Evaluation of Microstrain Generation in NMC622 Cathodes after the Hydraulic Pressure Compaction Process

Abstract

Cracking in lithium nickel manganese cobalt oxide (NMC) particles is one of the factors in reduced cyclic stability of NMC cathodes. While most studies focus on cracks caused by volume changes during electrochemical cycling, fewer address manufacturing-induced cracks formed during electrode fabrication. In this study, we investigated how an isolated and well-defined parameter applied during electrode calendering impacts the electrochemical performance of the LiNi0.6Mn0.2Co0.2O2 (NMC622) cathode material and proposed the underlying mechanism. We examined how applied pressure impacts the electrochemical, structural, and morphological properties of pressed compared to unpressed electrodes. Electrodes pressed at 3 tons (462 MPa) achieved the highest discharge capacities with optimal grain-to-grain contact and minimal cracking, while unpressed electrodes showed poor performance due to loosely packed particles. In contrast, 5 t (770 MPa) and 7 t (1078 MPa) electrodes exhibited fractured particles embedded in the aluminum collector, exceeding the material’s stress limit. Quantitative analysis of unit cell parameters, microstrain, and crystallite sizes correlated the electrode morphology and post-fabrication structure with electrochemical performance. The 3 t electrodes experienced slight unit cell expansion and the highest microstrain, while exceeding this threshold at 5−7 t caused particle cracking, structural relaxation causing crystal defects, and ∼10% crystallite size reduction. The investigation extends to the morphological implications of electrode compression and crystal defects, fracture mechanisms, and the final cracking types within polycrystalline NMC. This work demonstrates that selecting optimal calendering pressure positively influences electrochemical performance by introducing beneficial strain and compaction level.