Why Lithium-Ion Battery Capacity Decreases in Winter
Since entering the market, lithium-ion batteries have gained widespread use due to their long lifespan, high specific capacity, and lack of memory effect. However, when used in low temperatures, lithium-ion batteries exhibit issues such as reduced capacity, severe degradation, poor cycling performance, pronounced lithium plating, and unbalanced lithium stripping.
Despite these challenges, the expanding application areas of lithium-ion batteries have made the limitations of their poor performance in cold temperatures increasingly apparent.
At -20°C, the discharge capacity of lithium-ion batteries is only about 31.5% of that at room temperature. Traditional lithium-ion batteries typically operate within a temperature range of -20 to +55°C. However, in industries such as aerospace, defense, and electric vehicles, there is a demand for batteries to function normally at -40°C. Therefore, improving the low-temperature performance of lithium-ion batteries holds significant importance.
Factors Constraining the Low-Temperature Performance of Lithium-Ion Batteries:
In low-temperature environments, the viscosity of the electrolyte increases, and in some cases, it may even partially solidify, leading to a decrease in the conductivity of lithium-ion batteries.
The compatibility between the electrolyte and the negative electrode, as well as the separator, deteriorates in low-temperature conditions.
Severe lithium deposition occurs on the negative electrode of lithium-ion batteries in low-temperature environments. The reaction between deposited lithium metal and the electrolyte results in the deposition of products, leading to an increase in the thickness of the solid electrolyte interface (SEI).
In low-temperature environments, the internal diffusion system of active materials in lithium-ion batteries is hindered, and the charge transfer impedance (Rct) significantly increases.
Discussion on Factors Affecting the Low-Temperature Performance of Lithium-Ion Batteries:
The electrolyte has the most significant impact on the low-temperature performance of lithium-ion batteries. The composition and physicochemical properties of the electrolyte play a crucial role in the low-temperature performance of the battery. At low temperatures, the increased viscosity of the electrolyte leads to slower ion conduction, causing a mismatch in electron migration rates in the external circuit. This results in severe polarization and a sharp decrease in charge-discharge capacity. Particularly during low-temperature charging, lithium ions tend to form dendrites on the negative electrode surface, leading to battery failure.
The low-temperature performance of the electrolyte is closely related to its own electrical conductivity. A higher electrical conductivity allows for faster ion transport, enabling the battery to deliver more capacity at low temperatures. The dissociation of lithium salts in the electrolyte contributes to a higher number of migrating ions and, consequently, a higher electrical conductivity. Thus, higher electrical conductivity is a necessary condition for achieving good low-temperature performance in lithium-ion batteries.
The electrical conductivity of the electrolyte is influenced by its composition. Reducing the viscosity of the solvent is one way to improve the electrical conductivity of the electrolyte. The good flowability of the solvent at low temperatures ensures the transport of ions. Additionally, the solid electrolyte film formed on the negative electrode in low-temperature electrolytes is a critical factor affecting lithium-ion conduction, with RSEI being the primary impedance in lithium-ion batteries under low-temperature conditions.
The main factor limiting the low-temperature performance of lithium-ion batteries is the sharp increase in Li+ diffusion impedance at low temperatures, rather than the SEI film.
Low-Temperature Characteristics of Lithium-Ion Battery Cathode Materials
Low-Temperature Characteristics of Layered Cathode Materials
Layered cathode materials, possessing both the unmatched rate performance of one-dimensional lithium ion diffusion channels and the structural stability of three-dimensional channels, were the earliest commercially used lithium-ion battery cathode materials. Representative substances include LiCoO2, Li(Co1-xNix)O2, and Li(Ni,Co,Mn)O2. Taking LiCoO2/MCMB as the research subject, its low-temperature charge-discharge characteristics were tested. The results showed that as the temperature decreased, the discharge plateau decreased from 3.762V (0°C) to 3.207V (-30°C). The total capacity of the battery also sharply reduced from 78.98 mA·h (0°C) to 68.55 mA·h (-30°C).
Low-Temperature Characteristics of Spinel Cathode Materials
Spinel structure LiMn2O4 cathode materials, due to the absence of Co elements, offer advantages such as low cost and non-toxicity. However, the multivalent nature of Mn and the Jahn-Teller effect of Mn3+ result in issues such as structural instability and poor reversibility for this component. Different preparation methods have a significant impact on the electrochemical performance of LiMn2O4 cathode materials, as observed in terms of Rct (charge transfer resistance). LiMn2O4 synthesized using high-temperature solid-phase methods exhibits significantly higher Rct compared to that synthesized using sol-gel methods. This phenomenon is also reflected in the lithium ion diffusion coefficient. The main reason for this difference is the substantial influence of different synthesis methods on the crystallinity and morphology of the products.
Low-Temperature Characteristics of Phosphate System Cathode Materials
LiFePO4, due to its excellent volumetric stability and safety, has become a mainstay in current power battery cathode materials, alongside ternary materials. However, the poor low-temperature performance of lithium iron phosphate is primarily attributed to its insulating nature, low electronic conductivity, poor lithium ion diffusion, and reduced conductivity at low temperatures, leading to increased internal resistance and significant polarization effects, thus compromising the ideal low-temperature performance.
When studying the charge-discharge behavior of LiFePO4 at low temperatures, it was found that its Coulombic efficiency decreased from 100% at 55°C to 96% at 0°C and 64% at -20°C. The discharge voltage decreased from 3.11V at 55°C to 2.62V at -20°C. Researchers, such as Xing et al., have improved the low-temperature performance of LiFePO4 by modifying it with nano-carbon. After adding nano-carbon conductive agents, the sensitivity of LiFePO4’s electrochemical performance to temperature decreased, and its low-temperature performance improved.
The discharge voltage of modified LiFePO4 decreased only by 9.12%, from 3.40V at 25°C to 3.09V at -25°C. Moreover, the cell efficiency at -25°C was 57.3%, higher than the 53.4% of LiFePO4 without nano-carbon conductive agents. Recently, LiMnPO4 has attracted significant interest.
tudies have shown that LiMnPO4 possesses advantages such as a high voltage (4.1V), no pollution, low cost, and a large specific capacity (170mAh/g). However, due to the lower ion conductivity of LiMnPO4 compared to LiFePO4, Fe is often used to partially replace Mn to form a LiMn0.8Fe0.2PO4 solid solution in practice.
Low-Temperature Characteristics of Lithium-Ion Battery Anode Materials
Compared to cathode materials, the low-temperature deterioration of lithium-ion battery anode materials is more severe, primarily due to the following three reasons:
Severe polarization occurs during high-rate charge-discharge at low temperatures, leading to significant deposition of metallic lithium on the anode surface. Additionally, the reaction products of lithium metal with the electrolyte generally lack 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 the anode material. The formed solid electrolyte interface (SEI) film is more susceptible to low-temperature effects.
Lithium-ion diffusion in carbon anodes is challenging at low temperatures, leading to asymmetric charge-discharge behavior.
These factors contribute to the pronounced degradation of lithium-ion battery anode materials in low-temperature conditions.
Research on Low-Temperature Electrolytes
The electrolyte in lithium-ion batteries plays a crucial role in facilitating the transfer of Li+ ions. Its ion conductivity and the formation of the solid electrolyte interface (SEI) significantly impact the low-temperature performance of the battery. To assess the quality of electrolytes for low-temperature applications, three main indicators are considered: ion conductivity, electrochemical window, and electrode reaction activity.
The levels of these three indicators largely depend on the composition of the electrolyte, including solvents, electrolytes (lithium salts), and additives. Therefore, studying the low-temperature performance of each component of the electrolyte is of great significance for understanding and improving the low-temperature performance of batteries.
EC-Based Electrolyte Low-Temperature Characteristics: Compared to linear carbonates, cyclic carbonate structures in EC-based electrolytes are more closely packed, have higher melting points, and exhibit greater viscosity.
However, the large polarity introduced by cyclic structures often results in a high dielectric constant. EC solvents, with their large dielectric constant, high ion conductivity, and excellent film-forming properties, effectively prevent solvent molecules from co-inserting. Consequently, EC-based electrolyte systems are commonly used for low-temperature applications, often mixed with low-melting-point small-molecule solvents.
Lithium Salt as an Essential Component: Lithium salt is a crucial component of the electrolyte. It not only enhances the ion conductivity of the solution but also reduces the diffusion distance of Li+ ions in the solution. Generally, the higher the Li+ concentration in the solution, the greater the ion conductivity.
However, the relationship between lithium ion concentration in the solution and lithium salt concentration is not linear but rather follows a parabolic trend. This is because the lithium ion concentration in the solvent depends on the strength of the dissociation and complexation of lithium salt in the solvent.
Research on Low-Temperature Electrolytes
In addition to the battery components, process factors during actual operation also significantly impact battery performance.
(1) Preparation Process: Studies have shown that the electrode loading and coating thickness significantly affect the low-temperature performance of LiNi0.6Co0.2Mn0.2O2 / Graphite batteries. Smaller electrode loading and thinner coating layers lead to better low-temperature performance in terms of capacity retention.
(2) Charge-Discharge State: Research on the influence of low-temperature charge-discharge states on battery cycle life has found that larger discharge depths result in significant capacity loss and reduced cycle life.
(3) Other Factors: Electrode surface area, pore size, electrode density, wettability of the electrode with the electrolyte, and separator properties all influence the low-temperature performance of lithium-ion batteries. Additionally, defects in materials and processes should not be overlooked for their impact on low-temperature battery performance.
To ensure the low-temperature performance of lithium-ion batteries, the following aspects need attention:
(1) Formation of a thin and dense SEI film.
(2) Ensuring that Li+ ions have a large diffusion coefficient in the active material.
(3) The electrolyte should exhibit high ion conductivity at low temperatures.
Furthermore, an alternative approach in research could focus on a different type of lithium-ion battery—solid-state lithium-ion batteries. Compared to conventional lithium-ion batteries, especially thin-film solid-state lithium-ion batteries, these have the potential to completely address issues of capacity decay and cycling safety during low-temperature usage.