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, LiMnO delivers high capacity over 200 mAh g but suffers from poor structural stability and electronic conductivity. Replacing Mn ions by relatively larger Sn ions is regarded as a possible strategy to improve structural stability and thus cycling performance of LiMnO material. However, large difference in ionic radii of Mn and Sn ions leads to phase separation of LiMnO and LiSnO during high-temperature synthesis. To prepare solid-solution phase of LiMnO-LiSnO, a buffer agent of Ru, whose ionic radius is in between that of Mn and Sn ions, is introduced to assist the formation of a single solid-solution phase. The results show that the LiRuO-LiMnO-LiSnO 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 LiRuO-LiMnO-LiSnO ternary system has also been studied. It is revealed that higher Sn content benefits cycling stability of the system because Sn ions with larger sizes could partially block the migration of Mn and Ru 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.