Cathode Materials Produced by Spray Flame Synthesis for Lithium Ion Batteries
Lithium ion batteries are one of the most enthralling rechargeable energy storage systems for portable application due to their high energy density. Nevertheless, with respect to electromobility innovation towards better electrochemical properties such as higher energy and power density is required. Altering the cathode material used in Li-ion batteries is favorable since the mass- and volume performance is closely related to the cathode electrode mass. Instead of using LiCoO2 as cathode electrode, LiFePO4 has gained serious attention as this material owns a high theoretical capacity of 170 mAh g-1. It is non-toxic, cheap and consists of abundant materials but suffers from low electronic and ionic conductivity. Utilization of nanotechnology methods in combination with composite formation is known to cure this problem effectively. In this work, a new combination of techniques using highly scalable gas-phase synthesis namely spray-flame synthesis and subsequent solid-state reaction has been used to synthesize nanocomposite LiFePO4/C. At first this work deals with the formation and characterization of nanosize FePO4 from a solution of iron(III)acetylacetonate and tributyl phosphate in toluene using spray-flame synthesis. It was shown that a subsequent solid state reaction with Li2CO3 and glucose yielded a LiFePO4/C nanocomposite with very promising electrochemical properties. Based on these initial findings the influence of two synthesis parameter - carbon content and annealing temperature - was investigated towards the physicochemical properties of LiFePO4/C. It was shown that an annealing temperature of 700°C leads to high purity composite materials consisting of crystalline LiFePO4 with crystallite sizes well below 100 nm and amorphous carbon consisting of disordered and graphite-like carbon. Variation of glucose amount between 10 and 30 wt% resulted in carbon contents between 2.1 and 7.3 wt%. In parallel the specific electric conductivity increased by about three orders of magnitude and formation of aggregates could be oppressed at higher carbon content. Materials with high carbon content provided the best electrochemical properties and a stable capacity of up to 120 mAh g-1 when discharged within one hour (1 C). An additional way to improve the electrochemical properties of LiFePO4/C was investigated by doping FePO4 with manganese. Due to its higher redox potential, substitution of iron by manganese can lead to an increase in energy density. Fe(1 x)MnxPO4 was synthesized by partly substituting the iron precursor used for spray-flame synthesis by manganese(III)acetylacetonate. Manganese concentrations of up to 30 mol% were used and identical crystal structure compared to undoped FePO4 could be obtained. High purity nanocomposite LiFe(1 x)MnxPO4/C could be synthesized by the same method as described before. It was verified that manganese is quantitatively electrochemically active thus enabling the expected increase in energy density. Significant improvement in electrochemical properties of doped LiFePO4/C was observed with respect to very high discharge currents of up to 16 C and related to the fact that the substitution of iron by manganese widens the crystal structure in accordance with Vegard's law. In summary it is proven that scalable spray-flame synthesis in combination with subsequent solid-state reaction is most suitable for the formation of nanostructured high performance LiFe(1 x)MnxPO4/C composite material.
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