(Invited) An Overview of the Behavior of the LixNiO2 Electrode at High Voltage

Abstract

Nickel-rich layered oxides Li(Ni,M)O2 (M = Co,Mn, ..) are widely adopted as electrode materials for electric vehicles. They suffer from a loss of capacity when the cells are charged above 4.15 V vs Li/Li+. This limitation is generally attributed to: (i) sharp decrease of the interslab distance at the end of charge which induces cracks in the particles, (ii) slab gliding leading to the formation of the H4 phase, (iii) reaction with the liquid electrolyte, which leads to formation of a rocksalt type structure. The cycling behavior can be improved by texture optimization (concentration gradient, structural orientation). Doping, coating with various cations lead, in some cases, to significant improvement of the cycling properties when the cells are cycled at high voltage. However, the true mechanisms involved at high voltage are not fully understood. In order to overcome the effect of M cations our work was focused on LiNiO2.GITT and in situ XRD experiments were performed at very low rate in various voltage range (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. When the cell voltage is extended to 4.6V the H4 phase (NiO2) is formed. Cells were charged up to 4.3, 4.5 and 4.6V with a long potentiostatic step (144 hours).The recovered materials were characterized by S-XRD and HAADF electron microscopy. In all cases a mixture of R3 and H4 phases were obtained. The electron microscopy show that both phase R3 an H4 are mixed inside the whole particles. NiO2 is expected to be an ionic insulator (no Li+) and an electronic insulator (d6 low spin). Since there is no formation of a shell of NiO2 around the particles, the difficulty to deintercalation Li suggests that at high voltage another mechanism must be considered.To try to have a better understanding of the involved process several electrochemical studies were performed in various experimental conditions. As the charge proceeds above 4.17 V the cell polarization dramatically increases, hindering further Li deintercalation. In discharge, such polarization decreases immediately. Also and 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 did not undergo significant structural defect.This study shows that the change in polarization between the charge and the discharge results from the electrochemical behavior of the grain surface rocksalt type phase having very low conductivity above 4.17 V and high conductivity below this threshold. The results presented here open a new approach to monitor the electrochemical behavior of the rocksalt type shell and manage its electrochemical properties, at high voltage, by cationic doping of the surface layer. It can explain the behavior observed with some dopants (usually referred as “infused” coatings) like tungsten.