Elucidating the Electrochemical Activation Mechanism of a Li-Rich Layered Oxide Cathode for All-Solid-State Battery using 4D-STEM

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Currently, the cathode materials used in commercial lithium-ion batteries (LIBs) are based on layered oxide structures. With the growing demand for high-capacity batteries, such as for electric vehicles, research has increasingly focused on Li-rich cathode materials, which incorporate additional lithium into transition metal sites. [ 1] Despite their high capacity, these materials face challenges within conventional LIB systems due to Mn dissolution in liquid electrolytes, leading to capacity fading and voltage decay. To address these challenges, research has been conducted on applying Li-rich cathodes to all-solid-state batteries. Unlike liquid electrolytes, solid electrolytes can suppress the Mn dissolution phenomenon in cathode materials. As a result, when Li-rich cathodes are used in all-solid-state batteries, they exhibit improved battery performance with high coulombic efficiency and mitigated voltage decay. [ 2] We synthesized a Li 1.2Ni 0.13Co 0.13Mn 0.54O 2 cathode, equivalent to 0.5 LiNi 1/3Co 1/3Mn 1/3O 2 (NCM) + 0.5 Li 2MnO 3 (LMO), as a Li-rich layered oxide cathode. Using this material, we fabricated an all-solid-state battery (ASSB) system with an LPSCl solid electrolyte, and a conventional lithium-ion battery (cLIB) system with a liquid electrolyte composed of 1 M LiPF₆ in a 1:1 mixture of EC and DMC. When comparing the battery performance of these two systems, we observed that, unlike in cLIB, the ASSB showed an increase in reversible capacity during the initial cycles. [ 3] This increase corresponds to the well-known electrochemical activation process in Li-rich cathodes, occurring in the LMO region within the material. [ 4] The electrochemical activation of LMO is crucial for achieving the high capacity characteristic of Li-rich cathodes and, therefore, is an important process during the initial cycles of ASSBs. However, the exact mechanism by which this activation process occurs remains unclear, primarily due to the inability to clearly distinguish between the NCM nano-domains and LMO nano-domains within the Li-rich particles.

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