LiMnPO4 Li-ion battery cathodes: Synthesis, processing and characterization of aqueous solution precursors for powders and depositions

Abstract

Phospho-olivine cathode materials (LiMPO4) with M = Fe, Mn, Co, Ni are promising for improved Li-ion batteries. LiFePO4 is commercially already applied and has a potential of 3.4 V versus Li/Li+. The Mn, Co and Ni analogues have a higher potential (4.1, 4.8 and 5.2 V versus Li/Li+, resp.) which allows larger energy storage capacities (1). However, the potentials of the latter two are outside the range of the stability window of currently used electrolytes. LiMnPO4 on the other hand suffers both from low electronic conductivity and low ionic conductivity, which results in lower than theoretically expected electrode capacities, especially at high (dis)charging rates (1-3). Aforementioned issues can on one hand be circumvented by reducing the particle sizes to nano dimensions and structures (nanospheres, rods, sheets, thin films, …), which results in shorter Li-ion diffusion distances. On the other hand, amorphous conducting carbon is also introduced into the electrode material in order to improve electronic conductivity (2). In this work, the synthesis of aqueous solution precursors for LiMnPO4, with different complexing ligands is presented. This includes the EDTA ligand, which to our knowledge has not been reported before as being used in (aqueous) LiMnPO4 precursors. After drying, the gels are structurally characterized by FTIR and their thermal decomposition behavior is studied by thermogravimetrical analysis (TGA/DTA). Phase formation starts between temperatures as low as 350-400°C, as is shown by high temperature XRD. X-ray diffractograms show that annealing in argon or air can both produce phase-pure LiMnPO4. Characterization of residual (amorphous conducting) carbon is done by means of Raman spectroscopy, while morphological characterization is carried out by SEM. The suitability of these precursors for deposition methods like spin coating and spray coating for obtaining (thin) cathode layers on substrates is investigated. SEM shows that dense layers are indeed achievable.