Improvement methods for low-temperature performance of lithium-ion batteries

Ionic batteries have been 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, such as a significant decrease in energy density under low temperature conditions and a corresponding impact on cycle life, which seriously limits the scale of use of lithium-ion batteries.
At present, there is still debate among researchers about the main factors that cause poor low-temperature performance of lithium-ion batteries, but there are three reasons for this:
1. At low temperatures, the viscosity of the electrolyte increases and the conductivity decreases;
2. The facial mask impedance and charge transfer impedance of electrolyte/electrode interface increase;
3. The migration rate of lithium ions in the active substance decreases As a result, electrode polarization intensifies at low temperatures and the charging and discharging capacity decreases.
In addition, during low-temperature charging, especially during low-temperature high rate charging, lithium metal will precipitate and deposit on the negative electrode. The deposited metal lithium will easily react irreversibly with the electrolyte and consume a large amount of electrolyte. At the same time, the thickness of the SEI film will further increase, leading to the further increase of the impedance of the facial mask on the negative electrode of the battery, and the further enhancement of the battery polarization, which will greatly damage the low-temperature performance, cycle life and safety performance of the battery.
This article systematically explores the main influencing factors on the low-temperature performance of lithium-ion batteries from three aspects: positive electrode materials, electrolytes, and negative electrode materials, and proposes effective methods to improve the low-temperature performance of lithium-ion batteries.

1、 Positive electrode material
The positive electrode material is one of the key materials for manufacturing lithium-ion batteries, and its performance directly affects the various indicators of the battery. The structure of the material has an important impact on the low-temperature performance of lithium-ion batteries.  
The LiFePO4 with olivine structure has the advantages of high discharge specific capacity, stable discharge platform, stable structure, excellent cycling performance, and abundant raw materials, making it the mainstream positive electrode material for lithium-ion power batteries. However, lithium iron phosphate belongs to the Pnma space group, with P occupying tetrahedral positions and transition metal M occupying octahedral positions. Li atoms form one-dimensional migration channels along the [010] axis. This one-dimensional ion channel results in lithium ions only being able to escape or embed in an ordered manner in a single way, severely affecting the diffusion ability of lithium ions in the material. Especially at low temperatures, the diffusion of lithium ions in the body is further hindered, leading to an increase in impedance and more severe polarization, resulting in 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 similar to LiCoO2. This material has important advantages such as high reversible specific capacity, good cycling stability, and moderate cost. It has also been successfully applied in the field of power batteries, and its application scale has rapidly developed. However, there are also some urgent problems that need to be solved, such as low electronic conductivity, poor high rate stability, especially with the increase of nickel content, the high and low temperature performance of the material deteriorates, and so on.
Rich lithium manganese based cathode materials have higher discharge specific capacity and are expected to become the next generation of lithium-ion battery cathode materials. However, there are many problems with the practical application of lithium rich manganese based materials: firstly, the irreversible capacity is high, and during the charging and discharging process, it is easy to transform from a layered structure to a spinel structure, causing the diffusion channels of Li+to be blocked by transition metal ions that migrate, resulting in severe capacity decay. At the same time, the ion and electronic conductivity of the material itself is poor, leading to poor rate performance and low-temperature performance.

The mainstream ways to improve the ion diffusion performance of cathode materials at low temperatures are:
The method of using materials with excellent conductivity to surface coat the active substance body enhances the conductivity of the positive electrode material interface, reduces interface impedance, and reduces side reactions between the positive electrode material and electrolyte, stabilizing the material structure.
Rui et al. studied the low-temperature performance of carbon coated LiFePO4 using cyclic voltammetry and AC impedance methods, and found that its discharge capacity gradually decreased with decreasing temperature. At -20 ° C, the capacity was only 33% of the room temperature capacity. The author believes that as the temperature decreases, the charge transfer impedance and Weber impedance in the battery gradually increase, and the difference in redox potential in the CV curve increases. This indicates that the diffusion of lithium ions in the material slows down at low temperatures, and the Faraday reaction kinetics rate of the battery weakens, resulting in a significant increase in polarization (Figure 1).

                                               Figure 1 CV (A) and EIS (B) curves of LFP/C at different temperatures

Lv et al. designed and synthesized a composite cathode material with fast ion conductors coated with nickel cobalt manganese oxide lithium. The composite material exhibited superior low-temperature and rate performance, maintaining a reversible capacity of 127.7mAh · g-1 at -20 ° C, far superior to the nickel cobalt manganese oxide lithium material of 86.4mAh · g-1. The introduction of fast ion conductors with excellent ionic conductivity effectively improves the Li+diffusion rate, providing a new approach for improving the low-temperature performance of lithium-ion batteries.
By doping the material body with elements such as Mn, Al, Cr, Mg, F, etc., the interlayer spacing of the material is increased to improve the diffusion rate of Li+in the body, reduce the diffusion impedance of Li+, and thereby improve the low-temperature performance of the battery.
Zeng et al. prepared carbon coated LiFePO4 cathode materials using Mn doping. Compared with the original LiFePO4, its polarization was reduced to a certain extent at different temperatures, significantly improving the electrochemical performance of the material at low temperatures. Li et al. doped LiNi0.5Co0.2Mn0.3O2 material with Al and found that Al increased the interlayer spacing of the material, reduced the diffusion impedance of lithium ions in the material, and greatly improved its gram capacity at low temperatures.
The phase transition of lithium iron phosphate cathode material from lithium iron phosphate phase to lithium iron phosphate phase during charging is slower than that during discharge. Cr doping can promote the phase transition from lithium iron phosphate phase to lithium iron phosphate phase during discharge, thereby improving the rate performance and low-temperature performance of LiFePO4.
Reduce material particle size and shorten Li+migration path. It should be pointed out that this method will increase the specific surface area of the material, thereby increasing the side reactions with the electrolyte.
Zhao et al. investigated the effect of particle size on the low-temperature performance of carbon coated LiFePO4 materials and found that the discharge capacity of the material increased with the decrease of particle size at -20 ° C. This is because the diffusion distance of lithium ions is shortened, making the process of lithium deintercalation easier. Research by Sun et al. has shown that the discharge performance of LiFePO4 significantly decreases with the decrease of temperature, and materials with smaller particle sizes have higher capacity and discharge plateau.
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 films, playing a crucial role in the performance of SEI films. At low temperatures, the viscosity of the electrolyte increases, the conductivity decreases, the SEI film impedance increases, and the compatibility with positive and negative electrode materials deteriorates, greatly deteriorating the energy density and cycling performance of the battery.
At present, there are two ways to improve low-temperature performance through electrolytes:
1. By optimizing the solvent composition and using new electrolyte salts, the low-temperature conductivity of the electrolyte can be improved;
2. Use new additives to improve the properties of the SEI membrane, making it conducive to Li+conduction at low temperatures.

1. Optimize solvent composition
The low-temperature performance of an electrolyte is mainly determined by its low-temperature melting point. If the melting point is too high, the electrolyte is prone to crystallization and precipitation at low temperatures, seriously affecting its conductivity. Ethylene carbonate (EC) is the main solvent component in electrolytes, but its melting point is 36 ° C. Its solubility decreases or even precipitates in the electrolyte at low temperatures, which has a significant impact on the low-temperature performance of batteries. By adding low melting point and low viscosity components and reducing the solvent EC content, the viscosity and co melting 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, EC and poly (ethylene glycol) dimethyl ether. Only a glass transition temperature point appeared around -90 ° C, which greatly improved the performance of the electrolyte at low temperatures; At -60 ° C, its conductivity can still reach 0.014mS · cm-1, providing a good solution for the use of lithium-ion batteries at extremely low temperatures.
Chain carboxylate solvents have lower melting points and viscosity, and their dielectric constant is moderate, which has a good impact on the low-temperature performance of electrolytes. Dong et al. used ethyl acetate (EA) as a co solvent and lithium bis (trifluoromethyl) sulfonate as an electrolyte salt. The theoretical melting point of the electrolyte reached -91 ° C and the boiling point reached 81 ° C. The results showed that even at the extreme low temperature of -70 ° C, the ionic conductivity of the electrolyte still reached 0.2mS · cm-1. Combined with an organic electrode as the positive electrode and polyimide derived from 1,4,5,8-naphthalene anhydride as the negative electrode, the battery still had 70% of its room temperature capacity at -70 ° C.
Smart et al. have conducted extensive research on using chain carboxylic acid esters as electrolyte co solvents to improve the low-temperature performance of batteries. Research has shown that using ethyl acetate, ethyl propionate, methyl acetate, and methyl butyrate as co solvents for electrolytes is beneficial for improving the low-temperature conductivity of electrolytes, greatly improving the low-temperature performance of batteries.
2 New electrolyte salts
Electrolyte salts are an important component of electrolytes and a key factor in achieving excellent low-temperature performance. At present, the commercial electrolyte salt is lithium hexafluorophosphate, which forms a SEI film with high impedance, resulting in poor low-temperature performance. The development of new lithium salts is urgent. Lithium tetrafluoroborate anion has a small radius, is easy to associate, and has a lower conductivity than LiPF6. However, its charge transfer impedance is small at low temperatures, making it an electrolyte salt with good low-temperature performance.
Zhang et al. used LiNiO2/graphite as electrode material and found that the conductivity of LiBF4 was lower than that of LiPF6 at low temperatures. However, its capacity at -30 ° C was 86% of the room temperature capacity, while the LiPF6 based electrolyte was only 72% of the room temperature capacity. This is because the charge transfer impedance of LiBF4 based electrolyte is small, and the polarization at low temperatures is small, so the low-temperature performance of the battery is better. However, LiBF4 based electrolytes are unable to form a stable SEI film at the electrode interface, resulting in severe capacity degradation.
Lithium difluorooxalate borate (LiODFB), as an electrolyte of lithium salts, has high conductivity under high and low temperature conditions, allowing lithium-ion batteries to exhibit excellent electrochemical performance over a wide temperature range. Li et al. found that LiODFB/LiBF4 EC/DMS/EMC electrolyte has good low-temperature performance at low temperatures. Tests have shown that the capacity retention rate of graphite/Li button batteries after 20 weeks of cycling at -20 ° C and 0.5C is: LiODFB/LiBF4 EC/DMS/EMC (53.88%)>LiPF6 EC/DEC/DMC/EMC (25.72%), with the former having a much higher capacity retention rate than the latter. This electrolyte has good application prospects in low-temperature environments.
As a new type of lithium salt, LiTFSI has high thermal stability, low degree of association between anions and cations, and high solubility and dissociation in carbonate systems. At low temperatures, the higher conductivity and lower charge transfer impedance of the LiFSI 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 base solvent. The resulting electrolyte still had a high conductivity of 2mS · cm-1 at -40 ° C.

3 Additives
The SEI film has a significant impact on the low-temperature performance of batteries. It is an ion conductor and electronic insulator, serving as 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 sharply decreases, deepening the accumulation of surface charges on the electrode, leading to a decrease in graphite lithium insertion ability and enhanced polarization. By optimizing the composition and film-forming conditions of SEI films, improving the ion conductivity of SEI films at low temperatures is beneficial for improving the low-temperature performance of batteries. Therefore, developing film-forming additives with excellent low-temperature performance is currently a research hotspot.
Liu et al. studied the effect of FEC as an electrolyte additive on the low-temperature performance of graphite/Li half cells. The results showed that at a low temperature of -20 ° C, the electrolyte with 2% FEC added increased the capacity by 50% compared to the base electrolyte during initial discharge at -20 ° C, and the charging platform decreased by about 0.2V. XPS testing shows that the LiF content in the SEI film formed by adding FEC electrolyte is higher than that in the SEI film formed by not adding FEC electrolyte, which is beneficial for reducing the impedance of the SEI film at low temperatures and improving the low-temperature performance of the battery.
Yang et al. found that adding LiPO2F2 can significantly improve the low-temperature performance of LiNi0.5Co0.2Mn0.3O2/graphite soft pack batteries. The capacity retention rates of LiPO2F2 containing electrolyte batteries after 100 cycles at 0 ° C and -20 ° C were 96.7% and 91%, respectively, while the capacity retention rates of the base electrolyte were only 20.1% and 16.0% after 100 cycles. EIS tests were conducted on LiNi0.5Co0.2Mn0.3O2/Li and full cells as well as graphite/Li half cells. The results showed that the addition of LiPO2F2 can significantly reduce the SEI film impedance and charge transfer impedance of graphite negative electrodes, and reduce polarization at low temperatures.
Liao et al.'s research has shown that the addition of BS (butyl ketone, BS) in electrolytes is beneficial for improving the discharge capacity and rate performance of batteries at low temperatures. They have conducted in-depth discussions on the mechanism of BS using methods such as EIS and XPS. At -20 ° C, after adding BS, the impedance RSEI and Rct decreased from 4094 Ω and 8553 Ω to 3631 Ω and 3301 Ω, respectively, indicating that the addition of BS improved the charge transfer rate of lithium ions and greatly reduced polarization at low temperatures. XPS testing shows that BS is beneficial for the formation of SEI films, as it can form sulfur-containing compounds with low impedance. At the same time, it reduces the content of Li2CO3 in the SEI film, reduces the impedance of the SEI film, and improves the stability of the SEI film.
In summary, the conductivity and film-forming impedance of the electrolyte have a significant impact on the low-temperature performance of lithium-ion batteries. For low-temperature electrolytes, optimization should be comprehensively carried out from three aspects: electrolyte solvent system, lithium salts, and additives. For electrolyte solvents, a solvent system with low melting point, low viscosity, and high dielectric constant should be selected. Linear carboxylic acid ester solvents have excellent low-temperature performance, but they have a significant impact on cycling performance. It is necessary to match cyclic carbonates with high dielectric constant, such as EC and PC, for blending; For lithium salts and additives, the main consideration is to reduce the film-forming impedance and improve the migration rate of lithium ions In addition, increasing the lithium salt concentration appropriately at low temperatures can improve the conductivity of the electrolyte and enhance its low-temperature performance.
3、 Negative electrode material
The deterioration of diffusion kinetics of lithium ions in carbon negative electrode materials is the main reason limiting the low-temperature performance of lithium-ion batteries. Therefore, during the charging process, the electrochemical polarization of the negative electrode is significantly intensified, which can easily lead to the precipitation of metallic lithium on the surface of the negative electrode.
Luders et al. found that at -20 ° C, a charging rate exceeding C/2 significantly increases the amount of lithium metal precipitated. At C/2, the amount of lithium precipitated on the negative electrode surface is about 5.5% of the entire charging capacity, but at 1C, it will reach 9%. The precipitated lithium metal may further develop and eventually become lithium dendrites. Therefore, when the battery must be charged at low temperatures, it is necessary to choose a small current to charge the lithium-ion battery as much as possible, and fully store the lithium-ion battery after charging to ensure that the metal lithium precipitated from the negative electrode can react with graphite and be re embedded into the graphite negative electrode.
Zinth et al. conducted a detailed study on the lithium evolution behavior of NMC111/graphite 18650 lithium-ion batteries at low temperatures of -20 ° C using neutron diffraction and other methods. The battery was charged and discharged as shown in Figure 2, and Figure 3 shows a comparison of the phase changes of the graphite negative electrode when charged at C/30 and C/5 rates, respectively.
                                          

Figure 2: Relationship between Δ Q and time during charge discharge process at low temperature -20 ° C in neutron diffraction experiment

Figure 3 Comparison of phase changes in the negative electrode after charging at different rates (A) and leaving it idle for 20 hours (B)
From the graph, it can be seen that for two different charging rates, the lithium poor phase Li1-xC18 is very similar. The difference is mainly reflected in the LiC12 and LiC6 phases. In the early stage of charging, the phase change trend in the negative electrode under the two charging rates is relatively close. For the LiC12 phase, when the charging capacity reaches 95mAh, the change trend begins to appear different. When it reaches 1100mAh, there is a significant difference in the LiC12 phase under the two rates. When charging at a small rate of C/30, the decline rate of the LiC12 phase is very fast, but at a low rate of C/5, the decline rate of the LiC12 phase is very fast. It should be much slower, that is to say, due to the deterioration of the lithium insertion kinetics of the negative electrode at low temperatures, the rate of further lithium insertion into LiC12 to generate LiC6 phase decreases, which corresponds to this, The LiC6 phase increases very quickly at a low C/30 ratio, but it is much slower at a low C/5 ratio. This indicates that at a C/5 ratio, less Li is embedded in the crystal structure of graphite. However, at a C/5 ratio, the charging capacity of the battery is slightly higher than that at a C/30 ratio. The additional Li that is not embedded in the graphite negative electrode is likely to precipitate on the graphite surface in the form of metallic lithium, and the resting process after charging also indirectly confirms this
Zhang et al. used the EIS method to measure the trend of impedance parameters Re, Rf, and Rct of graphite/Li half cells with temperature changes, and found that all three increased with decreasing temperature. The growth rates of Re and Rf were roughly the same, while Rct grew faster. When the temperature dropped to -20 ° C, Rct became the main component of the total impedance of the battery, indicating that the deterioration of electrochemical reaction kinetics conditions was the main factor causing the deterioration of low-temperature performance.
Choosing appropriate negative electrode materials is a key factor in improving the low-temperature performance of batteries. Currently, the optimization of low-temperature performance is mainly achieved through negative electrode surface treatment, surface coating, doping to increase interlayer spacing, and controlling 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 generate more micro nano structured pores, which is beneficial for Li+transport and reduces impedance.
Zhang Lijin et al. treated the graphite with an oxide micro expansion layer, resulting in a decrease in the average grain size of the graphite and an increase in the amount of lithium ions embedded on the surface and edges of the carbon layer. The introduction of nanoscale pore structures on the graphite surface further increases the lithium ion storage space. Wu et al. used 5at% fluorine gas to fluorinate natural graphite at 550 ° C, and the electrochemical and cycling properties of the treated material were greatly improved.
2 Surface coating
Surface coating such as carbon coating and metal coating can not only avoid direct contact between the negative electrode and electrolyte, improve the compatibility between electrolyte and negative electrode, but also increase the conductivity of graphite, provide more embedded lithium sites, and reduce irreversible capacity. In addition, the interlayer spacing of soft or hard carbon materials is larger than that of graphite. Coating a layer of soft or hard carbon materials on the negative electrode is conducive to the diffusion of lithium ions, reducing the SEI film impedance, and thus improving the low-temperature performance of the battery. The conductivity of the negative electrode material was improved by surface coating with a small amount of Ag, resulting in excellent electrochemical performance at low temperatures.  
The Fe/Fe3C-CNF composite material developed by Li et al. exhibits excellent low-temperature performance, maintaining a capacity of 250mAh · g-1 even after 55 cycles at -5 ° C. Ohta et al. studied the effect of different negative electrode materials on the performance of lithium-ion batteries and found that both carbon coated artificial graphite and natural graphite have significantly reduced irreversible capacity compared to uncoated ones. At the same time, carbon coated graphite negative electrode can effectively improve the low-temperature performance of the battery. The discharge capacity retention rate of 5% coated graphite at -5 ° C is 90% of that at room temperature. Nobili et al. used graphite coated with metallic tin as the negative electrode material. At -20 ° C, the SEI film impedance and charge transfer impedance of the material were reduced by 3 and 10 times, respectively, compared to the uncoated material. This indicates that tin coating can reduce the polarization of the battery at low temperatures, thereby improving the low-temperature performance of the battery.
3. Increase the interlayer spacing of graphite
The interlayer spacing of graphite negative electrode is small, and the diffusion rate of lithium ions between graphite layers decreases at low temperatures, leading to increased polarization. Introducing elements such as B, N, S, and K during graphite preparation can modify the structure of graphite, increase its interlayer spacing, and improve its lithium removal/insertion ability. The atomic radius of P (0.106pm) is larger than that of C (0.077pm), and doping P can increase the interlayer spacing of graphite, enhance the diffusion ability of lithium ions, and possibly increase the content of graphite microcrystals in carbon materials. The introduction of K into carbon materials will form an insertion compound KC8. When potassium is removed, the interlayer spacing of the carbon material increases, which is conducive to the rapid insertion of lithium and thus improves the low-temperature performance of the battery.
4. Control the size of negative electrode particles
Huang et al. studied the effect of negative electrode particle size on low-temperature performance and found that coke negative electrodes with average particle sizes of 6 μ m and 25 μ m respectively have the same reversible charge discharge capacity at room temperature. However, at -30 ° C, coke electrodes with particle sizes of 25 μ m can only release 10% of room temperature capacity, while coke electrodes with particle sizes of 6 μ m can release 61% of room temperature capacity.
From this experimental result, it can be concluded that the larger the negative electrode particle size, the longer the lithium ion diffusion path and the greater the diffusion impedance, resulting in increased concentration polarization and poorer low-temperature performance. Therefore, appropriately reducing the particle size of the negative electrode material can effectively shorten the migration distance of lithium ions between graphite layers, reduce diffusion impedance, increase the electrolyte infiltration area, and thereby improve the low-temperature performance of the battery. In addition, graphite negative electrodes made by small particle size single particle granulation have high isotropy, which can provide more lithium insertion sites, reduce polarization, and significantly improve the low-temperature performance of batteries.
4、 Conclusion
In summary, the low-temperature performance of lithium-ion batteries is a key factor restricting their application, and how to improve the low-temperature performance of lithium-ion batteries is still a hot and difficult research topic.
The reaction process of the battery system mainly includes four steps: Li+transport in the electrolyte, passing through the electrolyte/electrode interface facial mask, charge transfer and Li+diffusion in the active substance body. At low temperatures, the rate of each step decreases, resulting in an increase in the impedance of each step, which intensifies electrode polarization and leads to a decrease in low-temperature discharge capacity and lithium deposition on the negative electrode.
Improving the low-temperature performance of lithium batteries should comprehensively consider the influence of various factors such as the positive electrode, negative electrode, and electrolyte in the battery. By optimizing the composition of electrolyte solvents, additives, and lithium salts, the conductivity of the electrolyte can be improved while reducing the film-forming impedance; Doping, coating, and small particle modification are carried out on the positive and negative electrode materials to optimize the material structure, reduce the interface impedance and Li+diffusion impedance in the active material body. By optimizing the overall battery system and reducing the polarization of lithium batteries at low temperatures, the low-temperature performance of the battery is further improved.