Amorphous nickel fluorosulfate cathode achieving a high-capacity by an additional electrochemical reaction mechanism

Abstract

Although lithium-ion cathode materials with crystal structure are extensively applied, many intrinsic problems are inevitable, such as the low capacity ascribed to the need to sacrifice a large amount of active Li to maintain the crystal structure, and the spontaneous irreversible phase transition of the crystal during the cycles, which will lead to the deterioration or even inactivation of the crystal structure, resulting in poor cyclic stability of the crystal material. Herein, an amorphous nickel fluorosulfate cathode material (a-LNSF) is fabricated by a simple mechanical ball grinding method and compared with the electrochemical properties of LNSF with crystal structure. Comparing with LNSF, a-LNSF exhibits a high discharge specific capacity of 238.5 mAh/g at a current density of 0.1 C, and the specific capacity is still 118.3 mAh/g under a high current density of 10 C. Moreover, the capacity retention ratio is 85.7 % after 300 cycles at the current density of 1 C. a-LNSF achieves satisfactory electrochemical properties attributed to its inherently advantages of the amorphous structure. Theoretical calculations coupled with experiments to further explore the reason of superior electrochemical properties of a-LNSF, and an additional electrochemical reaction mechanism for capacity accumulation were discovered. Interestingly, the directional reversible migration of F-ions (stemming from the a-LNSF) between bulk material and carbon nanotubes is accompanied by the process of charge and discharge, the C-F ionic bonds with excellent reversibility and electronic conductivity are formed to improve the comprehensive electrochemical properties of a-LNSF during the charge/discharge processes.