稀有金属(英文版) 2020,39(03),205-217

Atomic-scale structural evolution of electrode materials in Li-ion batteries:a review

Yi-Ru Ji Su-Ting Weng Xin-Yan Li Qing-Hua Zhang Lin Gu

Beijing National Laboratory for Condensed Matter Physics,Institute of Physics,Chinese Academy of Sciences

School of Physical Sciences,University of Chinese Academy ofSciences

Songshan Lake Materials Laboratory

作者简介：Lin Gu,e-mail:l.gu@iphy.ac.cn Researcher at the Institute of Physics,Chinese Academy of Sciences,has been engaged in the field of electron microscopy for nearly 20 years. In 2002,he graduated from Tsinghua University and was enlightened by Academician Jing Zhu.In 2005,he received his PhD from Arizona State University.He then worked as a postdoctoral researcher at the Max-Planck Metal Institute in Stuttgart,Germany,and the Northeastern University in Japan.Since the end of 2010,he has been a researcher of the "Hundred Talents Program" at the Institute of Physics,Chinese Academy of Sciences.After returning to China,he overcame the problems of weak image contrast caused by small scattering cross section of lithium and oxygen atoms and the atomic-scale instability in the in situ experiments.Besides,the lithium ions and oxygen vacancies under external field control are observed by in situ microscopy.And on this basis,the lithium-ion migration is also observed in three-dimensional by in situ microscopy.In recent years, Dr.Lin Gu has been engaged in research on the atomic-scale structure and electronic structure of functional oxide materials,energy storage materials,and nanocatalysts.So far,he has published more than 400 SCI papers (including 9 papers of Science/Nature,more than 40 papers of Science/Nature subjournals,and more than 160 papers whose IF>10),and these papers have been cited more than 16,000 times.He has been awarded the IMC16/IFSM Young Scientists Award (2006),Young Investigator Award from the International Meeting On Lithium Batteries (2012),"Lu Jia-Xi" Young Talents Award from the Chinese Academy of Sciences (2013),Outstanding Science and Technology Achievements Award from the Chinese Academy of Sciences (2013),"Outstanding Youth Fund" from the Fund Committee and the "Young Talents" Program of the Organization Department of the CPC Central Committee (2015),Outstanding Evaluation of the "Hundred Talents Program" of the Chinese Academy of Sciences (2016) and the "Youth Yangtze River Scholar" program of the Ministry of Education in the same year,Youth Science and Technology Award from Chinese Crystallographic Society (2018),and selected as a highly cited researcher by Clarivate Analytics in the field of Materials Science (2018-2019) and Chemistry (2019).;

收稿日期：6 May 2019

基金：financially supported by the National Natural Science Foundation of China (Nos.51672307 and 51421002);the Strategic Priority Research Program of Chinese Academy of Sciences (CAS) (No.XDB07030200);the Key Research Program of Frontier Sciences,CAS (No.QYZDB-SSWJSC035);

Atomic-scale structural evolution of electrode materials in Li-ion batteries:a review

Yi-Ru Ji Su-Ting Weng Xin-Yan Li Qing-Hua Zhang Lin Gu

Beijing National Laboratory for Condensed Matter Physics,Institute of Physics,Chinese Academy of Sciences

School of Physical Sciences,University of Chinese Academy ofSciences

Songshan Lake Materials Laboratory

Abstract：

Owing to the high spatial resolution at the atomic scale,the transmission electron microscopy(TEM)or scanning transmission electron microscopy is demonstrated as a promising characterization method to unveil the charge storage mechanism of electrode materials in Li-ion batteries.The structural evolution of electrode materials during charge/discharge process can be directly observed by using TEM.The detailed analysis establishes a relationship between the structure of electrode material and battery performance.Herein,we present a brief review of the atomic-scale characterization in Li-ion batteries,including Li(de)insertion mechanism(both cations and anions charge-compensation mechanism),migration of transition metal ions,and surface phase transition.The indepth microscopic analysis reveals the detailed structural characteristics,which influence the properties of LIBs,establish the structure-function relationship,and facilitate the development of Li-ion batteries.

Keyword：

Li(de)insertion mechanism; Migration of transition metal ions; Surface structural evolution;

Received： 6 May 2019

1 Introduction

Since the invention of commercial lithium-ion batteries(LIBs) by Sony in 1991[1],LIBs have been widely used in various portable electronic devices due to their unique performance advantages.Furthermore,the increasing demand for large-scale energy storage from renewable energy sources,such as solar and wind energy,and environmental concerns have fueled the research interest in next-generation LIBs with enhanced energy density and excellent cyclic performance.Therefore,several research groups aimed to improve the energy density of LIBs by developing novel electrode materials.It is worth mentioning that the structure of electrode material plays a critical role in determining the lithium (de)insertion behavior and overall performance of LIBs.

Over the last ten years of intense investigation,many key events in electrode characterization have been documented at the atomic level by transmission electron microscopy (TEM),giving rise to an unprecedented level of clarity in aspects of the Li-ion diffusion and (de)intercalation processes in a wide range of electrode materials.In particular,high-angle annular dark-field (HAADF-STEM)and annular bright-field scanning transmission electron microscopy (ABF-STEM) have enormously facilitated the structural analysis of electrode materials during different stages of the charge/discharge process[2,3].Even though nuclear magnetic resonance (NMR) and Raman spectroscopy provide the structural details,such as the environment of an atom and chemical bonding,TEM presents a straightaway picture by actually“visualizing”the atoms.Hence,it is of utmost significance to understand and advance the role of TEM-based characterization techniques to design novel electrodes materials for next-generation LIBs.In the current review,we focus on the atomic-scale characterization of Li-ion migration in the electrode material,including Li (de)insertion mechanism (both cations and anions charge-compensation mechanism),migration of transition metal ions,and surface phase transitions.

2 Li (de)insertion mechanism

The detailed understanding of electrode structure,at the atomic scale,is required to develop advanced high energy density batteries.However,imaging of light elements is a challenging task due to the limitation of HAADF-STEM.The development of annular bright-field scanning transmission electron microscopy (ABF-STEM) has solved this problem.In the case of LIBs,lithium (Li) and oxygen(O) are the light elements,which need to be imaged to unveil the migration of lithium ions and distortion of the host structure.In 2010,Oshima et al.[4]first observed Li ions by using ABF-STEM in LiV₂O₄ crystal.Then,the insertion of Li ions in different electrode materials,including TiNb₂O[5],LiFePO₄[6],LiMn₂O₄[7],LiCoO₂[8],and La_2/3-xLi_3xTiO₃[9],has been observed at the atomic scale by using ABF-STEM.As a result,Li (de)intercalation channel and Li migration mechanism have been established for a wide variety of electrode materials.For instance,Gu et al.[10]have observed lithium staging structure in partially delithiated LiFePO₄ single-crystal nanowires,as shown in Fig.1.The Li staging structure,where Li ions preferably occupied every second layer in partially delithiated LiFePO₄,along the b-axis (Fig.lc),is analogous to the staging phenomenon in layered intercalation compounds.To unveil the formation mechanism of Li staging structure,Suo et al.[11]have studied a highly ordered interface between LiFePO₄ phase and FePO₄ phase with staging structure,along the a-axis,in partially delithiated Li_0.90Nb_0.02FePO₄.They have observed that the Li staging structure is an intermediate phase or an intrinsic metastable phase at the LiFePO₄/FePO₄ interface.Furthermore,Sun et al.[12]have proposed the dual-interface structure to describe the delithiation mechanism of LiFePO₄,which can reproduce the LiFePO₄/staging/FePO₄three-phase configuration[11,13].However,Niu et al.[14]have observed a Li sublattice disordered solid solution zone (SSZ),without lithium staging structure,by using in situ high-resolution TEM (HRTEM),which confirms that the Li staging structure during the delithiation of LiFePO₄ can be ascribed to the equilibrium relaxation.These results reveal the importance of in situ structural observation to understand the electrochemical reactions[15-21].

Moreover,the existence of multi-phases during charge/discharge process of LIBs,as reported for various materials,has also been demonstrated by TEM-based techniques.For instance,Lu et al.[22]have observed two different phases at the half-charged state of electrochemically lithiated Li₄Ti₅O₁₂,as shown in Fig.2.It can be readily observed that Li₄Ti₅O₁₂ (Region 1) and Li₇Ti₅O₁₂ (Region2) phases coexist,as demonstrated by different lattice spacings of two phases (Fig.2a).The profiles,corresponding to different regions (Fig.2b,c),clearly demonstrate that Li ions migrated from 8 a site to 16c site without displacing Ti and O atoms,which confirms the coexistence of the initial Li₄Ti₅O₁₂ phase and the following Li₇Ti₅O₁₂phase.The yellow curve in Fig.2a indicates the sharp phase boundary between the Li₄Ti₅O₁₂ and Li₇Ti₅O₁₂phases without any obvious displacement of Ti and O atoms,which implies the zero-strain characteristic during lithiation/delithiation process[23,24].Moreover,the coexistence of both phases is also supported by first-principles calculation,as shown in Fig.2e.

Fig.1 ABF-STEM images of partially delithiated LiFePO₄,where Li ions are present at every other row:a pristine material with atomic structure of LiFePO₄ in inset;b fully charged state and atomic structure of FePO₄;and c half-charged state and atomic structure of Li_0.5FePO₄,showing Li staging (Li sites marked by yellow circles and delithiated sites marked by orange circles).Reproduced from Ref.[10]by permission from American Chemical Society

Fig.2 Interfacial structure in chemically lithiated Li₄Ti₅O₁₂ with～0.15 mol of Li inserted per fomula unit along[110]direction:a ABF image near interface between Li₄Ti₅O₁₂ phase (Region 1) and Li₇Ti₅O₁₂ phase (Region 2),where yellow line indicates interfacial boundary;ABF line profiles of b Region 1 and c Region 2;d colored ABF image of two phases near interface,where 8a sites,occupied in Li₄Ti₅O₁₂,and 16c sites,occupied in Li₇Ti₅O₁₂,are marked as yellow and black dots,respectively;e relaxed interfacial structure simulated by DFT calculations,where Li_16d-O octahedrons are shown in green,and remarkable shift of Li ions at 16c sites in Li₇Ti₅O₁₂ region is indicated by black arrows.Reproduced from Ref.[22]by permission from John Wiley and Sons

In addition,Li₄Ti₅O₁₂ anode has also been explored for Na-ion battery (NIB),which is a promising alternative to LIBs due to the abundant Na resources and low cost[25,26].It has been reported that Li₄Ti₅O₁₂ anode in NIBs experiences three different phase transitions during electrochemical sodiation[27].Later,Sun et al.[28]have visualized the three phases by using STEM in a half-discharged Li₄Ti₅O₁₂ nanoparticle (Fig.3),including Li₄Ti₅O₁₂ (Li4) phase (Fig.3b-d),Li₇Ti₅O₁₂ (Li7) phase(Fig.3e-g),and Na₆LiTi₅O₁₂ (Na6Li) phase (Fig.3h-g).As shown in Fig.3,Li ions migrated from 8 a site (Fig.3d)to 16c site (Fig.3g),forming the Li7/Li4 phase,and Na ions occupied 16c site (Fig.3j) and formed Na6Li phase.It can be readily observed that the two-phase boundary of Li7/Li4 and Li7/Na6Li phases is sharp and dislocation free(Fig.31,m),which is consistent with other reports (Fig.2).These studies emphasize the importance of directly observing the interfacial structure between different phases at the atomic scale,which plays a critical role in understanding the ionic transport and movement of phase boundaries during phase transition.

In general,Li charge-compensation mechanism in welldeveloped materials is elucidated by cations redox reactions.Nevertheless,in recent years,anions charge-compensation mechanism has gradually aroused researchers'attention.For instance,partial oxidation of oxygen has been speculated due to the reversibility of anionic redox processes (20^2-→ ,where n=1,2 or 3)[29-31].However,there was no direct evidence for the presence of peroxo-like structure in layered oxides until the development of ABF-STEM technique,which can visualize the light elements,such as Li and O ions.McCalla et al.[32]have directly visualized the O-O dimers in Li₂IrO₃ via ABF-STEM,as shown in Fig.4.The highly ordered honeycomb structure of the Ir cations has been clearly observed in charged Li₂IrO₃,and the symmetry of LiO₆octahedra is expected to be sixfold with the same length of O-O bonds (Fig.4a).These results reveal that the (O-O)ⁿperoxo-like dimers,visualized at the atomic scale by ABF-STEM,are responsible for the enhanced capacity of Li-rich layered electrode materials (Fig.4b,c).However,instead of expected sixfold symmetry,the shape of LiO₆ octahedra exhibited threefold symmetry due to the shortened (black dumbbells) and lengthened (left blank) O-O bonds(Fig.4d).The distinct difference in the short and long O-O bonds is presented in Fig.4e.This result established the fundamental relationship between the anionic redox process and evolution of the O-O bonding in layered oxides.

Fig.3 STEM images of a three-phase coexistence region:a crystal structure of spinel Li₄Ti₅O₁₂ viewed from[110]crystallographic direction,showing separated Ti and O columns,where Ti1 and Ti2 columns,with different atomic densities,are represented by balls of different sizes;b HAADF,c ABF images and d ABF line profile of Li₄Ti₅O₁₂ (Li4) phase;e HAADF,f ABF images and g ABF line profile of Li₇Ti₅O₁₂ (Li7)phase;h HAADF,i ABF images and j ABF line profile of Na₆LiTi₅O₁₂ (Na6Li) phase,k ABF image of a half-charged Li₄Ti₅O₁₂ nanoparticle;line profiles crossing I Li7/Li4 (Line A) and m Li7/Na6Li (Line B) boundaries(in ABF line profile,contrast is inverted to assist convenient visualization with scale bar=2 nm).Reproduced from Ref.[28]by permission from Springer Nature

Recently,disordered structure of electrode materials has garnered research interest due to its structural stability.In general,the electrodes,with well-ordered structure,are expected to deliver excellent cyclic performance.However,Ceder et al.have demonstrated that the Li-rich cathodes,with cation-disordered rock salt structure,exhibit high specific capacity,which led to increasing research efforts in designing high energy density LIBs[33].Zhao et al.[34]have designed a novel cation-disordered Li-rich cathode,LiTiNiNbOF,which exhibited high capacity due to the structural stability and immutability of Li local environment upon Li (de)intercalation.Moreover,they have utilized in situ X-ray diffractometer (XRD) and ex situ X-ray absorption spectroscopy (XAS) characterization techniques to demonstrate the oxidation of cationic ions (Ni²⁺) and anionic ions (O^2-) at the atomic scale,which provides charge compensation for Li (de)intercalation in LTNNbOF during the first charge cycle.These results demonstrate the unique advantages of ABF-STEM in direct imaging of lightweight elements,such as Li and O,at the atomic scale(red) projected distances.Reproduced from Ref.[32]by permission from The American Association for the Advancement of Scienceand providing convincing answers to several unresolved research questions related to the charge storage mechanism of the electrode materials.Hence,the development of nextgeneration LIBs is significantly promoted by the advancement of atomic structural characterization techniques[35-40].

Fig.4 Structural changes in oxygen sublattice:a[010]HAADF-STEM image of charged Li_0.5IrO₃ electrode,demonstrating ordered sequence of Ir layers corresponding to O1-type structure (hexagonal close packing is evident from absence of lateral displacement of layers;moreover,virtually no migration of Ir cations to Li layers is observed,whereas few anti-site point defects are marked with arrowheads);[001]b HAADF-STEM and c ABF-STEM images of same sample (taken from different areas;noise in ABF-STEM image is suppressed by applying a low-pass Fourier filter);d enlarged ABF-STEM image (O-O pairs with short projected distances are marked with dumbbells;O-O pairs arise from twisting opposite triangular faces of IrO6 octahedra,as shown in yellow);e ABF intensity profiles,along O-O pairs,with long (blue) and short

3 Migration of transition metal ion

In the electrode material,the migration of transition metal(TM) ions during the charge/discharge process is inevitable,which damages the electrode structure and leads to rapid capacity decay.Therefore,it is necessary to directly visualize the migration of TM ions,at the atomic scale,to further understand the structural dynamics during electrochemical cycling.In the case of Li₂MnO₃,Wang et al.[41]have discovered that few manganese atoms can migrate into the Li layer after electrochemical delithiation.One should note that the Li atoms,occupying 2c and 4h sites,constitute the Li plane in Li₂MnO_3.In pristine Li₂MnO₃,the line profiles and images of Li plane (Line 2 in Fig.5a,d) exhibit the uniform existence of lithium at these sites.After electrochemical delithiation,the line profiles of lithium plane (Line 2 and 4 in Fig.5b,e) exhibit some gray spots,marked by blue circles,due to the occupancy of Mn with strong contrast.These results confirm the atomic-sc ale migration of Mn into lithium planes.Then,after discharge,the line profiles (Line 2 in Fig.5c,f) show the same image contrast of lithium due to the disappearance of migrated Mn from the lithium layer and lithium atoms have been restored at their original positions,indicating the reversible migration of Mn into Li layers.It is worth noting that these observations are significantly different from the previous research[42].

Furthermore,Sathiya et al.[31]have utilized TEM and X-ray photoelectron spectroscopy (XPS) to reveal that the migration of cations between metal and Li layers is an intrinsic and inseparable part of charge/discharge process.As a result,the voltage fading can be also correlated with the intrinsic feature,which increased the trapping of metal ions in interstitial tetrahedral sites.However,a large amount of transition metal migration in different electrodes is not an intrinsic feature and causes severe capacity fading due to its irreversibility.

In recent years,layered lithium transition metal oxides (LTMO) have gained considerable research attention as one of the promising cathode materials for next-generation LIBs [43,44].However,LTMO suffers from severe capacity and voltage decay due to the migration of transition metal ions.Yan et al.[45] have discovered a direct correlation between ionic migration and lattice disorder in LiNi1/3Mn1/3Co1/3O2.As shown in Fig.6,Ni ions preferentially migrated into the Li layer rather than Mn and Co, which mainly contributed to the lattice disordering.To further explore the mechanism of Ni-rich Ni-Co-Mn(NMC) ternary material,Lin et al.[46] have investigated the property of Ni-rich LiNi0.8Mn0.1Co0.1O (NMC811) cathode.It has been demonstrated that Ni migrated from bulk to surface and dissolved into the electrolyte,resulting in a continuous electrochemical performance decay due to the low diffusion barrier of Li layer and Ni concentration gradient in lattice.These results reveal the unstable electrochemical performance of Ni-rich NMC electrode during charge/discharge process.Hence,even though Ni-rich NMCs exhibit the highest theoretical energy density among NMC-based ternary materials,the poor cyclic stability hinders their practical utilization in LIBs [44].The detailed investigation of TM migration mechanism can assist in improving the cyclic performance by suppressing the migration of transition metal ions.

Fig.5 STEM images and corresponding line profiles of Li ions in Li₂MnO₃ after electrochemical lithiation and delithiation:a,d pristine Li₂MnO₃;b,e Li₂MnO₃ charged to 4.8 V;and c,f Li₂MnO₃ discharged to 2 V after being charged to 4.8 V.Reproduced from Ref.[41]by permission from John Wiley and Sons

Fig.6 Atomic resolution STEM-EELS mapping:a pristine NMC333 without cycling;NMC333 after 100 charge/discharge cycles with a high cutoff voltage of 4.8 V:b well-preserved layered region, c disordered region,and d heavily disordered region (dashed blue frame at bottom right highlights TM maps with disordered structure; white arrows in b indicate a correspondence between STEM-HAADF image and EELS map to identify migrated Ni atoms to Li layer). Reproduced from Ref.[45] by permission from American Chemical Society

4 Surface structural evolution

It is worth mentioning that the surface structure of the electrode material is different from the bulk structure due to the occurrence of broken symmetry,which significantly influences the electrochemical performance.Moreover,the electrode surface reacts with the electrolyte during the charge/discharge process of LIBs.Some of these reactions result in the formation of stable structures,i.e.,the formation of SEI,whereas others cause damage to the electrode material,which is harmful to the battery performance and raises safety concerns.Therefore,the influence of electrochemical cycling on the surface morphology of the electrode material should be systematically studied to better understand the performance degradation of LIBs.

In 2012,Lu et al.[47]have investigated a notable cation rearrangement at the surface of LiCoO₂.When a fully charged (4.5 V) electrode was discharged to 3.0 V,the LiCo anti-sites (rock salt structure) appeared in the outermost surface region.The occupation of lithium sites with Co ions hinders the lithium diffusion channel and results in an increased charge-transfer resistance,leading to severe capacity decay.In addition to LiCoO₂,several layered cathode materials also suffer from the presence of Li-TM anti-sites,which is often accompanied by irreversible reactions with a layered structure,resulting in spinel or rock salt-like structure.In order to illustrate the rearranged surface structure,Feng et al.have reported the structural reconstruction and chemical evolution at the surface of LiNi_xMn_xCo_1-2xO₂ particles,which resulted in the formation of a surface reduced layer, transition and a surface reaction layer,respectively[48].The structural reconstruction and chemical evolution are primarily responsible for the prevailing capacity decay and higher impedance under high-voltage cycling conditions.As shown in Fig.7,a surface reconstruction layer has been observed after electrolyte exposure (Fig.7a).After one complete charge/discharge cycle,the thickness of the surface reconstruction layer increased (Fig.7b).These observations indicate that the surface reconstruction layer originated from both electrode/electrolyte reaction and electrochemical activation.By analyzing a large number of ADF-STEM images,the authors have demonstrated that the surface reconstruction layer has been significantly influenced by crystal orientation.The thicker surface reconstruction layer has been observed along the Li diffusion channels,whereas a relatively thinner surface reconstruction layer has been found along other orientations,i.e.,(001),as shown in Fig.7e.In addition,few dangling layers (loose atomic layers) have also been on the external surface of the reconstruction layers (Fig.7f).These loosely attached atomic layers dissolve into the electrolyte,which may be ascribed to the Mn²⁺dissolution in Mn-containing cathodes[49].The similar phenomenon has also been observed in Ni-rich NMC electrode,which suffered extremely severe surface distortion,with the formation of cracks[50,51].

Fig.7 Atomic resolution ADF-STEM images of NMC particles:a after electrolyte exposure (exposure time was～30 h,which is equivalent to time required for one complete charge/discharge cycle);b after 1 cycle (2.0-4.7 V),where blue arrow indicates surface reconstruction layer;c,d FFT images from b,showing surface reconstruction layer (Fm3m[110]zone axis) and NMC layered structure (R3m[100]zone axis),respectively;e variation of surface reconstruction layer thickness with respect to orientation after 1 cycle (2.0-4.7 V);and f loose atomic layers on outermost surface of reconstruction layer after 1 cycle (2.0-4.7 V)(blue lines indicate boundaries between NMC layered structure and surface reconstruction layer).Reproduced from Ref.[48]by permission from Springer Nature

In the case of spinel LiMn₂O₄,it is a typical commercial cathode material for LIBs,which suffers from Mn²⁺dissolution on the LiMn₂O₄ surface.Several studies have demonstrated that Mn dissolution rate increases with an increase in the state of charge[52,53].Tang et al.[54]have investigated the surface structure of LiMn₂O₄ at different charge and discharge states.In the pristine LiMn₂O₄,the surface structure is identical to the bulk structure.During charging,a spinel Mn₃O₄ phase emerged on the surface of LiMn₂O₄,which is a mixed valence compound with Mn²⁺and Mn³⁺,occupying the tetrahedral and octahedral sites,respectively.Since the Mn₃O₄ phase contains 1/3 Mn²⁺,the increased amount of Mn₃O₄resulted in Mn dissolution.Interestingly,when the electrode material was charged to 5.1 V,instead of spinel Mn₂O₃,a layered-like phase has appeared on the surface of LiMn₂O₄[55]and further damaged the surface structure.Hence,the voltage window of spinel LiMn₂O₄ is limited.

To further illustrate the phase transitions on the surface of LiMn₂O₄,Ben et al.[56]have directly observed an unusual layered Li₂MnO₃ (C2/m) phase (surface region5-6 nm) on LiMn₂O₄ cathode after cycling (3.0-4.9 V),as shown in Fig.8.In the normal voltage window(3.0-4.3 V),the subsurface of LiMn₂O₄ maintained the spinel structure after electrochemical cycling (Fig.8a,a₁)and the surface is converted into defective LiMn₃O_4-like structure (Fig.8a,a₂).However,after cycling in a larger voltage window (3.0-4.9 V),the surface is converted into layered Li₂MnO_3-like structure (Fig.8b,b₂),whereas the subsurface of LiMn₂O₄ is transformed into defective LiMn₃O₄-like structure (Fig.8b,b₁).Based on Fig.8a,b,the possible diffusion pathways of Mn-ion on LiMn₂O₄surface are discussed in Fig.8c-f.During initial cycling between 3.0 and 4.3 V,the migration of Mn ions from Mn octahedral sites (Fig.8c) to Li tetrahedral sites (Fig.8d)induced surface distortion,forming defective LiMn₃O₄-like structure.On the surface of LiMn₃O₄-like structure,Mn ions are present at Mn tetrahedral and Mn octahedral sites.Once the voltage window is further increased to 3.0-4.9 V,Mn ions partially migrate from tetrahedral and octahedral sites and form an Mn-deficient LiMn₃O₄-like structure(Fig.8e) and a new layered-like structure (Fig.8f).Moreover,the existence of Mn³⁺on the surface of fully charged LiMn₂O₄ particles suggests that oxygen loss might occur on the surface of LiMn₂O₄ to compensate for the reduction of Mn⁴⁺to Mn³⁺.The increase in Mn³⁺with cycling voltage increasing indicates that more oxygen might be lost at a higher voltage.In conclusion,Mn³⁺plays an important role in an unusual structural transformation in spinel LiMn₂O₄ cathode,which may lead to theloss of surface oxygen due to the specific environment.The unusual spinel to the layered-like transformation of spinel LiMn₂O₄ determines the rapid capacity decay in LIBs.

Fig.8 a HAADF-STEM images of LiMn₂O₄ (3-4.3 V),where a₁ and a₂ refer to surface and subsurface regions,corresponding to red and blue boxes in a,respectively;b HAADF-STEM images of LiMn₂O₄ (3-4.9 V),where b₁ and b₂ represent surface and subsurface regions,corresponding to red and blue boxes in b,respectively.HAADF-STEM images and corresponding simulated crystal structures,showing an increased distortion of spinel LiMn₂O₄ in surface and subsurface regions with cycling voltage window increasing from 4.3 to 4.9 V:c₁ HAADF image of standard LiMn₂O₄ spinel (e.g.,subsurface region in a) and c₂ corresponding simulation of standard LiMn₂O₄ spinel crystal structure;d₁HAADF image of defect-spinel LiMn₃O₄-like structure,cycled in voltage window of 3-4.3 V,(e.g.,surface region in a) and d₂ correspondingsimulation of defect-spinel LiMn3O4-like crystal structure;e1 HAADF image of defect-spinel LiMn3O4-like structure,cycled in voltage window of 3-4.9 V,(e.g, subsurface region in b) and e2 corresponding simulation of Mn-deficient LiMn3O4-like crystal structure (herein,contrast associated with Mn(t)(purple cycles) and Mn2 (red cycles) is much weaker than the standard defect-spinel LiMn3O4,suggesting Mn deficiency in structure;f1 HAADF image of layered-like structure,cycled in voltage window of 3-4.9 V,(e.g.,surface region in b) and f2 corresponding simulated layered-like structure.Reproduced from Ref.[56] by permission from American Chemical Society

To avoid the severe surface problems of cathodes,a wide range of methods,such as coating and doping,have been adopted to impede the surface reconstruction.Carbon coating of electrode material is a commonly used strategy to enhance the capacity and cyclic stability of battery materials.Hu et al.[57]have elucidated the mechanism of high-capacity graphene-modified LiFePO_4.It has been observed that graphene layers exhibited a more disordered structure after discharging than the charging state (Fig.9a,b).For instance,a large number of layers are randomly oriented with respect to the surface of the carbon-coated LiFePO₄ (cLFP) particle after discharge.Moreover,the interlayer distance between graphene sheets increased from0.32 nm (charged state) to 0.38 nm (discharged state),which indicates the intercalation of Li ions between fewlayer electrochemically exfoliated graphene (EG) after discharge.In addition,a hexagonal periodicity of the graphitic layers,at the edge of the cLFP particles,has been observed,as shown in Fig.9c.The electrochemical data demonstrate that Li ions can be reversibly intercalated into highly disordered graphene nanosheets,which results in a higher capacity of 208 mAh·g-1.One should note that the achieved specific capacity of LiFePO₄ is significantly higher than the theoretical value of 170 mAh g^-1.

These results indicate that the active cathode material,incorporated into EG flakes,renders enhanced Li-ion storage capacity,which is reversible and results in improved cyclic performance.Nevertheless,the ability of coating is not limited to the enhanced capacity of the electrode material;Jung et al.[58]have converted lithiumfree transition metal monoxides,without a lithium-conducting path,into a high-capacity positive electrode by coating with nanosized lithium fluoride.This unusual electrochemical behavior can be attributed to the reaction mechanism at the electrode surface,which offers a novel pathway to design high-performance electrodes for LIBs.In conclusion,the TEM technique of atomic observation can be used to design a highly effective method of surface modification.

It is also worth noticed that not all of the reconstruction on electrode surface is due to the reaction with electrolyte.Electron beam irradiation is also an important factor that cannot be ignored during the TEM imaging.Lozano et al.[59]have studied reconstruction in the Li-rich cathode surface caused by electron beam irradiation.It has been observed that after long irradiation,the surface of cathode is shown as an amorphized surface,and several artifacts in the most damaged areas of the particles (Fig.10a,b).In addition,the electron beam-induced reconstruction is easier to happen in charged Li-ion batteries.This reminds us that STEM measurement could induce the structure reconstruction unless the irradiation dose is controlled at a gentle level.

Fig.9 High-resolution TEM images of carbon-coated LiFePO₄ cathode material with 0.8 wt%graphene a after charging (Li⁺extraction),where yellow-colored square indicates amorphous carbon coating and white-colored square represents added graphene layers,and b after discharging(Li⁺insertion)(scale bar=1 nm),where interlayer distance between graphene sheets increased and led to disordered stacking;c in-plane view of graphitic layers at edge of carbon-coated LiFePO₄ particles,which shows a possible indication of hexagonal lattice that is typically formed due to intercalation of Li ions into graphite sheets.Reproduced from Ref.[57]by permission from Springer Nature

5 Conclusion

The establishment of structure-functionality relationship in battery materials is advanced due to the development of several atomic-scale characterization techniques,such as transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM),including high angular annular dark-field (HAADF) image and annular bright-field (ABF) image.Moreover,the utilization of these material characterization techniques improved the understanding of relationship between structure and functionality,which led to the design and development of novel electrode materials for next-generation Li-ion batteries.Herein,we have reviewed the utilization and role of electron microscopy,in general,TEM,HAADF-STEM,and ABF-STEM,in particular,in the development of Li-ion batteries.TEM-based techniques have been used to elucidate the mechanism of lithium intercalation/deintercalation,transition metal ion migration,and surface transformation during the charge/discharge process.In traditional electrode materials,such as LiFePO₄,Li₄Ti₅O₁₂,Li₂MnO₃,LiMn₂O₄,and LiCoO₂,two-or threephase interfaces and surface structure have been clearly observed at the atomic scale,which provided useful insights into the charge storage behavior and possible capacity degradation.

Throughout the discovery of lithium-ion migration at the atomic scale,the working mechanism of these electrodes is clearly emerged.For instance,the atomic-scale visualization of reversible Li-ion (de)insertion channel and staging Li_0.5FePO₄ structure explained the high capacity and stable performance of commercially used LiFeO₄ cathode material.Moreover,the undesirable capacity decay in LiFeO₄ cathode can be ascribed to the observed strained interface.On the other hand,a strain-free interface has been observed in Li₄Ti₅O₁₂,which explains the stable structure for lithium (de)insertion process.In the case of Li₂MnO₃,LiMn₂O₄,and LiCoO₂,transitional mental (TM) migration and the presence of Li-TM antisites structure increased the charge-transfer resistance and led to rapid capacity decay during the charge/discharge process.It is worth mentioning that the structure-functionality relationships in traditional electrode materials have rarely been directly visualized until the realization of STEM.

Furthermore,several research groups aimed to investigate ternary transition metal oxides (NCMs),instead of only one transition metal,due to their high working voltage and high theoretical capacity,which are highly desirable for the next-generation high energy density LIB s.The atomic-scale observations play a critical role in determining the structure-functionality relationship between different proportions of NCM.For instance,the actually poor cyclic performance of Ni-rich NCM cathode has been unveiled at atomic scale,which clearly demonstrated that the intercalation/deintercalation of Li ions destroyed the atomic structure and formed surface cracks.In addition,the electrode materials,with anions charge-compensation mechanism and disordered structure,have also garnered significant research interest due to its superior performance.

Fig.10 ADF-STEM images of beam-induced damage a before,b after,and c reconstructed phase image after long irradiation using a beam current of 6 pA (grayscale:0 to-1.33 rad).Reproduced from Ref.[59]by permission from American Chemical Society

In general,we showed how the atomic-scale observation techniques TEM could be used to understand the charge storage mechanism and elucidate the structural changes and valence-state evolution during the charge/discharge process in LIBs.At the same time,new methods in TEM have developed to better understand the deeper mechanism in LIBs.(1) In situ TEM is a critical method to show the real status during the reaction,including in situ electricity,in situ heating,and in situ gas reaction,which is more persuasive than ex situ TEM by showing direct evidence.(2) Developing a brand-new technology of TEM,for example,convergent beam electron diffraction (CBED),is also an important way to further understand the information of atom orbital and acquire the electronic structure in reciprocal space.(3) Cryo-TEM is also an important way to decrease the electron beam damage.In future,combining with the above-mentioned technologies,the atomic-scale observations by using TEM,HAADF-STEM,and ABF-STEM,can be better applied to discover higher-performance electrode materials for next-generation Li-ion batteries.

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