Wet-CO2 Pretreatment Process for Reducing Residual Lithium in High-Nickel Layered Oxides for Lithium-Ion Batteries

Reinforcement of Positive Electrode-Electrolyte Interface without Using Electrolyte Additives Through Thermoelectrochemical Oxidation of LiPF6 for Lithium Secondary Batteries

Owing to the limited electrochemical stability window of carbonate electrolytes, the initial formation of a solid electrolyte interphase and surface film on the negative and positive electrode surfaces by the decomposition of the electrolyte component is inevitable for the operation of lithium secondary batteries. The deposited film on the surface of the active material is vital for reducing further electrochemical side reactions at the surface; hence, the manipulation of this formation process is necessary for the appropriate operation of the assembled battery system. In this study, the thermal decomposition of LiPF6 salt is used as a surface passivation agent, which is autocatalytically formed during high-temperature storage. The thermally formed difluorophosphoric acid is subsequently oxidized on the partially charged high-Ni positive electrode surface, which improves the cycleability of lithium metal cells via phosphorus- and fluorine-based surface film formation. Moreover, the improvement in the high-temperature cycleability is demonstrated by controlling the formation process in the lithium-ion pouch cell with a short period of high-temperature storage before battery usage.

Air- and Moisture Robust Surface Modification for Ni-Rich Layered Cathode Materials for Li-Ion Batteries

The mainstream of high-energy cathode development is focused on increasing the Ni-ratio in layered structured cathode materials. The increment of the Ni portion in the layered cathode material escalates not only the deliverable capacity but also the structural degradation. High-Ni layered cathodes are highly vulnerable to exposure to air that contains CO₂ and H₂ O, forming problematic residual lithium compounds at the surface. In this work, a novel air- and moisture robust surface modification is reported for LiNi_0.8 Co_0.1 Mn_0.1 O₂ (NCM811) via the sol-gel coating method that selectively coats the internal surface area of the polycrystalline morphology secondary particles. Bare-, Li₂ SnO₃ -coated and LiCoO₂ -coated NCM811 are exposed to different ambient environments (air, hot-air, and moisture-air) to systematically investigate the correlation between the internal/external coating morphology and performance degradations. The LiCoO₂ -coated NCM811s exhibit high-capacity retention after exposure to all environments, due to the internal surface coating that prevents the penetration of harmful compounds into the polycrystalline NCM811. On the other hand, the Li₂ SnO₃ -coated NCM811s exposed to the ambient environments show gradual capacity fading, implying the occurrence of internal degradation. This paper highlights the impact of the internal degradation of polycrystalline NCM811 after environmental exposure and the correct coating mechanisms required to successfully prevent it.

Beneficial Role of Inherently Formed Residual Lithium Compounds on the Surface of Ni-Rich Cathode Materials for All-Solid-State Batteries

This study validates the beneficial role of residual Li compounds on the surface of Ni-rich cathode materials (LiNi_xCo_yMn_zO₂, NCM). Residual Li compounds on Ni-rich NCM are naturally formed during the synthesis procedure, which degrades the initial Coulombic efficiency and generates slurry gelation during electrode fabrication in Li-ion batteries (LIBs) using liquid electrolytes. To solve this problem, washing pretreatment is usually introduced to remove residual Li compounds on the NCM surface. In contrast to LIBs, we found that residual Li compounds can serve as a functional layer that suppresses the interfacial side reactions of the NCM in all-solid-state batteries (ASSBs). The formation of resistive phosphate-based compounds from the undesirable side reaction during the initial charging step is suppressed by the residual Li compounds on the surface of the NCM, thereby reducing polarization growth in ASSBs and enhancing rate performances. The advantageous effects of the intrinsic residual Li compounds on the NCM surface suggest that the essential washing process of the NCM for the liquid-based LIB system should be reconsidered for ASSB systems.

Towards commercialization of fluorinated cation-disordered rock-salt Li-ion cathodes

Cation-disordered rock-salt cathodes (DRX) are promising materials that could deliver high capacities (>250 mAh g^-1) with Earth abundant elements and materials. However, their electrochemical performances, other than the capacity, should be improved to be competitive cathodes, and many strategies have been introduced to enhance DRXs. Fluorination has been shown to inhibit oxygen loss and increase power density. Nevertheless, fluorinated cation-disordered rock-salts still suffer from rapid material deterioration and low scalability which limit their practical applications. This mini-review highlights the key challenges for the commercialization of fluorinated cation-disordered rock-salts, discusses the underlying reasons behind material failure and proposes future development directions.