Bi-Based Electrode Materials for Alkali Metal-Ion Batteries

Advanced Bismuth-Based Anode Materials for Efficient Potassium Storage: Structural Features, Storage Mechanisms and Modification Strategies

Potassium-ion batteries (PIBs) are considered as a promising energy storage system owing to its abundant potassium resources. As an important part of the battery composition, anode materials play a vital role in the future development of PIBs. Bismuth-based anode materials demonstrate great potential for storing potassium ions (K+) due to their layered structure, high theoretical capacity based on the alloying reaction mechanism, and safe operating voltage. However, the large radius of K+ inevitably induces severe volume expansion in depotassiation/potassiation, and the sluggish kinetics of K+ insertion/extraction limits its further development. Herein, we summarize the strategies used to improve the potassium storage properties of various types of materials and introduce recent advances in the design and fabrication of favorable structural features of bismuth-based materials. Firstly, this review analyzes the structure, working mechanism and advantages and disadvantages of various types of materials for potassium storage. Then, based on this, the manuscript focuses on summarizing modification strategies including structural and morphological design, compositing with other materials, and electrolyte optimization, and elucidating the advantages of various modifications in enhancing the potassium storage performance. Finally, we outline the current challenges of bismuth-based materials in PIBs and put forward some prospects to be verified.

Deconstruction Engineering of Lignocellulose Toward High-Plateau-Capacity Hard Carbon Anodes for Sodium-Ion Batteries

Biomass-derived hard carbon is a promising anode material for commercial sodium-ion batteries due to its low cost, high capacity, and stable cycling performance. However, the intrinsic tight lignocellulosic structure in biomass hinders the formation of sufficient closed pores, limiting the specific capacity of obtained hard carbons. In this contribution, a mild, industrially mature pretreatment method is utilized to selectively regulate biomass components. The hard carbon with a rich closed pore structure is prepared by optimizing the appropriate ratio of biomass composition. Optimized etching conditions enhanced the closed pore volume of hard carbon from 0.15 to 0.26 cm3 g-1. Consequently, the engineered hard carbon exhibited excellent electrochemical performance, including a high reversible capacity of 346 mAh g-1 with a high plateau capacity of 254 mAh g⁻¹ at 50 mA g⁻¹, robust rate capability, and cycling stability. The optimized hard carbon shows an 88 mAh g⁻¹ increase in plateau capacity compared to hard carbon from directly carbonizing bamboo fibers. This mature approach provides an easy-to-operate industrial pathway for designing high-capacity biomass-based hard carbons for sodium-ion batteries.

Constructing Three-Dimensional Architectures to Design Advanced Copper-Based Current Collector Materials for Alkali Metal Batteries: From Nanoscale to Microscale

Alkali metals (Li, Na, and K) are deemed as the ideal anode materials for next-generation high-energy-density batteries because of their high theoretical specific capacity and low redox potentials. However, alkali metal anodes (AMAs) still face some challenges hindering their further applications, including uncontrollable dendrite growth and unstable solid electrolyte interphase during cycling, resulting in low Coulombic efficiency and inferior cycling performance. In this regard, designing 3D current collectors as hosts for AMAs is one of the most effective ways to address the above-mentioned problems, because their sufficient space could accommodate AMAs' volume expansion, and their high specific surface area could lower the local current density, leading to the uniform deposition of alkali metals. Herein, we review recent progress on the application of 3D Cu-based current collectors in stable and dendrite-free AMAs. The most widely used modification methods of 3D Cu-based current collectors are summarized. Furthermore, the relationships among methods of modification, structure and composition, and the electrochemical properties of AMAs using Cu-based current collectors, are systematically discussed. Finally, the challenges and prospects for future study and applications of Cu-based current collectors in high-performance alkali metal batteries are proposed.

Band Structure Engineering Promotes Anionic Redox Reversibility of Cobalt-Free Li-Rich Layered Oxides Cathodes

Li-rich layered oxides cathodes (LLOs) have prevailed as the promising high-energy-density cathode materials due to their distinctive anionic redox chemistry. However, uncontrollable anionic redox process usually leads to structural deterioration and electrochemical degradation. Herein, a Mo/Cl co-doping strategy is proposed to regulate the relative position of energy band for modulating the anionic redox chemistry and strengthening the structural stability of Co-free Li1.16Mn0.56Ni0.28O2 cathodes. The incorporation of Mo with high d state orbit and Cl with low electronegativity can narrow the band energy gap between bonding and antibonding bands via increasing the filled lower-Hubbard band (LHB) and decreasing the non-bonding O 2p energy bands, promoting the anionic redox reversibility. In addition, strong covalent Mo─O and Mn─Cl bonding further increases the covalency of Mn─O band to further stabilize the O2 n- species and enhance the reversible distortion of MnO6 octahedron. The strengthening electronic conductivity, together with the epitaxial structure Li2MoO4 facilitates the fast Li+ kinetics. As a result, the dual doping material exhibits enhanced anionic redox reversibility and suppressed oxygen release with increased cyclic stability and excellent rate performance. This strategy provides some guidance to design high-energy-density LLOs with desirable anionic redox reversibility and stable crystal structure via band structure engineering.

KF-Containing Interphase Formation Enables Better Potassium Ion Storage Capability

Rechargeable potassium ion batteries have long been regarded as one alternative to conventional lithium ion batteries because of their resource sustainability and cost advantages. However, the compatibility between anodes and electrolytes remains to be resolved, impeding their commercial adoption. In this work, the K-ion storage properties of Bi nanoparticles encapsulated in N-doped carbon nanocomposites have been examined in two typical electrolyte solutions, which show a significant effect on potassium insertion/removal processes. In a KFSI-based electrolyte, the N-C@Bi nanocomposites exhibit a high specific capacity of 255.2 mAh g-1 at 0.5 A g-1, which remains at 245.6 mAh g-1 after 50 cycles, corresponding to a high capacity retention rate of 96.24%. In a KPF6-based electrolyte, the N-C@Bi nanocomposites show a specific capacity of 209.0 mAh g-1, which remains at 71.5 mAh g-1 after 50 cycles, corresponding to an inferior capacity retention rate of only 34.21%. Post-investigations reveal the formation of a KF interphase derived from salt decomposition and an intact rod-like morphology after cycling in K2 electrolytes, which are responsible for better K-ion storage properties.

Ultra-Thin Hydrogen-Organic-Framework (HOF) Nanosheets for Ultra-Stable Alkali Ions Battery Storage

Organic frameworks-based batteries with excellent physicochemical stability and long-term high capacity will definitely reduce the cost, carbon emissions, and metal consumption and contamination. Here, an ultra-stable and ultra-thin perylene-dicyandiamide-based hydrogen organic framework (HOF) nanosheet (P-DCD) of ≈3.5 nm in thickness is developed. When applied in the cathode, the P-DCD exhibits exceptional long-term capacity retention for alkali-ion batteries (AIBs). Strikingly, for lithium-ion batteries (LIBs), at current of 2 A g-1, the large reversible capacity of 108 mA h g-1 shows no attenuation within 5 000 cycles. For sodium-ion batteries (SIBs), the related capacity retains 91.7% within 10 000 cycles compared to the initial state, significantly much more stable than conventional organic materials reported previously. Mechanism studies through ex situ and in situ experiments and theoretical density functional theory (DFT) calculations reveal that the impressive long-term performance retention originates from the large electron delocalization, fast ion diffusion, and physicochemical stability within the ultra-thin 2D P-DCD, featuring π-π and hydrogen bonding stacking, nitrogen-rich units, and low impedance. The advantageous features demonstrate that rationally designed stable and effective organic frameworks pave the way to utilizing complete organic materials for developing next-generation low-cost and highly stable energy storage batteries.

Structure and Interface Engineering of Ultrahigh-Rate 3D Bismuth Anodes for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have attracted tremendous attention as promising low-cost energy storage devices in future grid-scale energy management applications. Bismuth is a promising anode for SIBs due to its high theoretical capacity (386 mAh g-1 ). Nevertheless, the huge volume variation of Bi anode during (de)sodiation processes can cause the pulverization of Bi particulates and rupture of solid electrolyte interphase (SEI), resulting in quick capacity decay. It is demonstrated that rigid carbon framework and robust SEI are two essentials for stable Bi anodes. A lignin-derived carbonlayer wrapped tightly around the bismuth nanospheres provides a stable conductive pathway, while the delicate selection of linear and cyclic ether-based electrolytes enable robust and stable SEI films. These two merits enable the long-term cycling process of the LC-Bi anode. The LC-Bi composite delivers outstanding sodium-ion storage performance with an ultra-long cycle life of 10 000 cycles at a high current density of 5 A g-1 and an excellent rate capability of 94% capacity retention at an ultrahigh current density of 100 A g-1 . Herein, the underlying origins of performance improvement of Bi anode are elucidated, which provides a rational design strategy for Bi anodes in practical SIBs.

Graphene-Based Materials for the Separator Functionalization of Lithium-Ion/Metal/Sulfur Batteries

With the escalating demand for electrochemical energy storage, commercial lithium-ion and metal battery systems have been increasingly developed. As an indispensable component of batteries, the separator plays a crucial role in determining their electrochemical performance. Conventional polymer separators have been extensively investigated over the past few decades. Nevertheless, their inadequate mechanical strength, deficient thermal stability, and constrained porosity constitute serious impediments to the development of electric vehicle power batteries and the progress of energy storage devices. Advanced graphene-based materials have emerged as an adaptable solution to these challenges, owing to their exceptional electrical conductivity, large specific surface area, and outstanding mechanical properties. Incorporating advanced graphene-based materials into the separator of lithium-ion and metal batteries has been identified as an effective strategy to overcome the aforementioned issues and enhance the specific capacity, cycle stability, and safety of batteries. This review paper provides an overview of the preparation of advanced graphene-based materials and their applications in lithium-ion, lithium-metal, and lithium-sulfur batteries. It systematically elaborates on the advantages of advanced graphene-based materials as novel separator materials and outlines future research directions in this field.

Selenide-doped bismuth sulfides (Bi2S3-xSex) and their hierarchical heterostructure with ReS2for sodium/potassium-ion batteries

In this work, selenide-doped bismuth sulfides (Bi₂S_3-xSe_x) was successfully prepared through Se doping Bi₂S₃ Se to improve the electronic conductivity and increase the interlayer spacing. Then the anisotropic ReS₂ nanosheet arrays were grown on the surface of Bi₂S_3-xSe_x to form a hierarchical heterostructure (Bi₂S_3-xSe_x@ReS₂). The doping and construction of heterostructure processes can greatly improve the electrochemical conductivity of electrode materials and relieve the volume expansion during the continuous charge/discharge processes. While applied as SIBs anode, the specific capacity of 330 mAh g^-1 was maintained after 450 cycles at the current density of 1.0 A g^-1. It can also keep 200 mAh g^-1 specific capacity after 900 cycles at 1.0 A g^-1 for the anode of PIBs. This heterogeneous engineering and doping dual strategies could provide a good idea for the synthesis of new bimetallic sulfides with outstanding battery performance for SIBs and PIBs.

Vapor-phase derived ultra-fine Bismuth nanoparticles embedded in carbon nanotube networks as anodes for sodium and potassium ion batteries

Bismuth (Bi) is a promising material as the anode for sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) due to its characteristics such as reasonable price and high theoretical volumetric capacity (3800 mAh cm^-3). Nevertheless, considerable drawbacks have hindered the practical applications of Bi, including its relatively low electrical conductivity and inevitable volumetric change during the alloying/dealloying processes. To solve these problems, we proposed a novel design:Bi nanoparticles were synthesized via a single-step low-pressure vapor-phase reaction and embedded onto the surfaces of multi-walled carbon nanotubes (MWCNTs). After being vaporized at 650℃ and 10^-5 Pa, Bi nanoparticles less than 10 nm were uniformly distributed in the three-dimensional (3D) MWCNT networks to form a Bi/MWNTs composite. In this unique design, the nanostructured Bi can reduce the risk of structural rupture during cycling, and the structure of the MWCMT network is beneficial in shortening the electron/ion transport path. In addition, MWCNTs can improve the overall conductivity of the Bi/MWCNTs composite and prevent particle aggregation, thus improving the cycling stability and rate performance. As an anode material for SIB, the Bi/MWCNTs composite has demonstrated excellent fast charging performance with a reversible capacity of 254 mAh/g at 20 A/g. A capacity of 221mAhg^-1 after cycling at 10 A/g for 8000 cycles has also been achieved for SIB. As an anode material for PIB, the Bi/MWCNTs composite has delivered excellent rate performances with a reversible capacity of 251 mAh/g at 20 A/g. A specific capacity of 270mAhg^-1 after cycling at 1Ag^-1 for 5000 cycles has also been achieved for PIB.

Novel Preoxidation-Assisted Mechanism to Preciously Form and Disperse Bi2O3 Nanodots in Carbon Nanofibers for Ultralong-Life and High-Rate Sodium Storage

Metal oxides, as promising electrode materials for sodium-ion batteries, usually need to be formed by exposure to oxygen, which usually thermally corrodes the carbon material with which they are compounded, reducing their flexibility and electrical conductivity. Herein, we present for the first time a preoxidation-assisted mechanism to prepare bismuth oxide and carbon nanofibers (Bi₂O₃@C-NFs) by electrospinning, using Bi₂S₃ nanorods as multifunctional templates. The bismuth could be oxidized by C═O bonds formed through the cyclization reaction in the high-temperature calcination process, effectively avoiding thermal corrosion of carbon in oxygen atmosphere at high temperature. More importantly, the uniformly distributed Bi₂O₃ nanodots and longitudinal tunnels are formed inside the S- and N-doped carbon nanofibers with the continuous diffusion of Bi generated from the decomposition of Bi₂S₃ nanorods and the conversion to Bi─O bonds with C═O bonds being broken. Benefiting from the structural and composition merits arising from preoxidation, Bi₂O₃@C-NFs self-supporting anodes show high specific capacity (439 mAh g^-1 at 0.05 A g^-1), superior rate performance (243 mAh g^-1 at a current density of 20 A g^-1), and outstanding cycling stability (211 mAh g^-1 after 2000 cycles at 5 A g^-1). The effective combination of the well-established electrospinning technology and the preoxidation assisted mechanism provides a new way for the preparation of metal oxide and carbon composites.

Emerging Chalcohalide Materials for Energy Applications

Semiconductors with multiple anions currently provide a new materials platform from which improved functionality emerges, posing new challenges and opportunities in material science. This review has endeavored to emphasize the versatility of the emerging family of semiconductors consisting of mixed chalcogen and halogen anions, known as "chalcohalides". As they are multifunctional, these materials are of general interest to the wider research community, ranging from theoretical/computational scientists to experimental materials scientists. This review provides a comprehensive overview of the development of emerging Bi- and Sb-based as well as a new Cu, Sn, Pb, Ag, and hybrid organic-inorganic perovskite-based chalcohalides. We first highlight the high-throughput computational techniques to design and develop these chalcohalide materials. We then proceed to discuss their optoelectronic properties, band structures, stability, and structural chemistry employing theoretical and experimental underpinning toward high-performance devices. Next, we present an overview of recent advancements in the synthesis and their wide range of applications in energy conversion and storage devices. Finally, we conclude the review by outlining the impediments and important aspects in this field as well as offering perspectives on future research directions to further promote the development of chalcohalide materials in practical applications in the future.

Porous Metal Current Collectors for Alkali Metal Batteries

Alkali metals (i.e., Li, Na, and K) are promising anode materials for next-generation high-energy-density batteries due to their superior theoretical specific capacities and low electrochemical potentials. However, the uneven current and ion distribution on the anode surface probably induces undesirable dendrite growth, which leads to significant safety hazards and severely hinders the commercialization of alkali metal anodes. A smart and versatile strategy that can accommodate alkali metals into porous metal current collectors (PMCCs) has been well established to resolve the issues as well as to promote the practical applications of alkali metal anodes. Moreover, the proposal of PMCCs can meet the requirement of the dendrite-free battery fabrication industry, while the electrode material loading exactly needs the metal current collector component as well. Here, a systematic survey on advanced PMCCs for Li, Na, and K alkali metal anodes is presented, including their development timeline, categories, fabrication methods, and working mechanism. On this basis, some significant methodology advances to control pore structure, surface area, surface wettability, and mechanical properties are systematically summarized. Further, the existing issues and the development prospects of PMCCs to improve anode performance in alkali metal batteries are discussed.

Emerging carbon-based flexible anodes for potassium-ion batteries: Progress and opportunities

In recent years, carbon-based flexible anodes for potassium-ion batteries are increasingly investigated owing to the low reduction potential and abundant reserve of K and the simple preparation process of flexible electrodes. In this review, three main problems on pristine carbon-based flexible anodes are summarized: excessive volume change, repeated SEI growth, and low affinity with K⁺, which thus leads to severe capacity fade, sluggish K⁺ diffusion dynamics, and limited active sites. In this regard, the recent progress on the various modification strategies is introduced in detail, which are categorized as heteroatom-doping, coupling with metal and chalcogenide nanoparticles, and coupling with other carbonaceous materials. It is found that the doping of heteroatoms can bring the five enhancement effects of increasing active sites, improving electrical conductivity, expediting K⁺ diffusion, strengthening structural stability, and enlarging interlayer spacing. The coupling of metal and chalcogenide nanoparticles can largely offset the weakness of the scarcity of K⁺ storage sites and the poor wettability of pristine carbon-based flexible electrodes. The alloy nanoparticles consisting of the electrochemically active and inactive metals can concurrently gain a stable structure and high capacity in comparison to mono-metal nanoparticles. The coupling of the carbonaceous materials with different characteristics can coordinate the advantages of the nanostructure from graphite carbon, the defects and vacancies from amorphous carbon, and the independent structure from support carbon. Finally, the emerging challenges and opportunities for the development of carbon-based flexible anodes are presented.

Flat-Zigzag Interface Design of Chalcogenide Heterostructure toward Ultralow Volume Expansion for High-Performance Potassium Storage

Heterostructure construction of layered metal chalcogenides can boost their alkali-metal storage performance, where the charge transfer kinetics can be promoted by the built-in electric fields. However, these heterostructures usually undergo interface separation due to severe layer expansion, especially for large-size potassium accommodation, resulting in the deconstruction of heterostructures and battery performance fading. Herein, first a stable interface design strategy where two metal chalcogenides with totally different layer-morphologies are stacked to form large K⁺ transport channels, rendering ultralow interlayer expansion, is presented. As a proof of concept, the flat-zigzag MoS₂ /Bi₂ S₃ heterostructures stacked with zigzag-morphology Bi₂ S₃ and flat-morphology MoS₂ present an ultralow expansion ratio (1.98%) versus MoS₂ (9.66%) and Bi₂ S₃ (9.61%), which deliver an ultrahigh potassium storage capacity of above 600 mAh g^-1 and capacity retention of 76% after 500 cycles, together with the built-in electric field of heterostructures. Once the heterostructures are used as an anode for potassium-based dual-ion batteries (K-DIBs), it achieves a superior full-cell capacity of ≈166 mAh g^-1 with a capacity retention of 71% after 400 cycles, which is an outstanding performance among the reported K-DIBs. This proposed interface stacking strategy may offer a new way toward stable heterostructure design for metal ions storage and transport applications.

Boosting Interfacial Ion Transfer in Potassium-Ion Batteries via Synergy Between Nanostructured Bi@NC Bulk Anode and Electrolyte

Using high-capacity alloy-type anodes can greatly advance potassium-ion batteries (PIBs). However, the primary limits are unstable solid electrolyte interphase (SEI) and tough interfacial ion transfer associated with large-size K⁺ during electrochemical (de)alloy reactions. Here, we achieve excellent energy storage performance of PIBs via the synergy between a nanostructured Bi@N-doped carbon (Bi@NC) bulk anode and a KPF₆-dimethoxyethane (DME) electrolyte. The Bi@NC material with a high tap density of 3.81 g cm^-3 is prepared by simply pyrolyzing a commercial Bi salt yet affords a favorable nano/microstructure consisting of Bi nanograins confined in 3D ultrathin N-doped carbon shells, facilitating electron/ion transport and structural integrity. Detailed impedance spectroscopy investigation unveils that K⁺ transport through SEI at the Bi@NC anode, rather than the desolvation of K⁺, dominates the interfacial K⁺ transfer. More importantly, spectroscopic and microscopic characterizations provide clear evidence that the interplay between Bi@NC anode and optimized KPF₆-DME electrolyte can produce a unique SEI layer containing Bi³⁺-solvent complex that enables the activation energy of interfacial K⁺ transfer as low as 25.9 kJ mol^-1, thereby ultrafast charge transfer at Bi@NC. Consequently, the Bi@NC anode in half cells achieves exceptional rate capability (206 mAh g^-1 or 784 mAh cm^-3 at 120C) accompanied by high specific capacity (331 mAh g^-1 or 1261 mAh cm^-3) and long cycle life (running 1400 cycles at 15C with a tiny capacity fading rate of 0.013% per cycle). Moreover, the Bi@NC anode and KPF₆-DME electrolyte are also compatible with a potassium Prussian blue cathode and assembled full PIBs achieve stable cyclability (87.3% capacity retention after 100 cycles at 2.5C) and excellent rate performance (65.1% capacity retention upon increasing rates from 1 to 20C).

Stabilizing BiOCl/Ti3C2Tx Hybrids for Potassium-Ion Batteries via Solid Electrolyte Interphase Reconstruction

The synergistic innovation from reasonable material design to electrolyte optimization is the key to improve the performance of anode materials for potassium ion batteries (PIBs). In this work, a two-dimensional...

Bi@C Nanospheres with the Unique Petaloid Core-Shell Structure Anchored on Porous Graphene Nanosheets as an Anode for Stable Sodium- and Potassium-Ion Batteries

Bismuth (Bi) has emerged as a prospective candidate as Na-ion and potassium-ion battery anodes because of its unique advantages of low cost, high theoretical gravimetric capacity (386 mAh g^-1), and superior volumetric capacity (3800 mAh cm^-3). However, the low electronic conductivity and the huge volume expansion of Bi during the alloying/dealloying reactions are extremely detrimental to cycling stability, which seriously hinder its practical application. To overcome these issues, we propose a rational design: Bi@C nanospheres with the unique petaloid core-shell structure are synthesized in one step for the first time and then combined with different contents of graphene (GR) nanosheets to form the composites Bi@C@GR. The Bi@C nanospheres with a core-shell structure are beneficial to shortening the transmission path of electrons/ions and reducing the risk from structural rupture of the particles during cycling. In addition, the combination of Bi@C nanospheres and porous GR could greatly improve the conductivity and prevent the aggregation of particles, which is conducive to better cycling stability and rate performance. Consequently, Bi@C@GR-2 presents a superior reversible capacity for sodium storage (300 mAh g^-1 over 80 cycles) and potassium storage (200 mAh g^-1 over 70 cycles) at 0.1 A g^-1. Furthermore, in situ electrochemical impedance spectroscopy and ex situ transmission electron microscopy are carried out to analyze and reflect the kinetic reaction mechanism and the phase change of the Bi@C@GR-2 electrode during the charge/discharge processes.

An advanced BiPO4/super P anode material for high-performance potassium-ion batteries

Dispersed BiPO₄ nanoparticles loaded on the surface of a super P conducting network (BiPO₄/SP) were fabricated and investigated as a novel anode for PIBs. The BiPO₄/SP electrode demonstrates high rate capability (97.1 mA h g^-1 at 500 mA g^-1) and good long-term cycling performance (116 mA h g^-1 at 200 mA g^-1 over 100 cycles).

A Low-Cost Na-Ion and K-Ion Batteries Using a Common Organic Cathode and Bismuth Anode

Molecule-aggregation organic electrodes in principle have the capability for "single-molecule-energy-storage" in metal-ion rechargeable batteries, which indicates that the same organic electrode can be simultaneously applied to multiple metal-ion rechargeable batteries. In this study, the polyanionic organic compound 9,10-anthraquinone-2,6-disulfonate (Na₂ AQ26DS, 130 mAh g^-1 ) is used as a common cathode and metal bismuth (Bi) as a common anode to simultaneously assemble low-cost Na-ion and K-ion full cells. The Na-ion full cells can deliver the peak discharge capacity of 139 mAh g^-1 _cathode at 0.5-3.0 V, and the K-ion full cells can show the peak discharge capacity of 130 mAh g^-1 _cathode at 0.5-3.0 V. These results are comparable to the best organic-based Na-ion and K-ion full cells reported to date.

Recent Advances and Perspectives on the Polymer Electrolytes for Sodium/Potassium-Ion Batteries

Owing to the low cost of sodium/potassium resources and similar electrochemical properties of Na⁺ /K⁺ to Li⁺ , sodium-ion batteries (SIBs) and potassium-ion batteries (KIBs) are regarded as promising alternatives to lithium-ion batteries (LIBs) in large-scale energy storage field. However, traditional organic liquid electrolytes bestow SIBs/KIBs with serious safety concerns. In contrast, quasi-/solid-phase electrolytes including polymer electrolytes (PEs) and inorganic solid electrolytes (ISEs) show great superiority of high safety. However, the poor processibility and relatively low ionic conductivity of Na⁺ and K⁺ ions limit the further practical applications of ISEs. PEs combine some merits of both liquid-phase electrolytes and ISEs, and present great potentials in next-generation energy storage systems. Considerable efforts have been devoted to improving their overall properties. Nevertheless, there is still a lack of an in-depth and comprehensive review to get insights into mechanisms and corresponding design strategies of PEs. Herein, the advantages of different electrolytes, particularly PEs are first minutely reviewed, and the mechanism of PEs for Na⁺ /K⁺ ion transfer is summarized. Then, representative researches and recent progresses of SIBs/KIBs based on PEs are presented. Finally, some suggestions and perspectives are put forward to provide some possible directions for the follow-up researches.

Bismuth Nanoparticles Confined in Carbonaceous Nanospheres as Anodes for High-Performance Potassium-Ion Batteries

Bismuth (Bi) has been considered as a promising alloying-type anode for potassium-ion batteries (PIBs), owing to its high theoretical capacity and suitable working voltage plateaus. However, Bi suffers from dramatic volume fluctuation and significant pulverization during the discharge/charge processes, resulting in fast capacity decay. Herein, we synthesize Bi nanoparticles confined in carbonaceous nanospheres (denoted as Bi@C) for PIBs by first utilizing BiOCl nanoflakes as a hard template and a Bi precursor. The construction of the loose structure buffers the mechanical stresses resulting from the volume expansion of Bi during the alloying reaction and avoids the fracture of the electrode structure, thus improving the cycling performance. Moreover, the carbonaceous layers increase the electronic conductivity and disperse the Bi nanoparticles, enhancing the charge transportation and ionic diffusion, which further promotes the rate capability of Bi@C. It exhibits a superior capacity (389 mAh g^-1 at 100 mA g^-1 after 100 cycles), excellent cycling stability (206 mAh g^-1 at 500 mA g^-1 over 1000 cycles), and an improved rate capability (182 mAh g^-1 at 2.0 A g^-1). This work provides a new structuring strategy in alloying materials for boosting reversible and stable potassium-ion storage.

Zn, Co, and Fe Tridoped N-C Core-Shell Nanocages as the High-Efficiency Oxygen Reduction Reaction Electrocatalyst in Zinc-Air Batteries

Transition metal-nitrogen-carbon (TM-N-C) nanomaterials are promising platinum-based substitutes for the oxygen reduction reaction (ORR). However, large-scale commercial production of high-efficiency, durable TM-N-C catalysts remains a formidable challenge. In this work, a facile ″ZIF-on-ZIF″ strategy is first adopted to design ZIF-8@ZIF-67 core-shell polyhedral nanocages, and then, ferrocene (Fc) is added to form ZIF-8@ZIF-67@Fc double-layer encapsulating polyhedral nanocages. Finally, Zn, Co, and Fe tridoped N-C nanocages (ZnCoFe-N-C) as the high-efficiency ORR electrocatalyst are prepared through high-temperature annealing. Benefiting from the trimetal, nitrogen and carbon species bond to each other to form highly efficient active sites, and the material exhibits outstanding performance in 0.1 M KOH, onset potential and half-wave potential of up to 0.95 and 0.878 V (vs RHE), respectively, and long-term durability and methanol tolerance. Furthermore, when utilizing as a zinc-air battery (ZAB) air electrode, it exhibits wonderful indicators, reflected in an open circuit voltage of 1.525 V, power density of 350.2 mW cm^-2, and specific capacity of 794.7 mAh g_zn^-1, which outperforms the benchmark Pt/C catalyst. This work provides a facile and effective strategy to obtain a highly efficient and stable TM-N-C electrocatalyst for the ORR in ZABs.

Yolk-Shell-Structured Nanospheres with Goat Pupil-Like S-Doped SnSe Yolk and Hollow Carbon-Shell Configuration as Anode Material for Sodium-Ion Storage

Rationally nanostructured electrode materials exhibit excellent sodium-ion storage performance. In particular, yolk-shell configurations of metal chalcogenide@void@C are introduced in various synthetic strategies for use as superior anode materials. Herein, yolk-shell-structured nanospheres, with goat pupil-like configuration of S-doped SnSe yolks and hollow carbon shells, are synthesized by salt-infiltration and a simple post-treatment procedure. Impressively, the co-infiltration of thiourea and selenium oxide enables the doping of sulfur into SnSe (SnSeS) and carbon shells, as well as the formation of a goat pupil-like yolk-shell architecture. High-reactivity thiourea-derived H₂ S gas forms nanocrystals inside the carbon nanospheres. The nanocrystals act as seeds for the crystal growth of SnSeS through Ostwald ripening. The unique yolk-shell structure and composition with a heterointerface provide not only structural stability but also fast electrode reaction kinetics during repeated cycling. The SnSeS@C electrode shows an excellent cycle life (186 mA h g^-1 for 1000 cycles at 0.5 A g^-1 ) and rate capability (112 mA h g^-1 at 5.0 A g^-1 ).