Self-discharge of lithium-ion batteries refers to the natural reduction of charge capacity when the battery is not in use. This phenomenon is unavoidable, but an excessively high self-discharge rate will impair battery performance and cycle life. Therefore, cells with a high self-discharge rate must be screened out before delivery. At present, the K-value is generally used to measure the self-discharge rate in mass-produced batteries.
Self-discharge is usually divided into two categories:
① Reversible self-discharge (temporary capacity loss, which can be recovered by recharging);
② Irreversible self-discharge (permanent capacity loss).
The main causes of self-discharge are as follows:
- Chemical Side Reactions
- Electrolyte decomposition
Solvents in the electrolyte (such as carbonates) may react slowly with cathode and anode materials, consuming active lithium ions. Especially at a high state of charge (SOC), the high potential on the cathode surface easily oxidizes carbonate solvents into CO₂ and various intermediate polymers, resulting in Li⁺ loss. Although the anode is protected by the SEI film, slight reduction may still occur, continuously consuming reversible lithium. - Instability of the SEI film (Solid Electrolyte Interphase)
Poorly formed or damaged SEI film on the anode surface allows the electrolyte to continue reacting with the anode (e.g., graphite), consuming lithium ions. SEI film formation mainly occurs during the formation process. Improper formation temperature, current, or time will lead to an unsatisfactory SEI film, which continuously decomposes and regenerates during subsequent charge-discharge cycles. This persistent consumption of reversible lithium ions causes abnormal cell behavior. - Redox reactions of electrode materials
① Cathode materials (such as NCM and LFP) may oxidize due to trace moisture or impurities in the electrolyte.
② Anode materials (such as graphite and silicon-based materials) may consume lithium ions by reacting with trace moisture or oxygen in the electrolyte.
- Micro Short Circuits
- Internal impurities or dendrites
Metal impurities (such as Cu, Fe particles) remaining during manufacturing or lithium dendrites formed during cycling may pierce the separator, causing local short circuits between the cathode and anode. - Separator defects
Non-uniform pores or damage in the separator create electronic leakage paths, generating small short-circuit currents.
Such abnormal self-discharge often causes the battery K-value (self-discharge rate) to jump by 1–2 orders of magnitude, which is an early sign of potential failure.
- Battery Aging
Battery aging is inevitable, and every battery has a designed service life. With increasing cycle numbers, the electrode structure gradually degrades, the SEI film thickens, side reactions intensify, and the self-discharge rate gradually increases.
II. Hazards of Self-Discharge
- Capacity Loss and Shortened Lifespan
Irreversible self-discharge directly reduces usable capacity and affects device runtime. Meanwhile, continuous side reactions accelerate the degradation of electrodes and electrolyte, shortening cycle life.
- Voltage Drop
Self-discharge causes battery voltage to drop, which may trigger the protection circuit to cut off discharge in advance, resulting in unexpected shutdown of the device.
- Safety Risks
- Thermal runaway risk: Micro-short circuits or local reactions may induce local overheating, especially at high temperatures or high SOC, amplifying local reactions and causing safety hazards.
- Cell imbalance: In series battery packs, inconsistent self-discharge rates lead to increased voltage differences among cells, which may cause overcharge or over-discharge.
- Storage and Transportation Issues
During long-term storage, high self-discharge batteries may become over-discharged due to full capacity depletion, leading to irreversible damage (e.g., dissolution of the copper current collector).
In summary, the self-discharge of lithium-ion batteries results from a combination of factors, and its hazards are mainly reflected in capacity fade, safety risks, and shortened service life. The self-discharge rate can be effectively controlled and battery reliability improved by optimizing materials, processes, and storage conditions. For users, avoiding extreme temperatures and properly controlling the state of charge are key measures to slow down self-discharge.
