Dislocation and oxygen-release driven delithiation in Li2MnO3

Oxygen vacancy chemistry in oxide cathodes

Secondary batteries are a core technology for clean energy storage and conversion systems, to reduce environmental pollution and alleviate the energy crisis. Oxide cathodes play a vital role in revolutionizing battery technology due to their high capacity and voltage for oxide-based batteries. However, oxygen vacancies (OVs) are an essential type of defect that exist predominantly in both the bulk and surface regions of transition metal (TM) oxide batteries, and have a crucial impact on battery performance. This paper reviews previous studies from the past few decades that have investigated the intrinsic and anionic redox-mediated OVs in the field of secondary batteries. We focus on discussing the formation and evolution of these OVs from both thermodynamic and kinetic perspectives, as well as their impact on the thermodynamic and kinetic properties of oxide cathodes. Finally, we offer insights into the utilization of OVs to enhance the energy density and lifespan of batteries. We expect that this review will advance our understanding of the role of OVs and subsequently boost the development of high-performance electrode materials for next-generation energy storage devices.

Localizing Oxygen Lattice Evolutions Eliminates Oxygen Release and Voltage Decay in All-Mn-Based Li-Rich Cathodes

All-Mn-based Li-rich cathodes Li₂ MnO₃ have attracted extensive attention because of their cost advantage and ultrahigh theoretical capacity. However, the unstable anionic redox reaction (ARR), which involves irreversible oxygen releases, causes declines in cycling capacity and intercalation potential, thus hindering their practical applications. Here, it is proposed that introducing stacking-fault defects into the Li₂ MnO₃ can localize oxygen lattice evolutions and stabilize the ARR, eliminating oxygen releases. The thus-made cathode has a highly reversible capacity (320 mA h g^-1 ) and achieves excellent cycling stability. After 100 cycles, the capacity retention rate is 86% and the voltage decay is practically eliminated at 0.19 mV per cycle. Attributing to the stable ARR, samples show reduced stress-strain and phase transitions. Neutron pair distribution function (nPDF) measurements indicate that there is a structure response of localized oxygen lattice distortion to the ARR and the average oxygen lattice framework is well-preserved which is a prerequisite for the high cycle reversibility.

Understanding the different effects of 4d-transition metals on the performance of Li-rich cathode Li2MnO3 by first-principles

The poor cycling performance of Li-rich cathode Li₂MnO₃, a promising cathode for next-generation Li-ion batteries, limits its commercial applications. Transition metal (TM) doping is widely applied to optimize the electrochemical performance of Li₂MnO₃, where the d valence electrons of the TM play a crucial role. Nevertheless, the rule of the doping effect of TM with various numbers of d electrons has not been well summarized. In this work, 4d-TMs (Zr, Nb, Mo, Ru and Rh) are selected as dilute doping elements for Li₂MnO₃ to evaluate their effect on the performance of Li₂MnO₃ through first-principles calculations. The calculations indicate that as the number of 4d electrons increases, the doped TM transforms from an electrochemically inert state (Zr and Nb) to an electrochemically active state (Mo, Ru and Rh) in Li₂MnO₃. Meanwhile, the orbital hybridization between the 4d electrons of the TM and the 2p electrons of O becomes stronger from Zr to Rh, which promotes the co-oxidation of the TM and O for charge compensation and alleviates the excessive oxidation of O, thus enhancing the stability of O. Moreover, the oxidation of the doped TM and lattice Mn during charging can trigger a decrease in the initial average delithiation potential. Although the 4d-TMs exhibit slight promoting or inhibiting effects on Li diffusion, no obvious rule related to the number of d electrons has been found. Our work highlights the rule of the doping effect of TMs with different 4d electrons on the electrochemical performance of Li₂MnO₃ and would facilitate a better design of Li₂MnO₃ cathode materials.

Single-Crystalline Cathodes for Advanced Li-Ion Batteries: Progress and Challenges

Single-crystalline cathodes are the most promising candidates for high-energy-density lithium-ion batteries (LIBs). Compared to their polycrystalline counterparts, single-crystalline cathodes have advantages over liquid-electrolyte-based LIBs in terms of cycle life, structural stability, thermal stability, safety, and storage but also have a potential application in solid-state LIBs. In this review, the development history and recent progress of single-crystalline cathodes are reviewed, focusing on properties, synthesis, challenges, solutions, and characterization. Synthesis of single-crystalline cathodes usually involves preparing precursors and subsequent calcination, which are summarized in the details. In the following sections, the development issues of single-crystalline cathodes, including kinetic limitations, interfacial side reactions, safety issues, reversible planar gliding and micro-cracking, and particle size distribution and agglomeration, are systematically analyzed, followed by current solutions and characterization techniques. Finally, this review is concluded with proposed research thrusts for the future development of single-crystalline cathodes.

Atomic-Scale Observations of Oxygen Release Degradation in Sulfide-Based All-Solid-State Batteries with Layered Oxide Cathodes

All-solid-state batteries exhibit considerable potential for applications in electric vehicles. Understanding and controlling the oxygen release from the layered cathode active material are essential in achieving long-term operation of all-solid-state batteries because the oxygen release degrades the cathode material and the solid electrolyte, triggering capacity degradation. In this study, we verified the specific interface where the oxygen release is accelerated by atomic-scale scanning transmission electron microscopy and electron energy loss spectroscopy analyses. Oxygen release is suppressed at the interface where the LiNbO₃ coating layer is sufficiently formed. Decomposition products on the solid electrolyte and the antisite defect layers on the cathode surface are formed by oxygen release at the interface where the Li₂S-P₂S₅ solid electrolyte and Li(Ni_1/3Mn_1/3Ni_1/3)O₂ cathode are in direct contact. These irreversible passivation layers lead to capacity degradation. In addition, we found that exfoliation of the LiNbO₃ coating from the cathode not only physically breaks the Li conduction path but also results in oxygen release and the deterioration of the cathode. These atomic-scale insights can further advance the development of all-solid-state batteries by suppressing oxygen release.

Atomic-Resolution STEM Image Denoising by Total Variation Regularization

Atomic-resolution electron microscopy imaging of solid state material is a powerful method for structural analysis. Scanning transmission electron microscopy (STEM) is one of the actively used techniques to directly observe atoms in materials. However, some materials are easily damaged by the electron beam irradiation, and only noisy images are available when we decrease the electron dose to avoid beam damages. Therefore, a denoising process is necessary for precise structural analysis in low-dose STEM. In this study, we propose total variation (TV) denoising algorithm to remove quantum noise in a STEM image. We defined an entropy of STEM image that corresponds to the image contrast to determine a hyperparameter and we found that there is a hyperparameter that maximize the entropy. We acquired atomic resolution STEM image of CaF2 viewed along the [001] direction, and executed TV denoising. The atomic columns of Ca and F are clearly visualized by the TV denoising, and atomic position of Ca and F are determined with the error of ± 1 pm and ± 4 pm, respectively.

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 density^1-3. However, although voltage decay issues cause continuous energy loss and impede commercialization, the prerequisite driving force for this phenomenon remains a mystery^3-6 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.

koLayered Oxide Cathode-Electrolyte Interface towards Na-Ion Batteries: Advances and Perspectives

With the ever increasing demand for low-cost and economic sustainable energy storage, Na-ion batteries have received much attention for the application on large-scale energy storage for electric grids because of the worldwide distribution and natural abundance of sodium element, low solvation energy of Na⁺ ion in the electrolyte and the low cost of Al as current collectors. Starting from a brief comparison with Li-ion batteries, this review summarizes the current understanding of layered oxide cathode/electrolyte interphase in NIBs, and discusses the related degradation mechanisms, such as surface reconstruction and transition metal dissolution. Recent advances in constructing stable cathode electrolyte interface (CEI) on layered oxide cathode are systematically summarized, including surface modification of layered oxide cathode materials and formulation of electrolyte. Urgent challenges are detailed in order to provide insight into the imminent developments of NIBs.

Facilitating Reversible Cation Migration and Suppressing O2 Escape for High Performance Li-Rich Oxide Cathodes

High-capacity Li-rich Mn-based oxide cathodes show a great potential in next generation Li-ion batteries but suffer from some critical issues, such as, lattice oxygen escape, irreversible transition metal (TM) cation migration, and voltage decay. Herein, a comprehensive structural modulation in the bulk and surface of Li-rich cathodes is proposed through simultaneously introducing oxygen vacancies and P doping to mitigate these issues, and the improvement mechanism is revealed. First, oxygen vacancies and P doping elongates OO distance, which lowers the energy barrier and enhances the reversible cation migration. Second, reversible cation migration elevates the discharge voltage, inhibits voltage decay and lattice oxygen escape by increasing the Li vacancy-TM antisite at charge, and decreasing the trapped cations at discharge. Third, oxygen vacancies vary the lattice arrangement on the surface from a layered lattice to a spinel phase, which deactivates oxygen redox and restrains oxygen gas (O₂ ) escape. Fourth, P doping enhances the covalency between cations and anions and elevates lattice stability in bulk. The modulated Li-rich cathode exhibits a high-rate capability, a good cycling stability, a restrained voltage decay, and an elevated working voltage. This study presents insights into regulating oxygen redox by facilitating reversible cation migration and suppressing O₂ escape.

Reaction inhomogeneity coupling with metal rearrangement triggers electrochemical degradation in lithium-rich layered cathode

High-energy density lithium-rich layered oxides are among the most promising candidates for next-generation energy storage. Unfortunately, these materials suffer from severe electrochemical degradation that includes capacity loss and voltage decay during long-term cycling. Present research efforts are primarily focused on understanding voltage decay phenomena while origins for capacity degradation have been largely ignored. Here, we thoroughly investigate causes for electrochemical performance decline with an emphasis on capacity loss in the lithium-rich layered oxides, as well as reaction pathways and kinetics. Advanced synchrotron-based X-ray two-dimensional and three-dimensional imaging techniques are combined with spectroscopic and scattering techniques to spatially visualize the reactivity at multiple length-scales on lithium- and manganese-rich layered oxides. These methods provide direct evidence for inhomogeneous manganese reactivity and ionic nickel rearrangement. Coupling deactivated manganese with nickel migration provides sluggish reaction kinetics and induces serious structural instability in the material. Our findings provide new insights and further understanding of electrochemical degradation, which serve to facilitate cathode material design improvements.

Electrochemical degradation is the most critical challenge for Li-rich materials. Here, the authors reveal that manganese related phase reaction inhomogeneity coupling with transition metal rearrangement triggers electrochemical degradation in lithium-rich layered cathode.

First-principles study of Mn antisite defect in Li2MnO3

Lithium-rich layered Li₂MnO₃is regarded as a new generation cathode material for lithium-ion batteries because of its high energy density. Due to the different preparation methods and technological parameters, there are a lot of intrinsic defects in Li₂MnO₃. One frequently observed defect in experiments is Mn antisite defect (Mn_Li). In this work, we study the energetics and electronic properties involving Mn_Liin Li₂MnO₃through first-principles calculations. We find that Mn_Lican reduce the formation energy of Li vacancies around it, but increase that of O vacancies, indicating that Mn_Licould suppress the release of O around it and facilitate capacity retention. Both O and Mn near the Mn_Lican participate in charge compensation in the delithiation process. Furthermore, the effect of Mn_Lion the migration of Li and Mn is investigated. All possible migration paths are considered and it is found that Mn_Limakes the diffusion energy barrier of Li increased, but the diffusion energy barriers of Mn from transition metal layer to Li layer are decreased, especially for the migration of the defect Mn. The insight into the defect properties of Mn_Limakes further contribution to understand the relationship between intrinsic defects and electrochemical properties of Li₂MnO₃.