High Pressure Effect on Structural and Electrochemical Properties of Anionic Redox-Based Lithium Transition Metal Oxides

Abstract

•Structural flexibility and metastability of anionic redox-based materials•Structure reordering and strain accumulation under high pressure•Negative bulk compressibility induced by the defect formation Transition metal oxide is one of the most interesting classes of solids, exhibiting many physical properties, including magnetism, piezoelectricity, and superconductivity. Lithium transition metal oxide is the cornerstone for energy storage through Li intercalation. This work demonstrates that the formation of structural defects during the electrochemical activation process results in metastable states in lithium transition metal oxides. The unique metastability provides an opportunity to reveal the unusual properties of complex oxide materials, such as negative thermal expansion and negative bulk compressibility. Predictive design for new materials with extraordinary combinations of physical characteristics can be realized by electrochemically controlling the lithium intercalation process. The richness of anionic redox chemistry in the solid state offers new opportunities and a possible paradigm shift in energy storage. The excess capacity that goes beyond conventional theoretical values is attributed to the anionic redox in Li-rich transition metal oxide cathodes for Li-ion batteries. Their electrochemical behavior is thermodynamically determined by structural evolution. To better understand the electrochemical dependence on structural factors, we have induced structural modifications in pristine and electrochemically activated Li1.144Ni0.136Co0.136Mn0.544O2 through high-pressure treatment. A unique cyclical change of structural reordering is observed in the anionic redox-activated material during operando pressure sweep, characterized by a periodic evolution of superlattice peak intensity in synchrotron X-ray diffraction patterns. During the structural reordering period, the bulk compressibility of the material decreases, even becoming negative. These insights elucidate the structural flexibility and metastability of anionic redox-based materials, which can undergo large compressions and structural modifications while delivering good electrochemical properties. The richness of anionic redox chemistry in the solid state offers new opportunities and a possible paradigm shift in energy storage. The excess capacity that goes beyond conventional theoretical values is attributed to the anionic redox in Li-rich transition metal oxide cathodes for Li-ion batteries. Their electrochemical behavior is thermodynamically determined by structural evolution. To better understand the electrochemical dependence on structural factors, we have induced structural modifications in pristine and electrochemically activated Li1.144Ni0.136Co0.136Mn0.544O2 through high-pressure treatment. A unique cyclical change of structural reordering is observed in the anionic redox-activated material during operando pressure sweep, characterized by a periodic evolution of superlattice peak intensity in synchrotron X-ray diffraction patterns. During the structural reordering period, the bulk compressibility of the material decreases, even becoming negative. These insights elucidate the structural flexibility and metastability of anionic redox-based materials, which can undergo large compressions and structural modifications while delivering good electrochemical properties. In classical cathode intercalation compounds for Li-ion batteries, such as LiCoO2 and Li(Ni1–x–yMnxCoy)O2, the electric charge is stored by reversible Li intercalation coupled to transition metal (TM) cation redox.1Radin M.D. Hy S. Sina M. Fang C. Liu H. Vinckeviciute J. Zhang M. Whittingham M.S. Meng Y.S. Van der Ven A. Narrowing the gap between theoretical and practical capacities in Li-ion layered oxide cathode materials.Adv. Energy Mater. 2017; 7: 1602888Crossref Scopus (274) Google Scholar The charge storage capacity is thus restricted by Li stoichiometry and the formal oxidation state of the TM in classical compounds. There has been increasing interest in introducing an over-stoichiometry of Li to surpass the conventional mechanism of formal TM redox (Figure S1).2Johnson C.S. Kim J.-S. Lefief C. Li N. Vaughey J.T. Thackeray M.M. The significance of the Li2MnO3 component in “composite” xLi2MnO3 · (1 − x)LiMn0.5Ni0.5O2 electrodes.Electrochem. 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Saubanère M. Doublet M.L. Unified picture of anionic redox in Li/Na-ion batteries.Nat. Mater. 2019; 18: 496-502Crossref PubMed Scopus (181) Google Scholar, 7Zhao E. Zhang M. Wang X. Hu E. Liu J. Yu X. Olguin M. Wynn T.A. Meng Y.S. Page K. et al.Local structure adaptability through multi cations for oxygen redox accommodation in Li-rich layered oxides.Energy Storage Mater. 2020; 24: 384-393Crossref Scopus (66) Google Scholar We propose that an additional unhybridized O 2p orbital, derived from the excess Li in the TM layer, causes oxygen oxidation, which contributes to the extra capacity.8Okubo M. Yamada A. Molecular orbital principles of oxygen-redox battery electrodes.ACS Appl. Mater. Inter. 2017; 9: 36463-36472Crossref PubMed Scopus (74) Google Scholar After oxygen redox activation, either localized electron holes or oxygen dimers tend to form in the bulk structure while oxygen gas evolution occurs at the particle surface, leaving oxygen vacancies and an under-coordinated TM.9Qiu B. Zhang M. Xia Y. Liu Z. Meng Y.S. Understanding and controlling anionic electrochemical activity in high-capacity oxides for next generation Li-ion batteries.Chem. Mater. 2017; 29: 908-915Crossref Scopus (78) Google Scholar The exact mechanism at the molecular/atomic scales remains largely under debate. It is well documented that the structural evolution plays a critical role in determining the reversibility of oxygen redox, thereby influencing the electrochemical properties. Work by Gent et al. 10Gent W.E. Lim K. Liang Y. Li Q. Barnes T. Ahn S.-J. Stone K.H. McIntire M. Hong J. Song J.H. et al.Coupling between oxygen redox and cation migration explains unusual electrochemistry in lithium-rich layered oxides.Nat. Commun. 2017; 8: 2091Crossref PubMed Scopus (292) Google Scholar ascribed drastic change in the local oxygen coordination environments associated with the migration of the TM interlayer to the cycling voltage hysteresis. Sequence changes of oxygen stacking induced by the dislocation network were also observed in nanoparticles of lithium-rich layered oxide material, which contributes to the voltage decay in long-term cycling.11Singer A. Zhang M. Hy S. Cela D. Fang C. Wynn T.A. Qiu B. Xia Y. Liu Z. Ulvestad A. et al.Nucleation of dislocations and their dynamics in layered oxide cathode materials during battery charging.Nat. Energy. 2018; 3: 641-647Crossref Scopus (158) Google Scholar Hu et al.12Hu E. Yu X. Lin R. Bi X. Lu J. Bak S. Nam K.W. Xin H.L. Jaye C. Fischer D.A. et al.Evolution of redox couples in Li- and Mn-rich cathode materials and mitigation of voltage fade by reducing oxygen release.Nat. Energy. 2018; 3: 690-698Crossref Scopus (377) Google Scholar observed large pores in the interior of the electrochemically cycled particle due to oxygen release within particles, leading to attenuation of energy during battery cycling. Our recent work has demonstrated various structural defects, including lithium vacancies in the TM layer, stacking faults in the TM layer, and local distortion of the oxygen framework, resulting in the unique metastable state, which is responsible for the voltage hysteresis and decay.13Qiu B. Zhang M. Lee S.-Y. Liu H. Wynn T.A. Wu L. Zhu Y. Wen W. Brown C.M. Zhou D. et al.Metastability and reversibility of anionic redox-based cathode for high-energy rechargeable batteries.Cell Rep. Phys. Sci. 2020; 1: 100028Abstract Full Text Full Text PDF Scopus (28) Google Scholar Assat et al.14Assat G. Glazier S.L. Delacourt C. Tarascon J.M. Probing the thermal effects of voltage hysteresis in anionic redox-based lithium-rich cathodes using isothermal calorimetry.Nat. Energy. 2019; 4: 647-656Crossref Scopus (62) Google Scholar investigated the thermal effects of voltage hysteresis in anionic redox-based cathodes and found that metastable electrochemical paths persist even under quasi-static conditions. The cycled state is a metastable state because of the relatively higher energy and the large energy barrier that the system cannot overcome to relax toward the stable state through electrochemical processes at room temperature. In our previous work, the thermal effect on cation and anion redox-based cathode material was directly compared with show that the metastable state is the key feature of Li-rich layered nanoparticles.13Qiu B. Zhang M. Lee S.-Y. Liu H. Wynn T.A. Wu L. Zhu Y. Wen W. Brown C.M. Zhou D. et al.Metastability and reversibility of anionic redox-based cathode for high-energy rechargeable batteries.Cell Rep. Phys. Sci. 2020; 1: 100028Abstract Full Text Full Text PDF Scopus (28) Google Scholar Thermal energy can effectively eliminate defects in the cycled structure so that the structure and voltage can both be recovered. More intriguingly, we reported negative thermal expansion of the cycled Li-rich layered materials within a specific temperature range while preserving the original layered phase. It is proposed that materials that contract on heating, in general, can also expand under hydrostatic pressure.15Goodwin A.L. Keen D.A. Tucker M.G. Large negative linear compressibility of Ag3[Co(CN)6].Proc. Natl. Acad. Sci. U S A. 2008; 105: 18708-18713Crossref PubMed Scopus (182) Google Scholar This rule inspires one to ponder whether mechanical energy (high pressure treatment in the scale of GPa) can potentially serve as a driving force, similar to thermal energy, to recover the cycled state. It has been reported previously that high pressure treatment of electrode materials is a powerful tool to induce structural modifications that are not typically possible at atmospheric pressure and which result in different electrochemical characteristics.16Arroyo y de Dompablo M.E. Amador U. Gallardo-Amores J.M. Morán E. Ehrenberg H. Dupont L. Dominko R. Polymorphs of Li3PO4 and Li2MSiO4 (M = Mn, Co). The role of pressure.J. Power Sources. 2009; 189: 638-642Crossref Scopus (42) Google Scholar, 17Arroyo-De Dompablo M.E. Gallardo-Amores J.M. Amador U. Lithium insertion in the high-pressure polymorph of FePO4 computational predictions and experimental findings.Electrochem. Solid State Lett. 2005; 8: A564Crossref Scopus (37) Google Scholar, 18García-Moreno O. Alvarez-Vega M. García-Alvarado F. García-Jaca J. Gallardo-Amores J.M. Sanjuán M.L. Amador U. Influence of the structure on the electrochemical performance of lithium transition metal phosphates as cathodic materials in rechargeable lithium batteries: a new high-pressure form of LiMPO4 (M = Fe and Ni).Chem. Mater. 2001; 13: 1570-1576Crossref Scopus (172) Google Scholar For compressible cathode materials, the high pressure treatment combined with high temperature may induce severe structural phase transformations (for instance, olivine-spinel LiCoPO4).19Amador U. Gallardo-Amores J.M. Heymann G. Huppertz H. Morán E. Arroyo y de Dompablo M.E. High pressure polymorphs of LiCoPO4 and LiCoAsO4.Solid State Sci. 2009; 11: 343-348Crossref Scopus (27) Google Scholar Layered cathode materials are materials that are not very compressible (bulk modulus in the range of 120–150 GPa), yet important structural modifications have been reported. Zhang et al.20Zhang Y. Wang L. Yang J. He X. Wang J. Zhang Y. Jin Y. Li Y. Xiong L. Effect of pressure on the structural properties of Li[Li0.1Ni0.35Mn0.55]O2.AIP Adv. 2015; 5: 047106Crossref Scopus (2) Google Scholar studied the high pressure behavior of pristine Li-rich layered oxide Li[Li0.1Ni0.35Mn0.55]O2 up to 19.7 GPa. The material remained in the layered phase and an obvious structure compression was observed. The electrochemistry of the pressure-treated materials was not explored. Fell et al.21Fell C.R. Lee D.H. Meng Y.S. Gallardo-Amores J.M. Morán E. Arroyo-De Dompablo M.E. High pressure driven structural and electrochemical modifications in layered lithium transition metal intercalation oxides.Energy Environ. Sci. 2012; 5: 6214-6224Crossref Scopus (31) Google Scholar compared structural modifications in LiNi0.5Mn0.5O2 and Li[NixLi1/3–2x/3Mn2/3–x/3]O2 (this is an Li-excess material with x = 1/4 and 1/2) using high pressure–high temperature treatment. After the treatment, the Li-excess materials displayed superior electrode characteristics to LiNi0.5Mn0.5O2 whose electrochemical characteristics deteriorated due to the structural modifications. Despite the above high pressure research on layered cathode materials, little is known about the effect of high pressure on their electrochemically cycled structure. The aim of this research is to investigate high pressure effects on the electrochemically cycled structure of anionic redox-based oxides. For this, a pristine and a cycled Li-excess material (Li[Li0.144Ni0.136Co0.136Mn0.544]O2, denoted as LR-NCM) were subjected to static pressure treatments in two types of high pressure cells: diamond anvil cells (DACs) up to 15 GPa and large anvil cells (LACs) in a Belt-type press up to 4 GPa. The combination of the two approaches in junction with molecular dynamics (MD) simulations reveal the occurrence of structural reordering upon pressure, although with incrementally increasing antisite defects and microstrain. We show that all of these structural modifications have a profound impact on the electrochemical properties of the material after high pressure treatment. Larger Li (de)intercalation capacity is obtained for the pressure-treated sample, yet with lower average discharge voltage. These results demonstrate the flexibility and metastability of the cycled anionic redox-based cathode material, in which crystal defects are of great importance. The same batch of cycled LR-NCM, as applied in our previous work,13Qiu B. Zhang M. Lee S.-Y. Liu H. Wynn T.A. Wu L. Zhu Y. Wen W. Brown C.M. Zhou D. et al.Metastability and reversibility of anionic redox-based cathode for high-energy rechargeable batteries.Cell Rep. Phys. Sci. 2020; 1: 100028Abstract Full Text Full Text PDF Scopus (28) Google Scholar was loaded into a DAC for a high pressure sweep. Figure 1A illustrates the DAC working mechanism, in which a transmitting medium allows isotropic pressure to be applied to the sample. Operando synchrotron X-ray diffraction (SXRD) data were then collected to reveal the structural evolution during the pressure treatment. As shown in Figure 1B, the cycled structure remains phase pure with an R-3m space group up to ~15 GPa at room temperature. Obvious peak broadening is observed in the SXRD patterns with the increase of applied pressure, which is a strong indication of continuous formation of microstrain in the bulk structure. The peak at ~2θ = 8.4° highlighted in the enlarged region of Figure 1B is attributed to Li and TM honeycomb superstructure ordering within the TM layers.22Zhang M. Liu H. Liu Z. Fang C. Meng Y.S. Modified coprecipitation synthesis of mesostructure-controlled Li-rich layered oxides for minimizing voltage degradation.ACS Appl. Energy Mater. 2018; 1: 3369-3376Crossref Scopus (13) Google Scholar It is well documented that the superstructure gradually disappears, and that the material becomes partially disordered during the initial electrochemical cycle.11Singer A. Zhang M. Hy S. Cela D. Fang C. Wynn T.A. Qiu B. Xia Y. Liu Z. Ulvestad A. et al.Nucleation of dislocations and their dynamics in layered oxide cathode materials during battery charging.Nat. Energy. 2018; 3: 641-647Crossref Scopus (158) Google Scholar One remarkable change in the SXRD patterns is that the superlattice peak intensity becomes intensified and weakened in a periodic pattern during the high pressure processing. The peak intensity reaches its maxima at pressure values of 2.73, 5.29, 8.38, and 11.78 GPa. The evolution of the superlattice peak indicates that the structure ordering can be recovered under certain pressure conditions. A high pressure sweep in the same range was also conducted on the pristine LR-NCM electrode and the SXRD patterns recorded are shown in Figure S2. The results show that all diffraction peaks shift to larger angles with increasing pressure and that the superlattice peaks in the 2θ range of 8°–9° are retained. These results for the pristine electrode are consistent with previous work by Zhang et al.20Zhang Y. Wang L. Yang J. He X. Wang J. Zhang Y. Jin Y. Li Y. Xiong L. Effect of pressure on the structural properties of Li[Li0.1Ni0.35Mn0.55]O2.AIP Adv. 2015; 5: 047106Crossref Scopus (2) Google Scholar and follow the normal pressure effect on inorganic ceramic materials. Rietveld refinements of the patterns for the pristine material demonstrate that the material is maintained in a layered structure. The pressure dependence of lattice parameters of pristine and initially cycled LR-NCM is presented in Figure 1C. For the pristine sample, the c lattice parameter almost linearly decreases by ~4% from 14.237(8) to 13.680(1) Å as the pressure increases. After electrochemical cycling, the unit cell volume is known to expand, with lattice parameters increasing.23Liu H. Chen Y. Hy S. An K. Venkatachalam S. Qian D. Zhang M. Meng Y.S. Operando lithium dynamics in the Li-rich layered oxide cathode material via neutron diffraction.Adv. Energy Mater. 2016; 6: 1502143Crossref Scopus (84) Google Scholar The c lattice parameter for the cycled sample reduces more than 5% in the full range of the pressure sweep. The larger reduction found for the initially cycled LR-NCM suggests that the electrochemical reaction produces a softer and more compressible material. Unlike the linear decrease for the pristine sample, the c lattice parameter of the cycled sample shows an obvious expansion period as the superstructure ordering is recovered (see the shaded area in Figure 1C). The sharp contrast between the pristine and cycled samples is also observed in the change of the a lattice parameter as the pressure increases (Figure S3). Note that the reduction percentage of the a lattice parameter is smaller than that of the c lattice parameter for both the pristine (2.3%) and cycled samples (2.9%). The lattice is more susceptible to deform along the c axis under pressure and this is correlated to the different compressions of the inter-plane and in-plane bonds between adjacent metal ions.21Fell C.R. Lee D.H. Meng Y.S. Gallardo-Amores J.M. Morán E. Arroyo-De Dompablo M.E. High pressure driven structural and electrochemical modifications in layered lithium transition metal intercalation oxides.Energy Environ. Sci. 2012; 5: 6214-6224Crossref Scopus (31) Google Scholar The effect of pressure on unit cell volume for the pristine and initially cycled samples is calculated in Figure 1D. The anomalous expansion behavior of the cycled sample under increasing pressure results from the evolution of the lattice parameters. The P–V data up to 15 GPa are fitted to the third-order Birch-Murnaghan equation of the state.24Birch F. Finite elastic strain of cubic crystals.Phys. Rev. 1947; 71: 809-824Crossref Scopus (4175) Google Scholar The pristine and cycled LR-NCM exhibits bulk moduli of 117 and 111 GPa, respectively. These values are lower than the bulk modulus found for classical layered oxide LiCoO2 (149 GPa) and LiNi0.5Mn0.5O2 (125 GPa).21Fell C.R. Lee D.H. Meng Y.S. Gallardo-Amores J.M. Morán E. Arroyo-De Dompablo M.E. High pressure driven structural and electrochemical modifications in layered lithium transition metal intercalation oxides.Energy Environ. Sci. 2012; 5: 6214-6224Crossref Scopus (31) Google Scholar The LR-NCM material is more compressible than the stoichiometric layered oxide due to the presence of Li ions in the TM layer. The material compressibility, reciprocal of the bulk modulus, is calculated based on the expression in Figure 1E. The absolute compressibility value of the cycled sample is much larger than that of the pristine sample. The compressibility of the pristine sample is positive for all the testing pressure and does not vary obviously as pressure increases, whereas the cycled material displays a negative bulk compressibility in the pressure range as anomalous unit cell volume expansion takes place. This negative compressibility effect once again manifests in the unique cycled structure response to the applied pressure. Similar to the thermal effect, as reported previously,13Qiu B. Zhang M. Lee S.-Y. Liu H. Wynn T.A. Wu L. Zhu Y. Wen W. Brown C.M. Zhou D. et al.Metastability and reversibility of anionic redox-based cathode for high-energy rechargeable batteries.Cell Rep. Phys. Sci. 2020; 1: 100028Abstract Full Text Full Text PDF Scopus (28) Google Scholar the cycled LR-NCM structure is induced to oppose the volume change as the external force increases. The sinusoidal waveform of the evolution of compressibility implies that the structure reordering is sensitive and recurrent to the applied mechanical force. It has been reported that, during electrochemical cycling, lithium from the TM layer is largely irreversible with formation of vacancies, which could induce TM ions in-plane migration.23Liu H. Chen Y. Hy S. An K. Venkatachalam S. Qian D. Zhang M. Meng Y.S. Operando lithium dynamics in the Li-rich layered oxide cathode material via neutron diffraction.Adv. Energy Mater. 2016; 6: 1502143Crossref Scopus (84) Google Scholar The in-plane TM migration does not alter the global layered structure, while largely disturbing the honeycomb superstructure within the TM layers. It is also demonstrated that TM in-plane reordering accompanies lithium reinsertion in the TM layer due to the energy penalty.13Qiu B. Zhang M. Lee S.-Y. Liu H. Wynn T.A. Wu L. Zhu Y. Wen W. Brown C.M. Zhou D. et al.Metastability and reversibility of anionic redox-based cathode for high-energy rechargeable batteries.Cell Rep. Phys. Sci. 2020; 1: 100028Abstract Full Text Full Text PDF Scopus (28) Google Scholar To investigate the superstructure reordering process induced by high pressure, the cycled LR-NCM sample was exposed to different hydrostatic pressure using a Belt-type press in the range of 1.0–4.0 GPa for 0.5 or 1.5 h at room temperature. The pressure was then slowly released, and the obtained powder was collected under argon atmosphere for further testing at ambient pressure. The use of the LAC in the Belt apparatus allows the preparation of sufficient samples (~0.5 g) for multiple characterization tasks with a better signal-to-noise ratio. The high pressure range (1–4 GPa) where the first superstructure reordering occurs was selected based on operando pressure sweep analysis. The ex situ SXRD data for the pristine sample are shown in Figure S4. The detail refinement results using both the R-3m and C2/m symmetry were summarized in Table S1. Note that the major difference between the two structure models is the ideal Li/TM ordering assumption in the C2/m symmetry. The results show that atomic occupancy is independent of selected symmetry between R-3m and C2/m. Compared with that using R-3m symmetry, a worse R factor is obtained for the C2/m symmetry due to the ideal ordering assumption. After cycling, the Li/TM ordering in the TM layer is mostly lost, which makes R-3m symmetry more appropriate for structural refinement. Figure S5 shows the refined ex situ SXRD patterns using R-3m symmetry for pristine and initially cycled samples treated with different Belt-type pressures for 0.5 or 1.5 h. After electrochemical cycling, the lattice parameters a and c increase to 2.8582(3) and 14.320(4) Å, respectively (Table S2). All the Belt-type pressure-treated samples display pure layered phases, and the a lattice parameter is observed to increase after pressure treatment, except for the sample that underwent a pressure of 4.0 GPa for 1.5 h. This lattice expansion phenomenon is consistent with the observations from the operando pressure sweep. For the atomic occupancy refinement, the pristine sample composition is determined by inductively coupled plasma measurement and the composition of the cycled sample is inferred from the electrochemical testing results. Oxygen vacancies were identified in the cycled sample and showed no obvious change after the pressure treatment. The migration barrier can be as high as 2 eV, which makes oxygen vacancy migration nearly impossible even under high pressure.25Lee E. Persson K.A. Structural and chemical evolution of the layered Li-excess LixMnO3 as a function of Li content from first-principles calculations.Adv. Energy Mater. 2014; 4: 1400498Crossref Scopus (130) Google Scholar Oxygen vacancies result in a large fraction of under-coordinated cations, which can potentially migrate to fully coordinated octahedral sites nearby due to different driving forces.26Qian D. Xu B. Chi M. Meng Y.S. Uncovering the roles of oxygen vacancies in cation migration in lithium excess layered oxides.in: Physical Chemistry Chemical Physics. Royal Society of Chemistry, 2014: 14665-14668Crossref Google Scholar Rietveld analysis indicates that, after the initial cycle, the lithium occupancy in the TM layer decreases to 0.059(1) from 0.173(3) of the pristine sample (Figure 2A), while the sample treated with 2 GPa for 0.5 h has ~30% more lithium in the TM layer compared with the cycled sample. The lithium migration into the TM layer is not observed for the sample with further increase of Belt-type pressure to 4 GPa. The trend in lithium migration is confirmed by the other set of samples pressure treated for 1.5 h, where the maximum lithium occupancy in the TM layer reaches 0.083(1) for the sample treated with 2.7 GPa (Figure 2B). The elimination of lithium vacancies in the TM layer recovers local Li-excess environments around oxygen, which is crucial for oxygen redox activity.4Seo D.-H. Lee J. Urban A. Malik R. Kang S. Ceder G. The structural and chemical origin of the oxygen redox activity in layered and cation-disordered Li-excess cathode materials.Nat. Chem. 2016; 8: 692-697Crossref PubMed Scopus (654) Google Scholar In addition, occupancy of TM ions in the lithium layer (antisite defects NiLi) starts to increase for the sample treated with a pressure greater than 2 GPa (Table S2), which could hinder lithium diffusion through the layer. The effect of pressure on lithium migration was then modeled by MD solid-state simulation using density functional theory (DFT). Isotropic pressure and room temperature, close to experimental conditions, were applied to isolate the mechanical effect from the thermal effect on lithium migration. The simulations were performed on a supercell model composed of two-formula units of Li14Ni3Mn7O24. In this model, there are two “excess” Li ions located in the TM layer. A specific Li11/14 concentration with one oxygen vacancy was chosen to simulate the discharged state (Li11Ni3Mn7O23), in which both Li ions are absent in the TM layer. The external pressure applied and the temperature as a function of the simulation time are shown in Figure S6. The external pressure and temperature were stabilized after 20 ps. Lithium migration to the TM layer began to take place at ~22 ps in the structure that underwent a pressure of 2 GPa (Video S1), while no lithium migration was observed for the MD simulation with 4 GPa (Video S2), which is consistent with the ex situ SXRD refined results. The energetically most favorable route for lithium migration proceeds through the oxygen layer, as shown in Figure 2C, with the migrated Li-ion highlighted in yellow. Note that TM migration from the TM layer to the Li layer was not captured in the MD simulation due to the higher activation barrier (1 eV for Ni2+ and 2.6 eV for Mn4+) compared with Li.26Qian D. Xu B. Chi M. Meng Y.S. Uncovering the roles of oxygen vacancies in cation migration in lithium excess layered oxides.in: Physical Chemistry Chemical Physics. Royal Society of Chemistry, 2014: 14665-14668Crossref Google Scholar Thus, it will take a much longer simulation time to capture TM migration, in particular under room temperature conditions. https://www.cell.com/cms/asset/45dff0d3-50e2-4e90-ac8a-217c39b03751/mmc2.mp4Loading ... Download .mp4 (0.91 MB) Help with .mp4 files Video S1. Molecular Dynamics Simulation Result under Isotropic Pressure of 2 GPa for the Cycled LR-NCM Structure(Green, Li; Purple, Mn; Blue, Ni; Red, Oxygen). https://www.cell.com/cms/asset/2d43858c-8adc-45a0-8fce-c16a5eda1608/mmc3.mp4Loading ... Download .mp4 (0.96 MB) Help with .mp4 files Video S2. Molecular Dynamics Simulation Result under Isotropic Pressure of 4 GPa for the Cycled LR-NCM Structure(Green, Li; Purple, Mn; Blu