A Study on the Capacity Degradation in Na3.2V1.8Zn0.2(PO4)3 Cathode and Hard Carbon Anode Based Sodium-Ion Cells

Abstract

In this manuscript, the impact of operating conditions such as voltage window, and operating temperature on electrochemical performance and cycle life of Zn-substituted Na3.2V1.8Zn0.2(PO4)3 (NVZP) vs hard carbon (HC) coin cells filled with 1 mol dm−3 NaBF4 in tetraglyme is presented. Initially, the cells are cycled for 500 times at C/2 charge and 1 C discharge in three different voltage windows (4.20–1.00 V, 4.05–1.00 V and 4.05–1.50 V) and at two temperatures (28 °C and 40 °C) and are subjected to periodic internal resistance and impedance measurements. The elemental composition of the electrodes harvested after cycling reveals that vanadium dissolution with accompanying deposition on the HC electrode and irreversible loss of sodium causes increased cell impedance. The identified degradation mechanisms, which causes severe capacity fade, are found to be accelerated in the cells cycled over wider voltage windows, particularly at elevated temperature. The best cycling performance and lowest impedance are recorded for the cells cycled within 4.05–1.50 V at 28 °C owing to negligible vanadium dissolution. Under these optimized testing conditions, a prototype 18650 cell, shows impressive capacity retention of 77% after 1000 cycles.