This article provides its reader with information on the design of secondary lithium-ion batteries and some of the reasons why a lithium-ion battery may fail in an unsafe manner.
I. Battery design
Secondary lithium-ion batteries have a complex design due to the addition of safety circuitry and precise control of voltage, current and temperature. A case or a wrapper houses the battery cells and usually also the circuit board (also known as battery protection circuit or BMU). The latter protects the cells against variables such as over-current charge, over-voltage charge and over-temperature charge. Since secondary batteries are rechargeable, there is also a charger system. The charger system is an electrical device controlling the amount of charge going into a lithium-ion battery and prevents overcharging. Overcharging may cause certain reliability and safety issues within the battery cell. E.g., lithium plating, degradation of the electrolyte, thermal runaway and gas generation.
II. Lithium-ion battery failure causes
Lithium-ion battery failure may be due to several reasons. The below list provides some of the most significant causes for safety-related failure.
- Electrical over-stress
Various components (e.g. transient suppressors and battery cells) are sensitive to electrical overstress and may fail thermally. An element can heat or ignite adjacent surfaces, creating electrical product hazards such as fire. Overheating is possible to occur during the charge or discharge of the lithium-ion cell. Installing a control circuit in the battery pack can help avoid such hazards and ensure the cell’s correct operation.
- Thermal over-stress
Lithium-ion batteries are sensitive to temperature. Because of that, the battery cell must always operate within a specific temperature range. When the temperature is below the recommended, undesirable lithium plating may occur. Furthermore, venting of the battery cell can cause the electrolyte to migrate to the circuit board of the battery pack or the product device. Consequently, the electrolyte may cause propagating circuit board failures, leading to external heating of the cell and forcing the cell into thermal runaway.
- Mechanical over-stress
Safety issues can occur when the battery cell or the circuit is mechanically stressed or damaged. Damage to any of these two components may lead to immediate battery failure or induce a defect before the failure occurs. The cell’s failure mode depends on factors such as the extent of mechanical damage, age of the cell, ambient temperature, and the state of charge. If there is mechanical stress on the cell, the separator within the cell may fail, resulting in an anode-to-cathode short circuit. If the damage is to the circuit board, the result could be malfunctioning.
- Cell internal fault
An internal cell fault can occur due to manufacturing defects, contaminated raw materials, improper cell design and failed separators, among many. To minimize internal cell faults, the cell’s design and manufacturing need to pass conformity evaluation. The cell design must comply with the technical specifications of the relevant battery safety standards and receive safety certification. The cell’s manufacturing needs to be managed by safety audits and certification. Implementing a quality management system will help ensure the consistency of the manufacturing process.
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The above-listed information can be used in the safety failure modes and effects analysis (SFMEA). During the safety evaluation, it is best to follow a systems approach based on relevant secondary battery safety standards (e.g. IEEE 1625). Such an approach will help evaluate the battery system from the perspectives of the users, environment, power supply, host device, battery pack and the battery cell.