Based on Li 4Ti 5O12Material battery because of high security, fast charging and other characteristics, has a very good impact on the prospects, but the use of LTO material battery is also facing more problems of gas production, LTO material on the gas production mechanism is now a variety of views, of which One is that the adsorbed moisture and the Lewis acid in the electrolyte cause an increase in gas production. According to this theory, the H 2Will dominate the resulting gas, and another view that the surface of the LTO material will react with the electrolyte to produce H 2, CO 2And CO, which can be used to suppress the occurrence of side reactions by coating a layer of carbon, AlF3 and other materials on the surface of the LTO material, and the view that the gas production behavior is mainly related to the LTO potential, since the graphite material 1.55V near the also produce a lot of gas.
In fact, LTO material gas production behavior is more complex, in practice, we not only detected the H 2, CO 2, CO, also detected C 2H4These gases are related to the decomposition of the electrolyte caused by the formation of the negative SEI film, so the gas production behavior of the LTO material is a complex process. Wei Liu et al. Of the Shanghai Institute of Industrial Technology have studied the aerobic behavior of LTO materials It is believed that the electronic structure of Ti ions and the formation of SEI films have a crucial effect on their gas production behavior.
The positive electrode material of the flexible battery used in the study was NMC111 and the negative electrode was Li 4Ti 5O12, The following figure shows a battery with a different SoC status of the battery aging at 55 ° C for 24 hours. You can see that the battery with gas production at 100% SoC is significantly more than 50% SoC and 0% SoC, As can be seen from Figure b, the battery is produced at the end of the very little gas production, but at 55 ℃ aging 24h, the battery gas production increased significantly.For example, 50% SoC battery before and after aging, the volume of air bags from 4.2ml Increased to 18.7ml, and 100% SoC, the volume of the bag increased from 3.9ml to 48.8ml. The cause of this phenomenon may be related to the electronic structure of Ti ions, Lu and others believe that there is spontaneous in the LTO material Ti 3+To Ti 4+, In this process will release an electron, which has an impact on the oxidation / decomposition of organic electrolytes, and in the higher SoC, LTO material will have more Ti 3+, So there will be more Ti 3+Transition to Ti 4+, Thus also means that the release of more charge, thereby increasing the decomposition of the electrolyte.
In the different SoC state, the surface morphology of the negative electrode is shown in the following figure. The graphs a and b are the original LTO materials. The particle size of the material is 0.2-1um, the surface of the LTO material is relatively smooth and the electrode surface exists More holes in the battery charge to 50% SoC, the electrode surface of some holes have disappeared, while LTO material particle surface also began to become rough, the surface of the electrolyte in the negative surface of the decomposition. When the battery charge To 100% SoC, the electrode surface covered with a layer of thick electrolyte decomposition products, while the electrode surface of all the holes are gone.With the previous gas research, basically can be judged, LTO battery gas production behavior Because the electrolyte in the LTO negative surface caused by decomposition.
In order to study the interfacial reaction characteristics of LTO / electrolyte, Wei Liu studied LTO by XAES, and the results are shown in the following figure: Figure a is a characteristic map of Ti L2 and 3-edge, where P3 and P4 peak represent L3 -edge, P3 and P4 represent L2-edge, corresponding to Ti 2P3 / 2 and Ti P1 / 2 excited states respectively. We can see that all the characteristic peaks are reduced when the battery is charged to 50% SoC, and P1 Peak and P2 peak intensity ratio t2g / eg also decreased, while Ti 4+Reduced to Ti 3+Will reduce t2g / eg, which indicates more Ti in LTO 4+Transition to Ti 3+At the same time we also found that almost all of the characteristic peaks disappeared after the battery was charged to 100% SoC. Since the XAES detection depth was only 5-10 nm, Wei Liu thought that this was mainly the surface of the LTO particles More than 10nm thick electrolyte decomposition products, resulting in the LTO material itself can not be detected.This is also from the O K-edge feature spectrum (Figure b) has been verified, from the figure can be seen in the battery charge to 100 % Of SoC, the electronic structure of O changes from 1s to p. The oxygen of this electron structure mainly appears in the C-OH structure, such as COOH functional group, so it also shows that the electrolyte has decomposed on the surface of LTO particles.
The following figure shows two times after 0.5C charge and discharge cycle after the battery performance and cycle performance test, from Figure a can be seen at 0.5C rate, the initial discharge capacity of the battery 5.27Ah, the voltage platform in the 2.2V or so, The specific capacity of LTO is about 144.4mAh / g, which is lower than that of the button cell, which is mainly due to the first efficiency of the positive and negative electrodes, as well as the formation of SEI film, etc. In 1, 3, 5 and 10C The discharge capacity of the battery is 4.91, 4.41, 4.05 and 3.77Ah, respectively, and the capacity retention rate of the 1C battery is 76.8% at 10C, which shows the good magnification performance of the NMC111 / LTO battery. Performance, after the cycle 100 times, the use of epoxy resin plate holding the battery capacity retention rate of 99.1%, without the use of epoxy resin battery capacity retention rate of only 93.2%, which may be because the battery in the cycle of production Gas caused by the positive and negative distance increases, causing some of the active substances can not participate in the charge and discharge reactions, resulting in decreased capacity.
The following figure shows the cycle 100 times, the battery inflatable bag volume, we can note that the battery in the cycle of gas production is very obvious, but compared to the chemical process, the cycle of the battery because the battery temperature is relatively low, so the gas or More moderate.
The following figure shows the battery after the completion of the cycle and the main component of gas production, you can see the chemical phase, the gas is mainly H 2, CO 2/C3H8And CO, their volume fraction were 30.6%, 14.2% and 19.6%, respectively 2Mainly in the electrolyte in the water, the electrode material on the adsorption of moisture caused by the decomposition of the battery in the process of the composition of the gas has changed, we see the CO 2/C3H8, CO and CH 4The proportion of gas in the battery was 20.6%, 41.4% and 7.3%, we can see that the gas is mainly due to the decomposition of the electrolyte, and SEI film dissolution and re-growth.
Wei Liu analysis shows that the NMC111 / LTO battery in the chemical phase of the gas production mechanism shown in the following formula, the gas production with the temperature and SoC increased speed, resulting in the battery at high SoC and high temperature gas production is more serious.
Wei Liu's research reveals the mechanism of LTO battery gas production, in the battery into the higher SoC state, due to LTO in the Ti 3+The amount of more, and Ti 3+There is spontaneous to Ti 4+This process will release an electron, which leads to the decomposition of the electrolyte.Often we believe that the LTO material due to the relatively high potential, so the use of the process does not produce SEI film, but Wei Liu found that the actual LTO surface Will still be covered by the decomposition products of the electrolyte, the thickness of more than 20nm, which also proved that the electrolyte and LTO side effects is the main cause of gas production.
