New insights into the electrochemical performance of Li2MnSiO4: effect of cationic substitutions

Abstract

The performance of the Li₂MnSiO₄ cathode material is hindered by voltage decay and capacity fading caused by structural instability. To rationalize the origin of such structural instability, we have investigated a total of 142 Li_2yMnSiO₄ configurations at y = 0.125, 0.25, 0.333, 0.375, 0.417, 0.5, 0.625, 0.666, 0.75 and 0.875 by density functional theory methods. It is found that the most stable Li_2yMnSiO₄ configurations with y ≤ 0.5 consist of Mn⁴⁺ and Mn³⁺ in octahedral or five-fold coordination. This induces a crystal deformation, loss of the orthogonal symmetry, and a notorious volume decrease (7% for LiMnSiO₄ and 14% for Li_0.5MnSiO₄). The effect of Mn substitution on the crystal structure of the delithiated silicates Li_0.5Mn_0.75M_0.25SiO₄ is computationally investigated for M = Mg, Fe, Co and Ni. The most stable configurations for Mg, Fe and Co substitutes possess Mn⁴⁺ in octahedral coordination, sharing edges with the adjacent Si and Mn polyhedra. DFT results suggest that among the studied substituents, only Ni could help to maintain the structural integrity of the delithiated samples. Experimentally, Li₂Mn_1−xNi_xSiO₄ samples with x = 0, 0.1 and 0.2 were synthesized and electrochemically tested.