Understanding sodium-ion diffusion in layered P2 and P3 oxides via experiments and first-principles calculations: a bridge between crystal structure and electrochemical performance

Shaohua Guo,Jin Yi,Guo-Zhen Zhu,Haoshen Zhou,Yang Sun,Kai Zhu,Mingwei Chen,Pan Liu,Masayoshi Ishida,Yanbei Zhu

doi:10.1038/am.2016.53

Abstract

Abstract Layered Na x MeO2 (Me=transition metal) oxides, the most common electrode materials for sodium-ion batteries, fall into different phases according to their stacking sequences. Although the crystalline phase is well known to largely influence the electrochemical performance of these materials, the structure–property relationship is still not fully experimentally and theoretically understood. Herein, a couple consisting of P2-Na0.62Ti0.37Cr0.63O2 and P3-Na0.63Ti0.37Cr0.63O2 materials having nearly the same compositions is reported. The atomic crystal structures and charge compensation mechanism are confirmed by atomic-scale characterizations in the layered P2 and P3 structures, respectively, and notably, the relationship of the crystal structure–electrochemical performance is well defined in the layered P-type structures for the first time in this paper. The electrochemical results suggest that the P2 phase exhibits a better rate capability and cycling stability than the P3 phase. Density functional theory calculations combined with a galvanostatic intermittent titration technique indicates that the P2 phase shows a lower Na diffusion barrier in the presence of multi-Na vacancies, accounting for the better rate capability of the P2 phase. Our results reveal the relationship between the crystal structure and the electrochemical properties in P-type layered sodium oxides, demonstrating the potential for future electrode advancements for applications in sodium-ion batteries.

Highlights

To smoothly integrate renewable energy into a smart grid system, an inexpensive and efficient energy storage device is urgently needed for large-scale applications.[1]
Structural characterization The crystal structures of the samples annealed at different temperatures (900, 950 and 1000 °C) for 20 h were recorded by X-ray diffraction (XRD) in Supplementary Figure S1
It has been demonstrated that a P3–P2 phase transition could be achieved in high-temperature conditions rather than through an electrochemical reaction because of the high energy required to break/reform the Me-O bonds for this transformation.[26]

Summary

Introduction

To smoothly integrate renewable energy into a smart grid system, an inexpensive and efficient energy storage device is urgently needed for large-scale applications.[1]. Rechargeable sodium-ion batteries have chemical storage mechanisms similar to their lithium-ion counterparts and are expected to be low cost and chemically sustainable as a result of an almost infinite supply of sodium.[7,8,9,10,11,12,13,14,15] the feasible replacement of Cu with Al current collectors Layered sodium oxide NaxMeO2 (Me = 3d transition metal) materials, owing to their large specific capacity and reversible insertion/. Extraction, have been studied intensely for sodium-ion battery applications for decades. Layered NaxMeO2 polytypes obtained through chemical solid-state synthesis mainly show two different structures: P2 and O3 (Figure 1).[16] They have been extensively studied as electrode materials for rechargeable sodium-ion batteries.[17,18]

Methods

Results

Conclusion