Coupling between oxygen redox and cation migration explains unusual electrochemistry in lithium-rich layered oxides

Wanli Yang,Qinghao Li,Michael F Toney,David Prendergast,William E Gent,Yufeng Liang,Kevin H Stone,D J Vine ,Sungyoon Ahn ,Jihyun Hong,Mitchell Mcintire,Tolek Tyliszczak,Taylor Barnes,Seok Gwang Doo ,A L D Kilcoyne ,Stefano Ermon,Jay Hyok Song,Jin-Hwan Park,William C Chueh,Apurva Mehta,Yiyang Li,Ki Moo Lim

doi:10.1038/s41467-017-02041-x

Abstract

Lithium-rich layered transition metal oxide positive electrodes offer access to anion redox at high potentials, thereby promising high energy densities for lithium-ion batteries. However, anion redox is also associated with several unfavorable electrochemical properties, such as open-circuit voltage hysteresis. Here we reveal that in Li1.17–xNi0.21Co0.08Mn0.54O2, these properties arise from a strong coupling between anion redox and cation migration. We combine various X-ray spectroscopic, microscopic, and structural probes to show that partially reversible transition metal migration decreases the potential of the bulk oxygen redox couple by > 1 V, leading to a reordering in the anionic and cationic redox potentials during cycling. First principles calculations show that this is due to the drastic change in the local oxygen coordination environments associated with the transition metal migration. We propose that this mechanism is involved in stabilizing the oxygen redox couple, which we observe spectroscopically to persist for 500 charge/discharge cycles.

Highlights

Lithium-rich layered transition metal oxide positive electrodes offer access to anion redox at high potentials, thereby promising high energy densities for lithium-ion batteries
Upon charging to 4.60 V, the LMR-NMC electrode loses the honeycomb-like in-plane transition metal (TM) ordering in the TM layer, which manifests as the disappearance of the 9–11 ̊ (20–23 ̊ in Cu Kα) superstructure peaks in the X-ray diffraction (XRD) pattern (Fig. 1b)[48]
While our treatment here is by no means exhaustive, we find that TM migration in all cases results in a shift to higher energy of the O projected density of states (pDOS) regardless of the pathway examined and the identity of the migrating TM, suggesting that this observation is general and largely independent of the migration mechanism

Summary

Introduction

Lithium-rich layered transition metal oxide positive electrodes offer access to anion redox at high potentials, thereby promising high energy densities for lithium-ion batteries. First principles calculations show that this is due to the drastic change in the local oxygen coordination environments associated with the transition metal migration. We propose that this mechanism is involved in stabilizing the oxygen redox couple, which we observe spectroscopically to persist for 500 charge/discharge cycles. Bruce and coworkers recently reported evidence for the localized O2–/O– mechanism in Li- and Mn- rich Ni/Mn/Co layered oxides (LMR-NMC) based on O K edge XAS6,7, yet similar observations have led others to conclude a 2O2–/O2n– redox couple in the same system[12,13]. A clear consensus on which mechanism prevails in which materials is lacking

Methods

Results

Conclusion