Design and regulation of high-performance catalysts in cathodes and separators for alkali metal-sulfur batteries

Abstract

[eng] Alkali metal-sulfur batteries (MSBs) are considered to be the most promising materials to replace lithium-ion batteries (LIBs) in next-generation energy storage systems. Compared to LIBs, lithium sulfur batteries (LSBs) exhibit a six-fold higher theoretical energy density, reaching up to 2500 Wh kg -1, sodium sulfur batteries (SSBs) also demonstrate a theoretical gravimetric energy density of 1274 Wh kg -1, while potassium sulfur batteries (PSBs) achieve a theoretical weight-specific energy density of up to 1023 Wh kg -1. Furthermore, their commercialization costs and environmental impact may be significantly lower. Despite these attractive advantages, the electrical insulating properties of sulfur and Li2S/Na2S/K2S, as well as the shuttle effect of intermediate metal polysulfides (MPS) greatly limit the practical application of AMSBs. In addition, the severe volume change (>80%) and slow redox kinetics during charge and discharge also reduce the cycle life and power density. The rational design and engineering of the catalysts can effectively overcome the above challenges. The state of the art of AMSBs and the targeted requirements from two points of view: physical adsorption and chemical catalysis, are summarized in Chapter 1. Within the results part of this thesis, in Chapter 4, I explore the effects of metal phosphide and heteroatom doping on the separator on the performance of LSBs. In this study, I detail a one- step approach to growing tungsten phosphide (WP) nanoparticles on the surface of nitrogen and phosphorus co-doped carbon nanosheets (WP@NPC). I further demonstrate that this material provides outstanding performance as a multifunctional separator in LSBs, enabling higher sulfur utilization and exceptional rate performance. These excellent properties are associated with the abundance of lithium polysulfide (LiPS) adsorption and catalytic conversion sites and rapid ion transport capabilities. Experimental data and density functional theory (DFT) calculations demonstrate tungsten to have a sulfophilic character while nitrogen and phosphorus provide lithiophilic sites that prevent the loss of LiPSs. Furthermore, WP regulates the LiPS catalytic conversion, accelerating the Li-S redox kinetics. As a result, LSBs containing a polypropylene separator coated with a WP@NPC layer show capacities close to 1500 mAh g-1

at 0.1C and coulombic efficiencies above 99.5 % at 3C. Batteries with high sulfur loading, 4.9 mg cm−2, are further produced to validate their superior cycling stability. Overall, this work demonstrates the use of multifunctional separators as an effective strategy to promote LSB performance. This work was published in Journal of Colloid and Interface Science in 2024. In Chapter 5, I demonstrate the design and production of multifunctional SnSe as a separator for MSBs. More in detail, SnSe nanosheets are introduced as additive into the cathode side of the glass microfiber (GF) separator of the MSB. Taking LSBs as an example, it is demonstrated that the GF-SnSe separator (GF@SnSe) shows strong chemical affinity to LiPSs and superior catalytic activity, inhibiting the transport of LiPSs to the anode and accelerating their conversion. Combining experimental and calculation results, the SnSe additive is shown to decrease the Li2S decomposition energy barrier. Overall, GF@SnSe separators provide significantly improved LSB rate performance and cycling stability with a 0.049% capacity decay per cycle. Besides, the GF@SnSe separator promotes the electrochemical performance of SSBs and PSBs. Overall, this work presents a significant advancement in the development of multifunctional separators in LSBs as well as the emerging Na-S and K-S systems. This work was published in Advanced Energy Materials in 2024. Since two-dimensional (2D) nanocarbon-based materials with controllable pore structures and hydrophilic surfaces have shown great potential in sulfur host materials, in Chapter 6, I present a scalable approach for the preparation of porous ultrathin nitrogen-doped carbon nanosheets decorated with ultrafine FeTe2 nanoparticles (FeTe2/CN), derived from metal–organic frameworks (MOFs) through a mild and modifier-free synthesis strategy. This graphene-like structure serves as a promising cathode material to address complex challenges in LSBs. Experimental results and DFT calculations highlight the distinct advantages of this structure: (1) synergistic adsorption occurs through the lithiophilic sites of CN and the sulfiphilic sites of FeTe2, efficiently capturing LiPS; (2) enhanced conductivity of the CN nanosheets, combined with the robust spin state effect of FeTe2, accelerates electron transfer and reduces energy barriers, thereby improving sulfur redox reaction (SRR) kinetics; (3) the graphene-like CN nanosheets provide numerous active sites and mitigate volume expansion during cycling.

Consequently, LSBs based on S@FeTe2/CN cathodes exhibit high initial capacity, exceptional rate performance, and outstanding stability. This work offers a novel strategy for preparing 2D nanocarbon-based materials with highly exposed active sites to enhance SRR efficiency. This work has been published in Energy & Environmental Science in 2025. Building on my previous works, in Chapter 7, I further explore the feasibility of nanomaterials inSSBs. In this study, I demonstrate a heteronuclear diatomic catalyst featuring Fe and Co bimetallic sites embedded in nitrogen-doped hollow carbon nanospheres (Fe–Co/NC) as an effective sulfur host at the cathode of Na–S batteries. Aberration-corrected high-angle annular dark field scanning transmission electron microscopy demonstrates the presence of isolated Fe– Co atomic pairs, while synchrotron radiation X-ray absorption fine structure analysis confirms the (Fe–Co–N6) coordination structure. DFTcalculations show that the introduction of Fe atoms induces electron delocalization in Co(II), shifting the electronic configuration from a low-spin to a higher-spin state. This shift enhances the hybridization of the Co dz2 orbitals with the antibonding π orbitals of sulfur atoms within the sodium sulfide species that accelerates their catalytic conversion. As a result, Fe–Co/NC-based cathodes exhibit excellent cycling stability (378 mAh g–1 after 2000 cycles) and impressive rate performance (341.1 mAh g–1 under 5 A g– 1). This work has been published in the Journal of the American Chemical Society in 2025.

[cat] Les bateries de sofre amb metalls alcalins (AMSBs), com les bateries de sofre-liti (Li-S) i sofre-sodi (Na-S), han emergit com una alternativa molt prometedora a les tradicionals bateries d’ió liti (Li-ion). Aquest interès creixent es deu principalment a la seva alta densitat energètica teòrica, el baix cost dels materials utilitzats (especialment el sofre, abundant i econòmic), així com als seus avantatges ambientals, ja que poden contribuir a una tecnologia de bateries més sostenible. Tanmateix, malgrat aquests avantatges inherents, les AMSBs encara es troben en fase de desenvolupament i presenten diversos desafiaments tècnics que han limitat la seva comercialització a gran escala. Entre els principals problemes que cal abordar destaquen l’efecte conegut com a “shuttle” dels polisulfurs de liti (LiPS), que provoca pèrdues de capacitat i eficiència, la baixa conductivitat elèctrica del sofre elemental i la seva natura isolant, així com els grans canvis de volum que es produeixen durant els cicles de càrrega i descàrrega, els quals poden comprometre la integritat estructural dels elèctrodes. Per superar aquestes limitacions, s’estan duent a terme nombrosos estudis que proposen solucions innovadores, tant en el disseny dels materials de càtode com en la modificació dels separadors. Algunes de les estratègies més recents i prometedores inclouen: Separadors modificats amb WP@NPC (òxid de tungstè fosfatat incrustat en carboni nitrogenat porós): Aquesta estructura millora considerablement l’adsorció dels polisulfurs de liti i en facilita la conversió redox, la qual cosa condueix a una millor eficiència electroquímica. En proves de laboratori, s’han assolit valors destacats de 636 mAh g⁻¹ a una elevada velocitat de 5C, mantenint una eficiència colòmbica del 99,6% després de 1000 cicles. Separadors amb nanosheets de diseleniur d’estany (SnSe): Aquests nanosheets actuen com a catalitzadors efectius per a la conversió de LiPS, accelerant les reaccions redox i estabilitzant el procés de cicle. Això ajuda a minimitzar la difusió dels polisulfurs cap a l’ànode, un dels principals responsables de la pèrdua de capacitat en aquestes bateries. Càtodes amb nanosheets porosos de carboni nitrogenat (CN) decorats amb nanopartícules de FeTe₂: Aquesta combinació permet una captura eficaç dels polisulfurs i una millora de la conductivitat elèctrica del sistema. A més, ofereix estabilitat estructural durant els cicles, fet que redueix la degradació de l’elèctrode al llarg del temps. Catalitzadors diatòmics Fe-Co incrustats en carboni dopat amb nitrogen: Aquests catalitzadors mostren una capacitat única per modular l’estat d’espín durant la conversió del Na₂S, la qual cosa accelera les reaccions redox i millora la reversibilitat del procés. Amb aquest enfocament, s’ha aconseguit una capacitat de 378 mAh g⁻¹ després de 2000 cicles, demostrant una excel·lent estabilitat a llarg termini. En conjunt, aquestes innovacions representen un pas endavant significatiu cap a l’optimització de les AMSBs, aportant solucions funcionals als problemes estructurals i químics que han obstaculitzat el seu desenvolupament. Tot i que encara queda camí per recórrer abans d’arribar a aplicacions comercials generalitzades, els resultats fins ara indiquen un gran potencial per a ús en sistemes d’emmagatzematge d’energia estacionària, vehicles elèctrics i altres aplicacions exigents. Amb la recerca contínua en materials, arquitectura cel·lular i disseny d’electròdes, les bateries de sofre amb metalls alcalins podrien esdevenir una peça clau en el futur panorama energètic global.