In Situ Monitoring the Potassium-Ion Storage Enhancement in Iron Selenide with Ether-Based Electrolyte

Synthesis of a hollow MoSe2@MXene anode material for sodium-ion batteries

MXene and MoSe2 as a novel two-dimensional transition metal carbide and selenide, respectively, are widely used in battery electrodes. In this study, MoSe2 nanoflakes were grown in situ on hollow MXene spheres via a hydrothermal method. The obtained hollow 3D spherical MoSe2@MXene composite exhibits outstanding rate performance and cycling stability as the sodium-ion battery anode. At a current density of 1 A g-1, it achieves a specific capacity of 350.7 mA h g-1 after 1000 cycles with a retention rate of 93.7%. Even at a high current density of 10 A g-1, the specific capacity remains at 224.2 mA h g-1. The outstanding electrochemical performance of the MoSe2@MXene composite is primarily attributed to the synergistic interaction between MoSe2 and the MXene, which significantly reduces the self-aggregation of the MXene and MoSe2 and effectively enhances the diffusion and migration rates of sodium ions.

Operando Optical Imaging of Cathode Swelling Process Inside Lithium Primary Batteries: Comparative Studies between Different Structured CFx

As a crucial cathode material with ultrahigh theoretical capacity (865 mAh/g) and energy density (>2100 Wh/kg), fluorinated carbon (CFx) is promising for lithium primary batteries. However, the operating performance of CFx cathode is hindered by nonuniform volume expansion during discharging. To investigate this, we used operando confocal microscopy to visualize the thickness evolution of two different CFx cathodes and quantified their swelling ratios as a function of the discharge depth. Our findings show that CFx synthesized from hard carbon (FHC), featuring a disordered structure and abundant pores, exhibits a swelling ratio lower than that of conventional layered fluorinated graphite (FG). This is due to different electrochemical mechanisms: lithium enters FG interlayers nonuniformly through "edge propagation" pathways while lithiation occurs homogeneously in FHC, unraveled by single-particle Raman and photoluminescence measurement. This work enhances our understanding on CFx volume expansion, offering important opportunities to address the cathode swelling issue and optimize electrochemical performance in Li/CFx batteries.

Layered Potassium Titanium Niobate/Reduced Graphene Oxide Nanocomposite as a Potassium-Ion Battery Anode

With graphite currently leading as the most viable anode for potassium-ion batteries (KIBs), other materials have been left relatively under-examined. Transition metal oxides are among these, with many positive attributes such as synthetic maturity, long-term cycling stability and fast redox kinetics. Therefore, to address this research deficiency we report herein a layered potassium titanium niobate KTiNbO5 (KTNO) and its rGO nanocomposite (KTNO/rGO) synthesised via solvothermal methods as a high-performance anode for KIBs. Through effective distribution across the electrically conductive rGO, the electrochemical performance of the KTNO nanoparticles was enhanced. The potassium storage performance of the KTNO/rGO was demonstrated by its first charge capacity of 128.1 mAh g-1 and reversible capacity of 97.5 mAh g-1 after 500 cycles at 20 mA g-1, retaining 76.1% of the initial capacity, with an exceptional rate performance of 54.2 mAh g-1 at 1 A g-1. Furthermore, to investigate the attributes of KTNO in-situ XRD was performed, indicating a low-strain material. Ex-situ X-ray photoelectron spectra further investigated the mechanism of charge storage, with the titanium showing greater redox reversibility than the niobium. This work suggests this low-strain nature is a highly advantageous property and well worth regarding KTNO as a promising anode for future high-performance KIBs.

Built-In Electric Field-Driven Ultrahigh-Rate K-Ion Storage via Heterostructure Engineering of Dual Tellurides Integrated with Ti3C2Tx MXene

Exploiting high-rate anode materials with fast K+ diffusion is intriguing for the development of advanced potassium-ion batteries (KIBs) but remains unrealized. Here, heterostructure engineering is proposed to construct the dual transition metal tellurides (CoTe2/ZnTe), which are anchored onto two-dimensional (2D) Ti3C2Tx MXene nanosheets. Various theoretical modeling and experimental findings reveal that heterostructure engineering can regulate the electronic structures of CoTe2/ZnTe interfaces, improving K+ diffusion and adsorption. In addition, the different work functions between CoTe2/ZnTe induce a robust built-in electric field at the CoTe2/ZnTe interface, providing a strong driving force to facilitate charge transport. Moreover, the conductive and elastic Ti3C2Tx can effectively promote electrode conductivity and alleviate the volume change of CoTe2/ZnTe heterostructures upon cycling. Owing to these merits, the resulting CoTe2/ZnTe/Ti3C2Tx (CZT) exhibit excellent rate capability (137.0 mAh g-1 at 10 A g-1) and cycling stability (175.3 mAh g-1 after 4000 cycles at 3.0 A g-1, with a high capacity retention of 89.4%). More impressively, the CZT-based full cells demonstrate high energy density (220.2 Wh kg-1) and power density (837.2 W kg-1). This work provides a general and effective strategy by integrating heterostructure engineering and 2D material nanocompositing for designing advanced high-rate anode materials for next-generation KIBs.

Trimming the Degrees of Freedom via a K+ Flux Rectifier for Safe and Long-Life Potassium-Ion Batteries

High degrees of freedom (DOF) for K+ movement in the electrolytes is desirable, because the resulting high ionic conductivity helps improve potassium-ion batteries, yet requiring support from highly free and flammable organic solvent molecules, seriously affecting battery safety. Here, we develop a K+ flux rectifier to trim K ion's DOF to 1 and improve electrochemical properties. Although the ionic conductivity is compromised in the K+ flux rectifier, the overall electrochemical performance of PIBs was improved. An oxidation stability improvement from 4.0 to 5.9 V was realized, and the formation of dendrites and the dissolution of organic cathodes were inhibited. Consequently, the K||K cells continuously cycled over 3,700 h; K||Cu cells operated stably over 800 cycles with the Coulombic efficiency exceeding 99%; and K||graphite cells exhibited high-capacity retention over 74.7% after 1,500 cycles. Moreover, the 3,4,9,10-perylenetetracarboxylic diimide organic cathodes operated for more than 2,100 cycles and reached year-scale-cycling time. We fabricated a 2.18 Ah pouch cell with no significant capacity fading observed after 100 cycles.

CO2-Activation Nanofiber Carbon Paper as a High-Performance Interlayer for Trapping Polysulfides in Li-S Batteries

Lithium-sulfur (Li-S) batteries have high theoretical energy density but low sulfur utilization due to the inherent insulating nature of sulfur and the shuttle effect of polysulfides. Herein, the CO₂-activation carbon paper was prepared by poly(p-phenylenebenzobisoxazole) (PBO) nanofiber and was first applied as an interlayer for efficiently alleviating the shuttle effect of polysulfides in Li-S batteries. This interlayer exhibits good flexibility and strength with rich -C═O and -COOH functional groups on the three-dimensional porous structure, which improves chemical adsorption on Li₂S_x species and ion rapid diffusion via interconnected diffusion channels and thus enhances the electrochemical kinetics. The initial specific capacity is 1367.4 mAh g^-1 and remains 999.8 mAh g^-1 after 200 cycles at 0.2C and 780.1 mAh g^-1 at 5C, and the Coulombic efficiency is high, up to 99.8%, which is much better than that for the carbon paper without CO₂ activation. The highly conductive flexible PBO carbon paper may bring breakthroughs in performance and thus lead to more practical applications of Li-S batteries.

Comprehensively Understanding the Role of Anion Vacancies on K-Ion Storage: A Case Study of Se-Vacancy-Engineered VSe2

Anion vacancy engineering (AVE) is widely used to improve the Li-ion and Na-ion storage of conversion-type anode materials. However, AVE is still an emerging strategy in K-ion batteries, which are promising for large-scale energy storage. In addition, the role of anion vacancies on ion storage is far from clear, despite several proposed explanations. Herein, by employing VSe₂ as a model conversion-type anode material, Se vacancies are intentionally introduced (labeled as P-VSe_2-x ) to investigate their effect on K⁺ storage. The P-VSe_2-x shows excellent cyclability in half cells (143 mA h g^-1 at 3.0 A g^-1 after 1000 cycles) and high energy density in coin-type full cells (206.8 Wh kg^-1 ). By applying various electrochemical techniques, the effects of Se vacancies on the redox potentials of K-ion insertion/extraction and the K-ion diffusion in electrodes upon cycling are uncovered. In addition, the structural evolution of Se vacancies during potassiation/de-potassiation using various operando and ex characterizations is revealed. Moreover, it is demonstrated that Se vacancies can facilitate the breaking of VSe bonds upon the P-VSe_2-x conversion using theoretical calculations. This work comprehensively explains the role of anion vacancies in ion storage for developing high-performance conversion-type anode materials.

A Review of In-Situ Techniques for Probing Active Sites and Mechanisms of Electrocatalytic Oxygen Reduction Reactions

Electrocatalytic oxygen reduction reaction (ORR) is one of the most important reactions in electrochemical energy technologies such as fuel cells and metal-O2/air batteries, etc. However, the essential catalysts to overcome its slow reaction kinetic always undergo a complex dynamic evolution in the actual catalytic process, and the concomitant intermediates and catalytic products also occur continuous conversion and reconstruction. This makes them difficult to be accurately captured, making the identification of ORR active sites and the elucidation of ORR mechanisms difficult. Thus, it is necessary to use extensive in-situ characterization techniques to proceed the real-time monitoring of the catalyst structure and the evolution state of intermediates and products during ORR. This work reviews the major advances in the use of various in-situ techniques to characterize the catalytic processes of various catalysts. Specifically, the catalyst structure evolutions revealed directly by in-situ techniques are systematically summarized, such as phase, valence, electronic transfer, coordination, and spin states varies. In-situ revelation of intermediate adsorption/desorption behavior, and the real-time monitoring of the product nucleation, growth, and reconstruction evolution are equally emphasized in the discussion. Other interference factors, as well as in-situ signal assignment with the aid of theoretical calculations, are also covered. Finally, some major challenges and prospects of in-situ techniques for future catalysts research in the ORR process are proposed.

Filling Selenium into Sulfur Vacancies in Ultrathin Tungsten Sulfide Nanosheets for Superior Potassium Storage

The development of WS₂ as an anode for potassium-ion batteries (PIBs) is severely confined by the low K⁺ storage capacity and poor intrinsic electrical conductivity. Our previous study demonstrated that the creation of sulfur vacancies (V_S) in WS₂ can enhance its K⁺ storage capability. However, it is a big challenge to keep the stability of V_S while reserving the excellent activity. Herein, we design Se-filled WS₂ nanosheets with V_S (V_S-WS₂-Se NS) for PIBs. The Se heteroatom filling into the V_S can not only stabilize and activate them, rendering more active sites to adsorb K⁺, but also further enhance the electrical conductivity. Consequently, the V_S-WS₂-Se NS anode presents significantly promoted storage capacity and reaction kinetics, superior to the pristine WS₂ and WS₂ with only V_S. Remarkably, the V_S-WS₂-Se NS anode exhibits the highest specific capacity of 363.9 mA h g^-1 at 0.05 A g^-1. Simultaneously, a high reversible capacity of 144.2 mA h g^-1 after 100 cycles at 2.0 A g^-1 is shown. Ex situ analyses demonstrated that the potassium storage mechanism involves the intercalation and conversion reaction between WS₂ and K⁺. Moreover, DFT calculations revealed that the Se filling into V_S can further enhance the electrical conductivity and reduce the K-insertion energy barriers of WS₂ and thus account for the outstanding electrochemical performance. This study demonstrates that engineering the vacancies by the heteroatom filling strategy offers a novel and feasible route for designing high-performance electrode materials in various energy-storage systems.

New enhancement mechanism of an ether-based electrolyte in cobalt sulfide-containing potassium-ion batteries

The performance of potassium (K)-ion batteries (KIBs) is not only dependent on electrode materials but also highly related to the electrolyte. In this work, we obtained a cobalt sulfide (CoS)-containing hybrid by the hydrothermal method and subsequent thermal treatment for K-ion storage. After ether-based electrolyte matching, the CoS-containing hybrid achieves a specific capacity of 229 mA h g^-1 at 1 A g^-1 after 300 cycles, and presents enhanced performance in the ether-based electrolyte. According to our measurement and calculation, the CoS-containing hybrid in the ether-based electrolyte promotes the formation of a highly anionic coordination solvated structure, which contributes to the enhancement of the stability of the electrolyte for K-ion storage. In addition, the strong coordination of anions also facilitates the rapid separation of the solvent during the potassiation process, which is also in favor of the decrease of the side reaction of the CoS@RGO hybrid for KIBs. We believe that our work will provide a new perspective on electrolyte engineering to boost the electrode material performance for K-ion storage.

Design of reduced graphene oxide coating carbon sub-microspheres hierarchical nanostructure for ultra-stable potassium storage performance

The development of high-performance carbon-based anode materials is still a significant challenge for K-ion storage. In our work, we designed reduced graphene oxide coating carbon sub-microspheres hierarchical nanostructure (CS@RGO) hierarchical nanostructure via a simple freeze-drying and subsequent pyrolysis as anode for K-ion batteries (KIBs), which presented an excellent electrochemical performance for K-ion storage, with a reversible specific capacity of 295 mAh g^-1 after 100 cycles at 100 mAh g^-1. Even at a high current density of 1 A g^-1, our CS@RGO still achieves ultra-stable K-ion storage of 200 mAh g^-1 at 1 A g^-1 after 5000 cycles almost without capacity fade. According to the galvanostatic intermittent titration technique result, the CS@RGO hybrid receives a high average diffusion coefficient of 7.35 × 10^-8 cm² s^-1, contributing to the rapid penetration of K-ion, which facilitates the enhancement of electrochemical performance for KIBs. Besides, we also use Raman spectra to investigate the electrochemical behavior of our CS@RGO hybrid for K-ion storage and confirm the reaction process. We believe that our work will offer the opportunity to enable ultra-stable carbon-based materials by the structure design in the K-ion battery field.

Recent progress on Sb- and Bi-based chalcogenide anodes for potassium ion batteries

Potassium ion batteries (PIBs) are potential alternative energy storage systems to lithium ion batteries (LIBs), due to elemental abundance of potassium, low cost and similar working principle to LIBs. Recently, metal chalcogenides (MCs) have gained enormous interests, especially antimony (Sb)-, bismuth (Bi) -based chalcogenides because they were able to undergo alloying/conversion dual mechanism, which can provide higher specific capacity and energy density (K 3 Sb~660 mA h g -1 , K 3 Bi~385 mA h g -1 ). However, several challenges hinder the development of Sb-, Bi-based chalcogenide anode materials for PIBs , such as huge volume expansion during potassiation, unstable solid-electrolyte interface (SEI), slow reaction kinetics, and polychalcogenide-induced shuttle effect . In this review, the current state-of-the-art Sb-, Bi-based chalcogenides are comprehensively summarized, including the reaction mechanism, electrochemical performance, ingenious nanostructures, electrolyte systems, and prospects for future development. This review contributes to understanding the K + storage mechanism and the interaction between active materials and electrolytes, providing guidance and foundation for the design of next-generation high-performance PIBs.