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What are the series of solid-state lithium battery electrolytes?

by:dcfpower     2021-03-24

Solid-state lithium batteries have the concept to the output of products. Until now, the technology has slowly matured, but there is still a long way to go before the real mass commercial use. The solid-state lithium battery is mainly based on the difference in the electrolyte morphology of the liquid lithium battery pack, that is, the electrolyte is solid, so what are the solid-state lithium battery pack electrolytes?

1. Organic polymer system of solid-state lithium battery electrolyte

The electrolyte and separator used in conventional liquid lithium-ion batteries are mainly organic components, so Organic polymers, which are also organic substances, are the natural choice for solid electrolyte matrix. The organic polymer electrolyte system includes polyethylene oxide (PEO) and polymers with a certain similarity to its structure (polypropylene oxide, polyvinylidene chloride, polyvinylidene fluoride), etc.

Polyethylene oxide has become the mainstream choice for organic polymer solid electrolytes due to its good compatibility with lithium anodes. In view of the fact that polyethylene oxide does not inherently contain lithium, it is necessary to dope the aforementioned lithium salt first; its lithium-conducting mechanism is the induction of lithium ions by ether oxygen bonds/other atoms with higher electronegativity, and subsequent lithium-rich chains in the amorphous region The segment movement realizes the transfer of lithium ions to the neighbors, and the final effect is that lithium ions enter from one side of the polymer layer and exit from the other side, realizing the charge and discharge transportation of lithium ions. The higher the crystallinity of polyethylene oxide doped with lithium salt, the higher its strength but the lower the lithium ion conductivity. Therefore, the method of reducing the moderate crystallinity such as doping of inorganic particles, polymer grafting, copolymerization, and cross-linking modification is also researched. They are widely adopted. So far, the lithium ion conductivity of the polyethylene oxide solid electrolyte under slightly higher temperature conditions has been practically acceptable, and its density is low, the interface impedance is low, and it is easy to thin the layer and perform mechanical processing.

However, the polyoxyethylene solid electrolyte doped with lithium salt has poor high-voltage resistance, and the ternary material of normal voltage can make it oxidized, which limits the choice of cathode material to a large extent. Limits the energy density of the final battery. In addition, the strength of polyethylene oxide is relatively low, and its resistance to puncture and short circuit is weaker than other solid electrolyte systems.

2. The oxide system of solid-state lithium battery electrolyte

The solid electrolyte of the oxide system mainly contains lithium steel titanium oxide (LLTO) with a perovskite structure and a garnet structure. Lithium steel zirconium oxide (LLZO), fast ion conductors (LISICON, NASICON), etc., the lithium-conducting mechanism is mostly that the material forms a structurally stable lithium ion transport channel at the microscopic level. The biggest advantage of solid oxide electrolyte comes from the intrinsic properties of inorganic oxides: high mechanical strength, high physical and chemical stability, strong pressure resistance, and low manufacturing complexity. At the same time, after some elements are doped, the lithium ion conductivity of the oxide solid electrolyte under slightly higher temperature conditions (such as 800C) can also be accepted in practice.

The shortcomings of oxide solid electrolytes are also due to the intrinsic properties of inorganic oxides: for the electrode-electrolyte interface, the interface contact ability is poor, and the interface stability during cycling is also poor, resulting in poor interface stability during cycling. The interface impedance increases faster, the effective capacity of the positive and negative electrodes is insufficient, and the battery life decays faster; thinning is also difficult. Therefore, oxide solid electrolytes often need to add some polymer components and mix with trace ionic liquid/high-performance lithium salt-electrolyte, or use auxiliary in-situ polymerization to manufacture quasi-solid batteries to retain some of the safety advantages and improve the electrolyte- The interface of the electrode contacts.

3. The sulfide system of solid-state lithium battery electrolyte

The solid electrolyte of the sulfide system can be considered to be composed of lithium sulfide and aluminum, phosphorus, silicon, titanium, aluminum, tin and other elements. The multi-element composite material composed of sulfide, which covers the crystalline state and the amorphous state when the material is the same. The large ionic radius of sulfur makes the lithium ion transmission channel larger; the electronegativity is also suitable, so the sulfide solid electrolyte has the best lithium ion conductivity among all solid electrolytes. Among them, the Li-Ge-

PS system The conductivity of lithium ions at room temperature can be directly compared with that of electrolyte. In addition, the sulfide solid electrolyte has greater mechanical strength, and its compatibility with high-capacity sulfur cathodes is the best.

The main disadvantages of sulfide solid electrolytes include: the electronegativity of sulfur is not as good as that of oxygen, which makes the electrolyte layer partly depleted in lithium when matched with a high-voltage positive electrode, which increases the interface resistance; and the SEI generated when matched with a metal lithium negative electrode The membrane impedance is also large; the sulfide is an inorganic non-metallic particle, and there is a relatively serious problem of electrolyte-electrode interface degradation during the cycle. In addition, the material system is very sensitive to water, oxygen, etc., and is also flammable once an accident occurs; thinning is also difficult. These make its manufacturing process requirements very high.

To sum up, different solid electrolyte material systems have different performance advantages and disadvantages, and solid electrolytes with excellent comprehensive performance have not yet appeared; the precise control of composite and composition and structure across basic types of materials may be a breakthrough key.

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