A review of the development of flow battery systems based on polysulfides
Classification：Industrial News
- Author：ZH Energy
- Release time：Oct-10-2024
【 Summary 】
There are two main types of PSRFB systems: all liquid systems and mixed systems. For a fully liquid system, the electroactive substance is dissolved in a solvent to produce an anode electrolyte and a cathode electrolyte, as shown in Figure 1a. Many organic solvents have been proven to have good solubility for polysulfides, such as DOL/DME, DMSO, and THF, which can be used as solvents for polysulfide reactions. The solution chemistry and potential window highly depend on the inherent properties of polysulfides and their interactions with solvent molecules.
Mixed PSRFB can have three subcategories: solid/liquid (Figure 1b), semi-solid (Figure 1c), and liquid/gas (Figure 1d) forms. In solid/liquid systems, lithium or sodium pure metal electrodes are commonly used as solid electrodes, which face the problem of dendrite formation. The characteristic of liquid electrodes is that polysulfides dissolve in liquid solvents. Although the energy density increases with the concentration of polysulfides in the solvent, the high viscosity of the solution leads to greater flow resistance, resulting in a decrease in energy efficiency. Additionally, due to slow diffusion of the bulk or slow interfacial reaction rate, the reaction rate will also decrease. Therefore, some researchers have turned to semi-solid systems to achieve highly distributed electrochemical reactivity throughout the electroactive region of flow batteries by using conductive materials in polysulfide solutions.
The capacity and theoretical energy density are determined by the reactions between S2-n species. Taking lithium polysulfide flow batteries as an example (Table 4), the actual energy density depends on the polysulfide conversion reaction used and the concentration of soluble polysulfides in the system. The concentration of polysulfides in the solution also affects the conversion reaction and the solubility of different polysulfides.
The author then provided a detailed overview of aqueous polysulfide flow batteries. Bromine polysulfide flow batteries were first reported by Remick et al. in 1983. During the discharge process, short chain polysulfides and sulfide solutions are pumped into the anode half cell and oxidized to high chain polysulfides. For cathodic reactions, bromine is reduced to bromide, and Na+ions perform charge compensation.
In order to alleviate the slow reaction kinetics of polysulfides/sulfides, nickel or cobalt is coated on the carbon electrode, as the metal coated carbon felt electrode exhibits better performance than the uncoated electrode. However, the metal coated carbon electrode only showed improved electrochemical performance in the initial short-term cycle, with an energy efficiency of over 80% at 40 mA/cm2. After about 50 cycles, significant capacity degradation was observed, and the main reason for the performance decline was the excessive accumulation of sulfur in the metal coated carbon felt electrode. In many systems (such as Li-S batteries), the conversion reaction between polysulfides, sulfides, and sulfur is difficult to control, and the irreversible conversion reaction of elemental sulfur can lead to capacity loss and increased mass transport resistance. Recently, multiple research groups have reported on polysulfide/iodide flow batteries. In Figure 2, KI and K2S2 with high solubility are used as redox active substances in the cathode electrolyte and anode electrolyte, respectively, with an estimated theoretical energy density of up to 85.4Wh/L. In the laboratory prototype, the energy density of the all liquid PSRFB is approximately 43.1 Wh/L, with a chemical cost as low as $85.4 kW/h. By using 4 M KI-3M K2S2, the battery can maintain stable performance at a current density of 15mA/cm2 in the first 50 cycles, with a Coulombic efficiency of up to 97%. However, the aqueous system of PSRFB still faces issues with stable cycling and control of polysulfide reactions.
The author provides an in-depth overview of solid-liquid mixed PSRFB. Lithium polysulfide redox flow batteries utilize the high energy density of lithium metal anodes and the scalable structure of flow batteries, with the characteristics of low chemical cost and high solubility of polysulfides. Long chain polysulfides are typically dissolved in organic solvents such as DOL/DME, THF, and DMSO to form catholytes, while the precipitation of short chain lithium sulfide often leads to loss of active materials and capacity degradation during long-term cycling. The Cui research team reported a membrane free hybrid PSRFB, in which long-chain polysulfides in non-aqueous solvents are used as the cathode electrolyte and lithium metal is used as the negative electrode, as shown in Figure 3. By controlling the cyclic reaction between Li2S8 and Li2S4, the two electron reaction avoids the precipitation problem associated with Li2S2/Li2S short chain polysulfides by using a narrow voltage window, exhibiting high energy densities of 97 Wh/kg and 108 Wh/L. Due to the good passivation of lithium anode by LiNO3 additive in the electrolyte, the shuttle effect related to polysulfides is alleviated. Batteries with low polysulfide concentration exhibit stable cycling of over 200 cycles, while batteries with high polysulfide concentration show significant capacity degradation.
Researchers have also developed an aqueous system using short chain lithium polysulfides as redox active substances. In order to protect the lithium metal anode from contact with the aqueous catholyte, LISICON ceramic film (LATP) was used, utilizing the high solubility of short chain lithium sulfides in aqueous solution (>5 M), resulting in a battery with a high energy density of 387 Wh/L. The application of LATP solid electrolyte suppresses the shuttle effect of polysulfides, but LATP is unstable when in direct contact with lithium metal anode, so multiple layers of solid electrolyte or liquid electrolyte are usually required to stabilize the interface.
The author also introduced the semi-solid mixed PSRFB. Chiang et al. first proposed the concept of semi-solid flow batteries and used a flowable suspension of redox active materials as the electrode for flow batteries (Figure 4). They found that it could significantly improve the energy density of flow batteries and fully utilize the solubility limit of redox active materials in solution. Chiang et al. reported a semi-solid electrode composed of solid active lithium-ion materials (such as LiFePO4, LiNi0.5Mn1.5O4, or LiCoO2) suspended in an electrolyte solution, which can improve the reaction rate of polysulfide redox substances by introducing conductive nanomaterials into polysulfide solutions. In addition, Chiang's research team reported a non-aqueous PSRFB, in which nanoscale conductive particles are mixed with a polysulfide solution to form a flowable electrode with an embedded current collector network. The polysulfide positive electrode electrolyte exhibits higher electrochemical activity, achieving a high reversible capacity of 600 mAh/g at an average voltage of 1.9 V. However, in lithium/polysulfide suspension batteries, the capacity retention rate after 100 cycles of cycling polysulfide suspension between 2.55-2.00 V is only 56%. When cycled between 1.90-2.50 V, including the Li2S precipitation region, a high initial specific capacity of 1200 mAh/g was achieved, but faster capacity decay was observed, with only 610 mAh/g after 100 cycles.
Recently, Li's research group reported a semi-solid mixed Na-S flow battery that utilizes molten sodium metal as the anode, β '' - Al2O3 as the membrane, and a flowable sulfur based suspension as the cathode liquid (Figure 5). Unlike existing high-temperature Na-S batteries, this flow battery decouples the thermal management system by controlling the different operating temperatures of the power stack and storage tank. In order to improve the conductivity of the catholyte and promote redox reactions, 2 vol% Ketjen and 1 M NaI were added to the TEGDME solution. At low discharge current density, a high storage capacity of approximately 864 mA h/g was obtained, which is much higher than traditional high-temperature Na-S batteries. When the Na-S flow battery is cycled at a current density of 0.5 mA between 1 and 2.5 V at 150 ° C, the reversible capacity reaches 705 mA h/g in the first cycle, but rapidly decays to approximately 484 mA h/g after 55 cycles. The estimated cost of the system is between $50-100 per kilowatt hour, which is highly likely to achieve the cost target set by DOE.
Finally, the author introduced the liquid gas mixture PSRFB. Inspired by fuel cells, vanadium air RFB was proposed in 1994 to further reduce the weight and size of flow batteries. It utilized 1.2 MV2 (SO4) 3 in 2 MH2SO4 as the anode electrolyte and a dual function cathode based on IrO2 catalyst. However, the energy efficiency of this battery was only 41.6%, with a short cycle life. Its low efficiency and rapid capacity degradation were mainly due to the loss of active materials, corrosion of the current collector, and cross effects. Recently, oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) have also been introduced into aqueous sulfur systems, with polysulfides and oxygen-containing/gas filled salt solutions used as anode electrolytes and cathode electrolytes, respectively (Figure 6).
