Relevance of LiPF6 as Etching Agent of LiMnPO4 Colloidal Nanocrystals for High Rate Performing Li-ion Battery Cathodes

LiMnPO4 is an attractive cathode material for the next-generation high power Li-ion batteries, due to its high theoretical specific capacity (170 mAh g-1) and working voltage (4.1 V vs Li+/Li). However, two main drawbacks... [ view full abstract ]

LiMnPO₄ is an attractive cathode material for the next-generation high power Li-ion batteries, due to its high theoretical specific capacity (170 mAh g^-1) and working voltage (4.1 V vs Li⁺/Li). However, two main drawbacks prevent the practical use of LiMnPO₄: its low electronic conductivity and the limited lithium diffusion rate, which are responsible for the poor rate capability of the cathode. The electronic resistance is usually lowered by coating the particles with carbon, while the use of nanosize particles can alleviate the issues associated with poor ionic conductivity. It is therefore of primary importance to develop a synthetic route to LiMnPO₄nanocrystals (NCs) with controlled size and coated with a highly conductive carbon layer. We report here the combination of a new colloidal approach to LiMnPO₄ NCs and an effective surface etching process (using LiPF₆)that makes the NCs miscible in the aqueous glucose solution used as carbon source for the carbon coating step. Also, it is likely that the improved exposure of the NC surface to glucose facilitates the formation of a conductive  carbon layer that is in intimate contact with the inorganic core, resulting in a high electronic conductivity of the electrode, as observed by us. The carbon coated etched LiMnPO₄-based electrode exhibited a specific capacity of 118 mAh g^-1 at 1C, with a stable cycling performance and a capacity retention of 92% after 120 cycles at different C-rates. The delivered capacities were higher than those of electrodes based on not etched carbon coated NCs, which never exceeded 30 mAh g^-1. The rate capability here reported for the carbon coated etched LiMnPO₄ nanocrystals represents an important result, taking into account that in the electrode formulation 80 % wt. is made of the active material and the adopted charge protocol is based on reasonable fast charge times.