Polytrimethylene carbonate-based solid polymer electrolyte

Polytrimethylene carbonate-based solid polymer electrolyte

Because of its good biocompatibility and biodegradability, polytrimethylene carbonate (PTMC) is widely used in the fields of drug controlled release materials, in vivo implant materials and in vivo support materials. PTMC is an amorphous polymer in a rubbery state at room temperature. The temperature at which it begins to lose weight can reach above 280 °C, and its dimensional thermal stability is good. The research on PTMC is mainly in biomedicine, and the research on solid polymer electrolyte is less. Figure 1 shows the ionic conductivity and electrochemical stability window data of different polytrimethylene carbonate solid polymer electrolytes. It can be seen from the table that the electrochemical stability window of the PTMC-based polymer electrolyte is above 4.5V, which can be considered to be applied in the field of high-voltage lithium batteries. However, the room temperature ionic conductivity is relatively low.

Polytrimethylene carbonate-based solid polymer electrolyte
4Figure 1 – Ionic conductivity and electrochemical stability window of different polytrimethylene carbonate solid polymer electrolytes

Sun et al. prepared polytrimethylene carbonate (PTMC) by ring-opening polymerization using trimethylene carbonate (TMC) as the monomer and stannous octoate as the catalyst. The prepared PTMC is an amorphous polymer, the glass transition temperature is much lower than the asphyxiation temperature, and it has strong mechanical strength. The study also found that when [Li+]:[carbonate] is 1:13 and 1:8, the conductivity can reach 10-7S/cm at 60℃, and the ionic conductivity can reach 10-9S/cm at 25℃. In order to study the effect of different lithium salts on the properties of polytrimethylene carbonate polymer electrolytes, Silva et al. mixed polytrimethylene carbonate with lithium salt to obtain a transparent solid polymer electrolyte, and found that adding lithium hexafluoroarsenate (LiSbF6) had better plasticizing effect than lithium perchlorate (LiCIO4), and with the increase of LisbF concentration, the glass transition temperature and ionic conductivity will gradually decrease, and the electrochemical stability window is greater than 5V. However, the low ionic conductivity of this solid polymer electrolyte at room temperature (Figure 2) limits its use and promotion in high-rate battery devices.

Polytrimethylene carbonate-based solid polymer electrolyte
Figure 2 – Ionic conductivity of PTMC-LisbF6 solid electrolyte systems with different ratios of TMC and Li at different temperatures

The amount of lithium salt added will affect many properties of the solid polymer electrolyte. Barbosa et al. dissolved TMC and lithium hexafluorophosphate (LiPF6) in a solvent, and then dried the film to obtain a solid polymer electrolyte with high mechanical strength, wide electrochemical stability window (4.5V), transparent and completely amorphous. The glass transition temperature was studied and found that when n>15{P(TMC)nLiPF6}, the glass transition temperature did not change significantly, and only when n<15, the glass transition temperature increases significantly with the increase of lithium salt content, and when n=5, the glass transition temperature (Tg) can reach a minimum value. The low glass transition temperature implies that the polymer electrolyte reaches amorphous state at lower temperature, and suggests that the interaction between lithium ions and polymer chains mainly occurs at higher concentrations of lithium salts. From the thermogravimetric curve, it can be concluded that as the lithium salt increases (n decreases), the thermal stability of the polymer electrolyte will gradually decrease, indicating that the lithium salt plays an unstable role. Barbosa et al. studied its ionic conductivity and found that when the temperature reaches 98 ℃, the ionic conductivity can reach the maximum value, that is, 4.79×10-6S/cm, and it can reach 1.78×10-8S/cm at room temperature. Compared with the PEO-nLiPF-based electrolyte system reported in the literature, the ionic conductivity is significantly higher than that of the PEO-based electrolyte at room temperature.

PTMC can be copolymerized with other polyesters to improve ionic conductivity. A new polymer electrolyte was obtained by ring-opening polymerization of trimethylene carbonate (TMC) and caprolactone, and a certain amount of polycaprolactone (PCL) was added in this study. Because the glass transition temperature (-60℃) of PCL is lower than that of PTMC, the flexibility of the PTMC chain can be increased, thereby achieving the effect of increasing the ionic conductivity, and the resulting material is completely amorphous, and its mechanical properties are better than those of pure PTMC. When 60% trimethylene carbonate (TMC) monomer and 40% caprolactone (CL) monomer (28% lithium salt) were added, the ionic conductivity was the highest, which was 7.9×10-7S/cm at 25℃ and 1.65×10-5S/cm at 60℃.

The copolymerized electrolyte is amorphous, and the mechanical properties, ionic conductivity, and charge-discharge performance are significantly improved compared with pure PTMC electrolytes.

The coordination-decoordination process and ion transport mechanism of polyether (such as PEO) solid polymer electrolytes have been extensively studied. However, there are few studies on the ion transport of aliphatic polycarbonate-based solid polymer electrolytes. Therefore, Sun et al. deeply explored the ion transport properties of PTMC-PCL solid polymer electrolytes through a combination of theoretical calculations and experiments.

Figure 3 shows the measured FTIR spectra of different ratios of PTMC and PCL mixed with LiTFSI. It can be seen that lithium ions will preferentially coordinate with the carbonyl oxygen on PCL, which is also demonstrated by the author’s analysis from the atomic-scale perspective of molecular dynamics simulations.

Polytrimethylene carbonate-based solid polymer electrolyte
Figure 3 – FTIR Spectrum

In summary, the mechanical strength and dimensional thermal stability of polytrimethylene carbonate (PTMC) solid polymer electrolytes are relatively high, but the low room temperature ionic conductivity seriously inhibits its practical application. In the following work, in order to further improve the ionicity of PTMC solid polymer electrolyte at room temperature, on the one hand, inorganic nanoparticles such as aluminum oxide (Al2O3), silicon oxide (SiO2), titanium dioxide (TiO2) and inorganic fast ion conductors, such as lithium copper lead oxygen (LLZO), can change the transport path of lithium ions; on the other hand, functionalized PTMC solid polymer electrolytes can be constructed by copolymerization and other methods.