A New Class of Ternary Compound for Lithium Ion Battery
Authors: Cui Yanhua , Wang Jiali , Wu Hailong , Liu Shengzhou , Yang Yin 
Published: February 2020

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)LiFeO2
2 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.

Figure 1