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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]

The TiO2 anode for sodium-ion batteries (SIBs) is currently drawing increased attention owing to its stable cycling performance. However, the storage performance is still extremely inadequate, especially for its low initial coulombic efficiency (ICE). In order to further improve the ICE, a good match of the electrolyte and binder is crucial, due to the fact that a solid electrolyte interface (SEI) is irreversibly formed on the surface, which will be very important for its reversible capacity. As an attempt, TiO2 nanocrystallites are prepared by an electrochemical method. When applied as the anode for SIBs, the TiO2 electrode with the sodium carboxymethylcellulose binder and ether-based electrolyte presents an impressive capacity of 166.23 mA h g−1 at 2C after 4800 cycles, much higher than that with the polyvinylidene fluoride binder and carbonate-based electrolyte. A series of characterization studies reveal that the charge transfer at the interface between the electrolyte and electrode and the SEI film are the key factors determining the electrochemical properties. Importantly, the dual optimization of the binder and electrolyte is revealed to be an effective means to enhance the ICE of TiO2 up to 81.6%, thus greatly contributing to better guidance on the design of TiO2 anodes with ultra-high ICE for SIBs. [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]

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]

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]

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]

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]