The development of electric vehicles is in full swing, and the power battery is one of the most important parts. Its development has a decisive effect on the battery life and safety of electric vehicles. Recently, we often hear some terms such as solid-state batteries, SVOLT’s jelly batteries, NIO’s Nickel 55 ternary Cell, IM Motors doped with silicon to supplement lithium, and CTP/CTC technology. In fact, with so many technical directions, the fundamental purpose is to improve the energy density and safety of the battery. In this article, the editor will take you to sort out the technical paths related to it.

Ways to improve energy density and safety

Engineers racked their brains in order to increase the energy density of the battery pack, using similar two paths: increasing the density of the battery cell and increasing the density of the system (battery pack). Of course, while improving energy density, safety is always the top priority. In order to improve the energy density and safety of the battery, what efforts have the majority of engineers made and what new technologies are currently emerging? Now we will discuss with the latest news.

Increase the energy density of batteries

The battery core is composed of three parts, the positive electrode, the negative electrode and the electrolyte between the positive and negative electrodes. To increase the energy density, we start with these three aspects. Let's look at them one by one.

Cathode-Nickel 55 single crystal material

The 100kWh battery pack recently released by NIO, which is the “only smoke but no fire” battery previously announced by the CATL, has increased its energy density by 37% without changing the size of the battery pack shell and almost no increase in weight. recharge mileage. The nickel 55 ternary cell used in the new battery is an important factor in increasing the energy density. Its cathode material is a high-voltage single crystal material. What is single crystal? Before answering this question, let's take a look at the technical direction of cathode materials.

The so-called "ternary" lithium battery refers to the three elements of nickel, cobalt and manganese (NCM) in the positive electrode material. Nickel is used to increase the capacity, and cobalt is used to stabilize the structure. The role of manganese is to reduce costs and improve the structural stability of the material. The higher the nickel ratio and the lower the cobalt and manganese ratio, the greater the energy density, but the safety is reduced.

In order to increase the energy density, the NCM ratio has been increased from "111 (N:C:M=1:1:1)" to "523" and then to "811". This route has always been the mainstream direction for the development of ternary cathode materials. The other direction corresponds to the single crystal route (the key point is here). The newly released battery cathode uses single crystal 5 series materials. Single crystal materials are more suitable for high voltage. At present, most of the commercialized ternary cathode materials are secondary spherical polycrystalline materials of about 10 microns formed by agglomeration of nano-level primary particles. For those who have no concept of polycrystalline and single crystal, please refer to quartz sand and glass. Both are silica. Quartz sand is a polycrystalline material, while glass can be regarded as a single crystal material.

There are a large number of grain boundaries in the polycrystalline NCM. During the charging and discharging process of the battery, due to the anisotropic crystal lattice change, the polycrystalline NCM is prone to grain boundary cracking, causing the secondary particles to break, the specific surface area and the interface pair The response increases rapidly, which leads to a rise in battery impedance and a rapid decline in performance. There is no grain boundary inside the single crystal ternary material, which can effectively deal with the problem of grain boundary fracture and the performance degradation caused by it. Therefore, the single crystal structure can achieve higher voltage, not only that, but also improves the cycle stability of the ternary material, and greatly improves the safety of the battery. This is the cathode material, let's look at the anode.

What is the "silicon-doped lithium battery cell" technology?

The density of graphite negative electrodes of traditional lithium-ion batteries is low. In order to pursue high density, new negative electrode materials silicon carbon and silicon oxygen have become new hotspots pursued by enterprises. However, silicon-oxygen will have the problem of low efficiency for the first time and the need to supplement lithium. During the first charge and discharge of liquid lithium-ion batteries, the electrode material and electrolyte react at the solid-liquid interface to form a passivation layer covering the surface of the electrode material. This passivation layer is an interface layer with the characteristics of a solid electrolyte. It is an electronic insulator but an excellent conductor of Li+. Li+ can be freely embedded and extracted through the passivation layer. Therefore, this passivation film is called " "Solid electrolyte interface" (solid electrolyte interface) is abbreviated as SEI film (the positive electrode also has layers of film formation, but at this stage it is believed that its impact on the battery is far less than the SEI film on the surface of the negative electrode ). The silicon carbon negative electrode lithium supplement process is to pre-coat a layer of lithium metal on the surface of the silicon carbon negative electrode. The coating is in close contact with the negative electrode. After the electrolyte is poured into the negative electrode, it will react with the negative electrode and be embedded in the negative electrode particles. Make up for the Li ions needed to form or repair the SEI film during the first charge and discharge or cycle. Compared with the difficult and high-input negative electrode lithium supplement process, the positive electrode lithium supplement process is much simpler. The typical positive electrode lithium supplement process is to add a small amount of high-capacity positive electrode material to the positive electrode homogenization process. During the charging process, the excess Li element is extracted from these lithium-rich positive electrode materials and inserted into the negative electrode to supplement the irreversible capacity of the first charge and discharge. Through this complicated process of replenishing lithium, the density of the negative electrode material can be increased. At present, it is not known what kind of technology is IM Motors, but it is basically a foregone conclusion that IM Motors will use this high-end lithium battery. Finally, look at the last link in the improvement of cell energy density-electrolyte.

Electrolyte—Solid State Battery

On December 8, local time, Quantum Scape announced the news of its latest solid-state battery and stated that the battery will be put into production in 2024. This kind of solid-state batteries has a significant improvement over traditional lithium-ion batteries: they can increase the cruising range of electric vehicles by 80%. Let's discuss what is a solid-state battery and what are its benefits.

While increasing the energy density of the battery, the safety of the battery is an issue that has to be considered. The fundamental elimination of the safety hazards of lithium-ion batteries is still in the improvement of the safety of battery materials. But for cathode materials, these two aspects are contradictory. For example, as mentioned earlier, increasing the nickel content can increase the energy density, but the increase in the nickel content means lower safety. Is there any way to enhance the safety of the battery from other aspects, so as to increase the energy density more assuredly? At this time, it is necessary to consider from the perspective of electrolyte. A large number of studies have shown that the liquid electrolyte participates in most of the reactions in the thermal runaway process of the battery, and greatly reduces the initial reaction temperature of the battery, which means that the threshold for thermal runaway becomes lower. Therefore, improving electrolyte safety is one of the most effective ways to achieve battery safety. The physical properties of the liquid electrolyte determine that it cannot always avoid leakage, and it is also not conducive to reducing the volume of the battery and thus increasing the energy density. Therefore, in order to improve the energy density and safety, the solidification of the electrolyte has become a trend. We call a battery in which the electrodes and electrolyte are both solid-state batteries. The solid-state battery cell does not contain liquid, which is not only safer, but also can be assembled in series and parallel first, reducing the material used for the packaging shell, and greatly simplifying the PACK design, which also improves the energy density of the battery after it is assembled.

Similar to traditional lithium batteries, solid-state batteries consist of a positive electrode, a negative electrode, and an electrolyte. Its structure is simpler than traditional lithium batteries, and the solid electrolyte acts as the dual function of electrolyte and separator. There is no essential difference between the positive electrode material and the traditional lithium battery. The anode materials are lithium metal anode materials, carbon group anode materials and oxide anode materials. For solid-state batteries, the research and development of solid-state electrolytes are the most important. There are many types of materials, including oxides, sulfides, polymers, and composite solid electrolytes.

In addition to large-scale liquid lithium batteries and solid-state batteries under research, a semi-solid battery-jelly battery-has entered people's field of vision. In December 2020, Honeycomb Energy took the lead in releasing the jelly battery and accepting orders. The jelly battery is a lithium battery that uses a new type of jelly-like electrolyte. This gel-type electrolyte can better fit the surface of the electrode material. It has the characteristics of self-healing and flame retardant. At the same time, heat diffusion is prevented. Jelly batteries can be said to be a transition from liquid batteries to solid-state batteries.

Increased System Density-New Battery Pack Technology

In addition to increasing the energy density of battery cells, it is also a way to increase the energy density of batteries by having more batteries in a battery pack of the same size and weight. Here is a brief introduction to the current relatively new battery pack technology.

Remove the internal packaging-Cell to Pack (CTP) technology:

Generally, a battery not only has a battery pack on the outermost part, but also a group of "modules" formed by a group of cells inside. The so-called CTP is non-modularization, and the cells are directly packaged. It is currently a major choice for companies to increase energy density. CATL, BYD, and Honeycomb Energy have all launched module-less battery pack technology. The BYD blade battery, which was relatively popular a while ago, is based on lithium iron phosphate batteries, and uses a module-free design to improve space utilization.

All internal packaging and outsourcing are removed-Cell to Chassis (CTC) technology:

On Tesla’s battery day, a structural battery solution was proposed, in which the battery is directly built into the car structure (see Long Ge’s previous article "Interpretation of Tesla Battery Day Information"). This structured battery technology is similar to the CTC technology previously proposed by the CATL. This technology integrates the battery cell and the chassis, and then integrates the motor, electronic control, and vehicle high-voltage system through an innovative architecture. The power domain controller optimizes power distribution and reduces energy consumption.

Concluding remarks

Through the above introduction, We believe that everyone has a certain understanding of new battery-related technologies. Although the commercialization of all-solid-state batteries still requires us to wait patiently, we believe that semi-solid-state batteries, positive monocrystalline materials, and silicon-doped lithium-supplement technology will be experienced by us in the near future.