Li10GeP2S12-type sulfide solid electrolytes possess an obvious advantage in Li-ion conductivity, while show rather poor interfacial stability and compatibility against electrodes. Herein, the compositional tune via incorporating LiBr into Li10GeP2S12 matrix has been conducted to obtain a novel LiBr-Li10GeP2S12 composite electrolyte. It is interestingly found that high-conducting Li6PS5Br and Li10GeP2S12 co-exist, and Br substitution at the partial S sites of Li10GeP2S12 phase occurs in the composite electrolyte through the combination of Rietveld refinement against X-ray diffraction with Raman spectroscopy. In comparison with Li10GeP2S12 electrolyte itself, no appreciable change in ionic conductivity and activation barrier can be observed with respect to LiBr-Li10GeP2S12 composite electrolyte. While there is obvious improvement in interfacial conduction properties and chemical compatibility between cathode/anode and LiBr-Li10GeP2S12 electrolyte in terms of room-temperature interfacial resistances and temperature-dependence exchange current densities. Moreover, an all-solid-state cell containing the LiBr-Li10GeP2S12 composite electrolyte exhibits the distinct enhancement in discharge capacity and cycling stability during room-temperature galvanostatic cycling. In-situ electrochemical impedance spectroscopy measurements are discussed, suggesting that favorable interfacial modification of the cells can be provided in an internal way of the emergence of argyrodite Li6PS5Br in the new Li10GeP2S12-based electrolyte materials.