Towards a Sealed Rechargeable Li-SO₂ Battery: Overcoming Slow Diffusion Kinetics and Side Reactions

Rechargeable lithium-sulfur dioxide (Li-SO₂) batteries are potentially of low-cost and high-energy density. The high SO₂ solubility in organic solvents has enabled cell operation under non-pressurized conditions. For example, superior electrochemical stability has been observed in carbonate-based electrolytes. However, most of the reports so far employ an “open cell configuration”, where continuous SO₂ supply is provided. The needed accessories, such as gas diffusion layers and Swagelok-type cell construction with heavy steel frames, and gas handling systems, greatly impede the development of practical high-energy-density batteries.<br/>Herein, we report our progress on understanding the behavior of electrolytes containing SO₂ within a closed system and to maximize the utilization of the active material. In comparison to other redox-active gases, SO₂ molecules, with their relatively high molecular weight (twice as heavy as O₂) and polarity, not only suffer from slow diffusion (~0.7 times lower diffusion coefficient compared to that of O₂) within the electrolyte but also exhibit a peculiar reverse osmotic behavior. When SO₂ is consumed in the electrolyte volume between the two electrodes, dissolved SO₂ in electrolyte outside the current path does not diffuse in. Instead, organic solvents are driven into the current path, leading to an evolution of SO₂ gas outside a cell stack. Our investigation confirmed that this behavior is driven by the molecular interactions between SO₂ and the organic solvents.<br/>Based on the above insights, we have designed electrochemical cells to strategically place all SO₂ containing electrolyte within the current path, leading to maximize utilization. Our optimized bobbin-type battery, in which the electrolyte containing SO₂ is entirely contained within the cell stack, has proven highly effective in maximizing discharge capacity. This configuration allowed us to achieve exceptional utilization (~73%) of SO₂ along with an acceptable E/C ratio (~13 g/Ah, with discharge capacities of 2639 mAh/g_KB and 7.9 mAh/cm²). To further enhance the performance, we have developed a nanoporous lithium protective layer made of a composite of Nafion and alumina nanopowder, capable of retaining SO₂ reactant within the cell stack while minimizing continuous reactant loss from undesirable reactions with Li metal anode. In addition, we have achieved remarkable cycling stability by identifying catalysts to reduce side reactions and polarization on the cathode. This research sheds new light on the working mechanisms of the Li-SO₂ chemistry and points to its potential as a low-cost, sustainable battery.