High-voltage pyrophosphate cathode: Insights into local structure and lithium-diffusion pathways

Rechargeable lithium batteries have helped power the consumer revolution in portable electronic devices. The search for alternative cathode materials to replace layered LiCoO2, because of cost and safety issues, has generated considerable research activity, particularly for large-scale applications (such as hybrid and pure electric vehicles). An avenue that has been investigated involves the combination of low-cost and abundant iron and phosphate groups (PO4 3-). Owing to the strong binding of the oxygen in the polyanion groups, these materials are more stable and safer than layered transition-metal oxides. To date, most interest has focused on the olivine-structured LiFePO4, which has been studied extensively. Other polyanion-type compounds based on silicates, borates,fluorosulfates, and fluorophosphates have also received attention as alternative cathodes.