Construction and Theoretical Calculation of an Ultra-High-Performance LiVPO4F/C Cathode by B-Doped Pyrolytic Carbon from Poly(vinylidene Fluoride).

Abstract

B-doped pyrolytic carbon from poly(vinylidene fluoride) (PVDF) was used to enhance the performance of a LiVPO4F/C cathode, which is much cheaper than carbon nanotubes and graphene. The carbon layer in LVPF/C-B3 becomes more and more regular compared with the undoped sample. The electronic conductivity, diffusion coefficient, and rate and cycle performance of the B-doped cathode are greatly improved. The capacities of LVPF/C-B3 at 0.2C, 5C, and 15C are 148.1, 132.9, and 125.6 mAh·g-1, which may be the best reported magnitude. The crystallite structure of LiVPO4F/C is well maintained after 300 charge and discharge cycles. The carbonization process of PVDF is greatly accelerated. These improvements are attributed to the changes in chemical and electronic structures. The generation of BC2O and BCO2 results in many defective active sites, and BC3 promotes the growth of a six-membered carbon ring. According to the first-principles approach based on density functional theory, the state density around the Fermi level of the B-doped pyrolytic carbon is increased. The electronic structure of pyrolytic carbon is transformed from a P-type semiconductor to a metal-like structure through the generation of pyridinic-like and graphitic-like B. Therefore, the electronic conductivity of LiVPO4F/C is increased.