Polyvinylcarbonate based solid polymer electrolyte

Polyvinylcarbonate based solid polymer electrolyte

Ethylene carbonate (EC) is an organic solvent with excellent performance, and is often used as an excellent solvent for electrolytes in the battery industry. Polyethylene carbonate obtained by polymerization of ethylene carbonate and lithium salt can be used as a solid polymer electrolyte.

Polyethylene carbonate is a macromolecular compound PEOEC obtained by anionic ring-opening polymerization of ethylene carbonate (EC), and modified to obtain a macromonomer M-PEOEC. PEOEC is mainly obtained by the ring-opening polymerization of EC, then PEOEC, lithium salt (LiCIO4), photoinitiator (HMPP), polysilsesquioxane acrylate (OA-POSS) and ETPTA are mixed, and HIPE is obtained by photocuring and crosslinking (Figure 1). It was found that the addition of OA-POSS could not only improve the ionic conductivity but also increase the dimensional thermal stability of the polymer electrolyte. When adding 10% OA-POSS and low molecular weight PEOEC, HIPE can achieve higher ionic conductivity (3.74×10-5S/m at 30℃). To characterize the dimensional thermal stability of the polymer electrolyte, the rheological properties of the polymer electrolyte were measured at 30~80 °C, and it was found that G′ (dynamic storage modulus) increased with the increase of the amount of OA-PC. This shows that if HIPE has higher dimensional thermal stability, it must contain more OA-POSS, and it is found that low molecular weight PEOEC has less influence on the dimensional stability of HIPE. Therefore, in order to keep the solid polymer electrolyte with high dimensional thermal stability, the timing should contain a high content of OA-POSS and low molecular weight PEOEC.

Polyvinylcarbonate based solid polymer electrolyte
Figure 1 – Photoinitiated formation of composite solid polymer electrolytes based on PEOEC and POSS

At the same time, Kimura et al. mixed polyethylene carbonate (PEC) obtained by alternating copolymerization of CO2 and epoxy compounds with lithium bistrifluoromethanesulfonimide (LiTFS), and then a certain amount of ionic liquid N-methyl-N-propylpyrrole bis(trifluoromethylsulfonyl)imide (Pyr14TSI) was added to obtain a polymer electrolyte with higher ion mobility. A certain amount of Pyr14TFSI was chosen as plasticizer because Pyr14TFSI has ideal ionic conductivity and wide electrochemical stability window. When the temperature is 80℃, the ionic conductivity of the polymer electrolyte is 10-5S/cm, and the ion migration number can reach 0.66. The high ion migration number can reduce the concentration polarization during the charging and discharging of the battery, thus providing a larger power density. Thermogravimetric analysis (TGA) test results found that the polymer electrolyte had almost no mass loss at 150℃, indicating that the polymer electrolyte has excellent thermal stability.

Under certain conditions, high concentrations of lithium salts are beneficial to the improvement of the ionic conductivity of solid-state electrolytes. Kimura’s group reported a polyimide porous membrane-supported polyvinyl carbonate/high-concentration LiTFSI lithium salt (mass ratio of 1:4) solid-state polymer electrolyte and used it in polymer solid-state lithium batteries. The solid polymer electrolyte has an ionic conductivity of 10-5 S/cm at room temperature. The LiFePO4 half-cell assembled with this solid electrolyte has a specific discharge capacity of 120~130 mA·h/g at a rate of C/20 (Figure 2).

Polyvinylcarbonate based solid polymer electrolyte
Figure 2 – Rate charge-discharge curves of lithium iron phosphate/lithium metal batteries assembled with polyvinylcarbonate/high-concentration LiTFSI lithium salt

Profound analysis of the relationship between the lithium ion transport mechanism and the aggregated structure of solid polymer electrolytes will help to rationally design and synthesize ideal polymer electrolyte matrix materials. Figure 3 is a schematic diagram of the mechanism of PEC transporting Li+. In the PEC solid-state polymer electrolyte system, lithium ions and intercalation oxygen are coordinated, and it is the presence of carbonyl oxygen that makes the chain segment movement of PEC more active, resulting in higher ionic conductivity. This is also one of the reasons for the higher Li-ion conductivity at room temperature compared to polyether-based solid polymer electrolytes.

Polyvinylcarbonate based solid polymer electrolyte
Figure 3-PEC transport mechanism diagram of Li+

From the molecular dynamics point of view of new polymer electrolytes, broad-spectrum dielectric constant spectroscopy is one of the effective tools to study ion transport properties. Motomatsu et al. investigated the relationship between dielectric relaxation and ionic conduction in PEC solid-state polymer electrolytes using broad-spectrum dielectric constant spectroscopy. There are two relaxation behaviors in PEC solid polymer electrolytes, namely α and β, which correspond to the motion of the segment and the local motion of the PEC chain, respectively. The ionic conductivity of the system increases exponentially with the concentration of lithium salt, which is due to two aspects: one is that the increase of lithium salt concentration reduces the intermolecular interaction; the other is that the intramolecular interaction reduces the dipole moment ( Figure 4).

Polyvinylcarbonate based solid polymer electrolyte
Figure 4 – Schematic diagram of dielectric constant and local structure of polyvinyl carbonate-lithium bistrifluoromethanesulfonimide (LiTFSI)

mol % means mole fraction

Although the ionic conductivity of PEC is higher than that of PEO, it still cannot meet the needs of lithium batteries. Therefore, PECs are often modified or inorganic fillers are added to improve the comprehensive properties of polymer electrolytes. For example, PEOBC is cross-linked with polysilsesquioxane acrylate (OA-POSS) and ethoxylated trimethylolpropane triacrylate (ETPTA) to prepare a polymer electrolyte with a semi-interpenetrating network structure. The ionic conductivity of the electrolyte is 3.74×10-5 S/cm at 30°C and 3.26×10-4 S/cm at 60°C, while the electrochemical stability window is 5.0V at 25°C and 4.7V at 60°C. And the mechanical properties and thermal properties are also significantly improved. At 60℃, the ionic conductivity of the electrolyte increased from 2.2×10-4S/cm to 4.3×10-4S/cm after adding inorganic material TiO2 to the PEC0.53LiFSI electrolyte. The ionic conductivity of PEC-based solid polymer electrolytes increases with the increase of lithium salt concentration, however, its mechanical properties also decrease, which seriously affects the safety performance of the lithium batteries it constitutes. How to further improve the ionic conductivity of PEC-based solid polymer electrolytes without affecting their mechanical properties will become the key to the study of PEC-based solid polymer electrolytes.