UC Davis

Lithium ion batteries are a critical part of the energy storage infrastructure and their utilization has expanded significantly over the past few decades. Batteries have improved greatly over that time; however, capacity, electrochemical performance, and charge/discharge rates are still insufficient for the growing energy needs. Nanoscale materials provide some enhancement in these areas due to the shorter diffusion path lengths and increased rates of lithium intercalation. Coarsening of nanomaterials during processing and operation is one of the critical problems for the implementation of novel technologies based on ‘nanoscale’ effects. In the context of batteries, nanomaterials inherently come with benefits, but also excess interfacial energies that cause parasitic reactions, intergranular cracking, and high surface reactivity that lead to accelerated cathode degradation. Additionally, the excess energies cause coarsening during the manufacturing of the nanoparticles as well as during battery operation through cycle induced coarsening.This work studied nanoscale LiCoO2 (LCO) particles to help improve the overall interfacial stability through dopant segregation. Currently, dopant selection can be an arduous experimental process to select the best dopant for segregation and lowering the interfacial energies. Molecular static calculations were used to screen a variety of dopants with differing ionic sizes and oxidations states. The simulations demonstrated a clear trend of increasing segregation energy with size and oxidation state for two surface planes, {001} and {104}, and two grain boundary structures, Sigma 3 and Sigma 5. From the model, lanthanum exhibited strong segregation to all interfaces and was selected as the best candidate for experimental synthesis. La-doped LCO and undoped LCO nanoparticles were synthesized through a hydrothermal synthesis method and produced nanoplatelet layered structures of LCO. STEM-EELS was used to confirm the presence of segregated La to grain boundaries and surfaces of the nanoplatelet morphology. Calorimetric sintering studies revealed that the La segregation lowers the average surface energy of the particles. The La also inhibited coarsening and grain growth in the nanoparticles during sintering resulting in doped nanoparticles with high surface areas and smaller crystallite sizes. In addition, water adsorption microcalorimetry was used to study chemically delithiated structures of the LCO particles. Water adsorption showed that delithiation results in a decrease of surface energy in the undoped particles, but the La stabilizes the surface energy of the nanoparticle. These results demonstrate a computational and thermodynamic framework for improving the interfacial stability of nanoscale cathode materials and could be used to optimize morphology of particles for battery operation.