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It is well known that monolayer transition-metal dichalcogenides (MX$_2$, M = Mo, W and X = S, Se, Te) could exist in three common structures, i.e. 1$T$, 1$T'$, and 1$H$ phases. In order to reveal their possible phase transitions driven by biaxial strain, we use first-principles calculations to determine the energy landscapes associated with these three phases. Due to its intrinsic dynamical instability, the centrosymmetric 1$T$ phase is known to be metastable and will transform into the 1$T'$ phase where pairs of metal atoms pull together toward each other. Moreover, controlling the metallic 1$T'$ and semiconducting 1$H$ phases is of particular interest as this can introduce novel opportunities in a series of applications. When a biaxial strain is simultaneously compressed along the zigzag direction and stretched along the armchair direction，phase transitions from 1$H$ to 1$T'$ have occurred in MSe$_2$ and MoTe$_2$, but for MSe$_2$ the biaxial strain is much difficult to realize in experiments. For WTe$_2$, the 1$T'$ structure will transform into the 1$H$ form when a biaxial strain is just compressed along the armchair direction. The transitions between 1$H$ and 1$T'$ phases could be attributed to the changes of metal-chalcogen bonds along the armchair direction by analyzing the Crystal Orbital Hamilton Population. Only half M-X bonds along the armchair direction is the main factor, because their lengths are robust in 1$T'$ phase and decrease in 1$H$ form with the tensile strain applied along the armchair direction. Our findings provide a guideline for the phase engineering of transition-metal dichalcogenides with biaxial strain.

Skyrmion bags are spin structures with arbitrary topological charges, which consist of a large skyrmion and multiple small skyrmions. In this work, by applying in-plane ac magnetic fields, we investigate the spin-wave modes of skyrmion bags, which behave differently from the clockwise (CW) and counterclockwise (CCW) rotation modes of skyrmions because of their complex spin topological structure. The in-plane excitation power spectral density shows that there are four resonance frequencies for each skyrmion bag. By further calculating the spin dynamics of a skyrmion bag at each resonant frequency, the four spin-wave modes appear as the composition modes of outer skyrmion-inner skyrmions, i.e., a CCW-CW mode, two CW-breathing modes with different resonance strength, and an inner CCW mode. Our results provide an understanding of the in-plane spin excitation of skyrmion bags, which may contribute to the characterization and detection of skyrmion bags, as well as the applications in logic devices.

Electrochromic batteries (EBs) have been developed as a technical breakthrough to solve the energy issues of storage and saving. Multivalent-ions (Zn2+, Mg2+ and Al3+) have recently demonstrated attractive properties for EBs due to their multiple-electron redox nature. However, still now, reported multivalent-ion EBs are typically assembled with a small-area metallic anode and a large-area electrochromic cathode. Non-uniformity of coloration and unstable metal plating/stripping hinder the developments of these devices. Additionally, insufficient energy/power density of EBs is a huge technical challenge needed to be overcome. In this work, we demonstrated a new aqueous aluminum-ion electrochromic full battery (AIEFB) to overcome the challenges. Systematic studies of density functional theory calculation, molecular dynamics simulation, electrochemical analysis, and mechanical measurements were conducted to optimize electrode materials and electrolyte. A water-in-salt (WIS) Al(OTF)3 electrolyte and a new electrochromic material couple of anodic amorphous WO3 (a-WO3) and cathodic indium hexacyanoferrate (InHCF) were exploited for AIEFB. The AIEFB demonstrated advantages of a high average discharge potential (1.06 V), an attractive energy density of 62.8 mWh m−2 at a power density of 2433.8 mW m−2, a high transmittance modulation of 63% at 600 nm, and a distinct transparent-to-deep blue coloration during the Al-ion shuttling processes.

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.

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

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.

Two-dimensional transition metal dichalcogenides are promising anode materials for Na ion batteries (NIBs). In this study, we carried out a comprehensive investigation to analyze the structural, electrochemical characteristics, and diffusion kinetics of bulk WX

Graphdiyne (GDY) has been considered as an appealing anode candidate for K-ion storage since its triangular pore channel, alkyne-rich structure, and large interlayer spacing would endow it with abundant active sites and ideal diffusion paths for K-ions. Nevertheless, the low surface area and disordered structure of bulk GDY typically lead to unsatisfied K storage performance. Herein, we have designed a GDY/graphene/GDY (GDY/Gr/GDY) sandwiched architecture affording a high surface area and fine quality throughout a van der Waals epitaxy strategy. As tested in a half-cell configuration, the GDY/Gr/GDY electrode exhibits better capacity output, rate capability, and cyclic stability as compared to the bare GDY counterpart.

MXenes have been used as substrate materials for single-atom catalysts (SACs) due to their unique two-dimensional (2D) structure, high surface area, and high electronic conductivity. Oxygen is the primary terminating group of MXenes; however, all of the reported Pt SACs till now are fabricated with F-terminated MXenes. According to the first-principles calculations of this work, the failure of using O-terminated MXenes as substrates is due to the low charge density around Pt and C, which weakens the catalytic activity of Pt. By adjusting the electronic structure of M2C using a second submetal with a lower work function than M, 18 potential bifunctional Pt SACs are constructed based on O-terminated bimetal MXenes. After further consideration of some important practical application factors such as overpotential, solvation effect, and reaction barriers, only four of them, i.e., Cr2Nb2C3O2-VO-Pt, Cr2Ta2C3O2-VO-Pt, Cr2NbC2O2-VO-Pt, and Cr2TaC2O2-VO-Pt, are screened as bifunctional oxygen reduction reaction/oxygen evolution reaction (ORR/OER) catalysts. All of these screened SACs are originated from Cr-based MXenes, implying the significance of Cr-based MXenes in designing bifunctional Pt SACs.

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.

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.

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.

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 Mg

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 (Ti

Facing low reactivity/selectivity of oxygen reduction reaction (ORR) in electro-Fenton (EF), N, S atoms were introduced into carbon-based cathode. "End-on" O

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.

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 of Mg

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.

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-Ti