1. Ionic conductivity
Compared with liquid electrolytes, low ionic conductivity is the main reason why the development of lithium secondary batteries with polymer electrolytes is difficult. For example, in practice, the ionic conductivity of 1M LiPF6-EC/DMC electrolyte at room temperature is about 10-2 S/cm, whereas thin-film polymer electrolyte applications require a conductivity greater than 10-3 S/cm. Figure 1 shows the variation of ionic conductivity with temperature for liquid electrolytes, gel polymer electrolytes and solid electrolytes. Solid polymer electrolytes have low ionic conductivity at room temperature, and we can also see that the ionic conductivity decreases rapidly with decreasing temperature. Because of this, solid polymer electrolytes are considered for lithium secondary batteries operating at high temperatures. Meanwhile, gel polymer electrolytes and liquid electrolytes have high ionic conductivity (above 10-3 S/cm) at room temperature and a wide temperature range, so they can be widely used.
2. Electrochemical stability
The electrochemical stability of the polymer electrolyte affects the operating voltage range. Within the given operating voltage range of the positive and negative electrodes, the polymer electrolyte cannot participate in the decomposition reaction caused by oxidation or reduction. The polymer electrolyte should be electrochemically stable up to 4.5 V, since lithium secondary batteries use 4 V metal oxide cathodes, such as LiCoO2, LiNiO2, and LiMn2O4, up to 4.3 V at full charge. Since the development of high-voltage lithium-ion secondary batteries is required to increase energy density, the polymer electrolyte is required to have oxidation stability and not cause side reactions with electrode active materials.
3. Cation migration number
Lithium cations are charge carriers in lithium-ion secondary batteries. Therefore, cations need to have higher mobility than anions, contributing significantly to ionic conductivity. The cation migration number of the polymer electrolyte is generally in the range of 0.2 to 0.5. For hydrogen ion conductors such as perfluorosulfonic acid used as a polyelectrolyte in fuel cells, the migration number is close to 1.0. Polyelectrolytes are powerful alternatives to electrolytes under investigation for use in lithium secondary batteries.
4. Electrode-electrolyte interface reaction
The main problems caused by the use of polymer electrolytes are low ionic conductivity and poor interfacial contact between electrodes. Unlike liquid electrolytes, polymer electrolytes can be divided into regions that have sufficient contact with electrodes and regions that do not. The former actively participates in charge transfer reactions, whereas the latter cannot utilize electrodes due to uneven current distribution. For example, when charging and discharging under high current, the active material in the electrode is distributed differently according to local conditions. Non-uniform expansion and contraction lead to rapid heating of graphite electrodes or transition metal oxide electrodes. In order to be able to form a uniform surface, an electrode in a secondary battery should be composed of a negative electrode material or a positive electrode material mixed with a polymer electrolyte. For example, monomers are injected into the pores of the negative or positive electrode, and then the polymerization reaction occurs, enabling intimate contact between the polymer electrolyte and the electrode. This electrochemical reaction between the polymer electrolyte and the electrode can generate products similar to those in liquid electrolytes. Compared with liquid electrolytes, polymer electrolytes have high molecular weight, lower mobility and lower surface activity. In particular, the electrode-electrolyte interface impedance between the lithium metal and the polymer electrolyte gradually increases, indicating that the interfacial reaction continues due to instability.
5. Mechanical properties
The biggest advantage of polymer electrolytes is that the membrane provides a large surface area. The thinner the film, the higher the energy density because more active species can be embedded in the battery. The film thickness is a very important factor affecting the battery performance, which is determined by the mechanical strength of the polymer electrolyte. The mechanical strength of electrolyte membranes is related to defect rate and production capacity. Increasing the glass transition temperature of the polymer or increasing the content of organic solvent can increase the ionic conductivity but decrease the mechanical strength. Considering this relationship, the ionic conductivity and mechanical strength need to be optimized within a suitable range. In order to improve the interfacial properties between electrodes, polymer electrolytes need to have sufficient adhesion and fluidity.
