Electrochemically Driven Transformation of Olivine LiFePO₄ to Olivine NaFePO₄ in Cells with Sodium Anode and Carbonate Electrolyte

Abstract The direct electrochemical transformation of commercial LiFePO₄ to Na_xFePO₄ olivine in a Na||LiFePO₄ electrochemical cell with sodium electrolyte has been studied. Electrolytes containing 1 M of NaClO₄ in ethylene carbonate (EC), dimethyl carbonate (DMC) and fluoroethylene carbonate (FEC) have been probed, and in the solvents with the ratio of EC:DMC:FEC=0.4:0.5:0.1 by volume, maximal values of the specific capacity (113 mAh g⁻¹) with good cycling stability and maximal discharge currents are achieved upon sodiation. To determine the composition of the cathode material and utilization factor of LiFePO₄, the dependence of open circuit voltage on specific capacity has been investigated. The general formula of the sodiation product can be written as Na_0.88FePO₄. Its theoretic specific capacity may be estimated as 135 mAh g⁻¹. The composition of the products obtained is supported by XRD analysis, which suggests that in discharged cathodes, Coulombic influence of LiFePO₄ crystallites on neighboring Na_xFePO₄ crystallites may enlarge the unit cell of the former and reduce its size of the latter. Probably, shrinkage in the presence of LiFePO₄ stabilizes the olivine structure of Na_xFePO₄ and prevents its transformation from electrochemically active olivine to inactive maricite.