ABSTRACT

During the last decade transition metal-based nanomaterials have received a lot of attention because of their wide field of potential applications like sensors, solar cells, and battery materials. From a fundamental point of view, this potential arises not only from the mere size reduction but from fundamentally novel properties appearing upon size reduction which may yield interesting new applications for well-known bulk materials. High surface area and reduced ionic or/and electronic diffusion lengths as well as less tendency to structural changes compared to the bulk form render nanoscaled materials beneficial for electrochemical energy storage. In this respect, olivine nanostructures are particularly promising because in olivinetype lithium iron phosphates the underlying FePO4-network is stable up to high temperatures and offers cycling stability at high cell voltages (Padhi, Nanjundaswamy, and Goodenough 1997, Goodenough 2007). However, ionic and electronic conductivity are relatively low in this material. Detailed studies of the intercalation process show that only the phases LiFePO4 and FePO4 where the Fe-ions exhibit the single oxidation states 2+ and 3+, respectively, are present in the cathodes which are bad electrical conductors (Thackeray 2002). The kinetics can be enhanced e.g. by doping, size-tailoring and/or “carbon nanopainting”. This can yield capacities of about 160 mAh/g which is near the theoretical value of 170 mAh/g (Armand, Gauthier, Magnan, and Ravet 2002). Although controversially discussed, the olivine member LiMnPO4 might be an even more promising cathode material than LiFePO4 due to the higher operational voltage of 0.6-0.7 V since it offers a rather flat discharge voltage curve at 4.1 V (Padhi, Nanjundaswamy, and Goodenough 1997). However, this material particularly suffers from poor electronic and ionic conductivity and increasing research activities aim to overcome this problem, e.g., by synthesis of carbon-coated nanostructured material with stable reversible capacity up to 145 mAh/g (Wang, Buqa, Crouzet, Deghenghi, Drezen, Exnar, Kwon, Miners, Poletto, and Grätzel 2009). In this respect, it is a promising task to synthesize nanoscaled LiMPO4 with various transition metal oxides, such as M = Mn, Co. The low intrinsic electronic and ionic conductivity of these materials in bulk requires particles with a large surface to volume ratio in order to reduce ionic or/and electronic diffusion lengths, and possibly optimized post-synthesis treatment like carbon coating.