The influence of vanadium doping lithium iron phosphate on the low temperature properties and the diffusion mechanism of lithium ion

Lithium iron phosphate (LiFePO4) is one of the most important cathode materials for high-performance lithium-ion batteries in the future, due to its incomparable cheapness, stability and cycle life. However, low Li-ion diffusion and electronic conductivity, which are related to the charging rate and low-temperature performance, have become the bottleneck problem. This article begins with doping Mn/Ti/V elements to study the effect of LiFePO4 low temperature discharge, LiFe0.95V0.05PO4, LiFe0.95Ti0.05PO4, LiFe0.95Mn0.05PO4 has been prepared easily and discharge capacity at -20℃ has reached 88%,80%, 76% respectively compared with that at 25 ℃. V-doped mode promotes the spherical growth of lithium iron phosphate material, and the particles are nano-spherical whose transverse and longitudinal growth rate is relatively consistent combined with the two diffusion models: “Mosaic models” and “Radial models”. Moreover, much smaller primary particle size (390 nm) LiFe0.95V0.05PO4 material whose lattice parameters of a and b reduced while c parameter enlarged with the increase of V doping content, which finally resulted in an overall decreased cell volume promote lithium ion diffusion. The (101) interplanar spacing of LiFePO4, LiFe0.95Mn0.05PO4, LiFe0.95Ti0.05PO4 and LiFe0.95V0.05PO4 was 4.274 Å, 4.279 Å, 4.282 Å, and 4.291 Å respectively, V-doped LiFePO4 has good low temperature performance, and the low temperature discharge performance is the best when the doping ratio is LiFe0.95V0.05PO4, and the which discharge capacity ratio is 87.99% and 67.69% at -20 ℃ and -40 ℃ respectively contrast with that at 25 ℃. The structural electron and ion transport properties of V-doped LiFePO4 are calculated by density functional theory. It has a low formation energy at the Fe site, which can be stably doped at the Fe site and form Mn-O /Ti-O /V-O chemical bonds. It is obvious that after doping vanadium, the migration energy barrier of lithium ions and the activation energy decreases even greater, and the transmission rate of lithium ions increases, which can improve the low-temperature discharge performance of LiFePO4 cathode materials.