Cryo-TEM Study of High-Performance Iron Difluoride Cathode Enabled by Low Temperature CVD Carbon Coating

Metal fluorides (MFs) are regarded as high-capacity conversion cathode materials for next-generation lithium-ion batteries with high energy density. However, these cathodes suffer from poor electronic conductivity, sluggish reaction kinetics, and deleterious cathode solid electrolyte interface (CEI) formation, which may cause rapid cell degradation upon cycling. Herein, a low temperature chemical vapor deposition (CVD) carbon coating technology is successfully achieved to coat MFs-FeF2 with a thin amorphous carbon layer, which promotes the formation of a stable CEI with improved electronic and ionic transport property. Consequently, a discharge capacity higher than 450 mAh g-1 of the CVD coated FeF2 is achieved with a capacity retention at about 75% after 2500 cycles in a saturated electrolyte. Moreover, an unprecedented cycling performance with the discharge capacity of 350 mAh g-1 after 500 cycles in a super diluted electrolyte is also achieved. Advanced cryo-electron microscopy reveals that the carbon coating significantly suppresses the undesirable CEI formation and promotes the formation of a thin CEI with excellent chemo-mechanical property. This work provides a feasible technology to stabilize the important interface on MFs and offers a new strategy to accelerate the commercial adoption of such cathodes. By successfully adopting the low temperature chemical vapor deposition carbon coating, the important cathode/electrolyte interface of iron difluoride (FeF2) in lithium-ion batteries is significantly stabilized. The cycle life of FeF2 in both concentrated and diluted ether-based electrolyte is remarkably improved, which can be attributed to the suppression of the serious electrolyte reduction during cycling.image

Automated summary

This study successfully applies the low temperature chemical vapor deposition carbon coating technology to stabilize the important cathode/electrolyte interface of iron difluoride (FeF2) in lithium-ion batteries. The cycle life of FeF2 in both concentrated and diluted ether-based electrolyte is remarkably improved.