Electrochemical and Electrocatalytic Behavior of Phosphate Family of Battery Insertion Materials: Few Case Studies
Authors: Barpanda Prabeer 
Published: February 2020

Exploring post Li-ion battery systems, various monovalent (Na+, K+) and multivalent (Mg2+, Ca2+, Zn2+, Al3+) chemistry have been intensely investigated over the last decade. Among them, sodium-ion based insertion compounds have emerged as front-runners owing to their operational similarity with Li-ion systems, low cost/ abundance and high rate kinetics [1]. Various oxides and oxyanionic compounds have been reported with great success delivering high capacity and/or redox potential leading to promising energy density. In addition, some of these systems can be exploited as electrocatalysts with bi-functional activity.
Over the past two years, our group has investigated various phosphate (PO4
3-) based insertion compounds for electrochemical and electrocatalytic activity [2-9]. In the current work, we will summarize these work to give some insights on sodium intercalation properties and bifunctional (oxygen evolution/ reduction reaction) electrocatalytic activity of following PO4 materials.
(i) Solution combustion synthesis was used to prepare phase-pure Na2FePO4F fluorophosphate compound involving low cost Fe(III) precursor. The target phase was obtained by annealing the intermediate complex in short duration of 1 minute. The electrochemical performance (3 V vs Na, 100 mAh/g) and one dimensional Na+ diffusional mechanism will be demonstrated.
(ii) The electrochemical and diffusional activity of novel sodium metaphosphate [NaM(PO3)3] class of cathodes will be shown with a 2.8 V and 3.2 V (vs. Na) activity for Fe- and Co-based metaphosphate respectively. The electrocatalytic (oxygen reduction reaction) behavior of NaM(PO3)3 compounds will be demonstrated in comparison to Pt/C system.
(iii) The bifunctional activity of various phosphate (PO4
3-) and pyrophosphate (P2O7
2-) materials (e.g. NaFePO4, KFePO4, NaCoPO4, Na2CoP2O7, K2CoP2O7) will be described and compared to Pt/C system. Most of these materials were found to be bifunctional in nature with potential application in Na-air batteries.
(iv) The electrochemical and electrocatalytic performance of phosphate based alluaudite materials [NaMFe2(PO4)3] will be reported. An overall 3 V Na (de)intercalation was noticed with reversible capacity over 70 mAh/g along with efficient oxygen reduction reaction activity.
(v) The entire work on phosphate based insertion materials will be shown using various experimental tools in synergy with computational investigation. It will demonstrate some examples of half-cell batteries, thin-film micro-batteries and metal-air batteries.

Reference
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