The research on the stability of oxides has actually been done a long time ago. This is the research of J.R. Dahn 20 years ago. He found that oxides are materials that easily react with water to form a hydrated phase. He found that adjusting the elements of the transition metal layer, for example, when nickel is doped to a certain extent, this hydration phase can be obviously suppressed, and the stability of crystal water can be greatly improved.
Study the specific mechanism of action of water in oxides. Everyone found that this water can enter between the layers and come out reversibly. The composition of transition metals also has a relatively large impact on air sensitivity.
In addition to water in the air, carbon dioxide also has a very large impact. Many studies have found that transition metal oxides will be removed from sodium to form sodium carbonate. Our own research has also found that sodium carbonate will grow out (placed in the air for a period of time). There are also some studies that found the formation of sodium bicarbonate or sodium hydroxide. However, there is still considerable controversy as to how sodium carbonate is produced. L Nazar proposed that sodium carbonate anions enter the interlayer to form this. Yang Yong found that this process is more likely to be the exchange of sodium and protons, which is responsible for the main charge compensation process. , And there is no obvious compensation factor for transition metal price change. Of course, there is still controversy about the specific microscopic process of sodium carbonate formation.
We explored this micro process because it has a very important impact on SEI. We use a relatively common ternary cathode material. This material is very clean in the initial state, and the lattice stripes can be seen very clearly. The high-resolution electron microscope can see that there is a layer of amorphous phase on the surface, which is mainly carbonic acid. sodium. For this microscopic exploration process, we mainly control the deterioration process very finely. This requires very control of the humidity conditions, because as long as the humidity is high, the material will deteriorate quickly, and it is impossible to explore the specific deterioration process. We found that 15% humidity actually has no effect. When the humidity reaches 30%, we can see that the O3 layered oxide begins to remove sodium to produce P3 phase, and sodium carbonate appears. It can be seen more clearly after one month, and it can be seen on the surface. Sodium carbonate has grown.
We found that this process is the generation of O3 phase, P3 phase transition, and some sodium carbonate. For the specific process, we found that it has a lot to do with the morphology and crystal orientation of the particles, which is important for us to design materials. It still has a big impact. It was found that cracks appeared on the vertical 003 edges, but not on the parallel edges. If the particles are set in a spherical shape, there will be no cracks, so we found that the air sensitivity is very closely related to the crystal phase of the particles. In addition, we found that the failure process of the oxide positive electrode is also related to humidity. When the humidity is relatively high, the hydrated phase will be preferentially formed instead of the sodium carbonate phase. In general, the process of oxide failure depends on the specific situation. First, it depends on the shape of the particles, the crystal phase and the specific air environment, as well as the failure time. The level of humidity and temperature are closely related.
Let me share some research on oxide electrolyte. In lithium-ion batteries, the dissolution of transition metals and the phase change of surface structure, especially the phase change from layered to spinel and salt rock structure, are also involved in lithium ion batteries. This is very common in lithium battery oxide cathode materials, and electrolyte It is also closely related. We found that this problem also exists in sodium-ion batteries, but we are not sure whether it has undergone the spinel phase change process, but we can see that there is also a layer of salt rock structure on the electrode surface after the cycle. This is quite interesting. . Because the sodium ions are relatively large, it is not clear whether they can occupy the position of the crystal lattice structure in the spinel.
We found that in different electrolytes, the stability of the sodium ion interface, including the cycle performance, is very different. The picture on the left is in the traditional carbonate electrolyte, the transition metal oxide has an obvious dissolution process, and the diaphragm has obvious discoloration after the cycle. The test found that the main dissolution of iron ions, but the nickel and manganese did not dissolve much, including The coulombic efficiency is relatively low in carbonates. We designed a new flame-retardant triethyl phosphate-based flame-retardant electrolyte that can significantly inhibit the dissolution of transition metals. It can be seen that the solubility of iron is significantly inhibited, and the cycle stability is also good. Obviously improved.
Including interface studies, it can also be seen that SEI is formed in carbonate, but this SEI is unstable, it will always grow, and it will also induce the phase change of the layered structure on the surface of the oxide positive electrode to the salt rock structure. The layer will continue to grow with the cycle. In the new electrolyte, the interface will be stable, the phase change layer can be significantly suppressed, and the cycleability can also be improved. But with this system, we can see that the ratio is 1:1.5:2. This concentration is actually relatively high. Next, when further designing this electrolyte, I wonder if there are any new electrolyte ideas? Especially for the flame-retardant type, everyone has a very high demand for safety. In traditional electrolyte design ideas, especially for solvents that are unstable to the negative electrode, such as phosphate esters, the most common method is to increase the salt. Concentration to change the local coordination structure of the solvent and ions, reduce the number of molecules in the free solvent, so as to improve the window and stability. However, this is faced with many problems, including rising costs, rising viscosity, and decreasing electrical conductivity, so we want to develop some different ideas when designing a new type of flame-retardant electrolyte. If our solution does not depend on the salt concentration, and the salt concentration can be very low, and the positive and negative electrodes are compatible, it should be more practical.
This is our new flame-retardant electrolyte. It can be seen that when the salt concentration is only about 0.2 mol, it has obvious flame retardancy. This is a new flame-retardant electrolyte battery performance test. It is based on a full battery test, or in a button battery, but the surface capacity of the positive and negative electrodes and the N/P ratio are strictly in accordance with the ratio of the soft pack. 2mAh/square centimeter, at 60 degrees, it can be seen that the cycle of the full battery is relatively stable. We also conducted a test at 80 degrees, and we can see that our battery can still cycle normally. Our new design can make the salt concentration very low and the ion conductivity can be kept relatively high.
Back to SEI, what kind of SEI is a good SEI? I don't think it is very clear. The useful components reported by everyone, including lithium fluoride, lithium oxide, sodium fluoride, etc., have significantly improved the performance of the battery. However, SEI membrane properties, structural composition, solubility, etc., as well as the structure and composition of SEI membranes, still lack a certain understanding of the specific structure-activity relationship of battery performance, especially for sodium ion batteries. . We also try to get a little understanding on this. Based on the hard carbon anode, we have done some research on the relationship between the structure and properties of SEI.
This is a simple example, hard carbon anode, which is made from commercial hard carbon in two electrolytes, one is carbonate and the other is ether, which can show significantly different rate performance, and the cycle stability is also very different. It can be seen that even if the type of salt is changed in ethers, its circulation is very stable, and it can circulate thousands of times, indicating that the electrolyte does affect it.
Although the electrochemical properties of hard carbon anodes are very different in carbonate and ether electrolytes, there is no difference in SEI components, and they are basically the same. The difference may be that the ether electrolyte has slightly more sodium fluoride, that is, more inorganic components, while the outer layer is mainly organic. It can be seen from the transmission electron microscope photos that although the components are the same, the structure is still very different. The SEI is not very stable in the carbonate, and the thinner and denser SEI is formed in the ether.
We found that in ether electrolytes, in addition to the surface SEI film on the hard carbon surface, an intermediate transition phase will also be formed, which we call pseudo-SEI. Because we found that in this area, solvent molecules enter between the graphite layers to increase the order of graphite and promote the diffusion and transport of surface ions to the bulk phase. We also found the same phenomenon in other ether electrolytes. After charging and discharging, you can see the solvent molecules embedded in the surface layer, especially the mapping on the right, you can see that the surface layer does have solvent molecules entering the hard carbon layers. In addition to the electrolyte itself, the structure of the hard carbon itself, including the doping of elements, will have an effect on the structure of the SEI, which in turn has a very important impact on the battery performance.
Today I mainly want to share with you the influence of the electrolyte and the SEI film formed on the electrode surface on the battery performance from the electrode material as the starting point, as well as the research on the stability of the oxide cathode surface. We found that the phase transition of the oxide positive electrode structure is very related to the shape of the particles, crystal orientation and environmental humidity. In addition, it was found that the local high salt concentration electrolyte has a good inhibitory effect on the dissolution of transition metals and the phase change. The SEI structure ratio of the hard carbon surface has a greater impact on the battery rate and cycle stability.