Li1.5Al0.5Ge1.5(PO4)(3) (LAGP)-PEO composite electrolytes are unstable in LiMn2O4. In addition, the discharge platform potential (2.8 and 4.0 V) difference of LiMn2O4 is relatively large, whereas the discharge platform potential (3.5 V) of LiFePO4 is between 2.8 and 4.0 V. Thus, LiMn2O4 and LiFePO4 can be compounded together to reduce the material platform voltage difference and obtain the advantages of both materials at the same time. Here, LiMn2O4/LiFePO4 composite cathodes were applied in solid-state batteries. LAGP-PEO(LiTFSI) was used as the electrolyte. The Li/composite cathode battery using composite electrolytes has a reversible capacity of 192.8 mAh g(-1) at 50 degrees C and 0.1 C. It possesses favorable rate performance and exhibits very good cycling stability. In addition, the composite electrolytes can prevent the further occurrence of the Jahn-Teller effect. Meanwhile, the charge-transfer resistance slightly decreases in 10 cycles. The excellent capacity retention of the battery is connected with the excellent electrochemical stability and the well-interfaced contacts of the composite electrolytes with electrodes.