A high-energy and low-cost polysulfide/iodide redox flow battery

Highlights

• The polysulfide/iodide redox flow battery (PSIB) achieved one of the highest energy densities for all-liquid aqueous RFBs (43.1 W h L⁻¹_{Catholyte+Anolyte}) with high coulombic efficiency (93–95%) and stable cycle life.

• Owing to the high achieved energy density and the inherent low materials cost of sulfur and iodine compared to vanadium, the PSIB system demonstrates a significantly lower materials cost per kilowatt hour ($85.4 kW h⁻¹) compared to the state-of-the-art and best reported VRBs ($152.0–154.6 kW h⁻¹).

• This study provides mechanistic understanding of the newly proposed battery chemistry using operando UV-Visible spectroscopy and reveals superior electrochemical reversibility of polysulfide and iodide redox chemistries.

Abstract

Redox flow batteries (RFBs) have been limited by low energy density and high cost. Here, we employ highly-soluble, inexpensive and reversible polysulfide and iodide species to demonstrate a high-energy and low-cost all-liquid polysulfide/iodide redox flow battery (PSIB). In contrast to metal-hybrid or semi-solid approaches that are usually adapted for high-energy RFBs, the all-liquid characteristic of the PSIB is crucial to enable practical scale-up development. Combining the achieved energy density (43.1 W h L⁻¹_{Catholyte+Anolyte}) and the inherent low materials cost of sulfur and iodine compared to vanadium, the PSIB system demonstrates a significantly lower materials cost per kilowatt hour ($85.4 kW h⁻¹) compared to the state-of-the-art vanadium-based redox flow batteries ($152.0–154.6 kW h⁻¹). Operando UV-Visible spectroscopy reveals superior electrochemical reversibility of both electrolytes. With its demonstrated energy density, inherent low material cost and benign chemical natures, the all-liquid PSIB offers a promising solution for high-energy-density and low-cost energy storage applications.

Introduction

Energy storage technologies are critical enablers for effective utilization of intermittent renewable resources such as solar and wind powers [1], [2], [3], [4], [5]. Redox flow batteries (RFBs) are one of the most important technologies for grid-scale energy storage applications, owing to its design flexibility in decoupling power and energy [6], [7], [8], [9], [10], [11], [12], [13], [14]. Recent development in nonaqueous RFBs has demonstrated numerous high-energy-density flow battery systems [15], [16], [17], [18], [19], [20], [21], [22]. However, the power densities of nonaqueous RFBs are typically 1–2 orders lower than those of aqueous RFBs [6], [23], [24], [25] In addition, the cost of the nonaqueous electrolyte is significantly higher than that of aqueous electrolytes [23].

Aqueous vanadium redox flow batteries (VRBs) exhibit high power capability but have been suffering from high material cost [26] and low energy density [27], [28], [29], [30] (e.g., state-of-the-art aqueous VRBs give 25 W h L⁻¹_{Catholyte+Anolyte} [10]), which significantly reduce their competitiveness in both stationary and mobile applications. To decrease the cost of RFBs, aqueous RFBs employing low-cost redox organic compounds have been developed recently [5], [31], [32], [33]. However, the energy densities of the organic compound based RFBs are limited by the solubility of organic molecules (4.1–12.7 W h L⁻¹_{Catholyte+Anolyte}) [5], [31], [32], [34]. To increase the energy density of VRB, Li et al. [35], [36] have successfully increased the solubility of vanadium ions up to 3.0 M in mixed-acid (sulfuric acid and hydrochloric acid) supporting electrolyte, demonstrating a new generation of VRB achieving a superior energy density of 43.1 W h L⁻¹_{Catholyte+Anolyte}. Recently, Skyllas-Kazacos and co-workers [37] have applied precipitation inhibitors to stabilize a 3 M vanadium electrolyte for use in a high energy density VRB. Alternatively, developing catholyte with multiple redox couples (H₂BQ[38]; Fe²⁺/Fe³⁺ and Br⁻/Br₂ [39]; V⁴⁺/V⁵⁺ and Mn²⁺/Mn³⁺ [40]; and V⁴⁺/V⁵⁺, V³⁺/V⁴⁺ and V²⁺/V³⁺ [41]) has been shown a promising approach to increase the energy density of the aqueous RFBs.

Metal-hybrid and semi-solid approaches [42] have been proposed to increase the energy density of aqueous RFBs. Lu et al. [43] have proposed to employ alkaline metal (like lithium and sodium) as the anode and redox couples with high solubility in aqueous media (such as Fe³⁺/Fe²⁺, [44] halogen, [45], [46] polysulfide [47] and multiple redox couples [41]) as the cathodes. This unique approach achieves high cell voltage and high electrolyte concentration [48]. One major challenge is the need for the crack-free glass ceramic membrane, which reduces the power capability of the aqueous RFBs. Replacing alkaline metal electrode, aqueous metal-hybrid RFBs employing zinc (Zn) metal such as Zn/Br flow battery (ZBB) [10], [11] and Zn/I flow battery (ZIB) [49] demonstrated both high power and superior energy density (65 W h L⁻¹_Catholyte for ZBB [9], and 167 W h L⁻¹_Catholyte for ZIB [49]). However, the use of solid metal electrode sacrifices the intrinsic merits of RFBs, i.e., the capability to scale up energy and power independently. Alternatively, Li et al. [50] reported the use of solid intercalation materials in a carbon percolating conducting network, forming aqueous semi-solid RFBs. This approach effectively increases the energy density of aqueous flow batteries but has been facing challenges including low electrical conductivity and high viscosity [51].

All-liquid aqueous polysulfide/halide redox flow batteries have been described in the 1980s [52] owing to the high solubility of polysulfide, where the aqueous polysulfide/bromine battery (PSB) was demonstrated [52], [53], [54]. To achieve the desired all-liquid characteristic with high energy density and low cost, we employ highly-soluble, inexpensive and reversible polysulfide and iodide species to demonstrate a high-energy and low-cost all-liquid polysulfide/iodide redox flow battery (PSIB) (Fig. 1a). We here compare the conventional polysulfide bromide (PSB) and the proposed polysulfide iodide (PSIB) in aspects including energy/power density, kinetics, and safety. First, PSB has lower energy density (demonstrated 14.52 W h L⁻¹_{catholyte+anolyte}) [55] due to the low solubility of bromine (0.21 M) in aqueous environment (Table S1); second, the kinetics of Br⁻/Br₃⁻ is much lower than that of I⁻/I₃⁻ (Fig. S1), and thus the total power density will be largely hindered; third, PSB exhibits safety concerns involving the evaporation of corrosive and hazardous bromine gas [10], [55] and high oxidizing power of bromine (the potential is 1.09 V vs. SHE (V_SHE)). For the PSIB, while the I⁻/I₃⁻ couple could potentially corrode metal current collectors [56], its long-term maintenance regarding the chemical stability could be less demanding than the VRB systems due to: (1) no concentrated acidic electrolyte is needed for the polysulfide/iodide redox couples as opposed to the concentrated acidic solution used for the VRB (e.g., 3.0 M H₂SO₄ and 6.0 M HCl) [36]; (2) the VO²⁺/VO₂⁺ couple (1.0 V_SHE) exhibits higher oxidizing power than the I⁻/I₃⁻ couple (0.536 V_SHE).

The PSIB system achieves a high energy density (43.1 W h L⁻¹_{Catholyte+Anolyte}) with stable cycling stability and high coulombic efficiency. We developed four-electrode PSIB cell to reveal the source of overpotential during cell operation and performed continuous flow cell test to evaluate the influence of flow rate and current density on the performance of PSIB flow cell. Finally, we exploited operando UV-Vis spectroscopy to investigate the reaction mechanism and reaction intermediate species of polysulfide/iodide redox reactions. Combining the achieved energy density and the inherent low materials cost of sulfur and iodine compared to vanadium, the PSIB system demonstrates a significantly lower materials cost per kilowatt hour ($85.4 kW h⁻¹) compared to the state-of-the-art vanadium-based redox flow batteries ($152.0–154.6 kW h⁻¹) [26], providing a promising candidate for high-energy-density, low-cost and large-scale energy storage applications.