Lithium-ion batteries are widely used in consumer electronics, electric vehicles and energy storage due to their high specific energy and power density, long cycle life, and environmental friendliness. As the power source of new energy vehicles, lithium-ion batteries still have many problems in practical applications. For example, the energy density is significantly reduced under low temperature conditions, and the cycle life is also affected accordingly, which also severely limits the large-scale use of lithium-ion batteries.

At present, researchers are still arguing about the important factors that cause the poor low-temperature performance of lithium-ion batteries, but the reasons are as follows:

1. The viscosity of the electrolyte increases at low temperatures and the conductivity decreases;

2. The membrane impedance and charge transfer impedance of the electrolyte/electrode interface increase;

3. The migration rate of lithium ions in the body of the active material is reduced. As a result, the electrode polarization is increased at low temperatures and the charge and discharge capacity is reduced.

In addition, during low-temperature charging, 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 is further increased, and the polarization of the battery is increased again, which will greatly destroy the low-temperature performance, cycle life and safety performance of the battery.

This article reviews the research progress of low-temperature performance of lithium-ion batteries, and systematically analyzes the important limiting factors of low-temperature performance of lithium-ion batteries. From the three aspects of positive electrode, electrolyte and negative electrode, the modification methods that researchers have used to improve the low-temperature performance of the battery in recent years are discussed.

1. Cathode material

The cathode material is one of the key materials for the manufacture of lithium-ion batteries, and its performance directly affects the various indicators of the battery, and the structure of the material has an important impact on the low-temperature performance of the lithium-ion battery.

LiFepO4 with olivine structure has the advantages of high discharge specific capacity, stable discharge platform, stable structure, excellent cycle performance, and abundant raw materials. It is the mainstream cathode material for lithium-ion power lithium batteries. However, lithium iron phosphate belongs to the pnma space group, p occupies the tetrahedral position, the transition metal M occupies the octahedral position, and the Li atom forms a migration channel along the [010] axis in a one-dimensional direction. This one-dimensional ion channel causes the lithium ion only The orderly extraction or insertion in a single way seriously affects the diffusion ability of lithium ions in the material. Especially at low temperatures, the diffusion of lithium ions in the body is further hindered, resulting in an increase in impedance, resulting in more serious polarization and poor low-temperature performance.

Nickel-cobalt-manganese-based LiNixCoyMn1-x-yO2 is a new type of solid solution material developed in recent years, which has a single-phase layered structure of α-NaFeO2 similar to LiCoO2. The material has important advantages such as high reversible specific capacity, good cycle stability, and moderate cost. It has also been successfully applied in the field of power lithium batteries, and its application scale has been rapidly developed. However, there are some problems that need to be solved urgently, such as low electronic conductivity and poor stability of large rates, especially as the nickel content increases, the high and low temperature performance of the material deteriorates.

The lithium-rich manganese-based layered cathode material has a higher discharge specific capacity and is expected to become the next generation of lithium-ion battery cathode materials. However, there are many problems in the practical application of lithium-rich manganese bases: the first time the irreversible capacity is high, and the layered structure is easily transformed into the spinel structure during the charge and discharge process, which makes the Li+ diffusion channel blocked by the migrated transition metal ions. It causes serious capacity degradation, and poor ion and electronic conductivity, resulting in poor rate performance and low temperature performance.

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

1. The method of surface coating 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.

Rui et al. used cyclic voltammetry and AC impedance methods to study the low-temperature performance of carbon-coated LiFepO4 and found that the discharge capacity gradually decreases as the temperature decreases, and the capacity at -20°C is only 33% of the normal temperature capacity. The author believes that as the temperature decreases, the charge transfer impedance and the Weber impedance in the battery gradually increase, and the difference in the redox potential in the CV curve increases, which indicates that the diffusion of lithium ions in the material slows down at low temperatures, and the Faraday of the battery The weakening of the reaction kinetic rate caused a significant increase in polarization (Figure 1).

Lv et al. designed and synthesized a composite cathode material coated with lithium nickel cobalt manganate with a fast ion conductor. The composite material shows superior low temperature performance and rate performance, and maintains a reversible capacity of 127.7mAhg-1 at -20°C. It is much better than the 86.4mAhg-1 of nickel-cobalt-manganese lithium material. The introduction of fast ion conductors with excellent ion conductivity can effectively improve the Li+ diffusion rate, which provides new ideas for improving the low-temperature performance of lithium-ion batteries.

2 The material body is doped in bulk with Mn, Al, Cr, Mg, F and other elements, and the layer spacing of the new material is added 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 .

Zeng et al. used Mn doping to prepare carbon-coated LiFepO4 cathode material. Compared with the original LiFepO4, its polarization at different temperatures is reduced to a certain extent, which significantly improves the electrochemical performance of the material at low temperatures. Li et al. doped the LiNi0.5Co0.2Mn0.3O2 material with Al and found that Al increases the interlayer spacing of the material, reduces the diffusion resistance of lithium ions in the material, and greatly increases the gram capacity at low temperatures.

The phase transition of the lithium iron phosphate cathode material from the lithium iron phosphate phase to the iron phosphate phase during the charging process is slower than the phase transition from the iron phosphate phase to the lithium iron phosphate phase during the discharge process, and Cr doping can promote the discharge process from the iron phosphate phase To the phase transition between lithium iron phosphate phases, thereby improving the rate performance and low temperature performance of LiFepO4.

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

Zhao et al. studied the effect of particle size on the low-temperature performance of carbon-coated LiFepO4 materials, and found that the discharge capacity of the material at -20°C increases with the decrease of the particle size. This is because the diffusion distance of lithium ions is shortened, which makes The process of deintercalating lithium becomes easier. Research by Sun et al. has shown that the discharge performance of LiFepO4 decreases significantly as the temperature decreases, and materials with small particle sizes have higher capacity and discharge platforms.

2. Electrolyte

As an important component of lithium-ion batteries, electrolyte not only determines the migration rate of Li+ in the liquid phase, but also participates in the formation of SEI film, which plays a key role in the performance of 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 the conduction of Li+ 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 will easily crystallize out at low temperatures, which will seriously affect the conductivity of the electrolyte. Ethylene carbonate (EC) is an important 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.

Kasprzyk et al. obtained an amorphous electrolyte by mixing two solvents of EC and poly(ethylene glycol) dimethyl ether. Only a glass transition temperature point appeared near -90°C. This amorphous electrolyte was extremely Dadi improves the performance of the electrolyte at low temperatures; at -60°C, its conductivity can still reach 0.014mScm-1, which provides a good solution for the use of lithium-ion batteries at extremely low temperatures.

Chain carboxylic acid ester solvents have lower melting point and viscosity, and their dielectric constant is moderate, which has a good influence on the low temperature performance of the electrolyte. Dong et al. used ethyl acetate (EA) as the co-solvent and lithium bistrifluoromethanesulfonate as the electrolyte salt. The theoretical melting point of the electrolyte reached -91°C and the boiling point reached 81°C. The results show that even at the extreme low temperature of -70°C, the ionic conductivity of the electrolyte can still reach 0.2mScm-1, combined with an organic electrode as a positive electrode and 1,4,5,8-naphthalene anhydride-derived polyimide As a negative electrode, the battery still has 70% of its normal temperature capacity at -70°C.

Smart and others have done a lot of research on the use of chain carboxylic acid esters as electrolyte co-solvents to improve the low-temperature performance of batteries. Studies have shown that using ethyl acetate, ethyl propionate, methyl acetate, and methyl butyrate as the electrolyte co-solvent is beneficial to the improvement of the low-temperature conductivity of the electrolyte and greatly improves the low-temperature performance of the battery.

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 low charge transfer resistance at low temperatures, and has good low temperature performance as an electrolyte salt.

Zhang et al. used LiNiO2/graphite as the electrode material and found that the conductivity of LiBF4 at low temperature is lower than that of LipF6, but its capacity at -30°C is 86% of the capacity at room temperature, while the LipF6 based electrolyte is only 72% of the capacity at room temperature. This is due to the low charge transfer resistance of LiBF4-based electrolyte and low polarization at low temperatures, so the low temperature performance of the battery is better. However, the LiBF4-based electrolyte cannot form a stable SEI film on the electrode interface, causing serious capacity degradation.

Lithium difluorooxalate borate (LiODFB) as the electrolyte of the lithium salt has high conductivity under high and low temperature conditions, so that the lithium ion battery exhibits excellent electrochemical performance in a wide temperature range. Research by Li et al. found that LiODFB/LiBF4-EC/DMS/EMC electrolyte has good low temperature performance at low temperatures. Tests show that the capacity retention rate of graphite/Li button batteries at a low temperature of -20°C and 0.5C for 20 weeks is as follows: LiODFB/LiBF4EC/DMS/EMC (53.88%) LipF6EC/DEC/DMC/EMC (25.72%), the former has a much higher capacity retention rate than the latter. This electrolyte has a good application prospect in low temperature environments.

As a new type of lithium salt, LiTFSI has high thermal stability, a small degree of association of anion and cation, and high solubility and dissociation in carbonate systems. Under low temperature conditions, the higher conductivity and lower charge transfer resistance of the LiFSI system electrolyte ensure its low temperature performance. Mandal et al. used LiTFSI as the lithium salt and EC/DMC/EMC/pC (mass ratio 15:37:38:10) as the basic solvent. The resulting electrolyte still has a high conductivity of 2mScm-1 at -40°C.