Solid-state batteries are seen as the power source for the next generation of the most promising alternative to liquid lithium batteries. Solid-state batteries use solid electrolytes compared to existing mass-produced power batteries. Different from the flammable characteristics of liquid electrolyte, solid electrolyte is non-flammable, non-corrosive, non-volatile, and has no leakage problem. It is more stable under high pressure, allowing the battery to work at high voltage, which will greatly improve the lithium battery. Specific energy and safety.
Problems with all solid state batteries
At present, the main problem limiting the application of all-solid lithium battery is that the energy and power density of the battery are low, and the main factors determining the energy and power density of the battery include the characteristics of the electrode material, the electrolyte material and the interface between the two. In the field of inorganic chemistry, many masters have studied inorganic electrolytes, which laid a solid foundation for the selection of lithium battery electrolytes.
For example, recently, inorganic sulfide solid electrolytes have attracted attention because of their high ionic conductivity. Its ionic conductivity is comparable to that of organic liquid electrolytes. However, the interface problem in all-solid-state batteries has not been effectively solved.
After the electrolyte is changed from a liquid to a solid, the lithium battery system is converted from a solid-liquid interface of the electrode material-electrolyte to a solid interface of the electrode material-solid electrolyte. The difference is that there is no wettability between the solids, and the interface is more likely to form higher contact resistance. At the solid electrolyte/electrode interface, there is a phenomenon that it is difficult to fully contact, components interdiffuse or even react, and a space charge layer is formed, resulting in a sharp increase in internal resistance of the all-solid lithium ion battery and deterioration of battery cycle performance.
There are currently three ways to establish a tight bond between living matter and solid electrolytes:
First, the use of pulsed laser deposition, although the method is good, but in the laboratory stage, and it is not practical to use this method for large-scale production.
The second is the planetary ball milling technology. Although this method can achieve mass production, the powders rub each other, the particle damage is inevitable, and the negative impact of the material structure damage on the battery is self-evident.
The third is hot pressing technology, heat treatment will destroy the solid electrolyte, so there is no particularly ideal way.
Solution of interface problems in solid state batteries
Recently, an article was published in Powder Technology, and Takashi Kawaguchi used an impact-mixing device to study the interface contact between electrolytes and living materials. The principle of the device is shown in Figure 1.
Figure 1. Schematic of the impact-mixing device, a front view b side view
The purpose is to use a mixing device to dry the coated form, using a larger particle cathode material (NCM11) as a host particle, and a smaller particle electrolyte particle as a host particle to coat the small particle on the surface of the large particle. Taking into account economic issues, the developer used a model of the sulphide electrolyte, sodium sulphate. The appearance of the two raw materials is like this:
Figure 2. (a) SEM of NCM111 (b) SEM of Na2SO4
The results of the particle analysis showed that the NCM particle had a median diameter of 5.4 μm and sodium sulfate of 0.95 μm. NCM particles are rigid and brittle, while sodium sulfate particles have the opposite toughness. NCM acts as a mixed conductor and is electrically conductive while sodium sulfate is not electrically conductive. At the same time, it was confirmed by indentation experiments that the mechanical properties of sodium sulfate and the sulfide electrolyte ((75 mol% Li2S ̇25 mol% P2S5) are similar.
2. Experimental methods and results
a. Differences in topographical features
Comparisons were made using three mixing methods: A. Simple vibrating mixing B. Grinding mixing C. Grinding and mixing and coating with impact-mixing dry method. As shown in Figure 3.
Figure 3. Experimental approach
After the NCM ternary material and the sulfide solid electrolyte simulated particles are mixed by the above three methods, the particles obtained by the three methods are subjected to corresponding field emission scanning electron microscope analysis and energy dispersive X-ray spectrometer analysis, and the analysis results are shown in FIG. Show, you can see:
(1) The first line is an electron micrograph of the three mixtures. It can be seen that after the vibration mixing, the ternary material and sodium sulfate are not well mixed, and a large agglomerate appears in the sodium sulfate. After grinding and mixing, although the agglomerates were reduced, they were still deposited on the surface of the ternary material particles and were not well coated thereon. The best coating effect is the third dry coating method, in which sodium sulfate is uniformly wrapped on the surface of the ternary particles.
(2) The second line picture and the third line picture respectively represent the mapping pictures of S and Mn in the mixed particles, which represent the distribution state of sodium sulfate and ternary materials in the mixed particles. It can be seen that the conclusion of the coating is consistent with (1). The surface of the dry-coated particles subjected to impact-mixing after grinding has uniform continuous sodium sulfate particles present.
Figure 4. FESEM and EDX images of three different powders
Then, through the above analysis results, the third method can well solve the problem of close contact between the electrolyte and the living material. In order to verify that such mixing strength will cause damage to the active material particles, the sodium sulfate in the C powder is washed away, as shown in the following figure, it can be seen that the surface of the ternary particles is as intact as the original state.
At the same time, by analyzing the profile of powder C, it can be seen that there is a continuous layered substance sodium sulfate on the surface of the NCM particles, and the thickness is about 0.5 μm. All the results show that such a mixing device does not destroy the particle integrity and topography of the electrode material. The structure is not destroyed after the electrolyte and the electrode material are mixed, which is important for battery performance.
Figure 5. SEM picture of C powder after washing off sodium sulfate
(a) at low magnification (b) at high magnification
Figure 6. Profile FESEM and EDX images of powder C particles
Based on the above analysis results, a model of dry-coated active material was proposed: Initially, after grinding, the sodium salt of the host particles adhered to the host particle NCM111. After impact, friction, mixing, and subsequent plastic deformation and aggregation of the host particles, the structure of the NCM is not damaged during this process. This is mainly determined by the difference in properties of the two types of grinding. The toughness and ductility of sodium sulfate are just suitable for coating the particles of living matter with rigidity and brittleness.
b. Comparison of resistivity
At the beginning of the article, the properties of electrolytes and active material particles were introduced. The electrolyte is not electrically conductive, while the living material is electrically conductive. This means that the better the particles of the active material are coated with the sodium sulfate particles, the greater the resistivity of the particles. The powders A, B, and C were pressed into a sheet shape under a pressure of 360 MPa, and the resistivity analysis was performed. The specific analysis results are shown in Fig. 7.
Figure 7. Resistance coefficient analysis results
It can be clearly seen from Fig. 7 that the sodium sulfate particles have the highest electrical resistivity, followed by the powder C-powder B-powder A, and preferably the NCM particles have the lowest electrical resistivity, which means that the powder C has the most coating effect. Preferably, the results are the same as the SEM, EDX, and FESEM results described above.
The powders A, B, and C were pressed into a sheet at a pressure of 360 MPa, and subjected to FESEM and EDX analysis, respectively. The photograph is shown in Fig. 8, in which dark gray is sodium sulfate and light gray is NCM. It can be seen that in the powder A, a large area of NCM is agglomerated together, and the sodium sulfate particles are not wrapped on the surface of the NCM. Also in the EDX analysis results, the sodium powder in the C powder was tightly wrapped on the NCM surface in the three powder samples, which also verified the feasibility of this mixing method.
Figure 8. Cross-section of three powders for FESEM and EDX analysis
In order to quantify the degree of mixing of the three types of powders, rather than simply visual observation, statistical methods were used to cumulatively compare the number of contacts between NCM-NCMs in the compressed sheets to obtain the conclusion of FIG. It can be seen that for powder C, 60% of NCM is not in direct contact with NCM, and the contact effect of the electrolyte with the electrode material is much better than that of powders A and B.
Figure 9. NCM-NCM contact point for each NCM particle in the compressed pellet section
A continuous, uniform electrolyte coating can be prepared by dry coating the host particles (live matter) by the host particles (electrolytes). At the same time, this method does not cause damage to the host particles, the electrolyte particles can be uniformly dispersed, and the porosity of the mixed powder is lowered, and the close contact between the electrolyte and the electrode material is completed. The close combination of the two particles can effectively reduce the interface resistance and increase the migration rate of lithium ions, which also means that the all solid state battery can have better electrochemical performance.