Review of Emerging Potassium–Sulfur Batteries

Abstract

This is the first review on potassium–sulfur (K–S) batteries (KSBs), which are emerging metal battery (MB) systems. Since KSBs are quite new, there are fundamental questions regarding the electrochemistry of S‐based cathode and of K metal anode, as well as the holistic aspects of full‐cell performance. The manuscript begins with a critical discussion regarding the potassium–sulfur electrochemistry and on how it differs from the much better‐known lithium–sulfur. Cathodes are discussed next, focusing on the role of sulfur structure, carbon host chemistry and porosity, and electrolytes in establishing the reversible potassium sulfide K2Sn phase sequence, the parasitic polysulfide shuttle, pulverization‐driven capacity fade, etc. Following is a discussion of solid‐state electrolytes (SSEs), including of hybrid solid–liquid systems that show much promise. Potassium metal anodes are then critically reviewed, emphasizing electrolyte reactions to form stable versus unstable solid electrolyte interphase (SEI), covering the current understanding of potassium dendrites, and highlighting the deep‐eutectic K–Na alloying approaches for room temperature liquid anodes. The manuscript concludes with K–S batteries, focusing on cell architectures and providing quantitative performance comparisons as master plots. Unanswered scientific/technological questions are identified, emerging research opportunities are discussed, and potential experimental and simulation‐based studies that can unravel these unknowns are proposed.

Electrochemical Insights, Developing Strategies, and Perspectives toward Advanced Potassium–Sulfur Batteries

“…[21][22][23] Therefore, the feasible integration of the potassium anode and the sulfur cathode in novel K-S batteries can generate promising large-scale stationary power applications with the merits of encouraging energy density and cost-effectiveness. [24,25,38,41] Nevertheless, despite these superior virtues and significant progresses, the evolution of K-S battery is still at a preliminary period due to tremendous challenges. Comparing with the most studied metal-sulfur systems, particularly lithiumsulfur (Li-S) batteries, rechargeable K-S batteries are not competitive with their rivals.…”

Potassium‐sulfur batteries: Status and perspectives

“…106 Although a little excess amount of K anode is required to offset the anode loss owing to the reaction between K and the electrolyte to form the SEI, 106 nearly the entire K metal anode would be stripped and deposited during each discharge-charge cycle, associated with huge volume changes at every cycle. 17 In these circumstances, the internal stress within the K metal caused by such a large volume change is likely to induce more severe cracking and reforming of the SEI, making problems such as low CE and short lifespan more pronounced. Therefore, thin K metal anodes that are well matched to the sulfur cathode (areal capacity ratio between them approaching to 1) should be used in future K-S research to obtain a better understanding of the K-S electrochemistry under practical conditions and to accelerate its practical applications.…”

“…[12][13][14] Having sulfur as the cathode material provides a theoretical energy density of 2600 Wh kg −1 or 2800 Wh L −1 on coupling with lithium-metal anode, far exceeding those of state-of-the-art LIBs. [15][16][17] In addition, the commercial potential of the lithium-sulfur (Li-S) system has been demonstrated in some niche applications. For example, some companies have announced prototype Li-S batteries operating in an unmanned system, electric bicycles, and test applications, while more are rushing to develop sulfur batteries.…”

“…The electrochemical advantages of K in organic electrolytes, for instance, the weaker Lewis acidity of K + than Li + and Na + results in a smaller Stokes' radius (3.6 Å vs 4.8 and 4.6 Å) in carbonate solvents ( Figure 1A), 46 endowing K + with the highest ion mobility, ion conductivity, and ion transport number of the three ions. 17 Also, theoretical calculations and experiments have demonstrated that the K + /K redox couple exhibits the lowest reduction potential in organic solvents as compared to Li + / Li and Na + /Na, 44,46,47 implying that a relatively high voltage for a full battery may be possible. Furthermore, enhanced electrolyte/electrode interfacial diffusion capacity of K + is expected due to its low desolvation energy, which has enabled the insertion of large-radius K + in graphite.…”

Alkali‐Metal Sulfide as Cathodes toward Safe and High‐Capacity Metal (M = Li, Na, K) Sulfur Batteries

“…The K–S phase diagram, including a series of stable phases of K 2 S x ( x = 1, 2, 3, 4, 5, and 6), provides a new direction to study the mechanism of K–S batteries by applying pure‐phase polysulfides. [ 46 ] Unlike lithium and sodium polysulfides, the short‐chained K 2 S x ( x ≤ 4) cannot dissolve in the ether‐base electrolytes, such as diethylene glycol dimethyl ether (DEGDME). According to the previous studies on RT K–S batteries, there is a K 2 S “dead” sulfur species, which are not able to be charged during cycling; K 2 S 3 was usually detected as the major final discharge product in K–S batteries.…”