Synergistic effects of carbon dots and heterojunctions to enable Li–Fe–F all-solid-state ceramic batteries with high cathode loading and cumulative capacity

Garnet-based all-solid-state lithium batteries encountering high interfacial resistance and lithium dendrite growth face challenges to their practical application. Here, we design a carbon dot (CD) decorated ZnOHF composite interlayer (ZnOHF@CDs), which can not only homogenize the distribution and diffusion of Li ions and electrons driven by the built-in electric field from the LiZn/LiF heterojunction, but also alleviate the diffusion of Li ions to the hot spots (with uneven charge accumulation) mediated by CDs with multiple edge groups. Moreover, the pinned CDs facilitate the additional Li-ion diffusion pathways and mitigate the volume change of LiZn alloy interlayers (even after 7400 h cycling), thereby inhibiting Li dendrite growth. The ZnOHF@CD modified garnet symmetric cell demonstrates an ultra-long cycling stability over 14 000 h with a small overpotential of ∼10 mV. Combined with the hot melt-penetration-bonding technology on the cathode side, a Li-Fe-F all-solid-state ceramic battery (without adding any wetting agent) is achieved with high FeF₃ cathode loading (∼8 mg cm^-2) and high cumulative capacity (487 mAh g^-1 at 140 mA g^-1 and ∼300 mAh g^-1 even after an ultralong process of 800 cycles). The synergistic effect of CDs and heterojunctions provides a solution to the high reversibility challenge of conversion-type all-solid-state lithium batteries.