Unraveling the Intricacies of Residual Lithium in High-Ni Cathodes for Lithium-Ion Batteries

High-nickel layered oxide cathodes suffer seriously from the formation of residual lithium on the surface, which causes notorious issues, such as slurry gelation and gas evolution. Due to the use of water for the titration to determine the residual lithium content, certain practical issues remain unresolved. We present here, for the first time, a thorough study of residual lithium that reveals the following. (1) Li2CO3 impurity in lithium raw materials contributes to an increase in residual lithium in high-Ni cathodes after synthesis. (2) LiOH formed due to a leaching of Li from high-Ni cathodes during analyte preparation in water exaggerates the LiOH content in residual lithium (employing a new titration method). (3) A dry cobalt hydroxide coating on high-Ni cathodes not only effectively reduces residual lithium content but also leads to the formation of a Co-rich concentration gradient layer on the surface that suppresses Li leaching when in contact with water.