Mn-rich Cathode Materials Research Articles

Redox Couple Strategy for Improving the Oxygen Redox Activity and Reversibility of Li- and Mn-Rich Cathode Materials.

The specific capacity of Li- and Mn-rich layered oxide (LMROs) cathodes can be enhanced by the oxidation of lattice oxygen at high voltages. Nevertheless, an irreversible oxygen loss emerges with cycling, which triggers interlocking surface/interface issues and results in the fast deterioration of cycling performance. Herein, we prepare a surface modified LMRO electrode by one step doctor-blade casting and introducing a benzoquinone species DBBQ redox couple. The electrochemical test shows that the DBBQ-modified electrode has a high reversible capacity (>320 mAh g-1) and excellent rate performance, while the cyclic stability has been significantly improved as well. The capacity retention reaches as high as 93.3% after 500 cycles at 1 C. Mechanism analysis shows that DBBQ can not only play a redox couple in LMROs which achieves the adsorption and reduction of surface oxygen gas but also significantly enhance anionic redox in the bulk, thus realizing extraordinary capacity.

Rapid in-Situ Electrochemical Formation of Partially Disordered Spinel from Mn-Rich Disordered Rocksalt Cathodes

We present the development of an electrochemical formation method that enables rapid and in-situ formation of a partially disordered spinel phase from an initially disordered rocksalt (DRX) cathode. Recent studies on Mn-rich DRX materials have demonstrated the emergence of a spinel-like phase during cycling, resulting in improved capacity retention, energy density, and rate capability compared to materials which do not transform1,2,3. However, this transformation process typically requires weeks of cycling at relatively low rates, posing significant challenges for the commercialization of such materials.To gain a deeper understanding of this transformation process, we carry-out a systematic study to determine whether the formation process of these spinel-like materials can be accelerated through a precycling formation process. By employing a novel electrochemical approach to purposefully stimulate the transformation, we successfully reduce the time required from several weeks to as little as one day for Mn-rich DRX. Extended cycling and synchrotron X-ray diffraction confirm that these materials exhibit both equivalent performance and resulting structure to those obtained through conventional cycling over a much longer duration. Further characterization of this material using HAADF and 4-D STEM reveals the complex local structure of the spinel-like partial ordering. It is hoped that such a practical electrochemical formation method will allow for an acceleration of research on these earth-abundant and energy-dense materials. Li, L.; Lun, Z.; Chen, D.; Yue, Y.; Tong, W.; Chen, G.; Ceder, G.; Wang, C. Fluorination-Enhanced Surface Stability of Cation-Disordered Rocksalt Cathodes for Li-Ion Batteries. Adv Funct Mater 2021, 31 (25), 2101888. https://doi.org/10.1002/adfm.202101888.Ahn, J.; Giovine, R.; Wu, V. C.; Koirala, K. P.; Wang, C.; Clément, R. J.; Chen, G. Ultrahigh-Capacity Rocksalt Cathodes Enabled by Cycling-Activated Structural Changes. Adv. Energy Mater. 2023. https://doi.org/10.1002/aenm.202300221. Cai, Z.; Ouyang, B.; Hau, H.-M.; Chen, T.; Giovine, R.; Koirala, K. P.; Li, L.; Ji, H.; Ha, Y.; Sun, Y.; Huang, J.; Chen, Y.; Wu, V.; Yang, W.; Wang, C.; Clément, R. J.; Lun, Z.; Ceder, G. In Situ Formed Partially Disordered Phases as Earth-Abundant Mn-Rich Cathode Materials. Nat. Energy 2023, 1–10. https://doi.org/10.1038/s41560-023-01375-9.

Imaging Structural Evolution on Cycling of Li- and Mn-Rich Cathode Materials Using Combined ADF and Ptychography in STEM

Imaging Structural Evolution on Cycling of Li- and Mn-Rich Cathode Materials Using Combined ADF and Ptychography in STEM

Constructing partially spinel phase in Mn-rich cathode material

As a cathode with earth-abundant element, Mn-rich oxide has been explored to enable the scaling of the Li-ion battery. However, the irreversible transition metals migration and phase transformations in Mn-rich materials have been proven to be detrimental, which trigger capacity degradation, voltage decay, and poor kinetics. In this work, using Li2MnO3 as model, spinel-layered biphase was constructed via Ti doping and de-lithiation modification, which exhibits higher available capacity of 236.9 mAh/g (55.6 mAh/g for pristine material) and improved rate performance. Compared with pristine Mn-based cathodes, the new phase delivers neglectable voltage degradation during electrochemical cycling. Based on systematic characterization, the new phase exhibits higher oxygen redox reversibility and accelerated kinetics of lithium ions transformation. This work provides a new strategy for designing high-energy density Mn-rich cathodes.

Engineering Transition Metal Layers for Long Lasting Anionic Redox in Layered Sodium Manganese Oxide

Oxygen-redox-based layered cathode materials are of great importance in realizing high-energy-density sodium-ion batteries (SIBs) that can satisfy the demands of next-generation energy storage technologies. However, Mn-based layered materials (P2-type Na-poor Nay[AxMn1−x]O2, where A=alkali ions) still suffer from poor reversibility during oxygen-redox reactions and low conductivity. Herein, we introduce P2-Na0.75[Li0.15Ni0.15Mn0.7]O2 and P2-Na0.6[Li0.15Co0.15Mn0.7]O2, an oxygen-redox-based layered oxide cathode materials for SIBs. The effect of Ni and Co doping on the electrochemical performance was investigated by comparison with Ni-free and Co-free P2-Na0.67[Li0.22Mn0.78]O2 material. Combined experiments (galvanostatic cycling, neutron powder diffraction (NPD), X-ray absorption spectroscopy (XANES), X-ray photoelectron spectroscopy (XPS), and nuclear magnetic resonance (7Li NMR)) and theoretical studies (density functional theory, DFT calculations) confirmed that Ni substitution not only increases the operating voltage and decreases voltage hysteresis but also improves the cycling stability by reducing Li migration from transition metal (TM) to Na layers. It is also anticipated that having Na–O–Li configuration in a Mn4+-based layered material is important to active oxygen redox and that Co doping is crucial for improving the electrical conductivity. This research demonstrates the effect of Ni and Co doping in P2-type layered materials and suggests a new strategy of using Mn-rich cathode materials via oxygen redox with optimization of doping elements for SIBs.

Optimizing Dopant for Enhanced Oxygen-Redox Performance in P2-Na Cathode Materials for Sodium-Ion Batteries

Recently, there has been a lot of interest in using oxygen-redox-based cathode materials for sodium-ion batteries (SIBs) due to their potential for providing additional capacity in the high-voltage range. However, they have some matter, including fast capacity fading and poor rate capability. Herein, P2-Na0.7[Li0.1Ni0.1Mg0.1Mn0.7]O2 is introduced, an layered oxide cathode material for Na-ion batteries based on oxygen-redox. The effect of Ni doping is improved electrochemical performance. Mg2+ in transition metal layer delivered relatively higher capacity via oxygen redox in high voltage region by comparison with Mg-free P2-Na0.75[Li0.15Ni0.15Mn0.7]O2, showing a specific discharge capacity of 175mAh g-1 with 83% of capacity retention after 100 cycles. This study show how the co-doping of Li, Ni and Mg can impact P2-type layered materials and proposes a novel approach to utilizing Mn-rich cathode materials through oxygen redox, with a focus on optimizing the dopant for sodium-ion batteries.

In situ formed partially disordered phases as earth-abundant Mn-rich cathode materials

AbstractEarth-abundant cathode materials are urgently needed to enable scaling of the Li-ion industry to multiply terawatt hours of annual production, necessitating reconsideration of how good cathode materials can be obtained. Irreversible transition metal migration and phase transformations in Li-ion cathodes are typically believed to be detrimental because they may trigger voltage hysteresis, poor kinetics and capacity degradation. Here we challenge this conventional consensus by reporting an unusual phase transformation from disordered Li- and Mn-rich rock salts to a new phase (named δ), which displays partial spinel-like ordering with short coherence length and exhibits high energy density and rate capability. Unlike other Mn-based cathodes, the δ phase exhibits almost no voltage fade upon cycling. We identify the driving force and kinetics of this in situ cathode formation and establish design guidelines for Li- and Mn-rich compositions that combine high energy density, high rate capability and good cyclability, thereby enabling Mn-based energy storage.

(Invited) Confirmation of Na-O-Li Configuration for P2-Type Cathode Materials

Oxygen-redox-based cathode materials for sodium-ion batteries (SIBs) have attracted considerable attention in recent years owing to their possibility of delivering additional capacity in the high-voltage region. However, they still suffer from not only fast capacity fading but also poor rate capability. Herein, we introduce P2-Na0.75[Li0.15Ni0.15Mn0.7]O2, an oxygen-redox-based layered oxide cathode material for SIBs. The effect of Ni doping on the electrochemical performance was investigated by comparison with Ni-free P2-Na0.67[Li0.22Mn0.78]O2. The Na0.75[Li0.15Ni0.15Mn0.7]O2 delivered a specific capacity of ~160 mAh g-1 in the voltage region of 1.5–4.6 V at 0.1C in Na cells. Combined experiments (galvanostatic cycling, neutron powder diffraction (NPD), X-ray absorption spectroscopy (XANES), X-ray photoelectron spectroscopy (XPS), and nuclear magnetic resonance (7Li NMR)) and theoretical studies (density functional theory, DFT calculations) confirmed that Ni substitution not only increases the operating voltage and decreases voltage hysteresis but also improves the cycling stability by reducing Li migration from transition metal (TM) to Na layers. This research demonstrates the effect of Li and Ni co-doping in P2-type layered materials and suggests a new strategy of using Mn-rich cathode materials via oxygen redox with optimization of doping elements for SIBs.

Nanocomposite Engineering of High-Capacity Partially Ordered Cathode Materials for Li-Ion Batteries

Understanding the local cation order in the crystal structure and its correlation with cycling performances has promoted the development of high-energy Mn-rich cathode materials for Li-ion batteries, such as Li- and Mn-rich layered cathodes (LMR, e.g., Li1.2Ni0.2Mn0.6O2) that are considered as nanocomposite layered materials with C2/m Li2MnO3-type medium-range order (MRO). Also, Li diffusivity in high-capacity Mn-based disordered rock-salt (DRX) cathodes (e.g., Li1.2Mn0.6Nb0.2O2) was found to be influenced by the short-range order (SRO) of cations, highlighting the criticality of engineering the local cation order in designing high-energy materials. In this presentation, we will discuss the nanocomposite, heterogeneous nature (like MRO found in LMR) of ultrahigh-capacity partially ordered cathodes (e.g., Li1.68Mn1.6O3.7F0.3) made of distinct domains of spinel-, DRX- and layered-like phases, which is different from conventional single-phase DRX cathodes. This multi-scale understanding of ordering informs engineering the nanocomposite material via Ti doping, altering the intra-particle characteristics to increase the content of the rock-salt phase and heterogeneity within a particle. This strategy greatly improves the reversibility of Mn- and O-redox processes to enhance the cycling stability of the partially ordered DRX cathodes (nearly ~30 % improvement of capacity retention). Overall, our work sheds light on the importance of nanocomposite engineering to develop ultrahigh-performance, low-cost Li-ion cathode materials.

Nanocomposite Engineering of a High-Capacity Partially Ordered Cathode for Li-Ion Batteries.

Understanding the local cation order in the crystal structure and its correlation with electrochemical performances has advanced the development of high-energy Mn-rich cathode materials for Li-ion batteries, notably Li- and Mn-rich layered cathodes (LMR, e.g., Li1.2 Ni0.13 Mn0.54 Co0.13 O2 ) that are considered as nanocomposite layered materials with C2/m Li2 MnO3 -type medium-range order (MRO). Moreover, the Li-transport rate in high-capacity Mn-based disordered rock-salt (DRX) cathodes (e.g., Li1.2 Mn0.4 Ti0.4 O2 ) is found to be influenced by the short-range order of cations, underlining the importance of engineering the local cation order in designing high-energy materials. Herein, the nanocomposite is revealed, withaheterogeneous nature (like MRO found in LMR) of ultrahigh-capacity partially ordered cathodes (e.g., Li1.68 Mn1.6 O3.7 F0.3 ) made of distinct domains of spinel-, DRX- and layered-like phases, contrary to conventional single-phase DRX cathodes. This multi-scale understanding of ordering informs engineering the nanocomposite material via Ti doping, altering the intra-particle characteristics to increase the content of the rock-salt phase and heterogeneity within a particle. This strategy markedly improves the reversibility of both Mn- and O-redox processes to enhance the cycling stability of the partially ordered DRX cathodes (nearly ≈30% improvement of capacity retention). This work sheds light on the importance of nanocomposite engineering to develop ultrahigh-performance, low-cost Li-ion cathode materials.

Sulfuretted Li- and Mn-rich cathode material with epitaxial spinel stabilizer for ultra-long cycle Li-ion battery

The utilization of Li- and Mn-rich layered cathode materials is an effective route to break the energy density bottleneck of lithium-ion batteries for practical applications. However, these materials always suffer from severe structural degradation and oxygen loss during long-term cycling process. Herein, the integrated modification was realized by epitaxial spinel phase film grown on the surface of Li1.2Mn0.54Ni0.13Co0.13O2 (LMR) and proper S elements doping to obtain outstanding electrochemical performance. The introduced spinel phase stabilizer can enhance Li ion conductivity and reduce the particle cracks and phase collapse during repeated cycling processes. The formation of TM-S bond configuration induced by S incorporation can effectively accelerate the lithium ions diffusion and suppress the undesired oxygen redox. Therefore, the LMRS@S cathode displays an excellent electrochemical performance: ultra-long-cycling stability with capacity retention of 82.1% after 600 cycles at 1 C, higher initial Coulomb efficiency (84.7% vs 76.3%) and excellent full cell performance (81.7% capacity retention after 140 cycles). The synergistic strategies with surface stabilizer and S-incorporation affords a promising way to design superior cycling performance LMR cathode material.

(Invited) Non-Conventional, Multimodal Strategies to Create Synergistic Effects on Electrode-Electrolyte Interphase Stabilities in Lithium-Ion Batteries

Ni- or Mn-rich cathode materials have attracted great interest for electric vehicle (EV) applications due to their high gravimetric and volumetric energy densities and low- or cobalt-free compositions. For example, Ni-rich LiNi1-xCox/2Mnx/2O2 (NMC) layered cathodes can deliver > 200 mAh/g when they operate at above 4.3 Vvs.Li. Cobalt-free LiNi0.5Mn1.5O4 (LNMO) spinel operates at around 4.75 Vvs.Li and delivers an excellent rate capability due to three-dimensional Li-diffusion pathways in its lattice. However, due to lack of high-voltage stabilities of conventional, carbonate-based electrolytes, such high-voltage operation (> 4.3 Vvs.Li) led to unwanted electrolyte oxidation and simultaneous parasitic reactions occurring at CEI. Different types of cathodes have their own failure mechanisms which are often associated with side reactions occurring at solid-electrolyte interphase (SEI) on graphite anodes. This was possible because the reaction by-products from CEI could migrate and attack SEI layers on anodes.To address the high-voltage related issues, various approaches have been proposed such as tuning chemical compositions of cathode active materials, surface modification of cathodes or anodes, and stabilizing the CEI or SEI using electrolyte additives. In literature, however, there was no single solution that can resolve all the complex issues occurring in the high-voltage battery cells. For example, coated layers on active materials are prone to fail during long-term cycling due to mechanical failure (e.g., crack and pulverization) of active materials. In addition, not only active materials but carbon conductors (e.g., acetylene black) inside a cathode leads to the electrolyte oxidation, prompted by its high electronic conductivity and large surface area. Considering that such parasitic reactions originate from CEI and propagate to anode SEI, there is a dire need of multimodal strategies that can create synergistic effect on stabilizing the electrode-electrolyte interphases in cell level.In this presentation, I will introduce non-conventional strategies that can effectively enhance electrode-electrolyte interphase stabilities and improve full-cell (i.e., using graphite anodes) performances. First, formation of self-healing CEI on cathode active materials will be demonstrated. The interface of LNMO spinel oxide will be passivated by Ti-enriched shell via sacrificial transition metal dissolutions and suppress the side reactions occurring at CEI. Second, a strategy for passivating both cathode active materials and carbon black conductor by using functional binders will be demonstrated. Since the binder coating will be performed in-situ during a slurry preparation, no extra process will be required while implementing this approach to a cell manufacturing. Finally, solid-electrolyte implemented NMC or LNMO cathodes will be demonstrated as an effective approach to stabilize the high-voltage performance by offering good Li-ion conduction pathways at CEI layer.Improvement mechanism of each approach will be also presented based on physical and electrochemical characterization data. Electrochemical impedance spectroscopy (EIS) revealed a suppression of CEI interfacial resistance during cycling, suggesting the enhanced stability of the multifunctional CEI. Various microscopy and spectroscopy data acquired from cycle-aged CEI and graphite SEI will be corelated to the cell performance data and identify the CEI property-performance relationship. These unique multifunction CEI strategies will be applicable to various cathode materials for the next-generation Li-ion batteries. Figure 1

Origin of structural degradation in Li-rich layered oxide cathode

Li- and Mn-rich(LMR) cathode materials that utilize both cation and anion redox can yield substantial increases in battery energy density1-3. However, although voltage decay issues cause continuous energy loss and impede commercialization, the prerequisite driving force for this phenomenon remains a mystery3-6 Here, with in situ nanoscale sensitive coherent X-ray diffraction imaging techniques, we reveal that nanostrain and lattice displacement accumulate continuously during operation of the cell. Evidence shows that this effect is the driving force for both structure degradation and oxygen loss, which trigger the well-known rapid voltage decay in LMR cathodes. By carrying out micro- to macro-length characterizations that span atomic structure, the primary particle, multiparticle and electrode levels, we demonstrate that the heterogeneous nature of LMR cathodes inevitably causes pernicious phase displacement/strain, which cannot be eliminated by conventional doping or coating methods. We therefore propose mesostructural design as a strategy to mitigate lattice displacement and inhomogeneous electrochemical/structural evolutions, thereby achieving stable voltage and capacity profiles. These findings highlight the significance of lattice strain/displacement in causing voltage decay and will inspire a wave of efforts to unlock the potential of the broad-scale commercialization of LMR cathode materials.

Improved Electrochemical Behavior and Thermal Stability of Li and Mn-Rich Cathode Materials Modified by Lithium Sulfate Surface Treatment

High-energy cathode materials that are Li- and Mn-rich lithiated oxides—for instance, 0.35Li2MnO3.0.65LiNi0.35Mn0.45Co0.20O2 (HE-NCM)—are promising for advanced lithium-ion batteries. However, HE-NCM cathodes suffer from severe degradation during cycling, causing gradual capacity loss, voltage fading, and low-rate capability performance. In this work, we applied an effective approach to creating a nano-sized surface layer of Li2SO4 on the above material, providing mitigation of the interfacial side reactions while retaining the structural integrity of the cathodes upon extended cycling. The Li2SO4 coating was formed on the surface of the material by mixing it with nanocrystalline Li2SO4 and annealing at 600 °C. We established enhanced electrochemical behavior with ~20% higher discharge capacity, improved charge-transfer kinetics, and higher rate capability of HE-NCM cathodes due to the presence of the Li2SO4 coating. Online electrochemical mass spectrometry studies revealed lower CO2 and H2 evolution in the treated samples, implying that the Li2SO4 layer partially suppresses the electrolyte degradation during the initial cycle. In addition, a ~28% improvement in the thermal stability of the Li2SO4-treated samples in reactions with battery solution was also shown by DSC studies. The post-cycling analysis allowed us to conclude that the Li2SO4 phase remained on the surface and retained its structure after 100 cycles.

Enhanced Li-storage performance of In-doped Li1.21[Mn0.54Ni0.125Co0.125]O2 as Li- and Mn-rich cathode materials for lithium-ion batteries

Enhanced Li-storage performance of In-doped Li1.21[Mn0.54Ni0.125Co0.125]O2 as Li- and Mn-rich cathode materials for lithium-ion batteries

Sulfuretted Li- and Mn-Rich Cathode Material with Epitaxial Spinel Stabilizer for Ultra-Long Cycle Li-Ion Battery

Sulfuretted Li- and Mn-Rich Cathode Material with Epitaxial Spinel Stabilizer for Ultra-Long Cycle Li-Ion Battery

Topologically protected oxygen redox in a layered manganese oxide cathode for sustainable batteries

Manganese could be the element of choice for cathode materials used in large-scale energy storage systems owing to its abundance and low toxicity levels. However, both lithium- and sodium-ion batteries adopting this electrode chemistry suffer from rapid performance fading, suggesting a major technical barrier that must be overcome. Here we report a P3-type layered manganese oxide cathode Na0.6Li0.2Mn0.8O2 (NLMO) that delivers a high capacity of 240 mAh g−1 with outstanding cycling stability in a lithium half-cell. Combined experimental and theoretical characterizations reveal a characteristic topological feature that enables the good electrochemical performance. Specifically, the -α-γ- layer stack provides topological protection for lattice oxygen redox, whereas reversibility is absent in P2-structured NLMO, which takes an -α-β- configuration. The identified new order parameter opens an avenue towards the rational design of reversible Mn-rich cathode materials for sustainable batteries.

P2-Na2/3Mn0.8M0.1M′0.1O2 (M = Zn, Fe and M′ = Cu, Al, Ti): A Detailed Crystal Structure Evolution Investigation

Incorporation of various transition metals has been shown to improve the electrochemical performance of Mn-rich Na-ion cathode materials. A greater comprehension of the role of dopant ions, particularly with regard to Mn-rich layered oxides as materials for the positive electrode of Na-ion batteries, is required for their continual development. Here two similar series of Mn-rich P2 cathode materials P2-Na2/3Mn0.8M0.1M′0.1O2 (M = Fe, Zn and M′ = Cu, Al, Ti) are explored, focusing on structural analysis using high-resolution operando synchrotron X-ray diffraction. Notably, under the cycling conditions employed, no P2 to O2 phase transitions toward the charged state were identified for any of the materials investigated. Particularly stable solid solution evolution was observed for P2-Na2/3Mn0.8Zn0.1Cu0.1O2 and P2-Na2/3Mn0.8Zn0.1Al0.1O2 when cycled at 40 mA.g–1 which reflects the electrochemical properties of the materials investigated herein and illustrates that Zn is an excellent choice of dopant for Mn-rich cathode materials. Moreover, the better cyclability of P2-Na2/3Mn0.8Zn0.1Al0.1O2 compared with P2-Na2/3Mn0.8Zn0.1Cu0.1O2 is in keeping with the minimal structural changes observed. This demonstrates that although oxidation state predictions to optimize the initial Mn oxidation state are a good way of initially selecting materials, to truly exploit Mn-rich P2-type materials it is necessary to build up an in-depth understanding of both oxidation states and the associated Jahn–Teller distortion as well as the subtle interplay of synergistic and antagonistic interactions between dopants. Overall, this study illustrates the value of structural investigations to assist in the rational design and validation of novel high-performance materials; the results highlight that the interplay between dopants in addition to the average Mn oxidation state are both crucial considerations when designing high-performance Mn-rich layered oxide materials.

Resolving atomic-scale phase transformation and oxygen loss mechanism in ultrahigh-nickel layered cathodes for cobalt-free lithium-ion batteries

Summary Doped LiNiO2 has recently become one of the most promising cathode materials for its high specific energy, long cycle life, and reduced cobalt content. Despite this, the degradation mechanism of LiNiO2 and its derivatives still remains elusive. Here, by combining in situ electron microscopy and first-principles calculations, we elucidate the atomic-level chemomechanical degradation pathway of LiNiO2-derived cathodes. We uncover that the O1 phase formed at high voltages acts as a preferential site for rock-salt transformation via a two-step pathway involving cation mixing and shear along (003) planes. Moreover, electron tomography reveals that planar cracks nucleated simultaneously from particle interior and surface propagate along the [100] direction on (003) planes, accompanied by concurrent structural degradation in a discrete manner. Our results provide an in-depth understanding of the degradation mechanism of LiNiO2-derived cathodes, pointing out the concept that suppressing the O1 phase and oxygen loss is key to stabilizing LiNiO2 for developing next-generation high-energy cathode materials.

Oxygen-redox reactions in LiCoO2 cathode without O–O bonding during charge-discharge

Summary Oxygen activity in highly delithiated LiCoO2 is critical to fully utilizing the energy density of this high-tap-density cathode but still lacks a clear understanding. In this work, we combined the results of several experimental techniques, especially resonant inelastic X-ray scattering (RIXS) and neutron pair distribution function (NPDF) analysis, together with theoretical calculations to study this topic. Our results conclude that oxygen redox takes place globally in the lattice, rather than forming localized dimerization as previously thought. RIXS results directly reveal the reversible oxygen redox, and NPDF results show that the O–O pair distance is considerably shortened in the highly delithiated LiCoO2. Theoretical calculations indicate that no O–O bonding is formed in LiCoO2, in sharp contrast to the lithium-rich system in which O–O bonding does form. These results provide the rationale for achieving a reversible deep delithiation and high energy density for LiCoO2-based electrodes.