Why does the capacity of lithium batteries decrease in winter? Finally, someone can explain!

Why does the capacity of lithium batteries decrease in winter? Finally, someone can explain!

Since entering the market, lithium-ion batteries have been widely used due to their advantages such as long lifespan, large specific capacity, and no memory effect. Lithium ion batteries used at low temperatures have problems such as low capacity, severe attenuation, poor cycling performance, obvious lithium evolution, and imbalanced lithium removal and insertion. However, with the continuous expansion of application fields, the constraints caused by the poor low-temperature performance of lithium-ion batteries are becoming increasingly apparent.

According to reports, the discharge capacity of lithium-ion batteries at -20 ℃ is only about 31.5% of that at room temperature. Traditional lithium-ion batteries operate at temperatures between -20~+55 ℃. However, in fields such as aerospace, military, and electric vehicles, batteries are required to operate normally at -40 ℃. Therefore, improving the low-temperature properties of lithium-ion batteries is of great significance.

Factors restricting the low-temperature performance of lithium-ion batteries

• In low temperature environments, the viscosity of the electrolyte increases and even partially solidifies, leading to a decrease in the conductivity of lithium-ion batteries.
• The compatibility between electrolyte, negative electrode, and separator deteriorates in low temperature environments.
• Under low temperature conditions, the negative electrode of lithium-ion batteries experiences severe lithium precipitation, and the precipitated metal lithium reacts with the electrolyte, resulting in the deposition of products that increase the thickness of the solid-state electrolyte interface (SEI).
• In low-temperature environments, the diffusion system inside the active material of lithium-ion batteries decreases, and the charge transfer impedance (Rct) significantly increases.

Discussion on factors affecting the low-temperature performance of lithium-ion batteries

Expert viewpoint 1: Electrolyte has the greatest impact on the low-temperature performance of lithium-ion batteries, and the composition and physicochemical properties of the electrolyte have a significant impact on the low-temperature performance of the battery. The problem faced by the cycling of batteries at low temperatures is that the viscosity of the electrolyte will increase, the ion conduction speed will slow down, causing a mismatch in the electron migration speed of the external circuit, resulting in severe polarization of the battery and a sharp decrease in charge discharge capacity. Especially when charging at low temperatures, lithium ions can easily form lithium dendrites on the negative electrode surface, leading to battery failure.

The low-temperature performance of electrolytes is closely related to the conductivity of the electrolyte itself. Electrolytes with high conductivity transport ions quickly and can exert more capacity at low temperatures. The more lithium salts in the electrolyte dissociate, the more they migrate and the higher their conductivity. The higher the conductivity and the faster the ion conduction rate, the smaller the polarization, and the better the performance of the battery at low temperatures. Therefore, high conductivity is a necessary condition for achieving good low-temperature performance of lithium-ion batteries.

The conductivity of the electrolyte is related to its composition, and reducing the viscosity of the solvent is one of the ways to improve the conductivity of the electrolyte. The good flowability of solvents at low temperatures is a guarantee for ion transport, and the solid electrolyte film formed by the electrolyte on the negative electrode at low temperatures is also a key factor affecting lithium ion conduction, and RSEI is the main impedance of lithium-ion batteries in low-temperature environments.

Expert 2: The main factor limiting the low-temperature performance of lithium-ion batteries is the rapidly increasing Li+diffusion impedance at low temperatures, rather than SEI membranes.

Low temperature characteristics of positive electrode materials for lithium-ion batteries

1. Low temperature characteristics of layered positive electrode materials

Layered structure, with unparalleled rate performance compared to one-dimensional lithium-ion diffusion channels and structural stability of three-dimensional channels, is the earliest commercially available cathode material for lithium-ion batteries. Its representative substances include LiCoO2, Li (Co1-xNix) O2, and Li (Ni, Co, Mn) O2.
Xie Xiaohua et al. tested the low-temperature charging and discharging characteristics of LiCoO2/MCMB as the research object.
The results show that as the temperature decreases, the discharge plateau decreases from 3.762V (0 ℃) to 3.207V (-30 ℃); The total battery capacity has also sharply decreased from 78.98mA · h (0 ℃) to 68.55mA · h (-30 ℃).

2. Low temperature characteristics of spinel structure positive electrode materials

The spinel structured LiMn2O4 cathode material has the advantages of low cost and non-toxicity due to its absence of Co element.
However, the variable valence states of Mn and the Jahn Teller effect of Mn3+result in structural instability and poor reversibility of this component.
Peng Zhengshun et al. pointed out that different preparation methods have a great impact on the electrochemical performance of LiMn2O4 cathode materials. 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 for this is that different synthesis methods have a significant impact on the crystallinity and morphology of the products.

3. Low temperature characteristics of phosphate system positive electrode materials

LiFePO4, along with ternary materials, has become the main cathode material for power batteries due to its excellent volume stability and safety. The poor low temperature performance of Lithium iron phosphate is mainly because its 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 charge and discharge of the battery. Therefore, the low temperature performance is not ideal.
Gu Yijie et al. found that the Coulombic efficiency of LiFePO4 decreased from 100% at 55 ℃ to 96% at 0 ℃ and 64% at -20 ℃, respectively, when studying its charge discharge behavior at low temperatures; The discharge voltage decreases from 3.11V at 55 ℃ to 2.62V at -20 ℃.
Xing et al. used nano carbon to modify LiFePO4 and found that adding nano carbon conductive agents reduced the sensitivity of LiFePO4's electrochemical performance to temperature and improved its low-temperature performance; The discharge voltage of modified LiFePO4 decreased from 3.40V at 25 ℃ to 3.09V at -25 ℃, with a decrease of only 9.12%; And its battery efficiency is 57.3% at -25 ℃, higher than 53.4% without nano carbon conductive agents.
Recently, LiMnPO4 has aroused strong interest among people. Research has found that LiMnPO4 has advantages such as high potential (4.1V), no pollution, low price, and large specific capacity (170mAh/g). However, because LiMnPO4 has lower ionic conductivity than LiFePO4, it is often used in practice to partially replace Mn with Fe to form LiMn0.8Fe0.2PO4 Solid solution.

Low temperature characteristics of negative electrode materials for lithium-ion batteries

Compared to positive electrode materials, the low-temperature deterioration of negative electrode materials in lithium-ion batteries is more severe, mainly due to the following three reasons:

• During low temperature and high rate charging and discharging, the battery polarization is severe, and a large amount of lithium metal deposits on the negative electrode surface, and the reaction products between lithium metal and electrolyte generally do not have conductivity;
• From a thermodynamic perspective, the electrolyte contains a large number of polar groups such as C-O and C-N, which can react with negative electrode materials, resulting in SEI films that are more susceptible to low temperatures;
• It is difficult to embed lithium in carbon negative electrodes at low temperatures, resulting in asymmetric charging and discharging.

Research on Low Temperature Electrolytes

Electrolyte plays a role in transmitting Li+in lithium-ion batteries, and its ion conductivity and SEI film forming performance have a significant impact on the low-temperature performance of the battery. There are three main indicators for judging the quality of low-temperature electrolyte: ion 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 study of the low-temperature performance of various parts of the electrolyte is of great significance for understanding and improving the low-temperature performance of batteries.

• Compared to chain carbonates, EC based electrolytes have a compact structure, high force, and high melting point and viscosity. However, the large polarity brought about by the circular structure often leads to a large dielectric constant. The high dielectric constant, high ionic conductivity, and excellent film-forming performance of EC solvents effectively prevent the co insertion of solvent molecules, making them indispensable. Therefore, most commonly used low-temperature electrolyte systems are based on EC and mixed with low melting point small molecule solvents.
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• Lithium salts are an important component of electrolytes. Lithium salts in electrolytes can not only improve the ionic conductivity of the solution, but also reduce the diffusion distance of Li+in the solution. Generally speaking, the higher the concentration of Li+in a solution, the greater its ionic conductivity. However, the concentration of lithium ions in the electrolyte is not linearly correlated with the concentration of lithium salts, but rather in a parabolic shape. This is because the concentration of lithium ions in the solvent depends on the strength of the dissociation and association of lithium salts in the solvent.

In addition to the battery composition itself, process factors in practical operation can also have a significant impact on battery performance.

(1) Preparation process. Yaqub et al. studied the effects of electrode load and coating thickness on the low-temperature performance of LiNi0.6Co0.2Mn0.2O2/Graphite batteries and found that in terms of capacity retention, the smaller the electrode load, the thinner the coating layer, and the better its low-temperature performance.

(2) Charging and discharging status. Petzl et al. studied the effect of low-temperature charging and discharging conditions on the cycle life of batteries and found that when the discharge depth is large, it will cause significant capacity loss and reduce the cycle life.

(3) Other factors. The surface area, pore size, electrode density, wettability between electrode and electrolyte, and separator of electrodes 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 batteries cannot be ignored.

Summarize

To ensure the low-temperature performance of lithium-ion batteries, it is necessary to do the following:

(1) Forming a thin and dense SEI film;

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

(3) Electrolytes have high ionic conductivity at low temperatures.

In addition, research can also explore new avenues and focus on another type of lithium-ion battery - all solid-state lithium-ion batteries. Compared to 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 cycling safety issues of batteries used at low temperatures.