Electrochemical and Electrocatalytic Activity in Two Phosphate-Based Systems: Pyrophosphate and Metaphosphate for Sodium-Based Batteries
Authors: Gond Ritambhara , Pralongb Valerie , Barpanda Prabeer 
Published: February 2020

Keywords: Energy storage devices, polyanionic materials, pyrophosphate, metaphosphate, cathodes, electrocatalysts.
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(Fe1-yMny)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(Fe1-xZnx)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(Fe1-xZnx)P2O7 (0 ≤ x ≤ 1) binary pyrophosphates. Further, we will describe the Na-insertion mechanism in Na2(Fe1-xZnx)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(Fe1-xMnx)(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.

Figure 1