A New Class of Ternary Compound for Lithium-Ion Battery: from Composite to Solid Solution

Searching for high-performance cathode materials is a crucial task to develop advanced lithium-ion batteries (LIBs) with high-energy densities for electrical vehicles (EVs). As a promising lithium-rich material, Li₂MnO₃ delivers high capacity over 200 mAh g^-1 but suffers from poor structural stability and electronic conductivity. Replacing Mn⁴⁺ ions by relatively larger Sn⁴⁺ ions is regarded as a possible strategy to improve structural stability and thus cycling performance of Li₂MnO₃ material. However, large difference in ionic radii of Mn⁴⁺ and Sn⁴⁺ ions leads to phase separation of Li₂MnO₃ and Li₂SnO₃ during high-temperature synthesis. To prepare solid-solution phase of Li₂MnO₃-Li₂SnO₃, a buffer agent of Ru⁴⁺, whose ionic radius is in between that of Mn⁴⁺ and Sn⁴⁺ ions, is introduced to assist the formation of a single solid-solution phase. The results show that the Li₂RuO₃-Li₂MnO₃-Li₂SnO₃ ternary system evolves from mixed composite phases into a single solid-solution phase with increasing Ru content. Meanwhile, discharge capacity of this ternary system shows significantly increase at the transformation point which is ascribed to the improvement of Li⁺/e^- transportation kinetics and anionic redox chemistry for solid-solution phase. The role of Mn/Sn molar ratio of Li₂RuO₃-Li₂MnO₃-Li₂SnO₃ ternary system has also been studied. It is revealed that higher Sn content benefits cycling stability of the system because Sn⁴⁺ ions with larger sizes could partially block the migration of Mn⁴⁺ and Ru⁴⁺ from transition metal layer to Li layer, thus suppressing structural transformation of the system from layered-to-spinel phase. These findings may enable a new route for exploring ternary or even quaternary lithium-rich cathode materials for LIBs.