In-Situ Formation of Low-Strain and Defect-Free Single Crystal NMC in Electrode Slurries

Abstract

Synthesis of cathode materials for lithium-ion batteries usually results in agglomerations of single crystals.[1][2] Jet milling is commonly used in industry to deagglomerate cathode material powders.[3] However, dry grinding techniques can often lead to the introduction of defects, requiring a reheating step to reform pristine NMC. Wet milling in water has also been proposed, however, this results in lithium loss and crystal defects, again requiring additional drying and reheating steps are required to reform pristine NMC.[4][5] Here we report a low-strain and defect-free NMC deagglomeration method that is performed in-situ during the electrode slurry dispersion process. In this method, cathode material, conductive additive and binder are planetary milled in N-methyl-2-pyrrolidone (NMP). This results in both the deagglomeration of cathode secondary particles and the formation of a well-dispersed electrode slurry. Figure 1(a-d) compares the SEM images of NMC samples deagglomerated by different dry grinding techniques and the in-situ slurry milling method. Figure 1(e) and (f) show lattice strains and the first cycle voltage curves of the same samples. In comparison to the dry grinding techniques, in-situ slurry milling was found to be more effective in secondary particle deagglomeration than dry methods and resulted in a single crystal NMC with the lowest lattice strain, lowest irreversible capacity, and highest coulombic efficiency. In fact, in-situ milling was found to reduce lattice strain compared to the original agglomerated cathode material, by relieving internal strains created at the grain boundaries of secondary particles. We believe that in-situ slurry milling is an effective method for NMC particle deagglomeration and improving NMC performance.