Exploring Phosphate-Based Mixed Polyanionic Materials for Enhanced Metal-Ion Rechargeable Batteries
Authors: Lochab Shubham , Bharathraj Sagar , Adiga Shashishekara Parampalli , Barpanda Prabeer 
Published: December 2024

Affordable and high-performance energy storage solutions are essential to meet the growing energy demands of society. Rechargeable batteries employing cost-effective and readily scalable battery materials offer promising prospects for fulfilling these needs. Mixed polyanionic insertion materials offer a rich treasure house for designing robust cathode materials for rechargeable batteries, leveraging properties such as chemical/thermal stability, tunable redox voltage, and excellent electrochemical performance.1,2 Among these, PO4-based materials stand out as exceptional systems, offering scalable synthesis, safe storage/handling, and high energy density. Here, we delve into three distinct classes of mixed polyanionic materials, elucidating their electrochemical behavior and mechanism.
(1) Mixed Phosphates: Nanostructured carbon-coated Na4Co3(PO4)2P2O7 and Na4Ni3(PO4)2P2O7 were explored as bifunctional electrocatalysts for oxygen evolution and reduction reactions (OER and ORR).3 Both materials showed bifunctional behavior, suggesting potential use in sodium-air batteries. For practical applications, nanoscale in-situ carbon-coated Na4Fe3(PO4)2P2O7 (NFPP) material was utilized for the fabrication of sodium ion full cells using hard carbon (HC) as the anode.4 Reduced order modeling (ROM) was employed to accurately monitor cycling-related degradation. A stable NFPP/HC full cell exhibited a robust specific capacity for over 200 cycles with minimum degradation was achieved.
(2) NASICON-type Phosphate-sulfate: Mixed polyanionic NaFe2PO4(SO4)2 was explored for Li- and Na-ion batteries, leveraging the Fe+3/+2 redox couple within a voltage range of 2.0 V to 4.5 V with excellent capacity and cycling stability. Spray-drying synthesized material with uniform spherical particles was studied using powder X-ray diffraction data followed by various physical characterizations (SEM, TEM, FTIR, Raman, UV vis and calorimetry, etc.).5,6 A single particle-based model (SPM) was employed to analyze the system for predictions of alterations associated with different geometric and physical parameters.
(3) Fluorophosphates: The electrochemical properties of Na2FePO4F fluorophosphate (3 V vs Na) were also investigated using a single particle model, validated using the half-cell data and various geometric and physical parameters. The magnetic structure and properties of fluorophosphate Na2MnPO4F sodium insertion material were explored using magnetic susceptibility measurements and neutron powder diffraction.7 A long-range antiferromagnetic ordering was exhibited below the Néel temperature (TN) ~10.4 K.

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[1] B. Senthilkumar, C. Murugesan, L. Sharma, S. Lochab, P. Barpanda, Small Methods 3 (2019) 1800253
[2] S. Lochab, S. Singh, S. P. Vanam, M. Fichtner, P. Barpanda, Elsevier Publications (2023) 00714
[3] S. Lochab, B. Senthilkumar, D. Singh, R. Ahuja, R.K. Rai, H.N. Alshareef, P. Barpanda, Nanoletters (Manuscript submitted)
[4] S. Lochab, S. Bharathraj, K. S. Mayya, P. Barpanda, S. P. Adiga (under review)
[5] S. Lochab, K. Jayanthi, A. Navrotsky, P. Barpanda (to be submitted)
[6] S. Lochab, B. Senthilkumar, P. Barpanda, Patent, CS-MRC-2023-153
[7] S. Lochab, S. Rayaprol, M. Avdeev, L. Sharma, P. Barpanda, J. Solid State Chem 308 (2022) 122926