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High Li+conductivity, good interfacial compatibility, and nano-scale particle size have always been essential conditions for selecting inorganic fillers in high-performance composite solid electrolytes. In this study, non-milled in situ LLZO fillers with nanosize was synthesized via the sol–gel method by rapid heating sintering, which resulted in more surface defects and fewer impurities in LLZO. Compared with milled LLZO fillers, these non-milled LLZO fillers with more surface defects and fewer impurities can effectively reduce the crystallinity of PEO and agglomeration in PEO, which can form composite electrolytes with high Li+conductivity. Most importantly, the discharge capacity of the 7.5% non-milled LLZO–PEO-based LiFePO4/Li battery is about 135.5 mA h g–1at 1C and 60 °C. After 100 cycles, the discharge specific capacity remains at 99%. It is worth noting that nano-sized non-milled LLZO will improve the discharge capacity of LiFePO4/Li batteries to 122.1 mA h g–1at 0.2C and 30 °C.

Solid polymer electrolytes (SPEs) incorporating inorganic ceramic fillers are key solutions of all solid-state lithium battery technology applications. At present, the conductive mechanism of LLZO-PEO electrolyte is still controversial. Herein, the effect of nano and micron LLZO on the conductive mechanism of PEO matrix with different molecular weight is studied. The low molecular weight PEO electrolyte (LSPEs) containing nano LLZO exhibit higher conductivities than those with the micron ones, which is primarily caused by the percolation effect resulting from the difference in specific surface area. However, for high molecular weight PEO electrolytes (HSPEs), the addition of nano LLZO ceramic fillers can mainly lead to reduce the crystallinity of HSPEs to enhance the motion of polymer segment. Thus, compared with HSPEs alone, the nano LLZO-HSPEs show conductivities one order of magnitude larger. Besides, the discharge capacity of nano LLZO-HSPEs based LiFePO4/Li batteries at 1 C is around 131 mAh g−1at 60 °C, as well as discharge specific capacity holds 99.2% after 200 cycles. Notably, nano LLZO ceramic fillers will enhance cyclic performance of LiFePO4/Li batteries at 0.2 C and 40 °C, which still maintain 126.7 mAh g−1after 50 cycles.