Lithium-ion batteries have been widely used since they entered the market with the advantages of long life, high specific capacity and no memory effect. In the field of energy storage, which is a big hit in the clean energy industry, it has been a natural to use lithium batteries. However, as the application field continues to expand, the constraints brought about by the poor low temperature performance of lithium-ion batteries are becoming more and more obvious. For countries that are always in low temperature, the low temperature performance of lithium battery system is undoubtedly the key consideration.
According to the survey, the discharge capacity of lithium-ion battery at -20℃ is only about 31.5% of that at room temperature. Traditional lithium-ion battery operating temperature is between -20 ~ +55 ℃. However, in aerospace, military, electric vehicles and other fields, it is required that the battery can work normally at -40℃. Therefore, it is of great significance to improve the low-temperature properties of lithium-ion batteries.
Under low temperature environment, the viscosity of electrolyte increases and even partially solidifies, which leads to a decrease in the conductivity of lithium-ion batteries.
The compatibility between electrolyte and anode and diaphragm becomes poor in low temperature environment.
Lithium precipitation from the negative electrode of Li-ion battery in low temperature environment is serious, and the precipitated lithium metal reacts with the electrolyte, and its product deposition leads to the increase of solid electrolyte interface (SEI) thickness.
The charge transfer impedance (Rct) of lithium-ion batteries in low-temperature environments is significantly increased due to the reduced diffusion system within the active material.
An expert opinion in the lithium industry is that the electrolyte has the greatest influence on the low-temperature performance of lithium-ion batteries, and the composition and physical and chemical properties of the electrolyte have an important impact on the low-temperature performance of the battery. The problem faced by the battery cycle at low temperature is that the electrolyte viscosity will become larger and the ion conduction speed will slow down, resulting in a mismatch in the electron migration rate of the external circuit, so the battery becomes severely polarized and the charge and discharge capacity appears to be drastically reduced. Especially when charging at low temperature, lithium ions can easily form lithium dendrites on the surface of the negative electrode, leading to battery failure.
The low temperature performance of the electrolyte is closely related to the size of the electrolyte's own conductivity, and the large conductivity of the electrolyte transports ions faster and can bring out more capacity at low temperature. The more lithium salts dissociated in the electrolyte, the higher the number of migration and the higher the conductivity. The higher the conductivity, the faster the ion conduction rate, the smaller the polarization suffered, and the better the performance of the battery at low temperatures. Therefore, higher conductivity is necessary to achieve good low temperature performance of Li-ion battery.
The conductivity of the electrolyte is related to the composition of the electrolyte, and reducing the viscosity of the solvent is one of the ways to improve the conductivity of the electrolyte. The good fluidity of solvent at low temperature is the guarantee of ion transport, and the solid electrolyte film formed by the electrolyte at the negative electrode at low temperature is also the key to influence the conduction of lithium ions, and the RSEI is the main impedance of lithium ion battery at low temperature environment.
Another professional perspective is that the main factor limiting the low-temperature performance of Li-ion batteries is the dramatically increased Li+ diffusion impedance at low temperatures, not the SEI film.
1, the low-temperature characteristics of cathode materials with a layered structure
The laminate structure, which has the incomparable multiplicative performance of the one-dimensional lithium ion diffusion channel and the structural stability of the three-dimensional channel, is the earliest commercial lithium ion battery cathode material. The representative materials include LiCoO2, Li(Co1-xNix)O2 and Li(Ni,Co,Mn)O2.
Xie Xiaohua, a researcher from China, et al. tested the low-temperature charge/discharge characteristics of LiCoO2/MCMB as a research object.
The results show that its discharge plateau decreases from 3.762V (0°C) to 3.207V (-30°C) as the temperature decreases; its total battery capacity also decreases sharply from 78.98mA-h (0°C) to 68.55mA-h (-30°C).
2、The low temperature characteristics of spinel structured cathode materials
The spinel structure LiMn2O4 cathode material has the advantage of low cost and no toxicity because it does not contain Co elements.
However, the Mn valence state variability and the Jahn-Teller effect of Mn3+ lead to the problems of structural instability and poor reversibility of this component.
Some other researchers from China, Zhengshun Peng et al. pointed out that different preparation methods have a strong influence on the electrochemical performance of LiMn2O4 cathode materials, taking Rct as an example: the Rct of LiMn2O4 synthesized by high-temperature solid-phase method is significantly higher than that synthesized by sol-gel method, and this phenomenon is also reflected in the lithium ion diffusion coefficient. The reason for this is mainly due to the different synthesis methods on the crystallinity and morphology of the products have a greater impact.
3, phosphate system cathode material low-temperature characteristics
LiFePO4 for excellent volume stability and safety, and ternary materials together, the main body of the current power battery cathode material. Lithium iron phosphate poor low-temperature performance is mainly due to its material itself as an insulator, low electronic conductivity, poor diffusion of lithium ions, poor conductivity at low temperatures, making the battery resistance increases, the polarization affected by the large, battery charging and discharging hindered, so the low-temperature performance is not ideal.
Chinese researchers Yijie Gu et al. found that the charge and discharge behavior of LiFePO4 at low temperature decreased from 100% at 55°C to 96% at 0°C and 64% at -20°C, respectively, and the discharge voltage decreased from 3.11 V at 55°C to 2.62 V at -20°C.
Using nanocarbon to modify LiFePO4, it was found that the electrochemical performance of LiFePO4 was less sensitive to temperature and the low-temperature performance was improved after adding nanocarbon conductive agent; the discharge voltage of modified LiFePO4 decreased from 3.40V at 25℃ to 3.09V at -25℃, which was only 9.12%; and its cell efficiency at -25°C is 57.3%, which is higher than that of 53.4% without carbon nanoconducting agent.
Recently, LiMnPO4 has attracted a lot of interest. It was found that LiMnPO4 has the advantages of high potential (4.1 V), no pollution, low price, and high specific capacity (170 mAh/g). However, due to the lower ionic conductivity of LiMnPO4 than LiFePO4, it is often used in practice to partially replace Mn with Fe to form LiMn0.8Fe0.2PO4 solid solution.
Compared with the cathode material, the low-temperature deterioration of the negative electrode material of lithium-ion batteries is more serious, mainly for the following three reasons.
severe battery polarization at low temperature and large multiplier charge/discharge, massive deposition of lithium metal on the surface of the negative electrode, and the reaction products of lithium metal and electrolyte are generally not conductive.
From the thermodynamic point of view, the electrolyte contains a large number of C-O, C-N and other polar groups, which can react with the cathode material, and the formed SEI film is more susceptible to low temperature.
Carbon cathodes have difficulty in lithium embedding at low temperatures and suffer from charge/discharge asymmetry.
The electrolyte plays the role of Li+ transfer in Li-ion batteries, and its ionic conductivity and SEI film formation performance have a significant impact on the low-temperature performance of the battery. There are 3 main indicators to judge the advantages and disadvantages of electrolytes for low temperature: ionic conductivity, electrochemical window and electrode reactivity. And the level of these 3 indicators depends largely on its constituent materials: solvent, electrolyte (lithium salt), and additives. Therefore, the study of the low-temperature performance of each part of the electrolyte is of great importance to understand and improve the low-temperature performance of the battery.
EC-based electrolyte low-temperature properties compared to chain carbonate, ring carbonate has a tight structure and high force, and has a high melting point and viscosity. However, the large polarity brought by the ring structure makes it often have a large dielectric constant. the large dielectric constant, high ionic conductivity and excellent film-forming properties of EC solvents, which effectively prevent solvent molecules from co-insertion, make them indispensable, so most of the commonly used low-temperature electrolyte systems are EC-based and then mixed with low-melting point small molecule solvents.
Lithium salt is an important component of electrolyte. Lithium salts in electrolytes not only improve the ionic conductivity of the solution, but also reduce the diffusion distance of Li+ in the solution. In general, the greater the concentration of Li+ in the solution, the greater the ionic conductivity. However, the Li ion concentration in the electrolyte is not linearly related to the Li salt concentration, but parabolic. This is because the lithium ion concentration in the solvent depends on the strength of the dissociation and association of the lithium salt in the solvent.
In addition to the composition of the cell itself, the process factors in the actual operation will also have a great impact on the performance of the battery.
(1) Preparation process. Adnan Yaqub et al. studied the effects of electrode loading and coating thickness on the low temperature performance of LiNi0.6Co0.2Mn0.2O2/Graphite cells and found that the lower the electrode loading and the thinner the coating layer, the better the low temperature performance in terms of capacity retention.
(2) Mathias Petzl et al. studied the effect of low-temperature charge/discharge state on the cycle life of the battery and found that a larger depth of discharge caused a larger capacity loss and reduced the cycle life.
(3) Other factors. The surface area of electrode, pore size, electrode density, wettability of electrode and electrolyte, and diaphragm all affect the low-temperature performance of lithium-ion batteries. In addition, the impact of defects in materials and processes on the low temperature performance of the battery should not be ignored.
From the above scholars' research, it is known that in order to ensure the low-temperature performance of lithium-ion batteries, the following points need to be done.
(1) Forming thin and dense SEI films.
(2) Ensuring a large diffusion coefficient of Li+ in the active material.
(3) The electrolyte has a high ionic conductivity at low temperature.
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