Disclosing the interfacial electrolyte structure at Na-insertion electrode materials: origins of desolvation phenomenon

Among a variety of promising cathode materials for Na-ion batteries, polyanionic Na-insertion compounds are among the preferred choice due to known fast sodium transfer through the ion channels along their framework structures. The most interesting representatives are Na3V2(PO4)3 (NVP) and Na3V2(PO4)2F3 (NVPF), which display large Na+ diffusion coefficients and high voltage plateaux (up to 4.2V for NVPF). While the diffusion in the solid material is well-known to be the rate-limiting step during charging, already being thoroughly discussed in the literature, liquid-state transport of sodium ions towards the electrode was recently shown to be important due to complex ion desolvation effects at the interface. In order to fill the blanks in the description of the electrode/electrolyte interface in Na-ion batteries, we performed a molecular dynamics study of the local nanostructure of a series of carbonate-based sodium electrolytes at the NVP and the NVPF interfaces along with the careful examination of the desolvation phenomenon. We show that the tightness of solvent packing at the electrode surface is a major factor determining the height of the free energy barrier associated with desolvation, which explains the differences between the NVP and the NVPF structures. To rationalize and emphasize the remarkable properties of this family of cathode materials, a complementary comparative analysis of the same electrolyte systems at the carbon electrode interface was also performed.