Study On The Synthesis And Electrochemical Performances Of LiFePO₄/C

In recent years, as a type of cathode materials of Lithium-ion battery, LiFePO₄ has been attracting a lot of attentions by many investigators. Its theoretical capacity is 170mAh/g.Compared with Lithium metal, it has a more stable discharge platform of 3.4V.Although it has many advantages, its low electronic conduction rate and ionic conductivity limits its practical application.In this thesis, Carbon-coated lithium iron phosphate composite material was synthesized by a solid-liquid method. HNO₃ was added to improve this method. Carbon-coated lithium iron phosphate crystal structures were characterized by XRD technigue. Lithium iron phosphate particle sizes were measured by laser particle size analyzer. Its electrochemical properties were tested by charge-discharge instrument and cyclic voltammeter. The results obtained as following:Using H3PO₄ and NH₄H₂PO₄ as mixed phosphorus sources, Carbon-coated lithium iron phosphates were synthesized by solid-liquid methods. The effect of different particle sizes of Fe₂O₃ produced by different factories on their electrochemical properties were investigated.The results indicated that Carbon-coated lithium iron phosphate synthesized by D50 (particle size of Fe₂O₃ smaller than 0.542μm) has higher charge-discharge capacity. In contrast, Carbon-coated lithium iron phosphates sythesized with the larger sizes of Fe₂O₃ have lower charge-discharge capacity. However,carbon-coated lithium iron phosphates synthesized by larger particle sizes of Shanghai yipin Fe₂O₃(D50=0.291μm) not the smallest particle sizes (Zhejiang Deqing, D50=0.271μm) possess excellent discharge capacity, whose initial cycle of Carbon-coated lithium iron phosphate was 140mAh/g.The addition of carbon can prevent the oxidation of Fe²⁺ in heat treatment, which also can prevent particles growing up, and get Carbon-coated lithium iron phosphate materials in small uniform particle sizes. In synthesizing lithium iron phosphate material process, glucose, lactose, citric acid or solubility amylum were added as carbon sources respectively. Better electrochemical properties of LiFePO₄/C composite materials were obtained by adding lactose or glucose with 7.5% carbon content. However, LiFePO₄/C composite materials synthesized by adding solubility amylum with 7.5% carbon content , get the worst electrochemical properties. The infuences of sintering temperature and sintering time on the LiFePO₄/C composite materials'electrochemical properties were investigated.The precursors of LiFePO₄/C composite materials synthesized with the best ratio of gluose, were placed in tube furnace, sintered for 2-12 hours at the sintering temperature of 500 800℃and grinded to obtain LiFePO₄/C composite materials. The results showed that the materials obtained at the sintering temperature of 700℃, with sintering time of more than 6 hours have the best specific discharge capacity. The specific discharge capacity is 145.8 mAh/g discharging with 0.25C current,while with the current from 0.25C to 6C, the specific discharge capacity is still 110.4 mAh/g.The effects of doping 1%CuO, 1%CaCO₃, 1%Al₂O₃ on LiFePO₄/C composite materials'electrochemical properties were studied . The experimental results showed that LiFePO₄/C composite materials doped 1%CuO or 1%Al₂O₃ can improve their electrochemical properties.The specific capacity of LiFePO₄/C composite materials doped 1%CuO is 110.6 mAh/g with discharge 1C current after 91 cycles of constant charging and discharging. The specific capacity retention rate is 90.2%.