All-solid-state lithium secondary batteries are strongly desired as novel candidates for EV and HEV power source, since safety issue is much more serious in large-sized lithium batteries for such applications. Replacement of combustible non-aqueous liquid electrolytes with solid electrolytes is considered to be the ultimate solution to this issue. Inorganic solid-electrolytes have several advantages over liquid, polymer or gel electrolytes, including better thermal and chemical stability, preventing safety hazard issues for energy storage systems with high energy densities.

First of all, a lithium superionic conductor exhibiting sufficient ionic conductivity at room temperature (RT) is required in order to develop safe rechargeable lithium batteries. Generally, the electrolytes can only be used in chemical power source devices with lithium ionic conductivity around 10^-3 S/cm at RT. For oxide type lithium ion solid electrolytes, the ionic conductivity, mechanical and chemical stability of lithium ion conductor can be improved through the optimization of material formula, morphology and single crystal size control as well as formation of composite electrolyte. Oxide solid electrolytes with high stability, high density (density is greater than 96%), high ionic conductivity (~ 10^-3 S/cm at RT), uniform single crystal size and shape controllable has been developed. Meanwhile, a series of products, including powder, density ceramic disk and large size block, etc., has realized large-scale production. For sulfide type lithium ion solid electrolytes, the ionic conductivities for binary and ternary sulfide solid electrolytes could be as high as 2.06×10^-3 and 8.27×10^-3S/cm at room temperature. The details for the properties of solid-state electrolytes are listed in Table 1.

Recently, different sulfide electrolytes has been applied in all-solid-state lithium batteries with LiNi_0.8Co_0.15Al_0.05O₂ (NCA) positive materials. Two different solid electrolytes (Li_3.25Ge_0.25P_0.75S₄: 2.03×10^-3 S cm^-1; Li₁₀GeP₂S₁₂: 8.27×10^-3 S cm^-1) are used,and Li-In alloy is used as negative electrode. As shown in Fig. 1, NCA/sulfide electrolytes/Li-In cell exhibits good charge and discharge capacity, although the discharge capacity is lower compared with traditional liquide battery. The lower discharge capacity is attributed to the higher interface resistance. This experiment proves that Ni-based exhibits excellent reversible intercalation-deintercalation ability. Besides, M-NCA is prepared by surface modification on NCA. All-solid-state batteries with M-NCA positive electrode are assemblyed by the same method, and the discharge capacity is dramatically increased to 150 mAh/g, which is due to the strong decrease of interface resistance.

References:

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