Developing Ternary Transition Metal-Based Nasicon-Cathodes for High Performance Sodium-Ion Batteries Via Chemical Substitution
Authors: Patra Biplab , Kumar Keshav , Ghosh Subham , Mazumder Madhulika , Pati Swapan Kumar , Senguttuvan Premkumar 
Published: December 2024

Sodium ion batteries are emerging as promising alternative to lithium-ion batteries due to their abundance and low cost of sodium and are expected to be widely used in grid energy storage.1 The most extensively investigated cathodes for SIBs are layered metal oxides, polyanionic-type materials, and prussian blue analogues. Among polyanionic materials NASICON (Natrium Super-Ionic CONductor) type compounds are considered to be one of the most promising cathode materials due to its high structural and thermal stability, high insertion voltage and fast sodium ion conductivity but their low electronic conductivity and limited capacity prevent its wide utilization. To improve intrinsically low electronic conductivity several methods have been studied such as carbon coating, reducing particle size and chemical substitution.2 Na3V2(PO4)3 (NVP) is a representing NASICON-type cathode material and exhibits promising electrochemical performance.3 Recently, several Mn-based compounds NaxMMn(PO4)3 (M = V, Zr, Ti, Cr, etc.) have been thoroughly investigated to reduce the toxicity and cost that come with vanadium. Among them Na4VMn(PO4)3 is popular because of its higher voltage compared to NVP, but it suffers from capacity fade and limited rate performance because of intrinsically low electronic conductivity and formation of Jahn-Teller active Mn3+ during cycling. To solve this problem several attempts were made to replace V3+ and Mn2+ or both with iso/aliovalent metals (M' = Mg2+, Al3+, Ti4+, Fe2+, etc.) along with carbon coating and particle nanosizing to create Mn-rich or V-rich ternary-Nax(V/Mn/M’)2(PO4)3-type cathodes, respectively. In this approach the fundamental understanding of the effect of chemical substitutions cannot deconvoluted with other competing factors.

Herein, we examine a series of ternary-NASICON cathodes with end members Na2VZr(PO4)3-Na3ZrMn(PO4)3-Na4VMn(PO4)3 that are situated in the pseudo-ternary phase diagram.4 To know the actual impact of the chemical substitutions, we avoided particle nano-sizing and carbon coating approaches. We want to know what concurrent substitutions of Na/V/Mn/Zr are required to achieve high-performance NASICON cathodes. So, we have synthesized 16 cathodes via solid-state methods by varying Na/V/Mn/Zr in the pseudo ternary phase diagram and studied their electrochemical performances. Using chemical substitution approach, we investigate a series of micron-sized NASICON-Na(9−2x−3y−4z)MnxVyZrz(PO4)3 cathodes, where 0 ≤ (x, y, z) ≤ 1 and (x + y + z) = 2. Our local structural analysis and DFT calculation provide the complex evolution of (local) crystal and electronic structures, which affects electronic and Na-ion conductivities. Further electrochemical study reveals the Na/V-rich cathodes deliver higher capacities, cycling stabilities, and rate performances compared to those of Na/Mn-rich compositions. We got the best performance for Na3.5Mn0.75VZr0.25(PO4)3 cathode which retains 91.6% of its initial capacity after 200 cycles. This study shows the power of chemical substitution to achieve better electrochemical performance in the cathodes for SIBs.

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