Micro/Nanostructured Materials for Sodium Ion Batteries and Capacitors

Emerging Chemistry for Wide-Temperature Sodium-Ion Batteries

The shortage of resources such as lithium and cobalt has promoted the development of novel battery systems with low cost, abundance, high performance, and efficient environmental adaptability. Due to the abundance and low cost of sodium, sodium-ion battery chemistry has drawn worldwide attention in energy storage systems. It is widely considered that wide-temperature tolerance sodium-ion batteries (WT-SIBs) can be rapidly developed due to their unique electrochemical and chemical properties. However, WT-SIBs, especially for their electrode materials and electrolyte systems, still face various challenges in harsh-temperature conditions. In this review, we focus on the achievements, failure mechanisms, fundamental chemistry, and scientific challenges of WT-SIBs. The insights of their design principles, current research, and safety issues are presented. Moreover, the possible future research directions on the battery materials for WT-SIBs are deeply discussed. Progress toward a comprehensive understanding of the emerging chemistry for WT-SIBs comprehensively discussed in this review will accelerate the practical applications of wide-temperature tolerance rechargeable batteries.

Enabling an Excellent Ordering-Enhanced Electrochemistry and a Highly Reversible Whole-Voltage-Range Oxygen Anionic Chemistry for Sodium-Ion Batteries

Though considerable Mg-doped layered cathodes have been exploited, some new differences relative to previous reports can be concluded by doping a heavy dose of Mg via two rational strategies. Unlike the common unit cell of the P6₃/mmc group by X-ray diffraction, neutron diffraction reveals a large supercell of the P6₃ group and enhanced ordering for Na_11/18Mg_1/18[Ni_1/4Mg_1/9Mn_11/18]O₂ with Mg occupying both the Na and Mn sites. Compared with only one obvious voltage plateau of Na_0.5[Ni_0.25Mn_0.75]O₂ (NNM), Na_11/18Mg_1/18[Ni_1/4Mg_1/9Mn_11/18]O₂ (NMNMM) shows more severe voltage plateaus but with excellent electrochemical performance. Na_0.5[Mg_0.25Mn_0.75]O₂ (NMM) with Mg only occupying the Ni site displays a highly reversible whole-voltage-range oxygen redox chemistry and smooth voltage curves without any voltage hysteresis. Cationic Ni²⁺/Ni⁴⁺ couples are responsible for the charge compensations of NNM and NMNMM, while only the oxygen anionic reaction accounts for the capacity of NMM between 2.5 and 4.3 V. Interestingly, the Mn³⁺/Mn⁴⁺ pair contributes all capacity for all cathodes between 1.5 and 2.5 V. All cathodes undergo a double-phase mechanism: an irreversible P2-O2 phase transition for NNM, an enhanced reversible P2-O2 phase transition for NMNMM, and a highly reversible P2-OP4 phase transition for NMM. In addition, the designed cathodes display excellent rate capability and long-term cycling stability but with a large difference in the various voltage ranges of 2.5-4.3 and 1.5-2.5 V, respectively. This work provides a good understanding of ion doping and some new insights into exploiting high-performance materials.

Emerging 2D Copper-Based Materials for Energy Storage and Conversion: A Review and Perspective

2D materials have shown great potential as electrode materials that determine the performance of a range of electrochemical energy technologies. Among these, 2D copper-based materials, such as Cu-O, Cu-S, Cu-Se, Cu-N, and Cu-P, have attracted tremendous research interest, because of the combination of remarkable properties, such as low cost, excellent chemical stability, facile fabrication, and significant electrochemical properties. Herein, the recent advances in the emerging 2D copper-based materials are summarized. A brief summary of the crystal structures and synthetic methods is started, and innovative strategies for improving electrochemical performances of 2D copper-based materials are described in detail through defect engineering, heterostructure construction, and surface functionalization. Furthermore, their state-of-the-art applications in electrochemical energy storage including supercapacitors (SCs), alkali (Li, Na, and K)-ion batteries, multivalent metal (Mg and Al)-ion batteries, and hybrid Mg/Li-ion batteries are described. In addition, the electrocatalysis applications of 2D copper-based materials in metal-air batteries, water-splitting, and CO₂ reduction reaction (CO₂ RR) are also discussed. This review also discusses the charge storage mechanisms of 2D copper-based materials by various advanced characterization techniques. The review with a perspective of the current challenges and research outlook of such 2D copper-based materials for high-performance energy storage and conversion applications is concluded.

Study of Synergistic Effects of Cu and Fe on P2-Type Na0.67MnO2 for High Performance Na-Ion Batteries

P2-type Na_0.67MnO₂ with a stable structure and an open framework can provide numerous channels for fast Na⁺ de/intercalation, for which it is considered to be advantageous in application of the cathode material for Na-ion batteries. However, the complex phase transition occurring during cycling and the lattice distortion triggered by the Jahn-Teller effect severely restrict its development. Herein, the modified Na_0.67MnO₂ with Cu or Fe single-element doping as well as Cu and Fe double-element doping was synthesized by the sol-gel method, and the effects of doping on the crystal structure and electrochemical performances of Na_0.67MnO₂ were studied. It was demonstrated that the phase of the material did not change after the introduction of Cu and Fe elements, and the cycling stability and rate performance were greatly improved by Cu and Fe double-doping owing to their synergistic effect. The Na_0.67Mn_0.92Fe_0.04Cu_0.04O₂ (NMFCO) cathode delivers discharge specific capacities of 110.5 mA h g^-1 at 5 C and 91.8 mA h g^-1 at 10 C and exhibits the high-capacity retention of 94.35% at 1 C and 90.68% at 5 C after 100 cycles. Overall, this study offers a guiding direction for accelerating the modification of P2-type Na_0.67MnO₂ as a cathode active material for high performance Na-ion batteries.

In-situ fabrication of active interfaces towards FeSe as advanced performance anode for sodium-ion batteries

Transition metal selenides have gained enormous interest as anodes for sodium ion batteries (SIBs). Nonetheless, their large volume expansion causing poor rate and inferior cycle stability during Na⁺ insertion/extraction process hinders their further applications in SIBs. Herein, a confined-regulated interfacial engineering strategy towards the synthesis of FeSe microparticles coated by ultrathin nitrogen-doped carbon (NC) is demonstrated (FeSe@NC). The strong interfacial interaction between FeSeand NC endows FeSe@NC with fast electron/Na⁺ transport kinetics and outstanding structural stability, delivering unexceptionable rate capability (364 mAh/gat 10 A/g) and preeminent cycling durability (capacity retention of 100 % at 1 A/g over 1000 cycles). Furthermore, variousex situcharacterization techniques and density functional theory (DFT) calculations have been applied to demonstrate the Na⁺ storage mechanism of FeSe@NC. The assembled Na₃V₂(PO₄)₂F₃@rGO//FeSe@NC full cell also displays a high capacity of 241 mAh/gat 1 A/g with the capacity retention of nearly 100 % over 2000 cycles, and delivers a supreme energy density of 135 Wh kg^-1 and a topmost power density of 495 W kg^-1, manifesting the latent applications of FeSe@NC in the fast charging SIBs.

Stretchable sodium-ion capacitors based on coaxial CNT supported Na2Ti3O7 with high capacitance contribution

Stretchable sodium-ion capacitors (SSICs) are promising energy storage devices for wearable electronic devices, and the development bottleneck is the realization of stretchable battery-type electrodes with desirable electrochemical properties during dynamic deformation. Herein, we find that electrostatic modification of acidified carbon nanotubes with polyamines can introduce active sites and modulate the surface pH microenvironment, thereby developing a route to realize the in situ coaxial nanometerization of sodium titanate (nCNT@NTO). The nCNT@NTO anode material has a fast Na⁺ transport and the high capacitive contribution, which can deliver a high specific capacity (206.5 mA h g^-1 at 0.1 A g^-1) and high rate performance (maintain 51% capacity at 10 A g^-1), and the ideal cycle stability (∼93% capacity retention after 1000 cycles at 5 A g^-1). In addition, acrylate-rubber with high stickiness and stretchability are served as the elastic matrix both of the stretchable electrodes and quasi-solid-state electrolytes, which endows strong adhesion between electrodes and electrolytes. Thus, the accordingly assembled SSIC delivers high energy density of 8.8 mW h cm^-3 (at a power density of 0.024 W cm^-3), and excellent deformation stability (89% capacitance retention after 500 stretching cycles under 100% strain).

Porous Microspheres Comprising CoSe2 Nanorods Coated with N-Doped Graphitic C and Polydopamine-Derived C as Anodes for Long-Lived Na-Ion Batteries

Metal-organic framework-templated nitrogen-doped graphitic carbon (NGC) and polydopamine-derived carbon (PDA-derived C)-double coated one-dimensional CoSe₂ nanorods supported highly porous three-dimensional microspheres are introduced as anodes for excellent Na-ion batteries, particularly with long-lived cycle under carbonate-based electrolyte system. The microspheres uniformly composed of ZIF-67 polyhedrons and polystyrene nanobeads (ϕ = 40 nm) are synthesized using the facile spray pyrolysis technique, followed by the selenization process (P-CoSe₂@NGC NR). Further, the PDA-derived C-coated microspheres are obtained using a solution-based coating approach and the subsequent carbonization process (P-CoSe₂@PDA-C NR). The rational synthesis approach benefited from the synergistic effects of dual carbon coating, resulting in a highly conductive and porous nanostructure that could facilitate rapid diffusion of charge species along with efficient electrolyte infiltration and effectively channelize the volume stress. Consequently, the prepared nanostructure exhibits extraordinary electrochemical performance, particularly the ultra-long cycle life stability. For instance, the advanced anode has a discharge capacity of 291 (1000th cycle, average capacity decay of 0.017%) and 142 mAh g^-1 (5000th cycle, average capacity decay of 0.011%) at a current density of 0.5 and 2.0 A g^-1, respectively.

A quasi-3D Sb2S3/reduced graphene oxide/MXene (Ti3C2Tx) hybrid for high-rate and durable sodium-ion batteries

Antimony sulfide (Sb₂S₃) is a promising anode material for sodium-ion batteries (SIBs) owing to its high theoretical capacity and superior reversibility. However, its cycling life and rate performance are seriously impeded by the inferior inherent electroconductibility and tremendous volume change in the charging/discharging processes. Herein, a quasi three-dimensional (3D) Sb₂S₃/RGO/MXene composite, with Sb₂S₃ nanoparticles (∼15 nm) uniformly distributed in the quasi-3D RGO/MXene architecture, was prepared by a toilless hydrothermal treatment. The RGO/MXene conductive substrate not only alleviates the volume expansion of Sb₂S₃, but also promotes electrolyte infiltration and affords highways for ion/electron transport. More importantly, the synergistic effects between RGO and Ti₃C₂T_x MXene are extremely favourable to maintain the integrity of the electrode during cycling. As a result, the Sb₂S₃/RGO/MXene composite exhibits a high reversible capacity of 633 mA h g^-1 at 0.2 A g^-1, outstanding rate capability (510.1 mA h g^-1 at 4 A g^-1) and good cycling performance with a capacity loss of 16% after 500 cycles.

Targeted Positioning of Quantum Dots Inside 3D Silicon Photonic Crystals Revealed by Synchrotron X-ray Fluorescence Tomography

It is a major outstanding goal in nanotechnology to precisely position functional nanoparticles, such as quantum dots, inside a three-dimensional (3D) nanostructure in order to realize innovative functions. Once the 3D positioning is performed, the challenge arises how to nondestructively verify where the nanoparticles reside in the 3D nanostructure. Here, we study 3D photonic band gap crystals made of Si that are infiltrated with PbS nanocrystal quantum dots. The nanocrystals are covalently bonded to polymer brush layers that are grafted to the Si-air interfaces inside the 3D nanostructure using surface-initiated atom transfer radical polymerization (SI-ATRP). The functionalized 3D nanostructures are probed by synchrotron X-ray fluorescence (SXRF) tomography that is performed at 17 keV photon energy to obtain large penetration depths and efficient excitation of the elements of interest. Spatial projection maps were obtained followed by tomographic reconstruction to obtain the 3D atom density distribution with 50 nm voxel size for all chemical elements probed: Cl, Cr, Cu, Ga, Br, and Pb. The quantum dots are found to be positioned inside the 3D nanostructure, and their positions correlate with the positions of elements characteristic of the polymer brush layer and the ATRP initiator. We conclude that X-ray fluorescence tomography is very well suited to nondestructively characterize 3D nanomaterials with photonic and other functionalities.

Understanding sodium storage properties of ultra-small Fe3S4 nanoparticles - a combined XRD, PDF, XAS and electrokinetic study

Various electrode materials are considered for sodium-ion batteries (SIBs) and one important prerequisite for developments of SIBs is a detailed understanding about charge storage mechanisms. Herein, we present a rigorous study about Na storage properties of ultra-small Fe₃S₄ nanoparticles, synthesized applying a solvothermal route, which exhibit a very good electrochemical performance as anode material for SIBs. A closer look into electrochemical reaction pathways on the nanoscale, utilizing synchrotron-based X-ray diffraction and X-ray absorption techniques, reveals a complicated conversion mechanism. Initially, separation of Fe₃S₄ into nanocrystalline intermediates occurs accompanied by reduction of Fe³⁺ to Fe²⁺ cations. Discharge to 0.1 V leads to formation of strongly disordered Fe⁰ finely dispersed in a nanosized Na₂S matrix. The resulting volume expansion leads to a worse long-term stability in the voltage range 3.0-0.1 V. Adjusting the lower cut-off potential to 0.5 V, crystallization of Na₂S is prevented and a completely amorphous intermediate stage is formed. Thus, the smaller voltage window is favorable for long-term stability, yielding highly reversible capacity retention, e.g., 486 mAh g^-1 after 300 cycles applying 0.5 A g^-1 and superior coulombic efficiencies >99.9%. During charge to 3.0 V, Fe₃S₄ with smaller domains are reversibly generated in the 1^st cycle, but further cycling results in loss of structural long-range order, whereas the local environment resembles that of Fe₃S₄ in subsequent charged states. Electrokinetic analyses reveal high capacitive contributions to the charge storage, indicating shortened diffusion lengths and thus, redox reactions occur predominantly at surfaces of nanosized conversion products.

Recent advances and perspectives of 3D printed micro-supercapacitors: from design to smart integrated devices

3D-printed micro-supercapacitors (MSCs) have emerged as the ideal candidates for energy storage devices owing to their unique characteristics of miniaturization, structural diversity, and integration. Exploring the 3D printing technology for various materials and architectures of MSCs is key to realizing customization and optimizing the performance of 3D-printed MSCs. In this review, we summarize the latest progress in 3D-printed MSCs with regards to general printing approaches, printable materials, and rational design considerations. Specifically, several general types of 3D printing techniques (their working principles, available materials, resolutions, advantages, and disadvantages) and their applications to fabricate electrodes with different energy storage mechanisms, and various electrolytes, are summarized. We further discuss research directions in terms of integrated systems with other electronics. Finally, future perspectives on the research and development directions in this important field are further discussed.

Synergetic stability enhancement with magnesium and calcium ion substitution for Ni/Mn-based P2-type sodium-ion battery cathodes

The conventional P2-type cathode material Na0.67Ni0.33Mn0.67O2 suffers from an irreversible P2-O2 phase transition and serious capacity fading during cycling. Here, we successfully carry out magnesium and calcium ion doping into the transition-metal layers (TM layers) and the alkali-metal layers (AM layers), respectively, of Na0.67Ni0.33Mn0.67O2. Both Mg and Ca doping can reduce O-type stacking in the high-voltage region, leading to enhanced cycling endurance, however, this is associated with a decrease in capacity. The results of density functional theory (DFT) studies reveal that the introduction of Mg2+ and Ca2+ make high-voltage reactions (oxygen redox and Ni4+/Ni3+ redox reactions) less accessible. Thanks to the synergetic effect of co-doping with Mg2+ and Ca2+ ions, the adverse effects on high-voltage reactions involving Ni-O bonding are limited, and the structural stability is further enhanced. The finally obtained P2-type Na0.62Ca0.025Ni0.28Mg0.05Mn0.67O2 exhibits a satisfactory initial energy density of 468.2 W h kg-1 and good capacity retention of 83% after 100 cycles at 50 mA g-1 within the voltage range of 2.2-4.35 V. This work deepens our understanding of the specific effects of Mg2+ and Ca2+ dopants and provides a stability-enhancing strategy utilizing abundant alkaline earth elements.

Electronic Effect and Regiochemistry of Substitution in Pre-sodiation Chemistry

The low oxidation potential of a pre-sodiation cathode additive intrinsically prevents decomposition of the electrolyte. Although the introduction of electron-donating substitution reduces the oxidation potential, the additional molecular weight restricts the output capacity. Herein, as theroretically predicted, the electrochemical oxidation potential of sodium carboxylate is manipulated by the electronic effect and regiochemistry of the functionality, in which the stronger electron-donating substituent, p-π conjugation, and optimized regiochemistry can dramatically lead to the lower potential originated from the elevation of the highest occupied molecular orbital level. Thus, benefiting from the para-NH₂ unit accompanied by a conjugated aromatic architecture, molecularly engineered sodium para-aminobenzoate (PABZ-Na) presents a reduced oxidation plateau of 3.45 V. Triggered by the positive compensation merit, sodium-based electrochemical storage systems manifest excellent electrochemical performances. This breakthrough sheds light into the correlation between the electronic effect of the functional group and the oxidation potential of the organic additive, affording in-depth insights into the fundamental guidance of pre-sodiation chemistry.

Development and Evaluation of Sn Foil Anode for Sodium-Ion Batteries

Metal foil electrodes are simple to prepare and have a high active material loading, making them well suited for the fabrication of inexpensive high-energy-density batteries. Herein, Sn metal foil is used as a binder- and conductive additive-free anode for sodium-ion batteries, achieving a high reversible specific capacity of 692 mAh g^-1 and coulombic efficiency of 99% after 100 cycles at a rate of 0.1 C. During the first discharge process, the anode undergoes area expansion. It then splits into multiple parts during the first-charge process. Upon cycling, the separated parts reconnect and form a single piece with a porous and robust coral structure owing to the self-healing nature of the anode. A full cell with a Sn foil anode and Na₃ V₂ (PO₄ )₃ cathode shows a stable cycle life of 100 mAh g^-1 for 300 cycles. Thus, the cracking or pulverization of the Sn anode is not the principal origin of poor cycling properties. The adopted strategy will promote the development and commercialization of high-capacity metal foil anodes that undergo volume changes during charge/discharge cycling.

Suppressing the P2 - O2 phase transformation and Na+/vacancy ordering of high-voltage manganese-based P2-type cathode by cationic codoping

High-voltage and low-cost manganese-based P2-type oxides show real promise as promising cathode for sodium-ion batteries (SIBs). But the P2 - O2 phase transformation and Na⁺/vacancy ordering results in the inferior structural stability and Na⁺ diffusion coefficient, which further leads to rapid decay of capacity and poor rate capability. Herein, in consideration of the synergetic effects of dual cationic doping, electrochemically inactive Li⁺ and active Co³⁺ codoping are proposed to solve the above issues. The novel two-step doping strategy, Co doping during synthesis of precursors via coprecipitation reaction followed by Li doping during solid-state reaction, are rationally developed. As anticipated, the Li/Co codoped P2-type oxide exhibits the absence of P2 - O2 phase transformation and Na⁺/vacancy disordering, which gives rise to an outstanding cycling stability (86.7% capacity retention within 100 cycles at 0.1C) and high-rate capability (reversible capacity of 109 mAh g^-1 even at 10C). In addition, the full-cells composed of the codoped P2-type positive and hard carbon negative show high energy-density, good lifespan and high-rate property. This proposed cationic codoping provides an effective and scalable tactics for modulating the structural properties of high-voltage P2-type cathodes for advanced SIBs.

Co-Salen Complex-Derived CoP Nanoparticles Confined in N-Doped Carbon Microspheres for Stable Sodium Storage

The poor rate and cycle performance rooting from the inferior electrical conductivity and large volume change are bottlenecks for further application of the potential anode material in sodium-ion batteries. To address this problem, homogeneous CoP nanoparticles enwrapped in the N-doped carbon (CoP/NC) microspheres are synthesized by the simultaneous carbonization and phosphorization of Co-salen complex microspheres for the first time. The N-doped carbon enhances its conductivity and diminishes the volume stress, and the dispersed CoP nanoparticles in carbon provide more reaction sites, resulting in a superior sodium storage performance. CoP/NC microspheres exhibit the capacity of 373 mA h g^-1 at 0.1 A g^-1 after 100 cycles. Even at 2 A g^-1 for 2000 cycles, the capacity of 195 mA h g^-1 is also achieved. This work provides an excellent reference for the design and synthesis of sulfide, selenide, and other transition-metal composites. It is also beneficial to expand the application of salen complexes in the design and synthesis of catalysts and energy storage materials.

Superior Sodium Storage Properties in the Anode Material NiCr2 S4 for Sodium-Ion Batteries: An X-ray Diffraction, Pair Distribution Function, and X-ray Absorption Study Reveals a Conversion Mechanism via Nickel Extrusion

The pseudo-layered sulfide NiCr₂ S₄ exhibits outstanding electrochemical performance as anode material in sodium-ion batteries (SIBs). The Na storage mechanism is investigated by synchrotron-based X-ray scattering and absorption techniques as well as by electrochemical measurements. A very high reversible capacity in the 500th cycle of 489 mAh g^-1 is observed at 2.0 A g^-1 in the potential window 3.0-0.1 V. Full discharge includes irreversible generation of Ni⁰ and Cr⁰ nanoparticles embedded in nanocrystalline Na₂ S yielding shortened diffusion lengths and predominantly surface-controlled charge storage. During charge, Ni⁰ and Cr⁰ are oxidized, Na₂ S is consumed, and amorphous Ni and Cr sulfides are formed. Limiting the potential window to 3.0-0.3 V an unusual nickel extrusion sodium insertion mechanism occurs: Ni²⁺ is reduced to nanosized Ni⁰ domains, expelled from the host lattice, and is replaced by Na⁺ cations to form O3-type like NaCrS₂ . Surprisingly, the discharge and charge processes comprise Na⁺ shuttling between highly crystalline NiCr₂ S₄ and NaCrS₂ enabling a superior long-term stability for 3000 cycles. The results not only provide valuable insights for the electrochemistry of conversion materials but also extend the scope of layered electrode materials considering the reversible nickel extrusion sodium insertion reaction as new concept for SIBs.

Operando mechanistic and dynamic studies of N/P co-doped hard carbon nanofibers for efficient sodium storage

In situ Raman and electrochemical results reveal that Na⁺ adsorbs on the surface/defective sites of N/P-HCNF and inserts randomly into its turbostratic nanodomains in the dilute state without a staged formation, which can facilitate fast Na⁺ diffusion kinetics for efficient sodium storage.

Future directions of material chemistry and energy chemistry

Energy is an important substantial foundation for the survival and development of humans. However, the over-consumption of resources and environmental pollution have become more prominent. The key factors for solving energy problems are to increase energy utilization efficiency and optimize energy structure. The development of new materials is the research emphasis in the field of material chemistry all the time. For instance, developing new light-capture materials and catalysts to improve the efficiency of existing photovoltaic cells is one of the most effective approaches to increasing solar power capacity radically. The design of high-performance catalytic materials to make better use of energy from fossil fuels and biomass. In addition, it is an important research direction of material chemistry and energy chemistry to deeply understand the reaction mechanism of energy conversion.

3D Ag@C Cloth for Stable Anode Free Sodium Metal Batteries

While sodium metal anodes (SMAs) feature many performance advantages in sodium ion batteries (SIBs), severe safety concerns remain for using bulk sodium electrodes. Herein, a 3D Ag@C natrophilic substrate prepared by a facile thermal evaporation deposition method, which can be employed as a much safer "anode-free" SMA, is reported. Initially, there is no bulk sodium on the Ag@C substrate in the assembled SIBs. Upon charging, sodium will be uniformly deposited onto the Ag@C substrate and afterwards functions as a real SMA, thus inheriting the intrinsic merits of SMA and enhancing safety simultaneously. While cycling, the as-synthesized substrate demonstrates superior sodium plating/stripping cycling stability at 1, 2 and 3 mA cm^-2 with a capacity of 2 mAh cm^-2 . Theoretical simulations reveal that Na ions prefer to bind with Ag and form a Na-Ag network, thus clearly revealing uniform sodium deposition on the Ag@C substrate. More importantly, a full battery based on Ag@C and Prussian white with impressive Coulomb efficiency (CE), high rate capability (from 0.1 C to 5 C) and long-term cycling life is illustrated for the first time, thus making Ag@C feasible for the establishment of "anode-free" SIBs with reduced cost, high gravimetric/volumetric energy density and enhanced safety.

2D-VN2 MXene as a novel anode material for Li, Na and K ion batteries: Insights from the first-principles calculations

Developing two-dimensional (2D) materials as anode materials have been proved a promising approach to significantly improve the charge storage performances of alkali metal ion. Herein, we investigate mono-layered VN₂ as an anode material in Li, Na and K ion batteries. Firstly, the high stability of 2D-VN₂ has been demonstrated via calculating the phonon spectra. 2D-VN₂ is capable of delivering high capacities of 678.8, 339.4 and 1357.6 mAh g^-1 in Li⁺, K⁺ and Na⁺ storage, respectively. In addition, the metallic properties and corresponding high electrical conductivity and low diffusion barriers of 201.1 meV for Li atoms, 34.7 meV for K atoms and 84.1 meV for Na atoms on VN₂ surface, indicating good capacity and the superior rate performances of alkali metal atoms migration on VN₂. The calculated average voltage of Li, Na and K are respectively 0.81 V, 0.29 V and 0.77 V, suggesting a promising voltage behavior compared with other 2D materials.

Natural Cellulose Substance Based Energy Materials

Natural cellulose substances have been proven to be ideal structural templates and scaffolds for the fabrication of artificial functional materials with designed structures, psychochemical properties and functionalities. They possess unique hierarchically porous network structures with flexible, biocompatible, and environmental characteristics, exhibiting great potentials in the preparation of energy-related materials. This minireview summarizes natural cellulose-based materials that are used in batteries, supercapacitors, photocatalytic hydrogen generation, photoelectrochemical cells, and solar cells. When natural cellulose substances are employed as the structural template or carbon sources of energy materials, the three-dimensional porous interwoven structures are perfectly replicated, leading to the enhanced performances of the resultant materials. Benefiting from the mechanical strengths of natural cellulose substances, wearable, portable, free-standing, and flexible materials for energy storage and conversion are easily obtained by using natural cellulose substances as the substrates.

Thermophysical Characterization of a Layered P2 Type Structure Na0.53MnO2 Cathode Material for Sodium Ion Batteries

Over the last decade, the demand for safer batteries with excellent performance and lower costs has been intensively increasing. The abundantly available precursors and environmental friendliness are generating more and more interest in sodium ion batteries (SIBs), especially because of the lower material costs compared to Li-ion batteries. Therefore, significant efforts are being dedicated to investigating new cathode materials for SIBs. Since the thermal characterization of cathode materials is one of the key factors for designing safe batteries, the thermophysical properties of a commercial layered P2 type structure Na0.53MnO2 cathode material in powder form were measured in the temperature range between −20 and 1200 °C by differential scanning calorimetry (DSC), laser flash analysis (LFA), and thermogravimetry (TG). The thermogravimetry (TG) was combined with mass spectrometry (MS) to study the thermal decomposition of the cathode material with respect to the evolved gas analysis (EGA) and was performed from room temperature up to 1200 °C. The specific heat (Cp) and the thermal diffusivity (α) were measured up to 400 °C because beyond this temperature, the cathode material starts to decompose. The thermal conductivity (λ) as a function of temperature was calculated from the thermal diffusivity, the specific heat capacity, and the density. Such thermophysical data are highly relevant and important for thermal simulation studies, thermal management, and the mitigation of thermal runaway.

Carbon Nanofibers with Embedded Sb2 Se3 Nanoparticles as Highly Reversible Anodes for Na-Ion Batteries

As a promising candidate for large-scale energy storage, sodium-ion batteries (SIBs) have superiority due to their low cost and abundance as a resource. Herein, homogeneous Sb₂ Se₃ nanocrystallites embedded in carbon nanofibers (Sb₂ Se₃ /CNFs) by electrospinning and selenization treatment are prepared. The obtained Sb₂ Se₃ /CNFs exhibit good cycling performance, high reversible capacity, and excellent rate capability as anodes for SIBs. The outstanding performances are attributed to a combination of Na⁺ intercalation, conversion reaction, and alloying with Sb₂ Se₃ , disclosed through in-situ X-ray diffraction. Meanwhile, unique nanostructures provide large contact surface and tolerant accommodation to volume expansion which bring high reversibility and long cycle durability. This distinctive material shows prospective applications of SIBs especially under high current density.

Emerging Potassium-ion Hybrid Capacitors

As a new type of capacitor-battery hybrid energy storage device, metal-ion capacitors have attracted widespread attention because of their high-power density while ensuring energy density and long lifespan. Potassium-ion capacitors (KICs) featuring the merits of abundant potassium resources, lower standard electrode potential, and low cost have been considered as potential alternatives to lithium-/sodium-ion capacitors. However, KICs still face issues including unsatisfactory reaction kinetics, low energy density, and poor lifetime owing to the large radius of the potassium ion. In this Review, the importance of emerging potassium-ion capacitor is addressed. The Review offers a brief discussion of the fundamental working principle of KICs, along with an overview of recent advances and achievements of a variety of electrode materials for dual carbon and non-dual carbon KICs. Furthermore, electrolyte chemistry, binders as well as electrode/electrolyte interface, are summarized. Finally, existing challenges and perspectives on further development of KICs are also presented.

Beryllene: A Promising Anode Material for Na- and K-Ion Batteries with Ultrafast Charge/Discharge and High Specific Capacity

We predict two-dimensional Be materials, α- and β-beryllene. In α-beryllene each Be atom binds to six other Be atoms in a planar scheme, whereas β-beryllene consists of two stacked α-beryllene monolayers. Both α- and β-beryllene are found to be highly stable, as demonstrated by high cohesive energies close to that of bulk Be, an absence of imaginary phonon modes, and high melting points. Both materials are metallic, indicating potential applications in Na-ion and K-ion batteries, which are explored in detail. The diffusion barriers of Na (K) on α- and β-beryllene are found to be only 9 (3) and 4 (5) meV, respectively. In particular, the diffusion barrier of K on α-beryllene exhibits the lowest ever recorded value in two-dimensional materials, suggesting the possibility of ultrafast charge/discharge. As the theoretical specific capacities of Na/K on α- and β-beryllene are found to be 1487/1322 and 743/743 mA h g^-1, respectively, the storage capacity is ultrahigh.

Recent Advances on Mixed Metal Sulfides for Advanced Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have drawn enormous attention in the past few years from both academic and industrial battery communities in view of the fascinating advantages of rich abundance and low cost of sodium resources. Among various electrode materials, mixed metal sulfides (MMSs) stand out as promising negative electrode materials for SIBs considering their superior structural and compositional advantages, such as decent electrochemical reversibility, high electronic conductivity, and rich redox reactions. Here, a summary of some recent developments in the rational design and synthesis of various kinds of MMSs with tailorable architectures, structural/compositional complexity, controllable morphologies, and enhanced electrochemical properties is presented. The effect of structural engineering and compositional design of MMSs on the sodium storage properties is highlighted. It is anticipated that further innovative works on the material design of advanced electrodes for batteries can be inspired.

Porous BN Nanofibers Enable Long-Cycling Life Sodium Metal Batteries

Sodium metal anode, featuring high capacity, low voltage and earth abundance, is desirable for building advanced sodium-metal batteries. However, Na-ion deposition typically leads to morphological instability and notorious chemical reactivity between sodium and common electrolytes still limit its practical application. In this study, a porous BN nanofibers modified sodium metal (BN/Na) electrode is introduced for enhancing Na-ion deposition dynamics and stability. As a result, symmetrical BN/Na cells enable an impressive rate capability and markedly enhanced cycling durability over 600 h at 10 mA cm^-2 . Density functional theory simulations demonstrate BN could effectively improve Na-ion adsorption and diffusion kinetics simultaneously. Finite element simulation clearly reveals the intrinsic smoothing effect of BN upon multiple Na-ion plating/stripping cycles. Coupled with a Na₃ V₂ O₂ (PO₄ )₂ F/Ti₃ C₂ X cathode, sodium metal full cells offer an ultrastable capacity of 125/63 mA h g^-1 (≈420/240 Wh kg^-1 ) at 0.05/5 C rate over 500 cycles. These comprehensive analyses demonstrate the feasibility of BN/Na anode for the establishment of high-energy-density sodium-metal full batteries.

Research Progress on Na3V2(PO4)3 Cathode Material of Sodium Ion Battery

Sodium ion batteries (SIBs) are one of the most potential alternative rechargeable batteries because of their low cost, high energy density, high thermal stability, and good structure stability. The cathode materials play a crucial role in the cycling life and safety of SIBs. Among reported cathode candidates, Na₃V₂(PO₄)₃ (NVP), a representative electrode material for sodium super ion conductor, has good application prospects due to its good structural stability, high ion conductivity and high platform voltage (~3.4 V). However, its practical applications are still restricted by comparatively low electronic conductivity. In this review, recent progresses of Na₃V₂(PO₄)₃ are well summarized and discussed, including preparation and modification methods, electrochemical properties. Meanwhile, the future research and further development of Na₃V₂(PO₄)₃ cathode are also discussed.

Biomass-Derived P/N-Co-Doped Carbon Nanosheets Encapsulate Cu3P Nanoparticles as High-Performance Anode Materials for Sodium-Ion Batteries

Biomass-derived approaches have been accepted as a practical way for the design of transitional metal phosphides confined by carbon matrix (TMPs@C) as energy storage materials. Herein, we successfully synthesize P/N-co-doped carbon nanosheets encapsulating Cu₃P nanoparticles (Cu₃P@P/N-C) by a feasible aqueous reaction followed by a phosphorization procedure using sodium alginate as the biomass carbon source. Cu-alginate hydrogel balls can be squeezed into two-dimensional (2D) nanosheets through a freeze–drying process. Then, Cu₃P@P/N-C was obtained after the phosphorization procedure. This rationally designed structure not only improved the kinetics of ion/electron transportation but also buffered the volume expansion of Cu₃P nanoparticles during the continuous charge and discharge processes. In addition, the 2D P/N co-doped carbon nanosheets can also serve as a conductive matrix, which can enhance the electronic conductivity of the whole electrode as well as provide rapid channels for electron/ion diffusion. Thus, when applied as anode materials for sodium-ion batteries, it exhibited remarkable cycling stability and rate performance. Prominently, Cu₃P@P/N-C demonstrated an outstanding reversible capacity of 209.3 mAh g⁻¹ at 1 A g⁻¹ after 1,000 cycles. Besides, it still maintained a superior specific capacity of 118.2 mAh g⁻¹ after 2,000 cycles, even at a high current density of 5 A g⁻¹.

Advanced Materials for Sodium-Ion Capacitors with Superior Energy-Power Properties: Progress and Perspectives

Developing electrochemical energy storage devices with high energy-power densities, long cycling life, as well as low cost is of great significance. Sodium-ion capacitors (NICs), with Na⁺ as carriers, are composed of a high capacity battery-type electrode and a high rate capacitive electrode. However, unlike their lithium-ion analogues, the research on NICs is still in its infancy. Rational material designs still need to be developed to meet the increasing requirements for NICs with superior energy-power performance and low cost. In the past few years, various materials have been explored to develop NICs with the merits of superior electrochemical performance, low cost, good stability, and environmental friendliness. Here, the material design strategies for sodium-ion capacitors are summarized, with focus on cathode materials, anode materials, and electrolytes. The challenges and opportunities ahead for the future research on materials for NICs are also proposed.

Biomass-Derived Carbons for Sodium-Ion Batteries and Sodium-Ion Capacitors

In the past decade, the rapid development of portable electronic devices, electric vehicles, and electrical devices has stimulated extensive interest in fundamental research and the commercialization of electrochemical energy-storage systems. Biomass-derived carbon has garnered significant research attention as an efficient, inexpensive, and eco-friendly active material for energy-storage systems. Therefore, high-performance carbonaceous materials, derived from renewable sources, have been utilized as electrode materials in sodium-ion batteries and sodium-ion capacitors. Herein, the charge-storage mechanism and utilization of biomass-derived carbon for sodium storage in batteries and capacitors are summarized. In particular, the structure-performance relationship of biomass-derived carbon for sodium storage in the form of batteries and capacitors is discussed. Despite the fact that further research is required to optimize the process and application of biomass-derived carbon in energy-storage devices, the current review demonstrates the potential of carbonaceous materials for next-generation sodium-related energy-storage applications.

Engineering Hollow Porous Carbon-Sphere-Confined MoS2 with Expanded (002) Planes for Boosting Potassium-Ion Storage

Potassium-ion batteries (PIBs) are emerging as promising next-generation electrochemical storage systems for their abundant and low-cost potassium resource. The key point of applying PIBs is to exploit stable K-host materials to accommodate the large-sized potassium ion. In this work, a yolk-shell structured MoS₂@hollow porous carbon-sphere composite (MoS₂@HPCS) assembled by engineering HPCS-confined MoS₂ with expanded (002) planes is proposed for boosting potassium-ion storage. When used as a PIB anode, the as-synthesized MoS₂@HPCS composite shows superior potassium storage performance. It delivers a reversible capacity of 254.9 mAh g^-1 at 0.5 A g^-1 after 100 discharge/charge cycles and maintains 126.2 mAh g^-1 at 1 A g^-1 over 500 cycles. The superior potassium-ion storage performance is ascribed to the elaborate yolk-shell nanoarchitecture and the expanded interlayer of the MoS₂ nanosheet, which could shorten the transport distance, enhance the electronic conductivity, relieve the volume variation, prevent the self-aggregation of MoS₂, facilitate the electrolyte penetration, and boost the intercalation/deintercalation of K⁺. Moreover, the potential application of the MoS₂@HPCS composite is also evaluated by assembled K-ion full cells with a perylenetetracarboxylic dianhydride cathode. Accordingly, the as-developed synthetic strategy can be extended to manufacture other host materials for PIBs and beyond.

Constructing consistent pore microstructures of bacterial cellulose-derived cathode and anode materials for high energy density sodium-ion capacitors

Bacterial cellulose-derived cathode and anode with similar carbon microstructure are well match in kinetic for high energy density sodium-ion capacitor.

Engineering doping-vacancy double defects and insights into the conversion mechanisms of an Mn-O-F ultrafine nanowire anode for enhanced Li/Na-ion storage and hybrid capacitors

The behavior of Li/Na-ion capacitors (LICs/NICs) is largely limited by the low number of electroactive sites in conventional insertion-type anodes. In this work, we demonstrated a novel doping-vacancy double-defective and conversion-type Mn-O-F ultrafine nanowire (denoted as MnF2-E) anode to boost the number of electroactive sites for enhanced LICs/NICs. Owing to the unique hetero oxygen-doping and intrinsic fluorine-vacancy double defects, the Mn-O-F nanowires exhibited superior electroactive sites and thus dramatically enhanced Li/Na-ion storage capability than pristine MnF2 micro/nano-crystals. Both the optimal MnF2 screened by orthogonal experiments and derived Mn-O-F anodes and commercial activated carbon (AC) cathode were used to construct MnF2//AC and MnF2-E//AC LICs/NICs, which were optimized by tuning the active mass ratios of the cathode/anode and the working voltage windows of the hybrid capacitors. The LICs/NICs based on the Mn-O-F anode demonstrated a considerably superior performance than the devices based on the MnF2 anode under the optimal voltages of 0-4 V and 0-4.3 V. The Mn-O-F anode exhibited dominant diffusion/surface-controlled kinetics for Li/Na-ion storage, respectively, showing a major conversion mechanism for the charge storage processes. This work provides a new concept of double-defective and conversion-type electrode materials to improve the Li/Na-ion storage capability and will have a significant impact on the relevant fields.

All-Solid-State Planar Sodium-Ion Microcapacitors with Multidirectional Fast Ion Diffusion Pathways

With the relentless development of smart and miniaturized electronics, the worldwide thirst for microscale electrochemical energy storage devices with form factors is launching a new era of competition. Herein, the first prototype planar sodium-ion microcapacitors (NIMCs) are constructed based on the interdigital microelectrodes of urchin-like sodium titanate as faradaic anode and nanoporous activated graphene as non-faradaic cathode along with high-voltage ionogel electrolyte on a single flexible substrate. By effectively coupling with battery-type anode and capacitor-type cathode, the resultant all-solid-state NIMCs working at 3.5 V exhibit a high volumetric energy density of 37.1 mWh cm-3 and an ultralow self-discharge rate of 44 h from V max to 0.6 V max, both of which surpass most reported hybrid micro-supercapacitors. Through tuning graphene layer covered on the top surface of interdigital microelectrodes, the NIMCs unveil remarkably enhanced power density, owing to the establishment of favorable multidirectional fast ion diffusion pathways that significantly reduce the charge transfer resistance. Meanwhile, the as-fabricated NIMCs present excellent mechanical flexibility without capacitance fade under repeated deformation, and electrochemical stability at a high temperature of 80 °C because of using nonflammable ionogel electrolyte and in-plane geometry. Therefore, these flexible planar NIMCs with multidirectional ion diffusion pathways hold tremendous potential for microelectronics.

Hollow carbon nanofibers as high-performance anode materials for sodium-ion batteries

Hollow carbon nanofibers (HCNFs) are successfully fabricated by pyrolyzation of a polyaniline hollow nanofiber precursor. The as-prepared HCNFs as sodium storage anode materials exhibit a high reversible charge capacity of 326 mA h g-1 at 20 mA g-1, high rate capability (85 mA h g-1 at 1.6 A g-1) and superior cycling stability (a capacity retention of 70% even after 5000 cycles at 1.6 A g-1). Such a high performance of HCNFs can be ascribed to the special hollow structure characteristics; they possess a well fabricated electronic transport path and can shorten the ion diffusion distance. Therefore, the HCNFs can be regarded as promising anode materials for advanced sodium ion batteries (SIBs).

High-Performance Sodium-Ion Battery Anode via Rapid Microwave Carbonization of Natural Cellulose Nanofibers with Graphene Initiator

Cellulose is a promising natural bio-macromolecule due to its abundance, renewability and low cost. Here, a new method is developed to prepare pre-sodiated carbonaceous anodes for sodium-ion batteries (SIBs) from cellulose nanofibers (CNFs) under microwave irradiation for potential ultrafast and large-scale manufacturing. While direct carbonization of CNFs through microwave treatment is usually impossible due to the weak microwave absorption of CNFs, it is found that a small amount of reduced graphene oxide (rGO) can act as an effective initiator. Microwaving rGO releases extremely high energy, giving rise to local ultrahigh temperature as well as ultrahigh heating rate, which then induces the fast carbonization of CNFs and the production of pre-sodiated carbonaceous materials within seconds. The sodium in the carbonaceous materials, introduced from the carbonization of CNFs containing sodium-ion carboxyl, offer favorable spaces for sodiation/desodiation, which improves the electrochemical performance of the sodium-inserted carbonaceous anode. When the microwaved rGO-CNF (MrGO-CNF) is used as an anode for SIBs, a high initial capacity of 558 mAh g^-1 is delivered and the capacity of 340 mAh g^-1 remains after 200 cycles. The excellent reversible capacity and cycling stability indicate MrGO-CNF a promising anode for sodium-ion batteries.

Capacitive Sodium-Ion Storage Based on Double-Layered Mesoporous Graphene with High Capacity and Charging/Discharging Rate

Sodium-ion batteries (SIBs) are regarded as an ideal alternative to lithium-ion batteries, but the larger radius of Na⁺ compared with Li⁺ results in lower energy density, shorter cycle life, and sluggish kinetics of SIBs. Therefore, it is of significant importance to explore appropriate Na⁺ storage materials with high capacity and fast Na⁺ transport kinetics. Herein, doublelayered mesoporous graphene nanosheets codoped with oxygen and nitrogen (O,N-MGNSs) were developed as a new cathode material with high Na⁺ storage capacity and fast ion-transport kinetics for SIBs. The codoping of MGNSs with oxygen and nitrogen by in situ chemical vapor deposition endowed them with a hierarchical porous network, robust structures, good conductivity, and abundant functional groups. The O,N-MGNSs could host Na⁺ in two ways: surface adsorption and surface redox reaction, and this endowed them with high Na⁺ storage capacity and fast charging/discharging rates in SIBs. Electrochemical results revealed that the O,N-MGNSs delivered a reversible capacity of 156 mAh g^-1 at a current density of 0.5 A g^-1 (corresponding to a rate of 3 C) between 1.5 and 4.2 V and exhibited a high cycling stability (95 % capacity retention at 1 A g^-1 for more than 1000 cycles).