Active–Inactive Molten Salt Synthesis of Li- and Mn-Rich Layered Oxide Single Crystals as Cathode Materials for All-Solid-State Batteries

Abstract

Micrometer-sized layered oxide single crystals are considered promising cathode materials for all-solid-state batteries (ASSBs) due to their superior properties compared to those of polycrystalline forms. In addition, Li- and Mn-rich layered oxides (LMRO), represented by the formula xLi₂MnO₃·(1–x)LiTMO₂ (where TM = Ni, Co, and Mn), are noted for their higher energy densities and cost-effectiveness relative to conventional layered LiNi_1–y–zCo_yMn_zO₂ materials. In this regard, micrometer-sized, monodisperse, and discrete LMRO single crystals are synthesized using an active–inactive molten salt method that exploits the high reactivity of LiOH and the negligible reactivity of Li₂SO₄. This approach overcomes the challenges associated with conventional molten salt synthesis in controlling the structure and composition of LMRO. The chemical reactivities of lithium salts (LiOH, LiNO₃, and Li₂SO₄) with transition metal hydroxide precursors are examined to synthesize LMRO single crystals. Our findings show that LiOH and LiNO₃ are highly reactive, whereas Li₂SO₄ remains significantly inert. For this reason, the active–active LiOH–LiNO₃ system forms the LMRO structure (0.73Li₂MnO₃·0.27Li[Ni_0.37Co_0.63]O₂) with Mn-free NCM domains, regardless of the molar fractions of LiOH in LiOH–LiNO₃. In contrast, the active–inactive LiOH–Li₂SO₄ system undergoes significant transformations from spinel to layered structures upon variation of the molar fraction of LiOH. At a LiOH molar fraction of 0.82, this system ultimately forms monodisperse LMRO single crystals (0.65Li₂MnO₃·0.35Li[Ni_0.3Co_0.5Mn_0.2]O₂) with Mn-containing NCM domains. Moreover, LMRO single crystals are investigated as high-capacity cathode materials for ASSBs. In particular, LMRO single crystals (0.65Li₂MnO₃·0.35Li[Ni_0.3Co_0.5Mn_0.2]O₂) demonstrate excellent electrochemical performance in ASSBs, achieving high reversible capacities of 220 mA h g^–1 and stable capacity retention over 300 cycles. These findings underscore the critical role of lithium salt reactivity in determining the structural and compositional characteristics of LMRO single crystals during synthesis, providing valuable insights for improving the electrochemical performance of high-capacity LMRO cathode materials in ASSBs.