Calendar and cycle ageing of commercial sodium-ion cells with layered oxide cathodes: how does operation influence degradation modes?

Abstract

Despite the continuous success of lithium-ion batteries, sodium-ion batteries pose a relevant alternative to lithium-based storage systems. Abundant resources that can be obtained by more environmentally friendly processes make sodium-ion batteries a feasible alternative to lithium-ion batteries for large-scale applications. However, the ageing behaviour of sodium-ion batteries and their subsequent degradation modes have not yet been fully investigated. In this work, commercial sodium-ion batteries are aged under various conditions to study the impact of state-of-charge, depth of discharge, temperature, and current rate on ageing. For this study, we investigated the initial electrical and electrochemical parameters of 94 commercial cells with layered oxide cathodes and hard carbon anodes. From these cells, we selected and aged 70 cells for almost two years or up to 5000 equivalent full cycles. We demonstrate that sodium-plating on the anode surface poses a relevant failure mechanism in sodium-ion batteries throughout all operating conditions. We used degradation-mode analysis to identify the main drivers for degradation. Capacity fade is arguably driven by loss of active material at the cathode, which is more dominant when operating or storing in high state-of-charge ranges. In contrast to lithium-ion batteries, the loss of charge carrier inventory is not the main driver for capacity fade in sodium-ion batteries. Loss of active material from the anode occurs but it plays a minor role. However, anode material loss is more pronounced when operating or storing in low state-of-charge ranges.