Two-dimensional nanoarchitectures for lithium storage

2D Se-Rich ZnSe/CoSe2@C Heterostructured Composite as Ultrastable Anodes for Alkaline-Ion Batteries

2D transitional metal selenide heterostructures are promising electrode materials for potassium-ion batteries (PIBs) owing to the large surface area, high mechanical strength, and short diffusion pathways. However, the cycling performance remains a significant challenge, particularly concerning the electrochemical conversion reaction. Herein, 2D Se-rich ZnSe/CoSe2@C heterostructured composite is fabricated via a convenient hydrothermal approach followed by selenization process, and then applied as high-performance anodes for PIBs. For example, the capacity delivered by the heterostructured composite is mainly contributed to the synergistic effect of conversion and alloy/de-alloy processes aroused by K+, where K+ may highly insert or de-insert into Se-rich ZnSe/CoSe2@C. The obtained electrode delivers an outstanding reversible charge capacity of 214 mA h g-1 at 1 A g-1 after 4000 cycles for PIBs, and achieves 262 mAh g-1 when coupled with a PTCDA cathode in the full cell. The electrochemical conversion mechanism of the optimized electrode during cycling is investigated through in situ XRD, Raman, and ex situ HRTEM. In addition, the heterostructured composite as anodes also displays excellent electrochemical performances for sodium-ion batteries (SIBs) and lithium-ion batteries (LIBs). This work opens up a new window for investigating novel electrode materials with excellent capacity and long durability.

Iron-arsenide monolayers as an anode material for lithium-ion batteries: a first-principles study

This theoretical investigation delves into the structural, electronic, and electrochemical properties of two hexagonal iron-arsenide monolayers, 1T-FeAs and 1H-FeAs, focusing on their potential as anode materials for lithium-ion batteries. Previous studies have highlighted the ferromagnetic nature of 1T-FeAs at room temperature. Our calculations reveal that both phases exhibit metallic behaviour with spin-polarized electronic band structures. Electrochemical studies show that the 1T-FeAs monolayer has better ionic conductivity for Li ions than the 1H-FeAs phase, attributed to a lower activation barrier of 0.38 eV. This characteristic suggests a faster charge/discharge rate. Both FeAs phases exhibit comparable theoretical capacities (374 mA h g-1), outperforming commercial graphite anodes. The average open-circuit voltage for maximum Li atom adsorption is 0.61 V for 1H-FeAs and 0.44 V for 1T-FeAs. The volume expansion over the maximum adsorption of Li atoms on both phases is also remarkably less than the commercially used anode material such as graphite. Furthermore, the adsorption of Li atoms onto 1H-FeAs induces a remarkable transition from ferromagnetism to anti-ferromagnetism, with minimal impact on the electronic band structure. In contrast, the original state of 1T-FeAs remains unaffected by Li adsorption. To summarize, both 1T-FeAs and 1H-FeAs monolayers have potential as promising anode materials for lithium-ion batteries, offering valuable insights into their electrochemical performance and phase transition behaviour upon Li adsorption.

A first-principles study of the BC3N2 monolayer and a BC3N2/graphene heterostructure as promising anode materials for sodium-ion batteries

High-performance sodium-ion batteries (SIBs) require anode materials with high capacity and fast kinetics. Based on first-principles calculations, we propose BC3N2 and BC3N2/graphene (B/G) heterostructure as potential SIB anode materials. The BC3N2 monolayer exhibits intrinsic metallic behavior. In addition, BC3N2 possesses a low Na+ diffusion barrier (0.15 eV), a high storage capacity (777 mA h g-1), a low open-circuit voltage (0.72 V), and a tiny axial expansion (0.36%). Compared with the BC3N2 monolayer, the B/G heterostructure exhibits a lower diffusion barrier of 0.027 eV, suggesting a much faster diffusion. More importantly, although the B/G heterostructure possesses heavier molar weight, its theoretical capacity (689 mA h g-1) is comparable to that of the BC3N2 monolayer. Based on the above-mentioned properties, we hope both the BC3N2 monolayer and the B/G heterostructure would be promising anodes for SIBs.

High yield and wide lateral size growth of α-Mo2C: exploring the boundaries of CVD growth of bare MXene analogues

Synthesis of Mo2C bare MXenes, without surface terminations groups, via chemical vapor deposition (CVD) on metal foils is scientifically a very intriguing crystal growth process, and there are still challenges and limited fundamental understanding to overcome to obtain high yield and wide crystal size lateral growth. Achieving large area coverage via direct growth is scientifically vital to utilize the full potential of their unique properties in different applications. In this study, we sought to expand the boundaries of the current CVD growth approach for Mo2C MXenes and gain insights into the possibilities and limitations of large area growth, with a particular focus on controlling Mo concentration. We report a facile modification of their typical CVD growth protocol and show its influence on the Mo2C synthesis, with growth times spanning up to 3 h. Specifically, prior to initiating the CVD growth process, we introduced a holding step in temperature at 1095 °C. This proved to be beneficial in increasing the Mo concentration on the liquid Cu growth surface. We achieved an average Mo2C crystals coverage of approximately 50% of the growth substrate area, increased tendency of coalescence and merging of individual flakes, and lateral flake sizes up to 170μm wide. To gain deeper understanding into their CVD growth behavior, we conducted a systematic investigation of the effect of several factors, including (i) a holding step time on Mo diffusion rate through molten Cu, (ii) the Cu foil thickness over the Mo foil, and (iii) the CVD growth time. Phase, chemical and microstructural characterization by x-ray diffraction, x-ray photon spectroscopy, SEM and scanning/transmission electron microscopy revealed that the grown crystals are single phaseα-Mo2C. Furthermore, insights gained from this study sheds light on crucial factors and inherent limitations that are essential to consider and may help guide future research progress in CVD growth of bare MXenes.

Biotene: Earth-Abundant 2D Material as Sustainable Anode for Li/Na-Ion Battery

Natural ores are abundant, cost-effective, and environmentally friendly. Ultrathin (2D) layers of a naturally abundant van der Waals mineral, Biotite, have been prepared in bulk via exfoliation. We report here that this 2D Biotene material has shown extraordinary Li-Na-ion battery anode properties with ultralong cycling stability. Biotene shows 302 and 141 mAh g-1 first cycle-specific charge capacity for Li- and Na-ion battery applications with ∼90% initial Coulombic efficiency. The electrode exhibits significantly extended cycling stability with ∼75% capacity retention after 4000 cycles even at higher current densities (500-2000 mA g-1). Further, density functional theory studies show the possible Li intercalation mechanism between the 2D Biotene layers. Our work brings new directions toward designing the next generation of metal-ion battery anodes.

Sand-Fixation Model for Interface Engineering of Layered Titania and N/O-Doped Carbon Composites to Enhance Potassium/Sodium Storage

Layered titania (L-TiO2 ) holds great potential for potassium-ion batteries (PIBs) and sodium-ion batteries (SIBs) due to their high specific capacity. Synthesizing L-TiO2 functional materials for high-capacity and long cyclability battery remains challenging due to the unstable and poor conductivity of bare L-TiO2 . In nature, plant growth can stabilize land by preventing sands from dispersing following desertification. Inspired by nature's "sand-fixation model," Al3+ "seeds" are in situ grown on layered Ti3 C2 Tx "land." Subsequently, NH2 -MIL-101(Al) "plants" with Al as metal nodes are grown on the Ti3 C2 Tx "land" by self-assembly. After annealing and etching processes (similar to desertification), NH2 -MIL-101(Al) is transformed into interconnected N/O-doped carbon (MOF-NOC), which not only acts as a plant-like function to prevent the pulverization of L-TiO2 transformed from Ti3 C2 Tx but also improves the conductivity and stability of MOF-NOC@L-TiO2 . Al species are selected as seeds to improve interfacial compatibility and form intimate interface heterojunction. Systematic ex situ analysis discloses that the ions storage mechanism can be endowed by mixed contribution of non-Faradaic and Faradaic capacitance. Consequently, the MOF-NOC@L-TiO2 electrodes exhibit high interfacial capacitive charge storage and outstanding cycling performance. The interface engineering strategy inspired by "sand-fixation model" provides a reference for designing stable layered composites.

Toward MBenes Battery Electrode Materials: Layered Molybdenum Borides for Li-Ion Batteries

Lithium-ion and sodium-ion batteries (LIBs and SIBs) are crucial in our shift toward sustainable technologies. In this work, the potential of layered boride materials (MoAlB and Mo2 AlB2 ) as novel, high-performance electrode materials for LIBs and SIBs, is explored. It is discovered that Mo2 AlB2 shows a higher specific capacity than MoAlB when used as an electrode material for LIBs, with a specific capacity of 593 mAh g-1 achieved after 500 cycles at 200 mA g-1 . It is also found that surface redox reactions are responsible for Li storage in Mo2 AlB2 , instead of intercalation or conversion. Moreover, the sodium hydroxide treatment of MoAlB leads to a porous morphology and higher specific capacities exceeding that of pristine MoAlB. When tested in SIBs, Mo2 AlB2 exhibits a specific capacity of 150 mAh g-1 at 20 mA g-1 . These findings suggest that layered borides have potential as electrode materials for both LIBs and SIBs, and highlight the importance of surface redox reactions in Li storage mechanisms.

Investigating the local structure of Ti based MXene materials by temperature dependent X-ray absorption spectroscopy

The local structures of Ti based MXene-type electrode materials have been studied by Ti K-edge X-ray absorption fine structure measurements as a function of temperature to obtain direct information on the local bond lengths and their stiffness. In particular, the parent MAX phases Ti₂AlC and Ti₃AlC₂ and their etched MXene systems are characterized and their properties compared. We find that selective etching has a substantial effect on the local structural properties of the Ti based MXene materials. It leads to an increase in the interatomic distances, i.e. a decrease in the covalency, and corresponding bond stiffness, that is a likely cause of higher achievable performances. The obtained results underline the importance of the local atomic correlations as limiting factors in the diffusion capacity of ion batteries.

Intercalation of argon in honeycomb structures towards promising strategy for rechargeable Li-ion batteries

High-performance rechargeable batteries are becoming very important for high-end technologies with their ever increasing application areas. Hence, improving the performance of such batteries has become the main bottleneck to transferring high-end technologies to end users. In this study, we propose an argon intercalation strategy to enhance battery performance via engineering the interlayer spacing of honeycomb structures such as graphite, a common electrode material in lithium-ion batteries (LIBs). Herein, we systematically investigated the LIB performance of graphite and hexagonal boron nitride (h-BN) when argon atoms were sent into between their layers by using first-principles density-functional-theory calculations. Our results showed enhanced lithium binding for graphite and h-BN structures when argon atoms were intercalated. The increased interlayer space doubles the gravimetric lithium capacity for graphite, while the volumetric capacity also increased by around 20% even though the volume was also increased. Theab initiomolecular dynamics simulations indicate the thermal stability of such graphite structures against any structural transformation and Li release. The nudged-elastic-band calculations showed that the migration energy barriers were drastically lowered, which promises fast charging capability for batteries containing graphite electrodes. Although a similar level of battery promise was not achieved for h-BN material, its enhanced battery capabilities by argon intercalation also support that the argon intercalation strategy can be a viable route to enhance such honeycomb battery electrodes.

Transition Metal Carbo-Chalcogenide "TMCC:" A New Family of 2D Materials

Here, a new family of 2D transition metal carbo-chalcogenides (TMCCs) is reported, which can be considered a combination of two well-known families, TM carbides (MXenes) and TM dichalcogenides (TMDCs), at the atomic level. Single sheets are successfully obtained from multilayered Nb₂ S₂ C and Ta₂ S₂ C using electrochemical lithiation followed by sonication in water. The parent multilayered TMCCs are synthesized using a simple, scalable solid-state synthesis followed by a topochemical reaction. Superconductivity transition is observed at 7.55 K for Nb₂ S₂ C. The delaminated Nb₂ S₂ C outperforms both multilayered Nb₂ S₂ C and delaminated NbS₂ as an electrode material for Li-ion batteries. Ab initio calculations predict the elastic constant of TMCC to be over 50% higher than that of TMDC.

Prediction of the Be2B2 monolayer: an ultrahigh capacity anode material for Li-ion and Na-ion batteries

In the recent past, beryllium-boron compounds have been the focus of attention as a consequence of their extremely rich structures and potential applications. Nevertheless, their complex phase structures greatly hinder further exploration of the physical and chemical properties. To this end, we propose a novel beryllium-boron compound based on a two-dimensional (2D) structure, namely a Be₂B₂ monolayer. Using first-principles methods, we first prove that the structure of the 2D Be₂B₂ monolayer is energetically and thermodynamically stable. Subsequently, we systematically explored its potential as a high performance anode material for LIBs and NIBs. The results reveal that the Be₂B₂ monolayer exhibits an excellent, comprehensive performance with a strong adsorption energy, low diffusion barrier, suitable average open circuit voltage and outstanding electronic conductivity. More importantly, the extremely high theoretical specific capacity (1352 mA h g^-1 for both LIBs and NIBs) well meets our expectations, which is quite ideal for an anode material. Meanwhile, we hope and believe that this work can provide some new insights into the study of beryllium-boron compounds.

Towards a new packing pattern of Li adsorption in two-dimensional pentagonal BCN

Two-dimensional (2D) materials with a penta-atomic-configuration, such as penta-graphene and penta-B₂C, have received great attention as anodes in Li-ion batteries (LIBs). Recently, penta-BCN has been demonstrated to exhibit the highest theoretical capacity to date of 2183 mA h g^-1, corresponding to the composition Li₃BCN. Herein, we study the layer-by-layer Li adsorption on penta-BCN by explicitly and comprehensively considering its structure. We discover a new, more energetically favorable Li adsorption site that is distinct from the latest report by Chen et al. (Phys. Chem. Chem. Phys., 2021, 23, 17693). The possible migration pathway and the accompanying activation energy are also investigated. Full lithium adsorption leads to the formula Li₂BCN and the reduced theoretical capacity of 1455 mA h g^-1. Still, penta-BCN exhibits metallic conductivity during Li adsorption, and has a low open-circuit voltage, and a low ion-diffusion barrier, all being beneficial for anode materials. These observations imply that penta-BCN remains one of the most effective anode materials for LIBs with a quick charge/discharge rate.

Tailoring the Void Space Using Nanoreactors on Carbon Fibers to Confine SnS2 Nanosheets for Ultrastable Lithium/Sodium-Ion Batteries

Herein, a rational design of SnS₂ nanosheets confined into bubble-like carbon nanoreactors anchored on N,S doped carbon nanofibers (SnS₂ @C/CNF) is proposed to prepare the self-standing electrodes, which provides tunable void space on carbon fibers for the first time by introducing hollow carbon nanoreactors. The SnS₂ @C/CNF provides the stable support with greatly enhanced ion and electron transport, alleviates aggregation and volume expansion of SnS₂ nanosheets, and promotes the formation of abundant exposed edges and active sites. The volume balance between SnS₂ nanosheets and hollow carbon nanoreactors is reached to accommodate the expansion of SnS₂ during cycles by controlling the thickness of SnO₂ shells, which achieves the best space utilization. The doping of N,S elements enhances the wettability of the carbon nanofiber matrix to electrolyte and Li ions and further improves the electrical conductivity of the whole electrode. Thus, the SnS₂ @C/CNF benefits greatly in structural stability and pseudocapacitive capacity for improved lithium/sodium storage performance. As a result of these improvements, the self-standing SnS₂ @C/CNF film electrodes exhibit the highly stable capacity of 964.8 and 767.6 mAh g^-1 at 0.2 A g^-1 , and excellent capacity retention of 87.4% and 82.4% after 1000 cycles at high current density for lithium-ion batteries and sodium-ion batteries, respectively.

Microwave-Assisted Coprecipitation Synthesis and Local Structural Investigation on NiO, β-Ni(OH)2/Co3O4 Nanosheets, and Co3O4 Nanorods Using X-ray Absorption Spectroscopy at Co-Ni K-edge and Synchrotron X-ray Diffraction

Developing the most straightforward, cheapest, and eco-friendly approaches for synthesizing nanostructures with well-defined morphology having the highest possible surface area to volume ratio is challenging for design and process. In the present work, nanosheets of NiO and β-Ni(OH)₂/Co₃O₄, and nanorods of Co₃O₄ have been synthesized at a large scale via the microwave-assisted chemical coprecipitation method under low temperature and atmospheric pressure. X-ray absorption spectroscopy (XAS) measurements, which comprises both X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques, have been carried out at Co and Ni K-edges to probe the electronic structure of the samples. Also, the local atomic structural, chemical bonding, morphological, and optical properties of the sample were systematically investigated using XAS, synchrotron X-ray diffraction (SXRD), Raman spectroscopy, FTIR, transmission electron microscopy (TEM), and UV-visible spectroscopy. The normalized XANES spectra of the β-Ni(OH)₂/Co₃O₄ nanosheets show the presence of Ni²⁺ and a mixed oxidation state of Co. The disorder factor decreases from β-Ni(OH)₂/Co₃O₄ to Co₃O₄ with increasing Co-O bond length. The SXRD pattern analyzed using Rietveld refinement reveals that NiO has a face-centered cubic phase, Co₃O₄ has the standard spinal structure, and β-Ni(OH)₂/Co₃O₄ has a mixed phase of hexagonal and cubic structures. TEM images revealed the formation of nanosheets for NiO and β-Ni(OH)₂/Co₃O₄ samples and nanorods for Co₃O₄ samples. FTIR and Raman spectra show the formation of β-Ni(OH)₂/Co₃O₄, which reveals the fingerprints of Ni-O and Co-O.

Tungsten(VI) selenide tetrachloride, WSeCl4 - synthesis, properties, coordination complexes and application of [WSeCl4(SenBu2)] for CVD growth of WSe2 thin films

WSeCl₄ was obtained in good yield by heating WCl₆ and Sb₂Se₃in vacuo. Green crystals grown by sublimation were shown by single crystal X-ray structure analysis to contain square pyramidal monomers with apical WSe, and powder X-ray diffraction (PXRD) analysis confirmed this to be the only form present in the bulk sample. Density functional theory (DFT) calculations using the B3LYP-D3 functional replicated the structure, identified the key bonding orbitals, and were used to aid assignment of the IR spectrum of WSeCl₄. Reaction of WSeCl₄ with ligands L gave [WSeCl₄(L)] (L = MeCN, DMF, thf, py, OPPh₃, 2,2'-bipy, SeMe₂, SeⁿBu₂), whilst the dimers [(WSeCl₄)₂(μ-L-L)] were formed with L-L = Ph₂P(O)CH₂P(O)Ph₂, 1,4-dioxane and 4,4'-bipyridyl. The complexes were characterised by microanalysis, IR and ¹H NMR spectroscopy, and single crystal X-ray structures determined for [WSeCl₄(L)] (L = OPPh₃, MeCN, DMF) and [(WSeCl₄)₂(μ-L-L)] (L-L = 1,4-dioxane, 4,4'-bipyridyl). All except the 2,2'-bipy complex, which is probably seven-coordinate, contain six-coordinate tungsten with the neutral donor trans to WSe. Alkylphosphines, including PMe₃ and PEt₃, decompose WSeCl₄ upon contact, forming phosphine selenides (SePR₃). In contrast, the selenoether complexes [WSeCl₄(SeMe₂)] and [WSeCl₄(SeⁿBu₂)] were isolated and characterised. The crystal structure of the minor W(VI) by-product, [(WSeCl₄)₂(μ-SeMe₂)], was determined and using SMe₂, a few crystals of the W(V) species, [{WCl₃(SMe₂)}₂(μ-Se)(μ-Se₂)], were obtained and structurally characterised. The isolated W(VI) complexes are compared with those of WOCl₄ and WSCl₄ and the combination of experimental and computational data are consistent with WSeCl₄ being a weaker Lewis acid and its complexes significantly less stable than those of the lighter analogues, especially in solution. Low pressure chemical vapour deposition (LPCVD) using [WSeCl₄(SeⁿBu₂)] in the range 660-700 °C (0.1 mmHg) produced highly reflective thin films, which were identified to be WSe₂ by grazing incidence X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. XRD analysis of the thinner films revealed them to be highly oriented in the <00l> direction.

Promoting Electrochemical Performance of Ti3C2O2 MXene-Based Electrodes of Alkali-Ion Batteries via S Doping: Theoretical Insight

Ti₃C₂O₂ MXene has been proposed as a promising electrode material for alkali-ion batteries owing to its tunable physical and chemical properties without sacrificing the excellent metallic conductivity. However, it still suffers from low specific capacity due to its limited interlayer spacing, especially for a larger ion like sodium (Na). Sulfur doping was suggested as a viable strategy to improve the electrode's storage performance. Herein, first-principles calculations and kinetic Monte Carlo (kMC) simulations were carried out to study the role of S doping on Li/Na intercalation. Based on experimental findings, two different doping sites, C (S_C) and O (S_O), with various S concentrations were reported and therefore used as the models in this study. Computations reveal that S doping on both C and O sites improves the electronic conductivity of the MXenes as their densities of states at the Fermi level are increased. In addition, the doped MXenes reveal an expanded lattice parameter in the normal direction, which agrees with experimental observations. However, only the S_O-doped MXenes display an enlarged interlayer spacing, whereas doping at the C site only increases the layer thickness. The enlarged interlayer spacing in the S_O-doped MXenes improves stabilities and transport kinetics of ion intercalation as indicated by their significantly lower insertion energies and diffusion barriers when compared with those of the pristine system. The kMC simulations were carried out to account for anisotropic diffusion in the S_O-doped system. The obtained macroscopic properties of diffusion coefficients and apparent activation energies of the S_O-doped system clearly confirm its superior transport kinetics. The estimated diffusion coefficients of Li(Na) are improved by 4(8) orders of magnitude upon S_O doping. A fundamental understanding of the role of S doping on the improved capacitive kinetics serves as a good guide for developing MXene-based electrode materials for Li- and Na-ion batteries.

Two-dimensional metallic carbon allotrope with multiple rings for ion batteries

Two-dimensional (2-D) materials, especially carbon allotropes, have larger storage capacity and faster diffusion rate due to their unique structures and are usually used in ion batteries. Recently, a new stable two-dimensional carbon allotrope, namely PAI-graphene, was reported by first-principles calculations. Due to its lightweight and multiple-ring structure, great stability and excellent properties, here, we theoretically reveal the excellent performance of PAI-graphene as an anode material for Li-/Na-ion batteries. Our results show that PAI-graphene has intrinsic metallicity before and after adsorption of Li/Na, which ensures that it has good conductivity when working as an electrode material. In addition, PAI-graphene exhibits quite low open circuit voltage (0.342-0.190 V for Li, 0.339-0.233 V for Na) and diffusion barrier (0.34 eV for Li, 0.17 eV for Na), which indicates its superiority as an anode material. Most noteworthily, the Na storage capacity of PAI-graphene is up to 1674 mA h g^-1, which is much higher than that of most 2-D anode materials. Thus, we believe that PAI-graphene can be an outstanding anode material with outstanding performance.

Nanoarchitecturing Carbon Nanodot Arrays on Zeolitic Imidazolate Framework-Derived Cobalt-Nitrogen-Doped Carbon Nanoflakes toward Oxygen Reduction Electrocatalysts

Two-dimensional (2D) nanoporous heterostructured composites formed by uniformly coating individual monolayers with porous layers introduce unparalleled opportunities to improve and optimize the electrochemical performances of 2D materials. Here, an all-porous carbon heterostructure composed of 2D microporous carbon nanoflakes uniformly decorated with carbon nanodots has been developed. Interestingly, resol-F127 micelles self-assemble on the surface of zeolitic imidazolate framework (ZIF) nanoflakes in the form of a nanodot array, yielding a heterostructure. Hydrothermal treatment followed by carbonization under a nitrogen atmosphere causes conversion of the nanodot-nanoflake assembly into a carbon-based material composed of hollow carbon nanodots (CNDs) and microporous carbon nanoflakes (CNFs), that is, a CND@CNF composite. The combination of 2D microporous carbon nanoflakes with carbon hollow nanodots enhances exposure of the active sites and improves mass transfer in all directions (including through the nanoflakes). The use of cobalt (Co)-containing ZIF leads to the synthesis of a Co-N_x-doped CND@CNF composite, which exhibits oxygen reduction reaction electrocatalytic activity and long-term stability superior even to commercial Pt/C catalysts. This architecture-engineering strategy has been used to design and synthesize 2D heterostructures possessing high electrocatalytic efficiency and will be useful for future developments in important electrochemical energy storage and conversion applications.

Review of ZnO Binary and Ternary Composite Anodes for Lithium-Ion Batteries

To enhance the performance of lithium-ion batteries, zinc oxide (ZnO) has generated interest as an anode candidate owing to its high theoretical capacity. However, because of its limitations such as its slow chemical reaction kinetics, intense capacity fading on potential cycling, and low rate capability, composite anodes of ZnO and other materials are manufactured. In this study, we introduce binary and ternary composites of ZnO with other metal oxides (MOs) and carbon-based materials. Most ZnO-based composite anodes exhibit a higher specific capacity, rate performance, and cycling stability than a single ZnO anode. The synergistic effects between ZnO and the other MOs or carbon-based materials can explain the superior electrochemical characteristics of these ZnO-based composites. This review also discusses some of their current limitations.

Construction of two-dimensional bimetal (Fe-Ti) oxide/carbon/MXene architecture from titanium carbide MXene for ultrahigh-rate lithium-ion storage

The development of battery systems with high specific capacity and power density could fuel various energy-related applications from personal electronics to grid storage. (Fe_2.5Ti_0.5)_1.04O₄ possessing high theoretical specific capacity has been considered as a promising high rate anode material for lithium ion batteries due to the replacement of Fe³⁺ (0.64 Å) by Ti⁴⁺ (0.68 Å) with a larger radius to expand the interlayer space for ion intercalation. However, its extreme volume variation upon cycling as well as poor electrical conductivity hinder its further application. To tackle the above problems, in this work, we successfully synthesized two-dimensional (2D) (Fe_2.5Ti_0.5)_1.04O₄/C/MXene architecture derived from Ti₃C₂T_x MXene via solvo-hydrothermal, ultrasound hybridizing and high temperature annealing processes. The (Fe_2.5Ti_0.5)_1.04O₄/C/MXene shows a high discharge capacity of 757.2 mAh g^-1 after 800 cycles at a current density of 3 A g^-1 with excellent rate performance. The superior electrochemical performances are triggered primarily by the incorporation of carbon and MXene into (Fe_2.5Ti_0.5)_1.04O₄ moiety to construct a 2D layered structure, which can improve the ion diffusion and electron transport. In addition, the synergistic contributions from diffusion controlled and capacitive processes for (Fe_2.5Ti_0.5)_1.04O₄/C/MXene improve the ion diffusion rate and offer high specific capacity at high current density. The MXene-derived synthesis strategy in this work should be a promising pathway to synthesize other anode materials with 2D layered architecture for high performance lithium storage.

Pentagonal B2C monolayer with extremely high theoretical capacity for Li-/Na-ion batteries

Recently, two-dimensional (2-D) materials with a Penta-atomic-configuration such as Penta-graphene have received considerable attention because of their potential applications in electronics, spintronics and ion batteries. Previously, Penta-graphene has been proposed as an excellent anode material for Li-/Na-ion batteries with a high theoretical capacity (1489 mA h g-1). Here, based on the first-principles calculations, we report that a new 2-D material namely Penta-B2C can become another excellent anode material with even higher theoretical capacity for Li-/Na-ion batteries than Penta-graphene. Our results demonstrate that Li/Na atoms can be stably adsorbed on Penta-B2C. Meanwhile, Penta-B2C shows metallic conductivity during the adsorption. Most strikingly, the theoretical capacities of Penta-B2C are as high as 1594 for Li and 2391 mA h g-1 for Na, which are superior to those of the most known 2-D anode materials. Especially, the Na theoretical capacity of Penta-B2C sets a new record among known 2-D anode materials. In addition, Penta-B2C possesses relatively low open-circuit voltage and a low diffusion barrier for ions, which are vital for anode materials. These results highly promise that Penta-B2C can be an excellent anode material with a fast charge/discharge rate and extremely high theoretical capacity for Li-/Na-ion batteries.

Blue-AsP monolayer as a promising anode material for lithium- and sodium-ion batteries: a DFT study

Based on first-principle calculations, we proposed a one two-dimensional (2D) blue AsP (b-AsP) monolayer as an ideal anode material for lithium/sodium-ion (Li/Na-ion) batteries for the first time. The b-AsP monolayer possesses thermal and dynamic stabilities. The system undergoes the transition from semiconductor to metal after Li/Na atoms are embedded, which ensures good electric transportation. Most remarkably, our results indicate that the b-AsP monolayer exhibits high theoretical capacities of 1011.2 mA h g^-1 (for Li) and 1769.6 mA h g^-1 (for Na), low average open circuit voltages of 0.17 eV for Li₄AsP and 0.14 eV for Na₇AsP systems and ultrafast diffusivity with the low energy barriers of 0.17/0.15 eV and 0.08/0.07 eV of the P/As sides for Li and Na, respectively. Given these exceptional properties, the synthesis of a buckled b-AsP monolayer is desired to achieve a promising electrode material for Li- and Na-ion batteries.

Sandwich shelled TiO2@Co3O4@Co3O4/C hollow spheres as anode materials for lithium ion batteries

A sandwich shelled hollow TiO₂@Co₃O₄@Co₃O₄/C composite is synthesized by consecutive coating of Co₃O₄ nanosheets and TiO₂ particles on Co₃O₄/C hollow spheres. The composite delivers an excellent lithium storage performance, maintaining 1081.78 mA h g^-1 after 100 cycles at 0.2 A g^-1 and 772.23 mA h g^-1 after 300 cycles at 1 A g^-1, due to its superior structure combining the advantages of each component with favorable electron-transfer, Li⁺-diffusion properties, and distinguished stability.

Functionalized M2TiC2Tx MXenes (M = Cr and Mo; T = F, O, and OH) as high performance electrode materials for sodium ion batteries

First-principles calculations were performed to study the electrochemical performance of M2TiC2 (M = Cr or Mo) and M2TiC2Tx (T = O, F or OH) used as anode materials for sodium ion batteries (SIBs). The O functionalized MXenes (Cr2TiC2O2 and Mo2TiC2O2) are found to be more stable than F and OH terminated systems. The diffusion performance of sodium in MXene materials is mainly affected by the functional groups. The lowest diffusion barrier of functionalized MXenes is about one order larger in magnitude than that of bare MXenes. Although the introduction of O-groups hinders the diffusion of sodium, it can greatly improve the theoretical storage capacities. Meanwhile, the diffusion paths and diffusion energy barriers of Na are affected by Na concentration effects, while the interactions between terminations have little effect. Furthermore, multiple layers of sodium atoms are found to be adsorbed between the layers of M2TiC2O2, thus significantly increasing the theoretical capacities. The theoretical sodium storage capacities of M2TiC2O2 monolayers reach 515.70 mA h g-1 (M = Cr) and 362.46 mA h g-1 (M = Mo) and the OCVs can approach 0.034 V (M = Cr) and 0.042 V (M = Mo). Therefore, Cr2TiC2O2 and Mo2TiC2O2 are expected to be promising anode materials for SIBs due to their excellent properties, such as good electronic conductivity, low sodium diffusion barrier, and high theoretical sodium storage capacity.

Porous hydrogen substituted graphyne as a promising anode for lithium-ion batteries

Porous hydrogen substituted graphyne (HsGY) has been considered as a promising candidate for anode material due to its excellent electrochemical properties. In this work, we found that monolayer and bilayer HsGY are good electrodes for high charge capacity lithium-ion batteries based on density functional theory calculations. Mechanical tests reveal that monolayer and bilayer HsGY exhibit excellent mechanical properties, including large critical strains (>25%) and high in-plane stiffness (>200 N m⁻¹). The bilayer HsGY displays ultrahigh stiffness (400.27 N m⁻¹). Li adsorption on bilayer HsGY is stronger than that on the monolayer HsGY. Moreover, Li diffusion on the surfaces of monolayer and bilayer HsGY has low energy barriers (<0.5 eV). Our calculation results suggest that HsGY may contain the highest theoretical charge capacity among two-dimensional (2D) materials studied so far, with ultrahigh Li capacities of 3378 and 2895 mA h g⁻¹ for monolayer and bilayer HsGY, respectively. Given these advantages, including large critical strain, high mechanical stiffness, strong adsorption, low diffusive energy barrier, and high charge capacity, we conclude that both monolayer and bilayer HsGY could be promising anode materials for lithium-ion batteries.

Porous hydrogen substituted graphyne (HsGY) has been considered as a promising candidate for anode material due to its excellent electrochemical properties.

Y–F co-doping behavior of LiFePO4/C nanocomposites for high-rate lithium-ion batteries

Lithium iron phosphate (LFP) has become one of the current mainstream cathode materials due to its high safety and low price.

Metallic two-dimensional BP2: a high-performance electrode material for Li- and Na-ion batteries

Searching for high-performance electrode materials is an important topic in rechargeable batteries. Using first-principles calculations, we systematically explore the potential application of a two-dimensional BP2 monolayer as a cathode material for Li-ion and Na-ion batteries. The pristine BP2 monolayer exhibits metallic characteristics, which facilitate the transportation of electrons. The Li and Na atoms bind strongly to the BP2 monolayer, indicating a good structural stability. Furthermore, the geometrical structure of BP2 is well maintained during the adsorption process. The Li and Na ions prefer to move along the zigzag direction with relatively low energy barriers. Especially, the ultralow Na diffusion barrier (0.03 eV) implies that monolayer BP2 has an excellent charge/discharge capability. The specific capacity and average electrode potential of Li (Na) are 619.45 (279.93) mA h g-1 and 2.89 (2.49) V, respectively. These results reveal that the BP2 monolayer is an appealing cathode material for alkali-metal batteries.

Controlled synthesis of hierarchical porous carbons with different morphologies and their application for potassium and lithium ion batteries

N-doped activated carbon nanostructures with different morphologies were synthesized, which exhibited specific capacities of 1394 and 526 mA h g⁻¹ for LIBs and PIBs, respectively.

Prediction of two-dimensional CP3 as a promising electrode material with a record-high capacity for Na ions

Borophene with a maximum Li/Na capacity of 1984 mA h g^-1 (nanoscale 2016, 8, 15 340-15 347) has shown the highest capacity among two-dimensional (2-D) anode materials identified so far. Herein, we report the record break for Na-ion using a newly proposed 2-D material, namely, CP₃. We fully investigated Li- and Na-ion adsorption and diffusion processes on a CP₃ monolayer. We found that the material can enable stable Li/Na adsorption considering charge accumulation on CP₃ surfaces. The ion diffusion barriers for Li and Na were identified to be 98 meV and 356 meV, respectively. These values were comparable or smaller than those of the typical high-capacity electrode materials such as borophene. Most remarkably, the maximum Na capacity in CP₃ monolayer can reach up to 2298.9 mA h g^-1, which breaks the value recorded using borophene (1984 mA h g^-1). Our work highly promises that the 2-D CP₃ material could serve as an outstanding electrode material for Na-ion batteries with an extremely high storage capacity.

Lithiation Mechanism in High-Entropy Oxides as Anode Materials for Li-Ion Batteries: An Operando XAS Study

High-entropy oxides based on transition metals, such as Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O (TM-HEO), have recently drawn special attention as potential anodes in lithium-ion batteries due to high specific capacity and cycling reversibility. However, the lithiation/delithiation mechanism of such systems is still controversial and not clearly addressed. Here, we report on an operando XAS investigation into TM-HEO-based anodes for lithium-ion cells during the first lithiation/delithiation cycle. This material showed a high specific capacity exceeding 600 mAh g-1 at 0.1 C and Coulombic efficiency very close to unity. The combination of functional and advanced spectroscopic studies revealed complex charging mechanisms, developing through the reduction of transition-metal (TM) cations, which triggers the conversion reaction below 1.0 V. The conversion is irreversible and incomplete, leading to the final collapse of the HEO rock-salt structure. Other redox processes are therefore discussed and called to account for the observed cycling behavior of the TM-HEO-based anode. Despite the irreversible phenomena, the HEO cubic structure remains intact for ∼60% of lithiation capacity, so proving the beneficial role of the configuration entropy in enhancing the stability of the HEO rock-salt structure during the redox phenomena.

Ti2P monolayer as a high performance 2-D electrode material for ion batteries

Electrical conductivity, storage capacity and ion diffusion ability are three crucial parameters for battery electrode materials. However, rare existing two-dimensional (2-D) electrode materials can achieve high performances in all these parameters. Here, we report that a 2-D transition-metal phosphide, the Ti₂P monolayer, is a promising superior electrode material which realizes high performances in all the parameters mentioned above. The Ti₂P monolayer has a stable honeycomb crystal structure. It has a metallic electronic structure with Li/Na adsorption, which ensures good electrical conductivity during the battery operation. We find that Li/Na can chemically bond to the Ti₂P substrate, with specific charge exchanges. Our results show the Li/Na capacity in the Ti₂P monolayer is about 846 mA h g^-1, which is much higher than that of the graphite anode. Remarkably, the Li/Na diffusion barrier on the Ti₂P monolayer is only 12-16 meV, which is lower than that in all 2-D anode materials proposed till now. Our work highly promises that theTi₂P monolayer can serve as a superior anode material for Li-ion/Na-ion batteries by providing good electrical conductivity, high storage capacity and ultrafast ion diffusion.

Biomass-Derived Carbons for Sodium-Ion Batteries and Sodium-Ion Capacitors

In the past decade, the rapid development of portable electronic devices, electric vehicles, and electrical devices has stimulated extensive interest in fundamental research and the commercialization of electrochemical energy-storage systems. Biomass-derived carbon has garnered significant research attention as an efficient, inexpensive, and eco-friendly active material for energy-storage systems. Therefore, high-performance carbonaceous materials, derived from renewable sources, have been utilized as electrode materials in sodium-ion batteries and sodium-ion capacitors. Herein, the charge-storage mechanism and utilization of biomass-derived carbon for sodium storage in batteries and capacitors are summarized. In particular, the structure-performance relationship of biomass-derived carbon for sodium storage in the form of batteries and capacitors is discussed. Despite the fact that further research is required to optimize the process and application of biomass-derived carbon in energy-storage devices, the current review demonstrates the potential of carbonaceous materials for next-generation sodium-related energy-storage applications.

Evaporation-Induced Vertical Alignment Enabling Directional Ion Transport in a 2D-Nanosheet-Based Battery Electrode

2D nanosheets have been widely explored as electrode materials owing to their extraordinarily high electrochemical activity and fast solid-state diffusion. However, the scalable electrode fabrication based on this type of material usually suffers from severe performance losses due to restricted ion-transport kinetics in a large thickness. Here, a novel strategy based on evaporation-induced assembly to enable directional ion transport via forming vertically aligned nanosheets is reported. The orientational ordering is achieved by a rapid evaporation of mixed solvents during the electrode fabrication process. Compared with conventional drop-cast electrodes, which exhibit a random arrangement of the nanosheets and obvious decrease of rate performance with increasing thickness, the electrode based on the vertically aligned nanosheets is able to retain the original high rate capability even at high mass loadings and electrode thickness. Combined electrochemical and structural characterization reveals the electrode composed of orientation-controlled nanosheets to possess lower charge-transfer resistances, leading to more complete phase transformation in the active material.

Bulk-Like SnO2-Fe2O3@Carbon Composite as a High-Performance Anode for Lithium Ion Batteries

Boosted power handling and the reduced charging duration of Li ion cells critically rests with ionic/electronic mobility. Ion mobility in electrochemically relevant grains normally stands for an essential restriction of the velocity at which the energy of a cell can be stored/released. To offset sluggish solid-state ionic transport and achieve a superior power/energy density rating, electroactive grains often need exquisite nanoscaling, harming crucial virtues on volumetric packing density, tractability, sustainability, durability, and cost. Unlike elaborate nanostructuring, here we describe that a SnO₂-Fe₂O₃@carbon composite—which adopts a metal oxide particles-intercalated bulk-like configuration—can insert many Li⁺ ions at elevated speeds, despite its micro-dimensionality. Analysis of charge transport kinetics in this tailor-made architecture unveils both much improved ion travel through compact monolithic substances and the greatly enhanced ion access to surfaces of SnO₂/Fe₂O₃ grains. According to the well-studied battery degradation mechanism, it is that both the effective stress management and internal electric field in our as-prepared sample that result in recommendable capacity, rate behavior, and cyclic lifespan (exhibiting a high reversible capacity of 927 mAh g⁻¹ at 0.2 A g⁻¹ with a capacity retention of 95.1% after 100 cycles and an ultra-stable capacity of 429 mAh g⁻¹ even over 1800 cycles at 3 A g⁻¹). Unique materials and working rationale which ensure the swift (de)lithiation of such micrometer-dimensional monoliths may open a door for various high-power/density usages.

Strain-engineered BlueP-MoS2 van der Waals heterostructure with improved lithiation/sodiation for LIBs and SIBs

Innovative van der Waals (vdW) heterostructures formed from various monolayers exhibit exceptional physical properties relevant to their corresponding individual layers. In addition, the strain engineering of 2D materials is significantly exciting because they have the potential to sustain much larger strain in comparison to their bulk counterparts. In this work, the influence of strain on a BlueP-MoS2 van der Waals heterostructure was studied in order to explore its performance in LIBs/SIBs by first-principles DFT calculations. To ascertain the influence of strain on the performance of the BlueP-MoS2 van der Waals heterostructure for electrodes in LIBs/SIBs, we gathered vertically aligned monolayers of MoS2 and BlueP with different amounts of strain and studied the Li/Na storage properties of the said material. The application of strain could effectively enhance the adsorption capability of both Li/Na at the surfaces/interface of the BlueP-MoS2 heterostructure in comparison to that of the pristine BlueP-MoS2 heterostructure along with improved storage capacity. On the other hand, the application of strain is robust to the high mobility of both Li/Na inside and outside surfaces of BlueP-MoS2 heterostructure which ensures the fast charge/discharge process and improved rate performance. The calculated electronic structure revealed that the applied strain converted the BlueP-MoS2 heterostructure from a semiconductor to a metal, indicating enhanced conductivity compared to that for the pristine BlueP-MoS2 heterostructure. All the above-mentioned findings suggest the high potential application of the BlueP-MoS2 vdW heterostructures for flexible nanoelectronic devices.

Advances of 2D bismuth in energy sciences

Since graphene has been successfully exfoliated, two-dimensional (2D) materials constitute a vibrant research field and open vast perspectives in high-performance applications. Among them, bismuthene and 2D bismuth (Bi) are unique with superior properties to fabricate state-of-the-art energy saving, storage and conversion devices. The largest experimentally determined bulk gap, even larger than those of stanene and antimonene, allows 2D Bi to be the most promising candidate to construct room-temperature topological insulators. Moreover, 2D Bi exhibits cyclability for high-performance sodium-ion batteries, and the enlarged surface together with the good electrochemical activity renders it an efficient electrocatalyst for energy conversion. Also, the air-stability of 2D Bi is better than that of silicene, germanene, phosphorene and arsenene, which could enable more practical applications. This review aims to thoroughly explore the fundamentals of 2D Bi and its improved fabrication methods, in order to further bridge gaps between theoretical predictions and experimental achievements in its energy-related applications. We begin with an introduction of the status of 2D Bi in the 2D-material family, which is followed by descriptions of its intrinsic properties along with various fabrication methods. The vast implications of 2D Bi for high-performance devices can be envisioned to add a new pillar in energy sciences. In addition, in the context of recent pioneering studies on moiré superlattices of other 2D materials, we hope that the improved manipulation techniques of bismuthene, along with its unique properties, might even enable 2D Bi to play an important role in future energy-related twistronics.