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Magnesium ion batteries have attracted increasing attention as a promising energy storage device due to the high safety, high volumetric capacity, and low cost of Mg. However, the strong Coulombic interactions between Mg2+ ions and cathode materials seriously hinder the electrochemical performance of the batteries. To seek a promising cathode material for magnesium ion batteries, in this work, (NH4)2V6O16·1.5H2O and water-free (NH4)2V6O16 materials are synthesized by a one-step hydrothermal method. The effects of NH4+ and lattice water on the Mg2+ storage properties in these kinds of layered cathode materials are investigated by experiments and first-principles calculations. Lattice water is demonstrated to be of vital importance for Mg2+ storage, which not only stabilizes the layered structure of (NH4)2V6O16·1.5H2O but also promotes the transport kinetics of Mg2+. Electrochemical experiments of (NH4)2V6O16·1.5H2O show a specific capacity of 100 mA·h·g-1 with an average discharge voltage of 2.16 V vs Mg2+/Mg, highlighting the potential of (NH4)2V6O16·1.5H2O as a high-voltage cathode material for magnesium ion batteries.

The unique properties of MXenes that arise from terminating functional groups and oxidation of MXenes make them attractive for application in photovoltaic devices like perovskite solar cells (PSCs). Here, oxidation of Ti3C2Tx hydrocolloid was carried out to tune its properties desirable for an electron transport layer (ETL) in low-temperature processed PSCs. The calculations of the energy levels were carried out using the Vienna ab initio simulation package (VASP) code based on density functional theory (DFT). Oxidation of Ti3C2Tx can generate Ti–O bonds and effectively reduce the macroscopic defects of the film fabricated by spin-coating, while a transition from metallic material to semiconductor occurred after heavy oxidation. A better matching of energy levels between perovskite and ETL layer in the case of a hybrid of oxidized and pristine Ti3C2Tx renders a champion power conversion efficiency (PCE) of 18.29%. The improvement in PCE can be attributed to the increased electron mobility in the ETL, which promotes electron transport and reduces the electron–hole recombination. Hence, by presenting a simple method for high performance in PSCs by MXene-derived materials, this work demonstrates the great potential of these materials for applications in low-temperature processed PSCs and other photovoltaic technologies.

NH4+ preintercalated V2O5·nH2O nanobelts with a large interlayer distance of 10.9 A were prepared by the hydrothermal method. The material showed a large specific capacity of 391 mA·h·g-1 at the 500 mA·g-1 current density in aqueous rechargeable zinc batteries. In operando synchrotron X-ray diffraction demonstrated that the material experienced reversible solid-solution reaction and two-phase transition during charge-discharge cycling, accompanied by the reversible formation/decomposition of a ZnSO4Zn3(OH)6·5H2O byproduct. In operando X-ray absorption spectroscopy confirmed the reversible reduction/oxidation of V, together with small changes in the VO6 local structure. The formation of byproduct was attributed to the dehydration of [Zn(H2O)6]2+, which concurrently improved the desolvation of [Zn(H2O)6]2+ into Zn2+. Bond valence sum map analysis and electrochemical impedance spectroscopy demonstrated that the byproduct improved the charge transfer kinetics of the electrode. Cyclic voltammetry and galvanostatic intermittent titration technique showed that the electrode reaction was dominated by ionic intercalation where the discharge capacity in the voltage window of 1.4-0.85 V was attributed to the intercalation of [Zn(H2O)6]2+, followed by the intercalation of Zn2+ at 0.85-0.4 V.

Sodium (Na) metal batteries have attracted increasing attention and gained rapid development. However, the processing, storing, and application of Na metal anodes are restricted by its inherent stickiness and poor mechanical properties. Herein, an MXene (Ti3C2Tx)-coated carbon cloth (Ti3C2Tx-CC) host is designed and synthesized, which shows a highly metallic conductive and sodiophilic surface. After a thermal infusion treatment, a Na-Ti3C2Tx-CC composite with rigidity and bendability is obtained and employed as a metal anode for Na ion batteries. The Na-Ti3C2Tx-CC electrodes present stable cycling performance and high stripping/plating capacity in both an ether-based (up to 5 mA·h·cm-2) and a carbonate-based (up to 8 mA·h·cm-2) electrolyte. The fundamental protection mechanism of MXene Ti3C2Tx is investigated. Ti3C2Tx efficiently induces Na's initial nucleation and laterally oriented deposition, which effectively avoids the generation of mossy/dendritic Na. The arrangement of Na atoms deposited on the MXene surface inherits the MXene atomic architecture, leading to a smooth "sheet-like" Na surface. Meanwhile, a flexible Na-based Na-Ti3C2Tx-CC∥Na3V2(PO4)3 device is assembled and exhibits capable electrochemical performance.

Hydrogen evolution reaction (HER) is vital for sustainable energy production and plays a key role in achieving a hydrogen economy. Herein, density functional theory calculations are used to screen ...

Soluble redox mediators (RMs) are one of the most promising approaches for reducing charging overpotentials in Li–O2 batteries. However, this auspicious strategy still in its infancy and raises new scientific problems needing to be clarified, such as the influence of RMs with different charge–transfer or different molecular structure (same redox functional group) on Li2O2 oxidation behavior. Herein, the realities of Li2O2 oxidation by some RMs, including lithium bromide, tetrathiafulvalene, 2,2,6,6–tetramethyl–1–piperidinyloxy, and 2–azaadamantane–N–oxyl, were investigated using detailed experimental results and first–principles calculations. Among these RMs studied, single electron–reaction RMs exhibited a more stable charging curve at lower potential than that of multiple electron–reaction RMs. Besides, the RM molecular with smaller steric effects and higher electron–donating power exhibited higher catalytic activity thus a lower charging overpotential. These findings offered a guidance direction for subsequent explorations and optimization of high performance RMs, which might further facilitate development for Li–O2 batteries.

Magnesium ion batteries (MIBs) have attracted increasing attention due to their advantages of abundant reserves, low price, and high volumetric capacity. However, the large Coulombic interactions o...

The ORR and OER properties of a series of recombinant single atom catalysts (SACs) prepared by recombining Pt single atoms on 26 representative MXenes were comprehensively studied by first-principles calculations. The stability of Pt atoms on the MXene surface was studied using formation energies and diffusion energy barriers. Charge transfer analysis showed that the Pt atoms not only acted as the catalytic center of the SACs but also behaved as a charge transfer medium between the MXene substrate and the reactants. The catalytic properties of the recombinant SACs were dependent on several interacting factors including the Pt-5d states, the work functions of the recombinant systems, the electronegativity of the submetals, and the vacant electron orbitals of the C/N and O/F elements of the MXenes. In all the recombinant SACs under investigation, V-, Ti-, Nb-, and Cr-based MXenes, including Ti2CF2-VF-Pt, Ti3C2F2-VF-Pt, V2CO2-VO-Pt, Nb2CF2-VF-Pt, Nb4C3F2-VF-Pt, Cr2TiC2F2-VF-Pt, Ti3(C,N)2-CO2-VO-Pt, and Ti3(C,N)2-NO2-VO-Pt, were screened as promising ORR catalysts. In particular, three F-terminated ones (Nb2CF2-VF-Pt, Nb4C3F2-VF-Pt, and Cr2TiC2F2-VF-Pt) were proposed as effective ORR/OER bifunctional catalysts. The results revealed the highly active nature of the selected SACs and highlighted the great potential of MXenes as efficient ORR and OER catalysts.

NASICON-structured Na4MnV(PO4)3 has been recognized as a potential positive electrode material for sodium-ion batteries, but its electrochemical mechanism during de(sodiation) has not been well understood. In this work, the structural transformation, charge transfer, and ionic diffusion properties of Na4MnV(PO4)3 were comprehensively studied by first-principles calculations combined with experimental studies. The results revealed two independent Na sites, Na(1) and Na(2), in the structure of Na4MnV(PO4)3, but only Na(2) can be extracted between 2.5 and 3.8 V. Extraction of the first Na+ caused charge transfer on V3+ and was associated with a solid-solution reaction. In addition, Na+ migrated along the 3D channels in the NASICON structure with low energy barriers of

Lithium metal batteries are promising next generation energy storage devices. However, uncontrolled lithium dendrite growth and inevitable side reactions of traditional organic liquid electrolytes with electrodes are obstacles to their practical applications. Herein, a new ionogel electrolyte with an organic–inorganic semi-interpenetrating network is designed by the confinement of ionic liquid within a NH2 pendent group optimized cross-linked poly(ionic liquid) copolymer backbone and glass fiber scaffold. The ionogel electrolyte shows superior physicochemical properties, including improved lithium ion transmission, high mechanical strength, wide electrochemical window, non-leakage, non-volatility and fire resistance. In Li//Li symmetric cells fabricated with this ionogel electrolyte, repeated Li plating/stripping could last over 1800 h without significant dendrite formation. Besides, the full cells paired with a high-voltage Li3V2(PO4)3 cathode present excellent cycling stability with a capacity retention of 83% after 1000 cycles (0.5C rate, 3.0–4.3 V) and 91% after 100 cycles (0.2C rate, 3.0–4.8 V). This study presents a new strategy for the use of organic–inorganic semi-interpenetrating networks for designing new composite ionogel electrolytes with desirable properties for high-voltage LMBs.

Developing highly active, stable, and conductive bifunctional oxygen reduction (ORR) and oxygen evolution (OER) catalysts is a key step for fuel cells and metal–air batteries. Herein, an effective idea for designing bifunctional catalysts is presented by regulating the surface electronic structures of Nb2CT2 (T = O, F, and OH) using Pt/Pd single atoms. The results indicated that Pt-doped systems (Nb2CO2–VO–Pt, Nb2CF2–VF–Pt) were the most promising bifunctional ORR/OER catalysts. In particular, Nb2CF2–VF–Pt was even better than landmark Pt(111) and IrO2(110) catalysts, with relatively low overpotentials of 0.40 V and 0.37 V for ORR and OER, respectively. The high catalytic nature of Nb2CF2–VF–Pt was explained by electronic structures, volcano plots, and charge transfer mechanisms, which mainly depended on the electron donor capacity and synergistic effects from F-terminated groups and Pt noble metals. Moreover, 100% utilization of Pt was achieved for the designed bifunctional catalysts with a minimum radius between two adjacent active centers. This was the first design of a bifunctional ORR/OER catalyst based on Nb2CT2 and highlighted a new perspective on the application of MXenes.

A novel metallic carbon allotrope, Q-carbon, was discovered using first-principles calculations. The named Q-carbon possessed a three-dimensional (3D) cage structure formed by carbon atoms with three ligands. The energy distribution of electrons in different orbitals revealed that Q-carbon has a low degree of s-p orbital hybridization. The calculated Li

Metallic anodes have high theoretical specific capacities and low electrochemical potentials. However, short-circuit problems caused by dendritic deposition and low Coulombic efficiency limit the cyclic life and safety of metallic anode-based batteries. Herein, dendrite-free and flexible three-dimensional (3D) alkali anodes (Li/Na-Ti3C2Tx-rGO) are constructed by infusing molten lithium (Li) or sodium (Na) metal into 3D porous MXene Ti3C2Tx-reduced graphene oxide (Ti3C2Tx-rGO) membranes. First-principles calculations indicate that large fractions of functional groups on the Ti3C2Tx surface lead to the good affinity between the Ti3C2Tx-rGO membrane and molten alkali metal (Li/Na), and the formation of Ti-Li/Na, O-Li/Na, and F-Li/Na mixed covalent/ionic bonds is extremely critical for uniform electrochemical deposition. Furthermore, the porous structure in Li/Na-Ti3C2Tx-rGO composites results in an effective encapsulation, preventing dendritic growth and exhibiting stable stripping/plating behaviors up to 12 mA·cm-2 and a deeper capacity of 10 mA·h·cm-2. Stable cycling performances over 300 h (750 cycles) at 5.0 mA·cm-2 for Li-Ti3C2Tx-rGO and 500 h (750 cycles) at 3.0 mA·cm-2 for Na-Ti3C2Tx-GO are achieved. In a full cell with LiFePO4 cathodes, Li-Ti3C2Tx-rGO electrodes show low polarization and retain 96.6% capacity after 1000 cycles. These findings are based on 2D MXene materials, and the resulting 3D host provides a practical approach for achieving stable and safe alkali metal anodes.

Transition metal oxides and sulfides have been intensively investigated as host materials for the S cathode in lithium-sulfur (Li-S) batteries; however, the distinctions between them in battery operation have remained unclear. In this study, VO2 and VS2 nanosheets were systematically studied as host materials for Li-S batteries through theoretical calculations and experimental testing. First-principles calculations demonstrated that VS2 showed more favorable properties, including the inherent semi-metallic conductivity of VS2 , moderate adsorption strength for Li2 Sn , fast Li+ transport with a low diffusion barrier, and accelerated surface redox reactions with a low Li2 S decomposition barrier. In comparison, the low electronic conductivity and strong adsorption strength of VO2 increased Li+ diffusion as well as Li2 S decomposition barriers of the electrode, resulting in relatively poor rate capability and cycle stability. In experiments, the VS2 @S electrode exhibited superior electrochemical performance compared with VO2 @S, giving a large capacity of 713 mAh g-1 at 5 C and a low capacity fading rate of 0.13 % per cycle over 200 cycles at 1 C. The constructed relationships between S cathode and host materials could guide the future design of high-performance S cathodes for Li-S batteries.

A modified gel polymer electrolyte (GPE) with boron nitride (BN) additive was designed to boost the electrochemical performance of rechargeable lithium metal battery, which consists of Li-rich layered cathode and possesses a high energy density. BN, electron insulator and ion conductor, has excellent thermodynamic and electrochemical stability. BN particles were well dispersed in polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) polymer matrices and thus substantially enhanced the electrochemical and physical properties of the GPEs. With only 0.5 wt% BN additive, it remarkably improved the mechanical modulus and ionic conductivity (to 4.1 × 10−4 S∙cm−2) of GPEs, which was beneficial for suppressing lithium dendrite formation and fast ion transportation, respectively, thereby enabling high-performance lithium metal batteries with ultra-long cycle life and high safety at ambient temperature (25 °C) and high temperature (55 °C).

Multiple negative factors, including the poor electronic conductivity of sulfur, dissolution and shuttling of lithium polysulfides (Li2Sn), and sluggish decomposition of solid Li2S, seriously hinder practical applications of lithium-sulfur (Li-S) batteries. To solve these problems, a general strategy was proposed for enhancing the electrochemical performance of Li-S batteries using surface-functionalized Ti3C2 MXenes. Functionalized Ti3C2T2 (T = N, O, F, S, and Cl) showed metallic conductivity, as bare Ti3C2. Among all Ti3C2T2 investigated, Ti3C2S2, Ti3C2O2, and Ti3C2N2 offered moderate adsorption strength, which effectively suppressed Li2Sn dissolution and shuttling. This Ti3C2T2 exhibited effective electrocatalytic ability for Li2S decomposition. The Li2S decomposition barrier was significantly decreased from 3.390 eV to ∼0.4 eV using Ti3C2S2 and Ti3C2O2, with fast Li+ diffusivity. Based on these results, O- and S-terminated Ti3C2 were suggested as promising host materials for S cathodes. In addition, appropriate functional group vacancies could further promote anchoring and catalytic abilities of Ti3C2T2 to boost the electrochemical performance of Li-S batteries. Moreover, the advantages of a Ti3C2T2 host material could be well retained even at high S loading, suggesting the potential of surface-modified MXene for confining sulfur in Li-S battery cathodes.

Due to the high discharge voltage and a favorable theoretical capacity, Na2FePO4F have been attracted much attention as the viable cathode materials for sodium-ion storage. However, the low intrinsic electronic conductivity of Na2FePO4F suppresses its practical applications. Herein, ion doping strategy is employed to improve the electrochemical performance of Na2FePO4F as the cathode of sodium ion batteries. We first used density functional theory (DFT) calculation to screen the optimum dopants by evaluating the structural, electronic and electrochemical properties of metal-doped Na2FePO4F. Our calculation results indicate that the Co-doped Na2FePO4F is the most promising candidate for practical applications. To further prove the validity of metal doping, Na2Fe0.94Co0.06PO4F/C was synthesized by sol-gel method and showed superior electrochemical performance compared with the pristine material. The specific capacity of the Na2Fe0.94Co0.06PO4F/C is 99.93 mAh g−1 at 0.2 C and the discharge capacity retention is 62.1% after 400 cycles at 1 C.

High-performance solid-state electrolytes with healability to repair mechanical damages are important for the fabrication of Li-ion batteries (LIBs) with enhanced safety and prolonged service life. In this study, we present the fabrication of healable, highly conductive, flexible, and nonflammable ionogel electrolytes for use in LIBs by loading ionic liquids and Li salts within a hydrogen-bonded supramolecular poly(ionic liquid) copolymer network. The ionogel electrolytes exhibit ionic conductivities as high as 10

Two-dimensional (2D) metal carbides, MXene, present the promising application for the energy storage system. Among the MXene family, Ti2CTx as the lightest material, shows its unique electrochemical performance. Herein, Ti2CTx is synthesized by selective etching Al layer from the Ti2AlC. With the optimized HF treating condition, Ti2CTx displays high volumetric capacitance and remarkable rate ability. Moreover, the Ti2CTx//Ti2CTx symmetric supercapacitor is designed and assembled, which presents capable capacitance, outstanding rate performance and excellent cycling performance. The remarkable electrochemical performance is attributed to its 2D structure and high electronic conductivity. This work demonstrates the potential application of the Ti2CTx for the supercapacitors and provides a template to design high-performance supercapacitors with 2D electrode materials.

Cobalt sulfides based on conversion mechanisms are considered as promising anode materials for sodium-ion batteries due to their appropriate working voltage and high practical capacities. But the severe volume change and structure transformation make their cycle stability and rate capability unsatisfactory. In this study, metal-organic framework derived Co9S8@carbon yolk-shell nanocages (Co9S8@CYSNs) was prepared and its direct conversion mechanism was carefully demonstrated for the first time by various spectroscopic techniques and first-principles calculations. The unique hierarchical structure of Co9S8@CYSNs composed of Co9S8 nanoparticles dispersed in amorphous carbon matrix inside a rigid carbon shell was capable of accelerating the conversion reaction, shortening the Na+ diffusion distance and providing a fast electron transport channel. Benefiting from the accelerated electrochemical reactions and high activities of nanosized particles, the Co9S8@CYSNs exhibited a large discharge capacity of 549.4 mA h g-1 at 0.1 A g-1. In addition, a superior rate performance of 100 mA h g-1 at 10 A g-1 and excellent cycle stability with a very low capacity decay of 0.019% per cycle over 800 cycles at 10.0 A g-1 were achieved because of the confine effect of the carbon shell and improved charge transfer reactions of the electrode.

The insufficient charge extraction in chlorophyll-based solid-state solar cells (CSSCs) limits the photovoltaic performance, resulting in low photon-to-electron conversion efficiency. In this work, we employ poly(3-hexylthiophene) (P3HT) as a hole transporter to improve the charge extraction in CSSCs with a carboxylated chlorophyll sensitizer (H2Chl-1) adsorbed on mesoporous TiO2 as an electron acceptor and self-aggregates of a zinc chlorophyll derivative (ZnChl-2) as an electron donor. P3HT enhances the photon-to-electron conversion in both 300–540 nm and 660–725 nm wavelength regions. The charge recombination of CSSCs was suppressed by addition of P3HT to the ZnChl-2 aggregate layer that is spin-coated on H2Chl-1 adsorbed TiO2, as evidenced by the increased recombination resistance in the electrochemical impedance spectroscopy. As a result, the incident photon-to-electron conversion efficiency of the redmost peak of CSSCs with P3HT layer achieves a maximum value of 70.8%, and the power conversion efficiency is substantially enhanced from 2.1% to 3.1%.

Inspired by the mimic of dual charge separation in natural photosynthesis systems for biosolar cells (BSCs) applications where the charge extraction at the interface was insufficient, we report chlorophyll (Chl)-based trilayer cascade BSCs composed of Chl-C1 and/or a Chl-C2 (Chl-C)-sensitized mesoporous TiO2 layer (TiO2–Chl-C) acting as electron extraction layers as well as active layers to simulate the primary electron acceptor and two additional Chl derivatives layers (Chl-A/Chl-D) mimicking the reaction centers of photosystems I and II in the Z-scheme process of natural oxygenic photosynthesis. All of the Chls could be excited to generate photocurrent, which is similar to the natural photosynthesis of multipigment synergy. The charge extraction has a significant enhancement compared to bilayer BSCs. Co-sensitization of Chl-C1 with Chl-C2 could further enhance the electron injection efficiency at the TiO2–Chl-C interface, leading to a power conversion efficiency of 4.14%, which is more than 3 times larg...

First-principles calculations combined with density functional theory were performed to illuminate the electrochemical properties of NaVPO4F. During desodiation to VPO4F, a ∼11% volume change was observed, which was ∼2% greater than that from LiVPO4F to VPO4F. An intermediate phase was observed while examining the structural stability during Na+ extraction from NaVPO4F. The voltage profile showed a distinct charging plateau positioned at ∼4.0 V. Bader charge analysis elucidated the reduction of charge-oriented V cations during Na+ extraction. The achieved electron density profiles were examined to analyze the influence of Na+ extraction on V–F and V–O bonds during the desodiation process. The most facile diffusion pathway for Na+ was discerned, with a minimum energy barrier of 0.85 eV. On the basis of these results, NaVPO4F was suggested as a promising cathode material for Na-ion batteries.

VC2, a new two-dimensional transition metal carbide containing C2 dimers, was predicted by the swarm-intelligent global-structure search method. The structural properties and Li+ storage ability of VC2 monolayers and stacked VC2 multilayers were systematically investigated by first-principles calculations, and the high structural stability and electronic conductivity of the materials suggested promising Li+ storage properties. VC2 monolayers showed a theoretical capacity of 1073 mA h g−1 based on multilayer Li+ adsorption, while stacked VC2 showed an even larger theoretical capacity of 1430 mA h g−1. Intercalated Li+ formed ordered arrangements between VC2 layers, retaining a well-ordered layered structure. Li+ near the VC2 layer formed ionic bonds with the host material, while Li in middle layers formed metallic Li–Li bonds. All Li+ was stored in the interlayer space with low diffusion barriers, which demonstrated high rate capability of the material for lithium ion batteries. Remarkably, the predicted VC2 carbide achieved more than 1000 mA h g−1 capacity irrespective of being in monolayer or stacked layer structures, which rendered them very convenient for practical material preparation and battery applications.

We fabricated a chlorophyll (Chl)-based biosolar cell with H2Chl-sensitized TiO2 as an acceptor and (ZnChl)n as a donor. This solar cell gives a relatively high quantum yield from the absorption spectral contribution from both the donor and acceptor species. We employed subpicosecond time-resolved absorption spectroscopy (TAS) to study the excited state dynamics at the Chl interface. A charge transfer (CT) state between TiO2-H2Chl and (ZnChl)n was observed at 640 nm after excitation at the Qy peaks, 680 nm and 720 nm. This CT state is entirely different from the CT states observed for either TiO2-H2Chl (TiO2-H2Chl/spiro-OMeTAD) or TiO2-(ZnChl)n systems. Due to the slower charge transfer process from H2Chl+ to TiO2 as compared to that from (ZnChl)n+ to H2Chl, the CT lifetimes of H2Chl--(ZnChl)n+ (τ1 = 0.1 ps, τ2 = 1.4 ps) excited at 720 nm are slightly shorter than that excited at 680 nm (τ1 = 0.2 ps, τ2 = 5.6 ps). The TAS results suggest that the interface of TiO2-H2Chl and (ZnChl)n not only transfers holes as spiro-OMeTAD does, but also provides a built-in field for charge dissociation between the two Chl species.

Identifying high performance electrode materials particularly with a large capacity and appropriate working voltage is one of the most promising approaches for improving the energy density of rechargeable batteries. Herein, a tetra-VN2 monolayer with intrinsic thermal/dynamic stability and excellent electronic conductivity is described that was identified using energy and stability directed screening as a potential electrode material for rechargeable alkali ion batteries. The maximum alkali ion storage was found for Li2VN2, Na4VN2 and K4VN2, which corresponded to specific capacities of 679, 1358 and 1358 mA h g−1. The average working voltages of tetra-VN2 in Li-, Na-, and K-ion batteries were 2.59, 1.59, and 1.62 V, which produced specific energies of 1761, 2162, and 2206 W h kg−1, which were much larger than those of most well-known cathode materials. This suggested that the tetra-VN2 monolayer could be a promising alkali ion storage material for high energy rechargeable batteries. Interestingly, different from intercalation-type cathode materials, alkali ions were stored in the tetra-VN2 monolayer via an adsorption/desorption process. With this surface storage mechanism, the alkali ions could migrate in the electrode with low energy barriers, which were found to be 0.237, 0.018, and 0.075 eV for Li+, Na+, and K+, respectively. This feature was representative of the excellent rate capability of tetra-VN2 in rechargeable batteries.

Recently, a large family of 2D materials called MXenes have attracted much attention in the field of supercapacitors, thanks to the excellent performance demonstrated by Ti3C2 MXene-based electrodes. However, research on MXenes for supercapacitor applications has been primarily focused on Ti3C2, even though there are more than 20 other members of this large family of materials already available. Studies on other MXenes are emerging, with promising results already achieved by Ti2C, Mo2C, and Mo1.33C in aqueous electrolytes. Yet, many other MXenes remain unexplored in aqueous supercapacitor applications. In this work, we report on the electrochemical behavior of a vanadium carbide MXene, V2C, in three aqueous electrolytes. Excellent specific capacitances were achieved, specifically 487 F/g in 1 M H2SO4, 225 F/g in 1 M MgSO4, and 184 F/g in 1 M KOH, which are higher than previously reported values for few micrometer-thick delaminated MXene electrodes. This work shows the promise of V2C MXene for energy storage using aqueous electrolytes. Keywords: MXene, Supercapacitor, Vanadium carbide, Two-dimensional material, Aqueous electrolyte, Pseudocapacitance

Journal of materials chemistry / A 8(19), 9567 - 9578 (2020). doi:10.1039/D0TA00858C, Monoclinic VO$_2$(B) nanoflakes prepared by a hydrothermal method displayed superior electrochemical performance in 1 M ZnSO$_4$ electrolyte. The reaction mechanisms of VO$_2$(B) and the essential causes of byproduct formation in aqueous rechargeable zinc batteries (ARZBs) were comprehensively studied by electrochemical measurements combined with in operando synchrotron techniques and first-principles calculations. During the electrochemical processes, the electrode underwent a reversible solid-solution reaction between VO$_2$(B) and Zn$_{0.44}$VO$_2$ with the simultaneous formation/decomposition of the (Zn(OH)$_2$)$_3$(ZnSO$_4$)��5H$_2$O byproduct. Importantly, the formation of the byproduct was attributed to [Zn(H$_2$O)$_6$]$^{2+}$ dehydration, where the byproduct could protect the electrode material from the corrosion of H$_3$O$^+$ and facilitate the dehydration process of Zn$^{2+}$ on the electrode���electrolyte interface. The byproducts could facilitate the migration of Zn$^{2+}$ on the electrode surface due to their three-dimensional pathways. In addition, the electrochemical performance of VO$_2$(B) and the byproduct in ZnSO$_4$ electrolyte were compared with those in Zn(CF$_3$SO$_3$)$_2$ and Zn(NO$_3$)$_2$. An appropriate electrolyte (1 M Zn(CF$_3$SO$_3$)$_2$) to form a byproduct with largely expanded ionic pathways was proven to further improve the electrochemical performance of VO$_2$(B). This work not only provides a deep understanding of the Zn$^{2+}$ storage mechanism in VO$_2$(B) but also establishes a clear relationship between the byproducts and electrochemical performance of vanadium-based electrode materials in ARZBs., Published by RSC, London ��[u.a.]��

Nanosheet assembled hollow ZnFe2O4 microspheres have been successfully prepared using a general hydrothermal method together with post-annealed process. As the anode electrode of a lithium-ion battery, the high discharge capacity of 1200 mAh g−1 can be obtained after 120 cycles at a current density of 100 mA g−1. It is noted that the cell capacity still remains at 533 mAh g−1 after 250 cycles at a high current density of 500 mA g−1. The prominent electrochemical performance is related to the unique mesoporous hollow structure, which increases the contact areas of the electrolyte and active material resulting in well electrolyte permeation as well as providing abundant void space to suppress the volume expansion during the lithium-ion insertion and extraction process.

Nanostructures can improve battery capacity and cycle life, especially with sulfide electrodes. In this work, a freestanding flexible electrode, consisting of MnS nanoparticles embedded onto carbon nanofibers, was prepared by electrospinning. The produced hybrid was used as an electrode for lithium-ion and sodium-ion batteries. MnS nanoparticles have a size of about 5 nm and the particles are evenly distributed in the carbon nanofibers. Carbon nanofibers act as electronic conductors and buffer the volume change, while MnS nanoparticles react through rapid electrochemical reaction. As a Li-ion battery anode, this hybrid electrode exhibits specific capacities from 240 mAh g-1 at a high current density of 5 A g-1 , up to 600 mAh g-1 at 200 mA g-1 .

Chlorophylls are the most abundant bipolar organic semiconductor found in nature. We demonstrated novel bulk-heterojunction organic-inorganic solar cells (OISCs) in which a carboxylated chlorophyll derivative (H2Chl-1) sensitizing mesoporous TiO2 functioned as an n-type electron acceptor and self-aggregates of zinc chlorophyll (ZnChl-2) functioned as a p-type electron donor. In this work, we employed chloroform (CF), chlorobenzene (CB), and their mixtures as processing solvents to manage the morphology of ZnChl-2 aggregates and the pore-filling of ZnChl-2 into mesoporous TiO2. ZnChl-2 aggregates generated from a mixed solvents (CF:CB = 2:1) exhibited the smallest surface roughness and most efficient charge separation and the power conversion efficiency of OISCs was substantially improved to 2.13%. Importantly, the incident photon-to-current conversion efficiency profiles of these OISCs consisted of the spectral components of both TiO2-attached H2Chl-1 and self-aggregated ZnChl-2, indicating that efficient charge separation can occur at the interface between these two chlorophyll species.

Na2+2xFe2-x(SO4)3 (NFS) is recently reported as a promising cathode material for Na-ion batteries with high voltage generation and high energy density, making it competitive to sodium battery cathodes. Nevertheless, owing to its insulating nature, a formidable challenge remains as to realize high rate capability. Herein, Na2+2xFe2-x(SO4)3/reduced graphene oxide (rGO) has been designed and realized by a facile one-stepped synthesis. Its structural, morphological and electrochemical properties have been characterized and compared with those of the bare NFS material. As a result, NFS/rGO cathode demonstrates enhanced electrochemical performance, as reflected by the initial specific capacity of 90 mAh g−1 and a high specific capacity of 80 mAh g−1 over 100 cycles at a current density of 0.1 C (1 C = 120 mA g−1), an excellent rate capability with specific capacity of 60 mAh g−1 at high current density of 20 C (2.4 A g−1) and long-term cycle life with capacity retention of 84% over 500 cycles at 20 C rate (2.4 A g−1). This improvement may be attributed to the high electronic conductivity of rGO, which facilitates the charge transfer reactions between the active materials and electrolyte interface, and promotes the sodium ion diffusion in the electrode. Therefore, Na2+2xFe2-x(SO4)3/rGO is favorable to realizing superior electrochemical performance for the sulfate, which presents a significant step forward in the development of low-cost large-scale batteries.

Selenium (Se) is a prospective candidate of cathode material for high-energy batteries. However, the dissolution of high-order lithium selenides and the shuttle effect in electrolytes lead to low Se utilization, inferior capacity and poor cycling performance. Herein, a heteroatoms (N and O) dual-doped hierarchical porous carbon (HDHPC) with interconnected macro/meso/micropores is used to confine Se composite and developed as a cathode material for advanced Li/Na–Se batteries in low-cost carbonate electrolyte. For Li-Se batteries, Se/HDHPC cathode delivers an initial charge capacity of 613 mAh g−1 and maintains stable reversible capacity of 545 mAh g−1 with a capacity decay as low as 0.0074% per cycle after 1500 cycles at a current density of 0.5 C (1 C = 675 mA g−1). For Na-Se batteries, Se/HDHPC exhibits a high initial charge capacity of 619 mAh g−1 and retains as high as 402 mAh g−1 after 500 cycles at a current rate of 0.5 C. Moreover, Se/HDHPC composite also exhibits an excellent rate performance both in the Li-Se and Na-Se batteries even at a high rate up to 20 C (13.5 A g−1). Such excellent electrochemical properties of Se/HDHPC are mainly contributed from two aspects. One is the hierarchical porous structure of the HDHPC, which benefit the penetration of electrolyte and also act as micro reaction chambers to effectively suppress the volume change of the electrode. The other is the strong interaction between lithium/sodium selenide and carbon host induced by heteroatoms (N and O) dual-doping, which can strongly anchor the Se-contained species, thus preventing the side reaction of Se anions with electrolyte.

LiFePO4 @C/reduced graphene oxide (rGO) hierarchical microspheres with superior electrochemical activity and a high tap density were first synthesized by using a Fe3+ -based single inorganic precursor (LiFePO4 OH@RF/GO; RF=resorcinol-formaldehyde, GO=graphene oxide) obtained from a template-free self-assembly synthesis followed by direct calcination. The synthetic process requires no physical mixing step. The phase transformation pathway from tavorite LiFePO4 OH to olivine LiFePO4 upon calcination was determined by means of the in situ high-temperature XRD technique. Benefitting from the unique structure of the material, these microspheres can be densely packed together, giving a high tap density of 1.3 g cm-3 , and simultaneously, defectless LiFePO4 primary nanocrystals modified with a highly conductive surface carbon layer and ultrathin rGO provide good electronic and ionic kinetics for fast electron/Li+ ion transport.

The hole transport layer (HTL) plays an important role in perovskite solar cells (PSCs). We demonstrate the cupper iodide (CuI)/poly (3,4-ethylenedioxythiophene) poly (styrenesulphonate) (PEDOT:PSS) double layers as the anode buffer layer for efficient hole transport in PSCs. An obvious enhancement of open-circuit voltage and power conversion efficiency (PCE) is observed, and a final PCE of 14.3% is achieved. While the single PEDOT:PSS based device’s PCE is 12.9%. The inorganic CuI buffer layer significantly enhanced the hole extraction from the perovskite film, as revealed from the photoluminescence spectra. The average exciton lifetime is reduced to 2.7 ns. Moreover, the devices with double HTLs structure exhibit better long time stability. The PCE remains 88% of the initial value after 720 h’ storage.

The electrochemical properties of a Li-excess Li1.18Ni0.15Co0.15Mn0.52O2 cathode material in a lithium difluoro (oxalate) borate (LiDFOB, 20 wt%) and lithium hexafluoride phosphate (LiPF6, 80 wt%) dual-salt electrolyte are investigated. The use of a dual-salt electrolyte significantly improves the material's electrochemical performance, especially at elevated temperature. The capacity retention of the electrode increases from 24% to 92% after charging-discharging at 55 °C for 100 cycles. Spectroscopic analysis demonstrates that LiDFOB salt suppressed Mn2+ dissolution into the electrolyte and improved the electrode's electrochemical kinetic properties. In addition, the thermal safety of the fully-charged electrode is improved by the dual-salt electrolyte, which results in high onset temperature and reduces thermal release.

Hierarchical flower-like VS 2 nanosheets assemblies are successfully synthesized via a facile solvothermal method, and their Na + storage behavior is systematically studied with respect to the galvanostatic charge-discharge profiles, cyclic voltammograms, rate capability and long-term cycle stability. With a well-controlled cut-off voltage (3.0–0.3 V) and suitable electrolyte (1.0 M NaCF 3 SO 3 in diglyme), flower-like VS 2 delivers a high reversible capacity of around 600 mAh g −1 at 0.1 A g −1 and excellent cycle stability with 83% and 87% of its initial capacities retained after 700 cycles at 2 and 5 A g −1 , respectively. Remarkably, the VS 2 anode shows a high initial Coulombic efficiency of 94% and nearly 100% in the subsequent cycles, which points to the promising application of the present material in the commercial sodium-ion batteries. Moreover, VS 2 nanostructures also exhibit superior rate performance with a discharge capacity of 277 mAh g −1 at a current density of as high as 20 A g −1 . Quantitative kinetic analysis indicates that the sodium storage is governed by a pseudocapacitance mechanism, particularly at high current rates. Combined with ex-situ Raman, HRTEM and SAED characterizations further reveal that the Na + storage is based on electrochemical intercalation-de-intercalation reactions.

The size and conductivity of the electrode materials play a significant role in the kinetics of sodium-ion batteries. Various characterizations reveal that size-controllable VS4 nanoparticles can be successfully anchored on the surface of graphene sheets (GSs) by a simple cationic-surfactant-assisted hydrothermal method. When used as an electrode material for sodium-ion batteries, these VS4 @GS nanocomposites show large specific capacity (349.1 mAh g-1 after 100 cycles), excellent long-term stability (84 % capacity retention after 1200 cycles), and high rate capability (188.1 mAh g-1 at 4000 mA g-1 ). A large proportion of the capacity was contributed by capacitive processes. This remarkable electrochemical performance was attributed to synergistic interactions between nanosized VS4 particles and a highly conductive graphene network, which provided short diffusion pathways for Na+ ions and large contact areas between the electrolyte and electrode, resulting in considerably improved electrochemical kinetic properties.

First-principles calculations based on density functional theory were performed to investigate the electrochemical properties of K1−xVOPO4 in potassium-ion batteries (KIBs). The material showed multiple phase transitions during K ion extraction, which began with a two-phase transition (0 ≤ x ≤ 0.5), followed by a solid-solution transition (0.5 < x ≤ 0.625), another two-phase transition (0.625 < x ≤ 0.75), and finally a solid-solution transition (0.75 < x ≤ 1). These processes resulted in a small total unit cell volume variation of 6.6%, which was beneficial for the cycle stability of KIBs. Density of states and Bader charge analysis revealed that both V and O participated in the charge transfer process, where V acted as the redox center of KVOPO4 contributing to the K storage capacity, and O acted as a charge transfer medium between V and K. The stepwise increased repulsion between V cations caused three voltage plateaus for K1−xVOPO4. In addition, the one-dimensional diffusion pathway for K ions with low energy barriers of 0.214–0.491 eV ensured high K ion mobility resulting in superior high rate capability.

Although lithium–sulfur batteries are expected to be the promising next generation of energy storage systems, the shuttle effect of polysulfides severely hampers their practical application. In this study, we introduce mesoporous titanium nitride microspheres to decorate the commercial separator and effectively suppress the shuttle effect. Specifically, titanium nitride improves the utilization of sulfur as an upper current collector through its strong chemical adsorption for polysulfides and high conductivity. In addition, its mesoporous spherical structure also forms a favorable physical barrier to block the diffusion of polysulfides. With this titanium nitride modified separator, the cell exhibits an excellent capacity retention ratio of 76% after 200 cycles at 0.5C. A relatively high capacity of 672 mA h g−1 is obtained even at a high current density of 3C. These results suggest that the design of separators modified with transition metal nitride-based materials is a promising approach for developing high performance lithium–sulfur batteries.

Two-dimensional transition metal disulfides have attracted great attention as anode materials for sodium ion batteries (SIBs) due to their high capacities and long cycle life, but knowledge of the mechanisms for phase transitions, charge-transfer reactions, and ionic diffusion kinetics during Na+ insertion has been lacking. These properties were systematically investigated in this work via experimental testing and first-principles calculations using VS2 nanosheets as an example material. The material showed a stable discharge capacity of 386 mA h g−1 in the 0.3–3.0 V voltage window which then increased to 657 mA h g−1 with further discharging to 0.01 V. It was discovered that Na+ first intercalated into octahedral interstitial sites of NaxVS2, with 0 < x ≤ 1.0, accompanied by partial reduction of S anions. Afterwards, Na+ intercalated into tetrahedral interstitial sites of NaxVS2, with 1.0 < x ≤ 2.0, causing partial reduction of both V cations and S anions. The electrode was finally converted into a V/Na2S nanocomposite after insertion of 3.0 mol of Na+, giving rise to a large specific capacity. This work not only revealed the structural transformation and mixed anionic/cationic redox reactions of VS2 during Na+ intercalation, but also helped us to understand the electrochemical reaction mechanisms of transition metal disulfides in SIBs.

Discovering effective bifunctional catalysts to facilitate Li2O2 oxidation and prolong the discharge life to ease the “sudden death” of batteries is a key task for developing high performance Li–O2 batteries. Herein, an advanced aprotic Li–O2 battery is designed using Co9S8@carbon porous nanocages as a bifunctional catalyst derived from a metal–organic framework. It achieves superior electrocatalytic activity, resulting in a high-energy efficiency of 72.7% and a long cycle life of up to 110 cycles at 100 mA g−1 current density. Combined experimental studies and density functional theory calculations reveal that the promising electrochemical performance observed here could be attributed to the high catalytic activity of Co9S8. In addition, the open-framework porous structure of these carbon porous nanocages provides a facile mass transport pathway and fast charge transfer kinetics for the oxygen reduction/evolution reactions.

Li3V2(PO4)3 (LVP) has been highlighted as a promising cathode material for lithium ion batteries, but it suffers from poor rate capability and rapid capacity decay due to sluggish electrode kinetics and vigorous electrode/electrolyte side reactions at high voltage. In this study, an inexpensive aqueous lithium poly-acrylic acid (LiPAA) binder was developed to deftly solve the shortcomings of the LVP material by tailoring the functional groups in the binder. The good adhesion and cohesion properties of the LiPAA binder ensured a close linkage between the active LVP particles, conductive additives and current collector, which formed a stable and conductive network in the electrode. In addition, the reversible H+/Li+ exchange in LiPAA effectively assisted the transport of Li+ ions at the electrode interface, which allowed the establishment of a Li+ conductive pathway without considerable degradation of the electrolyte. Due to these advantages, the LVP electrode containing the LiPAA binder exhibited significantly improved electrochemical performance compared to the electrode that employed the traditional polyvinylidene fluoride binder. The new electrode configuration showed a large specific capacity of 107 mA h g−1 at 70C rate and a high capacity retention of 91% was obtained after 1400 cycles at 10C rate, showcasing the great potential of this aqueous binder in lithium ion batteries.

Li+/Mg2+ hybrid-ion batteries (LMIBs) have attracted intensive attention because they can circumvent some serious drawbacks of Li- and Mg-rechargeable batteries. In this work, a novel LMIB was proposed that uses a VS2 nanosheet-based cathode and an all-phenyl complex + LiCl/tetrahydrofuran hybrid electrolyte. Combined spectroscopic analysis and theoretical simulations revealed that (phenyl)2Mg and tetrahydrofuran inserted into the nanosheets at an early battery-cycling stage. The interlayer spacing of VS2 was expanded from 5.78 to 8.76 A by the inserted organic species, which significantly reduced the diffusion barrier of Li+. As a result, the LMIBs showed remarkable battery performance with a large discharge capacity (181 mA h g−1 at 50 mA g−1), high rate capability (93 mA h g−1 at 5 A g−1), and long cycle stability (0.04% capacity fading per cycle in 500 cycles).

Slow diffusion kinetics of Na+ in electrode is a major obstacle for the rate capability of Na-ion batteries. In this work, super-fast Na+ diffusion in a VS2 anode was achieved by creating ordered (OSV-VS2-x) or disordered S vacancies (DSV-VS2-x) in VS2 nanosheets under a H2/Ar reduction atmosphere. Both the disordered and ordered S vacancies can improve the Na+ diffusion kinetics in VS2. Especially, disordered S vacancies provided an extra ion diffusion pathway perpendicular to the VS2 plane, which significantly reduced the Na+ diffusion barrier of DSV-VS2-x to 0.229 eV, much smaller than the 0.473 eV for VS2 and 0.401 eV for OSV-VS2-x. Taking this advantage, the DSV-VS2-x nanosheets exhibited impressive rate capability (117 mA·h·g−1 at 100 C) and ultra-long cycle life (~100% capacity retention after 5000 cycles at 50 C) in the 1.0–3.0 V voltage window. About 95% of the capacity was attributed to a pseudocapacitive process. The vacancy engineering strategy proposed in this work could significantly improve the rate capability of VS2 and other transition metal dichalcogenides. This technology that combines the high rate performance of supercapacitors with the high energy density of rechargeable batteries would promote the development of high power energy storage devices.

Co9S8 has been regarded as a desirable anode material for sodium ion batteries because of its high theoretical capacity. In this study, a Co9S8 anode material containing 5.5 wt% Co (Co9S8/Co) was prepared by a solid-state reaction. The material's electrochemical properties were studied with carbonate-based and ether-based electrolytes (the latter, EBE), which showed that the material had a longer cycling life and better rate capability in the EBE. This excellent electrochemical performance was attributed to the low apparent activation energy and the low over-potential for Na deposition in the ether based electrolyte, which improved the electrode kinetic properties. Also, EBE suppressed side reactions of the electrode and electrolyte, which avoided the formation of a solid electrolyte interphase film.

Li2Fe1-xTixSiO4/C cathode materials are designed and realized by a facile sol-gel method. The effects of various Ti doping amounts on the microstructure and electrochemical performance of Li2FeSiO4/C are investigated comprehensively. X-ray powder diffraction result confirms the crystal structure of Li2Fe1-xTixSiO4/C to be monoclinic Li2FeSiO4 with minor decrease in the unit cell volume. X-ray photoelectron spectroscopy shows that Ti does successfully dope into the crystal structure of Li2FeSiO4. Galvonostatic charge-discharge experiments are used to study the electrochemical performance of Ti-doped Li2FeSiO4/C as a lithium-ion batteries cathode material. As results, Li2Fe0.98Ti0.02SiO4/C exhibits the best electrochemical performance with the discharge capacity of 102.8 and 91.1 mAh g−1 even at 5 and 10 C (1 C = 165 mA g−1) high rate after 1000 cycles. Additionally, the electrochemical kinetic performance of Li2Fe1-xTixSiO4/C is further determined by electrochemical impedance spectroscopy, cyclic voltammetry and galvanostatic intermittent titration technique. The results indicate that the enhanced electrochemical performance can be attributed to the effect of Ti doping, which not only enhances the structural stability, but also improves the kinetic properties of Li2FeSiO4/C.

Layered oxides based on abundant elements have been extensively studied as cathodes of sodium-ion batteries. Among them, birnessite-type sodium manganese oxide containing lattice water meets the low-cost and high-performance requirement for stationary batteries. Herein, we for the first time present the controllable states of lattice water via adjusting the cutoff voltages, effectively enhancing the reversible capacity, cycling stability, and rate ability of the materials. The current investigation not only highlights the significance of intercalated lattice water for reversible Na (de)insertion of birnessite as well as other similar compounds, but also opens up new opportunities for advanced cathode materials for sodium storage.

A micro-emulsion method has been introduced to synthesize spinel ZnMn 2 O 4 . By means of a simple adjustment of the reaction solvent or added surfactant, we successfully gain distinct micro-shapes as porous hexahedron, porous microsphere and porous core-shell microsphere. XRD test results validate the phase purity of resulting ZnMn 2 O 4 materials. Interestingly, the different structural characteristics result in significant disparate electrochemical performances. The porous core-shell microsphere (PCMZ) exerts surprisingly remarkable specific capacity and excellent rate performance. The observed enormous discharge capacity of synthesized PCMZ (∼1600 mAh g −1 for the 100th cycle) is the highest among the ever reported ZnMn 2 O 4 -based material. The long cycle performance at high current rate (1208 mAh g −1 after 250 cycles at 500 mA g −1 ) is also the best. Large amount of experimental results demonstrate that the resulted high specific capacity is mainly assigned to the generation of higher valence Mn during cycling and extra interfacial Li storage caused by its unique architecture.

Spinel ZnMn 2 O 4 is considered as a potentially high-capacity anode material for Li-ion batteries. However, its low conductivity causes poor rate capability and insufficient cyclability, thus turning into a challenge for battery applications. Hybrid inorganic/ordered mesoporous carbon (CMK-3) nanostructure represents a promising material platform for next-generation energy storage due to the presence of a large surface area, which facilitates ion transport and storage, and improves electron transport. Here we report a convenient and advanced architecture design of ultrathin 2D ZnMn 2 O 4 nanosheets integrated on 3D CMK-3 framework. Benefiting from the sufficient electrochemically available interfaces and abundant electronic/ionic pathways provided by the ultrathin ZnMn 2 O 4 and the interconnected channels of CMK-3 network, this ZnMn 2 O 4 @CMK-3 material provides a high reversible capacity of 997 mAh g −1 at 100 mA g −1 after 100 cycles, a strong rate capability with 693 mAh g −1 at 2 A g −1 , and a remarkable long-term cyclability with a capacity retention of 94% over 1600 cycles.