Peculiarities of Phase Formation in Mn-Based Na SuperIonic Conductor (NaSICon) Systems: The Case of Na1+2x Mn x Ti2-x (PO4)3 (0.0 ≤ x ≤ 1.5).

Gustautas Snarskis,Linas Vilčiauskas,Lina Mikoliu Naitė,Jurgis Pilipavičius,Denis Gryaznov

doi:10.1021/acs.chemmater.1c02775

Abstract

NAtrium SuperIonic CONductor (NASICON) structured phosphate framework compounds are attracting a great deal of interest as suitable electrode materials for “rocking chair” type batteries. Manganese-based electrode materials are among the most favored due to their superior stability, resource non-criticality, and high electrode potentials. Although a large share of research was devoted to Mn-based oxides for Li- and Na-ion batteries, the understanding of thermodynamics and phase formation in Mn-rich polyanions is still generally lacking. In this study, we investigate a bifunctional Na-ion battery electrode system based on NASICON-structured Na1+2xMnxTi2–x(PO4)3 (0.0 ≤ x ≤ 1.5). In order to analyze the thermodynamic and phase formation properties, we construct a composition–temperature phase diagram using a computational sampling by density functional theory, cluster expansion, and semi-grand canonical Monte Carlo methods. The results indicate finite thermodynamic limits of possible Mn concentrations in this system, which are primarily determined by the phase separation into stoichiometric Na3MnTi(PO4)3 (x = 1.0) and NaTi2(PO4)3 for x < 1.0 or NaMnPO4 for x > 1.0. The theoretical predictions are corroborated by experiments obtained using X-ray diffraction and Raman spectroscopy on solid-state and sol–gel prepared samples. The results confirm that this system does not show a solid solution type behavior but phase-separates into thermodynamically more stable sodium ordered monoclinic α-Na3MnTi(PO4)3 (space group C2) and other phases. In addition to sodium ordering, the anti-bonding character of the Mn–O bond as compared to Ti–O is suggested as another important factor governing the stability of Mn-based NASICONs. We believe that these results will not only clarify some important questions regarding the thermodynamic properties of NASICON frameworks but will also be helpful for a more general understanding of polyanionic systems.

Highlights

The search and design of suitable electrode materials remains one of the major challenges for the future development of battery technologies
We investigate the system by first constructing its composition−temperature phase diagram using the semigrand canonical Monte Carlo sampling based on an effective cluster expansion (CE) Hamiltonian derived from the density functional theory (DFT) calculations
Of phase formation and thermodynamic stability are analyzed by constructing a composition−temperature phase diagram based on extensive computational sampling using DFT, CE, and semigrand canonical Monte Carlo methods

Summary

Introduction

The search and design of suitable electrode materials remains one of the major challenges for the future development of battery technologies. Electrodes.[8,9] Presently, NASICON-structured phosphates with a general formula of Na1−4M′M′′(PO4)[3] are probably the most studied and applied polyanion electrode materials for NIBs.[8,9] They show a number of advantages over other materials by providing a very stable and robust framework with fast ion insertion kinetics and three-dimensional bulk mobility as well as wide selection of electrode redox potentials. The latter can be Received: August 11, 2021 Revised: October 7, 2021 Published: October 21, 2021

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Results

Conclusion