Highly reversible oxygen redox in layered compounds enabled by surface polyanions

Abstract

Oxygen-anion redox in lithium-rich layered oxides can boost the capacity of lithium-ion battery cathodes. However, the over-oxidation of oxygen at highly charged states aggravates irreversible structure changes and deteriorates cycle performance. Here, we investigate the mechanism of surface degradation caused by oxygen oxidation and the kinetics of surface reconstruction. Considering Li 2 MnO 3 , we show through density functional theory calculations that a high energy orbital (lO 2p') at under-coordinated surface oxygen prefers over-oxidation over bulk oxygen, and that surface oxygen release is then kinetically favored during charging. We use a simple strategy of turning under-coordinated surface oxygen into polyanionic (SO 4) 2− , and show that these groups stabilize the surface of Li 2 MnO 3 by depressing gas release and side reactions with the electrolyte. Experimental validation on Li 1.2 Ni 0.2 Mn 0.6 O 2 shows that sulfur deposition enhances stability of the cathode with 99.0% capacity remaining (194 mA h g −1) after 100 cycles at 1 C. Our work reveals a promising surface treatment to address the instability of highly charged layered cathode materials.

Challenges and Recent Advances in High Capacity Li‐Rich Cathode Materials for High Energy Density Lithium‐Ion Batteries

“…Therefore, combining surface coating with bulk doping has been proposed as an alternative approach for promoting both electronic/ionic conductivity and surface stability. [95,102,151] For example, Zheng et al [143] incorporated a nanoscale electrochemically active LiFePO 4 (LFP) layer on the surface of LRM layered oxides through a facile sol-gel method. As illustrated in Figure 15g, a thin layer of LFP is homogenously coated on the LRM surface.…”

Inhibiting Oxygen Release from Li‐rich, Mn‐rich Layered Oxides at the Surface with a Solution Processable Oxygen Scavenger Polymer

“…[ 10 ] For example, spinel Li 4 Mn 5 O 12 layers on LRLO have been introduced to suppress oxygen reactivity on the LRLO surface at high voltage (>4.5 V versus Li/Li + ). [ 11 ] Dielectric Mg 2 TiO 4 coatings to increase the M‐O bond energy on the surface of LRLO, [ 12 ] and (SO n ) m − ( n = 2, 3) polyanion coatings to stabilize the surface oxygen (by charge compensation from sulfur oxidation via formation of (SO 4 ) 2− ), [ 13 ] have been employed to reduce oxygen reactivity and suppress oxygen loss during the first charge. Surface layer modification by TM doping to enhance Li + ion intercalation kinetics and stabilize surface oxygen has also been invoked.…”

A Redox Couple Strategy Enables Long‐Cycling Li‐ and Mn‐Rich Layered Oxide Cathodes by Suppressing Oxygen Release

“…In the S 2p XPS spectra, the two doublet peaks present at 162.0 and 167.4 eV can be attributed to S 2− and SO 3 2− , respectively. [ 14,44 ] S 2− is the dominant species before cycling, and only a small portion of S 2− is oxidized to SO 3 2− when charging to 4.4 V. Then, SO 3 2− becomes dominant upon further charging to 4.8 V due to the substantial oxidation of S 2− to SO 3 2− . Furthermore, SO 3 2− is reduced back to S 2− and S 2− becomes dominant again in the following discharging process.…”

“…In the S 2p XPS spectra, the two doublet peaks present at 162.0 and 167.4 eV can be attributed to S 2− and SO 3 2− , respectively. [14,44] S 2− is the dominant species before cycling, and only a small portion of S 2− is oxidized to SO ), thereby suppressing the oxygen release and enhancing the cycling stability of the electrode. More importantly, the S 2− can be regenerated during discharging and will participate in the next charging process (Figure 3f and Figures S13 and S14, Supporting Information).…”