Electrolytes

Electrolytes play a critical role in lithium-ion batteries by enabling ion transport between the cathode and anode, which is crucial for battery performance and stability.

As battery technology has evolved, there has been a growing demand for more efficient electrolytes to boost lithium-ion battery performance (Kainat et al., 2023). This has led to the development of various new electrolytes, each with its advantages and challenges.

Electrolytes used for lithium-ion batteries must show high electrochemical and chemical stabilities.

Electrochemical stability is related to the stability of the electrolyte under high oxidative and reductive potentials at the cathode and anode, respectively. This refers to the electrolyte’s resistance against degradation and decomposition when exposed to the electrodes during charging and discharging. An electrolyte has high oxidation and reduction stability when it does not decompose and react with the cathode and anode during charge and discharge processes. The electrochemical stability window of the electrolyte is usually determined by the range between these oxidation and reduction potentials, ensuring it remains stable over the battery’s operational voltage range.

Chemical stability refers to the electrolyte’s compatibility with other components of the battery like current collectors and separators (Wieczorek & Płocharski, 2021).

Organic electrolytes

Organic electrolytes contain organic solvents like ethylene carbonate and dimethyl carbonate, which dissolve lithium salts and enable ion flow between cathode and anode electrodes.

The main problem of organic electrolytes is their flammability, which poses safety risks and limits their application in certain lithium-ion batteries (Kainat et al., 2023). When a lithium-ion battery is exposed to excessive heat and oxygen release from the cathode in a lithium-ion battery, the organic liquid electrolyte acts as a fuel and results in the release of flammable gases. This reaction increases internal pressure and can lead to a catastrophic battery explosion (Chung et al., 2020).

Additionally, leakage of organic electrolytes can lead to further safety concerns, as the leaked liquid can ignite upon exposure to heat or sparks, further compounding the risk of fires and explosions.

Aqueous electrolytes

Aqueous electrolytes use a concentrated saline solution (mixture of water and salt) to support the flow of lithium ions between the electrodes (cathode and anode) and generate electrical current in lithium-ion batteries (Dong et al., 2016; Kainat et al., 2023).

Compared to organic electrolytes (which use an organic solvent and a lithium salt) and non-aqueous electrolytes (which may be organic or inorganic solvents), aqueous electrolytes offer several advantages such as lower cost, reduced flammability, and no explosion risk. However, they are limited by a narrow electrochemical stability window of only 1.23 V, resulting in a much lower energy density than non-aqueous lithium-ion batteries.

Additionally, leakage of the aqueous solution poses a significant challenge, which can compromise battery performance and safety.

Investigations into various salt solutions have provided insight into how different salts affect both electrochemical and chemical stability and performance of aqueous electrolytes (Kainat et al., 2023).

Solid electrolytes

Solid electrolytes can be classified into two main types: organic and inorganic. Organic solid electrolytes are based on polymer composites, while inorganic solid electrolytes consist of ceramic materials like lithium superionic conductors (LISICON) (Barbosa et al., 2022).

Solid electrolytes offer several advantages over aqueous electrolytes including superior safety, reduced risk of leakage and improved stability. Additionally, solid electrolytes reduce side reactions associated with high-voltage cathodes, which contributes to higher energy density in the battery.

Solid electrolytes eliminate the need for a separator, resulting in a more compact design with thinner layers. This reduces the overall volume of the battery while maintaining the same amount of active materials, leading to higher volumetric energy density (Kainat et al., 2023; Li et al., 2014; Zhang et al., 2016).

However, solid-state electrolytes have lower ionic conductivity, which can result in reduced power output. This is because ion transport is slower compared to liquid electrolytes.

Ionic liquid electrolytes

Ionic liquids (ILs) are noted for their chemical and thermal stability, flame resistance, extremely low volatility, and safety. This makes positions them as eco-friendly solvents that have garnered significant interest.

The super-concentrated electrolytes derived from ILs have proven beneficial for lithium-ion batteries operating at high voltages. These with lower viscosity demonstrate excellent stability at elevated temperatures (Kainat et al., 2023).

Research and share

Research eco-friendly electrolyte options and analyse their environmental impact, safety profiles, and cost implications. Share some findings in the comments.

References

Barbosa, J. C., Goncalves, R., Costa, C. M., & Lanceros-Méndez, S. (2022). Toward sustainable solid polymer electrolytes for lithium-ion batteries. ACS omega, 7(17), 14457-14464.

Chung, G. J., Han, J., & Song, S.-W. (2020). Fire-preventing LiPF₆ and ethylene carbonate-based organic liquid electrolyte system for safer and outperforming lithium-ion batteries. ACS applied materials & interfaces, 12(38), 42868-42879.

Dong, X., Chen, L., Su, X., Wang, Y., & Xia, Y. (2016). Flexible aqueous lithium‐ion battery with high safety and large volumetric energy density. Angewandte Chemie International Edition, 55(26), 7474-7477.

Kainat, S., Anwer, J., Hamid, A., Gull, N., & Khan, S. M. (2023). Electrolytes in Lithium-Ion Batteries: Advancements in the Era of Twenties (2020’s). Materials Chemistry and Physics, 128796.

Li, J., Ma, C., Chi, M., Liang, C., & Dudney, N. J. (2014). Solid electrolyte: the key for high-voltage lithium batteries. Advanced Energy Materials, 5(4).

Wieczorek, W., & Płocharski, J. (2021). Designing electrolytes for lithium-ion and post-lithium batteries. CRC Press.

Zhang, J., Zhao, N., Zhang, M., Li, Y., Chu, P. K., Guo, X., Di, Z., Wang, X., & Li, H. (2016). Flexible and ion-conducting membrane electrolytes for solid-state lithium batteries: Dispersion of garnet nanoparticles in insulating polyethylene oxide. Nano Energy, 28, 447-454.

Electrolytes (cropped). Shown are two scanning electron microscope images that illustrate how a traditional electrolyte can cause dendrite growth (left), while PNNL’s new electrolyte instead causes the growth of smooth nodules that don’t short-circuit batteries (right). Pacific Northwest National Laboratory; CC BY-NC-SA 2.0