Exploring the Charge Compensation Mechanism of P2-Type Na0.6Mg0.3Mn0.7O2 Cathode Materials for Advanced Sodium-Ion Batteries

Chen Cheng,Kehua Dai,Manling Ding,Jing Mao,Liang Zhang,Nian Zhang,Jinghua Guo,Tianran Yan

doi:10.3390/en13215729

Abstract

P2-type sodium layered transition metal oxides have been intensively investigated as promising cathode materials for sodium-ion batteries (SIBs) by virtue of their high specific capacity and high operating voltage. However, they suffer from problems of voltage decay, capacity fading, and structural deterioration, which hinder their practical application. Therefore, a mechanistic understanding of the cationic/anionic redox activity and capacity fading is indispensable for the further improvement of electrochemical performance. Here, a prototype cathode material of P2-type Na0.6Mg0.3Mn0.7O2 is comprehensively investigated, which presents both cationic and anionic redox behaviors during the cycling process. By a combination of soft X-ray absorption spectroscopy and electroanalytical methods, we unambiguously reveal that only oxygen redox reaction is involved in the initial charge process, then both oxygen and manganese participate in the charge compensation in the following discharge process. In addition, a gradient distribution of Mn valence state from surface to bulk is disclosed, which could be mainly related to the irreversible oxygen activity during the charge process. Furthermore, we find that the average oxidation state of Mn is reduced upon extended cycles, leading to the noticeable capacity fading. Our results provide deeper insights into the intrinsic cationic/anionic redox mechanism of P2-type materials, which is vital for the rational design and optimization of advanced cathode materials for SIBs.

Highlights

The rapid development of electronic devices and electric vehicles calls for electrochemical energy storage with an ever-increasing energy density
The as-prepared Na0.6 Mg0.3 Mn0.7 O2 (NaMMO) cathode was synthesized by a traditional solid-state method and its crystal structure was studied by the powder X-ray diffraction (XRD)
The oxygen layers stack with the ABBA sequence, in which the cations orderly distribute in the form of honeycomb-type as aforementioned to minimize the electrostatic energy [27]

Summary

Introduction

The rapid development of electronic devices and electric vehicles calls for electrochemical energy storage with an ever-increasing energy density. Lithium-ion batteries (LIBs) are dominating the market because of their high operating voltages and superior energy densities [1]. From this point of view, sodium-ion batteries (SIBs) are the prospective candidates of the next-generation “beyond-lithium-ion” batteries because of the natural abundance of sodium resources and the similar chemical properties of sodium and lithium [2,3]. Significant efforts have been devoted to find suitable cathode materials with desired properties and outstanding electrochemical performance for SIBs. Generally, the cathode materials can be mainly categorized into three groups: layered transition metal oxides (Nax TMO2 , transition metal denotes as TM), polyanionic types and Prussian blue derivates [4,5,6].

Methods

Results

Conclusion