Rechargeable lithium batteries are key components of the telecommunication equipment and portable computer systems. The layered $LiCoO_2$ is one of the earliest developed cathode materials, and the good performance and the stability make it the main commercially used cathode material despite its toxicity and high cost. Since the commercialization of the $LiCoO_2$ by SONY in 1991, alternative cathode materials have been pursued to improve the battery performances and cut down the cost. Among the several cathode materials, $LiFePO_4$ of the phospho-olivine family proposed by Goodenough appears particularly interesting due to the low cost and the environmental compatibility. For $LiFePO_4$, the charge/discharge voltage is about 3.4V vs. $Li/Li^{+}$ and the capacity fade is very small even after several hundreds cycles. Its theoretical capacity is 170mAh/g, which is as large as those of present cathode materials.
Part 1. Synthesis of $LiFePO_4$ by co-precipitation, and surface modification by silver post-treatment
The main problem of this material is low electronic conductivity. The low conductivity leads to initial capacity loss and poor rate capability. The conductivity problems have been studied by particle optimizing, metal doping, and mixing with the electronically conductive materials like carbon, metal and metal oxide. Among these approaches, the particle optimizing can be easily achieved by solution methods like co-precipitation, sol-gel and hydrothermal synthesis. Recently, Franger and Arnold tried a co-precipitation method to synthesize $LiFePO_4$, but homogeneous powder was not prepared because of the reduction of phosphates entities into phosphides in the reductive conditions.
Another simple approach to enhance the conductivity is mixing the precursor with the electronically conductive material. Croce et al. increased capacity by adding metal powder. However coating or post-treatment on the synthesized $LiFePO_4$ surface with electronic conductive metal has not been reported.
In this research, fine particle $LiFePO_4$ of single olivine phase was synthesized by the simple co-precipitation method. Blowing of nitrogen gas is needed in the co-precipitation stage to prevent oxidation of iron. Co-precipitated precursor showed high reactivity with reductive gas, and this problem can be reduced by carbon black treatment. $LiFePO_4$ prepared at 600℃ for 5 hours shows best electrochemical properties (127mAh/g at C/5), and $MÖssbauer$ spectroscopy and XANES analysis have confirmed that the oxidation state of iron is 2+. In this co-precipitated $LiFePO_4$ shows dependence of electrochemical property on the particle agglomerates.
To improve the poor conductivity, surface modification of the $LiFePO_4$ by aqueous coating was successfully done using silver nitrate solution. The surface of $LiFePO_4$ is covered with fine Ag particles after post-treatment. The electrochemical properties of the silver coated $LiFePO_4$ with the various current densities are analogous to those of highly conductive $LiFePO_4$. The specific capacities of silver coated $LiFePO_4$ are 139mAh/g at C/5 and 121mAh/g at 1C rate. The silver treatment can be a promising method to preserve the capacity even at the high current densities.
Part 2. Novel synthesis of $LiFePO_4$ by microwave heating
The one of main problems of the $LiFePO_4$ is that synthesis is not easy because of the iron oxidation state. The oxidation state control has been usually done by the furnace heating with the reductive or inert gas flow for several hours. In addition, alternative synthetic processes have developed continually, Yang et al. used hydrothermal synthesis and Franger et al. tried mechanochemical activation and rapid heat treatment.
I have tried to use the microwave heating as an alternative synthetic method. The microwave irradiation has been shown to provide a novel, rapid and cheap method of preparing many important materials. Several materials including activated carbon are known to exhibit extremely high heating rates when subjected to microwave irradiation. In this work, the activated carbon is used as the heat source for calcining the precursor due to its availability, low cost, rapid heating and ability to make reductive atmosphere. The basic idea of this heat treatment is that the activated carbon is the microwave absorber. If the activated carbon absorbs microwave, the activated carbon will heat the precursor rapidly and make reductive atmosphere by carbon oxidation reactions simultaneously. Therefore the heat treatment time can be very short and no additional inert or reductive gas is needed.
$LiFePO_4$ is successfully synthesized by the simple microwave heating without reductive gas flow. However pure precursor didn't make pure $LiFePO_4$ by excess of oxidation or reduction, and this problem can be reduced by carbon black treatment. XANES analysis shows the oxidation state of iron after carbon black mixing is 2+, therefore this new calcination method is very powerful tool to reduce the oxidation state. With the microwave heating using the activated carbon, the heat treatment time is tremendously decreased and kind of precursors is not important because the microwave absorber is not the precursor itself. This process is also very simple, cheap and reproducible. $LiFePO_4$ after 4 minute-microwave heating showed the smallest over-potential, the largest specific capacity (151mAh/g at C/10) and the stable cycle property over 50 cycles.
