New insights into the electrochemical performance of Li2MnSiO4: effect of cationic substitutionsElectronic supplementary information (ESI) available. See DOI: 10.1039/c4ta03367a

The performance of the Li 2 MnSiO 4 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 2 y MnSiO 4 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 2 y MnSiO 4 configurations with y ≤ 0.5 consist of Mn 4+ and Mn 3+ in octahedral or five-fold coordination. This induces a crystal deformation, loss of the orthogonal symmetry, and a notorious volume decrease (7% for LiMnSiO 4 and 14% for Li 0.5 MnSiO 4 ). The effect of Mn substitution on the crystal structure of the delithiated silicates Li 0.5 Mn 0.75 M 0.25 SiO 4 is computationally investigated for M = Mg, Fe, Co and Ni. The most stable configurations for Mg, Fe and Co substitutes possess Mn 4+ 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, Li 2 Mn 1− x Ni x SiO 4 samples with x = 0, 0.1 and 0.2 were synthesized and electrochemically tested. We investigate Mn substitution for Mg, Fe, Co and Ni in the Li 2 MnSiO 4 cathode material. Among these substituents only Ni might help maintaining the structural integrity of the delithiated phases.