Understanding the High Voltage Behavior of LiNiO2 Through the Electrochemical Properties of the Surface Layer

• Published in: Small (Weinheim an der Bergstrasse, Germany) Germany), 2023-07, Vol.19 (30), p.e2300616-n/a
• Main Authors: Bautista Quisbert, Edgar, Fauth, François, Abakumov, Artem M., Blangero, Maxime, Guignard, Marie, Delmas, Claude
• Format: Article
• Language: English
• Subjects: cell polarization; Chemical Sciences; Crystal defects; Cycles; Discharge; Electrochemical analysis; Electrode materials; Electrode polarization; High voltages; lithium nickel oxide; lithium‐ion batteries; Low conductivity; Material chemistry; Nanotechnology; positive electrode materials; Stacking faults; Surface layers; Titration
• ISSN: 1613-6810; 1613-6829
• DOI: 10.1002/smll.202300616

Nickel‐rich layered oxides are adopted as electrode materials for EV's. They suffer from a capacity loss when the cells are charged above 4.15 V versus Li/Li+. Doping and coating can lead to significant improvement in cycling. However, the mechanisms involved at high voltage are not clear. This work is focused on LiNiO2 to overcome the effect of M cations. Galvanostatic intermittent titration technique (GITT) and in situ X‐ray diffraction (XRD) experiments are performed at very low rates in various voltage ranges (3.8–4.3 V,). On the “4.2–4.3 V” plateau the R2 phase is transformed simultaneously in R3, R3 with H4 stacking faults and H4. As the charge proceeds above 4.17 V cell polarization increases, hindering Li deintercalation. In discharge, such polarization decreases immediately. Upon cycling, the polarization increases at each charge above 4.17 V. In discharge, the capacity and dQ/dV features below 4.1 V remain constant and unaffected, suggesting that the bulk of the material do not undergo significant structural defect. This study shows that the change in polarization results from the electrochemical behavior of the grain surface having very low conductivity above 4.17 V and high conductivity below this threshold. This new approach can explain the behavior observed with dopants like tungsten. When the Li//LiNiO2 cell is charged above 4.17 V there is formation of a rocksalt thin layer on the particle surface. Upon cycling its thickness increases. The electrochemical study shows that the polarization increases continuously in charge. It decreases in discharge. This effect is attributed to the change in conductivity of the surface layer.