Since entering the market, lithium-ion batteries have obtained a wide range of applications with its advantages of long life, large specific capacity, no memory effect.. Low temperature of lithium-ion battery is low, severe attenuation, poor cycle magnification performance, obvious lithium phenomenon, deinterlaxing lithium imbalance, etc.
. However, with the continuous expansion of the application, the restriction of low temperature performance of lithium-ion batteries is more obvious..
According to reports, the discharge capacity of the lithium-ion battery is only about 31.5% at room temperature at -20 ° C..
Traditional lithium-ion battery operating temperature between -20 ~ + 55 ° C. But in the fields of aerospace, electric vehicles, etc., battery can be working properly at -40 ° C.
Therefore, it is of great significance to improve the low temperature properties of lithium ion batteries.. Factors constrained lithium-ion batteries low temperature performance, the viscosity of the electrolyte increases, even partially solidified, resulting in a low-conductivity of the lithium ion battery.
The compatibility between the electrolyte and the negative electrode and the diaphragm is deteriorated under low temperature environment.. The negative electrode of the lithium-ion battery under low temperature environments was severely precipitated, and the precipitated metal lithium and electrolytic solution were reacted, and the product deposition caused to solid state electrolyte interface (SEI) thickness.
. Lithium-ion batteries under low temperature environment reduce the internal diffusion system of the active substance, the charge transfer impedance (RCT) is significantly increased. Exploration on the influence of the decisive factors affecting the low temperature performance of lithium ion batteries: The electrolyte has an important effect on the low temperature performance of lithium ion batteries, the composition and materialization properties of the electrolyte have an important impact on battery low temperature performance.
. The problem in the surface of the battery is: the viscosity of the electrolyte will become large, the ion conductivity is slow, resulting in the electron migration speed of the external circuit, so the battery has severely polarized, and the charge and discharge capacity has a sharp decrease..
Especially when low-temperature charging, lithium ions can easily form lithium delegranes on the surface of the negative electrode, resulting in failure of the battery.. The low temperature performance of the electrolyte and the size of the electrolyte own conductivity are closely related, the transfer ion of the electrical conductivity is fast, and more capacity can be exhibited at low temperatures.
. The more lithium salts in the electrolyte, the more the number of migrations, the higher the conductivity..
High electrical conductivity, the faster the ion conductivity, the smaller the polarization, the better the performance of the battery at low temperature. Therefore, higher conductivity is a necessary condition for achieving good low temperature performance of lithium ion batteries..
The electrical conductivity of the electrolyte and the composition of the electrolyte are related to the reduction of the viscosity of the solvent is one of the ways to improve the electrolyte electrical conductivity.. The fluidity of solvent is good at low temperature of the solvent is the guarantee of ion transport, and the solid electrolyte membrane formed by the electrolyte in the low temperature electrolyte is also the key to affecting lithium ion conductance, and RSEI is an important impedance of a lithium ion battery in a low temperature environment.
. Expert point of view: Important factors to limit low temperature performance of lithium ion batteries are low temperatures, new Li + diffusion impedance, but not SEI film. Low temperature characteristics of a lithium-ion battery positive material, a low temperature characteristic laminar structure of the positive electrode material, which has both a one-dimensional lithium ion diffusion channel, and has a three-dimensional channel structure stability, which is the first commercial lithium ion.
Battery positive material. Its representative substances include LiCoO2, Li (CO1-XNIX) O2 and Li (Ni, Co, Mn) O2, etc..
Xie Xiaohua, etc. use LiCoo2 / MCMB as research objects, testing its low temperature charge characteristics. The results show that as the temperature decreases, the discharge platform drops from 3.
762V (0 ° C) to 3.207V (-30 ° C); its battery total capacity is also reduced from 78.98mA · h (0 ° C) to 68.
55mA · h (-30 ° C). The low temperature characteristics of the positive electrode material of the spinel structure spinel structure LiMn2O4 positive material, because there is no Co element, there is a low cost and non-toxic advantages..
However, the Mn valence gear and the JaHN-Teller effect of Mn3 +, resulting in problems such as structural unstable and reversible differences.. Peng Zhengshun, indicating that the electrochemical performance of LiMn2O4 positive electrode materials is large, and the RCT is used as an example: the RCT of LIMN2O4 synthesized by high temperature solid phase is significantly higher than the sol gel method, and this phenomenon is in lithium ion Implanted on diffusion coefficients.
The reason is important because different synthetic methods have a large number of crystallinity and morphology of product.. The low temperature characteristic of the phosphate system positive electrode material LIFEPO4 is the main body of the current power lithium battery positive material due to excellent volume stability and safety, with the ternary material.
. Lithium phosphate low temperature performance is important because its material itself is an insulator, low electron conductivity, poor conductivity in low temperature, so that the internal resistance of the battery increases, the polarization is high, and the battery charge and discharge is blocked. Low temperature is not ideal.
Valley Yidi, etc., when studying the charge and discharge behavior of LifePO4 at low temperatures, the Kulen efficiency is 64% at 96% and -20 ° C at 55 ° C to 0 ° C, and the discharge voltage is from 55 ° C 3.11V.
2.62V of delivering to -20 ° C. XING et al, discovery, after the addition of nanocarbon conductive agents, the electrochemical properties of LiFePO4 decreased, and the low temperature performance is improved; the discharge voltage of LiFePO4 after modification 3.
40 V fell to 3.09V at -25 ° C, the decrease was only 9.12%; and its battery efficiency was 57.
3%, higher than 53.4% of the non-nanocarbon electrical agent at -25 ° C. Recently, LIMNPO4 has attracted people’s interested interests.
. The study found that LIMNPO4 has high potentials (4.1V), no pollution, low price, large specific capacity (170mAh / g), etc.
. However, due to the lower ion conductivity of LIMNPO4 than LiFePO4, it is often used to replace Mn to form a LiMn0.8Fe0.
2PO4 solid solution in the actual use of the FE portion.. The low temperature characteristics of the lithium-ion battery negative electrode material are related to the positive electrode material, the low temperature deterioration of the lithium ion battery negative electrode material is more serious, and it is important to have the following three reasons: low temperature high magnification charge and discharge, battery polarization is severe, negative surface metal lithium A large amount of deposition, and the reaction product of the metal lithium and the electrolyte generally does not have electrical conductivity; from the thermodynamic angle, the electrolyte contains a large amount of C-O, C-N isolaty group, can be reacted with the negative electrode material, the formed SEI The film is more vulnerable to the low temperature; the carbon negative electrode is difficult to lithium under low temperature, and there is a charge and discharge asymmetry.
. The study of low temperature electrolyte is taken on the lithium ion battery, and its ionic conductivity and SEI film formation performance have significant effect on battery low temperature performance..
It is determined that the low temperature electrolytic solution is very detrurher, three important indicators: ionic conductivity, electrochemical window, and electrode reactivity. The level of these three indicators is largely dependent on its compositional materials: solvent, electrolyte (lithium salt), additive. Therefore, the study of the low temperature performance of each part of the electrolyte, is of great significance for understanding and improving the low temperature performance of the battery.
. The EC-based electrolyte low temperature characteristics compared to chain carbonate, the cyclic carbonate structure is tight and used, and has a high melting point and viscosity..
However, the large polarity of the annular structure makes it often have a large dielectric constant.. EC solvent has a large dielectric constant, high ion conductivity, perfect film formation performance, effectively prevents the solvent molecule from being co-inserted, so that it is an indispensable position, so that mostly low temperature electrolytic solution systems are large, and then mixed Low melting point of small molecule solvent.
Lithium salt is an important composition of electrolyte. The lithium salt can not only improve the ionic conductivity of the solution, but also reduce the diffusion distance of Li + in the solution..
In general, the larger the Li + concentration in the solution, the greater the ion conductivity.. However, the concentration of lithium ion concentration and lithium salt in the electrolyte is not linearly correlated, but is a parabolic.
This is because the lithium ion concentration in the solvent depends on the strength of the lithium salt in the solvent and the strength of the association.. The study of low temperature electrolyte except that the battery is composed of itself, and the process factors in the actual operation will also affect the performance of the battery.
. (1) Preparation Process. YaquB et al.
Research on electrode load and coating thickness on low temperature performance of lini0.6co0.2 mn0.
2O2 / graphite battery, in terms of capacity retention ratio, the smaller the electrode load, the thinner coating layer, the better the low temperature performance. (2) charge and discharge state. Petzl et al research on the effect of low temperature charge and discharge on battery cycle life, found that when the discharge depth is large, a large capacity loss is caused, and the loop life is reduced.
. (3) Other factors. The surface area of the electrode, the aperture, the electrode density, the electrode and the electrolyte wettability and the diaphragm, etc.
, which affect the low temperature performance of the lithium ion battery.. In addition, the impact of the defects of materials and processes on low temperature performance of the battery can not be ignored.
. Summarizing the low temperature performance of the lithium-ion battery, to do the following: (1) forming a thin and dense SEI film; (2) to ensure that Li + has a large diffusion coefficient in active substance; (3) Electrolyte High ion conductivity at low temperatures. In addition, the study can also take another approach, and the eye is turned to another kind of lithium-ion battery – full solid lithium ion battery.
Compared to conventional lithium-ion batteries, all solid-state lithium ion batteries, especially full solid thin film lithium ion batteries, is expected to completely solve the capacity attenuation problem and cycle safety issues used in battery low temperatures.