Why does the capacity of lithium battery become low in winter? Finally, someone can explain!

 Company news     |      2022-05-31 13:10:51    |      小编

Since lithium-ion battery entered the market, it has been widely used because of its long service life, large specific capacity and no memory effect. The low-temperature use of lithium-ion batteries has many problems, such as low capacity, serious attenuation, poor cycle rate performance, obvious lithium precipitation, imbalance of lithium removal and intercalation, etc. However, with the continuous expansion of application fields, the constraints caused by the poor low-temperature performance of lithium-ion batteries become more and more obvious.
It is reported that the discharge capacity of lithium-ion battery at -20 ℃ is only about 31.5% of that at room temperature. The operating temperature of traditional lithium ion batteries is between -20~+55 ℃. However, in aerospace, military industry, electric vehicles and other fields, the battery is required to work normally at -40 ℃. Therefore, it is of great significance to improve the low-temperature properties of lithium-ion batteries.
Factors restricting the low temperature performance of lithium ion batteries
At low temperature, the viscosity of electrolyte increases and even partially solidifies, resulting in the decrease of conductivity of lithium-ion batteries. At low temperature, the compatibility between electrolyte, cathode and diaphragm becomes worse. In low temperature environment, lithium is seriously precipitated from the cathode of lithium-ion battery, and the precipitated metal lithium reacts with the electrolyte, resulting in the increase of the thickness of solid electrolyte interface (SEI). At low temperature, the diffusion system of lithium-ion battery in the active material decreases, and the charge transfer impedance (RCT) increases significantly.
Discussion on the factors affecting the low temperature performance of lithium ion batteries
Expert opinion 1: electrolyte has the greatest impact on the low-temperature performance of lithium-ion batteries. The composition and physicochemical properties of electrolyte have an important impact on the low-temperature performance of batteries. The problem faced by the battery under low temperature circulation is that the electrolyte viscosity will become larger and the ion conduction speed will become slower, resulting in mismatching of the electron migration speed of the external circuit. Therefore, the battery will be severely polarized and the charge and discharge capacity will be sharply reduced. Especially when charging at low temperature, lithium ions are easy to form lithium dendrites on the surface of the negative electrode, resulting in battery failure.
The low-temperature performance of the electrolyte is closely related to the conductivity of the electrolyte itself. The electrolyte with high conductivity can transport ions quickly, and can play more capacity at low temperature. The more the lithium salt in the electrolyte dissociates, the more the migration number and the higher the conductivity. The higher the conductivity, the faster the ionic conduction rate, the smaller the polarization, and the better the performance of the battery at low temperature. Therefore, high conductivity is a necessary condition to achieve good low-temperature performance of lithium-ion batteries.
The conductivity of electrolyte is related to the composition of electrolyte. Reducing the viscosity of solvent is one of the ways to improve the conductivity of electrolyte. The good fluidity of the solvent at low temperature is the guarantee of ion transport, and the solid electrolyte membrane formed by the electrolyte at the negative electrode at low temperature is also the key to affect lithium ion conduction, and rsei is the main impedance of lithium-ion batteries at low temperature.
Expert 2: the main factor limiting the low-temperature performance of lithium-ion batteries is the rapidly increasing li+ diffusion resistance at low temperature, rather than SEI film.
Low temperature characteristics of cathode materials for lithium ion batteries
1. Low temperature characteristics of layered cathode materials
Layered structure is the earliest commercial cathode material for lithium-ion batteries, which not only has the incomparable magnification performance of one-dimensional lithium-ion diffusion channel, but also has the structural stability of three-dimensional channel. Its representative substances include LiCoO2, Li (CO1 xnix) O2 and Li (Ni, Co, Mn) O2.
Xiexiaohua et al. Took licoo2/mcmb as the research object and tested its low-temperature charging and discharging characteristics.
The results show that the discharge plateau decreases from 3.762v (0 ℃) to 3.207v (– 30 ℃) with the decrease of temperature; The total capacity of the battery also decreased sharply from 78.98ma · H (0 ℃) to 68.55ma · H (– 30 ℃).
2. Low temperature properties of Spinel Cathode Materials
Spinel LiMn2O4 cathode material has the advantages of low cost and non toxicity because it does not contain Co element.
However, the variable valence state of Mn and the Jahn teller effect of mn3+ lead to structural instability and poor reversibility of this component.
Pengzhengshun et al. Pointed out that different preparation methods have a great impact on the electrochemical properties of LiMn2O4 cathode materials. Taking RCT as an example: the RCT of LiMn2O4 synthesized by high-temperature solid-state method is significantly higher than that synthesized by sol gel method, and this phenomenon is also reflected in the lithium ion diffusion coefficient. The main reason is that different synthesis methods have great influence on the crystallinity and morphology of the products.
3. Low temperature characteristics of phosphate cathode materials
LiFePO4, together with ternary materials, has become the main body of cathode materials for power batteries due to its excellent volume stability and safety. The poor low-temperature performance of lithium iron phosphate is mainly due to the fact that the material itself is an insulator, with low electronic conductivity, poor lithium ion diffusion, and poor conductivity at low temperature, which increases the internal resistance of the battery, greatly affects the polarization, and impedes the charging and discharging of the battery. Therefore, the low-temperature performance is not ideal.
Guyijie et al. Found that the coulomb efficiency of LiFePO4 decreased from 100% at 55 ℃ to 96% at 0 ℃ and 64% at – 20 ℃ respectively when studying the charge and discharge behavior of LiFePO4 at low temperature; The discharge voltage decreases from 3.11v at 55 ℃ to 2.62v at – 20 ℃.
Xing et al. Modified LiFePO4 with nano carbon and found that after adding nano carbon conductive agent, the electrochemical performance of LiFePO4 was less sensitive to temperature and the low temperature performance was improved; After modification, the discharge voltage of LiFePO4 decreased from 3.40v at 25 ℃ to 3.09v at – 25 ℃, with a decrease of only 9.12%; The cell efficiency is 57.3% at – 25 ℃, which is higher than 53.4% without nano carbon conductive agent.
Recently, LiMnPO4 has aroused great interest. It is found that LiMnPO4 has the advantages of high potential (4.1V), no pollution, low price and large specific capacity (170mah/g). However, because LiMnPO4 has lower ionic conductivity than LiFePO4, in practice, Fe is often used to partially replace Mn to form limn0.8fe0.2po4 solid solution.
Low temperature characteristics of cathode materials for lithium ion batteries
Compared with the cathode materials, the low-temperature deterioration of the cathode materials of lithium-ion batteries is more serious, mainly due to the following three reasons:
·When the battery is charged and discharged at low temperature and high rate, the battery polarization is serious, a large amount of metal lithium is deposited on the surface of the negative electrode, and the reaction products of metal lithium and electrolyte generally do not have conductivity· From the perspective of thermodynamics, the electrolyte contains a large amount of C – o, C – n, etc
Polar groups can react with negative electrode materials, and the formed SEI film is more susceptible to low temperature· It is difficult to embed lithium in carbon anode at low temperature, and there is asymmetry between charge and discharge.
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Study on low temperature electrolyte
Electrolyte plays the role of transferring li+ in lithium-ion battery. Its ionic conductivity and SEI film-forming performance have a significant impact on the low-temperature performance of the battery. There are three main indexes to judge the quality of low-temperature electrolyte: ionic conductivity, electrochemical window and electrode reaction activity. The level of these three indicators largely depends on their constituent materials: solvents, electrolytes (lithium salts) and additives. Therefore, the research on the low-temperature performance of each part of the electrolyte is of great significance to understand and improve the low-temperature performance of the battery.
·Compared with the chain carbonate, the cyclic carbonate has a compact structure, high force, high melting point and viscosity. However, due to the large polarity brought by the ring structure, it often has a large dielectric constant. The large dielectric constant, high ionic conductivity and excellent film-forming performance of EC solvent effectively prevent the co insertion of solvent molecules, making it an indispensable position. Therefore, most common low-temperature electrolyte systems are based on EC and mixed with low melting point small molecular solvents· Lithium salt is an important component of electrolyte. Lithium salt in the electrolyte can not only improve the ionic conductivity of the solution, but also reduce the diffusion distance of li+ in the solution. Generally speaking, the greater the li+ concentration in the solution, the greater the ionic conductivity. However, the lithium ion concentration in the electrolyte is not linearly correlated with the lithium salt concentration, but in a parabolic shape. This is because the concentration of lithium ion in the solvent depends on the dissociation and association of lithium salt in the solvent.
Study on low temperature electrolyte
In addition to the battery composition itself, the process factors in actual operation will also have a great impact on the battery performance.
(1) Preparation process. Yaqub et al. Studied the influence of electrode load and coating thickness on the low-temperature performance of lini0.6co0.2mn0.2o2 /graphite battery and found that in terms of capacity retention, the smaller the electrode load, the thinner the coating, the better the low-temperature performance.
(2) Charge and discharge status. Petzl et al. Studied the impact of low-temperature charge and discharge state on the battery cycle life, and found that when the discharge depth is large, it will cause large capacity loss and reduce the cycle life.
(3) Other factors. The surface area, pore diameter, electrode density, wettability between electrode and electrolyte, and membrane all affect the low-temperature performance of lithium-ion batteries. In addition, the influence of material and process defects on the low-temperature performance of the battery can not be ignored.
To ensure the low-temperature performance of lithium-ion batteries, the following points should be done well:
(1) Forming thin and dense SEI film;
(2) Ensure that li+ has a large diffusion coefficient in the active substance;
(3) The electrolyte has high ionic conductivity at low temperature.
In addition, the research can also find another way to focus on another kind of lithium-ion battery - all solid-state lithium-ion battery. Compared with conventional lithium-ion batteries, all solid-state lithium-ion batteries, especially all solid-state thin-film lithium-ion batteries, are expected to completely solve the capacity degradation and cycle safety problems of batteries used at low temperatures.
This paper is edited and excerpted by Dr. Li in the article "research progress on low temperature characteristics of lithium ion batteries". The original authors: zhaoshixi, guoshuangtao, etc.