Construction of superior performance LiVPO4F/C cathode assisting by a regulating additive NH4F and the “Combination-Protection-Release” mechanism for lithium ion batteries

The commercialization of LiVPO 4 F/C cathode for lithium ion batteries is hindered by its low electrical conductivity and the presence of Li 3 V 2 (PO 4 ) 3 impurity. In this work, pure LiVPO 4 F/C with superior electrochemical performance is successfully prepared by adding regulating additive NH 4 F in one-step rapid reduction method. N-doped LiVPO 4 F/C without Li 3 V 2 (PO 4 ) 3 is synthesized by adding 0.025 mol NH 4 F, which possesses high electrical conductivity (0.896 S cm −1 ) and large Li-ion diffusion coefficient (3.65 × 10 −10 cm 2 s −1 ). It presents superior capacity of 144.8, 127.5 and 112.7 mAh g −1 at 1, 5 and 15 C and high capacity retention of 85.13% after 300 cycles at 1 C and excellent long term performance after 1000 cycles at 8 C. The in-situ X-ray diffraction analysis reveals the transformation process of the crystal structure of LiVPO 4 F. The added NH 4 F can effectively improve the electrochemical performance by adjusting the negative logarithm of the effective hydrogen-ion concentration to refine the grain and serve as nitrogen-doping source, leading to increased Li-ion diffusion coefficient and electrical conductivity. The precipitate LiHF 2 is formed after the addition of NH 4 F, which can suppress the volatilization of fluorine and supplement its loss in the precursor. The “Combination-Protection-Release” mechanism has been proposed to explain the preparation of LiVPO 4 F/C without impurity Li 3 V 2 (PO 4 ) 3 . • Pure phase LiVPO 4 F/C with superior performance is prepared by adding additive NH 4 F. • The regulating additive NH 4 F can refine the grain and serve as N doping source. • The new precipitate LiHF 2 is formed after the addition of NH 4 F. • NH 4 F can suppress the volatilization of fluorine and supplement its loss. • “Combination-Protection-Release” influencing mechanism on pure phase is proposed.