Effect of moisture exposure on electrochemical performance of LiNiO2 cathode active materials

Abstract

LiNiO2 is a promising cathode active material to solve the problem of high cost of Co and to realize high capacity in stoichiometric Ni-rich cathode materials. However, LiNiO2 reacts with H2O in the air to change the structure and shows deteriorated electrochemical performance. Regarding the degradation of LiNiO2 by moisture, a mechanism has been proposed in which impurities such as LiOH and Li2CO3 are generated by Li+/H+ exchange to form a phase such as Li1-xHxNiO2. On the other hand, as Li+ ions are extracted from the surface of LiNiO2 by moisture, a mechanism is also proposed to form Ni3O4-like spinel and NiO-like rock salt phases on the near-surface of the active material. In Li1-xHxNiO2, Ni always exists as trivalent, and in Ni3O4 and NiO, divalent Ni also exists. However, papers explaining both claims have not yet been reported. Herein, we investigated how Li+/H+ exchange and formation of Ni3O4-like spinel and NiO-like rock salt phases occur when LiNiO2 is exposed to moisture. As a result of performing the discharge process first in the electrochemical test, it was confirmed that there is a site where Li can be electrochemically inserted or exchanged. Recalling that spinel such as Ni3O4 and rock salt phases such as NiO are electrochemically inert, we suggest the possibility that LiNiO2 active materials exposed to moisture may contain H+. From the Raman spectrum and the calculated diffusion coefficient, the possibility of NiOOH phase formation is presented together. Additionally, analysis of phase transition and discharge capacity retention during charging suggested the possible presence of Ni3O4-like spinel and NiO-like rock salt phases on the LiNiO2 surface exposed to moisture.