Electrochemical data transferability within LiyVOXO4 (X = Si, Ge0.5Si0.5, Ge, Si0.5As0.5, Si0.5P0.5, As, P) polyoxyanionic compounds

Given the interest of silicates as potential electrode materials for lithium batteries, it is critical to fully understand the role that the inductive effect of the polyoxyanionic group, (XO4)(n-), plays on the electrochemical performance of polyoxyanionic compounds. In this work we have combined experiments and first-principles methods to investigate to which extent the inductive effect of the X-O bond within the XO4n- polyanion (X = Si, Ge0.5Si0.5, Ge, Si0.5As0.5, Si0.5P0.5, As, P) modifies the redox energy of the V5+/V4+ couple in the LiyVOXO4 family of compounds. Calculations using the GGA+U method evidence a nice correlation between the X electronegativity (i.e., the magnitude of the XO4 groups' inductive effect) and details of the crystalline and electronic structures of Liy+1V4+OXO4/ LiyV5+OXO4 phases such as bond distances or band gaps. Besides the inductive effect of the polyanionic group, we found that the chemistry of these 2D compounds is also correlated to the Li+ site occupancy in the interlayer space. The calculated lithium insertion voltages display an almost linear dependence on the Mulliken X electronegativity; this offers promising prospects in the design of novel polyoxyanionic electrode materials. The new electrode materials Li2VOGeO4 and Li2VOSi0.5Ge0.5O4 have been electrochemically tested, providing good agreement between experimental and calculated lithium insertion voltages. Activation energies of the prepared compounds follow the band gap trend provided by the calculated data as well. Thus, experimental evidence support the computational results and the conclusions presented here.