Non-Flammable Electrolyte Helps Prevent Thermal Runaway

Battery safety is a huge concern for lithium-ion batteries. Within this, thermal runaway is considered one of the biggest sources of failure, combustion, and explosion.

 

Lithium battery fire. Image used courtesy of Adobe Stock

 

To help counteract the impacts of thermal runaway, a group of researchers recently developed a new, non-flammable electrolyte for lithium-ion batteries. This article examines thermal runaway, the role of the electrolyte in the process, and the research that aims to solve this problem. 

 

Thermal Runaway

Thermal runaway is one of the biggest concerns concerning safety in lithium-ion batteries.

The process can be defined as a self-sustaining, exothermic reaction within a battery that leads to a rapid and uncontrollable increase in temperature. This phenomenon can result in catastrophic failure in lithium-ion batteries, including fire or explosion.

 

Stages of thermal runaway. Image used courtesy of Pfrang et al.

 

The process of thermal runaway typically begins with localized heating within the battery. This can be caused by internal or external short circuits, overcharging, or physical damage. As the temperature rises, the separator between the electrodes may melt, leading to further internal short circuits, causing additional heating and further exacerbating the condition. At elevated temperatures, the electrolyte and other materials within the battery can undergo exothermic reactions, releasing more heat and further fueling the process. 

Many lithium-ion batteries have built-in safety mechanisms to prevent overcharging or overheating. Still, in a thermal runaway scenario, these safety features may fail, allowing the reaction to continue unchecked. If left uncontrolled, the thermal runaway can lead to the rupture of the battery casing, release of toxic gases, and even fire or explosion.

 

The Role of the Electrolyte

In lithium-ion batteries, the electrolyte serves as the medium through which lithium ions move between the anode and cathode during charging and discharging. Its role in the battery's operation is vital, but it also plays a complex and critical part in the initiation and propagation of thermal runaway.

The electrolyte can contribute to the initiation of thermal runaway through several mechanisms. Impurities or contaminants in the electrolyte can lead to localized heating, and overcharging the battery can cause lithium plating on the anode, leading to internal short circuits. These factors can cause the electrolyte to heat up, setting the stage for thermal runaway.

 

The electrolyte can be a major contributor to thermal runaway. Image used courtesy of Tian et al.

 

Once thermal runaway has started, the electrolyte's role becomes even more significant. As the temperature within the battery rises, the electrolyte can undergo exothermic reactions, releasing more heat and further fueling the temperature increase. This creates a feedback loop where the heating of the electrolyte leads to more reactions, generating additional heat and exacerbating the condition.

At very high temperatures, the electrolyte can break down into constituent components, some of which may be flammable. This breakdown can lead to combustion, contributing to the catastrophic failure of the battery, including fire or explosion. The properties of the electrolyte, such as its thermal stability and conductivity, can also influence the effectiveness of the battery's safety mechanisms. 

 

Non-Flammable Electrolyte Alternative Research

A group of researchers recently developed a non-flammable electrolyte alternative to solve the issue of the electrolyte's role in thermal runaway.

The researchers achieved this through molecularly engineering the linear carbonates of the electrolyte. This approach decreases solvent volatility and increases solvation ability, leading to a thermally stable high-performance battery. The resulting molecule, bis(2-methoxyethyl) carbonate (BMEC), exhibits a flash point 90°C higher than conventional electrolytes, thus significantly reducing the risk of ignition at room temperature.

 

The molecular design strategy for a high-flashpoint electrolyte. Image used courtesy of Lee et al.

 

Unlike previous attempts to reduce flammability, which often compromised battery performance and cost, this new approach maintains the electrolyte quality and enables long-lasting cycling. The BMEC solution has a flash point of 121°C and retains more than 92% of the original rate capability of conventional electrolytes. It also alleviates 37% of combustible gas evolution and 62% of heat generation compared to conventional electrolytes.

With the new electrolyte, the researchers demonstrated the stable operation of 1Ah lithium-ion batteries over 500 cycles and confirmed suppressed thermal runaway through nail-penetration tests on 4Ah-level Li-ion batteries. The synthesis method of BMEC can be easily scaled up, and the research team is also exploring eco-friendly synthesis methods.