The main function of electrolyte additives is to improve ionic conductivity, battery life or safety. Most liquid electrolytes used in batteries contain small amounts of additives that have a large impact on battery performance and safety. Additives participate in the reaction between the electrode and the electrolyte. The global market is dominated by very few electrolyte manufacturers and there is very little information available on the technology.
Additives can be divided into SEI film-forming additives, anti-overcharge additives, conductive additives and flame retardant additives according to their functions.
1. SEI film-forming additives
Vinylene carbonate (VC) is a common additive used to form and stabilize the SEI film on the surface of the carbon negative electrode. During the first charging process, VC will produce a stable SEI film, by preventing the peeling of the carbon and avoiding the interaction with electrolysis. The liquid reacts directly to improve battery life. Figure 1 shows the chemical structural formula of VC.
Due to the instability of the carbocyclic ring (sp² hybrid orbital), VC will participate in the ring-opening reaction, resulting in a more stable structure. The presence of vinyl groups causes polymerization to form a stable protective film. With the addition of only a small amount of VC, the irreversible capacity of the carbon anode is reduced due to the presence of the SEI film. This is more effective in PC electrolytes. For example, LiMn20, when the cathode uses 1M LiPF6/PC/VC electrolyte, shows stable reversible capacity up to 4.3 V. Even with the addition of excess additives, the battery performance remains unchanged, which facilitates process control during manufacturing. VC does not affect the positive electrode at high temperature, keeping the SEI film stable. Although VC has good performance, other additives are also being researched and developed because VC is difficult to synthesize and expensive.
2. Anti-overcharge additives
Battery safety is very important, and lithium secondary batteries are often equipped with various safety devices such as positive temperature coefficient, protection circuit module (PCM), and safety outlet valve. But using these devices makes batteries more expensive. Batteries should have an internal safety device to suppress chemical reactions. Anti-overcharge inhibitors have been proposed as additives to solve safety problems caused by overcharge. The redox shuttling reaction limits high voltage by allowing excess charge to be wasted inside the cell, while the cathode film-forming reaction creates a protective film that prevents the flow of current and the diffusion of lithium ions.
3. Redox shuttle reaction
In 2V Li/TiS, the addition of n-butylferrocene to the cell is the first reported redox shuttle additive. When the cell exceeds the cut-off voltage, as shown in Figure 2, n-butylferrocene on the surface of the positive electrode is blocked. The oxidation is then transferred to the negative electrode. The n-butylferrocene is reduced on the negative electrode and then transferred back to the positive electrode, and the cycle is repeated. Voltage increase is suppressed and battery overcharging is prevented.
Recently, halogen-containing anisole structures have been used as additives for 3V batteries instead of benzene. When the voltage is higher than 4.3 V, the excess current is consumed by repeated redox reactions. These redox additives are effective for small excess currents and require very high concentrations to prevent overcharging with large excess currents. The use of redox additives can still cause overcharge to occur when the battery is subjected to large excess currents large enough to damage the battery.
4. Cathode film formation reaction
Cathode film-forming additive is a more stable additive than redox shuttle additive. When the cut-off voltage is exceeded, an insulating polymer film is formed at the positive electrode to prevent current flow and lithium ion diffusion. As shown in Figure 3, polymerized monomers such as biphenyl (BP) are decomposed at the cathode and then polymerized to form a polymer protective film. This film blocks the movement of lithium ions in the battery, limiting the flow of external current. These additives prevent overcharging from occurring by rendering the battery inoperable, which is also a disadvantage.
5. Conductive Additives
In order to improve the ionic conductivity, it is necessary to promote the dissociation of the lithium salt, and the ions obtained by the dissociation should exist in the form of ions. At the same time, the ions in the electrolyte need to have high mobility. Crown ether additives can greatly facilitate the dissociation of lithium salts by sequestering lithium ions and dissociating cations through ionic-dipole interactions. Examples of crown ether additives are 12-crown-4 and 15-crown-5. The application of these additives results in a slight increase in the ionic conductivity of organic solvents and polymer electrolytes with low dielectric constants. When these additives are used in polymer electrolytes, the glass transition temperature decreases. But the cation acceptors of crown ether compounds reduce battery performance by slowing the movement of lithium ions. Furthermore, the use of crown ether additives is limited because they are highly toxic and environmentally harmful. Anion acceptors can compensate for the deficiency of cation acceptors by increasing the mobility of cations by dissociation of anions such as PF6- or BF4- anions, and by recombination with lithium ions. The dissociation of cations is suppressed at the electrodes, and the battery cycle is more stable. Cation acceptors consist of substituents that maximize cation-to-cation interactions by attracting electrons into boron sites4). When boron is combined with polyethylene glycol (PEG), the ionic conductivity increases with the vigorous dissociation of lithium salts in the electrolyte. Figure 4 is a chemical structural formula of a representative boron-containing additive.
6. Flame Retardant Additives
Liquid electrolytes are mainly composed of organic solvents, which, once ignited, burn quickly even if the external current is cut off. In order to suppress the flammability of the electrolyte, the organic solvent should have a high boiling point and can form a protective film during thermal decomposition to prevent oxides and combustible gases. Among lithium salts, LiPF, is an effective flame retardant in liquid electrolytes. Among the polymer electrolytes, polypropylene wax (PAN) gel electrolytes have certain flame retardancy. The CN triple bond in PAN breaks with the carbonization reaction of thermal decomposition at a temperature of 200 °C, and the ladder becomes hard to form a graphite layer structure, and the carbon layer acts as a protective layer to isolate the combustible gas.
Liquid electrolytes composed of organic solvents and lithium salts are difficult to obtain flame retardancy, which requires the addition of flame retardants. These additives need to be compatible with the electrolyte, not affect electrochemical performance, and be affordable. Most flame retardant additives are phosphates such as trimethyl phosphate (TMP), TFP and hexamethoxycyclotriphosphazene (HTMP). Although the mechanism of action of these additives remains unclear, they are very effective in reducing heat generation in batteries. Similar to PC, TMP causes exfoliation upon insertion into the graphite layer. A small amount of HMTP can prevent thermal runaway, and fluorine-containing TFP can improve the electrochemical stability and cycling performance.