Modification On Lithium Vanadium Phosphate Cathode Materials For Lithium Ion Batteries

As polyanion cathode material for lithium ion batteries, lithium vanadium phosphate (Li3V2(PO4)3, LVP) has attracted much attention due to the stable structure, excellent electrochemical performance and high safety. However, the poor intrinsic electronic conductivity of LVP, resulting from the separation of VO6 octahedra by PO4 tetrahedra in their structures, limits its practical application in lithium-ion batteries. This paper presents a thorough study of modification on LVP by doping and coating, with the purpose to improve their electrochemical performances. The results are as follows:Li3V2(PO4)3/C (LVP/C) composites were successfully synthesized via solid-state reaction, sol-gel process and spray-drying process, and the process parameters had been optimized. For solid-state reaction, the LVP/C composite with 15 wt% glucose as carbon source and sintered at 700℃, showed the best electrochemical performance, and also exhibited a stable monoclinic structure after cycling. This cathode material exhibited good cycle stability but low capacity when cycling between 2.5-4.5 V and 3.0-4.5 V; whereas showed high capacity but poor cycle stability when cycling between 2.5-4.8 V and 3.0-4.8 V. For sol-gel process, glucose, as a second carbon sources, has significant influence on the electrochemical performance of LVP when oxalic acid and V2O5 are in a stoichiometric ratio. The LVP/C sample prepared with 15 wt% glucose exhibits the best electrochemical performance with discharge capacity as high as 171.2 mAh g-1 at 0.1 C and 118.9 mAh g-1 at 10 C, which is much higher than that for the LVP/C sample without glucose (145.3 mAh g-1/0.1 C and 13.7 mAh g-1/10C). For spray-drying process, the effect of the amount and addition sequence of CMC on the performance of LVP was investigated. The LVP/C sample prepared with 15 wt% CMC exhibits the best electrochemical performance. Controllable spherical LVP/C composites were obtained, i.e., solid spherical particles for LVP/C(A) prepared by adding CMC after pre-sintering, and hollow spherical particles for LVP/C(B) prepared by adding CMC before pre-sintering. XRD results reveal that some impurities, such as Na3V2(PO4)3, Li2NaV2(PO4)3 and Li4(P2O7), appear when sodium salt of caboxy methyl cellulose (CMC) acts as carbon source, but the monoclinic structure of LVP remains after cycling between 3.0 and 4.8 V. Compared to LVP/C(B), LVP(A) presents better electrochemical performance at any C-rate and higher tap density.The effect of niobium and silicon coating on the electrochemical performance and mechanism of LVP was studied. Our results show that, for Nb-incorporated LVP, (3-NbOPO4 existed on the surface of LVP particles, rather than doped into the crystal lattice. The particle size is reduced and the electrical conductivity is enhanced for LVP after Nb-incorporation, therefore, electrochemical performance of LVP is remarkably improved. Compared with pristine LVP/C, the Li3V1-xNbxPO4)3/C (x=0.03) composite shows significant improvement in capacity, rate capability and cyclability, which delivers an initial discharge capacity as high as 182 mAh g-1 at 0.1 C with a capacity retention ratio of 84.7% after 50 cycles, and an excellent rate-capability of 160 mAh g-1 at 1 C and 125 mAh g-1 at 5 C. For SiO2modified LVP, the valence of V in both the pristine and SiO2-coated LVP are close to+3.SiO2 coating on the surface of LVP particles does not change the monoclinic structure of LVP. Furthermore, the electrochemical performance of LVP/C, especially high C-rate performance, can be significantly improved by low-level (2 wt%) SiO2 coating, which can suppress vanadium dissolution in the electrolyte, improve structural stability and reduce charge-transfer resistance.The effect of Fe and Na doping on the electrochemical performance and mechanism of LVP was also investigated. The experimental results show that, for Fe-doped LVP, Li3-xV2-yFe2+y(PO4)3, LiFePO4 and FePO4 co-exist in the Fe-doped LVP/C composite. Compared with pristine LVP/C, significant improvement in capacity, cycling stability and rate capability in LVP/C-Fe are achieved, which is attributed to reduced particle size, decreased charge-transfer resistance and enhanced structural stability of LVP. For Na-doped LVP, Na shows a notable effect on electrochemical behavior of LVP cathode material. The charge/discharge plateaus and the anodic/cathodic peaks around 3.85 V for LVP/C(A) and LVPC(B) electrodes confirmed the existence of Li3-xNaxV2(PO4)3 or some other electrochemically-active composites containing Na-A series of LiFe1-xVxPO4/C(x= 0.0.03,0.05,0.07,0.10.0.20.0.50,1.00) samples have been successfully prepared using a two-step solid-state reaction route. It is found that V-incorporation significantly enhances the electrochemical performance of LFP. Particularly, the LFP/C sample with 5 at% vanadium exhibited the best performance, i.e. a specific discharge capacity of 129 mAh g-1 at 5.0 C after 50 cycles and all capacity retention ratio was above 97.5% at C-rate from 0.1 to 5.0 C. X-ray absorption spectroscopy (XAS) results showed that the valence of V in LiFe1-xVxPO4/C (x= 0.05) is between+3 and+4. It was confirmed that the samples with x≤0.03 are in single phase while the samples with 0.05≤x＜1.00 contain two impurity phases:Li3V2(PO4)3 and LiVOPO4.