Multi-active-site MOF modified PEO composite electrolyte for high-performance all-solid-state lithium batteries

The two-step method generates abundant unsaturated metal active sites on ZIF-8. • Multi-active-site MOF promotes block-region formation, increasing contact area. • CSE exhibits conductivity of 9.62 × 10 −5  S cm −1 (30 °C) and 5.52 × 10 −4  S cm −1 (60 °C). • The mechanisms involved in enhancing ionic conductivity of ACoZ-PEO is explained. In this study, annealed Co-ZIF-8 with the two-step method of Co doping and annealing was prepared, and then it was incorporated into PEO to fabricate composite solid electrolytes (CSEs). It is found that in Co-doped ZIF-8 annealed at 300 °C for 2 h the doping reduces the binding energy of the Zn N coordination bond in ZIF-8. This promotes the breaking of Zn N bonds during subsequent annealing, and thus exposes a large number of unsaturated metal active sites. The mechanisms involved in the interaction between annealed Co -doped ZIF-8 and lithium salt-containing polyethylene oxide have been discussed detailly. These active sites interact with the ether oxygen atoms of PEO and the anions of the lithium salt, inhibiting crystallization, enhancing ionic conductivity, and promoting the formation of block-like regions, thereby increasing the effective contact area between the electrolyte and the interfaces of electrodes. The ionic conductivity of this composite electrolyte increases to 5.52 × 10 −4  S cm −1 at 60 °C, and the Li + transference number rises to 0.36. The corresponding lithium symmetric battery can stably operate for over 700 h at 0.1 mA cm −2 . The initial discharge specific capacity of lithium iron phosphate battery at 60 °C and 0.5C is 158 mAh g −1 , and the capacity retention rate after 125 cycles is 92.5%.