The polymer solid electrolyte (SPE), which is composed of a polymer matrix (such as polyester, polymerase and polyamine, etc.) and a lithium salt (such as LiClO4, LiAsF4, LiPF6, LiBF4, etc.), has received extensive attention due to its light weight, good viscoelasticity, and excellent mechanical processing performance. Organic polymer-based lithium ion conductors were discovered in the 1970s. In this type of material, lithium ions are “dissolved” in the polymer matrix (“solid solvent”) in the form of a lithium salt, and the transmission rate is mainly affected by the interaction with the matrix and the ability of the chain segment. So far, common SPEs include polyethylene oxide (PEO), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polymethyl methacrylate (PMMA), polypropylene oxide (PPO), polyvinylidene chloride (PVDC) and single-ion polymer electrolytes and other systems.
At present, the commonly used polymer matrix is polyethylene oxide (PEO), and its room temperature conductivity is generally 10-5s/cm after adding various lithium salts. Increasing the mobility of the chain segment, that is, reducing the glass transition temperature of the matrix, is beneficial to increase the lithium ion conductivity. However, because the ion transport in the solid polymer electrolyte mainly occurs in the amorphous region, and the high crystallinity of unmodified PEO at room temperature results in low ion conductivity, which seriously affects the high-current charge and discharge capabilities. Blending, copolymerization, cross-linking and addition of inorganic particles are all currently used modification methods. Cui et al. obtained an organic composite all-solid polymer electrolyte with greatly improved electrochemical and mechanical properties by blending means. The specific method is to blend PEO, polycyanoacrylate (PCA) and lithium dioxalate borate (LiBOB) with a solvent in a mass ratio of 10:2:1, and then coat them on a self-made cellulose film. YOUNG et al. designed and synthesized a triblock copolymer (PEOP) with a PEO segment in the center and syndiotactic polypropylene (sPP) as the outer layer, mixed with lithium bistrifluoromethanesulfonimide (LiTFSI) to obtain an ASPE with a special morphology, as shown in Figure 1.
Researchers improved the mobility of the PEO segment by reducing the crystallinity, thereby increasing the electrical conductivity of the system. The simplest and most effective method is to hybridize the polymer matrix with inorganic particles.
At present, the most researched inorganic fillers include metal oxide nanoparticles such as MgO, Al2O3, and SiO2, as well as zeolite, montmorillonite and so on. The addition of these inorganic particles disturbs the order of the polymer segments in the matrix and reduces its crystallinity. The interaction between the polymer, lithium salt and inorganic particles increases the lithium ion transport channel and improves the conductivity and the number of ion migration. Inorganic fillers can also play a role in adsorbing trace impurities (such as moisture) in the composite electrolyte and improving mechanical properties. In order to further improve the performance, researchers have developed some new types of fillers. Among them, the transition metal ions of the unsaturated coordination sites and the organic linking chain (generally rigid) are self-assembled, and the metal organic framework (MOF) formed by it has attracted attention due to its porosity and high stability. Based on the good flexibility and processability of polymers, polymer electrolytes are particularly suitable for all-solid-state battery systems that power wearable devices. However, since the lithium salt is sensitive to humidity, the synthesis process needs to be carried out under dry conditions, which increases the production cost. In addition, the limited thermal stability of polymers still has strict requirements on the range of battery operating temperature. When using lithium metal as a battery negative electrode, the limited mechanical strength of some polymer electrolytes is often difficult to prevent the growth of lithium dendrites. These problems have restricted the wide application of polymer electrolytes.