Phosphate Electrodes with Olivine Structure

Due to its acceptable electrochemistry and low cost or toxicity, the compound LiFeP04 crystallizing with an olivine-related structure is one of the most successful cathode materials in the short history of Li-ion batteries [9]. This material is already present in different commercial products, including energy storage for power tools and HEV [10]. 

The main drawback of this material is a low intrinsic electronic conductivity, which may lead to poor electrochemical performances. The synthesis procedure is aiso a sensitive point, as the oxidation of iron has to be avoided. A relatively simple method to deal with both the reducing synthesis conditions and the low conductivity of the product is to use carbon additives, also known as carbothermal process. Carbon additives have a two-fold effect. On the one hand, iron is protected against oxidation during the heating process. On the other hand, the use of carbon excess may provide an intimate mixture of LiFeP04 and carbon that has a much higher electronic conductivity than the phosphate alone. 

The carbothermal synthetic route may also have undesirable side effects. Thus, the carbothermal reduction of iron could yield iron phosphide contaminants, which are difficult to separate and compromise the4V capacity of the material. At this point, MS has been shown to be particularly effective in detecting and quantifying iron phosphide impurities and monitoring changes in the oxidation state of iron. The difference profiles of the Mossbauer spectra provide unequivocal identification of FeP and FezP phases (Fig. 28.4a) and their relative contributions depending on the temperature and other experimental conditions in which the synthetic work is carried out [11,12]. 

Unfortunately, for related solids such as UC0PO4, an interesting 5V cathode MS is not applicable. Other instrumental techniques such as Co K-edge XANES are better suited for the study of UC0PO4, although they are outside the scope of this chapter. 

---