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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.

Anatase TiO2 is a promising anode material for sodium-ion batteries (SIBs). However, its sodium storage mechanisms in terms of crystal structure transformation during sodiation/de-sodiation processes are far from clear. Here, by analyzing the redox thermodynamics and kinetics under near-equilibrium states, we observe, for the first time, that upon Na-ion uptake, the anatase TiO2 undergoes a phase transition and then an irreversible crystal structure disintegration. Additionally, unlike previous theoretical studies which investigate only the two end points of the sodiation process (i.e., TiO2 and NaTiO2), we study the progressive crystal structure changes of anatase TiO2 upon step-by-step Na-ion uptake (NaxTiO2, x = 0.0625, 0.125, 0.25, 0.5, 0.75, and 1) for the first time. It is found that the anatase TiO2 goes through a thermodynamically unstable intermediate phase (Na0.25TiO2) before reaching crystalline NaTiO2, confirming the inevitable crystal structure disintegration during sodiation. These combined experimental and theoretical studies provide new insights into the sodium storage mechanisms of TiO2 and are expected to provide useful information for further improving the performance of TiO2-based anodes for SIB applications.

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.

In spite of the low-cost and abundant potassium resources, the potential commercialization of potassium-ion batteries (PIBs) is still confined by the large-sized K+ ions and sluggish kinetic process. A flexible free-standing advanced anode for PIBs is synthesized by tactfully incorporating dual anionic vacancies on MoSSe arrays in combination of carbon nanofiber membrane (v-MoSSe@CM). The vacancy-rich MoSSe arrays in v-MoSSe@CM dramatically enhance the adsorption of K+ ions, leading to a higher capacity of 370.6 ​mAh g−1 at 0.1 ​A ​g−1 over 60 cycles as compared with that 168.5 ​mAh g−1 of vacancy-free MoSSe@CM. Meanwhile, the density functional theory (DFT) calculations demonstrate a facilitated ability for K+ insertion into v-MoSSe interlayers with a much more negative adsorption value of −1.74 ​eV than that (0.53 ​eV) of vacancy-free MoSSe. The thousands of carbon nanofiber-supported three-dimensional frameworks can not only inhibit the agglomeration of MoSSe nanosheets, but also remit the volume expansion and avoid possible collapse of the nanostructures during cycling, resulting into a high capacity retention of 220.5 ​mAh g−1 ​at 0.5 ​A ​g−1 after 1000 cycles. Therefore, this work uncovers the relationship between vacancy engineering and potassium-ion storage performance, guiding a feasible route to develop potential materials for potassium-ion battery.

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.

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

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.

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.

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.

Hydrogen evolution reaction (HER) has been severely suppressed by the first proton adsorption step, due to the “shackles” of the surrounding water in the form of hydronium ions (H13O6+). Here, it has been found anionic vacancies in NiCo2A4 (A = O, S, Se) can weaken the shackles of surrounding water in H13O6+, resulting into efficient capture of H+ to form a H+-enriched field. Taking selenium vacancy-rich NiCo2Se4 nanowires on nitrogen-doped carbon nanofibers as an example, it displays higher electrocatalytic activities with low overpotential of 168 mV at 10 mA cm−2 and a Tafel slope of 49.8 mV dec−1. The HER performance is strongly related to the anionic vacancy size, the larger the anionic vacancy size is (VSe > VS > VO), the higher the HER activity does. Guided by density functional theory calculations, it is found that the point defect structures can not only strengthen its interfacial linkage force with H+ by weakening the hydration force of contiguous hydronium ion, but also increase its charge density for efficient electron transfer to adsorbed H+. Therefore, this work creates a useful strategy to optimize the HER performance of traditional bimetallic compounds by engineering the solid-liquid-gas three-phase interfacial interaction.

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.]��

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).

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.

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.

Inorganic-organic crystal materials such as K2C6O6 have received intensive interest as electrode materials for metal ion batteries. In order to gain deep understanding of the structural, electronic and kinetic properties of K2C6O6 during electrochemical processes, the K ion storage properties of K2C6O6 are comprehensively studied by first-principles calculations. The insertion of K begins with a two-phase transition from K2C6O6 to K3C6O6, followed by a homogeneous reaction from K3C6O6 to K4C6O6. The space group of the material successively changes with the potassiation process, which provides flexible structures for K ion storage. The material shows a first voltage plateau at 2.76 V, followed by stepwise voltage decrease from 1.06 V to 0.74 V, providing a theoretical capacity of 218 mA∙h∙g−1. Bader charge analysis demonstrates that C is the redox center of the electrode reactions and O acts as a charge transfer medium between C and K. The flexible structures provide feasible pathways for K ions, which allows fast K ion migration under low energy barriers. The calculated K ion diffusion coefficients between 7.25×10−9 and 9.34×10−8 cm2/s indicate that K2C6O6 can realize excellent rate capability in K ion batteries.

Ultrathin TiO2-B nanowires with a naked (−110) surface were prepared by a hydrothermal process and used as the anode material for Mg-ion batteries. The material delivered a reversible Mg2+ ion capacity of 110 mA h g−1 at the 0.1C rate. Excellent cycling stability was achieved with a small capacity-fading rate of 0.08% per cycle. In addition, a discharge capacity of 34 mA h g−1 was obtained at the 50C rate, demonstrating the material's excellent high rate capability. First-principles calculations showed that Mg2+ ions hardly penetrated into the TiO2-B lattice because of a very large Mg2+ ion diffusion barrier of 0.63 eV. Instead, the Mg2+ ions were stored at the 4-coordinated vacancies of TiO2-B nanowire (−110) surfaces. The adsorbed Mg2+ ions were bonded with unpaired surface oxygen atoms. Meanwhile, a small amount of electrons were transferred from the O-2p state to the Ti-3d state.

Polyanionic Na super ionic conductors (NASICON) have attained great attention as cathode materials for Na ion batteries. In this study, we have comprehensively investigated the structural and electronic properties, as well as the ionic diffusion kinetics of Na3Cr2(PO4)3 using first-principles calculations based on the density functional theory. Our calculations have figured out the R 3 ¯ c - P 3 ¯ c 1 - R 3 ¯ c phase transformation of Na3Cr2(PO4)3 during desodiation. The material shows a semi-conductive property with a band gap of ~ 1.0 V, which has turned into metallic during the charging process. Comparing with other transition metal polyanionic materials, Na3Cr2(PO4)3 exhibits relatively higher working voltage of 3.65-4.7 V due to the redox couple of Cr4+/Cr3+. Na ions have migrated in a zig-zag pathway with small energy barriers fell in the range of 0.17–0.42 eV, which validates the good rate capability of this NASICON-type cathode material in Na ion batteries.

Facing low reactivity/selectivity of oxygen reduction reaction (ORR) in electro-Fenton (EF), N, S atoms were introduced into carbon-based cathode. “End-on” O2 adsorption was achieved by adjusting electronic nature via N doping, while *OOH binding capability was tuned by spin density variation via S doping. Results showed the optimized N, S co-doped cathode presented a 42.47% improvement of H2O2 accumulation (7.95 ± 0.02 mg L−1 cm−2). According to density functional theory (DFT), N, S co-doped structure favored the “end-on” O2 adsorption as adsorption energy dropped to − 2.24 eV. Moreover, O-O/C-O bond lengths variation proved a possibility for *OOH desorption. The elaborated cathode was used in EF for sulfonamides (SAs) decay. A 100% removal rate of sulfadiazine (SDZ), sulfathiazole (STZ) and sulfadimethoxine (SDM) was achieved within 60 min, among which SDZ tended to be degraded easily. Because the absolute hardness (η) of those pollutants is ranked as follows: ηSDM> ηSTZ> ηSDZ. Degradation pathways were proposed based on the detected byproducts, along with toxicity was evaluated by ecological structure-activity relationship (ECOSAR) program. Results showed that toxic intermediates generated were reduced or even disappeared. EF with N, S co-doped cathode provides a promising process for antibiotics wastewater treatment.

Two-dimensional transition metal dichalcogenides (TMDs) have been intensively studied as electrode materials for lithium-ion batteries. But, most of TMDs are low electronic conductors, and there is lack of research on the distinct electrochemical mechanisms of monolayer and bulk TMDs. In this work, the Li+ storage properties of monolayer and bulk TiSe2 are studied by first-principles calculations based on the density functional theory. Calculations showed that bulk TiSe2 works on the intercalation mechanism, which results in a theoretical capacity of 260 mA·h·g−1 in the voltage window of 1.14–2.09 V. In comparison, monolayer TiSe2 works on the adsorption mechanism which gives a theoretical capacity of 780 mA·h·g−1 in 0.18–1.43 V. A two-stage redox process was revealed for both TiSe2 forms. In the initial stage, Se acted as the main redox species; then both Ti and Se participated in the redox reaction. In addition, the material possesses low Li+ diffusion barriers. Especially, a very small diffusion barrier of 163 meV at high Li+ concentration and 39 meV at low Li+ concentration was obtained for monolayer TiSe2, which provide great potential as a high rate anode material for lithium ion batteries.

The construction of advanced micro-supercapacitors (MSCs) with both wide working-voltage and high energy density is promising but still challenging. In this work, a series of nitrogen-doped, cross-coupled micro-mesoporous carbon-metal networks (N-STC/Mx Oy ) is developed as robust additives to 3D printing inks for MSCs fabrication. Taking the N-STC/Fe2 O3 nanocomposite as an example, both experimental results and theoretical simulations reveal that the well-developed hierarchical networks with abundantly decorated ultrafine Fe2 O3 nanoparticles not only significantly facilitate the ion adsorption at its three-phase boundaries (Fe2 O3 , N-STC, and electrolyte), but also greatly favor ionic diffusion/transport with shortened pathways. Consequently, the as-prepared N-STC/Fe2 O3 electrode delivers a high gravimetric capacitance (267 F g-1 at 2 mV s-1 ) and outstanding stability in a liquid-electrolyte-based symmetric device, as well as a record-high energy density of 114 Wh kg-1 for an asymmetric supercapacitor. Particularly, the gravimetric capacitance of the ionogel-based quasi-solid-state MSCs by 3D printing reaches 377 F g-1 and the device can operate under a wide temperature range (-10 to 60 °C).

Magnesium battery is a recently emerging energy storage system that has attracted considerable attention. However, its development is limited by the lack of proper electrode materials for reversible Mg2+ intercalation/de-intercalation with satisfied capacity. Here, we firstly report easy synthesis of Ti-deficient bronze titanium dioxide nanowires by topology transformation of H-titanate precursor. It's found OH− anions substitution of O2− supports the formation of Ti vacancies in TiO2(B) with a high concentration, denoted as (Ti0.91O1.64(OH)0.36), and can be utilized as a robust host for Mg-ion storage. Both the theoretical and experimental study revealed that Ti-deficient TiO2(B) exhibits much improved electronic properties with unpaired electrons. Density functional theory (DFT) calculations also reveal Ti vacancies provide more feasible binding sites for Mg-ion. More importantly, it's surprisingly found the presence of protons enables a suitable binding energy for Mg-ion intercalation and extraction. As a result, such material displays discharge and charge capacities of 217.3 and 165.3 mA h g−1 at 0.02 A g−1, representing the highest value among the reported Ti-based electrode materials as well as a high initial Columbic efficiency up to 76.1%. This study gives a new and in-depth view on how cation-deficient structure regulates and promotes the reversible energy storage.

It has been known that oxygen vacancy defects on the surface of electrocatalysts are beneficial for oxygen evolution reaction (OER), but the promotion effect for OER is still unclear and sometimes controversial. Here, we developed a new and facile method to introduce the oxygen vacancies (O-vacancies) to the surface of iron-nickel layered double hydroxide deposited on Ni foam (FeNi-LDH/NF) via the control of the NaCl contents in the initial FeNi precursor solution. O-vacancies are created by the intercalation and removal of Na+ cations in the FeNi-LDH crystalline structure, achieving the FeNi-LDH/NF electrodes with O-vacancy ratios of 1.3%, 4.2%, 6.8%, 14.2% and 27.4%. O-vacancies enhance the electronic conductivity, surface wettability and H2O adsorption/activation ability of the FeNi-LDH/NF electrodes. Most important, the results show a distinguished volcano-type dependence of the electrocatalytic activity of FeNi-LDH/NF on the O-vacancy ratios for OER. The best performance is obtained on FeNi-LDH/NF electrodes with 6.8% O-vacancies, achieving an overpotential of 177 mV to reach OER 10 mA cm−2 and excellent stability at a high current density of 400 mA cm−2 in 1.0 M KOH solution for over 60 h. The density functional theory (DFT) calculation indicates that excess O-vacancy defects on the surface of FeNi-LDH/NF catalysts would lead to too many delocalized electrons, which in turn deters the adsorption of the water molecule and thus reduces the activity for OER, while low O-vacancies would result in limited active sites for the H2O adsorption for OER. The results demonstrate the intrinsic relationship between the O-vacancy defects and electrocatalytic activity of FeNi-LDH catalysts for OER. The facile and controllable method to introduce O-vacancy defects in FeNi-LDH structure can also be utilized for other catalysts applications.

Interfacial engineering plays an important role in tuning the intrinsic property of electrode materials for energy applications such as lithium-ion batteries (LIBs), which however is rarely realized to amorphous electrode materials, despite a set of characteristics of amorphous materials desirable for LIBs. Here, Au atomic cluster layer-interfaced amorphous porous CoSnO3 nanocubes were fabricated by galvanic replacement and employed as a superior LIB anode, showing high reversible capacity (1615 mAh g–1 at 0.2 A g–1), good rate capability (1059 mAh g–1 with a 61.3% capacity retention upon the dramatic current variation from 0.1 to 5 A g–1), and excellent cycling stability. The amorphous nature, interconnected mesopores, and especially the thin Au atomic cluster layer on the surface/pore walls of CoSnO3 nanocubes can not only improve electron transport and ion diffusion in the electrode and electrolyte but also release the volume strain. Most significantly, density functional theory calculations reveal tha...

The use of free-standing carbon-based hybrids plays a crucial role to help fulfil ever-increasing energy storage demands, but is greatly hindered by the limited number of active sites for fast charge adsorption/desorption processes. Herein, an efficient strategy is demonstrated for making defect-rich bismuth sulfides in combination with surface nitrogen-doped carbon nanofibers (dr-Bi2 S3 /S-NCNF) as flexible free-standing electrodes for asymmetric supercapacitors. The dr-Bi2 S3 /S-NCNF composite exhibits superior electrochemical performances with an enhanced specific capacitance of 466 F g-1 at a discharge current density of 1 A g-1 . The high performance of dr-Bi2 S3 /S-NCNF electrodes originates from its hierarchical structure of nitrogen-doped carbon nanofibers with well-anchored defect-rich bismuth sulfides nanostructures. As modeled by density functional theory calculation, the dr-Bi2 S3 /S-NCNF electrodes exhibit a reduced OH- adsorption energy of -3.15 eV, compared with that (-3.06 eV) of defect-free bismuth sulfides/surface nitrogen-doped carbon nanofiber (df-Bi2 S3 /S-NCNF). An asymmetric supercapacitor is further fabricated by utilizing dr-Bi2 S3 /S-NCNF hybrid as the negative electrode and S-NCNF as the positive electrode. This composite exhibits a high energy density of 22.2 Wh kg-1 at a power density of 677.3 W kg-1 . This work demonstrates a feasible strategy to construct advanced metal sulfide-based free-standing electrodes by incorporating defect-rich structures using surface engineering principles.

External electric field control of topological surface states (SSs) is significant for the next generation of condensed matter research and topological quantum devices. Here, we present a first-principles study of the SSs in the magnetic topological insulator (MTI) Cr-doped Bi2Se3 under external electric field. The charge transfer, electric potential, band structure and magnetism of the pure and Cr doped Bi2Se3 film have been investigated. It is found that the competition between charge transfer and spin-orbit coupling (SOC) will lead to an electrically tunable band gap in Bi2Se3 film under external electric field. As Cr atom doped, the charge transfer of Bi2Se3 film under external electric field obviously decreases. Remarkably, the band gap of Cr doped Bi2Se3 film can be greatly engineered by the external electric field due to its special band structure. Furthermore, magnetic coupling of Cr-doped Bi2Se3 could be even mediated via the control of electric field. It is demonstrated that external electric field plays an important role on the electronic and magnetic properties of Cr-doped Bi2Se3 film. Our results may promote the development of electronic and spintronic applications of magnetic topological insulator.