First principles study of the electrochemical properties of Mg-substituted Li2MnSiO4

The phase stability of the Li2MnSiO4 during Li insertion/extraction, is a key requirement for acceptable cyclability and practical application as a cathode material for lithium batteries. Here we present first-principles calculations used to study the phase stability of Mg substituted Li2MnSiO4. The 137 structures of Li2Mn1-x MgxSiO4 (x = 0.25-0.50) were calculated based on 5 known polymorphs of Li2MnSiO4. Using three different functionals (PBE, PW91 and PBEsol), it is shown that the total-energy vs. distance between layers in the layered Pmn2(1) curve has a clear minimum and does not demonstrate the exfoliation of layers found previously. The amorphlization of Li2MnSiO4 is explained by high value of energy above Hull of its fully delithiated form. Crystal orbital Hamiltonian populations (COHP) revealed that the strength of Mn-O and Si-O bonds unchanged during the substitution with Mg, thus eliminating the concern about the safety. The pure Li2MnSiO4 in the P2(1)/n form was suggested as the most stable upon cycling. In the Mg substituted Li2-xMnSiO4 case, the Mg substitution is more beneficial for x in the range (0.0-2.0) than in the range (0.0-1.0). The increase in the performance for the x = 0.0-1.0 region, can be explained by the small particle size and the uniformity of nanoparticles distribution rather than the enhancement of the thermodynamic stability. (C) 2017 Elsevier B.V. All rights reserved.