Reasons for Self-Discharge in Lithium-Ion Batteries

Self-discharge in lithium-ion batteries refers to the phenomenon where the battery's charge/voltage naturally decreases when disconnected from an external circuit (i.e., in an open-circuit state). This is an inherent characteristic of all batteries, though the degree varies. Lithium-ion batteries exhibit a relatively low self-discharge rate but are still affected. The primary causes can be categorized as follows:

1. Inevitable Chemical Side Reactions (Normal Self-Discharge)

(1) SEI Layer Growth and Dissolution:

The anode (typically graphite) is coated with a Solid Electrolyte Interphase (SEI) layer formed during initial charging/discharging, which is essential for battery operation. However, the SEI layer is not perfectly stable. During storage, especially at elevated temperatures, it undergoes slow dissolution and reformation. This reformation consumes lithium ions and electrolyte, leading to capacity loss and voltage drop—a major contributor to self-discharge.

(2) Electrolyte Oxidation/Reduction:

Charged cathode materials (e.g., LiCoO₂, NCM, LiFePO₄) are highly oxidative. Electrolyte solvents (e.g., EC, DMC) and additives gradually decompose through oxidation when in prolonged contact with the high-potential cathode. Similarly, at the anode, despite SEI protection, minor reduction reactions of the electrolyte may occur. These parasitic reactions deplete active lithium ions, causing capacity fade.

(3) Impurity Reactions:

Trace impurities (e.g., Fe, Cu, Zn ions) in electrode materials or current collectors can create micro-shorts or participate in side reactions, consuming charge.

2. Internal Micro-Short Circuits (Caused by Manufacturing Defects or Aging)

(1) Separator Defects:

Microscopic pinholes, impurities, or weak spots in the separator may allow electronic conduction (micro-shorts) between electrodes after cycling or long-term storage, directly leaking charge. This is a primary cause of abnormally high self-discharge. While separators macroscopically block electrons, microscopic electron leakage paths can form via conductive networks or electrolyte.

(2) Dendrite Penetration:

Lithium dendrites may form unevenly on the anode due to overcharging, low-temperature charging, or aging. Sharp dendrites can penetrate the separator, bridging electrodes and causing internal shorts.

(3) Metal Dust Contamination:

Residual metal dust (e.g., from electrode cutting) trapped between electrodes or separators can cause micro-shorts. While absolute dust-free conditions are unattainable, minor dust has negligible impact. However, dust exceeding the threshold to pierce the separator significantly accelerates self-discharge. For high-quality separator solutions, see our Battery Production Line Equipment.

3. Temperature Effects

Temperature is a critical factor. Higher temperatures exponentially accelerate all self-discharge reactions (SEI evolution, electrolyte decomposition, impurity reactions). Thus, batteries should be stored at low temperatures (avoiding freezing) for long-term storage.

4. Impacts of Self-Discharge

• Capacity Loss: Reduced usable capacity.

• Voltage Drop: Decreased open-circuit voltage (OCV) over time.

• Accelerated Aging: Side reactions (e.g., SEI growth) consume active lithium/electrolyte, accelerating aging.

• SOC Estimation Challenges: Self-discharge complicates accurate state-of-charge (SOC) estimation via voltage.

• Safety Risks: Severe micro-shorts may cause localized heating or thermal runaway.

5. Mitigation Strategies

(1) Optimize Design & Materials:

Enhance SEI stability, develop oxidation-resistant electrolytes, use high-purity materials, and improve separator quality. Explore our Custom Battery Equipment for tailored solutions.

(2) Control Storage Conditions:

• Temperature: Store at 10°C–25°C (avoid <0°C).

• SOC: Maintain 40%–60% SOC for long-term storage. Full charge accelerates electrolyte oxidation; deep discharge risks anode damage.

(3) Periodic Recharging:

For idle batteries, monitor voltage/SOC and recharge to ~50% when low to prevent deep discharge.

(4) Strict Manufacturing Control:

Minimize impurities/metal dust and ensure separator integrity. Our Battery Material Supply offers high-purity materials to reduce contamination risks.

Conclusion