Aqueous Electrochemical Degradation of Phosphate Framework Materials: Mechanisms and Mitigation Strategies

Abstract

Phosphate framework materials are attracting a lot of interest as very stable, structurally and electronically diverse materials, suitable for various electrochemical applications. For example, Na SuperIonic Conductor (NASICON) structure type AxMe2(PO4)3 (A = Li, Na, K; Me = Ti, V, Mn etc.) compounds, in addition to be being archetypical ionic conductors for solid state batteries are also potent electrode active materials for various devices such as aqueous batteries and Faradaic deionization cells. However, there is still a number of issues to be solved before their full potential could be utilized. This involves not only finding the right compositions of transition metals but also improving the stability of aqueous electrolyte/electrode interface. Aqueous stability and electrochemical degradation of electrode materials is related to complex acid-base equilibria, metal leaching, material dissolution, corrosion of cell components etc.Some of the recent contributions in terms of understanding and enabling of Ti-, Mn- and V-based phosphate framework materials for the use in aqueous electrochemical systems is reviewed. The effects of parasitic reactions such as oxygen reduction (ORR) and hydrogen evolution (HER) and their relation to local pH changes towards the stability of NaTi2(PO4)3 (NTP) are determined. The results show that it is the local alkalinity which is responsible for most of the NTP degradation which is only accelerated at high currents. Several mitigation strategies by the use of conformal atomic layer deposited (ALD) coatings and oxygen management by the use of strongly reducing additives are suggested. The dissolution of transition metals as the main aqueous degradation mechanism for Na1+2xMnTi2-x(PO4)3 and Na3-xV2-xTix(PO4)3 positive electrodes is established. However, the results suggest that in Mn- and V-based positive aqueous battery electrodes it is not the material degradation which is responsible for most of the observed capacity fade per se but rather the loss of contact between particles and the conductive phase. The latter comes from the dissolution at the interfacial points but most of the electrode material, is shown to stay intact after extended cycling. Various metal substitution strategies leading to superior stability in half- and full-cells are presented.[1] Plečkaitytė et al. J. Mater. Chem A, 9, 12670-12683 (2021).[2] Snarskis et al. Chem. Mater., 33, 8394-8403 (2021).[3] Tediashvili et al. Electrochim. Acta, 417, 140294 (2022).[4] Petrulevičienė et al. Electrochim. Acta, 424, 140580 (2022). Figure 1