What is a solid polymer electrolyte?

Solid-state polymer electrolytes have been extensively studied since Wright discovered that ions could migrate in polymers and Armand discovered that they could be used in electrochemical devices including batteries. The advantages of all-solid-state batteries using solid-state polymer electrolytes are as follows:
1) Batteries using lithium metal anodes have high energy density;
2) Very reliable, there is no danger of leakage;
3) Can be manufactured into different shapes and designs;
4) Can make ultra-thin batteries;
5) No flammable gas will be released at high temperature;
6) The low cost of the battery is realized without the use of diaphragm and protection circuit.

Solid-state polymer electrolytes consist only of polymers and lithium salts, and research has focused on the molecular design and synthesis of polymers. In solid polymer electrolytes, the polymer should be amorphous and contain polar elements such as oxygen, nitrogen, and sulfur to facilitate the movement of polymer chains and dissociation of lithium salts at room temperature. In the past studies on derivatives such as polyethylene oxide (PEO), polypropylene oxide (PPO), polyphosphoronitrile, and polysiloxane, PEO-based polymers are the most studied.

The polymer matrix of solid polymer electrolytes is closely related to the ionic conductivity. For solid polymer electrolytes to have high ionic conductivity, the polymer matrix should have the following properties:
1) It contains polar groups with the ability of ionic recombination, and contains polar groups on the adjacent chains to participate in the recombination.
2) Sufficient steric conformation to allow dissociation of the lithium salt
3) Electron donating groups such as ethers, esters or amines should be included in polar groups for cationic solvation.
4) The glass transition temperature should be low so that the polymer chain has strong elasticity.

In the preceding items, 1)~3) are the requirements for dissociation of lithium salts, and 4) are related to the migration of ions.

PEO consists of repeating -CH2CH2O units in which oxygen atoms and basic metals form coordination bonds. This is because oxygen atoms have stronger electron donor properties than other polar elements with recombination ability. Therefore, the lithium salt dissociates when the alkali metal and the oxygen in the ether are coordinated. The migration of lithium ions in solid polymer electrolytes proceeds according to the mechanism shown in Figure 1.

What is a solid polymer electrolyte?
Figure 1. Lithium ion conduction mechanism in solid polymer electrolytes

Due to the low rotational energy barrier between oxygen and methylene groups in PEO, elastic polymer chains can easily conform to form cationic coordination bonds. Similar to crown ethers, ionic dipole interactions between oxygen electron pairs and cations in ethers generate a complex, and the lithium salt dissociates in PEO. In order to obtain high ionic conductivity, dissociated ions need to have better mobility in the polymer. The polymer segments participate in active thermal motion at room temperature, which is higher than the glass transition temperature of elastic polymers. Therefore, lithium ions change their positions by complexing with polymer segments. Therefore, lithium cations move freely inside and participate in the local structural change of the polymer. At the same time, with the redistribution of free space created by the relaxation motion of the polymer, the anions can move freely without restriction. The PEO-matrix composites have high crystallinity due to the strong O-Li+ interaction, so the ionic conductivity below room temperature is low. Most research has focused on synthesizing new polymers to improve ionic conductivity. Various approaches have been tried, including grafting a short EO unit into the side chain. This method lowers the glass transition temperature while maintaining the amorphous structure, resulting in ionic conductivity as high as 10-4 s/cm at room temperature. Another approach is to introduce a cross-linking structure to increase the amorphous region while improving the mechanical structure. The ionic conductivity, mechanical strength, and electrode/electrolyte surface properties are improved by adding inorganic particulates such as alumina, silica, and titania to solid polymer electrolytes. This is because the inorganic filler inhibits the crystallization of the polymer, and excess moisture and impurities are adsorbed to the surface of the inorganic particles. Ferroelectric inorganic materials can also facilitate the decomposition of lithium salts. Despite much research, polymer lithium batteries using solid-state polymer electrolytes have not yet been commercialized. Compared with liquid electrolytes, solid polymer electrolytes have lower ionic conductivity, inferior mechanical properties and interfacial properties at room temperature. Therefore, it is being studied for use in large secondary batteries in electric vehicles and energy storage devices at high temperatures.

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