Recent Advances in the Development of LiCoPO_4 as High Voltage Cathode Material for Li-Ion Batteries

摘要

Polyanion-type materials, like lithium iron phosphate, are one of the great success in the research field of applied electrochemistry. Within this family, LiFePO_4 is a now mature material and its properties have been largely optimized thus opening the door to commercial exploitation. Only in recent years the attention of the scientific community is focusing the great advantage of the substitution of iron with manganese, cobalt or nickel in the olivine lattice. In fact the Mn~(3+)/Mn~(2+), Co~(3+)/Co~(2+) and Ni~(3+)/Ni~(2+) couples show increasing redox potentials, all higher than the Fe~(3+)/Fe~(2+), thus opening the door to large improvements in the energy performances of post-LiFePO_4 olivine based Li-ion cells. This chapter reviews the most relevant reports about the LiCoPO_4 material and its use in lithium cells. The goal is to provide an overview of the reported advancements concerning LiCoPO_4 physical properties, synthesis routes, electrochemical lithium de-insertion/insertion mechanism, effect of doping/substitution and effect of coating on the battery performances. In summary LiCoPO_4 is a valuable candidate material for next generation high voltage and high energy Li-ion cell. Although the research efforts dedicated so far by the scientific community, some fundamental aspects of the material properties and reaction mechanisms in lithium cells are not completely understood. In particular the investigation of the possible occurrence of terminal solid solution in the first stages of de-lithiation of LiCoPO_4, the characterization of the intermediate Li_(0.7)CoPO_4 phase, the effect of doping as well as a more careful understanding of the parasitic interactions with electrolytes are key aspects that have not been successfully tackled yet. Moreover, although a large phenomenological description of the structure/morphology/performances relationship has been developed so far, there is a lack of synthetic and fundamental explanations concerning the role of defects, the ion conduction mechanism, the electronic conductivity and the microscopic effect of crystal growth conditions. Owing to this, large room for improvements in the performances in Li-cells of LCP is expected by assessing the synthesis route, the coatings, the doping/substitution and the electrolyte additives in order to boost its ability to reversibly cycle lithium and minimize the unavoidable parasitic interaction with electrolytes at high voltage.