A Sodium Polysulfide Battery with Liquid/Solid Electrolyte: Improving Sulfur Utilization Using P₂S₅ as Additive and Tetramethylurea as Catholyte Solvent

Abstract

Herein, the proof of concept of a sodium polysulfide battery consisting of two electrode chambers being separated by a solid electrolyte is described. The concept is suited for dissolved polysulfide cathodes and has the advantage that both half reactions can be optimized separately. The formation of solid sulfide discharge products is identified as the major limiting factor for cell cycling. This issue can be alleviated by adding solid P2S5. Further improvement can be achieved by replacing diglyme (2G) as the cathode compartment solvent with tetramethylurea (TMU). Using TMU, the cell cycles with Coulombic efficiencies >99% and capacities of 800 mAh g−1 are maintained for at least 30 cycles. Viscosity, density, conductivity, and the electrochemical stability window values of the 2G‐ and TMU‐based electrolytes are compared. The latter shows higher viscosity (2.806 vs 1.603 mPa s), higher density (1.016 vs 0.996 g cc−1), and higher conductivity (4.27 vs 1.45 mS cm−1). The oxidative stability limit of the TMU electrolyte is 3.2 V versus Na+/Na, which is sufficient for polysulfide redox reactions. Vis spectroscopy is used to follow the electrode reaction. In case of TMU, the reaction is based on the redox activity of S3−• radicals (blue coloration of the catholyte solution).

Copper Thiophosphate (Cu3PS4) as Electrode for Sodium‐Ion Batteries with Ether Electrolyte

“…This is not necessarily expected, as for example Na 2 S in combination with P 2 S 5 shows a high solubility in diglyme (the individual compounds do not). [ 41 ] Dissolution of the active material would lead to a rapid capacity fading. As XPS and XRD both provide evidence for Na 2 S formation, the long cycle life directly implies that P 2 S 5 does not occur in larger amounts during cycling.…”

A β”‐Alumina/Inorganic Ionic Liquid Dual Electrolyte for Intermediate‐Temperature Sodium–Sulfur Batteries

“…As a strategy for enhancing the kinetics of inert polysulfides, an electrolyte based on Na[OTf ] is adopted as the medium for ), it is postulated to promote the dissolution of polysulfides into the electrolyte, [12][13][14]34,[45][46][47][48] thereby improving the electrochemical performance. However, alkali metal OTf salts tend to have high melting points above 150 °C because of the low degree of freedom in the anionic structure.…”

“…The IL, which is placed in the positive electrode compartment, enhances the reactivities of the S and polysulfide active materials, thereby improving their utilization and consequently the energy density of the battery. In the proposed system, we select an IL incorporating the [OTf ] − counteranion due to its high solvation properties, particularly for polysulfides, boosting kinetic performance as has been reported for Na-S batteries and analogous Li-S batteries utilizing [OTf ] − -based organic electrolytes [12][13][14][45][46][47][48] and IL electrolytes. [34] However, this strategy of enhancing the solubility of intermediate polysulfide species also tends to shorten cycling lifetimes because of the simultaneously heightened polysulfide shuttling.…”

“…As a strategy to combat the issues associated with HT operations, Na–S batteries capable of room temperature (RT) operations have been proposed. Although the BASE exhibits a sufficient ionic conductivity for room‐temperature operation Na–S batteries, [ 12–14 ] the high resistance resulting from its thickness and interfacial behavior leads to poor kinetics. Reducing the operation temperatures also limits the electrochemical performance of RT Na–S batteries because of the formation of solid polysulfides with sluggish electrochemical reactivity.…”

Challenges of today for Na-based batteries of the future: From materials to cell metrics

“…Important strategies to improve the battery performance include nanostructuring of carbon/sulphur composites [315], implementation of membranes (inorganic, organic or mixed) [316][317][318][319] and optimization of the electrolyte composition [320][321][322]. Moreover, other cell concepts based on dissolved positive electrodes (solutions containing polysulfides) are explored, some of them being designed to operate at intermediate temperatures [323][324][325]. Very recently, solid-state configurations also became of interest [326][327][328].…”