Iron Phosphate Materialsas Cathodes for Lithium Batteries

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磷酸铁锂外文书
In the book, our efforts to make lithium iron phosphate (LiFePO4) a suitablematerial for lithium-ion batteries are presented. It was found that carbon, added before the formation of the crystalline phase, was effective on improving the electrochemical properties of the material in terms of practical capacity and charge/discharge rate. The full capacity (170 Ah kg-1) was attained when discharging the cell at 80C and C/10 rate. To evaluate the lithium chemical diffusion the lithium insertion in LiFePO4 was treated with a Frumkin-type sorption isotherm.
The diffusion coefficient was found lower than the theoretical value of seven orders of magnitude. The poor electrochemical performance exhibited from
the material was related to the relatively low value of the calculated diffusion coefficient. The reduction of the grain size was supposed to be one of the possible routes to enhance the performance of LiFePO4. Solution-based, low-temperature approaches can access metastable phases and unusual valence states that are otherwise inaccessible by conventional solid-state reactions. Amorphous FePO4 was prepared by sol-gel precipitation followed by air oxidation. Amorphous FePO4 was also prepared by spontaneous precipitation from equimolar aqueous solutions of iron and phosphate ions using hydrogen peroxide as an oxidizing agent. The material was able to reversibly intercalate lithium. Amorphous FePO4 was lithiated to obtain amorphous LiFePO4. Nanocrystalline LiFePO4 was prepared by thermal treatment of the amorphous material. Electrochemical tests have been done to evaluate factors affecting rate performance and long-term cyclability of the material. It was shown that nanocrystalline LiFePO4 showed good electrochemical
performance both at low- and high-discharge rate. At C/10 discharge rate the material delivered a specific energy close to the theoretical one. To test
both the effects of carbon coating and grain size reduction on the same sample, carbon covered nanocrystalline LiFePO4 was prepared. The so-obtained material showed the best electrochemical performance in terms of specific capacity, energy density, power density, and cyclability. A model to explain lithium insertion/extraction and predict the discharge curves at various rates was also illustrated.