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Why does the safe battery capacity become low in winter?

2022-06-13 15:45:25
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Why does the safe battery capacity become low in winter?

Since entering the market, lithium-ion safety battery has been widely used for its long life, large specific capacity and no memory effect. The low-temperature use of lithium-ion safety batteries has many problems, such as low capacity, serious attenuation, poor cycle rate performance, obvious lithium separation, and imbalance of lithium intercalation. However, with the continuous expansion of the application field, the low-temperature performance of lithium-ion safety batteries has become more and more obvious.

It is reported that the discharge capacity of lithium-ion safety battery at - 20 ℃ is only about 31.5% of that at room temperature. The working temperature of traditional lithium-ion safety battery is between - 20 ~ + 55 ℃. However, in aerospace, military industry, electric vehicles and other fields, it is required that the safety battery can work normally at - 40 ℃. Therefore, it is of great significance to improve the low-temperature properties of lithium-ion safety batteries.

Factors restricting the low temperature performance of lithium ion safe sub batteries

At low temperature, the viscosity of electrolyte increases, even partially solidifies, resulting in the decrease of conductivity of lithium-ion safety battery. The compatibility between the electrolyte, the negative electrode and the separator becomes poor in the low temperature environment. In the low temperature environment, the negative electrode of the lithium-ion safety battery precipitates lithium seriously, and the precipitated metal lithium reacts with the electrolyte, and the product deposition leads to an increase in the thickness of the solid electrolyte interface (SEI). In the low temperature environment, the internal diffusion system of lithium-ion safety battery in the active material decreases, and the charge transfer impedance (RCT) increases significantly.

Discussion on factors affecting low temperature performance of lithium ion safety battery

Expert opinion 1: electrolyte has a great impact on the low-temperature performance of lithium-ion safety battery. The composition and physicochemical properties of electrolyte have an important impact on the low-temperature performance of safety battery. The problems faced by the safety battery in low-temperature cycling are: the electrolyte viscosity will become large, the ion conduction speed will become slow, and the electron migration speed of the external circuit will not match. Therefore, the safety 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 the failure of the safety battery.

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 ion conduction rate, the smaller the polarization, and the better the performance of the long-life battery at low temperature. Therefore, high electrical conductivity is a necessary condition for achieving good low-temperature performance of lithium-ion batteries.

The conductivity of the electrolyte is related to the composition of the electrolyte. Reducing the viscosity of the solvent is one of the ways to improve the conductivity of the 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 the conduction of lithium ions, 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 long life batteries

1. Low temperature characteristics of layered cathode materials

The layered structure has the incomparable rate performance of one-dimensional lithium ion diffusion channels and the structural stability of three-dimensional channels. It is the earliest commercial cathode material for lithium ion batteries. Its representative materials include LiCoO2, Li (CO1 xnix) O2 and Li (Ni, Co, Mn) O2.

Xie Xiaohua et al. Tested the low-temperature charge and discharge characteristics of LiCoO2 / MCMB.

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 its long-life battery also decreased sharply from 78.98ma · H (0 ℃) to 68.55ma · H (– 30 ℃).

2. Low temperature characteristics of Spinel Cathode Materials

Spinel structure LiMn2O4 cathode material has the advantages of low cost and non toxicity because it does not contain Co element.

However, the variable valence of Mn and the Jahn teller effect of Mn3 + lead to the structural instability and poor reversibility of this component.

Peng Zhengshun and others pointed out that different preparation methods have a great impact on the electrochemical performance of LiMn2O4 cathode material. Take 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 main reason is that different synthesis methods have great influence on the crystallinity and morphology of the products.

3. Low temperature characteristics of phosphate system cathode materials

LiFePO4, together with ternary materials, has become the main cathode material of power long-life battery due to its excellent volume stability and safety. The poor low-temperature performance of lithium iron phosphate is mainly because the material itself is an insulator, with low electronic conductivity, poor diffusion of lithium ions, and poor conductivity at low temperature, which increases the internal resistance of long-life battery, greatly affected by polarization, and impedes the charge and discharge of long-life battery. Therefore, the low-temperature performance is not ideal.

Gu Yijie 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 charging and discharging 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. It was 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%; And its long-life battery efficiency is 57.3% at - 25 ℃, which is higher than 53.4% without nano carbon conductive agent.

Recently, LiMnPO4 has attracted 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, limn0.8fe0.2po4 solid solution is often formed by partially replacing Mn with Fe.

Compared with the positive electrode materials, the low-temperature deterioration of the negative electrode materials of lithium-ion long-life batteries is more serious, mainly for the following three reasons:

·During low-temperature high rate charge and discharge, the long-life battery has serious polarization, a large amount of metal lithium is deposited on the negative electrode surface, and the reaction products of metal lithium and electrolyte generally do not have conductivity· From the thermodynamic point of view, the electrolyte contains a large amount of C – o, C – n, etc

Polar groups can react with negative electrode materials, and the SEI film formed is more susceptible to low temperature· It is difficult to intercalate lithium into carbon anode at low temperature, and there is charge discharge asymmetry.

The electrolyte plays the role of transferring Li + in the lithium-ion composite titanium battery. Its ionic conductivity and SEI film-forming performance have a significant impact on the low-temperature performance of the composite titanium battery. There are three main indicators to judge the quality of low-temperature electrolyte: ionic conductivity, electrochemical window and electrode reaction activity. The level of these three indicators depends largely on their 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 significance for understanding and improving the low-temperature performance of the composite titanium battery.

·Compared with the chain carbonate, the cyclic carbonate has a compact structure, a large force, and a 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 indispensable. 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. The 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 concentration of Li + in the solution, the greater the ionic conductivity. However, the concentration of lithium ions in the electrolyte is not linearly related to the concentration of lithium salts, but is parabolic. This is because the lithium ion concentration in the solvent depends on the dissociation and association of the lithium salt in the solvent.

In addition to the composition of the composite peptide battery itself, the process factors in the actual operation will also have a great impact on the performance of the composite peptide battery.

(1) Preparation process. Yaqub et al. Studied the effect of electrode load and coating thickness on the low-temperature performance of lini0.6co0.2mn0.2o2/graphite composite peptide battery and found that the smaller the electrode load, the thinner the coating, and the better the low-temperature performance in terms of capacity retention.

(2) Charge and discharge state. Petzl et al. Studied the influence of low-temperature charge and discharge state on the cycle life of composite peptide battery, 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 separator 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 composite peptide batteries can not be ignored.

summary

In order to ensure the low-temperature performance of lithium-ion batteries, the following points need to be done:

(1) Forming a thin and dense SEI film;

(2) Ensure that Li + has a large diffusion coefficient in the active material;

(3) The electrolyte has high ionic conductivity at low temperature.

In addition, the research can also find a new way to focus on another type of lithium-ion batteries - all solid-state lithium-ion batteries. 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 composite peptide batteries used at low temperatures.


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