Robust SnO2-x Nanoparticle-Impregnated Carbon Nanofibers with Outstanding Electrochemical Performance for Advanced Sodium-Ion Batteries

Three-Dimensional Electronic Network Assisted by TiN Conductive Pillars and Chemical Adsorption to Boost the Electrochemical Performance of Red Phosphorus

The practical application of red phosphorus (P) for sodium-ion batteries (SIBs) is retarded by its poor reversibility and its unstable cycling life derived from its poor conductivity and huge volume expansion. Graphene is considered as an ideal matrix to remedy these weaknesses due to its excellent conductivity and two-dimensional structure. Its π-π restacking causes spatial collapse, however, meaning that graphene cannot effectively buffer volume expansion. Herein, multifunctional TiN is introduced into a P composite to fix this issue. TiN acts as conductive pillars, electron transfer bridges, and a chemical adsorbent of phosphorus in the composite, to prevent the graphene nanoplates from restacking, to bridge gaps between the graphene nanoplates, and to chemically adsorb the P, resulting in the formation of a three-dimensional electronic network and endowing the pulverized P particles with good contact with the conductive matrix to avoid forming insulating "dead P". Consequently, the P composite presents excellent performance for SIBs.

Cu2Se Nanoparticles Encapsulated by Nitrogen-Doped Carbon Nanofibers for Efficient Sodium Storage

Cu₂Se with high theoretical capacity and good electronic conductivity have attracted particular attention as anode materials for sodium ion batteries (SIBs). However, during electrochemical reactions, the large volume change of Cu₂Se results in poor rate performance and cycling stability. To solve this issue, nanosized-Cu₂Se is encapsulated in 1D nitrogen-doped carbon nanofibers (Cu₂Se-NC) so that the unique structure of 1D carbon fiber network ensures a high contact area between the electrolyte and Cu₂Se with a short Na⁺ diffusion path and provides a protective matrix to accommodate the volume variation. The kinetic analysis and D_Na+ calculation indicates that the dominant contribution to the capacity is surface pseudocapacitance with fast Na⁺ migration, which guarantees the favorable rate performance of Cu₂Se-NC for SIBs.

Double Morphology of Co9S8 Coated by N, S Co-doped Carbon as Efficient Anode Materials for Sodium-Ion Batteries

Co₉S₈ is a potential anode material for its high sodium storage performance, easy accessibility, and thermostability. However, the volume expansion is a great hindrance to its development. Herein, a composite containing Co₉S₈ nanofibers and hollow Co₉S₈ nanospheres with N, S co-doped carbon layer (Co₉S₈@NSC) is successfully synthesized through a facile solvothermal process and a high-temperature carbonization. Ascribed to the carbon coating and the large specific surface area, severe volume stress can be effectively alleviated. In particular, with N and S heteroatoms introduced into the carbon layer, which is conducive to the Na⁺ adsorption and diffusion on the carbon surface, Co₉S₈@NSC can perform more capacitive sodium storage mechanism. As a result, the electrode can exhibit a favorable reversible capacity of 226 mA h g^-1 at 5 A g^-1 and a favorable capacity retention of 83.1% at 1 A g^-1 after 800 cycles. The unique design provides an innovative thought for enhancing the sodium storage performance.

Multi-heteroatom-doped dual carbon-confined Fe3O4 nanospheres as high-capacity and long-life anode materials for lithium/sodium ion batteries

The lithium/sodium-ion storage properties of transition metal oxides often undergo startling volume variation and poor electrical conductivity. Herein, N, P and S doped dual carbon-confined Fe₃O₄ nanospheres (Fe₃O₄@C@G) are prepared by the multi-heteroatom-doped dual carbon-confined strategy. The first carbon layer results from multi-heteroatom-containing polymer derived N, P and S doped carbon to form Fe₃O₄@doped carbon core-shell nanostructure. And the second carbon layer results from the further encapsulated reduced graphene oxide (rGO) to form Fe₃O₄@doped carbon@graphene 3D architecture (Fe₃O₄@C@G). As expected, the resulting Fe₃O₄@C@G can be served as the universal anode materials towards lithium/sodium-ion batteries (LIBs/SIBs). Interestingly, Fe₃O₄@C@G delivers higher reversible capacity of 919 mAh g^-1 at 0.1 A g^-1 for LIBs. As for SIBs, Fe₃O₄@C@G also shows a high reversible capacity of 180 mAh g^-1 after 600 cycles at 0.1 A g^-1. Furthermore, the electrochemical reaction kinetics in LIBs/SIBs are investigated and Li⁺ full cells are also assembled to demonstrate its practical application.

Two-Dimensional Borophene: Properties, Fabrication, and Promising Applications

Monoelemental two-dimensional (2D) materials (Xenes) aroused a tremendous attention in 2D science owing to their unique properties and extensive applications. Borophene, one emerging and typical Xene, has been regarded as a promising agent for energy, sensor, and biomedical applications. However, the production of borophene is still a challenge because bulk boron has rather intricate spatial structures and multiple chemical properties. In this review, we describe its excellent properties including the optical, electronic, metallic, semiconducting, photoacoustic, and photothermal properties. The fabrication methods of borophene are also presented including the bottom-up fabrication and the top-down fabrication. In the end, the challenges of borophene in the latest applications are presented and perspectives are discussed.

Deciphering an Abnormal Layered-Tunnel Heterostructure Induced by Chemical Substitution for the Sodium Oxide Cathode

Demands for large-scale energy storage systems have driven the development of layered transition-metal oxide cathodes for room-temperature rechargeable sodium ion batteries (SIBs). Now, an abnormal layered-tunnel heterostructure Na_0.44 Co_0.1 Mn_0.9 O₂ cathode material induced by chemical element substitution is reported. By virtue of beneficial synergistic effects, this layered-tunnel electrode shows outstanding electrochemical performance in sodium half-cell system and excellent compatibility with hard carbon anode in sodium full-cell system. The underlying formation process, charge compensation mechanism, phase transition, and sodium-ion storage electrochemistry are clearly articulated and confirmed through combined analyses of in situ high-energy X-ray diffraction and ex situ X-ray absorption spectroscopy as well as operando X-ray diffraction. This crystal structure engineering regulation strategy offers a future outlook into advanced cathode materials for SIBs.

MoS2 nanoflowers encapsulated into carbon nanofibers containing amorphous SnO2 as an anode for lithium-ion batteries

SnO₂ with high abundance, large theoretical capacity, and nontoxicity is considered to be a promising candidate for use as advanced electrodes. However, the poor electronic conductivity and large volume variations hinder the practical applications of SnO₂-based electrodes for use in lithium-ion batteries (LIBs). Herein, the MoS₂-SnO₂ heterostructures were encapsulated into carbon nanofibers (CNFs) via facile solvothermal and electrospinning methods. Remarkably, when the binder-free and robust MoS₂-SnO₂@CNF is employed as the anode for LIBs, such a clever structure yields a discharge capacity of 983 mA h g^-1 at a current density of 200 mA g^-1 after 100 cycles and a capacity of 710 mA h g^-1 after 800 cycles at a current density of 2000 mA g^-1. Moreover, full cells and flexible full cells were constructed, which exhibited high flexibility and delivered a high reversible capacity of 463 mA h g^-1 after 100 cycles at 500 mA g^-1. The exceptional performance of MoS₂-SnO₂@CNF could be attributed to the rational design of the electrode structure. On one hand, the robust structure of the amorphous SnO₂ and MoS₂ nanoflowers in the conductive carbon network not only provides direct current pathways, but also enhances electron transfer. On the other hand, the abundance of p-n heterogeneous interfaces considerably reduces the charge transfer resistance and enhances the surface reaction kinetics. This work proposes a feasible strategy to enhance the capacity and stability of SnO₂-based electrodes and opens up a new avenue for the potential applications of SnO₂ anode materials.

Enabling Superior Electrochemical Properties for Highly Efficient Potassium Storage by Impregnating Ultrafine Sb Nanocrystals within Nanochannel-Containing Carbon Nanofibers

Sb-based nanocomposites are attractive anode materials for batteries as they exhibit large theoretical capacity and impressive working voltage. However, tardy potassium ion diffusion characteristics, unstable Sb/electrolyte interphase, and huge volume variation pose a challenge, hindering their practical use for potassium-ion batteries (PIBs). Now, a simple robust strategy is presented for uniformly impregnating ultrasmall Sb nanocrystals within carbon nanofibers containing an array of hollow nanochannels (denoted u-Sb@CNFs), resolving the issues above and yielding high-performance PIBs. u-Sb@CNFs can be directly employed as an anode, thereby dispensing with the need for conductive additives and binders. Such a judiciously crafted u-Sb@CNF-based anode renders a set of intriguing electrochemical properties, representing large charge capacity, unprecedented cycling stability, and outstanding rate performance. A reversible capacity of 225 mAh g^-1 is retained after 2000 cycles at 1 A g^-1 .

Ionic-liquid-bifunctional wrapping of ultrafine SnO2 nanocrystals into N-doped graphene networks: high pseudocapacitive sodium storage and high-performance sodium-ion full cells

Sodium ion batteries are in great need of electrode materials with high specificity and rate capability being developed. The sluggish reaction kinetics of SnO2-based materials has impeded their applications as anodes of SIBs. Designing electrode materials with high pseudocapacitive contribution can increase the near-surface faradaic reaction, which helps to improve their kinetics and achieve high rate capability. Here, we designed a high-pseudocapacitance sodium storage anode SnO2/N-rGO by downsizing the particle size of SnO2 and constructing an N-doped graphene wrapped structure. The ultrafine structure of SnO2 ensures the high faradaic near-surface reaction, while the N-doped graphene matrix guarantees the superior electron and Na+ diffusion. Meanwhile, the wrapped N-doped graphene acts as a buffer layer to alleviate the volumetric changes of the active SnO2. The obtained ultrafine SnO2/N-graphene composite exhibits a high capacity of 607.6 mA h g-1 at 50 mA g-1 with an impressive rate capability (261.8 mA h g-1 at 2 A g-1) in Na+ half-cells. Furthermore, a good performance with a capacity of 133.3 mA h g-1 at 2.4 A g-1 in Na+ full-cells can also be achieved, which makes it a promising anode material for SIBs.

Electrostatic Self-assembly of 0D-2D SnO2 Quantum Dots/Ti3C2Tx MXene Hybrids as Anode for Lithium-Ion Batteries

MXenes, a new family of two-dimensional (2D) materials with excellent electronic conductivity and hydrophilicity, have shown distinctive advantages as a highly conductive matrix material for lithium-ion battery anodes. Herein, a facile electrostatic self-assembly of SnO2 quantum dots (QDs) on Ti3C2Tx MXene sheets is proposed. The as-prepared SnO2/MXene hybrids have a unique 0D-2D structure, in which the 0D SnO2 QDs (~ 4.7 nm) are uniformly distributed over 2D Ti3C2Tx MXene sheets with controllable loading amount. The SnO2 QDs serve as a high capacity provider and the "spacer" to prevent the MXene sheets from restacking; the highly conductive Ti3C2Tx MXene can not only provide efficient pathways for fast transport of electrons and Li ions, but also buffer the volume change of SnO2 during lithiation/delithiation by confining SnO2 QDs between the MXene nanosheets. Therefore, the 0D-2D SnO2 QDs/MXene hybrids deliver superior lithium storage properties with high capacity (887.4 mAh g-1 at 50 mA g-1), stable cycle performance (659.8 mAh g-1 at 100 mA g-1 after 100 cycles with a capacity retention of 91%) and excellent rate performance (364 mAh g-1 at 3 A g-1), making it a promising anode material for lithium-ion batteries.

Flexible Conductive Anodes Based on 3D Hierarchical Sn/NS-CNFs@rGO Network for Sodium-Ion Batteries

Metallic Sn has provoked tremendous progress as an anode material for sodium-ion batteries (SIBs). However, Sn anodes suffer from a dramatic capacity fading, owing to pulverization induced by drastic volume expansion during cycling. Herein, a flexible three-dimensional (3D) hierarchical conductive network electrode is designed by constructing Sn quantum dots (QDs) encapsulated in one-dimensional N,S co-doped carbon nanofibers (NS-CNFs) sheathed within two-dimensional (2D) reduced graphene oxide (rGO) scrolls. In this ingenious strategy, 1D NS-CNFs are regarded as building blocks to prevent the aggregation and pulverization of Sn QDs during sodiation/desodiation, 2D rGO acts as electrical roads and "bridges" among NS-CNFs to improve the conductivity of the electrode and enlarge the contact area with electrolyte. Because of the unique structural merits, the flexible 3D hierarchical conductive network was directly used as binder- and current collector-free anode for SIBs, exhibiting ultra-long cycling life (373 mAh g-1 after 5000 cycles at 1 A g-1), and excellent high-rate capability (189 mAh g-1 at 10 A g-1). This work provides a facile and efficient engineering method to construct 3D hierarchical conductive electrodes for other flexible energy storage devices.

In-situ Grown SnO2 Nanospheres on Reduced GO Nanosheets as Advanced Anodes for Lithium-ion Batteries

Nanostructured tin dioxide (SnO2) has emerged as a promising anode material for lithium-ion batteries (LIBs) due to its high theoretical capacity (1494 mA h g-1) and excellent stability. Unfortunately, the rapid capacity fading and poor electrical conductivity of bulk SnO2 material restrict its practical application. Here, SnO2 nanospheres/reduced graphene oxide nanosheets (SRG) are fabricated through in-situ growth of carbon-coated SnO2 using template-based approach. The nanosheet structure with the external layer of about several nanometers thickness can not only accommodate the volume change of Sn lattice during cycling but also enhance the electrical conductivity effectively. Benefited from such design, the SRG composites could deliver an initial discharge capacity of 1212.3 mA h g-1 at 0.1 A g-1, outstanding cycling performance of 1335.6 mA h g-1 after 500 cycles at 1 A g-1, and superior rate capability of 502.1 mA h g-1 at 5 A g-1 after 10 cycles. Finally, it is believed that this method could provide a versatile and effective process to prepare other metal-oxide/reduced graphene oxide (rGO) 2D nanocomposites.

Tailoring Three-Dimensional Composite Architecture for Advanced Zinc-Ion Batteries

Rechargeable aqueous Zn-ion batteries (ZIBs) are of considerable interest for future energy storage. Their main limitation, however, is developing suitable cathode materials capable of sustaining the Zn²⁺ repeated intercalation/deintercalation. Herein, a three-dimensional polypyrrole (PPy)-encapsulated Mn₂O₃ composite architecture is developed for advanced ZIBs. The engineering can be easily realized via in situ phase transformation of MnCO₃ microboxes with subsequent self-initiated polymerization of PPy. The abundant open-up pores (∼30 nm) throughout the construction accelerate ion migration and provide a more active interface for Zn²⁺ storage in the Mn₂O₃@PPy bulk electrode. Meanwhile, the PPy skin uniformly wrapped on the Mn₂O₃ microbox not only guarantees a good conductive network for faster electron transport but also inhibits the dissolution of Mn₂O₃ and protects the integrity of the electrode from structural damage. As a result, the Mn₂O₃@PPy electrode can operate at reversible capacity exceeding those of most other cathode materials, but can still provide longer lifetime (no capacity decay over 2000 cycles at 0.4 A g^-1) and higher rate performance than others. Furthermore, theoretical studies show the H⁺ and Zn²⁺ coinsertion storage mechanism and reaction dynamics. The results show that this three-dimensional Mn₂O₃@PPy architecture is a promising cathode material for high-performance ZIBs.

Galvanic Replacement Synthesis of Highly Uniform Sb Nanotubes: Reaction Mechanism and Enhanced Sodium Storage Performance

One-dimensional nanotubes are very useful for achieving excellent performance in lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) due to the fact that tubular structures can effectively alleviate the structural strain and shorten the ion diffusion length during repeated cycling. In this work, we report a Cu₂Sb-mediated growth strategy to controllably fabricate highly uniform Sb nanotubes (NTs), as well as Cu@Cu₂Sb and Cu₂Sb@Sb composite NTs, via a facile galvanic replacement reaction using Cu nanowires (NWs) as sacrificial templates. Benefiting from their structural merits, the Sb NTs manifest excellent sodium storage performance with superior rate performance (286 mAh g^-1 at 10 A g^-1) and extraordinary cycling stability (342 mAh g^-1 after 6000 cycles at 1 A g^-1). Furthermore, a full cell with Sb NTs as anode and Na₃(VOPO₄)₂F as cathode exhibits a high energy density (252 Wh kg^-1) and high output voltage (2.7 V), revealing their significant application promise in the next-generation SIBs.

Carbon-Based Alloy-Type Composite Anode Materials toward Sodium-Ion Batteries

In the scenario of renewable clean energy gradually replacing fossil energy, grid-scale energy storage systems are urgently necessary, where Na-ion batteries (SIBs) could supply crucial support, due to abundant Na raw materials and a similar electrochemical mechanism to Li-ion batteries. The limited energy density is one of the major challenges hindering the commercialization of SIBs. Alloy-type anodes with high theoretical capacities provide good opportunities to address this issue. However, these anodes suffer from the large volume expansion and inferior conductivity, which induce rapid capacity fading, poor rate properties, and safety issues. Carbon-based alloy-type composites (CAC) have been extensively applied in the effective construction of anodes that improved electrochemical performance, as the carbon component could alleviate the volume change and increase the conductivity. Here, state-of-the-art CAC anode materials applied in SIBs are summarized, including their design principle, characterization, and electrochemical performance. The corresponding alloying mechanism along with its advantages and disadvantages is briefly presented. The crucial roles and working mechanism of the carbon matrix in CAC anodes are discussed in depth. Lastly, the existing challenges and the perspectives are proposed. Such an understanding critically paves the way for tailoring and designing suitable alloy-type anodes toward practical applications.

SnS2@C Hollow Nanospheres with Robust Structural Stability as High-Performance Anodes for Sodium Ion Batteries

Constructing unique and highly stable structures with plenty of electroactive sites in sodium storage materials is a key factor for achieving improved electrochemical properties through favorable sodium ion diffusion kinetics. An SnS₂@carbon hollow nanospheres (SnS₂@C) has been designed and fabricated via a facile solvothermal route, followed by an annealing treatment. The SnS₂@C hybrid possesses an ideal hollow structure, rich active sites, a large electrode/electrolyte interface, a shortened ion transport pathway, and, importantly, a buffer space for volume change, generated from the repeated insertion/extraction of sodium ions. These merits lead to the significant reinforcement of structural integrity during electrochemical reactions and the improvement in sodium storage properties, with a high specific reversible capacity of 626.8 mAh g^-1 after 200 cycles at a current density of 0.2 A g^-1 and superior high-rate performance (304.4 mAh g^-1 at 5 A g^-1).

A novel synthetic approach for designing metal-free, redox-active quinoxaline-benzimidazole-based organic polymers with high energy storage capacity

We established the first use of thiophene integrated with a quinoxaline-benzimidazole unit for energy storage applications and delivered strategies for further developments in the performance of such materials.

Pyridinic and pyrrolic nitrogen-enriched carbon as a polysulfide blocker for high-performance lithium–sulfur batteries

Pyridinic and pyrrolic nitrogen (16.95 atm%)-enriched carbon was simply prepared as an interlayer constructed in Li–S batteries.

A Multi-Ion Strategy towards Rechargeable Sodium-Ion Full Batteries with High Working Voltage and Rate Capability

Sodium-ion batteries (SIBs) are a promising alternative for the large-scale energy storage owing to the natural abundance of sodium. However, the practical application of SIBs is still hindered by the low working voltage, poor rate performance, and insufficient cycling stability. A sodium-ion based full battery using a multi-ion design is now presented. The optimized full batteries delivered a high working voltage of about 4.0 V, which is the best result of reported sodium-ion full batteries. Moreover, this multi-ion battery exhibited good rate performance up to 30 C and a high capacity retention of 95 % over 500 cycles at 5 C. Although the electrochemical performance of this multi-ion battery may be further enhanced via optimizing electrolyte and electrode materials for example, the results presented clearly indicate the feasibility of this multi-ion strategy to improve the electrochemical performance of SIBs for possible energy storage applications.

Oxygen Vacancy Engineering in Tin(IV) Oxide Based Anode Materials toward Advanced Sodium-Ion Batteries

A high theoretical capacity of approximately 1400 mA h g^-1 makes SnO₂ a promising anode material for sodium-ion batteries (SIBs). However, large volume expansion, poor intrinsic conductivity, and sluggish reaction kinetics have greatly hindered its practical application. The controlled creation of oxygen vacancy (OV) defects allows the intrinsic properties of SnO₂ to be effectively modulated, but related work concerning SIBs is still lacking. In this Minireview, the mechanism of failure of SnO₂ electrodes is discussed and an overview of recent progress in the general synthesis of OV-containing SnO₂ materials and the feasible detection of OVs in SnO₂ is presented. The use of OV-containing SnO₂ -based anode materials in SIBs is also reviewed. Finally, challenges and future opportunities to engineer OVs for semiconductor oxides are examined.

Flexible Freestanding Carbon Nanofiber-Embedded TiO2 Nanoparticles as Anode Material for Sodium-Ion Batteries

Sodium-ion batteries (SIBs), owning to the low cost, abundant resources, and similar physicochemical properties with lithium-ion batteries (LIBs), have earned much attention for large-scale energy storage systems. In this article, we successfully synthesize flexible freestanding carbon nanofiber-embedded TiO₂ nanoparticles (CNF-TiO₂) and then apply it directly as anode in SIBs without binder or current collector. Taking the advantage of flexible CNF and high structural stability, this anode exhibits high reversible capacity of 614 mAh·g^-1 (0.27 mAh·cm^-2) after almost 400 cycles and excellent capacity retention ability of ~100.

Low Interface Energies Tune the Electrochemical Reversibility of Tin Oxide Composite Nanoframes as Lithium-Ion Battery Anodes

The conversion reaction of lithia can push up the capacity limit of tin oxide-based anodes. However, the poor reversibility limits the practical applications of lithia in lithium-ion batteries. The latest reports indicate that the reversibility of lithia has been appropriately promoted by compositing tin oxide with transition metals. The underlying mechanism is not revealed. To design better anodes, we studied the nanostructured metal/Li₂O interfaces through atomic-scale modeling and proposed a porous nanoframe structure of Mn/Sn binary oxides. The first-principles calculation implied that because of a low interface energy of metal/Li₂O, Mn forms smaller particles in lithia than Sn. Ultrafine Mn nanoparticles surround Sn and suppress the coarsening of Sn particles. Such a composite design and the resultant interfaces significantly enhance the reversible Li-ion storage capabilities of tin oxides. The synthesized nanoframes of manganese tin oxides exhibit an initial capacity of 1620.6 mA h g^-1 at 0.05 A g^-1. Even after 1000 cycles, the nanoframe anode could deliver a capacity of 547.3 mA h g^-1 at 2 A g^-1. In general, we demonstrated a strategy of nanostructuring interfaces with low interface energy to enhance the Li-ion storage capability of binary tin oxides and revealed the mechanism of property enhancement, which might be applied to analyze other tin oxide composites.

In-situ wrapping of tin oxide nanoparticles by bacterial cellulose derived carbon nanofibers and its application as freestanding interlayer in lithium sulfide based lithium-sulfur batteries

Lithium-Sulfur (Li-S) batteries are mostly known for their high energy density and cost-effectiveness. However, their intrinsic problems hinder their implementation into the marketplace. The most pronounced problems are the parasitic reactions which occur between lithium polysulfides species and lithium metal anode, the volume expansion of sulfur (80%) at the end of discharge and the safety issues which are linked with the use of lithium metal. Herein this work, two approaches are applied to prevent these effects; one approach is the use of Li₂S as cathode material, instead of starting from sulfur powder, both to circumvent the volume expansion of sulfur taking place during discharge and to enable lithium-free anodes cell assembling (i.e. Si-Li₂S or Sn-Li₂S cell configurations). Second approach deals with the lithium anode protection by SnO₂ containing freestanding pyrolyzed bacterial cellulose interlayers located between anode and cathode electrodes. Since bacterial celluloses are formed in the presence of SnO₂ nanoparticles, the resulting structure enables intimate contact between carbon and SnO₂ nanoparticles. By employing Li₂S cathode and freestanding interlayer concurrently, 468 mAh g^-1 discharge capacity is obtained at C/10 current density over 100 cycles.

Thermal annealing assisted synthesis of Sb@C yolk–shell microspheres for sodium-ion batteries

The interior space of the Sb@C yolk–shell structure has a significant influence on the electrochemical performance of the electrode material.