Lithium-ion batteries have become an integral part of our daily lives, powering everything from smartphones to electric vehicles. Pouch cells, a popular lithium-ion battery format, have gained widespread recognition due to their flexibility, lightweight design, and high energy density. While these attributes make pouch cells a desirable choice for many applications, the type of electrolyte used within them must be considered to ensure their safety. In this article, we will explore how the choice of electrolyte influences the safety of pouch cells, with a focus on the tab and other important considerations.
The choice of electrolyte significantly impacts the thermal stability of a pouch cell. Electrolytes are typically composed of lithium salts dissolved in a solvent. Common electrolyte solvents include ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). These solvents have varying thermal stability properties. The wrong choice of solvent can result in thermal runaway, leading to fires or explosions. Manufacturers carefully select electrolytes to ensure they maintain stability under various conditions, such as high temperatures or overcharging.
Compatibility with Electrodes
The electrolyte must be compatible with the electrodes in pouch cells, which are usually made of lithium cobalt oxide, lithium iron phosphate, or other materials. An inappropriate electrolyte can react with the electrodes, compromising the structural integrity of the cell and potentially causing short circuits or other safety issues.
Pouch cells are particularly susceptible to electrolyte leakage because of their flexible packaging. A poorly chosen electrolyte can lead to swelling, cracking, or leakage, potentially exposing the internal components to the external environment. Leakage can result in corrosion, electrical short circuits, or chemical reactions that compromise safety. A strong tab bond to the flexible pouch cell will ensure safe operation.
During operation, lithium-ion batteries produce gases, especially when subjected to high stress, overcharging, or over-discharging. The type of electrolyte used can influence the nature and volume of gas evolution. An electrolyte that generates excessive gas can lead to pressure build-up within the pouch cell, increasing the risk of rupture and associated safety hazards.
Considerations Regarding the Tab
In addition to the electrolyte, the tab design and material are essential aspects of pouch cell safety:
The tab, which connects the electrode to the external circuit, should be made of a conductive, corrosion-resistant material, typically aluminium or copper. Some electrolyte compositions are not compatible with the conventional tab metals used due to their corrosive nature. Choosing the right tab material and/or treatments and ensuring it is securely connected to the electrode is vital to prevent resistance, heating, and potential safety issues.
Welding or Bonding
Proper welding or bonding of the tab to the electrode is crucial. Inadequate connections can lead to increased resistance, generating heat during charge and discharge cycles. This heat can, in turn, increase the risk of thermal runaway. The welded bond must also resist the chosen electrolyte. If the tab-electrode bond is broken, then the component will be free to move inside the pouch cell which may pierce the pouch foil and cause electrolyte leakage.
The choice of electrolyte is a critical factor in determining the safety of pouch cells. Manufacturers carefully select electrolytes to ensure thermal stability, compatibility with electrodes and tabs, and resistance to leakage and gas evolution. Simultaneously, the design and materials of the tab play an essential role in pouch cell safety, preventing electrical issues that could lead to thermal runaway or other hazards. As lithium-ion battery technology continues to advance, a strong focus on electrolyte selection and tab design remains paramount in ensuring the safe and reliable operation of pouch cells.