As an important part of the battery, the electrolyte plays a role in transmitting ions between the positive and negative electrodes. Choosing a suitable electrolyte is the key to improving battery power density, energy density, long cycle life, reducing battery internal resistance, and ensuring its safety Where. Sulfide-based solid electrolytes are currently the fastest-growing electrolyte system.

Sulfide solid electrolyte

The sulfide solid electrolyte is derived from the oxide solid electrolyte. The oxygen element in the oxide body is replaced by the sulfur element to form a sulfide solid electrolyte. The electronegativity of sulfur is smaller than that of oxygen, and its binding force to lithium ions is conducive to the migration of ions, so sulfides have higher ionic conductivity than oxides. The sulfide solid electrolyte mainly includes binary compounds such as Li2S-GeS2, Li2S-P2S5, Li2S-SiS2 and Li2S-MeS2-P2S5 (Me=Si, Ge, Sn, Al, etc.) ternary compounds.

Binary sulfide solid electrolyte

The most researched sulfide glassy solid electrolyte is the Li2S-P2S5 system. The glass-ceramic electrolyte formed after partial crystallization of Li2S-P2S5 based glass has higher ionic conductivity, is highly stable to metallic lithium, and has an electrochemical window of about 10V. However, the current Li2S-P2S5 electrolyte material still has some problems. The lithium ion conductivity of the material is still low, the chemical stability is slightly poor, the activation energy is higher, and the preparation cost is high, and the system is easy to react with water to generate hydrogen sulfide gas , It is difficult to realize industrialized production and utilization.

Ternary sulfide solid electrolyte

GeS2, SiS2, P2S5-based binary sulfide electrolytes generally have problems such as low conductivity, poor electrochemical stability or poor chemical stability. Therefore, it is commonly used to add another sulfide network modifier to improve The above situation. This is the ternary sulfide solid electrolyte. In 2011, Li10GeP2S12 with a room temperature ion conductivity of 1.2x10-2S/cm was prepared for the first time, leading to basic research on ion mobility on bulk materials and promoting the development of next-generation batteries. Because Ge is expensive, Roling et al. replaced Ge with Sn to triple the material cost. Experiments show that Li10SnPS12 has a large grain boundary resistance at 27℃, and it is expected to reduce the grain boundary resistance through optimization of synthesis conditions.

Preparation of sulfide solid electrolyte

Melting method

The starting materials are mixed uniformly according to a certain stoichiometric ratio to obtain the initial material. The initial material is processed at high temperature to melt the material, and the molten material is quenched to obtain a glassy sulfide solid electrolyte. The glass can be further obtained by crystallizing the glassy sulfide solid electrolyte. Ceramic sulfide solid electrolyte.

High energy ball milling

The mixed starting materials are processed by high-energy ball milling, and a glassy sulfide solid electrolyte is obtained after ball milling for a certain period of time, and a glass-ceramic sulfide solid electrolyte can be obtained after crystallization.

Liquid phase method

A certain stoichiometric ratio of starting materials is added to the organic solvent, the mixture is stirred at a certain temperature, the reacted solute is separated from it by centrifugation or rotary evaporation, and dried at a certain temperature to obtain a glassy sulfide solid electrolyte The material is further crystallized to obtain a glass-ceramic sulfide solid electrolyte.

Inorganic solid electrolyte materials have higher conductivity than polymer solid electrolytes. Reducing the cost of synthetic electrolytes, simplifying the synthesis steps, introducing more elements, and giving full play to the performance and coordination of each element is the future development direction of sulfide solid electrolytes.

Reference materials:

Ye Ming, Xie Jun, etc. Research progress of sulfide solid electrolytes.
 

Sun Yingzhi, Huang Jiaqi, etc. Research progress of solid-state lithium batteries based on sulfide solid electrolytes.

Information source: Powder Network

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