As for conventional cathode materials for lithium ion battery (LIB), such as LiMO₂ (M=Ni, Co, Mn), LiFePO₄, LiMn₂O₄, the ratio of Li to metal (Li/M) is 1 or less, which constrains their specific capacities to be in the range of 100-180 mAh/g. As an alternative, lithium rich material (LRM) possesses Li/M ratio of higher than 1, providing more than one reversible (de)intercalation Li⁺ ions per metal. LMR is able to deliver capacity of higher than 250 mAh/g, making it the most promising candidate for next generation high-energy LIBs. Li₂MnO₃, one of the most important family members of LRM, has attracted extensive interest in the research community. Li₂MnO₃ has been composited with layered compounds such as LiMO₂ (M=Ni, Co, Mn), LiFeO₂ and Li₂RuO₃ to form solid solutions of xLi₂MnO₃•(1-x)LiMO₂ (M=Ni, Co, Mn),¹ xLi₂MnO₃•(1-x)LiFeO₂² and xLi₂MnO₃•(1-x)Li₂RuO₃.³ These LMR layered solid solutions deliver high capacity of more than 250 mAh/g due to the involvement of anionic redox electrochemistry during charge/discharge process besides of cationic redox electrochemistry. However, LRM layered oxides suffer from the structural transformation from layered-to-spinel phase, leading to severe voltage decay and capacity loss upon cycling. Cation substitution has been reported as an effective strategy to suppress the structural transformation for lithium rich layered oxides. For example, Sathiya et al. investigated the effect of Sn⁴⁺ doping on electrochemical properties of Li₂RuO₃ LRM and it is revealed Sn⁴⁺ could slow down or prevent trapping of metal ions in interstitial tetrahedral sites due to its larger ionic radius, resulting in enhanced cycling performance and less voltage decay.⁴ Based on the aforementioned work, we propose to partially substitute Mn⁴⁺ ions in Li₂MnO₃ structure by Sn⁴⁺ ions to synthesize Li₂MnO₃- Li₂SnO₃ solid solution, in which case the Sn⁴⁺ ions could block migration pathway of Mn⁴⁺ ions from transition metal layer to Li layer and therefore suppress the structural transformation upon cycling.

Li₂MnO₃ shares a similar crystal structure as Li₂SnO₃ with the same oxide-ion lattice filled alternatively by Li layers and honeycomb LiM₂ layers. Due to the minor distortion of the oxygen stacking, C2/c space group is usually chosen to describe Li₂SnO₃ and C2/m is regarded better to describe Li₂MnO₃ structure. The similarity in structure implies the possibility to form Li₂MnO₃-Li₂SnO₃ solid solution. However, since Sn⁴⁺ ion (0.69 Å) has a much larger radius than Mn⁴⁺ ion (0.54 Å), the substitution of Sn⁴⁺ ions for Mn⁴⁺ ions in Li₂MnO₃ lattice cause severe lattice distortion and thus brings extra lattice energy, leading to phase separation once the substitution exceeds a certain limit. Attempt of synthesizing a single 0.5Li₂MnO₃-0.5Li₂SnO₃ solid solution phase always failed by high-temperature solid-state reaction. Instead, a composite of Li₂MnO₃ and Li₂SnO₃ mixture phase was obtained because of phase separation. In order to buffer the lattice distortion resulted from large radius difference between Mn⁴⁺ and Sn⁴⁺ ions, we introduce a third element of Ru, whose ionic radius (0.62 Å) is in between Mn⁴⁺ and Sn⁴⁺ ions, to assist the formation of 0.5Li₂MnO₃-0.5Li₂SnO₃ solid solution. It is found that, for the first time, with the increment of Ru content incorporated in the Li₂RuO₃-Li₂MnO₃-Li₂SnO₃ ternary system, it gradually evolves from composite into single-phase solid solution. Electrochemical performance of this ternary oxide series have been systematically investigated in this work. This novel strategy opens a new avenue to extend the limit for forming solid solutions and significantly enriches electrode family members for lithium ion batteries.