Preparation And Electrochemical Performance Of LiFePO₄/C Composites Cathode Materials For Lithium-ion Batteries

Olivine Li Fe PO₄（LFP）has become one of the most promising cathode materials for Lithium-ion batteries due to its environmental compatibility, excellent cycling performance, low cost, superior safety performance and thermal stability. However,the low ionic and electric conductivity limit the rate performance of LFP, seriously restrict the development of its commercialization. Porous carbon is used to coat LFP to improve the electrochemical performance. Electrospinning is a common method to obtain one-dimensional structure for different materials. Such as carbon fiber and carbon fiber based composites. In our work, we combined with electrospinning technology to obtain different LFP/C composites, and investigated its electrochemical performance in detail. Besides, we combined superior electrical conductivity of graphene with superior ion conductivity of Li3V2（PO₄）3（LVP） to collaboratively modify LFP. The obtained composite has a high rate capability, excellent cycling performace. The main content of this assay is listed below.（1）We combined electrospinning and low temperature hydrothermal technique to prepare LFP in situ coated with hierarchical porous nitrogen-doped graphene-like membrane. And used electrospun polymer membrane as the carbon source. During the low temperature hydrothermal, the electrospun polymer membrane as the soft template to form graphene-like structure. Compared with the LFP/C with untreated carbon source. The discharge capacity can be reached as high as 174.2 m Ah g-1, 178.4m Ah g-1, 156.9 m Ah g-1, and 102.2 m Ah g-1 could be obtained at 0.1 C, 0.5 C, 1 C and8 C, respectively. After 300 cycles, it also delivers a discharge capacity of 80 m Ah g-1at 10 C and maintains almost 100% capacity retention. The improved electrochemical performance can be ascribed to the N-doped graphene-like membrane improves the electronic conductivity, and the hierarchical porous structure facilitates transfer of lithium ion and the penetration of electrolyte.（ 2） Hierarchical LFP/C mesoporous microbelts were obtained byelectrospinning. The using of double solvent enhances the spinnability, which makes an increase in salts solubility and a decrease in the surface tension. Which plays a key role for the formation of the thoroughly one dimensional mesoporous structure.Hierarchical microbelts can provide bicontinuous electron/ion pathways, and thoroughly mesoporous structure facilitates electrolyte penetration. Compared with LFP/C with single solvent, this composite delivers a discharge capacity of 153 m Ah g-1, 140.5 m Ah g-1, 114.8 m Ah g-1, 101.3 m Ah g-1, 84.1 m Ah g-1, and 72.9 m Ah g-1 at 0.5C, 1C, 3C, 5C, 10 C, and 15 C, respectively. It also can delivers a discharge capacity of 76.6 m Ah g-1 after 100 cycles at 10 C, and maintains 89.9% capacity retention.（ 3） We successfully synthesized graphene modified LFP-LVP/C composite cathode material. We not only used advanced inorganic carbon additive of graphene,but also used organic sources of ethylenediamine and oxalic acid. On the one hand,the two organic sources can be the coupling agent of graphene. On the one hand, they can in situ reduce Fe3+ and V5+, releasing gases of CO2 and NH3, benefiting for the unfold of graphene, thus an effective conductive network formed. Besides, lithium ion can transfer fast for LVP’s open three-dimensional framework, which can enhance the ionic conductivity. Therefore, the composite has a high electronic and ionic conductivity, display an excellent rate performance. When compared with the graphene modified LFP/C composite, this composite has a super high rate performance. At a high current density of 8 A g-1, it delivers an initial discharge capacity of 97.2 m Ah g-1. Even at a higher current density of 10 A g-1, the initial discharge capacity can also reach to 91.7 m Ah g-1. After 300 cycles, it also remains a discharge capacity of 77.8 m Ah g-1.