Construction of Stable Oxygen Redox by Electrochemical Activation O–TM–Se in Nickel‐Rich Layered Oxides for Lithium‐Ion Batteries

The irreversible oxygen redox and structural degradation of LiNi0.8Co0.1Mn0.1O2 (NCM811) at a 4.5 V high voltage cause a severe decline in cycling performance for lithium‐ion batteries. In this study, a novel approach is proposed to enhance the anionic redox chemistry and stability of NCM811 cathode material by introducing gaseous selenium. Seβ+ species are selectively adsorbed within oxygen vacancies, leading to the continuous replacement of Oα− to form a stable O–TM–Se bond during deep charging. Furthermore, Selenium modification improves cationic redox efficiency and alleviates Oα− (α < 2) outward migration, increases oxygen vacancy formation energy. The redox activity of oxygen is diminished, facilitating improved reversibility of oxygen redox and effectively inhibiting irreversible oxygen escape. Additionally, Selenium increases the energy barrier for phase transition, effectively suppressing irreversible phase transition and Ni migration. Selenium reacts with escaping oxygen to form SeO2, effectively reducing side reactions during cycling. As a result, the proposed approach significantly inhibits irreversible oxygen release, leading to remarkable cyclic stability with 87.5% capacity retention after 300 cycles at 1C at 4.5 V and maintained 192.9 mAh g−1 after 150 cycles under 60C. The Se modification realizes stability anionic redox strategy to design novel high‐energy‐density cathode materials with superior cycling performance.


Introduction
The nickel-rich layered metal oxide cathode material, LiNi 1-x-y Co x Mn y O 2 (x þ y ≤ 0.4), particularly LiNi 0.8 Co 0.1 Mn 0.1 O 2 , has gained significant attention as a promising candidate for high-energy-density lithium-ion batteries due to its reversible capacity The irreversible oxygen redox and structural degradation of LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811) at a 4.5 V high voltage cause a severe decline in cycling performance for lithium-ion batteries.In this study, a novel approach is proposed to enhance the anionic redox chemistry and stability of NCM811 cathode material by introducing gaseous selenium.Se βþ species are selectively adsorbed within oxygen vacancies, leading to the continuous replacement of O αÀ to form a stable O-TM-Se bond during deep charging.Furthermore, Selenium modification improves cationic redox efficiency and alleviates O αÀ (α < 2) outward migration, increases oxygen vacancy formation energy.The redox activity of oxygen is diminished, facilitating improved reversibility of oxygen redox and effectively inhibiting irreversible oxygen escape.Additionally, Selenium increases the energy barrier for phase transition, effectively suppressing irreversible phase transition and Ni migration.Selenium reacts with escaping oxygen to form SeO 2 , effectively reducing side reactions during cycling.As a result, the proposed approach significantly inhibits irreversible oxygen release, leading to remarkable cyclic stability with 87.5% capacity retention after 300 cycles at 1C at 4.5 V and maintained 192.9 mAh g À1 after 150 cycles under 60C.The Se modification realizes stability anionic redox strategy to design novel high-energy-density cathode materials with superior cycling performance.materials, an effective strategy involves modifying the redoxactive centers by altering the TM-O interaction and oxygen redox behavior.For instance, the utilization of highly covalent TM ions such as Ru and Ir, as alternative cation redox-active centers can form more flexible covalent bonds with oxygen, thus enhancing structural reversibility. [17,18]However, the selection of suitable TM ions is limited to noble and rare elements.Hence, in the pursuit of a cost-effective and efficient approach for reversible anion redox during long-term cycling, the Ni-rich materials stabilizes O as a redox-active center, thereby improving energy density and battery stability. [19,20]As a monomer with a relatively low melting point (217 °C), selenium (Se) has the potential to permeate the interstices of secondary particles, occupying oxygen vacancies and forming metal-Se bonds.Chu et al. employed a combination of solid-phase mixing and low-temperature sintering to deposit a 35 nm thick Se coating onto LiNiO 2 cathode material. [21]By suppressing phase transitions, stabilizing interfaces, enhancing electrode kinetics, and mitigating particle pulverization, the Se coating elevates the overall structural integrity of the cathode materials.In a similar vein, Zhu et al. applied a Se coating to commercial LiCoO 2 particles, effectively curbing the release of oxygen from LiCoO 2 .During deep charging, the pre-coated selenium intervenes by substituting for mobile O αÀ species at the charged LiCoO 2 surface.This process transfers the accumulated charges from O αÀ , thereby reverting them back to O 2À .This serves to stabilize the oxygen lattice over extended cycling periods, contributing to enhanced stability and prolonged cycling performance. [19]ere, a stable O-TM-Se bond was formed through the adsorption of Se βþ on the oxygen vacancy by wet ball-milling and low-temperature calcination.During high-voltage charging, the charged Se particles (Se !Se βþ ) continuously replace O αÀ , effectively reducing oxygen vacancies and converting O αÀ into immobile O 2À .The process successfully inhibited oxygen migration, increased oxygen vacancy formation, and inhibited structural degradation during the long cycle.Additionally, the Se in the outer layer absorbs escaping oxygen, forming SeO 2 and preventing electrolyte decomposition.Benefit for this, under 4.5 V, the Se-modified Ni-rich cathode material maintains a capacity retention rate as high as 87.5% after 300 cycles at 1C, and a capacity retention rate of 89.3% after 150 cycles at 60C.

Results and Discussion
The NCMSe electrode material was prepared through a simple mechanical mixing process followed by low-temperature calcination (Figure S1, Supporting Information).Scanning electron microscopy (SEM) images of NCM811 (Figure 1a) and NCMSe (Figure 1c) samples clearly illustrate the morphological and microstructural differences between the particles before and after modification.Compared to the rough surface of NCM811 materials, the Se-modified material exhibits a coated surface layer.High-resolution transmission electron microscopy (HRTEM) images of NCM811 and NCMSe reveal that there is no coating present on the exterior of NCM811.The internal lattice fringes correspond to the (104) crystal plane with a plane spacing of 0.204 nm (Figure S2a, Supporting Information).In the case of NCMSe, the lattice fringes within both region 1 and region 2 display the same pattern, corresponding to the (101) crystal plane with a spacing of 0.245 nm (Figure S2b, Supporting Information).The crystal phase and microstructure of NCM811 (Figure 1b) and NCMSe (Figure 1d) were further characterized using high-angle angular dark field-scanning transmission electron microscopy (HAADF-STEM).The layered structures (spatial group: R-3m) of both samples remained intact, and atomic-level phases of the transition-metal (TM) and lithium (Li) layers were clearly observed. [22]Elemental mapping confirms the uniform distribution of Ni, Co, Mn, O, and Se on the surface of NCMSe.Ball-milling and high-temperature calcination were likely employed to distribute Se throughout the bulk, as evidenced by Se atoms approximately 20 nm inward (Figure 1e).It should be noted that the asymmetrical distribution of Se between the surface and the interior can be attributed to the residual Se present on the surface. [23]ietveld refinement analysis confirmed the presence of Se in the oxygen site and an increase in Ni 3þ concentration (Figure 1f, g and Table S1, Supporting Information).To further investigate the diffusion of Se into the bulk material, NCMSe was analyzed using X-ray photoelectron spectroscopy (XPS) etching. [24,25]The detection of the Se element extended up to 90 nm inside the material, with a gradual decrease in intensity, indicating a reduction of Se content within the material (Figure 1h).XPS etching of Ni revealed a gradual decrease in Ni 3þ concentration from the outer to the inner region, consistent with the refinement results (Figure S3, Supporting Information).Additionally, electron paramagnetic resonance (EPR) results indicated a significant reduction in oxygen vacancy content, [26,27] suggesting that Se-diffused and occupied oxygen vacancies during the calcination process, aligning with the Rietveld refinement results (Figure S4, Supporting Information).To further investigate whether Se occupies oxygen vacancies in NCMSe, density-functional theory (DFT) calculations were performed to determine the energy of Se occupying different oxygen layers (topmost, first, second, and third layers).The energy differences, shown in Figure S5 (Supporting Information), indicate the stability of the Se-doped structure, with values of 0, À0.27, À0.35, and 0.49 eV. [28,29]The DFT calculation results demonstrate that Se atoms have a favorable energy when occupying oxygen vacancies in NCM, with a slight energy difference, ensuring the successful occupation of these vacancies.
X-ray absorption spectroscopy (XAS) was employed to further investigate the influence of Se modification.The X-ray absorption near-edge structure (XANES) of NCMSe exhibited a slight shift to the right, indicating an improved valence state of Ni, while the extended X-ray absorption fine structure (EXAFS) spectrum features remained nearly unchanged (Figure 1i and S6, Supporting Information), indicating that the bulk structure was unaffected by Se modification.Typically, the first peak (1-2 Å) corresponds to the absorption of the nearest oxygen around the Ni atom (Ni-O bond), while the second peak (2-3 Å) corresponds to the absorption of the TM near the Ni atom (Ni-M bond). [29,30]Notably, the wavelet transform of the k 3 -weighted EXAFS signal further confirmed the successful diffusion of Se into the volume phase (Figure 1j,k, and S7, Supporting Information).Compared to NCM811, the peaks of NCMSe exhibited a slight shift toward higher wavenumbers, indicating the incorporation of Se atoms.Based on the above characterization and analysis, it can be concluded that Se effectively occupies the oxygen site, leading to the oxidation of Ni 2þ and the formation of a stable O-TM-Se bond, thereby enhancing the stability of the lattice oxygen skeleton.
To assess the electrochemical performance of the cathodes at high voltage, the half-cells were cycled within the range of 2.7-4.5 V. Simultaneously, the electrochemical stability was evaluated using cyclic voltammetry (CV) with a scanning rate of 0.1 mV s À1 .The NCMSe cathode (Δ EOR = 0.29 V) exhibited a smaller potential hysteresis compared to NCM811 (Δ EOR = 0.38 V), indicating that Se modification had a positive impact on interfacial polarization and electrochemical reversibility (Figure 2a).The initial charge-discharge curves of NCM811 and NCMSe showed similar shapes, with initial Coulombic efficiencies of 85.5% and 88.6%, respectively (Figure 2b).These results indicate that the modified sample demonstrated improved reversibility and reduced irreversible capacity losses, which can be attributed to the enhanced stability of the lattice oxygen after surface modification, consistent with the CV findings.Subsequently, the electrochemical performance at different rates was evaluated.NCMSe exhibited excellent rate capability, achieving a high discharge specific capacity of 167.6 mAh g À1 at 10 C, surpassing that of NCM811 (157.6 mAh g À1 ) (Figure 2c).Further investigation into the stability of electrochemical performance revealed that NCMSe maintained a specific capacity of 174.5 mAh g À1 after 300 cycles at 1 C, corresponding to a capacity retention of 87.5%.In contrast, NCM811 only retained 146.8 mAh g À1 , with a capacity retention rate of 73.9% (Figure 2d).To precisely examine the polarization evolution during cycling, differential capacity (dQ dV À1 ) versus voltage curves were derived from the charge/discharge profiles (Figure 2e,f ).The observed gradual peak shift and reduced peak intensity of the redox reactions in NCM811 indicated structural degradation and side reactions, leading to an increased capacity fading.In contrast, NCMSe exhibited significantly less attenuation of the reduction peaks, even after 200 cycles.Moreover, the Ni 3þ/ Ni 4þ peak in NCM811 around 3.75 V virtually disappeared compared to the stable oxidation peak in NCMSe.This observation suggests that the in-situ electrochemically activated O-TM-Se bond can stabilize the highly active Ni 4þ cation, thereby greatly enhancing the reversibility of the Ni 2þ/4þ redox reactions. [5,31]To further investigate the cycle stability of the modified sample under high current density, the cycle performance at 5C was evaluated (Figure 2g).NCMSe maintained a high capacity of 154.9 mAh g À1 after 300 cycles, whereas NCM exhibited a lower capacity of 93.9 mAh g À1 .Additionally, to examine the advantages of the Se modification strategy and assess the electrochemical performance at 60C, NCMSe maintained a high capacity of 192.9 mAh g À1 after 150 cycles at 1 C, while NCM only achieved 73.1 mAh g À1 (Figure 2h).
To further investigate the impact of Se modification on the local fine structure, we performed in situ EPR measurements during charging (Figure 3a 3g).These parasitic reactions during charging promote unfavorable phase transformations and reduce thermal stability.The oxidation of Ni 2þ at the Li layer causes a significant shrinkage of the octahedral cell due to the corresponding electron loss on the, e.g., orbit (Figure 3h).This unit cell shrinkage is closely associated with c-axis shrinkage, leading to anisotropic volume deformation of the primary particles and internal mechanical stress. [32]To investigate the inhibition of oxygen precipitation related to Se modification occupying oxygen sites, we performed DFT calculations to study the potential barrier of oxygen vacancy formation resulting from oxygen precipitation on the surface of NCM811 and NCMSe materials.The calculated oxygen vacancy formation energies were 0.4194 and 0.6056 eV for NCM811 and NCMSe, [33,34] respectively, indicating that Se modification can increase the stability of lattice oxygen and structural stability (Figure 3i-k).
To further investigate the impact of Se surface treatment on the structural changes of NCM during Li þ de/intercalation, we conducted in situ X-ray diffraction (XRD) analysis during the initial cycle (0.1C)(Figure 4a,b).The expansion of the c-axis at 4.18 V can be attributed to increased O-O repulsion resulting from Li þ removal in the Li-plate, leading to unit cell expansion along the c-axis.The pronounced contraction of the crystal lattice above 4.18 V can be explained by negative charge transfer from O ions to Ni ions in the highly charged state, reduction of the cubic repulsive force along the c-axis in the O-Li-O plate, and phase transitions of NCM811 material from H1 to M and from H2 to H3, resulting in lattice contraction along the c-axis.Upon discharge to 2.7 V, the (003) peak shifted to a lower angle, indicating lattice expansion along the c-axis and the a-b plane with Li þ insertion.Overall, the diffraction peaks of NCM811 and NCMSe exhibited similar changes during the discharge process, but the difference in diffraction peak displacement between NCM811 and NCMSe samples was significant during the charging process.After charging to 4.6 V, the maximum displacement of the (003) peak in NCMSe was smaller than that in NCM811.Additionally, the (003) peak shifts during the H2-H3 phase transition were determined to be 1.13°and 0.95°for NCM811 and NCMSe cathodes, respectively.During charging, the and (104) diffraction lines of all electrodes shifted monotonically toward higher angles, indicating a gradual decrease in the lattice parameter a, which can be attributed to the oxidation of Ni 2þ !Ni 4þ .Importantly, the more significant shrinkage of the a-axis in NCMSe electrode material further indicates that more Ni ions participate in the electrochemical reaction, reflecting a more effective inhibition of lattice oxygen escape on the NCM811 surface from another perspective. [35,36]Thus, Se surface treatment can stabilize the host and surface structure during cycling, improving the reversibility of the electrochemical reaction.To evaluate the effect of Se modification on oxygen precipitation in NCM811, we investigated the thermal stability of NCM811 and NCMSe electrodes in the charged state using differential scanning calorimetry (Figure S8, Supporting Information).After Se modification, the exothermic peak temperature related to oxygen evolution significantly increased, and the lattice shrinkage was noticeably inhibited, with the exothermic peak temperature increasing from 216.9 to 204.1 °C. [37]o verify the advantages of the structural design, XPS tests were performed on the electrodes after cycling (Figure S9, Supporting Information), confirming that Se modification effectively inhibits the formation of the solid-electrolyte interphase (SEI) film.To further elucidate these results, the metal content in the electrolyte was measured after 100 cycles of NCM811 and NCMSe (Figure S10, Supporting Information), revealing that the dissolution ratio of TMs, especially Ni, was higher in NCM811 than in NCMSe.These results indicate that Se modification inhibits the dissolution of TMs.The composition of the electrode surface after 100 cycles was further analyzed using time-of-flight secondary ion mass spectrometry (Figure 4c), providing a 3D reconstruction of NCM811 and NCMSe cathodes during longterm cycling, to help better understand the SEI film's growth process, lithium impurity decomposition, and oxygen loss.The uneven distribution of the C 2 HO À , CH 2 À , and PF 2 O 2 À layers in NCM811 results in an unstable SEI and continuous electrolyte consumption.Conversely, a stable SEI was observed on the surface of NCMSe after 100 cycles, with the particles maintaining their initial morphology, particularly NiO 2 À .The formation of a thinner layer of SEI on the surface of NCMSe particles is expected to enhance interfacial Li þ transfer. [38]Electrochemical impedance spectroscopy (EIS) testing after 100 cycles of NCM811 and NCMSe was conducted (Figure S10, Supporting Information).Notably, the charge-transfer impedance (Rct) of NCMSe (104 Ω) is smaller than that of NCM811 (133 Ω), indicating that the modification can reduce the resistance of the electrode at the electrolyte interface, thereby improving electrochemical performance.Furthermore, to understand the effect of Se modification on Li þ migration, the Li þ diffusion coefficient was determined using the galvanostatic intermittent titration technique (Figure S11, Supporting Information).The general trend for both materials was similar, but the Li þ diffusion coefficient of NCMSe was slightly higher than that of NCM811, consistent with the EIS and different-rate CV results (Figure S12, Supporting Information).These results indicate that Se modification has a positive effect on Li þ diffusion, mainly due to the inhibition of irreversible oxygen loss at the stable interface.Additionally, the replacement of some oxygen atoms on the material surface by Se atoms weakens the Li-Se bond, reducing the migration energy barrier for Li þ (Figure S13, Supporting Information).To confirm this, first-principles calculations were performed to compare the Li þ migration barriers for NCM811 and NCMSe, the migration energy barrier for lithium ions in NCMSe is significantly lower than that in NCM811. [39]o further analyze the reasons for the improved electrochemical properties of Ni-rich materials after Se surface treatment, we conducted a detailed study of SEM and HRTEM images of NCM and NCMSe after 200 cycles to evaluate the stabilizing effect of Se surface treatment on the structure.In NCM811, a large number of cracks were observed, attributed to residual stress, while NCMSe exhibited only a small number of microcracks due to structural stability (Figure 5a,d).After 200 cycles, NCM811 showed an increased presence of spinel phase, disordered mixed phase, rock salt phase, and cracks, which hindered the transport channel of lithium ions and resulted in a rapid increase in electrochemical impedance (Figure 5b,c).In contrast, NCMSe maintained the layered structure of the main regions even after high-pressure cycling, indicating that Se surface treatment could stabilize the layered structure and inhibit lattice oxygen loss during high-pressure cycling (Figure 5e,f ).To further investigate the structural changes, electron energy loss spectroscopy was used to scan the interior of NCM811 (Figure 5g).In the cationic mixing state, Ni exhibited a reduction to a lower valence state, as evidenced by the chemical shift of the Ni L-edges to a lower energy loss position.However, no chemical changes were observed in the Mn L-edges.Lattice oxygen precipitation was indicated by a further weakening of the O K-edges at the cationic mixing point.It is worth noting that the Ni L-edges and O K-edges on the surface of NCMSe were weaker, while the Mn L-edges remained unaffected, suggesting that Se modification improves structural stability and stabilizes lattice oxygen (Figure 5h).During long cycles, the Se layer on the material's surface captures escaping oxygen, forming an outer layer of SeO 2 , which prevents oxygen from reacting with the electrolyte.XPS analysis of this process demonstrated the gradual oxidation of Se to SeO 2 over 10 cycles (Figure S14, Supporting Information).Se modification effectively enhances structural stability during long cycles and plays a triple role: first, selenium vapor permeates inward along the gaps of secondary particles, providing a more effective protective effect; second, it forms 1D solid polymer oxide SeO 2 with the escaped oxygen, preventing electrolyte decomposition and exacerbating side reactions; third, Se occupies oxygen vacancies on the surface of NCM811, reducing the migration of O αÀ along the oxygen vacancy and decreasing oxygen mobility.Furthermore, selenium replaces O on the material's surface in a charged state, forming stable Se-TM-O compounds, which reduce the mobility of oxygen ions (O αÀ ) to nonmobile O 2À , thus decreasing the outward migration of oxygen anions.

Conclusion
To sum up, NCM811 was successfully changed by a simple wet ball-milling and low-temperature calcination process.Highvoltage charging effectively inhibits oxygen migration over the long period and improves crystal stability by substituting a charged particle (Se !Se βþ ) for the O αÀ , eliminating the oxygen vacancy and reducing the O αÀ to the immobile O 2À .The findings demonstrated that NCMSe samples exhibited remarkable electrochemical stability, with a capacity retention rate of up to 87.5% after 300 cycles at 1C and 89.3% after 150 cycles at 60C at 4.5 V for the Ni-rich cathode material.A reasonable way to developing improved cathode material for highperformance lithium-ion batteries is provided by the suggested Se-modified technique, which might be used to other high capacity cathode materials with similar results.

Experimental Section
Synthesis of Se Modification Ni-Rich Material: The synthesis of Se-modified Ni-rich material involved adding 10 g of LiNi 0.8 Co 0.1 Mn 0.1 O 2 (referred to as NCM811), 0.3 g of Se, and 30 mL of anhydrous ethanol into the ball mill tank, followed by ball-milling at 400 rpm 5 h.The resulting product was then dried at 100 °C for 12 h and calcined at 350 °C under an Ar atmosphere for 4 h to synthesize the Se-modified NCM811 sample, denoted as NCMSe (3 wt%).
Materials Characterization: Characterization of the materials involved using field-emission scanning electron microscopy (FESEM, JEOL JSM-7600F, 5 kV) and transmission electron microscopy (TEM, JEOL, JEM-2100F) to observe the morphologies of the active materials and battery electrodes.XRD data were obtained using a Bruker D8 Advance X-ray diffractometer equipped with a LynxEye 1D detector.Cu-Ka radiation at 40 kV and 40 mA (λ = 1.5418Å) was employed with a step increment of 0.02 and a duration per step of 0.8 s.Rietveld refinement of the powder diffraction patterns was conducted using the GSAS program, based on the crystal model of LiNi 0.8 Co 0.1 M n0.1 O 2 with a space group of R-3m.
Electrodes Fabrication and Testing: The composite electrodes were fabricated by using NCM811 and Se-modification Ni-rich as active materials (90 wt%), PVDF5130 as the binder (5 wt%), and Super P (5 wt%) as a conductive agent.The homogeneous slurry was applied to an aluminum sheet to remove the leftover solvent and dried overnight in a vacuum at 100 °C.The electrochemical characteristics of CR2025 coin-type batteries were examined.All cells were built in an argon-filled glove box with oxygen and water concentrations below 0.01 ppm.The anode was made of lithium metal foil.The battery electrolyte was a solution of 1.0 M LiPF 6 in ethylene carbonate: diethyl carbonate (EC: DEC, 3:7 by weight).A LAND (Wuhan Land Electronics Co., Ltd.) battery analyzer was employed to do the charging/discharging tests at various current rates.

Figure 1 .
Figure 1.a,c) Scanning electron microscopy (SEM) image and b,d) scanning transmission electron microscopy (STEM) images of NCM811 and NCMSe.e) Elemental mapping of Ni, Co, Mn, O, and Se for NCMSe.f,g) X-ray diffraction (XRD) Rietveld refinement results of NCM811 and NCMSe.h) X-ray photoelectron spectroscopy (XPS) spectra of Se 3d from NCMSe at different etching times.i) X-ray absorption spectroscopy (XAS) spectra of Ni K-edge and j,k) X-ray absorption fine structure (EXAFS) wavelet transform images for NCM811 and NCMSe.
,b).The initial states of NCM811 and NCMSe exhibited Lorentz linear Ni 3þ signals, indicating the presence of Ni 3þ in the TM layer.Consequently, the EPR signal exhibited a symmetric spherical electron motion orbit.As the charging progressed to a high voltage range, the Ni-O covalency became more prominent due to the transition of the nickel oxidation state from Ni 2þ -O to Ni 4þ -O and Li deintercalation.The increased covalency resulted in delocalized electrons across the

Figure 2 .
Figure 2. a) Cyclic voltammetry (CV) of the first cycle for NCM811 and NCMSe at a sweep rate of 0.1 mV s À1 within the potential range from 2.7 to 4.5 V versus Li/Li þ .b) The initial charge/discharge curves at 0.1C in the voltage range of 2.7-4.5 V. c) Rate performance of NCM811 and NCMSe.d) Cycling performance and e,f ) the corresponding dQ dV À1 curves at 1C in the voltage range of 2.7-4.5 V. g) Cycling performance at 5C in the voltage range of 2.7-4.5 V. h) Cycling performance at 60C.

Figure 3 .
Figure 3. a,b) In situ electron paramagnetic resonance (EPR) spectra of NCM811 and NCMSe collected upon charging from open circuit voltage (OCV) to 4.6 V. c) X-ray absorption near-edge structure spectra, d) EXAFS, and e,f ) 2D Fourier transform of Ni K-edges of NCM811 and NCMSe cathodes collected at 4.6 V. g) Ni 2þ transfer through tetrahedral interstice with loss of lattice oxygen after Li þ extraction.h) Shrinkage of the octahedral cell of nickel ions caused by electron loss of, e.g., orbit during oxidation.Red sphere for oxygen, blue sphere for bivalent nickel, purple sphere for tetravalent nickel, hollow sphere for Li vacancy, and blue arrow for shrinkage of the unit cell.i,j) Crystal structures and k) formation energy of oxygen vacancies of NCM811 and NCMSe.

Figure 5 .
Figure 5. a,d) Cross-section SEM images , b,e) high-resolution transmission electron microscopy (HRTEM), and c,f ) high-angle angular dark-field STEM (HAADF-STEM) images of NCM811 and NCMSe electrodes after 200 cycles between 2.7 and 4.5 V. Electron energy loss spectroscopy spectra of the surface region O K-edges, Mn K-edges, and Ni L-edges: g) NCM811 and h) NCMSe.