Low-temperature lithium ion battery performance improvement method

The main factors for the poor low-temperature performance of lithium-ion batteries are still debated. The main reasons are: the viscosity of the electrolyte increases at low temperatures and the conductivity decreases; the electrolyte/electrode interface membrane resistance and charge transfer resistance increase; lithium ions are in the active material body The migration rate in the medium decreases. As a result, the electrode polarization at low temperatures increases, and the charge and discharge capacity decreases.

During low-temperature charging of lithium-ion batteries, especially during low-temperature high-rate charging, lithium metal precipitation and deposition will occur in the negative electrode. The deposited metal lithium is easy to irreversibly react with the electrolyte and consumes a large amount of electrolyte. At the same time, the thickness of the SEI film is further increased, resulting in The impedance of the negative electrode surface film of the battery further increases, and the polarization of the battery increases again, which will greatly damage the low-temperature performance, cycle life and safety performance of the battery.

Low-temperature lithium-ion battery performance improvement method A modification method to improve the low-temperature performance of the battery from three aspects: positive electrode, electrolyte and negative electrode.

1. Cathode material

The mainstream ways to improve the ion diffusion performance of cathode materials at low temperature are:

1 The surface coating method of the active material body with excellent conductivity materials improves the conductivity of the positive electrode material interface, reduces the interface impedance, while reducing the side reactions of the positive electrode material and the electrolyte, and stabilizing the material structure.

2 The material body is doped in bulk with Mn, Al, Cr, Mg, F and other elements, and the layer spacing of the material is increased to increase the diffusion rate of Li+ in the body, reduce the diffusion resistance of Li+, and improve the low-temperature performance of the battery.

3 Reduce the material particle size and shorten the Li+ migration path. It should be pointed out that this method will increase the specific surface area of the material and increase the side reaction with the electrolyte.

2. Electrolyte

As an important part of the lithium ion battery, the electrolyte not only determines the migration rate of Li+ in the liquid phase, but also participates in the formation of the SEI film, which plays a key role in the performance of the SEI film. At low temperatures, the viscosity of the electrolyte increases, the conductivity decreases, the impedance of the SEI film increases, and the compatibility with the positive and negative materials deteriorates, which greatly deteriorates the energy density and cycle performance of the battery.

At present, there are two ways to improve low temperature performance through electrolyte:

(1) Improve the low-temperature conductivity of the electrolyte by optimizing the composition of the solvent and using new electrolyte salts;

(2) Use new additives to improve the properties of the SEI film, making it conducive to Li+ conduction at low temperatures.

1 Optimize solvent composition

The low-temperature performance of the electrolyte is mainly determined by its low-temperature eutectic point. If the melting point is too high, the electrolyte is easy to crystallize at low temperatures, which will seriously affect the conductivity of the electrolyte. Ethylene carbonate (EC) is the main solvent component of the electrolyte, but its melting point is 36°C, and its solubility in the electrolyte decreases or even precipitates at low temperatures, which has a greater impact on the low-temperature performance of the battery. By adding low melting point and low viscosity components to reduce the solvent EC content, the viscosity and eutectic point of the electrolyte at low temperatures can be effectively reduced, and the conductivity of the electrolyte can be improved.

2 New electrolyte salt

Electrolyte salt is one of the important components of electrolyte, and it is also a key factor to obtain excellent low temperature performance. At present, the commercial electrolyte salt is lithium hexafluorophosphate, and the formed SEI film has a large impedance, resulting in poor low-temperature performance. The development of a new type of lithium salt is imminent. Lithium tetrafluoroborate has a small anion radius, is easy to associate, and has a lower conductivity than LiPF6, but has a low charge transfer resistance at low temperatures, and has good low temperature performance as an electrolyte salt.

3 additives

The SEI film has a very important effect on the low temperature performance of the battery. It is an ionic conductor and an electronic insulator, and it is a channel for Li+ to reach the electrode surface from the liquid phase. At low temperatures, the impedance of the SEI film increases, and the diffusion rate of Li+ in the SEI film decreases sharply, which deepens the accumulation of charges on the surface of the electrode, resulting in a decrease in the lithium insertion capacity of the graphite and an increase in polarization. By optimizing the composition and film-forming conditions of the SEI film, improving the ionic conductivity of the SEI film at low temperatures is beneficial to the improvement of the low-temperature performance of the battery. Therefore, the development of film-forming additives with excellent low-temperature performance is a current research hotspot.

In summary, the conductivity and film-forming resistance of the electrolyte have an important influence on the low-temperature performance of lithium-ion batteries. For the low-temperature electrolyte, it should be comprehensively optimized from three aspects: electrolyte solvent system, lithium salt and additives. For the electrolyte solvent, a solvent system with low melting point, low viscosity and high dielectric constant should be selected. Linear carboxylate solvents have excellent low temperature performance, but they have a greater impact on the cycle performance, and need to match the cyclic carbonic acid with high dielectric constant Blending of esters such as EC and PC; for lithium salts and additives, the main consideration is to reduce the film-forming resistance and increase the migration rate of lithium ions. In addition, appropriately increasing the lithium salt concentration at low temperatures can increase the conductivity of the electrolyte and increase the low temperature performance.

3. Anode material

The deterioration of the diffusion kinetics conditions of lithium ions in carbon anode materials is the main reason that limits the low-temperature performance of lithium-ion batteries. Therefore, the electrochemical polarization of the anode is significantly intensified during the charging process, which easily leads to the precipitation of metallic lithium on the surface of the anode.

Choosing a suitable anode material is a key factor to improve the low-temperature performance of the battery. At present, the optimization of the low-temperature performance is mainly carried out through the methods of surface treatment of the anode, surface coating, doping to increase the layer spacing, and control of particle size.

1 Surface treatment

Surface treatment includes surface oxidation and fluorination. Surface treatment can reduce the active sites on the graphite surface, reduce irreversible capacity loss, and can generate more micro-nano structure pores, which is conducive to Li+ transmission and reduces impedance.

2 Surface coating

Surface coatings such as carbon coating and metal coating can not only avoid direct contact between the negative electrode and the electrolyte, improve the compatibility of the electrolyte and the negative electrode, but also increase the conductivity of graphite, provide more lithium insertion sites, and make Irreversible capacity reduction. In addition, the layer spacing of soft carbon or hard carbon material is larger than that of graphite. Coating a layer of soft carbon or hard carbon material on the negative electrode facilitates the diffusion of lithium ions and reduces the resistance of the SEI film, thereby improving the low temperature performance of the battery. The surface coating of a small amount of Ag improves the conductivity of the negative electrode material, making it have excellent electrochemical performance at low temperatures.

3 Increase the spacing between graphite layers

The graphite anode has a small interlayer spacing, and the diffusion rate of lithium ions between graphite layers at low temperatures decreases, resulting in increased polarization. The introduction of B, N, S, K and other elements in the graphite preparation process can modify the structure of graphite and increase The interlayer spacing of graphite improves its ability to release/intercalate lithium. The atomic radius of P (0.106pm) is larger than that of C (0.077pm). P doping can increase the interlayer spacing of graphite, enhance the diffusion ability of lithium ions, and possibly improve it. The content of graphite crystallites in carbon materials. The introduction of K into the carbon material will form the intercalation compound KC8. When the potassium is removed, the interlayer spacing of the carbon material will increase, which is beneficial to the rapid insertion of lithium, thereby improving the low-temperature performance of the battery.

4 Control the size of negative electrode particles

The larger the negative electrode particle size, the longer the lithium ion diffusion path and the greater the diffusion resistance, which leads to increased concentration polarization and poor low-temperature performance. Therefore, appropriately reducing the particle size of the negative electrode material can effectively shorten the migration distance of lithium ions between the graphite layers, reduce the diffusion resistance, increase the electrolyte infiltration area, and improve the low-temperature performance of the battery. In addition, the graphite negative electrode granulated by a small particle size single particle has higher isotropy, can provide more lithium insertion sites, reduce polarization, and can significantly improve the low-temperature performance of the battery.

The low-temperature performance of lithium-ion batteries is a key factor restricting the application of lithium batteries. How to improve the low-temperature performance of lithium batteries is still a hot and difficult point of current research. To improve the low-temperature performance of lithium batteries, the influence of comprehensive factors such as the positive electrode, negative electrode, and electrolyte in the battery should be considered. By optimizing the electrolyte solvent, additives and lithium salt composition, the conductivity of the electrolyte is improved, and the film formation resistance is reduced; The electrode material undergoes modification treatments such as doping, coating, and granulation to optimize the material structure and reduce the interface resistance and the diffusion resistance of Li+ in the active material body. Through the overall optimization of the battery system, the polarization of the lithium battery at low temperatures is reduced, and the low temperature performance of the battery is further improved.