Preparation, Modification And Electrochemical Lithium-storage Properties Of LiFePO₄ Cathode Materials

As a new generation of lithium ion battery cathode material, olivine-type lithium iron phosphate(Li Fe PO4) has been widely concerned because of its high safety, low cost, stable charge-discharge platform, stable cycle performance and environment-friendly. However, olivine structure materials universally have some deficiencies, such as low tap density, low electrical conductivity and the low diffusion rate of lithium ion, which seriously hinder their applications. In this paper, a novel polyol method was attempted to prepare Li Fe PO4 cathode materials under optimum conditions, and carbon materials were used to modify Li Fe PO4, with the aim of improving the electrochemical properties of Li Fe PO4 such as the electrical conductivity and the diffusion rate of lithium ion. The main research contents are given as follows.1. The influence of calcination on the as-prepared Li Fe PO4 cathode materialsLi Fe PO4 cathode materials were prepared via a polyol method, and the products were subsequently calcinated under inert atmosphere. It was found that the crystal defects of as-prepared Li Fe PO4 cathode materials were significantly reduced and their crystalinity was greatly increased after calcination. As a result, the electronic conductivity and electrochemical performance of the Li Fe PO4 cathode material was improved as well.2. The influence of reaction time on the as-prepared Li Fe PO4 cathode materialsLi Fe PO4 cathode materials were successfully prepared via a polyol method at 3, 6, 12 and 18 hour reaction time, respectively. The as-prepared Li Fe PO4 product at 12 hour reaction time exhibited the optimal electrochemical performance. The results indicated that the electrochemical performances lie on two main factors: the higer crystalinity of the product, the less crystal defects in the product and the better of the electrochemical performance; secondly, the electrochemical performance was increased with the decrease of grain size.3. Synthesis and characterizations of C-Li Fe PO4 compositesC-Li Fe PO4 composites were prepared by using ascorbic acid(Vc) as a carbon source via a polyol method. Experimental results demonstrated that C-Li Fe PO4 composites showed a similar grain size to pure Li Fe PO4 cathode material with the size of about 1.5 μm. However, the introduction of carbon material decreased the agglomerate degree of the Li Fe PO4 particles. Besides, the electronic conductivity and the reversibility of intercalation-deintercalation of Li ion were improved because of the introduction of carbon source. As a result, compared to pure Li Fe PO4, C-Li Fe PO4 composite exhibited higher specific capacity and cycle performances at the same rate.4. Synthesis and characterizations of Li Fe PO4-graphene compositesLi Fe PO4-graphene(Li Fe PO4-GE) composites were prepared via a two-step method. Firstly, graphite oxide(GO) was successfully synthesized by a modified Hummers method, and graphene(GE) were subsequently obtained by calcining GO at high temperature under inert atmosphere. Then, Li Fe PO4-GE composites were synthesized by using as-prepared GE as a carbon source via a polyol method. Experimental results showed that the introduction of GE did not destroy the olivine structure of Li Fe PO4, and significantly increased the electronic conductivity and the reversibility of intercalation-deintercalation of Li ion of Li Fe PO4, thus to some extent improving the electrochemical lithium-ion storage properties of Li Fe PO4 materials.