Understanding and mitigation of NaTi2(PO4)3 degradation in aqueous Na-ion batteries

Aqueous Na-ion batteries are deemed to be among the most suitable candidates for future large-scale stationary energy storage systems. However, there are still a number of problems to be solved before their full potential can be utilized such as instability of the aqueous electrolyte/electrode interface towards electrolyte decomposition, chemical dissolution, and other side reactions. The mechanism of electrochemically induced degradation in NASICON-structured NaTi2(PO4)3 is studied in detail using various chemical and electrochemical techniques. The results unambiguously show that irreversible capacity fade, especially severe at low charging rates, is an electrochemically induced chemical dissolution of the active material. The oxygen reduction induced self-discharge reaction leading to a local pH increase is indicated as the main culprit for the material degradation and capacity loss during charge–discharge cycling. Although the degradation products form an insoluble Ti-rich interphasial layer similar to non-aqueous solid-electrolyte interphases, in this case it is shown to be insufficient for providing even kinetic stability of the electrochemical interface. Artificial protective layers applied using atomic layer deposition of Al2O3 directly on the electrode surface are shown to be a simple and scalable approach to significantly stabilize the electrodes towards oxygen attack and limit the rate of negative electrode degradation. The new understanding of irreversible degradation and its mitigation techniques presented in this work offer a pathway to viable and scalable strategies that enable and exploit the high-performance potential of this and many other aqueous battery materials.