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Why lithium battery capacity will be reduced in winter?
Since entering the market, lithium-ion batteries have been widely used due to their long life, large specific capacity, and no memory effect. Low-temperature use of lithium-ion batteries has problems such as low capacity, severe attenuation, poor cycle rate performance, obvious lithium deposition, and unbalanced lithium dissociation. However, as the field of application continues to expand, the constraints imposed by the low temperature performance of lithium-ion batteries are becoming more apparent.
According to reports, the discharge capacity of lithium ion batteries at -20 ° C is only about 31.5% at room temperature. Traditional lithium-ion batteries operate at temperatures between -20 and +55 °C. However, in the fields of aerospace, military, electric vehicles, etc., the battery is required to work normally at -40 °C. Therefore, improving the low temperature properties of lithium ion batteries is of great significance.
Factors limiting the low temperature performance of lithium ion batteries.
• In low temperature environments, the viscosity of the electrolyte increases and even partially solidifies, resulting in a decrease in the conductivity of the lithium ion battery.
• The compatibility between the electrolyte and the negative electrode and separator is deteriorated in a low temperature environment.
• In the low temperature environment, the lithium anode of the lithium ion battery precipitates lithium seriously, and the precipitated metal lithium reacts with the electrolyte, and the product deposition causes the solid electrolyte interface (SEI) thickness to increase.
• Lithium-ion batteries in the low-temperature environment have a reduced internal diffusion system and a significant increase in charge transfer resistance (Rct).
Discussion on the decisive factors affecting the low temperature performance of lithium ion batteries.
• The electrolyte has the greatest influence on the low temperature performance of the lithium ion battery. The composition and physical and chemical properties of the electrolyte have an important influence on the low temperature performance of the battery. The problem of circulating the battery at low temperature is that the viscosity of the electrolyte becomes large, the ion conduction speed becomes slow, and the electron migration speed of the external circuit does not match, so the battery is severely polarized, and the charge and discharge capacity is drastically reduced. Especially when charging at a 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 conductivity of the electrolyte itself. The conductivity of the electrolyte is fast, and the electrolyte can exert more capacity at low temperatures. The more lithium salts are dissociated in the electrolyte, the greater the number of migrations and the higher the conductivity. The conductivity is high, the faster the ion conduction rate, the smaller the polarization, and the better the performance of the battery at low temperatures. Therefore, higher 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 increase 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 low temperature is also the key to the lithium ion conduction, and the RSEI is the main impedance of the lithium ion battery in the low temperature environment.
• The main factor limiting the low temperature performance of lithium ion batteries is the sharp increase in Li+ diffusion resistance at low temperatures, rather than the SEI film.
Low temperature characteristics of cathode materials for lithium ion batteries.
• Low temperature characteristics of layered structure cathode material
The layered structure, which has the incomparable rate performance of the one-dimensional lithium ion diffusion channel and the structural stability of the three-dimensional channel, is the earliest commercially available cathode material for lithium ion batteries. Representative materials are LiCoO2, Li(Co1-xNix)O2, and Li(Ni, Co, Mn)O2, etc.
The results show that as the temperature decreases, the discharge platform drops from 3.762V (0°C) to 3.207V (–30°C); the total battery capacity is also reduced from 78.98mA•h (0°C) to 68.55mA•h. (–30 ° C).
• Low temperature characteristics of spinel structure cathode materials.
The spinel structure LiMn2O4 cathode material has the advantages of low cost and no toxicity because it does not contain Co element.
However, the variability of Mn valence and the Jahn-Teller effect of Mn3+ lead to problems such as structural instability and poor reversibility.
Different preparation methods have great influence on the electrochemical performance of LiMn2O4 cathode material. Taking Rct as an example: the Rct of LiMn2O4 synthesized by high temperature solid phase method is obviously higher than that of sol gel method, and this phenomenon also has lithium ion diffusion coefficient. Reflected. The reason is mainly because different synthetic methods have a great influence on the crystallinity and morphology of the product.
• Low temperature characteristics of phosphate system cathode materials
LiFePO4, together with ternary materials, has become the main body of current battery cathode materials due to its excellent volume stability and safety. The low-temperature performance of lithium iron phosphate is mainly due to the fact that the material itself is an insulator, the electronic conductivity is low, the lithium ion diffusibility is poor, and the conductivity at low temperature is poor, so that the internal resistance of the battery increases, the polarization is greatly affected, and the battery is blocked by charging and discharging, so the low temperature Performance is not ideal.
The charge-discharge behavior of LiFePO4 decreases from 100% at 55°C to 96% at 0°C and 64% at –20°C; discharge voltage decreases from 3.11V at 55°C to 2.62 at –20°C. V.
Xing et al. modified LiFePO4 with nano-carbon. It was found that the sensitivity of LiFePO4 to temperature was reduced and the low-temperature performance was improved after the addition of nano-carbon conductive agent. The discharge voltage of LiFePO4 after modification was 3.40 from 25 °C. V dropped to 3.09V at –25°C, a reduction of only 9.12%; and its cell efficiency was 57.3% at –25°C, which was higher than 53.4% without nanocarbon conductive agent.
Recently, LiMnPO4 has aroused people's strong 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, since LiMnPO4 has a lower ionic conductivity than LiFePO4, it is often used in practice to replace the Mn with Fe to form a LiMn0.8Fe0.2PO4 solid solution.
Low temperature characteristics of anode materials for lithium ion batteries.
Compared with the positive electrode material, the low temperature deterioration phenomenon of the lithium ion battery anode material is more serious, mainly for the following three reasons:
• The battery polarization is severe at high temperature and large rate charge and discharge, and the lithium metal on the surface of the negative electrode is deposited in a large amount, and the reaction product of lithium metal 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;
• Carbon anodes are difficult to insert lithium at low temperatures, and there is a charge and discharge asymmetry.
Research on low temperature electrolyte.
The electrolyte plays a role in transferring Li+ in the lithium ion battery, and its ionic conductivity and SEI film forming performance have a significant effect on the low temperature performance of the battery. To judge the advantages and disadvantages of low temperature electrolytes, there are three main indicators: ionic conductivity, electrochemical window and electrode reactivity. The level of these three indicators depends to a large extent on their constituent materials: solvent, electrolyte (lithium salt), additives. Therefore, the study of the low temperature properties of various parts of the electrolyte is of great significance for understanding and improving the low temperature performance of the battery.
• Low-temperature properties of EC-based electrolytes Compared to chain carbonates, cyclic carbonates have a tight structure, high force, and high melting point and viscosity. However, the large polarity brought about by the ring structure tends to have a large dielectric constant. The large dielectric constant of EC solvent, high ionic conductivity, excellent film-forming property, and effective co-insertion of solvent molecules make it an indispensable position. Therefore, most commonly used low-temperature electrolyte systems are based on EC, and then mixed. Low melting point small molecule solvent.
• Lithium salts are an important component of the electrolyte. The lithium salt not only increases the ionic conductivity of the solution in the electrolyte, but also reduces the diffusion distance of Li+ in the solution. In general, the greater the Li+ concentration in the solution, the greater its ionic conductivity. However, the concentration of lithium ions in the electrolyte is not linearly related to the concentration of the lithium salt, but is parabolic. This is because the concentration of lithium ions in the solvent depends on the dissociation of the lithium salt in the solvent and the strength of association.
In addition to the battery composition itself, the process factors in actual operation can also have a significant impact on battery performance.
(1) Preparation process. Yaqub et al studied the effect of electrode loading and coating thickness on the low temperature performance of LiNi0.6Co0.2Mn0.2O2/Graphite battery. It is found that the smaller the electrode load, the thinner the coating layer, the better the low temperature performance in terms of capacity retention. .
(2) Charge and discharge status. Petzl et al. studied the effect of low temperature charge and discharge on battery cycle life and found that when the depth of discharge is large, it will cause large capacity loss and reduce cycle life.
(3) Other factors. The surface area, pore size, electrode density, wettability of the electrode and electrolyte, and the separator all affect the low temperature performance of the lithium ion battery. In addition, the effects of defects in materials and processes on the low temperature performance of the battery cannot be ignored.
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 temperatures.
In addition, the research can also take a different approach and focus on another type of lithium-ion battery - 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 problem of capacity attenuation and cycle safety of batteries used at low temperatures.
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