Degradation Mechanisms of NaTi2(PO4)3 Aqueous Anodes and Mitigation Techniques

Abstract

Aqueous rechargeable alkali-ion batteries offer an attractive alternative to Li-ion batteries for large scale installations because of their lower cost and higher degree of safety. However, with the exception of activated carbon anodes, demonstrated chemistries have generally showed poor cycle stability. Understanding the failure mechanisms of aqueous anode materials is of vital importance for commercialization of these batteries. In this study we have investigated three primary mechanism for capacity loss in NaTi2(PO4)3 (NTP) based anodes (1) dissolution of active material at high pH, (2) loss of active surface area due to gas entrapment from hydrogen evolution, and (3) alkaline oxidation of the carbon conductive additive. Inductively coupled mass spectrometry (ICP-MS) was used to quantify the solubility of NTP as a function of pH and helps explain the formation of a secondary reduction peak during extended low C-rate cycling. This degradation mechanism has been mitigated using a conformal polydopamine based carbon coating. Gas chromatography was used to quantify the production of H2, CO, and CO2 due to hydrogen evolution and alkaline oxidation of carbon. Electrochemical cycling followed by gas removal and re-infiltration of the porous electrodes was used to quantify the loss of electrochemical surface area due to gas entrapment and is found to be the primary cause of apparent capacity loss. A composite of the NTP electrode with a PTFE bound activated carbon was found to be effective in preventing gas entrapment with in the porous electrode.