Designing Novel Solvents for High Voltage Li-Ion Batteries
Authors: Su Chi Cheung , Amine Khalil 
Published: August 2024

The pursuit of higher energy density in lithium-ion batteries has led to the development of new cathode materials that can operate at elevated voltages and provide increased specific capacities. [1-2] One such class of materials is the nickel-rich layered oxide cathodes known as LiNixMnyCozO2 (NMC). These cathodes offer high specific capacities and demonstrate electrochemical stability at high cutoff voltages, making them promising candidates for advanced lithium-ion batteries. [3-4] However, NMC cathodes face a significant challenge in the form of capacity fading during cycling. This issue primarily arises from the voltage instability of the conventional carbonate-based electrolytes used in lithium-ion batteries, which are typically designed for the 4-V lithium-ion chemistry. [5-6] As a result, researchers and scientists have directed extensive efforts toward developing new and improved electrolyte systems that can withstand the high voltage requirements of NMC cathodes and other high-energy applications. [7-8]

Designing and synthesizing new molecules for use as electrolyte solvents in lithium-ion batteries is indeed a complex and challenging task. Researchers often focus on optimizing one specific property, which can lead to the development of molecules that excel in that aspect while potentially overlooking other crucial features that are necessary for stable battery cycling. For example, the development of α-fluorinated sulfones as electrolyte solvents was driven by their exceptional anodic stability. [9] This stability is achieved by introducing strong electron-withdrawing trifluoromethyl groups directly attached to the sulfonyl group. However, while this electron-withdrawing effect enhances anodic stability, it can also significantly increase the reduction potential of α-fluorinated sulfones. This heightened reduction potential renders them unstable when used with graphite anodes, highlighting the delicate balance required in designing electrolyte solvents that perform well across various aspects of battery operation.

In this presentation, we embraced the concept of a "golden middle way" when designing and synthesizing new electrolyte solvents. The recently developed β-fluorinated sulfone, TFPMS, doesn't claim to be the best in any single property, but it strikes a balance across various key characteristics. This equilibrium has proven to be highly effective in ensuring the long-term stability of high-voltage graphite||NMC622 full cells. Positioned at the β site of the sulfone molecule, the strong electron-withdrawing trifluoromethyl group renders β-fluorinated sulfone sufficiently stable against the high-voltage NMC622 cathode, even if it possesses a slightly lower oxidation potential compared to its α-fluorinated counterparts. Furthermore, its reduction potential is lower than that of α-fluorinated sulfone, making it considerably more stable when paired with the graphite anode. While it may not possess the same high lithium solvating power as the typical sulfone (EMS), the lithium solvating capacity of β-fluorinated sulfone falls somewhere in between, mitigating transition metal dissolution and deposition in the graphite||NMC622 full cell that utilizes a β-fluorinated sulfone-based electrolyte. As a result, the full cell equipped with TFPMS-based electrolyte demonstrates superior cycling performance, with a significantly higher average capacity than cells using regular sulfone or α-fluorinated sulfone-based electrolytes. In summary, our approach exemplifies the successful application of the "golden middle way" in designing and synthesizing new electrolyte solvents.

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