Improving the Stability of an RT‐NaS Battery via In Situ Electrochemical Formation of Protective SEI on a Sulfur–Carbon Composite Cathode

Abstract

lower than that of lithium. [3][4][5] Therefore, sodium has been considered appropriate for large-scale EESs as an alternative to lithium. Among the EESs using sodium, the high temperature sodium-sulfur battery (HT-NaS battery) is best known and operates at 300-350 °C with a molten electrode and a ß″-alumina solid electrolyte. At present, this technology is commercialized for stationary-energy-storage systems because of its reasonable energy density and cost. [6] However, the HT-NaS battery has limited capacity, with one third of the theoretical capacity due to solid phase products (Na 2 S 2 ) with a high melting point (T m = 470 °C) formed during the discharge process. [7] Additionally, the high-temperature operation results in safety, cost, and efficiency concerns. [8,9] In recent years, a room-temperature sodium sulfur battery (RT-NaS battery) that uses liquid organic electrolytes has attracted great interest due to the abundance of sulfur, low operating temperature, and high theoretical capacity (1672 mAh g −1 ). [9,10] However, RT-NaS still faces critical obstacles to practical use, such as the low electrical conductivity of sulfur, large volume expansion (≈170%), loss of active materials, etc. Among these concerns, the most critical problem is dissolution of sodium polysulfides into the electrolyte during electrochemical reactions. In particular, the high-ordered polysulfides (NaS x , 4 < x ≤ 8) move back and forth between the electrodes (known as the shuttle effect) and undergo unwanted redox reactions at the sodium metal surface. [9] This effect leads to high irreversible capacity, rapid capacity decay, and low Coulombic efficiency. [9][10][11][12] To prevent the shuttle effect, several research studies have reported various attempts: (i) encapsulation with/infiltration into porous carbon, [13] (ii) coating of the cathode with conductive polymers, [14] (iii) formation of multiple metal oxides, [15] and (iv) development of a membrane to impede polysulfide diffusion to the anode. [16,17] Most of these approaches have shown improved stability in RT-NaS. However, the additional treatment of the material and the complex process result in high cost, and the impediment to practical use remains. Therefore, it is necessary to develop a new and facile surface-treatment method to protect the sulfur-based electrode from polysulfide dissolution in RT-NaS.The solid electrolyte interphase (SEI) is a passive film that is formed mostly on the anode surface during batteryThe ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adsu.201800076. RT-NaS BatteriesCurrently, rechargeable lithium-ion batteries (LIBs) are the predominant electrochemical energy storage (EES) systems from small-portable electronic devices to electric vehicles (EVs) due to the higher energy density than other technologies. However, the lithium is relatively expensive and a finite resource hence the exploration of new batteries with other sources is necessary. [1][2][3] Specifically, sodium has ch...

Stable Room-Temperature Sodium–Sulfur Batteries in Ether-Based Electrolytes Enabled by the Fluoroethylene Carbonate Additive

“…Figure d displays the F 1s spectra of the discharged cathode in the FEC-containing electrolyte. The prominent peak at 684.5 eV corresponds to sodium fluoride (NaF). , The other three peaks at ∼685.9, 688.3, and 689.7 eV indicate the C–F in the decomposition products of FEC. Figure e shows the O 1s spectra, the intensity of the peaks that correspond to the NaCO 3 and alkyl carbonate of ROCO 2 Na at 532–532.8 eV and R–ONa at 533.7 eV increase with the addition of FEC in the electrolytes. , Consistently, the same tendency also appears in the C 1s spectra that NaCO 3 and ROCO 2 Na at ∼290 eV and O–CO at ∼288.5 eV get intensified with the FEC additive (Figure f) .…”

Electrolytes/Interphases: Enabling Distinguishable Sulfur Redox Processes in Room‐Temperature Sodium‐Sulfur Batteries

“…Furthermore, Lee and co‐workers found that FEC additive formed a protective CEI layer on the C/S cathode, suppressing the dissolution of polysulfides. [ 67 ] The FEC‐assisted CEI layer had a thickness of ≈20 nm and contained fluoride anions that improved the cell's electrical conductivity and mechanical stability. As is well known, dissolved polysulfides undergo severe side reactions with carbonate ester solvents.…”

“…Meanwhile, this passivated layer is helpful to suppress the polysulfide dissolution, leading to greatly enhanced stability and reversible capacity. [ 67 ]…”

“…Reproduced with permission. [ 67 ] Copyright 2018, Wiley‐VCH. b) Schematic illustrations of surface components of the YP50F/S electrode surface after the first discharge with (left) 1 m NaClO 4 ‐EC/PC and (right) 5 wt% FEC‐added 1 m NaClO 4 ‐EC/PC; c) High resolution XPS S 2p spectra of YP50F/S cathode at the pristine stage, after discharge to 0.1 V and after charge to 3.0 V (left) without FEC additive, (right) with 5 wt% FEC.…”

Beyond Lithium-Based Batteries

“…The energy storage mechanism for all metal-sulfur batteries follows a conversion type mechanism [13,[150][151][152][153][154]. A simple version of this reaction occurs for lithium, sodium and magnesium [151][152][153][154][155][156][157][158][159][160][161][162][163][164][165][166]. In this mechanism the sulfur in the cathode is reduced and forms a metal sulfide complex with the metal ions from the electrolyte solution, as in Equation 22.…”