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O3-type NaNi1/3Fe1/3Mn1/3O2 (NFM) is one of the most representative cathode materials with low cost and high capacity advantages. However, the sluggish diffusion kinetics and severe cathode–electrolyte interfacial reaction are serious roadblocks to commercialization. Hereby, a novel method of in situ V-modification is well-designed to play dual roles in reconstructing the crystal lattice and interface structure. Notably, the broadened layer spacing of O–Na–O and shortened TM–O bond are attributed to successful doping of V5+ into the bulk, accelerating the transmission of sodium ions and improving the structure stability. Concomitantly, the construction of a thin surface coating layer is beneficial for mitigating volume expansion and inhibiting structural degradation, which is validated by in situ X-ray diffraction coupled with the synchrotron X-ray absorption spectroscopy. Consequently, the rationally designed V-modified NFM cathode exhibits excellent cycling performances, exhibiting great capacity retention of 75.8% after 500 cycles at 2C in a half cell, and 85.6% capacity retention is observed after 150 cycles at 1C in the full cell. This work provides new insights into the development of O3-type layered oxide cathodes toward long-cycle life applications for large-scale energy storage systems. [ABSTRACT FROM AUTHOR]

Relaxor ferroelectric polymer P(VDF-TrFE-CTFE) is attracting increasing attention for electronic applications due to its high dielectric constant and relaxation characteristic. However, the low breakdown strength and premature saturation of polarization of this polymer have hampered its practical application severely. In this work, carbon quantum dots (CQDs) with functional groups and size of about 5 nm have been incorporated to overcome these issues. The nanocomposite with 0.1 wt% CQDs presents significantly improved breakdown strength and discharge energy density. Its breakdown strength (Eb) has been increased by 30.7% up to 340 kV mm−1. Due to the Coulomb blockade effect, the electrons can be restrained in the deep traps, which is favorable for the improvement of breakdown strength. Besides, the abundant functional groups around CQDs are beneficial to form adequate hydrogen bonds, resulting in increased crystallinity and breakdown strength as well. Additionally, the discharge energy density has been improved by 41.7% up to 6.8 J cm−3. The study demonstrates the effective enhancement of breakdown strength and energy density by ultralow loading of CQDs for P(VDF-TrFE-CTFE) polymer. [ABSTRACT FROM AUTHOR]

Limited by the relative radii of sodium ions (0.102 nm), sodium ion batteries suffer from sluggish kinetics. Owing to the high conductivity of selenium, Se-rich samples have captured plenty of attention. Herein, FePSe3 sheets were successfully prepared through vacuum tube sealing. Significantly, the as-resulted samples displayed considerable conductivity, further forming electrodes without conductive agents. Utilized as sodium ion storage anode with ether-based electrolytes, even at 20.0 A g−1, the ultra-stable rate abilities could be retained at about 480 mA h g−1 after 4000 cycles at room temperature (25 °C) and ∼210 mA h g−1 after 10 000 loops at low temperature (−30 °C). Assisted by a series of Cu2Se/Se samples, the strong coordination of Cu-atoms with DEGDME was noted, finally resulting in the formation of ultra-large coordination molecule Cu–(DEGDME)n. From the detailed kinetic analysis, the coordination could offer rich shared electron pairs, inducing the quickening adsorption/desorption of ions, finally resulting in the remarkable pseudo-capacitive behavior Moreover, a detailed phase transformation (Cu0 → Cu1+ → Cu2+) was carried out. Furthermore, the selection of glass fiber as separators was conducive to capturing NaPSes, improving the cycling stabilities. Given this, the work was anticipated to provide anion-rich materials for advanced SIBs anodes whilst shedding light on the in-depth reaction mechanism of metal-sulfide/selenides in ether-based electrolytes. [ABSTRACT FROM AUTHOR]

As a highly safe and low-cost cathode material for lithium-ion batteries, LiFePO4 predominately suffers from undesirable rate performance, arising from its inferior conductivity, during practical applications. Herein, LiFePO4 modified with a modulated band structure, as theoretically predicted, was successfully designed to overcome this deficiency. Notably, replacing the Li sites with Ti4+ led to tuned band structures, giving rise to an effective reduction of the forbidden bandwidth and the activated delocalization of the d-orbital electrons, as clearly observed using density functional theory calculations. Importantly, it also induces the shrinkage of the particle size, reducing the transport distance for Li+ inside the particles, as verified using Rietveld refinement of the X-ray diffraction spectra. As expected, electronic transfer and Li+ transport are synchronously accelerated thanks to the appropriate doping of Ti4+ in the olivine lattice, confirmed based on the measurement of the conductivity and calculations of the Li+ diffusion coefficient. Hence, the as-optimized cathode (0.5% Ti4+-LFP/C) exhibits an excellent rate capacity (135 mA h g−1 at 10C within 2.5–4.2 V) with superior long-term cycling stability (78% capacity retention after 3000 cycles). Based on these results, the dramatic improvement in the rate performance achieved via modulating the band structures and the ionic diffusion may provide novel opportunities for the development of olivine cathode materials. [ABSTRACT FROM AUTHOR]

Spinel LiMn2O4 is a promising cathode material for lithium-ion batteries ascribed to its steady bulk structure and abundant manganese sources. Nevertheless, severe capacity decay due to the Jahn–Teller effect and spontaneous disproportionation reaction seriously hampers its extensive application, especially at high-temperature or high-potential operation circumstances. Attributed to the strengthening of the crystallographic structure and the inhibition of Mn3+ side reactions, element substitution has become one of the most momentous tactics in the modification of LiMn2O4. Here, we comprehensively review the developments on restraining capacity fading and enhancing the performances of LiMn2O4 through element substitution. Primarily, the capacity fading mechanism of LiMn2O4 has been thoroughly recapitulated. Subsequently, our summary is proposed based on two categories, which are phase evolution and solid solution strengthening invoked by element substitution, and the latter can be subdivided into structure strengthening and Li+ diffusion kinetics boosting. More importantly, we put forward a methodological approach for future element substitution research studies. This report will shed light on the future direction of LiMn2O4 cathode material optimization. [ABSTRACT FROM AUTHOR]

As a promising cathode material for high performance lithium ion batteries, olivine LiMnxFe1−xPO4 (LMFP) combines the high safety of LiFePO4 and the high energy density of LiMnPO4. However, there are still obstacles to overcome for achieving higher rate performance, especially its inherent low electronic conductivity and Li+ diffusion coefficient. Here, the restricting factors for realizing high rate performance LMFP cathode materials are reviewed systematically. The bulk properties that affect the internal ion transport and electronic conduction are thoroughly expounded, particularly the phase transition mechanism, lattice distortion, point defects, element doping, Fe/Mn ratio and particle morphology. Moreover, the effect of utilizing a carbon-based/non-carbon material coating for improving the interface structure on rate performance is discussed comprehensively. A particular emphasis is placed on the design of LMFP-based batteries with high rate capability as well from the aspect of cell preparation technology, including the electrolyte selection and electrode design. Finally, on the basis of state-of-the-art understanding of bulk properties, the interface structure and cell preparation engineering, several technical challenges and research trends in improving the rate performance of LMFP cathode materials are proposed. [ABSTRACT FROM AUTHOR]

Rechargeable aqueous zinc-ion batteries (ZIBs) are attractive candidates for next-generation batteries. However, the challenge of uneven zinc electroplating/electrostripping on bare Zn anodes has severely restrained the practical application of ZIBs. To address this problem, a straightforward strategy of sliver particle-coated zinc plate (designated as Zn@Ag) via the metallic replacement reaction as the zinc metal anode has been employed to facilitate uniform and stable Zn-plating/stripping. The newly formed AgZn3 alloy from silver particles at the first zinc-plating cycle with good zinc affinity can effectively lower the energy barrier of zinc nucleation and promote uniform electric field distribution for the flow of Zn ions, resulting in stable zinc deposition. Benefiting from the in situ formed alloy phase, the Zn@Ag anode achieves stable cycling for over 1700 h with a very low polarization voltage of about 21 mV at 0.25 mA cm−2 and 0.25 mA h cm−2, while the bare Zn anode just exhibits less than 150 cycles with large voltage fluctuation and polarization under same conditions. As a consequence, greatly improved performance of the Zn@Ag//CNT/MnO2 full-cell with twice the capacity (177 mA h g−1) than that of bare Zn//CNT/MnO2 full-cell (71 mA h g−1) after 400 cycles at 0.6 A g−1 can be realized. This work provides a facile and effective approach to regulate Zn deposition for the achievement of long-life rechargeable ZIBs. [ABSTRACT FROM AUTHOR]

Two-dimensional transition metal dichalcogenides are promising candidates for lithium-ion and sodium-ion batteries. It is becoming more meaningful to explore the effective preparation method of these materials and study their surface, interfacial, and chemical characteristics for energy storage. Herein, low crystalline MoS2 nanoflakes embedded on N-doped CNT (MoS2/N-CNT) have been designed utilizing Mo metal as the raw material through alternating current and hydrothermal technique. Through this approach, a strong interaction has been formed between the few layers of MoS2 nanoflakes and N-CNT via chemical bonds (C–O–Mo, C–S–Mo, and C–O–S), which can shorten the ion diffusion and electronic transmission paths and even maintain the structural integrity. Meanwhile, the expanded interlayers, doped N, and the many defects in MoS2/N-CNT further make it favorable for effective energy storage. Therefore, MoS2/N-CNT presents high specific capacity, ultra-long cycling stability (1003 mA h g−1 after 800 cycles at 2 A g−1), and excellent rate capacity for lithium-ion batteries. The electrode also shows similar sodium storage performance (the capacity is 4.97 times higher than that of pure MoS2 at 10 A g−1). These results provide a novel strategy to enhance the synergistic effect of layered transition metal dichalcogenide/carbon composites with unique structural properties, which is beneficial for lithium and sodium storage. [ABSTRACT FROM AUTHOR]

In this study, N-rich carbon coated interconnected CoSnO3 nanoboxes (CoSnO3–NCs) with controllable amounts of N-rich carbon ranging from 7.5% to 24.6% were firstly prepared by the carbonization of CoSnO3–MOFs. Impressively, the preparation of CoSnO3–MOFs has been realized for the first time by directly utilizing CoSnO3 as the precursor under solvent-free conditions, bridging between the cobalt(ii) ion in the skeleton of CoSnO3 and the 2-methylimidazole. Interestingly, the growth of these CoSnO3–MOFs can be manipulated by changing the amount of 2-methylimidazole used, resulting in a tunable N-rich carbon content. Furthermore, the electrochemical storage behavior of these materials for SIBs was initially explored. Compared with the pure CoSnO3, the as-resulted CoSnO3–NC materials showed largely enhanced sodium storage performance with a reversible capacity of 493.9 mA h g−1 at a current density of 0.1 A g−1 after 100 cycles. Moreover, the as-prepared materials showed an excellent high rate storage performance with a remarkable capacity of 273.8 mA h g−1 at 1 A g−1 after 1000 cycles. This work provides a new approach for constructing Co–MOFs as well as providing an efficient N-rich carbon-coating route, which may be expanded to other Co-based oxides and can greatly expand the development of new species of Co-based MOFs. [ABSTRACT FROM AUTHOR]

Hierarchical anatase TiO2 homogeneously tuned by using carbon through Ti–C bonds has been designed, exploiting carbon quantum dots as uniform carbon additives and functionalization inducers for structure tailoring and surface modification. The fabricated pinecone-like structure constructed by ultrafine subunits presents a highly increased surface area (202.4 m2 g−1) and abundant mesopores. Surface bonded carbon significantly boosts its electronic conductivity derived from both the conductive carbon and accompanied oxygen vacancies. When utilized in sodium-ion batteries, it delivers a high reversible specific capacity of 264.1 mA h g−1 at a rate of 0.1C (33.6 mA g−1) and still maintains 108.2 mA h g−1 even after 2000 cycles at 10C with a retention of 94.7% outstandingly. Notably, its Na+ intercalation pseudocapacitive behavior is enhanced by the modulated TiO2/carbon interfaces, facilitating a fast (de-)sodiation process. Combining the elaborate hierarchical structure with the unique surface composition, synergetic merits are noticed when the promoted kinetics, improved electronic conductivity, increased electrolyte penetration areas and shortened Na+ diffusion length are achieved simultaneously, giving rise to remarkable high-rate capabilities and long-term cyclability. [ABSTRACT FROM AUTHOR]

A three-dimensional dandelion-shaped TiO2/multi-layer graphene compound (TiO2/MLG) composed of TiO2(B) fibrils and anatase pappus structures has been synthesized as a potential anode material for Li storage. Electron microscopy indicates that the composite contains triphase boundaries between anatase, TiO2(B) and graphene, which are responsible for the enhancement of energy storage and the decrease of electrode polarization. Cyclic voltammetric investigations indicate that both Li+ insertion and pseudocapacitance contribute to charge storage. Ultrahigh specific capacities of 243 and 182 mA h g−1 have been obtained at 0.1 and 1 A g−1, respectively. Moreover, the excellent capacity retention can reach 99.6% after 100 cycles with almost 100% coulombic efficiency at 0.1 A g−1. The importance of the triphase boundary in enhancing the storage of charge and transport of Li+ is demonstrated. [ABSTRACT FROM AUTHOR]

Carbon coated anatase TiO2 hollow spheres (CCAnTHSs) are prepared through the carbon wrapping of etched amorphous TiO2 solid spheres (AmTSSs). The as-obtained CCAnTHS composite is applied as an anode material for sodium-ion batteries (SIBs) for the first time, delivering excellent cycle stability. At a high current density of 5C, a reversible capacity of 140.4 mA h g−1 remained after 500 cycles. Especially, at a 25C rate it could still reach 84.9 mA h g−1 after 80 cycles. Briefly, the sodium storage performance of CCAnTHSs are superior to these of the amorphous TiO2 solid spheres and bare anatase TiO2 hollow spheres, mainly benefitting from the advantages of a unique hollow structure with a large specific surface area and a carbon coating with high electronic conductivity. [ABSTRACT FROM AUTHOR]

A C quantum dot coated Mn3O4 composite (Mn3O4/Cdots) has been obtained for the first time by a green alternating voltage electrochemical approach. It is interesting to note that the morphology of Mn3O4 particles in the composite can be induced to form an octahedral structure through the introduction of C quantum dots. In particular, the as-produced Mn3O4/Cdots composite utilized as an anode material for lithium ion batteries demonstrates excellent electrochemical performances, showing an enhanced reversible discharge capacity of 934 mA h g−1 after 50 cycles at a current density of 100 mA g−1 which is almost five times as much as that of pure Mn3O4. [ABSTRACT FROM AUTHOR]

With the aim of advancing anatase TiO2 anodes for sodium ion batteries, crystalline titania nanocubes were employed and they delivered a gradually increasing capacity during the initial cycles, termed as an activation process. The number of necessary discharge–charge loops for total activation is dependent on the galvanostatic current density (about 20 cycles at 0.2 C, or 90 cycles at 1 C). A percentage of Ti3+ was detected after the activation, indicating an amount of irreversibly trapped sodium ions in the lattice. After the activation process, an excellent rate capability and outstanding cycling stability were presented. The reversible capacity reached 174, 132, and 108 mA h g−1 at rates of 1 C, 5 C, and 10 C, respectively. The capacity was sustained with a loss of less than 10% after 1000 discharge–charge cycles at a rate of 2 C or 10 C. The superior battery performance achieved by the nanocubes is related to the encircled {100} facets that are more favorable for sodium ion attachment compared to the {001} and {101} facets, as supported by first-principles calculations. From this work we can see the feasibility of optimizing electrode materials via rational surface structure construction based on theoretical calculations. [ABSTRACT FROM AUTHOR]

N-doped TiO2 nanorods decorated with carbon dots with enhanced electrical-conductivity and faster charge-transfer have been fabricated utilizing a simple hydrothermal reaction process involving TiO2 powders (P25) and NaOH in the presence of carbon dots followed by ion exchange and calcination treatments. Due to the merits of the carbon dots, doping and nanostructures, the as-designed N–TiO2/C-dots composite utilized as anode materials for lithium-ion batteries can sustain a capacity of 185 mA h g−1 with 91.6% retention even at a high rate of 10 C over 1000 cycles. It is interesting to note that the ratios of capacitive charge capacity during such high rates for the N–TiO2/C-dots composite electrodes are higher than those at low rates, which likely explains the observed excellent rate capabilities. In contrast to lithium-ion batteries, sodium-ion batteries have gained more interest in energy storage grids because of the greater abundance and lower cost of sodium-containing precursors. The as-obtained N–TiO2/C-dots composites reported here and utilized as anode materials for sodium-ion batteries exhibit excellent electrochemical performances, including substantial cycling stabilities (the capacity retention ratios after 300 cycles at 5 C is 93.6%) and remarkable rate capabilities (176 mA h g−1 at 5 C, 131 mA h g−1 at 20 C); such performances are the greatest ever reported to date over other structured TiO2 or TiO2 composite materials. [ABSTRACT FROM AUTHOR]

Carbon quantum dots (CQDs) tuned porous NiCo2O4 sphere composites are prepared for the first time via a reflux synthesis route followed by a post annealing treatment. Benefiting from the advantages of the unique porous structure with a large specific surface area, high mesoporosity and superior electronic conductivity, the as-obtained CQDs/NiCo2O4 composite electrode exhibits high specific capacitance (856 F g-1 at 1 A g-1), excellent rate capability (83.9%, 72.5% and 60.8% capacity retention rate at 20, 50 and 100 A g-1, respectively) and exceptional cycling stability (98.75% of the initial capacity retention over 10 000 cycles at 5 A g-1). Furthermore, the assembled AC//CQDs/NiCo2O4 asymmetric supercapacitor manifests a high energy density (27.8 W h kg-1) at a power density of 128 W kg-1 or a high power density (10.24 kW kg-1) at the reasonable energy density of 13.1 W h kg-1 and remarkable cycling stability (101.9% of the initial capacity retention over 5000 cycles at 3 A g-1). The results above suggest a great potential of the porous CQDs/NiCo2O4 composites in the development of high-performance electrochemical energy storage devices for practical applications. [ABSTRACT FROM AUTHOR]

Nickel cobaltite (NiCo2O4), with excellent electrochemical performance, has become a new class of energy storage material for electrochemical supercapacitors, which facilitates to relieve the pressure of energy crisis and environmental pollution. It possesses richer electroactive sites and at least two magnitudes higher electrical conductivity than that of NiO and Co3O4, which exhibit not only large power density, but also high energy density of up to 35 W h kg-1 Furthermore, it shows comparable capacitive performances with noble metal oxides of RuO2, but with much lower cost and more abundant resources. This feature article briefly analyses the energy storage mechanism of NiCo2O4, summarizes the methodologies and nanostructures discovered in recent years, and points out the potential problems and future prospects of utilizing NiCo2O4-based materials as supercapacitor electrodes. Moreover, composite electrodes based on nickel cobaltite are also elaborated with considerable interest. Since the pioneering work of Hu and his group in 2010, numerous research studies have also demonstrated NiCo2O4 electrodes to show remarkable supercapacitive performances; however, more specialized work should be performed to further develop the potential of this novel electrode material so as to realize their massive commercial applications. [ABSTRACT FROM AUTHOR]

Layered Na2FePO4F is utilized as a cathode in hybrid-ion batteries in order to explore the ion migration and diffusion capability. It is the first time that the ion migration mechanism and capability in a hybrid-ion battery is investigated by considering the activation energies of different migration ways. It is proposed that a rapid ion exchange of Na+ ions on the Na(2) site of the crystal structure with Li+ ions can take place to produce the NaLiFePO4F phase and is firstly confirmed by first principle calculations. Li+ ion conduction in NaLiFePO4F is prone to be two-dimensional (2D) in the interlayer plane with an essentially restricted migration along the [010] direction for interlayer transport due to the much higher energy value (4.53 eV for sodium ion and 1.63 eV for lithium ion). Additionally, the 2D ways which need lower activation energies along [100] and [001] directions and the small volume variation during redox cycling are responsible for the large diffusion capability with a maximum magnitude of 10−10 cm2 s−1. [ABSTRACT FROM AUTHOR]