Improved charging lithium battery low temperature resistance from positive and negative electrode materials and electrolytic solution

Improved charging lithium battery low temperature resistance from positive and negative electrode materials and electrolytic solution

Lithium-ion batteries have been widely used in consumer electronics, electric vehicles and energy storage, etc.. As the power source of new energy vehicles, there is still a lot of problems in practical applications, such as low energy density under low temperature conditions, and the cycle life is also affected accordingly.

This also seriously limits the size of the lithium-ion battery.. At present, researchers have argued on the important factors that cause poor low temperatures in lithium-ion batteries, but they have the following factors: 1.

The viscosity of the electrolyte at low temperature increases, and the electrical conductivity is lowered; The film impedance and charge transfer impedance of liquid / electrode are increased; 3. The migration rate of lithium ions in the active material body is lowered. Thereby caused low temperature electrode polarization, and the charge and discharge capacitance is reduced.

. In addition, during the low temperature charging process, the negative electrode will cause lithium metal precipitation and deposition, deposition of metal lithium and electrolyte, and the electrolyte consume a large amount of electrolyte, and the thickness of the SEI film is further added, resulting in The impedance of the battery negative surface film is further increased, and the polarization of the battery is again enhanced, which will greatly destroy the low temperature performance, cycle life and safety performance of the battery..

This paper reviews the research progress of low temperature performance of lithium ion batteries, and systematically analyzes the important constraints of low temperature performance of lithium ion batteries.. From the three aspects of the positive, electrolyte, the negative electrode, researchers in recent years to improve the battery low temperature performance.

First, the positive material positive material is one of the key materials for producing a lithium ion battery, 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.. The LifePO4 discharge ratio of the olivine structure is high, the discharge platform is stable, the structure is stable, the cycle performance is excellent, and the raw material is rich, etc.

is the mainstream positive material of lithium-ion power lithium battery.. However, lithium lithium iron phosphate belongs to the PNMA space group, p occupying the tetrahedral position, the transition metal M occupy the octahedral position, the LI atom forms a migration passage in the [010] axis, which causes the lithium ion to only Optionally, it is detached or embedded, and the diffusion capability of lithium ions in the material is severe.

. Especially in the low temperature, the diffusion of lithium ions in the body causes an increase in impedance, resulting in a more severe polarization, poor low temperature performance..

Nickel-cobalt-yromide LiniXCoyMN1-X-YO2 is a new type of solid solution material developed in recent years, has a α-NAFEO2 single-phase layer structure similar to LiCoO2. The material has an important advantage of high inverse comparison capacity, good cycle stability, and cost-effective, and also achieved successful application in the field of power lithium battery, and application scale is rapidly developed..

However, there are some problems that need to be solved, such as low electron conductivity, poor magnification stability, especially with the improvement of nickel content, high and low temperature performance of materials, etc.. Lithium manganese-rich manganese-like positive material has a higher discharge ratio capacity, which is expected to become a lower generation lithium ion battery positive material.

However, lithium manganese groups have many problems in practical applications: the first non-reversible capacity is high, and it is easy to transition from the layered structure to the spine structure during the charge and discharge, so that the transition metal ion of the Li + diffuse channel is blocked. Creating capacity is serious, and at the same time it is poorly ion, and electron conductivity is poor, resulting in unparallel performance and low temperature performance..

Improve the mainstream mode of the positive electrode material at low temperature ion diffusion properties: 1 Method for surface coating of the active material body with material, the material body, to reduce the electrical conductivity of the positive electrode material, reduce the interface impedance, and reduce the positive material and electrolyte Sub-reactive, stable material structure. Rui et al. It has studied the low temperature performance of carbon-covered LIFEPO4 by cyclic voltammetry and AC impedance method.

It has found that the capacity of -20 ° C is only 33% of the temperature capacity when the discharge capacity is gradually decreased.. The authors believe that with the decrease in temperature, the charge transfer impedance and Weber impedance in the battery gradually becomes large.

The difference in oxidation potential in the CV curve increases, indicating that the diffusion of lithium ions in the material slow down at low temperatures, the Faraday of the battery The rate of reactive dynamics is reduced to increase (Figure 1). Figure 1LFP / C is designed with CV (a) and Eis (b) graph LV, etc. at different temperatures, and a composite positive material of lithium ion conductors covering nickel-cobalt-manganese acid is designed, and the composite material shows superior low temperatures.

Performance and magnification performance, in -20 ° C still maintains the reversible capacity of 127.7mAh · g-1, which is excellent in lithium nickel-cobalt-oxygen-yamate material 86.4mAh · g-1.

By introducing a fast ion conductor with excellent ionic conductivity to effectively improve the Li + diffusion rate, it is a new idea to improve the low temperature performance of lithium ion batteries.. 2 Take the material body in varying the material body by Mn, Al, Cr, Mg, and F, and the layer spacing of the material is increased by the layer spacing of the material to increase the diffusion rate of Li + in the body, reducing the diffusion impedance of Li +, and then enhances the low temperature performance of the battery.

. ZENG et al., Using Mn doping, prepared carbon-covered LIFEPO4 positive electrode material, which has a certain degree of decrease in the polarization of different temperatures, significantly lifting electrochemical properties under low temperature.

Li et al. For LINI0.5CO0.

2Mn0.3O2 material, discovered that Al increased the layer spacing of the material, reducing the diffusion impedance of lithium ions in the material, making it greatly improved at low temperatures..

Lithium iron phosphate positive electrode material is slowly changed from the phase transition phase between the lithium iron phosphate phase to the phosphoric acid iron phase to the phase transformation phase of the phosphate phase to the lithium iron phosphate phase, while Cr doping promotes discharge process from the phosphate phase To the phase transition between lithium iron phosphate, thereby improving the magnification performance and low temperature performance of LIFEPO4. 3 reduce material particle size, shorten Li + migration path. It should be pointed out that this method increases the specific surface area of ​​the material to increase the side reaction of the electrolyte.

. ZHAO et al, the effect of particle size on low temperature performance of carbon coated LiFePO4 materials was found that the discharge capacity of the material under -20 ° C increased as the particle size decreases, because the diffusion distance of the lithium ion is shortened, so that The process of deioning is easier. SUN et al.

Shows that as the temperature reduction is significantly reduced by LIFEPO4, the material having a small particle size has a high capacity and discharge platform.. Second, the electrolyte electrolyte is an important part of the lithium ion battery, but also determines the migration rate in Li + in the liquid phase, but also on the formation of SEI film, it is a key to the performance of SEI.

. The viscosity of the electrolyte at low temperature increases, the electrical conductivity is lowered, the SEI film impedance is increased, and the compatibility between the positive and negative electrode materials is deteriorated, and the energy density of the battery is greatly deteriorated, the cycle performance, etc..

At present, the low temperature performance by the electrolyte has the following route: (1) by optimizing the solvent, use a new type of electrolyte salt and other means to improve the low temperature conductivity of the electrolyte; (2) Improve the nature of the SEI film using the new additive It is conducive to Li + under low temperature. 1 Optimizing the low temperature performance of the solvent constitutive electrolyte is important to determine the low temperature melting point, if the melting point is too high, the electrolyte is easily obtained by crystal crystallization at a low temperature, and severely affects the electrical conductivity of the electrolyte..

The vinyl carbonate (EC) is an important solvent component of the electrolyte, but its melting point is 36 ° C, and the solubility in the electrolyte is low in electrolyte even precipitation, and the low temperature performance of the battery is large.. By adding low melting points and low viscosity components, the solvent EC content can be reduced, and the viscosity and total melting point of the electrolyte at low temperature can be effectively reduced, and the electrical conductivity of the electrolyte can be effectively reduced.

. Kasprzyk et al., By EC and poly (ethylene glycol) dimethyl ether, mixed with amorphous electrolyte, only a glass transition temperature point near -90 ° C, this amorphous electrolytic liquid The electrolyte increases the performance of the electrolyte at low temperature; at -60 ° C, its conductivity can still reach 0.

014 ms · cm-1, and supply a good solution for lithium ion batteries at very low temperatures.. The chain carboxylic acid ester solvent has a lower melting point and viscosity, while their dielectric constant has a good effect on the low temperature performance of the electrolyte.

. DONG et al. Is used as a co-solvent, and the metallic fluoromethylsulfonate is an electrolyte salt, the theoretical melting point of the electrolyte reaches -91 ° C, the boiling point reaches 81 ° C.

. The results show that even if the ionic conductivity is still 0.2 ms · cm-1 at a low temperature of -70 ° C, the ionic conductivity is still 0.

2 ms · cm-1, and the organic electrode is used as the positive electrode and 1,4,5,8-naphthal-derived polyachane. Impromine is a negative electrode, which still has 70% of normal temperature capacity at -70 ° C..

SMART et alternating chain carboxylic acid ester as an electrolytic solution to increase the low temperature performance of the battery.. Studies have shown that with ethyl acetate, ethyl propionate, methyl acetate, butyroate as an electrolyte coexisone is advantageous to improve the low temperature conductivity of electrolyte, greatly improved low temperature performance of the battery.

. 2 new electrolyte salt electrolyte salts is one of the important components of electrolyte, and it is also a key factor in obtaining excellent low temperature performance..

At present, commercial electrolyte salts are lithium hexafluorophosphate. The formation of SEI film impedance is large, resulting in poor low temperature performance, and the development of new lithium salts is imminent..

Lithium tetrafluoroborate is small, easy to associated, and the electrical conductivity is low, but the charge transfer impedance is small at low temperature, which has good low temperature performance as electrolyte salts.. ZHANG et al.

In LINIO2 / graphite, the study found that the electrical conductivity of LiBF4 at low temperatures was lower than LiPF6, but its low temperature -30 ° C was 86% of the normal temperature capacity, while LiPF6-based electrolyte was only 72% of normal temperature capacity. This is because the charge transfer impedance of the Libf4-based electrolyte is small, the polarization is small at low temperatures, so the low temperature performance of the battery is better..

However, the LiBF4-based electrolytic solution cannot form a stable SEI film in the electrode interface, resulting in severe capacity attenuation.. Lithium borophipate (LiODFB) as a lithium salt has a higher electrical conductivity under high and low temperature conditions, so that the lithium ion battery exhibits excellent electrochemical properties in the wide temperature range.

. Li et al. Found that LiODFB / LIBF4-EC / DMS / EMC electrolyte had good low temperature performance at low temperatures, indicating that the graphite / Li buckle battery at low temperature -20 ° C, 0.

5c cycle 20 weeks later capacity retention rate:: LIODFB / LIBF4EC / DMS / EMC (53.88%) LIPF6EC / DEC / DMC / EMC (25.72%), the former capacity retention rate is much higher than the latter, the electrolyte has good application prospects in a low temperature environment.

As a new lithium salt, Litfsi has a high thermal stability, the intensity of the intensity of the intensive, has a high solubility and degree of dissolution in the carbonate system.. In low temperatures, the higher electrical conductivity and lower charge transfer impedance of the LIFSI system electrolyte guarantees its low temperature performance.

. Mandal et al as a lithium salt, EC / DMC / EMC / PC (mass ratio 15: 37: 38: 10) is based on the solvent, and the resulting electrolyte still has a high conductivity of 2 ms · cm-1 under -40 ° C..

3 Additive SEI membranes have an important effect on the low temperature performance of the battery. It is an ionic conductor and an electron insulator, which is a channel from the surface of the liquid phase to the electrode..

At low temperatures, the SEI membrane impedance became large, and the diffusion rate of Li + in the SEI film decreased sharply, so that the amount of charge accumulation of electrode surface charges degraded, resulting in decreased graphite, polarization. By optimizing the composition of the SEI film and film formation conditions, the ion conductivity of the SEI film is advantageous for the low temperature energy of the SEI film facilitates the low temperature performance of the battery, and therefore the film formation additive excellent in development of low-temperature performance is the current research hotspot..

LiU et al, the results of FEC as the low temperature performance of the electrolytic solution additives were studied. The results showed that the graphite / Li semi-battery added 2% FEC electrolyte than the base electrolyte in -20 ° C at a low temperature of -20 ° C. When the first discharge is discharged, the capacity has increased by 50%, and the charging platform is reduced by about 0.

2V.. The XPS test showed that the content of the SEI film formed in the SEI film formed by the FEC electrolyte was high, which facilitates the reduction in the impedance of the SEI film at low temperatures, which in turn increases the battery.

Low temperature performance. Yang et al. Found that the addition of LIPO2F2 significantly improved the low temperature performance of lini0.

5co0.2mn0.3O2 / graphite soft boiler battery, which contains LiPO2F2 electrolyte batteries at low temperature 0 ° C and -20 ° C 100 weeks later, the capacity retention ratio is respectively 96.

7% and 91%, and the base electrolyte is only 20.1% and 16.0% after the cycle of 100 weeks.

. EIS testing for Lini0.5CO0.

2Mn0.3O2 / Li and a full battery and graphite / li semi-battery indicating that the addition of LiPO2F2 can significantly reduce graphite negative electrode SEI membrane impedance and charge transfer impedance, reduce the polarization of low temperature..

LIAO et al. Suggests that the addition of BS (BUTYLSULTONE, BS) in electrolyte is conducive to the improvement of battery discharge capacity and magnification performance at low temperatures, which uses EIS, XPS and other means to explore the use mechanism of BS..

At -20 ° C, the impedance RSEI and RCT were added after the BS, and the RCT was reduced from 4094Ω, 8553Ω, indicating that the addition of the BS increased the charge transfer rate of lithium ions, which greatly reduced the polarization of low temperature.. The XPS test indicates that the BS is conducive to the formation of the SEI film, which can form a sulfur-containing compound having a low impedance while reducing the content of Li2CO3 in the SEI film, reducing SEI membrane impedance while improving the stability of SEI membranes.

. In summary, the electrical conductivity of the electrolyte and film formation impedance impact on low temperature performance of lithium ion batteries..

About the low temperature type electrolyte, optimization should be optimized from three aspects of the electrolyte solvent system, lithium salt and additives.. Regarding the electrolyte solvent, the solvent system of low melting point, low viscosity, and high dielectric constant should be selected.

The linear carboxylate solvent is excellent, but the cycle performance has a large influence, a cyclic carbonate having high dielectric constant Esters such as EC, PC blending; about lithium salts and additives, important from reducing film-forming impedance, improve lithium ions migration rate. In addition, properly increase the lithium salt concentration at low temperatures to increase electrical conductivity of electrolyte, improve low temperature performance. Third, the diffusion kinetic conditions of the lithium ion in the carbon negative electrode material are important causes to limit the low temperature performance of lithium ion batteries, so the electrochemical polarization of the negative electrode in the process of charging, it is easy to cause the negative surface precipitation.

Metal lithium. Luders et al. Showed that at -20 ° C, the charging magnification exceeded C / 2 significantly added precipitation of metal lithium, at C / 2 ratio, the negative surface is about 5.

5% of the total charging capacity. However, it will reach 9% at 1C magnification, and the precipitated metal lithium may further develop, and finally become a lithium branch crystal..

Therefore, when the battery must be charged at low temperatures, the small current is selected to charge the lithium ion battery, and the lithium ion battery is sufficiently put on, thereby ensuring that the metal lithium of the negative electrode can be reacted, refunded. Embedded into the inner body of the graphite. ZINTH et al, etc.

is a detailed study of NMC111 / graphite 18650 lithium ion battery at low temperature -20 ° C by neutron diffraction, and the battery is charged and discharge, and Fig. 3 is in C. When charging is performed under / 30 and C / 5 times, the change in the phase change of the graphite.

Figure 2 Relationship between ΔQ and time of the low temperature -20 ° C under low temperature -20 ° C, Figure 3 Different magnification charging (a) and after 20 h (b) negative polarity phase change comparison can see two different different The charging magnification is very similar, and the difference is important. The difference is reflected in the two objects of LiC12 and LiIC6, and the change trend in the negative electrode in the negative electrode under the initial stage of charging is relatively close. Regarding the LIC12 phase, when the charging capacity reached 95 mAh, the trend of change begins with different trends.

When the 1100mAh is reached, the LIC12 object at the two magnifications begins a significant gap. When the C / 30 small rate is charged, the LIC12 object decreases. The speed is very fast, but the falling speed of the LiC12 phase at the C / 5 magnification is slow, that is to say, due to the deterioration of the lithium kinetic conditions of the negative electrode under low temperature, so that LiC12 is further incense to generate the speed drop of the LIC6 phase.

It corresponds to the corresponding, the LIC6 object has a very fast in C / 30 small magnification, but there is a slower in C / 5 times, which indicates that at C / 5 times, fewer Li Among the crystal structure of the graphite, but at the C / 5 charging magnification, the battery’s charging capacity is higher than the capacity of C / 30 charging magnification, which is not embedded to the graphite negative electrode. It may be in the form of a metal lithium in the form of graphite surface, and the standing process after the end of charging is also protected from the side ZHANG et al., The ES method, measures the impedance parameters of the graphite / Li semi-battery RE, RF and RCT with temperature changes with temperature change.

It is found that the three have increased with temperature, where the RE and RF rise rates are substantially the same, and the RCT rise rate is faster, and when the temperature is reduced to -20 ° C, the RCT has become an important part of the battery total impedance, this Indicates that the electrochemical reaction kinetic conditions deteriorate is an important factor in deterioration of low temperature performance.. Selecting a suitable negative electrode material is a key factor in increasing battery low temperature performance.

At present, it is important to optimize the optimization of low temperature performance through the surface treatment, surface coating, doping layer spacing, control particle size.. 1 Surface treatment surface treatment includes surface oxidation and fluorination.

Surface treatment can reduce the active site of the graphite surface, reduce non-reversible capacity loss, and can also generate more micro-tank structural apertures, which is advantageous for Li + transmission, reduce impedance. Zhang Lijin et alted, the grain size of the graphite is reduced, and the lithium ion is added to the surface of the carbon layer and the edge embedding amount, and the nano-stage pore structure introduced by the surface of the graphite surface has further increased lithium ion storage space..

WU et al. Takes 550% fluorine gas to treat natural graphite in 550 ° C, and the electrochemical performance and cycle performance of post-treatment materials are greatly improved..

2 Surface coating surfaces such as carbon cladding, metal cladding can not only prevent direct contact of the negative electrode and the electrolyte, improve the compatibility of the electrolyte and the negative electrode, but also new graphite conductivity, supply more embedded Lithium site, reduce non-reversible capacity. In addition, the layer spacing of soft carbon or hard carbon material is larger than graphite, and a layer of soft carbon or hard carbon material is conducive to the diffusion of lithium ions, reducing SEI membrane impedance, thereby increasing the low temperature performance of the battery..

The conductivity of the negative electrode material is improved by a small amount of Ag, which has excellent electrochemical properties at low temperatures.. The Fe / FE3C-CNF composites developed by Li et al.

Have good low temperature performance, and the capacity of 250 mAh · g-1 is maintained after 55 weeks of -5 ° C.. OHTA et al.

Studied the effects of different negative electrode materials on the performance of lithium ion battery. The study found that no matter whether carbon coated artificial graphite or natural graphite, its irreversible capacity is greatly reduced..

At the same time, the carbon cladding graphite negative electrode can effectively improve the low temperature performance of the battery, and 5% coated graphite at -5 ° C is 90% at normal temperature.. Nobili et al.

As a negative electrode material, at -20 ° C, its SEI film impedance and charge transfer impedance are reduced by three and 10 times respectively, which indicates the coating of tin compared to uncoated materials. It can reduce the polarization of the battery low temperature, thereby increasing the low temperature performance of the battery..

3 Increase the level spacing of the graphite layer pitch of the graphite negative electrode, the diffusion rate of lithium ions in the graphite layer is lowered, resulting in an increase in polarization, introducing B, N, S, K and other elements, etc. Structural modification, new graphite layer spacing, improve its degreasing / in-cell lithium loss, P (0.077 pm) of atomic radius ratio C (0.

077 pm) large, increasing P can add graphite layer spacing, enhance lithium ion Diffusion ability, while it is possible to increase the content of graphite crystallization in carbon material. K is introduced into the carbon material to form an insertion compound KC8. When potassium derets the layer spacing of the carbon material, it is advantageous for rapid insertion of lithium, thereby increasing the low temperature performance of the battery.

. 4 Control the negative particle size Huang et al, study the effect of the negative electrode particles on the low temperature performance, and the average particle diameter is found that the coke negative electrode having a coke negative electrode of 6 μm and 25 μm respectively has the same reversible charge capacity at room temperature, while at -30 ° C, granules The coke electrode having a 25 μm of 25 μm can only discharge 10% of the room temperature capacity, and the coke electrode having a particle size of 6 μm can release 61% of the room temperature capacity..

From this experimental result, the larger the negative electrode particle size, the longer the lithium ion diffusion path, the larger the diffusion impedance, resulting in increased concentration, and the low temperature performance is deteriorated.. Therefore, the negative material particle size is appropriately reduced, and the migration distance between lithium ions can be effectively shortened, reducing the diffusion impedance, new electrolyte infiltration area, thereby improving the low temperature performance of the battery.

. Further, by small particle diameter single particulate granulation granulation, there is a high degree, and more of the lithium lithium sites can be supplied, and the polarization can be lowered, and the battery low temperature performance can be significantly improved..

IV. Conclusion, the low temperature performance of lithium-ion batteries is a key factor constraints of lithium-ion battery applications. How to improve the low temperature performance of lithium-ion batteries is still the hotspots and difficulties of current research.

. The battery system reaction process is important to include 2 steps of transferring Li + in electrolyte, through electrolyte / electrode interface, charge transfer, and Li + in active substance body..

At low temperatures, the rate of each step is lowered, thereby causing various step impedances, resulting in an intensification of electrode polarization, causing low temperature discharge capacity, negative electrode analysis. Improve the low temperature performance of the lithium-ion battery should consider the impact of integrated factors in the battery in the battery. The electrical conductivity of the electrolyte solvent, the additive and the lithium salt is improved, and the film impedance is reduced.

The negative electrode material is doped, coated, small particles, etc. modified treatment, optimized material structure, reducing interface impedance and diffusion impedance in active substance body. By optimizing the overall battery system, the polarization of the low temperature of the lithium ion battery is reduced, so that the low temperature performance of the battery is further improved.

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