Fast-charging lithium-ion batteries are necessary for a range of applications, including electric vehicles that need to be charged outside of the home. Pseudocapacitive charge storage is a potential solution to this need because it allows for intrinsically fast charging/discharging, while still retaining high energy density. Some traditional battery materials can transition from battery-like to pseudocapacitive charge storage behavior upon nanostructuring. This occurs when the lithium-ion diffusion lengths are shortened so that slow solid-state diffusion is no longer rate limiting. Here we synthesize a range of nanostructured cathode materials, including LiNi0.80Co0.15Al0.05O2 (NCA), which is a high-capacity layered transition metal oxide cathode material, and LiVPO4F (LVPF), which is a high voltage tunnel structured cathode material. We created materials with decreased particle sizes and nanoscale porosity, compared to bulk, using colloidal polymer templating. For the NCA system, particle size can be further controlled using heating conditions, where conventional heating results in medium sized particles (~300 nm), and rapid thermal annealing produces small particles (~150 nm). Bulk NCA already shows promise for fast cycling due to suppression of intercalation-induced phase transitions, and nanostructuring the NCA can potentiall further improve rate capabilities by decreasing diffusion lengths. We observed that nanoporous NCA shows pseudocapacitive behavior with good rate capabilities. The medium domain sized material has the highest capacity retention at high charging rate, while the smallest domain sized material showed slightly poorer performance, likely due to kinetic limitations at the highly reactive surface. In contrast, LVPF has a less reactive surface, and so excellent rate capability is observed in small nanoporous materials. Interestingly, despite very fast charging times, LVPF remains fundamentally a battery and does not show pseudocapacitive behavior, even at very small domain size. The results help us understand the role of pseudocapacitive charge storage in creating fast charging electrode materials.

Nanoporous cathode materials for high rate energy storage

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.