Recently, iphone 8 frequent battery safety issues, let us once again concerned about the safety of lithium-ion battery problems, causing lithium ion battery safety problems many factors, such as mechanical abuse, electrical abuse and design defects and so may cause safety problems. Is the most common electrical abuse, because unlike the NiMH and other batteries with anti-overcharge mechanism, lithium-ion battery in the event of overcharge will cause damage to the battery, and even lead to security problems, etc., for this lithium-ion batteries are generally equipped with Protect the circuit board in order to achieve the charge cut-off voltage when the lithium-ion battery can cut off the current protection of lithium-ion battery, but the circuit board is still there is damage, or the risk of error, so in order to protect the lithium-ion battery we will add some Anti-overcharge additives. There are two main types of anti-overcharge additives: redox additives and electric polymerization additives.
The redox pair works by the electrochemical reaction at the normal voltage of the lithium ion battery. However, when the battery is overcharged, the additive is oxidized near the positive electrode to form an active molecule and then diffuses to the negative electrode to be reduced to form a neutral Molecules, and then spread to the positive, so repeated to achieve the purpose of anti-overcharge. Commonly there are mainly metallocene compounds, polypyridine complexes, thianthracene and its derivatives and fennel benzene and its derivatives.
The working principle of the electric polymerization additive is different from the above-mentioned additive. Generally, when the battery is overcharged, the potential reaches the reaction potential of the polymer, the single molecule is oxidized to produce the radical ion, and the radical ion is polymerized in the electrolyte to form The polymer is deposited on the diaphragm near the positive electrode and extends to the positive and negative poles. In this case, there are two cases where the polymer forms a bridge between the positive and negative electrodes to produce a micro short circuit to prevent the voltage from rising. The other is completely blocked ion in the positive and negative between the shuttle, thus blocking the electrochemical reaction, so the electric polymerization additive is an irreversible additive, the common ingredients are biphenyl, cyclohexyl benzene, esters and Its derivatives and so on.
Oxidation and reduction of the additives generally have a fixed operating voltage, such as metallocene compounds oxidation potential is generally between 1.7-3.5V, polypyridine complex voltage of about 4.0V, etc., and recently the German University of Münster Pia Janssen The NHC-BF3 operating voltage is 4.4V, which is suitable for application on NMC111 material, and NHC-PF4CF3 can be used to change the working voltage of NHC-BF3. Voltage of 4.6V, can be used in high voltage NMC material, while the two additives in the normal circumstances will not have an impact on the electrical properties of the battery.
The molecular formula of the anti-overcharge additive used in the experiment is shown below
The following figure shows the addition of different additives and control group of electrolyte cyclic voltammetry, from the figure can be noted that the control group of electrolyte in 5.4V there is an oxidation current peak, which shows that the electrolyte in the beginning of the solvent But the electrolytic solution containing NHC-BF3, NHC-BF5 and NHC-PF4CF3 additive appeared two additional oxidation current peaks before the solvent was oxidized.The electrolyte containing NHC-BF3 additive had the lowest trigger voltage 4.6V, the second current peak is 5.1V, and the third current peak is 5.6V. The oxidation current peak of the electrolytic solution containing the -PF4CF3 functional group is shifted to the direction of the higher voltage, and the oxidation current of several electrolytes The voltage of the peak is shown in the following table: It can be seen that the different trigger voltages of the functional groups contained in the additive have also changed.
Pia Janssen used NMC111 / Li and NMC111 / graphite battery to evaluate the electrolyte, verify its constant current charge and discharge process of the battery protection function.The picture shows the use of several different electrolyte battery charge and discharge curve, From the figure we can note that the blank control group of batteries charged to 4.95V when the voltage did not show the phenomenon of overcharge, but containing NHC-BF3 electrolyte in 4.5V there is a voltage platform, and contains NHC-PF4CF3 And NHC-PF5 electrolyte there are two different voltage platform in the anti-overcharge additives after the oxidation of decomposition, the battery charge and discharge process are suppressed, lost the ability of charge and discharge, which may be additive decomposition After the cover on the surface of the cathode, inhibit the diffusion of Li, thus achieving the electrochemical reaction to block the battery.
Of course, as a anti-overcharge electrolyte solution, in the protection of the battery at the same time can not have a negative impact on battery performance, through the graphite / Li battery experiments found that the additive on the graphite anode cycle performance has no significant impact, but will The first efficiency of the batteries was 82.2%, and the first efficiencies of NHC-BF3, NHC-PF5 and NHC-PF4CF3 were 79.7%, 70.5% and 71.9%, respectively, for the first time, , Indicating that these additives on the stability of the graphite anode to be slightly worse. Additives on the first battery efficiency also have a certain impact, such as NMC / graphite battery using the blank control group when the first efficiency of 82%, but the use of added NHC-BF3, NHC-PF5 and NHC-PF4CF3 were 80%, 74.3% and 74.1%, respectively.
For the above additives, especially NHC-BF3 additive mechanism of the study found that its working principle, in the event of overcharge in the positive electrode oxidation, the formation of an insulating layer, hinder the Li + diffusion and charge exchange, so as to achieve blocking power Chemical reaction, to prevent the safety of lithium-ion battery safety purposes. Figure a for the new NMC111 surface, Figure b for the use of blank control group electrolyte overcharge after the NMC111 material, Figure c for the use of containing NHC-BF3 additive electrolyte And the surface of the electrode after the charge has occurred, we can see that graphs a and b are very smooth, but the figure of NMC111 in Figure c shows that there is a lot of uplift and roughness, which also shows that NHC-BF3 The surface of the positive electrode generates an insulating layer, which is the insulating layer that blocks the electrochemical reaction.
The thickness of the surface layer of the positive electrode material is shown in the following table when the NMC111 material of the electrolyte solution and the electrolyte containing NHC-BF3 additive is 4.5V. After the blank control group NMC111 material surface electrolyte decomposition product The thickness of the layer is significantly higher than that of the NHC-BF3-containing experimental group. It is clear that NHC-BF3 will form an inert layer on the surface of the positive electrode when the battery is overcharged to prevent the decomposition of the electrolyte from continuing.
Pia Janssen developed this anti-overcharge additive can be formed in the positive electrode to form an inert coating to block the electrochemical reaction to achieve the purpose of anti-overcharge, its prominent feature is to adjust the structure of functional groups, the trigger voltage regulation, To meet the needs of different batteries.At the same time the experiment also found that the additive on the electrochemical performance of the battery is relatively small, especially NHC-BF3 additives will only have a slight impact on the first efficiency of the battery, the battery will not form a negative performance The effect is very suitable for use on the NMC battery to improve the safety of the battery.
