Safety and high-capacity electrodes are the top priorities for lithium battery breakthroughs

　　At present, the industry generally believes that the short-term goal of lithium battery technology is to achieve 300wh/kg through high nickel ternary anodes and silicon carbon anodes; the medium-term (2025) goal is based on lithium-rich manganese-based/high-capacity Si-C anodes to achieve single The body is 400wh/kg; the long-term is to develop lithium-sulfur and lithium-air batteries to achieve a specific energy of 500wh/kg.

　　But in fact, in addition to the innovation of some core materials, there are still many technical problems in the development of lithium batteries. Such as battery safety issues, high-capacity electrode design technology, and so on.

　　1. Battery safety issues.

　　The three units that undertook the development of the 300Wh/kg battery project in 2016 still failed to meet the assessment requirements for battery safety. Whether the 300Wh/kg battery can be installed in vehicles in 2020 is actually not a performance issue, but a safety issue.

　　Among them, the dissociation heat of the positive electrode is an important cause of thermal runaway of the battery. Taking three raw materials as an example, whether it is high nickel ternary or general ternary, their thermal stability is much worse than that of lithium iron phosphate. Not only does it emit a large amount of heat, but also has a low decomposition temperature, which will lead to the safety of our future batteries. The problem will be more serious. Of course, to solve the safety problem, we must carry out the work in all aspects from three aspects: materials, monomers, and systems.

　　Ai Xinping believes that from the perspective of the overall solution to safety, the material is the foundation, and what material determines what kind of safety; the monomer is the key, good or bad is determined by the monomer; the system is the guarantee, and the lithium ion monomer is generated. If the heat is out of control, it won't cause everything else.

　　Here, only a few solutions from the monomer level are discussed.

　　The first idea is to develop battery self-excited thermal protection technology.

　　Lithium batteries have no temperature-sensitive features, and high temperature may cause thermal runaway. If there is a temperature-sensitive material in the battery that can effectively cut off the transmission of electrons and ions when the temperature is high, the battery will automatically shut down its reaction under abusive conditions to avoid further temperature rise.

　　The easiest way is to use PTC materials in batteries to achieve temperature sensitivity. In fact, PTC materials are used in many fields, but the batteries are not used. The main feature of PTC material is that it has good conductivity at room temperature; when a certain conversion temperature is reached, the resistance rises sharply, changing from a conductor to an insulator, which cuts off the electron transmission on the electrode.

　　Research also found that some conductive polymers have PTC effect and are soluble. This material can be used to prepare very thin coatings. For example: P3OT polymer, the conductivity is relatively high at 30-80 degrees, but there will be three orders of magnitude change immediately at 90-110 degrees, the coating is less than 1 micron, about 600 nanometers, so Will not affect the energy density of the battery. This material exhibits thermal shutdown properties at 120 degrees, which significantly improves the safety of the battery under conditions such as overcharging, hot box, and acupuncture.

　　In addition, thermally closing the diaphragm is also a feasible method. The existing three-layer diaphragms all have a thermal shutdown function. For conventional diaphragms, its closed cell temperature is determined by the melting point of PE, which is about 135 degrees; the melting temperature is determined by the melting point of PP, which is about 165 degrees. Because the closed cell temperature is too high, after the heat is closed, the thermal inertia can easily cause the battery temperature to continue to rise to 165 degrees, causing the diaphragm to melt and the battery to short-circuit. Therefore, the thermal protection effect of the conventional diaphragm is limited.

　　If a layer of plastic micropores are coated on the surface of the diaphragm, the surface microsphere layer will melt when the temperature reaches the melting point of the microspheres. After the ball is melted, the hole in the diaphragm is blocked. As a result, on which side of the electrode the microspheres are facing, the holes on the surface of the electrode are blocked. The effect is very obvious. Because the ion transmission is cut off, the battery reaction stops and the battery is safe.

　　The second idea to solve the safety problem is to develop all-solid-state batteries.

　　In fact, in terms of increasing volumetric energy density, all-solid-state batteries are very promising. As battery density increases, volumetric energy density becomes more and more important for passenger cars. Judging from the feedback at the 57th Japan Battery Conference, some R&D institutions in South Korea and Japan are conducting research on solid-state batteries, and some large domestic battery companies such as ATL are also doing research in this area.

　　Comparing all solids and liquids, the main advantage is high safety. Another feature is the ability to realize internal series connection, which is conducive to the improvement of module and system energy density. However, its interface stress is large and its stability is poor. The solid electrolyte must be in full contact with the active material particles, otherwise the transmission of lithium ions cannot be realized. However, any electrode material, whether it is graphite or ternary material, will change in volume during charging and discharging. Once the volume change causes solid/solid separation, the conduction of lithium ions will be hindered, and the battery performance will drop rapidly.

　　Therefore, one of the key points of the entire R&D of solid batteries is the selection of solid electrolytes. From now on, sulfides are more suitable because sulfides are relatively soft; the second is the construction technology and stabilization technology of the solid/solid interface. There is a knack, if you can't do it with pure solid electrolyte, the best way is the hybrid of inorganic and polymer; the third is the development of production technology and special equipment. The production process of solid-state batteries is definitely different from our current business format.

　　2. Design technology of high-capacity electrode.

　　After the energy density is increased, the problem of electrode design becomes more prominent. The proportion of active materials in the battery is an important factor that affects the specific energy of the battery. The same positive and negative materials, the same gram capacity, if the active material in a battery accounts for a relatively small mass, the energy density of the battery will be low. Therefore, to increase the energy density, it is necessary to fill as much active material as possible from the battery of the same weight. More active materials must mean fewer auxiliary materials, less copper foil and less aluminum foil; in fact, the most important thing is to make the electrode thicker. If the electrode is thicker, the amount of current collector and diaphragm will be reduced.

　　However, lithium ion electrodes cannot be made thick. After thickening, the electrode surface polarization becomes larger, the utilization rate of the electrode in the thickness direction is reduced, and it will cause problems such as lithium ionization in the negative electrode and decomposition of the positive electrode during the charging process. In terms of increasing energy density, it is hoped that the thicker the better; but the polarization theory tells us that the thinner the electrode, the better, the two are completely contradictory. As the energy density increases, for example, a single unit of 100wh/kg now becomes 300wh/kg, which means that the current per unit weight of the material increases simultaneously. Therefore, it is very difficult to maintain power performance for future high-energy-density batteries. Therefore, high-capacity electrode design technology is becoming more and more important.

　　There is actually a way to resolve this contradiction. The liquid phase current is greater as you get closer to the diaphragm, and this current is the external current; along the thickness of the pole piece, the liquid phase current gradually decreases, and the solid phase current gradually increases. Therefore, the closer the diaphragm to the electrode, the higher the pores, and the closer to the electrode's polar fluid, the lower the electrode pores can be. Therefore, in order to ensure high energy density and power performance, it is necessary to design an electrode with gradient pore distribution. As for how to do it, we must find a way. With the application of new materials and the improvement of battery energy density, the design of gradient pore electrodes has become more and more important. As for the extent of the gradient, it is difficult to find out by experiment, and it is necessary to establish a polarization model.

　　Finally is the summary of Professor Ai Xinping of Wuhan University:

　　1) Lithium-ion batteries are still the focus of power battery development. To solve the problems of low cycle coulombic efficiency of silicon anode and voltage attenuation of lithium-rich manganese-based, it is expected that advanced lithium-ion power batteries with specific energy exceeding 400wh/kg will be developed.

　　2) From a long-term perspective, innovative lithium-ion batteries are more realistic and feasible than lithium sulfur and lithium air. The development of a high-capacity lithium-rich oxide positive electrode based on the anion charge compensation mechanism can develop a power battery with a specific energy greater than 500wh/kg.

　　3) Safety determines the prospects of high-specific energy battery loading applications. The development of self-heating control technology and all-solid-state batteries is a feasible solution, and it is necessary to step up research.

　　4) High-capacity electrodes are the basis for achieving high specific energy of batteries. According to the new polarization, the development of gradient porosity electrodes has an important role and significance for the development of high specific energy batteries.