High performance sulfide all-solid-state batteries enabled by Li1.26Mg0.12Zr1.86(PO4)3 coating of iron fluoride cathodes

Integrating conversion-type cathodes (such as iron-based fluorides (IBFs)) into sulfide all-solid-state batteries (ASSBs) significantly enhances energy density but faces challenges including unstable cathode/electrolyte interfaces. To mitigate these issues, a near-amorphous Li1.26Mg0.12Zr1.86(PO4)3 (LMZP) coating technology is developed to protect nano-sized FeF2 (n-FeF2) and micro-sized FeF3 (m-FeF3) cathodes from reacting with the sulfide electrolyte for the first time. The LMZP coating approach offers simplicity, low energy consumption and scalability. With the LMZP-coated cathodes and lithium–indium (Li–In) anode, the ASSBs achieve a discharge capacity of 310 mAh g−1 after 600 cycles and a maximum areal capacity of 2 mAh cm−2 for n-FeF2, and 301 mAh g−1 after 700 cycles with a maximum areal capacity of 3.1 mAh cm−2 for m-FeF3. When using a lithium (Li) metal anode, the ASSBs maintain a discharge capacity of 310 mAh g−1 after 300 cycles for n-FeF2 and 300 mAh g−1 after 600 cycles for m-FeF3, demonstrating excellent cycling stability and capacity retention. Notably, a discharge capacity of 280 mAh g−1 is sustained after 200 cycles for n-FeF2 and 301 mAh g−1 after 180 cycles for m-FeF3, with a reduced stack pressure (10 MPa). These results are unprecedented in IBF-based ASSBs, highlighting the potential of advanced coating strategies to enable scalable, high-performance metal fluoride cathodes in ASSBs with higher energy densities.