Abstract:

FeF₃∙0.33H₂O possesses the characteristics of high theoretical capacity and high voltage, but its electrochemical cycling performance is unsatisfactory due to its poor conductivity and serious volume change during redox reaction, resulting in limited application. In this study, by using the strategies of dopamine self-assembly coating, carbonization, HCl etching and HF fluorination, the yolk-shell structured composite FeF₃∙0.33H₂O@carbon nanoboxes (FeF₃∙0.33H₂O@CNBs) composed of N-doped graphene shell and nanocube FeF₃∙0.33H₂O core was synthesized. Its particle size is about 250 nm and thickness of carbon shell is 30-40 nm. FeF₃∙0.33H₂O@CNBs displays an initial charge-discharge capacity of 208 mAh·g^-1 at a current density of 0.2C(1C=237 mA·g^-1). After 50 cycles, the capacity remains 173 mAh·g^-1, and the capacity attenuation rate per cycle is only 0.3%. In comparison, the initial capacity of bare FeF₃∙0.33H₂O is 112 mAh·g^-1, and after 50 cycles, only 95 mAh·g^-1 reserves, indicating superior cycle performance of FeF₃·0.33H₂O@CNBs. Furthermore, charging and discharging results at 0.1C-1C show that the rate performance is also significantly better than bare FeF₃∙0.33H₂O. It’s due to that N-doped graphene shell prepared by this strategy provides good electron/ion transport performance. At the same time, the carbon shell can not only buffer and inhibit the volume change of the core FeF₃∙0.33H₂O, but also shorten the ion migration distance and improve the Li⁺ migration rate on the electrolyte storage and retention performance of the electrolyte. As a result, the electrochemical performances are better than those of previous literature.

Key words: lithium ion battery, cathode material, iron fluoride, yolk-shell structure