Surface-redox sodium-ion storage in anatase titanium oxide

Constructing Low-Strain Cation Storage via High-Entropy Doping to Stabilize Layered Oxide Cathodes for Advanced Sodium-Ion Batteries

P2-type manganese-based layered cathode materials hold great promise for sodium-ion battery applications. However, their practical implementation is hindered by structural instability caused by Jahn-Teller distortion and complex phase transitions. Herein, this work proposes a simple and efficient high-entropy doping strategy by introducing redox-active and lattice-stabilizing cations to elaborate sodium-deficiency P2-type Na0.67Al0.02Fe0.02Ni0.02Cu0.02Zn0.02Mn0.9O2 cathodes with low-strain operation, which can significantly reduce the Mn3+ Jahn-Teller active centers and enhance structural stability. In situ X-ray diffraction results confirm that the high-entropy doping strategy effectively mitigates volume strain of 1.58% during cycling with increased interplanar spacing and fast Na+ transfer kinetics, further maintaining a capacity retention of 88.81% with an average Coulombic efficiency of 99.90% after 600 cycles. This work provides valuable insights into the design of advanced sodium cathode materials for future low-cost energy storage devices.

Boosting fast and stable sodium-ion storage in TiO2 by amorphization engineering and superstructure design

We introduce a hierarchical lamellar superstructure colloidally co-assembled from MXene-derived amorphous TiO2 nanosheets (am-TiO2 NSs) and carbon nanotubes (CNTs) as an anode for sodium-ion batteries (SIBs). Leveraging the synergistic effect of active amorphous NSs and an open-accessible architecture, the am-TiO2-CNT electrode demonstrates exceptional rate capabilities and cycling stabilities in SIBs.

Ultrathin Mesoporous Sandwiched Junctions with Monolayered Mesopores for Ultrahigh-Rate Sodium-Ion Storage

Rational nanostructural design for anode materials plays a crucial role in sodium-ion batteries (SIBs). Two-dimensional (2D) mesostructured composites capable of ideal diffusion kinetics and electronic conductivity are promising materials; however, difficulties remain in actual synthesis. Herein, we present a sequential monomicelle assembly strategy to synthesize uniform monolayered mesoporous TiO2-reduced graphene oxide sandwiched junctions (meso-TiO2/rGO sandwich), which enables superior pseudocapacitive sodium-ion storage. Such a sandwich structure has an ultrathin thickness, monolayered TiO2 mesopores at both sides of the rGO, a high surface area, and a uniform mesopore size. The combination of ultrathin 2D morphology, excellent mesoporosity, and electrical conductivity gives rise to comprehensive electrolyte access, reduced Na+ diffusion lengths, and promoted charge transfer. Remarkably, the sandwich anode exhibits a high reversible capacity, great rate capability, and cycling stability. Our study exemplifies the significance of constructing hybrid materials as an effective strategy for fast electrochemical sodium-ion storage.

In-situ conversion of BiOBr to Br-doped BiOCl nanosheets for "rocking chair" zinc-ion battery

Developing insertion-type anodes is essential for designing high-performance "rocking chair" zinc-ion batteries. BiOCl shows great potential as an insertion-type anode material for Zn2+ storage due to its high specific capacity and unique layered structure. However, the development of BiOCl has been significantly hampered by its poor stability and kinetics during cycling. In this study, Br-doped and carbon-coated BiOCl ultrathin nanosheets (Br-BiOCl@NC) are synthesized as high-performance anodes. The ultrathin nanosheet morphology facilitates Zn2+/H+ transfer and the Br doping reduces the Zn2+/H+ diffusion barrier. Additionally, the carbon coating enhances the electronic transfer. Furthermore, an insertion-conversion mechanism involving H+ and Zn2+ storage is revealed by ex-situ tests. Therefore, Br-BiOCl@NC exhibits a high discharge capacity of 174 mA h/g at 500 mA/g without capacity degradation after 1000 cycles. The Br-BiOCl@NC//MnO2 full cell presents a discharge capacity of ≈ 120 mA h/g at 200 mA/g. This work offers valuable insights for the design of high-performance insertion-type anode materials in zinc-ion batteries.

Unifying Electrochemically-Driven Multistep Phase Transformations of Rutile TiO2 to Rocksalt Nanograins for Reversible Li+ and Na+ Storage

Rutile titanium dioxide (TiO2(R)) lacks octahedral vacancies, which is not suitable for Li+ and Na+ intercalation via reversible two-phase transformations, but it displays promising electrochemical properties. The origins of these electrochemical performances remain largely unclear. Herein, the Li+ and Na+ storage mechanisms of TiO2(R) with grain sizes ranging from 10 to 100 nm are systematically investigated. Through revealing the electrochemically-driven atom rearrangements, nanosize effect and kinetics analysis of TiO2(R) nanograins during repeated cycling with Li+ or Na+, a unified mechanism of electrochemically-driven multistep rutile-to-rocksalt phase transformations is demonstrated. Importantly, the electrochemically in situ formed rocksalt phase has open diffusion channels for rapid Li+ or Na+ (de)intercalation through a solid-solution mechanism, which determines the pseudocapacitive, "mirror-like" cyclic voltammetry curves and excellent rate capabilities. Whereas, the nanosize effect determines the different Li+ and Na+ storage capacities because of their distinct reaction depths. Remarkably, the TiO2(R)-10 nm anode in situ turns into rocksalt nanograins after 30 cycles with Na+, which delivers a reversible capacity of ≈200 mAh g-1, high-rate capability of 97 mAh g-1 at 10 A g-1 and long-term cycling stability over 3000 cycles. The findings provide deep insights into the in situ phase evolutions with boosted electrochemical Li+ or Na+ storage performance.

Electrochemically in situ formed rocksalt phase in titanium dioxide determines pseudocapacitive sodium-ion storage

Earth-abundant TiO2 is a promising negative electrode for low-cost sodium-ion batteries (SIBs) owing to its high capacity, rapid (dis)charging capability, safe operation potential and nonflammability. Crystalline anatase TiO2 is not suitable for reversible Na+ (de)intercalation, but it displays pseudocapacitive response after repeated cycles. Herein, we find and demonstrate that ordered rocksalt (RS) NaTiO2 nanograins are in situ formed by electrochemically cycling with Na+ ions in anatase and amorphous TiO2. The in situ formed RS-NaTiO2 follows a solid-solution reaction with small volume changes of only 2.0%, that determines the pseudocapacitive "mirror-like" cyclic voltammetry curve with a couple of broad redox peaks at 0.75 V vs. Na+/Na, a high capacity of 253 mAh g-1, high-rate capability and thousands of stable cycles. The multistep crystalline-to-amorphous-to-RS transformations are able to be electrochemically activated during the aging process of assembled full cells. Our finding provides a direction for developing unconventional Ti-based high-performance active materials for SIBs with both high energy and power densities.

Implantable photoelectrochemical-therapeutic methotrexate monitoring system with dual-atomic docking strategy

The need for precise modulation of blood concentrations of pharmaceutical molecule, especially for high-risk drugs like Methotrexate (MTX), is underscored by the significant impact of individual variations on treatment efficacy. Achieving selective recognition of pharmaceutical molecules within the complex biological environment is a substantial challenge. To tackle this, we propose a synergistic atomic-molecular docking strategy that utilizes a hybrid-dual single-atom Fe1-Zn1 on a TiO2 photoelectrode to selectively bind to the carboxyl and aminopyrimidine groups of MTX respectively. By integrating this Fe1-Zn1-TiO2 photoelectrode with a microcomputer system, an implantable photoelectrochemical-therapeutic drug monitoring (PEC-TDM) system is developed for real-time, continuous in vivo MTX monitoring. This system facilitates personalized therapeutic decision-making and intelligent drug delivery for individualized cancer therapy, potentially revolutionizing oncological care and enhancing patient outcomes.

Anion-Tailored EDL Induced Triple-Layer SEI on High-Capacity Anodes Enabling Fast-Charging and Durable Sodium-Storage

High-capacity electrodes face a great challenge of cycling stability due to particle fragmentation induced conductive network failure and accompanied by sustained electrolyte decomposition for repeatedly build solid electrolyte interphase (SEI). Herein, Se-solubility induced Sex 2- as self-adjustment electrolyte additive to regulate electric double layer (EDL) for constructing novel triple-layer SEI (inner layer: Se; mediate layer: inorganic; outer layer: organic) on high-capacity FeS2 anode as an example for achieving stable and fast sodium storage. In detail, Sex 2- in situ generated at 1.30 V (vs. Na+/Na) and was preferentially adsorbed onto EDL of anode, then converted to Se0 as inner layer of SEI. In addition, the Sex 2- causes anion-enhanced Na+ solvation structure could produce more inorganic (Se0, NaF) and less organic SEI components. The unique triple-layer SEI with layer-by-layer dense structure alleviate the excessive electrolyte consumption with less gas evolution. As a result, the anode delivered long-lifespan at 10 A g-1 (383.7 mAh g-1, 6000 cycles, 93.1 %, 5 min/cycle). The Se-induced triple-layer SEI could be also be formed on high-capacity SnS2 anode. This work provides a novel SEI model by anion-tailored EDL towards stable sodium-storage of high-capacity anode for fast-charging.

Fe3N/Fe3O4 hetero-nanocrystals embedded in porous carbon fibers for enhanced lithium storage

Energy storage devices have applications in large-scale portable and smart devices due to their high energy density and long lifespan, but the limited theoretical capacity of the graphite anode in lithium-ion batteries has slowed the development of portable electronic devices. Herein, we prepared porous fibers with heterogeneous Fe3N/Fe3O4 nanocrystals wrapped by a carbon layer. A series of measurements, such as TEM images, Raman spectra, XRD pattern and XPS analysis, were used to unveil the formation of Fe3N/Fe3O4 nanocrystals. Due to the synergistic effect of the large specific surface area originating from the porous structure and the heterogeneous nanocrystals, the porous Fe3N/Fe3O4@C fibers exhibit a good electron/ion transmission route and rich active sites. As anode materials for lithium-ion batteries (LIBs), porous Fe3N/Fe3O4@C fibers delivered a reversible capacity of 964 mA h g-1 after 200 cycles at 2 A g-1 and long-term cycling stability (282 mA h g-1 after 2000 cycles at 5 A g-1). This work provides a method to regulate biphasic anode materials with desirable structures to enhance the reversibility of LIBs.

Single Atomic Cu-C3 Sites Catalyzed Interfacial Chemistry in Bi@C for Ultra-Stable and Ultrafast Sodium-Ion Batteries

Regulating interfacial chemistry at electrode-electrolyte interface by designing catalytic electrode material is crucial and challenging for optimizing battery performance. Herein, a novel single atom Cu regulated Bi@C with Cu-C3 site (Bi@SA Cu-C) have been designed via the simple pyrolysis of metal-organic framework. Experimental investigations and theoretical calculations indicate the Cu-C3 sites accelerate the dissociation of P-F and C-O bonds in NaPF6-ether-based electrolyte and catalyze the formation of inorganic-rich and powerful solid electrolyte interphase. In addition, the Cu-C3 sites with delocalized electron around Cu trigger an uneven charge distribution and induce an in-plane local electric field, which facilitates the adsorption of Na+ and reduces the Na+ migration energy barrier. Consequently, the obtained Bi@SA Cu-C achieves a state-of-the-art reversible capacity, ultrahigh rate capability, and long-term cycling durability. The as-constructed full cell delivers a high capacity of 351 mAh g-1 corresponding to an energy density of 265 Wh kg-1. This work provides a new strategy to realize high-efficient sodium ion storage for alloy-based anode through constructing single-atom modulator integrated catalysis and promotion effect into one entity.

Optimizing Reversible Phase-Transformation of FeS2 Anode via Atomic-Interface Engineering Toward Fast-Charging Sodium Storage: Theoretical Predication and Experimental Validation

Sodium-storage performance of pyrite FeS2 is greatly improved by constructing various FeS2-based nanostructures to optimize its ion-transport kinetics and structural stability. However, less attention has been paid to rapid capacity degradation and electrode failure caused by the irreversible phase-transition of intermediate NaxFeS2 to FeS2 and polysulfides dissolution upon cycling. Under the guidance of theoretical calculations, coupling FeS2 nanoparticles with honeycomb-like nitrogen-doped carbon (NC) nanosheet supported single-atom manganese (SAs Mn) catalyst (FeS2/SAs Mn@NC) via atomic-interface engineering is proposed to address above challenge. Systematic electrochemical analyses and theoretical results unveil that the functional integration of such two type components can significantly enhance ionic conductivity, accelerate charge transfer efficiency, and improve Na+-adsorption capability. Particularly, SAs Mn@NC with Mn-Nx coordination center can reduce the decomposition barrier of Na2S and NaxFeS2 to further accelerate reversible phase transformation of Fe/Na2S→NaFeS2→FeS2 and polysulfides decomposition. As predicted, such FeS2/SAs Mn@NC showcases outstanding rate capability and fascinating cyclic durability. A sequence of kinetic studies and ex situ characterizations provide the comprehensive understanding for ion-transport kinetics and phase-transformation process. Its practical use is further demonstrated in sodium-ion full cell and capacitor with impressive electrochemical capability and excellent energy-density output.

Electrochemically-Formed Disordered Rock Salt ω-Li x V9Mo6O40 as a Fast-Charging Li-Ion Electrode Material

Electrochemically-formed disordered rock salt compounds are an emerging class of Li-ion electrode materials for fast-charging energy storage. However, the specific factors that govern the formation process and the resulting charge storage performance are not well understood. Here, we characterize the transformation mechanism and charge storage properties of an electrochemically-formed disordered rock salt from V9Mo6O40 (VMO). The crystal structure of VMO has similar motifs to that of α-V2O5, a well-studied analogue, but VMO has less mechanical flexibility due to additional corner-sharing octahedra in its structure. As a result, VMO undergoes a single-step transformation pathway, which we characterize through operando X-ray diffraction, and forms an unusual highly distorted lamellar microstructure, as we show with high-resolution transmission electron microscopy. The resulting Li x VMO material shows fast charging and other electrochemical characteristics and performance typical of many nanomaterials, even though the material is composed of relatively large particles.

Deciphering the structural and kinetic factors in lithium titanate for enhanced performance in Li+/Na+ dual-cation electrolyte

The widespread application of Li4Ti5O12 (LTO) anode in lithium-ion batteries has been hindered by its relatively low energy density. In this study, we investigated the capacity enhancement mechanism of LTO anode through the incorporation of Na+ cations in an Li+-based electrolyte (dual-cation electrolyte). LTO thin film electrodes were prepared as conductive additive-free and binder-free model electrodes. Electrochemical performance assessments revealed that the dual-cation electrolyte boosts the reversible capacity of the LTO thin film electrode, attributable to the additional pseudocapacitance and intercalation of Na+ into the LTO lattice. Operando Raman spectroscopy validated the insertion of Li+/Na+ cations into the LTO thin film electrode, and the cation migration kinetics were confirmed by ab initio molecular dynamic (AIMD) simulation and electrochemical impedance spectroscopy, which revealed that the incorporation of Na+ reduces the activation energy of cation diffusion within the LTO lattice and improves the rate performance of LTO thin film electrodes in the dual-cation electrolyte. Furthermore, the interfacial charge transfer resistance in the dual-cation electrolyte, associated with ion de-solvation processes and traversal of the cations in the solid-electrolyte interphase (SEI) layer, are evaluated using the distribution of relaxation time, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Our approach of performance enhancement using dual-cation electrolytes can be extrapolated to other battery electrodes with sodium/lithium storage capabilities, presenting a novel avenue for the performance enhancement of lithium/sodium-ion batteries.

Bifunctional Fluorocarbon Electrode Additive Lowers the Salt Dependence of Aqueous Electrolytes

The solid electrolyte interphase (SEI) plays a crucial role in extending the life of aqueous batteries. The traditional anion-derived SEI formation in aqueous electrolytes highly depends on high-concentrated organic fluorinating salts, resulting in low forming efficiency and long-term consumption. In response, this study proposes a bifunctional fluorocarbon electrode additive (BFEA) that enables electrochemical pre-reduction instead of TFSI anion to form the LiF-rich SEI and in situ produce conductive graphite inside the anode before the lithiation. The BFEA lowers the salt dependence of aqueous electrolytes, enabling the inorganic LiCl electrolyte to work first, but also successfully achieves a high SEI formation efficiency in the relatively low 10 m LiTFSI without mass transfer concerns, suppressing the parasitic hydrogen evolution from 11.24 to 4.35 nmol min-1. Besides, BFEA strengthens the intrinsic superiority of Li storage reaction by lowering battery polarization resulting from the in situ production of graphite, promoting charge transfer of electrode kinetics. Compared with the control group, the demonstrated Ah-level pouch cell employing BFEA exhibits better cycle stability above 300 cycles with higher capacity retention of 78.2% and the lower decay of the round-trip efficiency (△RTE = 2%), benefiting for maintaining the high efficiency and reducing heat accumulation in large-scale electric energy storage.

Polar Covalent Triazine Frameworks as High-Performance Potassium Metal Battery Cathodes

The exploration of potassium metal batteries (PMBs) has been intensified, leveraging potassium's abundant availability, low redox potential, and small Stokes radius. Covalent triazine frameworks (CTFs) stand out for their accessible nitrogen sites and customizable structures, making them attractive electrode materials. Nonetheless, there is a lack of established design principles to guide the development of high-performance PMBs using CTFs. In this work, CTFs consisting of different monomers are used as PMB cathodes to investigate the structure-performance correlation. The electronic structure analysis reveals the polar characteristic of a CTF derived from the tetracyanoquinodimethane monomer, setting it apart with superior capacity (161 mAh g-1 at 0.1 A g-1), rate performance (85 mAh g-1 at 5 A g-1), and stability (capacity retention of 81% after 1000 cycles) over three non-polar counterparts in PMBs. Calculations based on density functional theory support the exceptional performance with increased K+ adsorption energy. Ultimately, among multifaceted factors, the polarity of CTF is the leading element that determines the K+ storage capability. These findings pave the way for the development of prudent CTF electrodes for high-performance PMBs.

Accelerating Li-Ion Diffusion in LiFePO4 by Polyanion Lattice Engineering

Despite the widespread commercialization of LiFePO4 as cathodes in lithium-ion batteries, the rigid 1D Li-ion diffusion channel along the [010] direction strongly limits its fast charge and discharge performance. Herein, lattice engineering is developed by the planar triangle BO3 3- substitution on tetrahedron PO4 3- to induce flexibility in the Li-ion diffusion channels, which are broadened simultaneously. The planar structure of BO3 3- may further provide additional paths between the channels. With these synergetic contributions, LiFe(PO4)0.98(BO3)0.02 shows the best performance, which delivers the high-rate capacity (66.8 mAh g-1 at 50 C) and long cycle stability (ultra-low capacity loss of 0.003% every cycle at 10 C) at 25 °C. Furthermore, excellent rate performance (34.0 mAh g-1 at 40 C) and capacity retention (no capacity loss after 2500 cycles at 10 C) at -20 °C are realized.

On-Demand Picoliter-Level-Droplet Inkjet Printing for Micro Fabrication and Functional Applications

With the advent of Internet of Things (IoTs) and wearable devices, manufacturing requirements have shifted toward miniaturization, flexibility, environmentalization, and customization. Inkjet printing, as a non-contact picoliter-level droplet printing technology, can achieve material deposition at the microscopic level, helping to achieve high resolution and high precision patterned design. Meanwhile, inkjet printing has the advantages of simple process, high printing efficiency, mask-free digital printing, and direct pattern deposition, and is gradually emerging as a promising technology to meet such new requirements. However, there is a long way to go in constructing functional materials and emerging devices due to the uncommercialized ink materials, complicated film-forming process, and geometrically/functionally mismatched interface, limiting film quality and device applications. Herein, recent developments in working mechanisms, functional ink systems, droplet ejection and flight process, droplet drying process, as well as emerging multifunctional and intelligence applications including optics, electronics, sensors, and energy storage and conversion devices is reviewed. Finally, it is also highlight some of the critical challenges and research opportunities. The review is anticipated to provide a systematic comprehension and valuable insights for inkjet printing, thereby facilitating the advancement of their emerging applications.

Eliminating Charge Transfer at Cathode-Electrolyte Interface for Ultrafast Kinetics in Na-Ion Batteries

Sodium-ion batteries suffer from kinetic problems stemming from sluggish ion transport across the electrode-electrolyte interface, causing rapid energy decay during fast-charging or low-temperature operation. One exciting prospect to enhance kinetics is constructing neuron-like electrodes that emulate fast signal transmission in a nervous system. It has been considered that these bioinspired designs enhance electron/ion transport of the electrodes through carbon networks. However, whether they can avoid sluggish charge transfer at the electrode-electrolyte interface remains unknown. By connecting the openings of carbon nanotubes with the surface of carbon-coated Na3V2O2(PO4)2F cathode nanoparticles, here we use carbon nanotubes to trap Na+ ions released from the nanoparticles during charge. Therefore, Na+ movement is confined only inside the neuron-like cathode, eliminating ion transport between the electrolyte and cathode, which has been scarcely achieved in conventional batteries. As a result, a 14-fold reduction in interfacial charge transfer resistance is achieved when compared to unmodified cathodes, leading to superior fast-charging performance and excellent cyclability up to 200C, and surprisingly, reversible operation at low temperatures down to -60 °C without electrolyte modification, surpassing other Na3V2O2(PO4)2F-based batteries reported to date. As battery operation has relied on charge transfer at the electrode-electrolyte interface for over 200 years, our approach departs from this traditional ion transport paradigm, paving the way for building better batteries that work under harsh conditions.

Unification of insertion and supercapacitive storage concepts: Storage profiles in titania

Insertion storage in battery electrodes and supercapacitive storage are typically considered to be independent phenomena and thus are dealt with in separate scientific communities. Using tailored experiments on titanium oxide thin films of various thicknesses, we demonstrate the simultaneous occurrence of both processes. For the interpretation of the entire storage profile encompassing both contributions, the (free) energies of the charge carriers in the mixed conductor and the neighboring phase are the only materials parameters required. The experimental results enable no less than a unification of insertion and supercapacitive storage, the first being dominant for thick films, the latter for thin films or negligible electronic conductivity. Therefore, the size of the storage medium and the nature of the current collecting phases can be used to tune power density versus energy density.

Heterogeneous MoS2 Nanosheets on Porous TiO2 Nanofibers toward Fast and Reversible Sodium-Ion Storage

2D layered molybdenum disulfide (MoS2) has garnered considerable attention as an attractive electrode material in sodium-ion batteries (SIBs), but sluggish mass transfer kinetic and capacity fading make it suffer from inferior cycle capability. Herein, hierarchical MoS2 nanosheets decorated porous TiO2 nanofibers (MoS2 NSs@TiO2 NFs) with rich oxygen vacancies are engineered by microemulsion electrospinning method and subsequent hydrothermal/heat treatment. The MoS2 NSs@TiO2 NFs improves ion/electron transport kinetic and long-term cycling performance through distinctive porous structure and heterogeneous component. Consequently, the electrode exhibits excellent long-term Na storage capacity (298.4 mAh g-1 at 5 A g-1 over 1100 cycles and 235.6 mAh g-1 at 10 A g-1 over 7200 cycles). Employing Na3V2(PO4)3 as cathode, the full cell maintains a desirable capacity of 269.6 mAh g-1 over 700 cycles at 1.0 A g-1. The stepwise intercalation-conversion and insertion/extraction endows outstanding Na+ storage performance, which yields valuable insight into the advancement of fast-charging and long-cycle life SIBs anode materials.

S/N Co-Doped Ultrathin TiO2 Nanoplates as an Anode Material for Advanced Sodium-Ion Hybrid Capacitors

Anatase titanium dioxide (TiO2) has emerged as a potential anode material for sodium-ion hybrid capacitors (SICs) in terms of its nontoxicity, high structure stability and cost-effectiveness. However, its inherent poor electrical conductivity and limited reversible capacity greatly hinder its practical application. Here, ultrathin TiO2 nanoplates were synthesized utilizing a hydrothermal technique. The electrochemical kinetics and reversible capacity were significantly improved through sulfur and nitrogen co-doping combined with carbon coating (SN-TiO2/C). Sulfur and nitrogen co-doping generated oxygen vacancies and introduced additional active sites within TiO2, facilitating accelerated Na-ion diffusion and enhancing its reversible capacity. Furthermore, carbon coating provided stable support for electron transfer in SN-TiO2/C during repeated cycling. This synergistic strategy of sulfur and nitrogen co-doping with carbon coating for TiO2 led to a remarkable capacity of 335.3 mAh g-1 at 0.1 A g-1, exceptional rate property of 148.3 mAh g-1 at 15 A g-1 and a robust cycling capacity. Thus, the SN-TiO2/C//AC SIC delivered an impressive energy density of 177.9 W h kg-1. This work proposes an idea for the enhancement of reaction kinetics for energy storage materials through a synergistic strategy.

Investigating the Electrochemical Performance of MnFe2O4@xC Nanocomposites as Anode Materials for Sodium-Ion Batteries

Transition metal oxides (TMOs) are important anode materials in sodium-ion batteries (SIBs) due to their high theoretical capacities, abundant resources, and cost-effectiveness. However, issues such as the low conductivity and large volume variation of TMO bulk materials during the cycling process result in poor electrochemical performance. Nanosizing and compositing with carbon materials are two effective strategies to overcome these issues. In this study, spherical MnFe2O4@xC nanocomposites composed of MnFe2O4 inner cores and tunable carbon shell thicknesses were successfully prepared and utilized as anode materials for SIBs. It was found that the property of the carbon shell plays a crucial role in tuning the electrochemical performance of MnFe2O4@xC nanocomposites and an appropriate carbon shell thickness (content) leads to the optimal battery performance. Thus, compared to MnFe2O4@1C and MnFe2O4@8C, MnFe2O4@4C nanocomposite exhibits optimal electrochemical performance by releasing a reversible specific capacity of around 308 mAh·g-1 at 0.1 A·g-1 with 93% capacity retention after 100 cycles, 250 mAh·g-1 at 1.0 A g-1 with 73% capacity retention after 300 cycles in a half cell, and around 111 mAh·g-1 at 1.0 C when coupled with a Na3V2(PO4)3 (NVP) cathode in a full SIB cell.

Fast-Charging Anode Materials for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have undergone rapid development as a complementary technology to lithium-ion batteries due to abundant sodium resources. However, the extended charging time and low energy density pose a significant challenge to the widespread use of SIBs in electric vehicles. To overcome this hurdle, there is considerable focus on developing fast-charging anode materials with rapid Na⁺ diffusion and superior reaction kinetics. Here, the key factors that limit the fast charging of anode materials are examined, which provides a comprehensive overview of the major advances and fast-charging characteristics across various anode materials. Specifically, it systematically dissects considerations to enhance the rate performance of anode materials, encompassing aspects such as porous engineering, electrolyte desolvation strategies, electrode/electrolyte interphase, electronic conductivity/ion diffusivity, and pseudocapacitive ion storage. Finally, the direction and prospects for developing fast-charging anode materials of SIBs are also proposed, aiming to provide a valuable reference for the further advancement of high-power SIBs.

Pressure-Induced Defects and Reduced Size Endow TiO2 with High Capacity over 20 000 Cycles and Excellent Fast-Charging Performance in Sodium Ion Batteries

Anatase TiO2 as sodium-ion-battery anode has attracted increased attention because of its low volume change and good safety. However, low capacity and poor rate performance caused by low electrical conductivity and slow ion diffusion greatly impede its practical applications. Here, a bi-solvent enhanced pressure strategy that induces defects (oxygen vacancies) into TiO2 via N doping and reduces its size by using mutual-solvent ethanol and dopant dimethylformamide as pressure-increased reagent of tetrabutyl orthotitanate tetramer is proposed to fabricate N-doped TiO2/C nanocomposites. The induced defects can increase ion storage sites, improve electrical conductivity, and decrease bandgap and ion diffuse energy barrier of TiO2. The size reduction increases contact interfaces between TiO2 and C and shortens ion diffuse distance, thus increasing extra ion storage sites and boosting ion diffusion rate of TiO2. The N-doped TiO2 possesses highly stable crystal structure with a slightly increase of 0.86% in crystal lattice spacing and 3.2% in particle size after fully sodiation. Consequently, as a sodium-ion battery anode, the nanocomposite delivers high capacity and superior rate capability along with ultralong cycling life. This work proposes a novel pressure-induced synthesis strategy that provides unique guidance for designing TiO2-based anode materials with high capacity and excellent fast-charging capability.

Ordered Vacancies as Sodium Ion Micropumps in Cu-Deficient Copper Indium Diselenide to Enhance Sodium Storage

Unordered vacancies engineered in host anode materials cannot well maintain the uniform Na+ adsorbed and possibly render the local structural stress intense, resulting in electrode peeling and battery failure. Here, the indium is first introduced into Cu2Se to achieve the formation of CuInSe2. Next, an ion extraction strategy is employed to fabricate Cu0.54In1.15Se2 enriched with ordered vacancies by spontaneous formation of defect pairs. Such ordered defects, compared with unordered ones, can serve as myriad sodium ion micropumps evenly distributing in crystalline host to homogenize the adsorbed Na+ and the generated volumetric stress during the electrochemistry. Furthermore, Cu0.54In1.15Se2 is indeed proved by the calculations to exhibit smaller volumetric variation than the counterpart with unordered vacancies. Thanks to the distinct ordered vacancy structure, the material exhibits a highly reversible capacity of 428 mAh g-1 at 1 C and a high-rate stability of 311.7 mAh g-1 at 10 C after 5000 cycles when employed as an anode material for Sodium-ion batteries (SIBs). This work presents the promotive effect of ordered vacancies on the electrochemistry of SIBs and demonstrates the superiority to unordered vacancies, which is expected to extend it to other metal-ion batteries, not limited to SIBs to achieve high capacity and cycling stability.

Structural Engineering of Anode Materials for Low-Temperature Lithium-Ion Batteries: Mechanisms, Strategies, and Prospects

The severe degradation of electrochemical performance for lithium-ion batteries (LIBs) at low temperatures poses a significant challenge to their practical applications. Consequently, extensive efforts have been contributed to explore novel anode materials with high electronic conductivity and rapid Li+ diffusion kinetics for achieving favorable low-temperature performance of LIBs. Herein, we try to review the recent reports on the synthesis and characterizations of low-temperature anode materials. First, we summarize the underlying mechanisms responsible for the performance degradation of anode materials at subzero temperatures. Second, detailed discussions concerning the key pathways (boosting electronic conductivity, enhancing Li+ diffusion kinetics, and inhibiting lithium dendrite) for improving the low-temperature performance of anode materials are presented. Third, several commonly used low-temperature anode materials are briefly introduced. Fourth, recent progress in the engineering of these low-temperature anode materials is summarized in terms of structural design, morphology control, surface & interface modifications, and multiphase materials. Finally, the challenges that remain to be solved in the field of low-temperature anode materials are discussed. This review was organized to offer valuable insights and guidance for next-generation LIBs with excellent low-temperature electrochemical performance.

Multiscale Interface Engineering of Sulfur-Doped TiO2 Anode for Ultrafast and Robust Sodium Storage

Sodium-ion batteries (SIBs) are a promising electrochemical energy storage system; however, their practical application is hindered by the sluggish kinetics and interfacial instability of anode-active materials. Here, to circumvent these issues, we proposed the multiscale interface engineering of S-doped TiO2 electrodes with minor sulfur/carbon inlaying (S/C@sTiO2), where the electrode-electrolyte interface (SEI) and electrode-current collector interface (ECI) are tuned to improve the Na-storage performance. It is found that the S dopant greatly promotes the Na+ diffusion kinetics. Moreover, the ether electrolyte generates much less NaF in the cycled electrode, but relatively richer NaF in the SEI in comparison to fluoroethylene carbonate-contained ester electrolyte, leading to a thin (9 nm), stable, and kinetically favorable SEI film. More importantly, the minor sodium polysulfide intermediates chemically interact with the Cu current collector to form a Cu2S interface between the electrode and the Cu foil. The conductive tree root-like Cu2S ECI serves not only as active sites to boost the specific capacity but also as a 3D "second current collector" to reinforce the electrode and improve the Na+ reaction kinetics. The synergy of S-doping and optimized SEI and ECI realizes large specific capacity (464.4 mAh g-1 at 0.1 A g-1), ultrahigh rate capability (305.8 mAh g-1 at 50 A g-1), and ultrastable cycling performance (91.5% capacity retention after 3000 cycles at 5 A g-1). To the best of our knowledge, the overall SIB performances of S/C@sTiO2 are the best among all of the TiO2-based electrodes.

MXene-Derived Na+ -Pillared Vanadate Cathodes for Dendrite-Free Potassium Metal Batteries

Cation-intercalated vanadates, which have considerable promise as the cathode for high-performance potassium metal batteries (PMBs), suffer from structural collapse upon K+ insertion and desertion. Exotic cations in the vanadate cathode may ease the collapse, yet their effect on the intrinsic cation remains speculative. Herein, a stable and dendrite-free PMB, composed of a Na+ and K+ co-intercalated vanadate (NKVO) cathode and a liquid NaK alloy anode, is presented. A series of NKVO with tuneable Na/K ratios are facilely prepared using MXene precursors, in which Na+ is testified to be immobilized upon cycling, functioning as a structural pillar. Due to stronger ionic bonding and lower Fermi level of Na+ compared to K+ , moderate Na+ intercalation could reduce K+ binding to the solvation sheath and favor K+ diffusion kinetics. As a result, the MXene-derived Na+ -pillared NKVO exhibits markedly improved specific capacities, rate performance, and cycle stability than the Na+ -free counterpart. Moreover, thermally-treated carbon paper, which imitates the microscopic structure of Chinese Xuan paper, allows high surface tension liquid NaK alloy to adhere readily, enabling dendrite-free metal anodes. By clarifying the role of foreign intercalating cations, this study may lead to a more rational design of stable and high-performance electrode materials.

Spontaneous redox reaction-mediated interfacial charge transfer in titanium dioxide/graphene oxide nanoanodes for rapid and durable lithium storage

Titanium dioxide (TiO2) anodes show significant advantages in ion storage owing to their low cost, abundant sources, and small volume change during cycling. However, their intrinsic low electronic conductivity and sluggish ion diffusion coefficient restrict the application of TiO2 anodes, especially at high current densities. The construction of a covalently-bonded interface in TiO2-based composite anodes is an effective approach to solve these issues. Covalent bonds are usually formed in situ during materials synthesis processes, such as high-energy ball milling, solvothermal reactions, plasma-assisted thermal treatment, and addition of a linking agent for covalent coupling. In this study, we demonstrate that a spontaneous redox reaction between defective TiO2 powder and an oxidative graphene oxide (GO) substate can be used to form interfacial covalent bonds in composites. Different structural characterization techniques confirmed the formation of interfacial covalent bonds. Electrochemical measurements on an optimized sample showed that a specific capacity of 281.3 mA h g-1 after 200 cycles can be achieved at a current density of 1 C (1 C = 168 mA g-1). Even at a high rate of 50 C, the electrode maintained a reversible capacity of 97.0 mA h g-1. The good lithium storage performance of the electrode is a result of the uniquely designed composite electrodes with strong interfacial chemical bonds.

Understanding How the Suppression of Insertion-Induced Phase Transitions Leads to Fast Charging in Nanoscale LixMoO2

Fast-charging Li-ion batteries are technologically important for the electrification of transportation and the implementation of grid-scale storage, and additional fundamental understanding of high-rate insertion reactions is necessary to overcome current rate limitations. In particular, phase transformations during ion insertion have been hypothesized to slow charging. Nanoscale materials with modified transformation behavior often show much faster kinetics, but the mechanism for these changes and their specific contribution to fast-charging remain poorly understood. In this work, we combine operando synchrotron X-ray diffraction with electrochemical kinetics analyses to illustrate how nanoscale crystal size leads to suppression of first-order insertion-induced phase transitions and their negative kinetic effects in MoO2, a tunnel structure host material. In electrodes made with micrometer-scale particles, large first-order phase transitions during cycling lower capacity, slow charge storage, and decrease cycle life. In medium-sized nanoporous MoO2, the phase transitions remain first-order, but show a considerably smaller miscibility gap and shorter two-phase coexistence region. Finally, in small MoO2 nanocrystals, the structural evolution during lithiation becomes entirely single-phase/solid-solution. For all nanostructured materials, the changes to the phase transition dynamics lead to dramatic improvements in capacity, rate capability, and cycle life. This work highlights the continuous evolution from a kinetically hindered battery material in bulk form to a fast-charging, pseudocapacitive material through nanoscale size effects. As such, it provides key insight into how phase transitions can be effectively controlled using nanoscale size and emphasizes the importance of these structural dynamics to the fast rate capability observed in nanostructured electrode materials.

Boosting Fe Cationic Vacancies with Graphdiyne to Enhance Exceptional Pseudocapacitive Lithium Intercalation

Modulating the electronic structure of electrode materials at atomic level is the key to controlling electrodes with outstanding rate capability. On the basis of modulating the iron cationic vacancies (IV) and electronic structure of materials, we proposed the method of preparing graphdiyne/ferroferric oxide heterostructure (IV-GDY-FO) as anode materials. The goal is to motivate lithium-ion batteries (LIBs) toward ultra-high capacity, superior cyclic stability, and excellent rate performance. The graphdiyne is used as carriers to disperse Fe3 O4 uniformly without agglomeration and induce high valence of Fe with reducing the energy in the system. The presence of Fe vacancy could regulate the charge distribution around vacancies and adjacent atoms, leading to facilitate electronic transportation, enlarge the lithium-ion diffusion, and decrease Li+ diffusion barriers, and thus displaying significant pseudocapacitive process and advantageous lithium-ion storage. The optimized electrode IV-GDY-FO reveals a capacity of 2084.1 mAh g-1 at 0.1 C, superior cycle stability and rate performance with a high specific capacity of 1057.4 mAh g-1 even at 10 C.

Advances in Mn-Based Electrode Materials for Aqueous Sodium-Ion Batteries

Aqueous sodium-ion batteries have attracted extensive attention for large-scale energy storage applications, due to abundant sodium resources, low cost, intrinsic safety of aqueous electrolytes and eco-friendliness. The electrochemical performance of aqueous sodium-ion batteries is affected by the properties of electrode materials and electrolytes. Among various electrode materials, Mn-based electrode materials have attracted tremendous attention because of the abundance of Mn, low cost, nontoxicity, eco-friendliness and interesting electrochemical performance. Aqueous electrolytes having narrow electrochemical window also affect the electrochemical performance of Mn-based electrode materials. In this review, we introduce systematically Mn-based electrode materials for aqueous sodium-ion batteries from cathode and anode materials and offer a comprehensive overview about their recent development. These Mn-based materials include oxides, Prussian blue analogues and polyanion compounds. We summarize and discuss the composition, crystal structure, morphology and electrochemical properties of Mn-based electrode materials. The improvement methods based on electrolyte optimization, element doping or substitution, optimization of morphology and carbon modification are highlighted. The perspectives of Mn-based electrode materials for future studies are also provided. We believe this review is important and helpful to explore and apply Mn-based electrode materials in aqueous sodium-ion batteries.

High-Performance Alkali Metal Ion Storage in Bi2Se3 Enabled by Suppression of Polyselenide Shuttling Through Intrinsic Sb-Substitution Engineering

Bismuth selenide holds great promise as a kind of conversion-alloying-type anode material for alkali metal ion storage because of its layered structure with large interlayer spacing and high theoretical specific capacity. Nonetheless, its commercial development has been significantly hammered by the poor kinetics, severe pulverization, and polyselenide shuttle during the charge/discharge process. Herein, Sb-substitution and carbon encapsulation strategies are simultaneously employed to synthesize Sb_xBi_2-xSe₃ nanoparticles decorated on Ti₃C₂T_x MXene with encapsulation of N-doped carbon (Sb_xBi_2-xSe₃/MX⊂NC) as anodes for alkali metal ion storage. The superb electrochemical performances could be assigned to the cationic displacement of Sb³⁺ that effectively inhibits the shuttling effect of soluble polyselenides and the confinement engineering that alleviates the volume change during the sodiation/desodiation process. When used as anodes for sodium- and lithium-ion batteries, the Sb_0.4Bi_1.6Se₃/MX⊂NC composite exhibits superior electrochemical performances. This work offers valuable guidance to suppress the shuttling of polyselenides/polysulfides in high-performance alkali metal ion batteries with conversion/alloying-type transition metal sulfide/selenide anode materials.

Compact TiO2@SnO2@C heterostructured particles as anode materials for sodium-ion batteries with improved volumetric capacity

Sodium-ion batteries (SIBs) are promising candidates for large-scale energy storage. Increasing the energy density of SIBs demands anode materials with high gravimetric and volumetric capacity. To overcome the drawback of low density of conventional nanosized or porous electrode materials, compact heterostructured particles are developed in this work with improved Na storage capacity by volume, which are composed of SnO2 nanoparticles loaded into nanoporous TiO2 followed by carbon coating. The resulted TiO2@SnO2@C (denoted as TSC) particles inherit the structural integrity of TiO2 and extra capacity contribution from SnO2, delivering a volumetric capacity of 393 mAh cm-3 notably higher than that of porous TiO2 and commercial hard carbon. The heterogeneous interface between TiO2 and SnO2 is believed to promote the charge transfer and facilitate the redox reactions in the compact heterogeneous particles. This work demonstrates a useful strategy for electrode materials with high volumetric capacity.

Understanding the origin of the improved sodium ion storage performance of the transition metal oxide@carbon nanocomposite anodes

Transition metal oxide (TMO) anodes show inferior sodium ion storage performance compared with that of lithium ion storage owing to the larger radium size and heavier elemental mass of Na+ than Li+. Effective strategies are highly desired to improve the Na+ storage performance of TMOs for applications. In this work, using ZnFe2O4@xC nanocomposites as model materials for investigation, we found that by manipulating the particle sizes of the inner TMOs core and the features of outer carbon coating, the Na+ storage performance can be significantly improved. The ZnFe2O4@1C with a diameter of the inner ZnFe2O4 core of around 200 nm coated by a thin carbon layer of around 3 nm shows a specific capacity of only 120 mA h g-1. The ZnFe2O4@6.5C with a diameter of the inner ZnFe2O4 core of around 110 nm embedding in a porous interconnected carbon matrix displays a significantly improved specific capacity of 420 mA h g-1 at the same specific current. Furthermore, the latter shows an excellent cycling stability of 1000 cycles with a capacity retention of 90% of the initial 220 mA h g-1 specific capacity at 1.0 A g-1. TEM, electrochemical impedance spectroscopy, and kinetic analysis show that the inner ZnFe2O4 core with reduced particle size and the outer thicker and interconnected carbon matrix synergistically improve the active reaction sites, integrity, electric conductivity, and pseudocapacitive-controlled contribution of ZnFe2O4@xC nanocomposites, thus leading to an overall enhanced Na+ storage performance. Our findings create a universal, facile, and effective method to enhance the Na+ storage performance of the TMO@C nanomaterials.

Pore-forming mechanisms and sodium-ion-storage performances in a porous Na3V2(PO4)3/C composite cathode

Na₃V₂(PO₄)₃ (NVP) is regarded as one of the most promising cathode materials for sodium-ion batteries (SIBs). However, it suffers from a dense bulk structure and low intrinsic electronic conductivity, which lead to limited electrochemical performances. Herein, we propose a surfactant-assisted molding strategy to regulate the pore-forming process in NVP/C composite cathode materials. More precisely, the forming process of the pores in NVP could be easily controlled by utilizing the huge difference in critical micelle concentration of a surfactant (cetyltrimethylammonium bromide, CTAB) in water and ethanol. By reasonably modulating the ratio of water and ethanol in the solution, the as-synthesized NVP/C sample exhibited a three-dimensional interconnected structure with hierarchical micro/meso/macro-pores. Benefiting from these hierarchical porous structures in NVP/C, the structural stability, contact surface with the electrolyte, and electronic/ionic conductivity were improved simultaneously; whereby the optimized porous NVP/C sample exhibited an excellent high-rate performance (61.3 mA h g^-1 at 10 C) and superior cycling stability (90.2% capacity retention after 500 cycles at 10 C).