Synthesis of LiFePO4/C composite cathode for lithium-ion batteries

O. CECH; J. E. THOMAS; A, VISINTIN; M. SEDLARIKOVA; J. VONDRAK

Brno, Rep. Checa

Congreso; 11th Advanced Batteries, Accumulators and Fuel Cells; 2010

11th Advanced Batteries, Accumulators and Fuel Cells

All conventional electrode materials used for lithium-ion cell cathode suffer from poor electronic conductivity, which is in range of 10-2 – 10-9 S/cm. Because electrode system of a lithium ion battery has to allow flowing of both lithium ions and electrons it is necessary to add some conductive agents to the mass of electrode active material. Many researchers have discovered different conductive materials for cathode electronic conductivity enhancement. Not only electronic conductivity but also ionic conductivity influences electrochemical performance of the composite cathode. This can be improved mostly by increasing electrode porosity, decrease the size of the particle to nanosize and thus contact surface between electrolyte solution and active material. This is best way to shorten ion diffusion lengths. Higher specific capacity and especially better high rate performance are achieved by template-synthesized hollow-sphere thin wall particles or nanotubes. Nowadays, olivine LiFePO4 is going to be the leading material for high-power li-ion rechargeable batteries because of its thermal stability, cost and environmental harmlessness. LiFePO4 unfortunately suffers from much poorer electronic conductivity than widely used layered LiCoO2 material. Improved electrochemical performance can be achieved by obtaining nanostructured active material particles together with highly conductive material – usually carbon black. Electronic conductivity is affected by carbon support and ionic conductivity rises due to higher porosity. Goodenough’s group start working in the LiFePO4 [1] to be used as the cathode material for secondary lithium batteries, many research groups aftener that have tried to improve the performance of this material. For LiFePO4, the discharge voltage is about 3.4 V vs. lithium. Its theoretical energy density is 161.4 Ah/kg, higher than obtained in commercial LiCoO2 cells, and comparable to stabilized LiNiO2, and moreover it is very stable during discharge/ recharge. LiFePO4 can be synthesized by high tempe rature reactions [1] or under hydrothermal conditions [2]. As this material has a very low conductivity at room temperature, it can achieve the theoretical capacity only at a very low current density [3] or at elevated temperatures [4], as suggested in [1] due to the low lithium diffusion at the interface. Ravet et al. [5] showed a carbon coating significantly improves the electrochemical performance of this material; sucrose was proposed as one carbon precursor, and we used it for hydrothermal samples [2]. In our experiment, sintering of lithium, iron and phosphate sources together with organic carbon precursor and little amount of Super P, was used. Organic precursor is carbonized during heating and is believed to be used for particle coating. The presence of Super P carbon in the base reaction mixture can prevent oxidation of Fe2+ to Fe3+ during synthesis [2].