Ionic liquids are salts that exist in a liquid state. In particular, salts that are in a liquid state at room temperature are called room temperature ionic liquids (RTILs). With the discovery of pyridinium or imidazolium compounds and aluminum chloride, ionic liquids began to be extensively studied in the 1950s. Compared with liquid electrolytes, ionic liquids have the following advantages:
1) Wide liquid range and low vapor pressure;
2) Not easy to burn, and heat resistant;
3) Good chemical stability;
4) Has quite high polarity and ionic conductivity.
However, the electrochemical performance of ionic liquids for battery applications is not ideal. This is due to the high viscosity due to ionic bonds, and the diffusion of lithium is hindered by other cations present.
1. The structure of ionic liquids
Ionic liquids include organic cations and inorganic anions. It can be seen from Figure 1 that the ionic liquids centered on N or P present various structures, such as alkyl imidazolium salts, alkyl pyridinium salts, alkyl ammonia, and alkyl phosphine salts. Even with the same cation, depending on the type of anion, ionic liquids are not necessarily in a liquid state at room temperature. For example, ionic liquids containing 1-ethyl-3-methylimidazolium salt (EMI) as cation, different anions have different melting points. If the anion is Br–, the ionic liquid is a white crystalline powder at room temperature (melting point is 78°C); if it is BF4 – and TFSI– anions, it becomes a colorless and transparent liquid with melting points of 15 and -16°C, respectively, containing Fluoride anions such as BF4–, PF6–, CF3SO3– and (CF3SO2)2– are commonly used in ionic liquids. As shown in Figure 1, ionic liquids with the following structural characteristics have lower melting points:
1) Large size of cation and anion;
2) have charge delocalized ions;
3) The cation and anion have considerable conformational freedom, and the melting entropy is large;
4) Asymmetric cationic structure.
2. Characteristics of ionic liquids
As mentioned above, ionic liquids have unique properties such as high ionic conductivity, non-volatile, non-flammable, and very stable thermodynamics. And it has high polarity, can dissolve inorganic and organic metal compounds, and they can be liquid in a wide temperature range. Table 1 shows the physical and chemical properties of imidazolium salt ionic liquids. We can see that the properties of ionic liquids vary with the structure of cations and anions.
3. Viscosity and ionic conductivity
Ionic liquids are special liquids composed only of ions. Because of the high ion concentration, ionic liquids have high ionic conductivity. The viscosity of ionic liquids varies with the combination of cations and anions, often more than ten times that of organic solvents. Inter-ionic interactions increase with the addition of lithium salts, which leads to an increase in viscosity but a decrease in ionic conductivity at the same time.
As an example, LiTFSI was added to an ionic liquid composed of TMPA and TFSI. The variation of viscosity and ionic conductivity with the change of lithium salt concentration is shown in Figure 2. As can be seen from the figure, the addition of 1M lithium salt results in a three-fold increase in viscosity and a reduction in ionic conductivity by a factor of four.
4. Density and Melting Point
Similar to organic liquid electrolytes, the density of ionic liquid-containing electrolytes increases with the addition of lithium salts. Since most lithium salts have melting points above 200 °C, this value increases with the concentration of lithium salts. For example, when 1.2 M LiTFSI was added to TMPA-TFSI, the electrolyte solidified at room temperature. Therefore, the melting point of the ionic liquid of the rechargeable battery should be below room temperature. The melting points of ionic liquids exhibit different values according to different combinations of cations and anions. As shown in Table 1, even in liquids containing the same cation, different types of anions cause different melting points.
5. Electrochemical stability
The electrochemical stability of ionic liquids can be determined by cyclic voltammetry using a three-electrode system. Figure 3 shows the CV detection results of TFSI as an anion combined with different types of cations for ionic liquids. In Figure 3, the anodic current rises at 2.5 V, while the cathodic current fluctuates in the range of -1.5 ~ 3.0 V, depending on different cations. The anti-reduction and anti-oxidative properties of ionic liquids are determined by the species of cations and species of anions, respectively.
Many known ionic liquids meet the oxidative stability conditions required for lithium rechargeable batteries. But they do not meet the requirement of resistance to reducibility. Considering the reduction potential of EMI is +1.1 V (vs Li/Li+), it is necessary to add a compound with a higher reduction potential than Li to form the SEI film, or the battery uses an anode with a higher potential than the Li electrode. In ionic liquids, Aliphatic quaternary ammonium cations have higher stability on electrochemical reduction than aromatic cations such as EMI.
6. Lithium rechargeable battery electrolyte
Having a high electromotive force (EMF) is a great advantage of lithium rechargeable batteries. This is due to the combined effect of the high potential of the transition metal oxide used in the positive electrode and the low potential of the negative electrode or metal negative electrode. Conventional organic liquid electrolytes cannot be simply applied to lithium rechargeable batteries because they cannot meet safety requirements such as flame retardancy and non-volatility. On the other hand, ionic liquids are not flammable, have low volatility and exhibit relatively high ionic conductivity. One of the most common ionic liquids uses EMI as the cation. EMI cations can be used to combine with a variety of anions to form a variety of ionic liquids with lower melting points and viscosities. However, one drawback of EMI is lower positive stability. Because of this, quaternary ammonium salts that can form SEI films without additives are considered as electrolyte materials for lithium rechargeable batteries. Aliphatic quaternary ammonia containing fluorine anion has lower viscosity and higher oxidation resistance. These ammoniums may contain methyl side chains, or exist with BF4–, or CLO4–, TFSI, or TSAC systems.
Figure 4 shows the results of charge and discharge of a lithium rechargeable battery using an ionic liquid containing a lithium salt. Among the three ionic liquids, (PP13)-TFSI exhibits the best charge-discharge cycling performance. TEA-TSAC and EMI-TSAC ionic liquids have higher reduction potentials than PP13-TFSI, and the capacity decays rapidly with cycling. From this, we can determine the relationship between ionic liquid reduction stability and cycling performance. In order to obtain good charge-discharge performance, it is better to use ionic liquids with high cathode stability. The viscosity of ionic liquids such as TFSI and TSAC ranges from 10 to 150 cP, which is 10 times that of PC organic solvents. At room temperature, the addition of lithium salts to ionic liquids increases the viscosity further. However, when the temperature is higher than 80 °C, the viscosity of the ionic liquid is similar to or slightly higher than that of the organic liquid electrolyte. If the non-volatility of ionic liquids is considered, they can also be used as electrolytes for lithium rechargeable batteries operating at high temperatures.
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