The concept of “redox shuttle” has been proposed before the advent of Tycorun Lithium Battery. Compound R with reversible redox potential is added to the electrolyte, R is oxidized to compound O on the positive electrode, and then O migrates to the negative electrode and is reduced For the initial form R, the properties required for this compound [130] are as follows:
① At the end of charging, the redox potential is slightly higher than the apparent potential of the positive electrode (0.1 ~ 0.2V);
② During overcharging, the redox reaction between the positive and negative electrodes should be kinetically reversible (the electrochemical reaction rate constant is higher than 10-5cm/s);
③ Redox substances should be chemically stable and not react with other components;
④ The diffusion coefficient and solubility of redox substances should be as high as possible.
Researchers have recommended the use of various metal complexes [132,133] and aromatic compounds [132-139] as overcharge protectors, however, their redox potentials are too low for 4V Li-ion batteries. Their onset oxidation potentials are clearly shown in Figure 1 (measured values are not in good agreement due to different measurement conditions) [132,136–139]. It is clear from these examples that the π-electron conjugation system is necessary for the redox reaction.
The equation for the limiting current density iim of a single-electron-transferred redox shuttle is as follows:
In the formula, F is the Ferrari constant; Co is the initial concentration; L is the thickness of the diaphragm; D is the diffusion coefficient [136]. Because the diffusion coefficient of cationic groups produced by oxidation reactions is usually an order of magnitude smaller than that of the original neutral compound, the limiting current density depends on the diffusion coefficient of cationic groups [135,136]. This is because charged particles are solvated and cationic groups tend to interact with a neutral compound. Sex molecules interact to form dimers. The diffusion coefficient of neutral molecules is inversely proportional to the viscosity of the electrolyte. In the electrolyte (1mol/L LiPF6/EC+2DMC) [136], their diffusion coefficient is 10-6 ~ 10-5cmz/s, which is different from that of solvated lithium ions. The diffusion coefficient values are similar. For example: Assuming Co = 0.2moI/dL, L = 25mm, D = 10_6cm2/s, 3% (mass fraction) of 2,4-difluoroanisole (DFA)[138] can withstand iiim = 8mA/cm2 the maximum limiting current density.
Experiments show that in Li/LiCoO2 2025 coin cell (25mA h), the redox strobilide compound 4-bromo-1,2-dimethoxy soluble in electrolyte (1mol/L LiPF6/PC+DMC) Benzene (0.1mol/L) achieves overcharge protection at a rate of 0.02C, as shown in Figure 2 [132]. When there is no additive, the voltage of the battery continues to rise, and the electrolyte decomposes and releases heat at 4.6V measured by a C80 calorimeter. On the other hand, when there is an additive, the battery voltage stops rising at 4.3V, and the input energy W = 4.3VX0. 15mA = 0.65mW is all converted into thermal energy and consumed by the redox shuttle reaction.
Another one added 2,7-dibromophosphorus (0.05mol/L) to the electrolyte of 1mol/L LiPF6/EC+PC+DMC+DEC (35:10:20:35, volume ratio) of prismatic battery (900mA h) reported [139]. The heat release during overcharging was measured with an acceleration calorimeter (ARC). When the battery under test is overcharged at a rate of 1C, the voltage of the battery rises to the preset 12V after 1.1h, and then keeps the voltage until the end of charging, as shown in Figure 2. Although the temperature T of the battery reached about 110°C, no thermal runaway occurred. However, at rates above 2C, no battery can pass similar tests. It should be emphasized that other methods (using safety devices) must be used to avoid overcharging at high current.
We found that alkylbenzene derivatives without H atoms in the benzyl position can act as mediators for the decomposition of carbonate solvents, as hypothesized in Figure 3. Because these derivatives undergo a reversible redox reaction, which increases the amount of CO2 released without being consumed by themselves. After the overcharge test at 60 °C, tert-butylbenzene and 1,3-di-tert-butylbenzene are more than toluene and ethylbenzene. and cumene have low open circuit voltage, they are very good overcharge protection additives.
