Effects of Crystallites Stabilization Under High Electrode Density in Ni-Rich Cathode Materials
Authors: Cho Jaephil , Kim Junhyeok 
Published: February 2020

For next generation of lithium-ion batteries, cathode materials require high energy density, long cycle life and battery safety for application in electric vehicles and energy storage system. The huge challenges are specific and volumertric energy and power density among the many of the aggressive requirements for PHEV and EV purposes. In electrode design, electrode thickness (active material loading), electrode porosity and chemical composition are important parameters affecting the energy and power capability of the cell. For a given active material, energy density of the electrode could be improved by decreasing electrode thickness, reducing electrode porosity and decreasing the content of inactive materials (polymeric binder and conductive carbon). Ni-rich cathode materials are promising candidates for PHEV and EV  due to its higher energy density and power density than cobalt-rich based layered cathode materials.  Among many strategies, electrode thickness control was carried out to get high volumetric energy and power density. 

In highly pressed electrode, surfaces and inner secondary particles are shattered and porosity is decreased. When electrode is pressed to 80 % (2.1 g/cc) and 67 % (2.7 g/cc) and 56 % (3.2 g/cc), volumetric energy density and porosity are 1530 Wh/l (67 %) and 1983 Wh/l (57 %) and 2304 Wh/l (43 %), respectively.  As increasing electrode density, energy density was increased and porosity was decreased. As increasing electrode density, surface secondary particle is shattered so primary particle is exposed. Therefore, phase transition from layered structure (R-3m) to rock-salt like structure (Fm-3m) and side reaction between electrolyte and electrode become severe.

Here in, we introduce effects of crystallites stabilization under high electrode density in Ni-rich cathode materials. Through every primary particle coating that is applicable to high electrode density spec, we clarify stabilization of crystallites based on microstructural and electrochemical view. To identify concretely effect, we compare pristine NCA with surface treated NCA sample and compare both samples from low electrode density to high electrode density. At high temperature (45 oC) cycle (0.5 C charge, 1 C discharge), the lower electrode density, the higher performance and 100th cycle retention is achieved that means ionic conductivity is critical parameter in low C-rate test, 81% (2.1 g/cc, pristine), 89% (2.1 g/cc, surface treated) and 75% (2.7 g/cc, pristine), 86% (2.7 g/cc, surface treated) and 53% (3.2 g/cc, pristine), 75% (3.2 g/cc, surface treated). In rate test (0.5 C charge, 7 C discharge), the higher electrode density, the higher rate capability (efficiency of 7 C/0.5 C) is acquired that means electronic conductivity is dominant factor in high C-rate test, 44% (2.1 g/cc, pristine), 60% (2.1 g/cc, surface treated) and 50% (2.7 g/cc, pristine), 66% (2.7 g/cc, surface treated) and 62% (3.2 g/cc, pristine), 71% (3.2 g/cc, surface treated). Surface treatment show superior ionic conductivity and electronic conductivity compared with pristine NCA so it has improved high temperature cycle life and rate capability. In other words, ionic conductivity is more dominant factor than electronic conductivity at low C-rate but electronic conductivity is more critical factor than ionic conductivity at high C-rate test. By differing electrode density, we introduce effects of crystallites stabilization under high electrode density in Ni-rich cathode materials.