Review of Multifunctional Separators: Stabilizing the Cathode and the Anode for Alkali (Li, Na, and K) Metal–Sulfur and Selenium Batteries

Abstract

Alkali metal batteries based on lithium, sodium, and potassium anodes and sulfur-based cathodes are regarded as key for next-generation energy storage due to their high theoretical energy and potential cost effectiveness. However, metal−sulfur batteries remain challenged by several factors, including polysulfides' (PSs) dissolution, sluggish sulfur redox kinetics at the cathode, and metallic dendrite growth at the anode. Functional separators and interlayers are an innovative approach to remedying these drawbacks. Here we critically review the state-of-the-art in separators/interlayers for cathode and anode protection, covering the Li−S and the emerging Na−S and K−S systems. The approaches for improving electrochemical performance may be categorized as one or a combination of the following: Immobilization of polysulfides (cathode); catalyzing sulfur redox kinetics (cathode); introduction of protective layers to serve as an artificial solid electrolyte interphase (SEI) (anode); and combined improvement in electrolyte wetting and homogenization of ion flux (anode and cathode). It is demonstrated that while the advances in Li−S are relatively mature, less progress has been made with Na−S and K−S due to the more challenging redox chemistry at the cathode and increased electrochemical instability at the anode. Throughout these sections there is a complementary discussion of functional separators for emerging alkali metal systems based on metal−selenium and the metal−selenium sulfide. The focus then shifts to interlayers and artificial SEI/cathode electrolyte interphase (CEI) layers employed to stabilize solid-state electrolytes (SSEs) in metal−sulfur solid-state batteries (SSBs). The discussion of SSEs focuses on inorganic electrolytes based on Li-and Na-based oxides and sulfides but also touches on some hybrid systems with an inorganic matrix and a minority polymer phase. The review then moves to practical considerations for functional separators, including scaleup issues and Li−S technoeconomics. The review concludes with an outlook section, where we discuss emerging mechanics, spectroscopy, and advanced electron microscopy (e.g. cryo-transmission electron microscopy (cryo-TEM) and cryo-focused ion beam (cryo-FIB))-based approaches for analysis of functional separator structure−battery electrochemical performance interrelations. Throughout the review we identify the outstanding open scientific and technological questions while providing recommendations for future research topics.

Molybdenum Carbide Electrocatalyst In Situ Embedded in Porous Nitrogen‐Rich Carbon Nanotubes Promotes Rapid Kinetics in Sodium‐Metal–Sulfur Batteries

“…Although much less is known for Na–S as compared to Li–S, the redox reactions with the former are well‐documented to be significantly more sluggish, posing a greater challenge for an effective electrocatalyst. [ 21 ] The volume expansion and associated stress of sulfur converting to Na 2 S versus to Li 2 S (171% vs 80%) create a larger nucleation barrier for the former redox reaction. Sodiation and desodiation reactions are also impeded by the slower solid‐state diffusivity of sodium ions within various carbon hosts as compared to lithium ions.…”

“…This is due to a combination of enhanced sodium polysulfide dissolution and possible crossover to the anode, accelerated loss of active S during cycling due to its repeated volume expansion, and because of the instability of the Na-metal anodes in both carbonate and ether electrolytes. [21,23] In carbonate electrolytes, Na-S batteries may display a "quasi-solid-state phase transformation" where the liquid-phase polysulfide redox reactions are suppressed. [2,24] This is desirable since it prevents polysulfide dissolution into the electrolyte.…”

Eliminating Lightning‐Rod Effect of Lithium Anodes via Sine‐Wave Analogous MXene Layers

“…[ 5–8 ] Moreover, the uncontrollable Li dendrites cause a series of issues such as severe volume changes, low Coulombic efficiency (CE) as well as poor cycle life, which impede the commercial application of Li anodes. [ 9–12 ]…”

“…[5][6][7][8] Moreover, the uncontrollable Li dendrites cause a series of issues such as severe volume changes, low Coulombic efficiency (CE) as well as poor cycle life, which impede the commercial application of Li anodes. [9][10][11][12] A large number of theoretical and experimental studies demonstrate that Li dendrite generation mainly originates from the inhomogeneous nucleation of metallic Li as well as the huge concentration gradient of lithium ion and nonuniform electric field. [13][14][15][16] Thus, Many strategies have been proposed to solve the intricate Li dendrite problems.…”

Phase Engineering of Metastable Transition Metal Dichalcogenides via Ionic Liquid Assisted Synthesis

“…But unlike graphene, group VIB TMDs such as MoS 2 , MoSe 2 , WS 2 , and WSe 2 exist in different polymorphs including 2H and 1T phase, depending on the electron filling state in the valence d-orbitals of transition metal atoms, which exhibit diverse chemical and electronic properties. − The thermodynamically stable 2H-TMDs possess two layers joined by weak van der Waals forces, where the transition metal atom is surrounded by six chalcogen atoms in the trigonal prismatic lattice. Instead, 1T-TMDs are a single-layer structure, in which the transition metal atoms are in octahedral coordination. , Notably, 2H-TMDs are typically semiconductors with bandgaps in the 1–2 eV range, whereas 1T-TMDs show metallic properties. , Therefore, 1T-TMDs exhibit a much superior performance in electrocatalysis, − batteries, − and supercapacitors, − because of the lower charge transfer resistance. Although 1T-TMDs have extraordinary performance in the field of electrochemistry, the thermodynamical metastability makes 1T-TMDs hard to synthesize directly, greatly restricting their wide applications …”