Interface Engineering Strategies for Shuttle Mitigation in Alkali Metal–Sulfur Batteries: A Comparative Review from Li–S to Na–S and K–S Systems

Abstract

Rechargeable alkali metal-sulfur (M–S) batteries, including Li/Na/K–S chemistries, have the potential to utilize abundant and low-cost sulfur cathodes yet offer high theoretical energy densities. However, their practical electrochemical performance is fundamentally limited by the polysulfide shuttle effect. This challenge is particularly exacerbated in Na–S and K–S systems owing to larger metal-ion radii, weaker solvation energies, slower redox kinetics, and greater electrolyte–electrode incompatibilities compared to Li–S batteries. This review presents a comparative analysis of interface engineering strategies designed to suppress the shuttle effect across these three systems. Following a summary of sulfur cathode properties and reaction mechanisms, we systematically examine the origins of polysulfide shuttling. Our analysis progresses from functional separator design and interlayer enhancements to the implementation of solid‑state electrolytes for root-cause inhibition. By evaluating interface engineering research specific to Na–S and K–S batteries, we elucidate both shared principles and unique challenges inherent to alkali M-S systems. Finally, we propose multifaceted solutions to achieve shuttle-free operation and enhance overall battery performance, thereby establishing a foundation for future advancements.

1. Introduction

With the rapid development of society and ever-increasing energy consumption, the demand for renewable energy has reached a high level. The rechargeable battery systems (lithium/sodium/potassium ion batteries or supercapacitors) play an important role in electrochemical energy conversion and storage devices. Lithium‐ion batteries (LIBs) have been commercialized due to their high energy density, long cycle life, low self‐discharge, and minimal environmental pollution. However, commercial alkali (Li/Na/K) ion batteries (AIBs) rely on the reversible insertion-extraction of ions between cathode and anode, which limits their practical cell-level energy densities to about 200–500 Wh kg−1, insufficient to meet the increasing energy demands of modern applications [1–3].

Rechargeable M-S batteries, such as lithium-sulfur (Li–S) batteries, sodium-sulfur (Na–S) batteries, and potassium-sulfur (K–S) batteries, use cost‑effective, earth‑abundant sulfur cathodes, in contrast to conventional AIB cathodes built from scarce metals like cobalt, manganese or nickel (Fig. 1). On an active-material basis, M-S batteries can achieve much higher theoretical energy densities. For example, Li–S batteries possess a high specific capacity of 1675 mAh g−1 and a theoretical energy density of approximately 2600 Wh kg−1 based on sulfur. The practical cell-level performance of M-S batteries depends strongly on several critical factors, including the electrolyte-to-sulfur ratio (E/S), the negative-to-positive capacity ratio (N/P), and the areal sulfur loading (Fig. 2d, f). However, in Li–S batteries, the cost advantage of the sulfur cathode is offset by the combination with lithium, an expensive and geologically scarce anode material. (Industrial-grade lithium metal is about 100 $ kg−1, with a low abundance of approximately 0.0065% in the Earth’s crust, and its concentration in seawater is about 0.17 mg L−1) In contrast, Na–S and K–S batteries leverage sodium and potassium resources, which are both inexpensive and abundant globally. (Industrial-grade sodium metal is 2–3 $ kg−1, with an abundance of approximately 2.36% in the Earth’s crust and an extremely high content in seawater, about 10,620 mg L−1. Industrial-grade potassium metal is 3–5 $ kg−1, with a content of approximately 2.09% in the Earth’s crust and about 380 mg L−1 in seawater.) [4–6]. It should be noted that the cost values in Table 1 refer to the prices of commonly used carbonate precursors (Li2CO3, Na2CO3, K2CO3), which are widely adopted as industry benchmarks for estimating raw material costs in battery manufacturing, rather than the direct prices of metallic lithium, sodium, or potassium [2]. Besides, we also summarize the corresponding crustal abundances, thereby providing a clear comparison of resource availability and raw material costs among the three alkali metal systems. Therefore, Na–S and K–S batteries emerge as desirable alternatives to conventional ion batteries, offering a more cost-effective edge and superior element.