Enabling Long-term Cycling Stability of Na3V2(PO4)3/C vs. Hard Carbon Full-cells

Sodium-ion batteries are becoming an increasingly important complement to lithium-ion batteries. However, while extensive knowledge on the preparation of Li-ion batteries with excellent cycling behavior exists, studies on applicable long-lasting sodium-ion batteries are still limited. Therefore, this study focuses on the cycling stability of batteries composed of Na3V2(PO4)3/C based cathodes and hard carbon anodes. It is shown that full cells with a decent stability are obtained for ethylene carbonate / propylene carbonate electrolyte and the conducting salt NaPF6. With cathode loadings of 1.2 mAh/cm², after cell formation discharge capacities up to 92.6 mAh/g are obtained, and capacity retentions > 90 % over 1000 charge / discharge cycles at 0.5 C / 0.5 C are observed. It is shown that both, the additive fluoroethylene carbonate and traces of water in the cell, negatively affect the overall discharge capacity and cycling stability and should therefore be avoided. Remarkably, the internal resistances of well-balanced and wellbuilt cells did not increase over 1500 cycles and 5 months of testing, which is a very promising result regarding the possible lifespan of the cells. The initial loss of active sodium in hard carbon remains a major problem, which can only be partially reduced by proper balancing.