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

At present, it is generally believed in the industry that the near-term goal of lithium battery technology is to achieve 300wh/kg through high nickel ternary positive pole and silicon carbon negative pole. Medium term (2025) target is to achieve 400wh/kg monomer based on lithium-manganese rich/high-capacity si-c negative electrode; In the future, lithium sulfur and lithium empty batteries will be developed to achieve the monomer specific energy of 500wh/kg.

In the article "why lithium sulfur/lithium empty battery does not have the application prospect of power battery", professor ai xinping from wuhan university has recognized the feasibility of short-term and medium-term goals and discussed in detail the reasons why lithium sulfur/lithium empty battery does not have the application prospect of power battery. In "lithium battery core material innovation, where is the way out?" In this paper, the solution of the next generation lithium electric materials is also discussed.

But in fact, in addition to some core material innovation, lithium battery development has many technical problems. Such as battery safety issues, high load electrode design technology and so on.

1. Battery safety.

In 2016, the three units that undertook the development of 300Wh/kg battery project failed to meet the assessment requirements for battery safety. However, whether 300Wh/kg battery can be loaded into the vehicle in 2020 is actually not a performance issue, but a safety issue.

Among them, the decomposition and exothermic of the positive electrode is an important inducement for the runaway of battery heat. Take three raw materials as an example, no matter it is high nickel ternary or general ternary, their thermal stability is much worse than lithium iron phosphate, not only the heat release is large, but also the decomposition temperature is low, which will lead to our future battery safety problems will be more serious. Of course, to solve the problem of safety, from three aspects, materials, monomer, system all-round work.

Ai xinping believes that from the security of the whole solution, the material is the basis, what material determines what kind of security; Monomers are the key, good and bad are determined by monomers; The system is the safeguard, the lithium ion monomer has a thermal runaway and does not cause everything else.

Only a few solutions from the monomer level are discussed here.

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

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

The easiest way to do this is to use PTC materials in batteries to achieve temperature sensitivity. PTC materials are used in many fields, but not in batteries. PTC materials are characterized by good conductance at room temperature; When a certain transition temperature is reached, the resistance rises sharply from a conductor to an insulator, thus cutting off the transmission of electrons from the electrode.

It was also found that some conductive polymers have PTC effect and are soluble. Very thin coatings can be prepared with this material. For example, P3OT, a polymer, has a relatively high conductivity at 30-80 degrees, but immediately changes by three orders of magnitude at 90-110 degrees. The coating is less than 1 micron and 600 nanometers, which will not affect the energy density of the battery. The material exhibits thermal closing properties at 120 degrees, significantly improving the safety of the battery under overcharge, hot box, acupuncture and other conditions.

In addition, thermal closure of the diaphragm is also a viable option. The existing three-ply diaphragm has a thermal shut-off function. The normal diaphragm, whose obturator temperature is determined by the melting point of PE, is about 135 degrees. The melting temperature is determined by the melting point of PP, about 165 degrees. Because the temperature of the obturator is too high, the thermal inertia will easily cause the battery temperature to continue to rise to 165 degrees after the thermal closure, resulting in the melting of the diaphragm and short circuit of the battery. Therefore, the thermal protection effect of the conventional diaphragm is limited.

If a layer of plastic micropores is applied to the surface of the diaphragm, the surface microsphere layer melts when the temperature of the microsphere melting point is reached. The ball melted and plugged the hole in the diaphragm. As a result, the holes on the surface of the electrodes on which the microspheres are facing are blocked, with remarkable results. As the ion transport is cut off, the battery's reaction stops and the battery is safe.

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

In fact, solid-state batteries are very promising in terms of increasing bulk energy density. As the battery density increases, the volume energy density becomes more and more important for passenger cars. According to the feedback from the 57th Japanese battery conference, some research institutions in South Korea and Japan are carrying out research on solid-state batteries, and some large battery companies such as ATL in China are also doing research in this field.

Compared with all solid and liquid, the main advantage is high safety, another feature is to achieve internal series, which is conducive to the improvement of module and system energy density. However, its interface stress is large and its stability is poor. Solid electrolyte must be in full contact with active material particles, otherwise the transmission of lithium ion cannot be realized. However, any electrode material, whether it is graphite or ternary material, will change in volume during charging and discharging. Once the solid/solid separation is caused by the volume change, the conduction of lithium ions will be blocked, and the battery performance will decline rapidly.

So one of the priorities of the whole development of solid batteries is the choice of solid electrolytes. Second, solid/solid interface construction technology and stabilization technology, there is a trick, if the pure solid electrolyte can not do, the best way is inorganic and polymer hybrid; Third, the development of production technology and special equipment. Solid-state batteries are certainly not made in the same way that we do today.

2. Design technology of high load electrode.

With the increase of energy density, the problem of electrode design becomes more serious. The proportion of active materials in the battery is an important factor affecting the specific energy of the battery. The same positive and negative materials, the same gram capacity, if a battery has a relatively small proportion of the active material mass, the battery energy density is low. So to increase energy density, make sure you fill as many active materials as possible from the same weight of the battery. More active materials must be auxiliary materials, copper foil to reduce, aluminum foil to reduce; In fact, the most important is to make the electrode thick, thick electrode, fluid collection and the amount of diaphragm is also reduced.

However, the lithium ion electrode cannot be made thick, and after the thickness, the electrode surface polarization will become larger, and the utilization rate of the electrode in the direction of thickness will be reduced, and it will cause problems such as lithium separation from the negative electrode and decomposition of the positive electrode in the charging process. In terms of increasing the energy density, hopefully the thicker the better; But the polarization theory tells us that the thinner the better. With the increase of energy density, such as a monomer of 100wh/kg, it now becomes 300wh/kg, which means the current borne by the material per unit weight increases synchronously. Therefore, it is very difficult to maintain the power performance for future high-energy density batteries, so the electrode design technology with high load becomes more and more important.

There are ways to resolve this contradiction. The closer you get to the membrane, the greater the liquid current is, and that current is the external current; Along the direction of the thickness of the plate, the liquid phase current decreases slowly and the solid phase current increases gradually. Therefore, the closer to the diaphragm electrode, the higher the porosity should be, and the closer to the polar fluid of the electrode, the lower the electrode porosity can be. Therefore, to ensure both high energy density and power performance, we must design an electrode with gradient pore distribution. With the application of new materials and the improvement of battery energy density, the design of gradient porosity electrode becomes more and more important. As for the gradient to what extent, not by trial and error, trial and error is very difficult to build a polarization model.

Finally, the summary of professor ai xinping from wuhan university:

1) lithium ion battery is still the focus of power battery development, which can solve the problems of low cycle coulomb efficiency of silicon negative electrode and voltage attenuation of lithium-manganese rich base, and is expected to develop advanced lithium ion power battery with specific energy exceeding 400wh/kg.

2) in the long run, innovative lithium-ion batteries are more feasible than lithium sulfur and lithium air. The development of high-capacity lithium-rich anode based on anionic charge compensation mechanism can develop power batteries with specific energy greater than 500wh/kg.

3) safety determines the prospect of high specific energy battery loading application. The development of spontaneous thermal control technology and all-solid-state battery is a feasible solution, which needs to be intensified.

4) high-load electrode is the basis to realize the high specific energy of the battery. According to the new polarization end, the development of gradient porosity electrode has an important role and significance for the development of high-specific energy battery.

The page contains the contents of the machine translation.