Insights into Changes of Lattice and Electronic Structure Associated with Electrochemistry of Li2CoSiO4 Polymorphs

First-principles calculations combined with XRD simulations are performed to systematically study crystal structures, bonding characteristics and electronic structures of LixCoSiO4 (x = 2.0, 1.5, 1.0) polymorphs with symmetries Pmn2(1)-DP and Pbn2(1). The calculated average voltages by lithium extraction agree well with available experiments. Coat tetrahedron is the key structural unit to track the process of delithiation. The oxidation of CoO4 tetrahedron results in a special pattern of bonding characteristic, which corresponds to spin ordering and may be observable in XRD spectra according to simulation. We find delithiated phases are intrinsic Mott insulators, electronic band gaps change from Mott-Hubbard-type to charge-transfer-type during lithium removing. The swapping of near-gap states is associated with the contraction of the oxidized CoO4 units, indicating Peierls distortions that may be the physical origin of capacity degrading of Co-silicate chemistry