Control of Crystallite Size of Cathode Material Via Artificial Organic Template
Authors: Cho Hyeon , Cho Jaephil 
Published: February 2020

Recent trend of cathode materials for high-performance lithium-ion batteries (LIB) has required high-energy density, high rate capability, and high stability in order to apply for electric vehicles (EV) and energy storage systems (ESS). Ni-rich cathode materials are promising candidates because of its high capacity. However, structural instability problem at a high temperature of 60 °C places an obstacle for their commercialization, causing poor cycling performance. Among the several issues, the main causes of structural instability are originated from cation mixing and microcracks inside the primary particles. The cation mixing occurs when de-intercalating the lithium ion from the Ni-rich material (charging process). Subsequently, Ni2+ ions migrate from transition metal sites (octahedral 3a) to lithium ion sites (octahedral 3b), which is ascribed to similar ionic radius between Ni2+ (0.69 Å) and Li+ (0.76 Å). Finally, Ni2+ ions take the lithium ion sites, and these lithium ions cannot be intercalated again during discharging, showing a severe capacity decrease. In addition, primary particles undergo continuous anisotropic lattice volume change during charging and discharging process because each particle has different crystallographic orientation and slip plane. This constant volume change imposes stress and strain between primary particles, resulting in ‘microcracks’ along the grain boundaries. Once occurrence of microcracks is started, they are drastically increased. Consequently, an electronic conductivity is deteriorated by separation of each primary particle from continuously increased SEI layer at the microcracks.

Here, we present a novel porous Ni-rich cathode layered material synthesis method that can control the grain size of primary particles. The difference between our method (co-precipitation) and reported method is usage of organic templates. Although organic template maintain its morphology (sphere) in co-precipitation, it is removed at high temperature sintering process. From the removal of organic template, Ni-rich cathode material grain size became smaller and alleviating voids were formed. Smaller grain size increase the efficiency and structural stability of Ni-rich layered materials. Alleviating voids inside the secondary particles compensate the volume change, improve the rate capability due to inside pores, and facilitating electrolyte penetration by improved diffusion pathways for rapid Li+ ions transport through the electrolyte/pores. Additionally, current density per unit surface area decreased, reducing electrode polarization, and that charge transfer at the interface is easier compared to dense NCM particle that is, Rct value is low at porous material. This porous material showed not only a very high specific capacity and coulombic efficiency of 208 mAh g−
1, 97 % based on the particle composition of Li[Ni0.6Co0.2Mn0.2]O2, but also superior cycling characteristics. Even at high temperature (60 °C), the difference of capacity retention between dense and porous material was about 20 % at 250 cycles. This superior stability could attributed to the alleviating voids which act as buffer space to suppress the microcracks caused by volume change of primary particle and smaller grain size which increase the stability of structure. This new synthesis method propose an alternative for advanced lithium-ion batteries to meet the demands of energy storage system.