Superior Cyclic Reversibility of Amorphous Lithium-Iron Fluorosulphate Based on Both Insertion and Conversion Reaction for High Energy Density Lithium-Ion Battery Cathode Material

As the era of electric devices, EV and renewable energy start to flourish, lithium ion battery(LIB) has been considered as one of the promising energy storage system. However, current status of LIBs has struggled to meet the increasing demand of cheaper, smaller and lighter energy storage systems. Since conventional cathodes utilize intercalation chemistry to accomodate charges in electrode materials, their theoretical maximum capacities are restricted by the number of interstitial sites of host structure. Unlike intercalation chemistry, conversion reaction accomodates lithium ions and electrons in sepereated phase whose capacity is not limited by interstitial sites. In this respect, exploiting both intercalation and converison reaction provide electrode material double or triple capacities of conventional cathode materials by taking advantage of multi redox(TM^0/2+/3+). However, conversion based cathode materials shows severely poor cyclic reversibility compared to commercialized cathode materials (LCO, LFP, NCA, ...). Conversion reaction carries substantial structural changes which is detrimental for maintaining facile diffusion path of intercalation reaction. Furthermore, conversion reaction has insufficient cycle reversibility of itself. Regarding these struggles, enlarging the range of various insertion/conversion type materials will give us in-depth understanding about those type material and effective cycle enhancing strategies. Herein, we report a mechanochemically derived iron fluorosulphate material, a-LiFeSO4F with high energy density, cycle stability and rate capability. This newly derived phase exploits both insertion and conversion reaction for charge storage mechanism with abnormal cycling performances compared to reported conversion type materials. a-LiFeSO4F demonstrates 370 mAh/g specific capacity with 89 % capacity retention after 200 cycles even at elevated temperature (40 mA/g, 333 K). These superior electrochemical performances are originated from its amorphous structure which induces facile kinetics of re/conversion reaction. Consequently, we provide a new insertion/conversion type electrode material as well as potentials of which a method for excavating new insertion/conversion type electrode materials which might lead us to comprehensive understanding of those type materials.