2, NECCES, Binghamton University, Binghamton, New York, United States
The market for lithium ion batteries (LIBs) has been rapidly grown with exponentially increasing demands for electric vehicles and portable information technology devices. However, the current materials are still limited in capacity and energy density. A feature common to the current materials is that less than one electron participates in the redox reaction. To increase the chemical energy stored, a multielectron redox reaction would be preferred. Recently, researchers have reported LiVOPO4 as a promising multi-electron material to incorporate two electrons. There are two redox couples in this material, V3+/V4+ and V4+/V5+, per vanadium, which enables two Li ions to be reversibly inserted/extracted, leading to a high theoretical capacity of 305 mAh g-1 based on the weight of Li2VOPO4. However, LiVOPO4 has both poor lithium diffusivity and electronic conductivity, which can cause polarization and poor utilization of active material. To overcome these challenges, we propose a mechanical method to nanosize LiVOPO4 and coat carbon on the surface of particles meanwhile, which shortens diffusion pathway for Li ions and increases the electronic conductivity. Whereas the structural disorder and poor crystallinity were detected in LiVOPO4 after the mechanical treatment, which impeded the migration of Li ions in the diffusion passages and resulted a poor cycling stability. A following thermal method was adopted to reconstruct the structure of LiVOPO4 under a controlled condition with keeping particles nanosizing and without reducing LiVOPO4. The resulting LiVOPO4 gave superior electrochemical performance in terms of rate capability and cycling stability, with the highest stable cycling capacity, 260 mAh g-1 at C/5, of any two electrons redox phosphate system reported to date.
This research is funded by (1) U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) program under BMR Award No. DE-EE0006852, and (2) NorthEast Center for Chemical Energy Storage (NECCES), an Energy Frontier Research Center supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award No. DE-SC0012583.