Voltage decay and redox asymmetry mitigation by reversible cation migration in lithiumrich layered oxide electrodes.

The electrochemical properties and performances of lithium-ion batteries are primarily governed by their constituent electrode materials, whose intrinsic thermodynamic and kinetic properties are understood as the determining factor. As a part of complementing the intrinsic material properties, the strategy of nanosizing has been widely applied to electrodes to improve battery performance. It has been revealed that this not only improves the kinetics of the electrode materials but is also capable of regulating their thermodynamic properties, taking advantage of nanoscale phenomena regarding the changes in redox potential, solid-state solubility of the intercalation compounds, and reaction paths. In addition, the nanosizing of materials has recently enabled the discovery of new energy storage mechanisms, through which unexplored classes of electrodes could be introduced. Herein, we review the nanoscale phenomena discovered or exploited in lithium-ion battery chemistry thus far and discuss their potential implications, providing opportunities to further unveil uncharted electrode materials and chemistries. Finally, we discuss the limitations of the nanoscale phenomena presently employed in battery applications and suggest strategies to overcome these limitations.

Lattice oxygen redox activity offers an unexplored way to access the latent superior electrochemical property of transition metal oxides for rechargeable batteries. However, the redox reaction of the lattice oxygen is often accompanied by unfavorable structural transformations and the corresponding degradation of electrochemical performances, precluding its practical application. Herein, we explore the close interplay between the local structural change during the dynamic intercalation process and the solid-state oxygen electrochemistry in the short-or long-term battery operation for layered transition metal oxides. By employing two model systems of the layered Na 0.6 (Li 0.2 Ti x Mn 0.8−x )O 2 with the oxygen redox capability, it is demonstrated that the substantially distinct evolutions in the oxygen redox activity and reversibility are caused by different cation migration mechanisms available in the system during the de/intercalation (i.e. out-of-plane and in-plane migrations of transition metals (TMs)). We show that the π stabilization upon the oxygen oxidation initially aids in the reversibility of the oxygen redox and is predominant in the absence of TM migrations, however, the π-interacting oxygens are gradually replaced by the σ-interacting oxygens that trigger the formation of O-O dimers and the structural destabilization over cycles. More importantly, it is revealed that the distinct TM migration paths available in the respective layered materials govern the conversion from π to σ interactions and its kinetics. It infers that regulating the dynamics of TMs in the layered materials can play a key role in delaying or inhibiting the deterioration of the oxygen redox reversibility. These ndings constitute a step forward in unraveling the correlation between the local structural evolution and the reversibility of solid-state oxygen electrochemistry, and provide a guidance for developing oxygen-redox layered electrode materials. Main TextThe use of reversible lattice oxygen redox has been a transformative strategy for accessing superior electrochemical activity of transition metal oxide-based materials such as in catalysts and battery electrodes. [1][2][3] In particular, with the growing demands for the next-generation battery technology, extensive efforts have been devoted to exploiting the lattice oxygen redox in developing novel electrode materials with higher energy densities. Lithium-rich layered oxides (Li 1 + x TM 1−x O 2 , TM: transition metal) are one of the examples, which could exhibit the remarkable oxygen redox activity. 4,5 The cumulative cationic and anionic redox activities from TM and oxygen, respectively, enable them to deliver energy and power densities that can surpass those of conventional lithium layered oxides (LiTMO 2 ). More recently, various transition metal oxides have been investigated as being capable of showing the anionic redox activity, which include not only lithium-rich layered compounds but also sodium layered oxides, disordered rocksalt phases, partially ordered spinels and...

Ni-rich layered LiNi x Co y Mn 1−x−y O 2 (LNCM) with Ni content over >90% is considered as a promising lithium ion battery (LIB) cathode, attributed by its low cost and high practical capacity. However, Ni-rich LNCM inevitably suffers rapid capacity fading at a high state of charge due to the mechanochemical breakdown; in particular, the microcrack formation has been regarded as one of the main culprits for Ni-rich layered cathode failure. To address these issues, Ni-rich layered cathodes with a textured microstructure are developed by phosphorous and boron doping. Attributed by the textured morphology, both phosphorous-and boron-doped cathodes suppress microcrack formation and show enhanced cycle stability compared to the undoped cathode. However, there exists a meaningful capacity retention difference between the doped cathodes. By adapting the various analysis techniques, it is shown that the boron-doped Ni-rich layered cathode displays better cycle stability not only by its ability to suppress microcracks during cycling but also by its primary particle morphology that is reluctant to oxygen evolution. The present work reveals that not only restraint of particle cracks but also suppression of oxygen release by developing the oxygen stable facets is important for further improvements in state-of-the-art Li ion battery Ni-rich layered cathode materials.

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The rampant generation of lithium hydroxidea nd carbonate impurities,commonly knownasresidual lithium, is apractical obstacle to the mass-scale synthesis and handling of high-nickel (> 90 %) layered oxides and their use as highenergy-density cathodes for lithium-ion batteries.H erein, we suggest as imple in situ method to control the residual lithium chemistry of ah igh-nickel lithium layered oxide,L i-(Ni 0.91 Co 0.06 Mn 0.03)O 2 (NCM9163), with minimal side effects. Based on thermodynamic considerations of the preferred reactions,w es ystematically designed as ynthesis process that preemptively converts residual Li 2 O(the origin of LiOH and Li 2 CO 3)into amore stable compound by injecting reactive SO 2 gas.T he preformed lithium sulfate thin film significantly suppresses the generation of LiOH and Li 2 CO 3 during both synthesis and storage,t herebym itigating slurry gelation and gas evolution and improvingthe cycle stability.

The anionic redox activity in lithium‐rich layered oxides has the potential to boost the energy density of lithium‐ion batteries. Although it is widely accepted that the anionic redox activity stems from the orphaned oxygen energy level, its regulation and structural stabilization, which are essential for practical employment, remain still elusive, requiring an improved fundamental understanding. Herein, the oxygen redox activity for a wide range of 3d transition‐metal‐based Li2TMO3 compounds is investigated and the intrinsic competition between the cationic and anionic redox reaction is unveiled. It is demonstrated that the energy level of the orphaned oxygen state (and, correspondingly, the activity) is delicately governed by the type and number of neighboring transition metals owing to the π‐type interactions between LiOLi and M t2g states. Based on these findings, a simple model that can be used to estimate the anionic redox activity of various lithium‐rich layered oxides is proposed. The model explains the recently reported significantly different oxygen redox voltages or inactivity in lithium‐rich materials despite the commonly observed LiOLi states with presumably unhybridized character. The discovery of hidden factors that rule the anionic redox in lithium‐rich cathode materials will aid in enabling controlled cumulative cationic and anionic redox reactions.

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