LiFePO_4−xN_y thin-film electrodes coated on carbon fiber-modified current collectors for pseudocapacitors

LiFePO_4−xN_y thin films were sputter-deposited on micron carbon fibers (MCFs) under a gas mixture of N₂/Ar/H₂ as electrode materials in pseudocapacitors. The MCFs were fabricated by thermal chemical vapor deposition on stainless steel substrates as current collectors. Various amounts of N₂ were introduced by controlling the flow ratios of N₂ to Ar/H₂. The LiFePO_4−xN_y thin films coated on the surfaces of MCFs were observed by field emission scanning electron microscopy. The electrochemical properties of the LiFePO_4−xN_y thin films were characterized using cyclic voltammetry and charge–discharge processes. The LiFePO_4−xN_y thin-film electrode deposited under the optimal N₂ contents exhibited a high specific capacitance of 722 F/g at 1 A/g. Even at a current of 20 A/g, the electrode delivered a capacitance of 298 F/g. The pseudocapacitors using LiFePO_4−xN_y thin-film electrodes showed no significant capacitance fading after 1000 cycles at 1 A/g. The results indicated that nitrogen doping improved the electrochemical performances of LiFePO₄, demonstrating the potential of LiFePO_4−xN_y as an active material in pseudocapacitors. - Highlights: • LiFePO_4−xN_y thin films were sputter-deposited on micron carbon fibers (MCFs). • MCFs only act as a three-dimensional current collector in this system. • The pseudocapacitor exhibits a high specific capacitance.