Electrochemical properties of Ni-rich LiNi Co Mn O2 materials for use in aqueous lithium-ion batteries: How do they differ from those in non-aqueous systems?

This study seeks to identify the electrochemical properties of Ni-rich LiNi x Co y Mn z O 2 (NCM) materials in aqueous systems with a special focus on how they differ compared to those in non-aqueous systems. Electrochemical tests show that most of the electrochemical properties of the Ni-rich materials are more severely deteriorated in the aqueous solution compared to those in non-aqueous systems, although their intrinsic properties, such as phase transitions and loss of capacity due to a decrease in the content of Ni in the structure, are maintained. Operando X-ray diffraction measurement reveals that the rapid phase transition during charging and discharging, which has been suggested to be an important factor for the loss of capacity in Ni-rich NCM materials in non-aqueous systems, is not the main determinant of the degradation of electrochemical properties in this aqueous system. Based on the results of transmission electron microscopy and inductively coupled‐plasma optical emission spectroscopy, Ni ions dissolve from the Ni-rich NCM surface into the aqueous solution during electrochemical cycling through direct contact with H 2 O molecules, and the most rapid dissolution of Ni ions among transition metal ions is the most important factor in determining the poor electrochemical properties in this aqueous system. • Electrochemical properties of Ni-rich NCM are investigated in aqueous systems. • Various properties are compared to those in non-aqueous systems. • Operando XRD is used to reveal the main determinant of the degradation. • Rapid dissolution of Ni ions into aqueous solution is observed by various analyses.