Present world needs to harvest energy in the form of electricity to power myriads of consumer electronic devices to hybrid vehicles. Electrical energy can be efficiently stored in electrochemical storage devices such as supercapacitors, batteries, and fuel cells. Among them, rechargeable (metal-ion/ metal-air) batteries form the most promising and commercially viable solution. Owing to the light weight and high energy density, Li-ion battery has seen unprecedented commercial success. For next generation Li-ion batteries, various phosphate-based polyanionic materials (e.g. LiFePO₄) have been proposed as robust and safe cathode materials [1]. In addition to efficient intercalation activity, various phosphate-based compounds have been reported to work as efficient bi-functional catalysts for water splitting showing good oxygen evolution reaction (OER) with appreciable oxygen reduction reaction (ORR) activity [2]. So, these phosphate compounds can be used to develop metal-air batteries.

Off-late, sodium-ion batteries (SIBs) are being extensively explored as economic alternatives to Li-ion batteries. Here, the electrochemical and electrocatalytic performance of two distinct class of PO₄-based compounds, pyrophosphates [Na₂MP₂O₇] and metaphosphates [NaM(PO₃)₃], have been explored to develop Na-ion and Na-air batteries (Figure 1). They can be easily synthesized by solution combustion synthesis as well as conventional solid-state synthesis involving moderate annealing at 600 ºC for 6 h.

Pyrophosphates: Among pyrophosphates, Na₂FeP₂O₇ has been reported as a low cost cathode with promising electrochemical performance and thermal stability. Binary Na₂(Fe_1-yMn_y)P₂O₇ (0 ≤ y ≤ 1) pyrophosphate family has been extensively studied as well as the best composition with highest stability has been investigated. We have investigated the binary Na₂(Fe_1-xZn_x)P₂O₇ (0 ≤ x ≤ 1) pyrophosphate family, where Na₂FeP₂O₇ (P-1, #2) and Na₂ZnP₂O₇ (P4₂/n, #86) end members are anisostructural. We will report the structural and electrochemical characterization of novel Na₂(Fe_1-xZn_x)P₂O₇ (0 ≤ x ≤ 1) binary pyrophosphates. Further, we will describe the Na-insertion mechanism in Na₂(Fe_1-xZn_x)P₂O₇ (x = 0, 0.25) electrodes obtained by a small-amplitude (incremental) techniques such as galvanostatic intermittent titration (GITT) and potentiostatic intermittent titration (PITT). Pursuing pyrophosphate chemistry, the superior bifunctional electrocatalytic activity of Na₂CoP₂O₇ will be demonstrated [3]. Na₂CoP₂O₇ form a potential catalyst to implement in Na-air batteries.

Metaphosphates: Metaphosphates have been recently reported by our group as electrochemically active compounds for reversible Na⁺ (de)intercalation. We will showcase (i) a 2.8 V Fe³⁺/Fe²⁺ redox activity in orthorhombic Na(Fe_1-xMn_x)(PO₃)₃ (x = 0-1) binary solid-solutions developed by combustion synthesis [4] and (ii) a 3.2 V Co³⁺/Co²⁺ redox activity in cubic polymorph of NaCo(PO₃)₃ [5]. However, their high molecular weight leads to limited reversible capacity. Nonetheless, the presence of metal redox centre can be exploited to harness electrocatalytic performance. We will demonstrate the electrocatalytic properties (e.g. oxygen evolution reaction, OER) of the Fe, Mn, and Co analogues of metaphosphate [NaM(PO₃)₃] family for Na-air batteries application. OER onset potential of NaCo(PO₃)₃ system was found to be 1.45 V, which is close to the conventional RuO₂ electrode [6]. Synergising experiments with computational modelling, we will showcase the electrochemical and electrocatalytic activity of NaFe(PO₃)₃ and NaCo(PO₃)₃ for the first time.

[1] A. K. Padhi, K. S. Nanjundaswamy, and J. B. Goodenough, J. Electrochem. Soc., 144 (1997), 1188.

[2] H. Kim et al, and K. Kang, Nat. Commun., 6 (2015), 8253.

[3] R. Gond et al, and P. Barpanda, ChemElectroChem., 5 (2018), 153.

[4] R. Gond et al, and P. Barpanda, Inorg. Chem., 56 (2017), 5918.

[5] R. Gond et al, and P. Barpanda, Inorg. Chem., (2018), in press.

[6] R. Gond et al, and P. Barpanda, manuscript submitted.