1. Ionic conductivity
The ionic conductivity of the electrolyte is an important property that enables the evaluation and measurement of battery performance. The ionic conductivity is proportional to the ionic charge z, the concentration c and the electron mobility μ.
σ=NAeΣ|ZiICiμi
Here, NA and e are Avogadro’s constant and elementary charge, respectively. The ionic conductivity increases as the number of dissociated free ions increases and the mobility of these ions increases. Normal battery reactions are difficult at low ionic conductivity because lithium ions inside one electrode cannot easily migrate to the other. At room temperature, the ionic conductivity of the electrolyte of the lithium secondary battery needs to be higher than 10-3 Scm. If the conductivity is low, the lithium ions cannot sufficiently migrate between the two electrodes, and the electrode active material cannot fully exert the capacity. Ionic conductivity can be measured with a conductivity meter or calculated from the impedance of the electrolyte with known cell constants. Another method is to first obtain the impedance of the electrolyte solvent and then calculate it using the following formula. Ionic conductivity = distance between electrodes / (impedance of solvent × electrode area)
2. Electrochemical stability
The electrochemical stability of the electrolyte is determined by the potential range that does not participate in redox reactions. The potential of the working electrode relative to the reference electrode was scanned using a potentiostat at a specific scan rate. The rapid increase and decrease of the current corresponds to the decomposition voltage. This value can also be determined by the potential at which the redox current reaches a certain value. The working electrode is a platinum electrode, a carbon electrode or a stainless steel electrode, while the reference electrode is composed of lithium metal or Ag/AgCl. This method is called linear sweep voltammetry. Since the decomposition voltage varies with different test conditions, the reference potential and scan rate need to be recorded. Slow scan speeds (eg, below 1 mV/s) can more accurately detect electrochemical stability.
Table 1 shows the oxidative decomposition potential (Eox) and various physical properties of representative organic solvents. These voltages were scanned at 5mV/s and translated into voltages for Li/Li with current densities above 1 mA/cm². The quaternary ammonium salt (C2H5)4NBF4 was used to replace the lithium salt to prevent the voltage drop caused by the desolvation and intercalation of lithium ions in the electrode. As shown in the table, alkyl carbonate or ester-based solvents can prevent oxidation at 1V, higher than ether-based solvents. For the same reason, low-viscosity ether solvents are widely used in 3V lithium primary batteries, while carbonate solvents are commonly used in 4V lithium secondary batteries. Using a similar approach described above, we can compare the electrochemical stability by dissolving different lithium salts in specific solvents. Table 2 shows the oxidative decomposition voltages of different lithium salts in PC. The order of oxidative stability of lithium salts is as follows:
LiAsF6 > LiPF6 > LiBF4 > Li(CF3SO2) 2N > LiClO4, LiCF3SO3
3. Interfacial properties of electrode/electrolyte
The SEI film is formed by the reaction between the liquid electrolyte and the electrode active material, which greatly affects the charge-discharge cycle characteristics of lithium secondary batteries. As the lithium diffuses during the cycle, the SEI film will have a direct impact on the side reactions between the electrolyte and the electrode. For graphite electrodes, ethylene carbonate is more favorable than propylene carbonate because the latter destroys the graphite layer and prevents the formation of SEI films. Lithium salts are also beneficial for the formation of protective films. Adding compounds such as vinylene carbonate (VC) to the organic electrolyte can promote the reduction reaction, thereby improving the properties of the negative SEI film. Although many studies on the surface reactions of carbon electrodes and organic electrolytes have been carried out, the relationship between additives and SEI films has not been established. Although the use of additives to inhibit the activity of electrolytes is under investigation, a more fundamental understanding of this is also required.
4. Operating temperature
Lithium-ion batteries operate in the temperature range of -20 to 60 °C, so the melting and boiling points of solvents must be carefully considered. For example, a solvent with a low melting point such as DEC, DME, PC is mixed with a mixed solvent of EC, DMC or other salts that exist in a solid state at 0°C. Precipitation occurs when the lithium salt is not readily soluble in the solvent, and this temperature becomes the lower limit of the electrolyte. In addition to this, the use of solvents with low boiling points is also limited because packaging materials such as aluminum expand when the vapor pressure increases. At higher temperatures, the thermodynamic and electrochemical stability of the liquid electrolyte decreases, while the ionic conductivity increases.
5. Cation migration number
Ionic conductivity is the sum of the cationic and anionic conductivities. In a lithium secondary battery, lithium ions participate in an electrochemical reaction that generates an electric current at the electrodes. Therefore, the conductivity of the cations in the electrolyte is crucial. The contribution of cations to the overall conductivity can be expressed as the cation transport number (t+).
T+=σ+/(σ++σ–)=μ+/(μ++μ–)
In the above equation, the conductance ratio can be expressed in terms of electron mobility (μ), since lithium salts decompose to give equal numbers of cations and anions. When the cation transfer number is small, the overall impedance of the cell increases due to the concentration polarization of anions in the electrolyte. The cation migration number can be calculated by using a variety of methods, such as the AC impedance method, the DC polarization method, the Tubandt method, the Hittoff method, and the pulsed gradient field NMR method. This number is affected by various factors such as temperature, salt concentration in the electrolyte, ionic radius and charge.
