Since lithium-ion batteries entered the market, they have been widely used due to their advantages of long life, large specific capacity, and no memory effect. Low temperature use of lithium-ion batteries has problems such as low capacity, serious attenuation, poor cycle rate performance, obvious lithium deposition, and unbalanced lithium extraction. However, with the continuous expansion of application fields, the constraints caused by the poor low-temperature performance of lithium-ion batteries are becoming more and more obvious.

According to reports, the discharge capacity of lithium-ion batteries at -20°C is only about 31.5% of that at room temperature. The operating temperature of traditional lithium-ion batteries is between -20 and +55 °C. However, in the fields of aerospace, military industry, electric vehicles, etc., the battery is required to work normally at -40°C. Therefore, it is of great significance to improve the low-temperature properties of Li-ion batteries.

Factors Restricting Low Temperature Performance of Li-ion Batteries

In a low temperature environment, the viscosity of the electrolyte increases and even partially solidifies, resulting in a decrease in the conductivity of lithium-ion batteries.

The compatibility between the electrolyte and the negative electrode and the separator becomes poor in a low temperature environment.

The negative electrode of lithium-ion battery has serious lithium precipitation under low temperature environment, and the precipitated metal lithium reacts with the electrolyte, and its product deposition leads to an increase in the thickness of the solid-electrolyte interface (SEI).

In low temperature environment, the diffusion system of Li-ion batteries in the active material decreases, and the charge transfer resistance (Rct) increases significantly.

Discussion on Factors Affecting Low Temperature Performance of Li-ion Batteries

Expert opinion 1: The electrolyte has the greatest impact on the low-temperature performance of lithium-ion batteries, and the composition and physicochemical properties of the electrolyte have an important impact on the low-temperature performance of the battery. The problems faced by the battery cycle at low temperature are: the viscosity of the electrolyte will increase, and the ion conduction speed will become slower, resulting in the mismatch of the electron migration speed of the external circuit, so the battery is severely polarized, and the charge and discharge capacity is sharply reduced. Especially when charging at low temperature, lithium ions easily 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 size of the conductivity of the electrolyte itself. The electrolyte with high conductivity transmits ions quickly and can exert more capacity at low temperature. The more dissociated the lithium salt in the electrolyte, the higher the number of migrations and the higher the conductivity. The higher the electrical conductivity, the faster the ion conduction rate, the less the polarization, and the better the performance of the battery at low temperature. Therefore, higher electrical conductivity is a necessary condition to achieve good low-temperature performance of lithium-ion batteries.

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 the 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 affecting the conduction of lithium ions, 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 sharply increased Li+ diffusion resistance at low temperatures, not the SEI film.

Low temperature properties of cathode materials for lithium ion batteries

1. Low temperature properties of layered cathode materials

The layered structure not only has the incomparable rate performance of one-dimensional lithium ion diffusion channels, but also has the structural stability of three-dimensional channels. It is the earliest commercial cathode material for lithium ion batteries. Its representative substances are LiCoO2, Li(Co1-xNix)O2 and Li(Ni, Co, Mn)O2 and so on.

Xie Xiaohua et al. took LiCoO2/MCMB as the research object and tested its low-temperature charge-discharge characteristics.

The results show that with the decrease of temperature, the discharge platform drops from 3.762V (0°C) to 3.207V (–30°C); the total battery capacity also decreases sharply from 78.98mA·h (0°C) to 68.55mA·h (–30°C).

2. Low-temperature characteristics of spinel-structured cathode materials

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

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

Peng Zhengshun et al. pointed out that different preparation methods have a great 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 of sol-gel method, and this phenomenon is not affected by lithium ions. The diffusion coefficient is also reflected. The reason is that different synthesis methods have great influence on the crystallinity and morphology of the products.

3. Low temperature characteristics of cathode materials of phosphate system

Due to its excellent volume stability and safety, LiFePO4, together with ternary materials, has become the main body of current power battery cathode materials. 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 diffusivity, and poor conductivity at low temperature, which increases the internal resistance of the battery, which is greatly affected by polarization, and the battery charge and discharge are hindered. Therefore, the low temperature Performance is not ideal.

When studying the charge-discharge behavior of LiFePO4 at low temperature, Gu Yijie et al. found that its coulombic efficiency dropped from 100% at 55°C to 96% at 0°C and 64% at -20°C, respectively; the discharge voltage decreased from 3.11V at 55°C. Decrease to 2.62V at –20°C.

Xing et al. modified LiFePO4 with nanocarbon and found that after adding nanocarbon conductive agent, the electrochemical performance of LiFePO4 was less sensitive to temperature, and the low temperature performance was improved; the discharge voltage of modified LiFePO4 increased from 3.40 at 25 °C V drops to 3.09V at –25°C, a decrease of only 9.12%; and its cell efficiency at –25°C is 57.3%, which is higher than 53.4% ​​without nano-carbon conductive agent.

Recently, LiMnPO4 has attracted a lot of interest. The study found that LiMnPO4 has the advantages of high potential (4.1V), no pollution, low price, and large specific capacity (170mAh/g). However, due to the lower ionic conductivity of LiMnPO4 than LiFePO4, Fe is often used to partially replace Mn to form LiMn0.8Fe0.2PO4 solid solution in practice.

Low temperature properties of anode materials for lithium ion batteries

Compared with the positive electrode material, the low temperature deterioration of the negative electrode material of lithium ion battery is more serious, mainly for the following three reasons:

When charging and discharging at low temperature and high rate, the battery is seriously polarized, and a large amount of metal lithium is deposited on the surface of the negative electrode, and the reaction product of metal lithium and electrolyte generally does not have conductivity;

From a thermodynamic point of view, the electrolyte contains a large number of polar groups such as C-O and C-N, which can react with the negative electrode material, and the formed SEI film is more susceptible to low temperature;

The carbon negative electrode is difficult to intercalate lithium at low temperature, and there is asymmetric charge and discharge.

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Research on Low Temperature Electrolyte

The electrolyte plays the role of transporting Li+ in lithium-ion batteries, and its ionic conductivity and SEI film-forming properties have a significant impact on the low-temperature performance of the battery. There are three main indicators for judging the pros and cons of low-temperature electrolytes: ionic conductivity, electrochemical window and electrode reactivity. The level of these three indicators depends to a large extent on its constituent materials: solvent, electrolyte (lithium salt), and additives. Therefore, the research on 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.

Compared with chain carbonates, the low temperature characteristics of EC-based electrolytes, cyclic carbonates have a compact structure, large acting force, and higher 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 effectively prevent the co-insertion of solvent molecules, making them indispensable. Therefore, most of the commonly used low-temperature electrolyte systems are based on EC, and then mixed Small molecule solvent with low melting point.

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. In general, 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 concentration of lithium ions in the solvent depends on the strength of the dissociation and association of lithium salts in the solvent.

Research on Low Temperature Electrolyte

In addition to the composition of the battery itself, process factors in actual operation will also have a great impact on the performance of the 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 batteries and found that in terms of capacity retention, the smaller the electrode load and the thinner the coating layer, the better the low temperature performance. .

(2) State of charge and discharge. Petzl et al. studied the effect of low-temperature charge-discharge state on battery cycle life, and found that when the depth of discharge is large, it will cause greater capacity loss and reduce cycle life.

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

Summarize

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

(1) Form 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 another way to look at another type 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 problem of capacity decay and cycle safety when batteries are used at low temperatures. c