the whole battery industry.

4.Conclusions and perspectives

There have been intensive R＆D efforts to characterize and improve the performance of secondary batteries through exploring new electrode materials,optimizing the electrolytes,adjusting charge-discharge parameters,etc.Compared with the well-focused SEI researches,there is a great deficiency in the optimization of the CEI.The main dilemma refers to the fundamental understandings of CEI,including components evolution and structural changes.Therefore,advanced characterization techniques are the key to unlock these puzzles to further improve battery performance.

In this literature,the progress of the latest characterization techniques such as synchrotron radiation,cryo-EM,in situ or operando diagnoses,ToF-SIMS,and other achievements in the field of the research on the CEI is summarized.For the characterization and mechanism analysis of the CEI,although there are many difficulties due to the characteristics (such as instability,nonuniformity,thin thickness,etc.),these dilemmas can be gradually broken by introducing new technologies and redeveloping existing technologies.For instance,by shortening the detection time and improving the electrochemical methods to solve and understand the problem of whether the formation of the CEI is dominated by chemical reaction or electrochemical reaction.Moreover,after more accurate identification and in situ characterization or tracking of the components of the SEI and the CEI,it may be possible to clarify the interaction and influence between the compositions.With the improvements of characterization and research methods,many obstacles faced in the research on the CEI have already been overcome.However,there is still significant room for current researches on the CEI to enhance the electrochemical performance of batteries.Thereby,for the development of the battery research,we should devote more attention to the research,characterization,analysis,and regulation of the CEI,such as(I)Improving the existing novel diagnosis methods(cryo-EM,nano-IR,etc.)by improving the resolution or reducing their requirements for samples in order to enhance their versatility in the field of the CEI research (II)Since the low proportion of the CEI to the electrode material(less than 1%),utilizing high-resolution MS (which basically does not meet this detection limit at present)to track and detect is an urgent need(III)Analyzing the mechanism of CEI often requires the collection and analysis of instantaneous states.As mentioned above,combining IR with synchrotron radiation can improve its time resolution to the nanosecond level,but many other spectroscopy techniques in this field have not been developed or even introduced to strengthen the clarification of composition information about the CEI (such as ultra-violet photoelectron spectroscopy (UPS) [9,22],fluorescence spectrum (FS) [93],NMR [226,227],etc.) (IV) Intensifying the analysis of the dynamics and thermodynamic mechanism of the CEI with the help of the thermal analysis (thermogravimetric analysis(TG),differential thermal analysis(DTA),accelerating rate calorimeter(ARC)[94,228],etc.)

In addition,after a preliminary understanding of the CEI,diversified modification methods such as optimizing electrolyte,modifying electrode surface and preparing artificial CEI,etc.have been developed.Although there are still many problems to solve,the ubiquitous adoption of safer,highlyefficient,and longer cycle life batteries benefiting from the CEI optimization are very much anticipated in the near future.But this also requires advanced characterization methods to provide suggestions (I) The optimization of the electrolyte will become more reasonable after a deeper understanding of the species relationship in the CEI (II) The diagnosis of the CEI morphology can provide more suggestions for surface modification (III) Most of the conclusion drawn from current CEI studies may not comprehensively elucidate the mechanism because the characterization methods that they implemented are limited.And it is believed that the underlying mechanism can be more complicated and can be step forward explained when advanced techniques are applied,such as Cryo-EM and in situ/operando observation.Therefore,characterization and optimization are inseparable in the battery industry.

The characterization and regulation of the SEI on anodes have already produced a driving effect on the battery industry,such as exploring the forming mechanism and function of the SEI of various new electrolyte components to point out the direction for the development of electrolyte formulations,improving the cycling performance of the battery by constructing and optimizing the SEI through manual regulation or control,etc.Therefore,the researches on characteristics,mechanism,and interaction relationship of the composition of the CEI on the cathode will definitely bring superior support to the battery industry,especially the gradually emerging advantages during the research of the CEI to inhibit the phase transition of the cathode materials,avoid dissolution of the transition metals and impede the consumption of electrolytes are of great importance for improving the performances of high voltage,long cycling and excellent safety of commercial batteries.Ultimately,the great breakthroughs will be made in the already difficult battery research by the advanced characterization and improved optimization towards the CEI.

Conflict of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This research is supported by the National Natural Science Foundation of China (51804290,22075025,21975026),the Beijing Natural Science Foundation (L182023),the Science and Technology Program of Guangdong Province (Grant No.2020B0909030004),and the Beijing Institute of Technology Research Fund Program for Young Scholars (2019CX04092).