Understanding the Risks: Thermal Runaway and Safety Mechanisms in Lithium-Ion Batteries

Research Background

The advent of lithium-ion batteries has been a game-changer for modern technology, providing efficient energy storage solutions for electronic products, electric vehicles, and a host of other applications. However, the rise of this technology has brought with it a number of safety concerns that have become increasingly apparent through a series of incidents.

Notable instances such as the Samsung Note 7 smartphone fires in 2016 and the Tesla car fires in Shanghai in 2017 have underscored the potential dangers of lithium-ion batteries. These incidents were characterized by smoke emission, spontaneous ignition, and even explosions. Such safety incidents have set off alarm bells within the industry and among consumers, highlighting the urgent need for a deeper understanding of the risks associated with these batteries.

The 'Made in China 2025' Vision and Safety

As a part of the "Made in China 2025" initiative, there is a clear technology roadmap for key areas, including the development of new energy vehicle power batteries. The goals set for 2020, 2025, and 2030 focus on increasing the specific energy of monomer batteries, extending service life, and reducing costs. These ambitious targets go hand-in-hand with the necessity for improved safety measures, as higher energy densities could potentially increase the risks of thermal runaway and other hazardous events.

The Science of Lithium-Ion Batteries and Risks

Lithium-ion batteries function through the movement of lithium ions between the cathode and anode. While this process is typically safe, it can become dangerous under certain conditions. For instance, if the battery is overcharged, it can lead to overheating or even thermal runaway—a condition where the battery generates heat faster than it can dissipate, potentially leading to a fire or explosion.

Factors Contributing to Thermal Runaway

The 'Technical Roadmap for Energy Saving and New Energy Vehicles' released by the Ministry of Industry and Information Technology in 2016 proposed specific energy targets for pure electric vehicle power battery cells, which present the challenge of ensuring safety while meeting these high energy density requirements.

Thermal runaway can be triggered by a variety of factors, including:

Process Factors: These include misalignment of poles, burrs on pole pieces, and uneven electrolyte distribution, which can lead to short circuits.

Material Factors: Impurities in materials and exothermic side reactions can cause battery temperature rise.

Application Process: Vibration, drop, collision, overcharge, and high current charging can all contribute to potential risks in the battery system.

The Anatomy of Thermal Runaway

Thermal runaway is a catastrophic event involving several exothermic reactions. When the temperature of a battery exceeds a critical level, it can lead to the decomposition of the solid electrolyte interface (SEI) film, catalyzing a series of reactions that generate significant heat. This includes the reduction and decomposition of the electrolyte, thermal decomposition of the charged positive electrode, and the reaction between the binder and the highly reactive negative electrode.

Safety Mechanisms in Lithium-Ion Batteries

To mitigate the risks, several safety mechanisms have been incorporated into the design of lithium-ion batteries:

Shutdown Separator: A safety feature that shuts off ion flow when the temperature reaches a certain threshold, preventing further chemical reactions.

Circuit Protection: Battery packs often include a control IC, MOSFET, and other devices to ensure operation within a safe voltage range.

Flame Retardant Additives: Certain compounds can be added to the electrolyte to reduce flammability.

Battery Management Systems (BMS): These systems monitor the state of the battery and can take action to prevent dangerous conditions such as overcharging or overheating.

Safety Testing and Standards

Safety testing is a critical part of battery development, ensuring that batteries can withstand a range of stressful conditions without failing. Tests include overvoltage charging, external short circuits, overcurrent charging, high-temperature exposure, and mechanical stress tests such as shock, vibration, and impact.

Outlook on Battery Safety Issues

Despite the risks, the continued evolution of lithium-ion battery technology is imperative. Safety issues remain a significant bottleneck in the development and energy density improvement of power batteries. It is crucial that the industry continues to develop and enhance safety technologies to prevent short circuits, overcharge, thermal runaway, and non-combustion incidents. Research into new lithium-ion systems, including aqueous lithium-ion and all-solid-state batteries, is progressing and may offer safer alternatives to current technologies.

Conclusion

Understanding the risks associated with lithium-ion batteries is essential for the continued safe use of these powerful energy storage devices. As we push the boundaries of battery technology to meet the growing demand for higher energy densities and longer life spans, we must also innovate and implement robust safety mechanisms to protect users and property from the potential dangers of thermal runaway and other related hazards. Only by balancing these two aspects can we secure the future of lithium-ion batteries in the new energy landscape.