Transition metal oxides as lithium-free cathodes for solid-state lithium metal batteries

Solid-state lithium metal batteries (LMBs) receive increasing attentions worldwide due to their safety and high energy density. Herein, the practical application of conversion reaction-type transition metal oxides (TMOs, FeO, MnO or CuO) as Li-free cathodes for solid-state LMBs is demonstrated for the first time, employing poly(ethylene oxide) (PEO), lithium garnet LiLaZrTaO or LiAlGe(PO) solid-state electrolytes (SSEs). The cathodes embody good interfacial compatibility with the SSEs, ensuring the excellent electrochemical performance of the TMOs/SSEs/Li solid-state cells. The as-prepared FeO/PEO/Li cell measured at 60 °C delivers a high discharge capacity of 990 mAh g at 10 mA g, which is comparable to the FeO liquid cell. When cycling at 200 mA g, a stable capacity of 733 mAh g is achieved at the 1,100 cycle for the FeO/PEO/Li cell, while the FeO liquid cell only sustains 860 cycles until failure. In order to further demonstrate the application feasibility, stable cycling of the FeO/PEO/Li cells with high mass loading of 2.83 mg cm and thus high areal capacity of 1.67 mAh cm or with limited Li usage (150% in excess) is preliminarily achieved. In addition, the FeO/PEO/Li pouch cells are assembled, which exhibit satisfied electrochemical performance. Moreover, poly(vinylidene difluoride) (PVDF)-based solid polymer electrolye is employed to make the FeO solid-state cell work at 30 °C successfully. These results convince the great promise of TMOs in future high-energy-density solid-state LMBs.