Editors' Choice—Washing of Nickel-Rich Cathode Materials for Lithium-Ion Batteries: Towards a Mechanistic Understanding

Abstract

Washing is a commonly used method to remove surface impurities of cathode materials for lithium-ion batteries. However, a clear mechanistic understanding of the washing process is missing in the literature. In this study, we will investigate the effect of washing and subsequent drying of nickel-rich NCM cathodes (85% nickel) with respect to gassing and impedance of the washed cathodes. By on-line electrochemical mass spectrometry (OEMS), we will show a drastic reduction of the O 2 release above 80% SOC for the NCM washed with deionized water, suggesting the formation of an oxygen-depleted surface layer on the NCM particle surface. The modification of the surface can be confirmed by a strong impedance buildup of cathodes composed of washed NCM (using a microreference electrode in a full-cell), revealing that the impedance increases strongly with increasing drying temperature after washing. Last, we will propose a comprehensive mechanism on the processes occurring during the washing/drying process of nickel-rich NCM materials and identify the drying temperature after washing as the dominant factor influencing the surface properties.

Strategies of Removing Residual Lithium Compounds on the Surface of Ni‐Rich Cathode Materials†

“…If the drying temperature was too high (more than 180 °C), the NiO phase would form on the surface, which would significantly increase the surface impedance of Ni‐rich cathode materials, reducing their capacity and cycling life. [ 69 ] Consequently, special attention should be paid to the design of drying temperature and other parameters.…”

Surface Modification Strategies for Improving the Cycling Performance of Ni‐Rich Cathode Materials

“…Washing using water as solvent is believed to not only remove impurities from the surface but also to modify the near‐surface region through Li + /H + exchange between the material and the washing solution , , . As a consequence, water‐processed materials (or materials stored under moist conditions) do not deliver the same specific charge/discharge capacities as pristine (unwashed) ones.…”

“…In a recent paper, Pritzl et al investigated the underlying mechanism of washing and subsequent drying conditions of CAMs with a Ni content of 85 % with respect to battery performance. Their results indicate the presence of an oxygen‐depleted rock salt‐like surface layer related to the increase in drying temperature after washing.…”

“…(b) Full‐cell voltage vs. capacity profiles of the 3 rd and of the 28 th cycle at C/2 charge and 1C discharge of the pristine NCM851005 cathode (in gray) and of the washed NCM851005 cathode dried at 300 °C. Reproduced from ref . (c) Long‐term cycling performance at C/3 rate of single‐layer pouch cells with graphite anode and aqueous‐processed NCM811 cathode and NMP‐processed NCM811 cathode.…”

“…These mechanisms apply both to liquid carbonate electrolytes, which evolve CO and CO 2 , and sulfur‐based solid electrolytes (lithium thiophosphates), which oxidize to SO 2 . Additional capacity losses from factors extrinsic to the CAM are induced by alkaline surface contaminations, such as Li 2 CO 3 or transition‐metal carbonates, which result from post‐synthetic reactant residues and/or suboptimal storage conditions and lead to CO 2 evolution . During assembly and operation of a battery, additional contaminants, such as HF, may be introduced, which contributes to cation dissolution at high voltages and also leads to anode capacity losses , .…”