Enabling Conversion‐Type Iron Fluoride Cathode by Halide‐Based Solid Electrolyte

Abstract

The practical application of low‐cost and energy‐dense iron fluoride cathodes is hindered by the first cycle electrochemical irreversibility, cycling instability, and large voltage hysteresis. Here, it is reported that these challenges may be overcome by the utilization of halide‐based solid electrolytes (SEs). The excellent electrochemical stability of halide‐based SEs enables a complete conversion and deconversion of FeF2 that cannot be achieved with sulfide‐based SEs. Due to restricted and reversible decomposition of SE, prevention of Fe dissolution, mechanical confinement of active material, as well as improved electrode kinetics, solid‐state FeF2 cathode with halide‐based SE demonstrates superior electrochemical performance compared with FeF2 electrodes in liquid electrolytes, with a high 1st cycle coulombic efficiency (≈100%), high specific capacity (≈600 mAh g−1), long cycle life (>100 cycles), and high‐rate performance (up to 2 C). The results suggest solidifying the batteries is a viable approach to addressing the long‐standing key challenges of iron fluoride cathodes.

Halide‐based solid electrolytes: The history, progress, and challenges

“…It was also reported that halide‐based SEs enable matching with conversion cathodes, such as FeF 2 . [ 99 ] Recently, Han et al reported that the FeF 2 cathode demonstrates superior electrochemical performance in the Li 3 YCl 6 ‐based system, compared with the conventional liquid electrolyte system. This effect is ascribed to the restricted and reversible decomposition of Li 3 YCl 6 SEs, prevention of transition‐metal dissolution, mechanical confinement of active materials, and improved kinetics.…”

“…Besides, they highlighted the significant effect of SE composition on the redox behavior of the FeF 2 cathode, and Li 3 YCl 6 is demonstrated to enable a complete conversion and reconversion of FeF 2 that cannot be achieved with sulfide‐based 75Li 2 S‐25P 2 S 5 SEs. [ 99 ]…”

Toward High Energy Density FeFx (x=2 and 3) Cathodes for Lithium‐Ion Batteries

“…The confinement and compaction of the multi‐phase reaction products by the garnet electrolyte enable a high‐rate performance (330 mAh g −1 even at 2 C). Moreover, the remarkable electrochemical stability of the halide‐based solid‐state electrolyte allows for the complete conversion of FeF 2 cathode [133] . It is shown that the same concept employing halide‐based solid‐state electrolytes can be extended to other conversion cathodes to overcome the challenges inherent in metal fluorides.…”

“…Moreover, the remarkable electrochemical stability of the halide-based solid-state electrolyte allows for the complete conversion of FeF 2 cathode. [133] It is shown that the same concept employing halide-based solid-state electrolytes can be extended to other conversion cathodes to overcome the challenges inherent in metal fluorides.…”

Electronic Conductivity of Lithium Solid Electrolytes

“…[26] While SSBs based on LiPON thin-film electrolyte have been demonstrated to have a calendar life of >10 years, [27] we propose that long calendar life cannot be assumed for bulk-type SSBs based on sulfide, Li-garnet oxide, and halide SEs due to the drastically different electronic conductivity of SEs (the electronic conductivity of LiPON is 10 −14 to 10 −13 S cm −1 ). [27][28][29][30] Whether the electronic conductivity of the bulk-type SEs is sufficiently low to enable a long calendar life remains unknown for the SSB community. If we do a similar rough estimate for a future 4 V SSB with a 5 mAh cm −2 areal capacity and a 30-µm-thick SE using Ohm's law, the electronic conductivity of the SE should be lower than 5.7×10 −12 S cm −1 to enable a long calendar life of 15 years.…”