Structural and Electrochemical Insights into Iron-Based Fluoro(hydroxy)Phosphate axFePO4y (A= Na, Li; Y= F, OH) As Cathode Materials for Aqueous Batteries

Abstract

In search for efficient cathode materials for batteries, the focus has shifted from oxides to Polyanion based materials. Polyanions not only impart thermal and structural stability to the material but also enhance the electronegativity of the central atom which in turn increases the voltage of the material. Hence, a lot of research has been done on PO4-based materials especially when combined with other anions like F- or OH-. However, safety and cost of existing organic electrolytes force us to revisit the possible application of aqueous electrolytes. Use of aqueous electrolytes reduces the toxicity and increases the ionic conductivity by two-fold. The limited working potential range of aqueous electrolytes to avoid water splitting makes it important to look for efficient and stable electrode materials. In this pursuit, we present two case studies using earth-abundant Fe-based compounds. First, Na2FePO4F is demonstrated as a cathode material for aqueous sodium-ion batteries. A discharge capacity of 85 mAh g-1 at 1 mA cm-2 current density with excellent rate kinetics was observed for half-cell configurations. [Figure 1(a)] A full cell was assembled with NaTi2(PO4)3 anode and the Na2FePO4F | NaTi2(PO4)3 full cell delivered a reversible capacity of 85 mAh g-1 working as a 0.8 V battery (ChemElectroChem, In Press, 2018). The second work is based on LiFePO4OH which works as an anode for aqueous lithium-ion batteries (LIBs). It is reported as a 2.5 V battery cathode material for LIBs in organic electrolytes. However, it was found to be working in the anodic range for aqueous batteries. In half-cell configuration, it delivered a discharge capacity of 153 mAh g-1 with very good rate kinetics. [Figure 1(b)] A full cell was assembled with LiFePO4 as a cathode and an energy density of 70 Wh kg-1 was observed in the first cycle. The structural characterization along with electrochemical studies will be showcased for both case studies. Figure 1: Galvanostatic (dis)charge profiles and cyclability of (a) Na2FePO4F, (b) LiFePO4OH in aqueous electrolytes in half-cell configuration. Figure 1