LiPF6 as Effective Etching Agent of LiMnPO4 colloidal Nanocrystals for High Rate Li-Ion Battery Cathodes

Monday, 29 May 2017: 15:40
Grand Salon C - Section 15 (Hilton New Orleans Riverside)


LiMnPO4 is an attractive cathode material for the next-generation high power Li-ion batteries, due to its high theoretical specific capacity (170 mA h g−1) and working voltage (4.1 V vs Li+/Li). Two main drawbacks prevent the practical use of LiMnPO4: (i) its low electronic conductivity and (ii) the limited lithium diffusion rate, responsible for the poor rate capability of the cathode. The use of nano-particles can alleviate the issues associated with poor ionic conductivity while the electronic resistance is usually lowered by coating the particles with a carbon layer. It is therefore of primary importance to develop a synthetic route to LiMnPO4 nanocrystals (NCs) with controlled size and coated with a highly conductive carbon layer. Here we report an effective surface etching process (using LiPF6) on colloidally synthesized LiMnPO4 NCs that makes the NCs more hydrophilic and dispersible in the aqueous glucose solution used as carbon source for the carbon coating step. The carbon coated etched LiMnPO4-based electrode exhibited a specific capacity of 118 mA h g−1 at 1C, with a stable cycling performance and a capacity retention of 92% after more than 100 cycles at different C-rates. The delivered capacities were higher than those of not etched carbon coated NCs, which never exceeded 30 mA h g−1. The adopted etching process allowed: (i) The efficient removal of the hydrophobic passivated surfactants shell, present on NCs surface after the colloidal synthesis. This increases the nanoparticles’ solubility in the aqueous glucose solution used as carbon source for NCs coating, enabling the formation of a good conductive carbon layer. (ii) The possibility to prepare composite electrodes with a reduced amount of carbon additive and polymeric binder (less than 20% wt. in total), with a consequent benefit on the energy density of LMP-based cathodes. The protocol reported here enabled the preparation of LMP/NCs-based cathodes with high rate capability and which can be charged with a fast CC−CV procedure (of maximum 2 h at 1C-rate), which is of paramount importance for the future development of high rate and high power Li-ion batteries.