Why Lithium-Ion Battery Capacity Decreases in Winter

Since entering the market, lithium-ion batteries have gained widespread use thanks to their long lifespan, high specific capacity, and lack of memory effect. However, in low-temperature environments, they face significant challenges such as:

• Reduced capacity
• Severe degradation
• Poor cycling performance
• Pronounced lithium plating
• Unbalanced lithium stripping

With the expanding applications of lithium-ion batteries, these limitations in cold environments have become increasingly apparent.

At -20°C, the discharge capacity of lithium-ion batteries drops to about 31.5% of that at room temperature. While traditional lithium-ion batteries typically operate between -20°C and +55°C, industries like aerospace, defense, and electric vehicles require normal operation at -40°C. Thus, improving low-temperature performance is crucial.

Factors Constraining Low-Temperature Performance

Several factors limit lithium-ion batteries at low temperatures:

• Increased electrolyte viscosity or partial solidification, reducing conductivity
• Poor compatibility between the electrolyte, anode, and separator
• Severe lithium deposition on the anode, thickening the solid electrolyte interface (SEI)
• Hindered ion diffusion in active materials and increased charge-transfer impedance (Rct)

Electrolyte Impact on Low-Temperature Performance

The electrolyte has the most significant influence on cold-weather performance. Its composition and physicochemical properties determine:

• Ion conduction speed
• Electron migration matching
• Polarization effects during charge-discharge

At low temperatures, slow ion conduction can lead to lithium dendrite formation on the anode, causing battery failure. Higher electrical conductivity of the electrolyte ensures faster ion transport, improving capacity retention.

Key strategies to improve electrolyte performance include:

• Reducing solvent viscosity to maintain flowability at low temperatures
• Optimizing SEI film formation to lower impedance (RSEI)
• Using lithium salts with good dissociation properties to increase migrating ions

Notably, the main limitation in low-temperature performance is increased Li+ diffusion impedance, rather than the SEI film itself.

Low-Temperature Characteristics of Cathode Materials

Layered Cathode Materials

Layered cathodes, like LiCoO2, Li(Ni,Co,Mn)O2, offer stable 3D structures and fast 1D lithium diffusion channels. Tests show that at -30°C, LiCoO2/MCMB cells drop from 3.762V (0°C) to 3.207V, and total capacity decreases from 78.98 mAh to 68.55 mAh.

Spinel Cathode Materials

LiMn2O4 offers low cost and non-toxicity but suffers from structural instability due to the Jahn-Teller effect of Mn3+. The synthesis method affects electrochemical performance, with high-temperature solid-phase methods yielding higher charge transfer resistance (Rct) than sol-gel methods.

Phosphate Cathode Materials

LiFePO4 provides excellent safety and stability but poor low-temperature performance due to low electronic conductivity and lithium-ion diffusion. Its discharge voltage drops significantly at -20°C, but adding nano-carbon conductive agents improves low-temperature behavior. For example:

• Discharge voltage decreases only 9.12% from 25°C (3.40V) to -25°C (3.09V)
• Cell efficiency at -25°C improves to 57.3% from 53.4% without nano-carbon

LiMnPO4 is promising due to high voltage (4.1V) and large specific capacity (170 mAh/g), though lower ion conductivity often requires partial replacement with Fe to form LiMn0.8Fe0.2PO4.

Low-Temperature Characteristics of Anode Materials

Anode degradation is more severe due to:

• Severe polarization causing lithium metal deposition
• SEI film vulnerability under low-temperature effects
• Difficult lithium-ion diffusion in carbon anodes, leading to asymmetric charge-discharge

These factors contribute to significant low-temperature performance decline in anode materials.

Low-Temperature Electrolyte Research

The electrolyte plays a critical role in facilitating Li+ transfer. Key performance indicators include:

• Ion conductivity
• Electrochemical window
• Electrode reaction activity

EC-Based Electrolytes

Cyclic carbonates like ethylene carbonate (EC) have high dielectric constants and viscosity but provide strong ion conductivity and stable film formation, making them suitable for low-temperature use when mixed with low-melting-point solvents.

Lithium Salts

Lithium salt concentration affects ion conductivity, but the relationship is parabolic, depending on dissociation and complexation in the solvent.

Process and Operational Factors

• Electrode preparation: Lower loading and thinner coatings improve low-temperature capacity retention
• Charge-discharge state: Deep discharges can reduce cycle life and capacity
• Other factors: Electrode surface area, porosity, density, wettability, and separator quality affect low-temperature behavior

Key strategies include:

1. Forming a thin, dense SEI film
2. Ensuring high Li+ diffusion coefficients in active materials
3. Using high-conductivity electrolytes at low temperatures

Future Outlook: Solid-State Lithium-Ion Batteries

Solid-state lithium-ion batteries, especially thin-film types, show potential to overcome capacity decay and cycling issues in low temperatures, offering a promising alternative to conventional lithium-ion batteries.