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.

References:

[1] N. Yabuuchi et al, S. Komaba, Chem. Rev. 114 (2014) 11636–11682.

[2] L. Sharma et al, P. Barpanda, ACS Appl. Mater. Interfaces. 9 (2017) 34961–34969.

[3] L. Sharma et al, P. Barpanda, Ionics. 24 (2018) In press.

[4] C. Murugesan et al, P. Barpanda, ChemCatChem. 10 (2018) In press.

[5] R. Gond et al, P. Barpanda, ChemElectroChem. 5 (2018) In press.

[6] R. Gond et al, P. Barpanda, Inorg. Chem. 56 (2017) 5918–5929.

[7] D. Dwibedi et al, P. Barpanda, ECS Trans. 80 (2017) 337–342.

[8] D. Dwibedi et al, P. Barpanda, MRS Adv. (2018) In press.

[9] D. Dwibedi et al, P. Barpanda, Electrochim. Acta. (2018) Submitted.