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, Fe 2 O 3 , Mn 2 O 3 or CuO) as Li-free cathodes for solid-state LMBs is demonstrated for the first time, employing poly(ethylene oxide) (PEO), lithium garnet Li 6.4 La 3 Zr 1.4 Ta 0.6 O 12 or Li 1.5 Al 0.5 Ge 1.5 (PO 3 ) 4 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 Fe 2 O 3 /PEO/Li cell measured at 60 °C delivers a high discharge capacity of 990 mAh g −1 at 10 mA g −1 , which is comparable to the Fe 2 O 3 liquid cell. When cycling at 200 mA g −1 , a stable capacity of 733 mAh g −1 is achieved at the 1,100 th cycle for the Fe 2 O 3 /PEO/Li cell, while the Fe 2 O 3 liquid cell only sustains 860 cycles until failure. In order to further demonstrate the application feasibility, stable cycling of the Fe 2 O 3 /PEO/Li cells with high mass loading of 2.83 mg cm −2 and thus high areal capacity of 1.67 mAh cm −2 or with limited Li usage (150% in excess) is preliminarily achieved. In addition, the Fe 2 O 3 /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 Fe 2 O 3 solid-state cell work at 30 °C successfully. These results convince the great promise of TMOs in future high-energy-density solid-state LMBs.