Tuning Transition Metal Oxides Towards Achieving Water-Stability and Electrochemical Stability As Electrode Materials for Sodium-Ion Batteries

Abstract

Transition metal (TM) oxides are a fascinating class of materials, whose properties can be suitably tuned in a variety of ways; such as by selecting TM-ions/dopants having preferred electronic configurations, engineering the crystallographic site occupancy by dopants, controlling/modifying the degree of covalence of TM-O bonds, modifying lattice spacing(s), tuning phase assemblage etc. Such modifications done from the fundamental perspectives influence the performances of TM-oxides for a variety of applications, including their widespread usage as electrode-active materials in alkali metal-ion batteries.In the context of the upcoming Na-ion battery system, O3-type ‘layered’ Na-TM-oxides are promising as cathode-active materials due to their inherently high initial Na-content (as compared to the P2 counterparts). However, they suffer from instabilities caused due to multiple phase transformations during Na-removal/insertion and sensitivity to air/moisture. Against this backdrop, with the help of a dopant, having d 0 electronic configuration (viz., no octahedral site preference energy), we have been able to tune the composition and structural features to suppress the phase transitions upon Na-removal/insertion and also improve the air/water-stability in significant terms; so much so that long-term cyclic stability has been achieved with health/environment-friendly ‘aqueous processed’ electrodes (sans, usage of toxic/hazardous/expensive chemicals like NMP and PVDF) [1]. As will be explained in more elaborate terms during the talk, the changes in structural features, which have led to such outstanding water-stability, include differential contraction/dilation of the Na-‘inter-slab’/TM-‘slab’ spacing and partial occupancy of the dopant at tetrahedral sites of the structure.On the anode front, successful development of Na-ion battery system necessitates looking beyond hard carbon based anodes, which possess safety hazards due to the Na-insertion potential being a bit too close to the Na-plating potential. In this context, Na-titanates promise to be ‘safe’ anode materials; but suffer from cyclic instability [2]. Against this backdrop, we have developed carefully tuned bi-phase Na-titanate based electrodes, having Na2Ti3O7 and Na2Ti6O13 as the primary and secondary phases, respectively. The as-developed phase assemblage has been able to address the cyclic instability of single-phase Na-titanate, leading to long-term cyclic stability even at high current densities (up to 50C!) [3]. These are important steps towards the development of health/environment-friendly, cost-effective, safe and high-performance Na-ion batteries.Acknowledgement to RRCAT, Indore, India, for enabling the usage of Synchrotron facility.Parts of the concerned works have been published as (i.e., the associated publications); B. S. Kumar, A. Pradeep, A. Dutta, and A. Mukhopadhyay; J. Mater. Chem. A 8 (2020) 18064H. S. Bhardwaj, T. Ramireddy, A. Pradeep, M. K. Jangid, V. Srihari, H. K. Poswal, and A. Mukhopadhyay; ChemElectroChem 5[8] (2018) 1219A. Pradeep, B. S. Kumar, A. Kumar, V. Srihari, H. K. Poswal, and A. Mukhopadhyay; Electrochim. Acta 362 (2020) 137122