Molecular-Level Interfacial Chemistry Regulation of MXene Enables Energy Storage beyond Theoretical Limit.

Ti3C2Tx MXene often suffers from poor lithium storage behaviors due to its electrochemically unfavorable OH terminations. Herein, we propose molecular-level interfacial chemistry regulation of Ti3C2Tx MXene with phytic acid (PA) to directly activate its OH terminations. Through constructing hydrogen bonds (H-bonds) between oxygen atoms of PA and OH terminations on Ti3C2Tx surface, interfacial charge distribution of Ti3C2Tx has been effectively regulated, thereby enabling sufficient ion-storage sites and expediting ion transport kinetics for high-performance energy storage. The results show that Li ions preferably bind to H-bond acceptors (oxygen atoms from PA), and the flexibility of H-bonds therefore renders their interactions with adsorbed Li ions chemically "tunable", thus alleviating undesirable localized geometric changes of the OH terminations. Meanwhile the H-bond-induced microscopic dipoles can act as directional Li-ion pumps to expedite ion diffusion kinetics with lower energy barrier. As a result, the as-designed Ti3C2Tx/PA achieves a 2.4-fold capacity enhancement compared with pristine Ti3C2Tx (even beyond theoretical capacity), superior long-term cyclability (220.0 mAh g-1 after 2000 cycles at 2.0 A g-1), and broad temperature adaptability (-20 to 50 °C). This work offers a promising interface engineering strategy to regulate microenvironments of inherent terminations for breaking through the energy storage performance of MXenes.

Conductive Ti3C2Tx networks to optimize Na3V2O2(PO4)2F cathodes for improved rate capability and low-temperature operation.

Na3V2O2(PO4)2F (NVOPF) is gaining attention as a high-energy cathode candidate for sodium-ion batteries owing to its wide operating voltage, high energy density and excellent thermal stability. However, its intrinsic poor electrical conductivity results in its current sodium-storage performance being far below expectations. Herein, two-dimensional Ti3C2Tx MXene nanosheets with excellent electrical conductivity are introduced to construct an interconnected conductive framework to tightly encapsulate NVOPF nanoparticles. The Ti3C2Tx nanosheets ensure superior electronic contacts, along with inhibiting the agglomeration of NVOPF nanoparticles, thus accelerating electron and ion transfer during sodium-ion de/intercalation and maximizing the storage capacity. As a result, the optimized NVOPF/Ti3C2Tx cathode exhibits high rate capabilities (111 mA h g-1 at 0.2 C and 78 mA h g-1 at 20 C), with an impressively high capacity retention of 74.8% over a wide temperature range (from -20 to 20 °C). Additionally, the assembled sodium-ion full cell provides a highly reversible capacity of 116 mA h g-1 at 1 C, with a capacity retention of 67.2% after 100 cycles. These inspiring results provide new insights for improving the charge-transfer kinetics of the NVOPF cathode and this methodology may be extended to other cathode materials.

Surface Charge Engineering for Covalently Assembling Three-Dimensional MXene Network for All-Climate Sodium Ion Batteries.

MXenes, as excellent candidate anode materials for sodium ion batteries (SIBs), suffer from sluggish ion-diffusion kinetics resulting from the anchoring effect of the negatively charged functional groups on their surface on sodium ions. Herein, we introduce positively charged conductive polyaniline (PANI) to induce self-assembly of Ti3C2Tx MXenes into a three-dimensional PANI/Ti3C2Tx network. In this PANI/Ti3C2Tx network, PANI not only intercalates into Ti3C2Tx nanosheets to enlarge the interlayer spacing, but also promotes negative-to-positive transition of the surface charges of the Ti3C2Tx nanosheets, significantly improving ion-diffusion kinetics. Electrochemical test results further confirm the superb ion-diffusion kinetics of the PANI/Ti3C2Tx network. Meanwhile, a covalent interaction (Ti-N) between PANI and Ti3C2Tx, proved by X-ray photoelectron spectra (XPS) and X-ray absorption near-edge structure (XANES) tests, plays a key role in stabilizing this network structure. Therefore, PANI/Ti3C2Tx exhibits excellent sodium storage performances with a high specific capacity, superior rate performance and ultralong lifespan at high current density. More importantly, when operated at rigorous temperatures from +50 to -30 °C, PANI/Ti3C2Tx also exhibits good electrochemical performances. The present work presents a simple strategy for designing 3D porous MXene-based materials to realize high rate performance and all-climate energy storage device.

Boosting the lithium-ion and sodium-ion storage performances of pyrite by regulating the energy barrier of ion transport.

Pyrite (FeS2) is a functional material of great importance for lithium/sodium ion batteries (LIBs/SIBs), but its sluggish dynamics greatly hinder its high performance. Here, we demonstrate an effective strategy of regulating the energy barrier of ion transport to significantly enhance the sluggish dynamics of FeS2 by Co doping. Compared to pristine FeS2, a series of Co-doped FeS2 shows enhanced alkali metal ion storage performance and most typically, the optimized Fe0.7Co0.3S2 sample displays high reversible capacities, of 1170 mA h g-1 for LIBs and 650 mA h g-1 for SIBs at a current density of 0.1 A g-1 as well as super long-life cycling stability for SIBs (1200 cycles at 5 A g-1). The evidently enhanced performances of Fe0.7Co0.3S2 for LIBs/SIBs can be attributed to its significantly decreased activation energy of ion transport, thus leading to greatly accelerated ion transport dynamics. Furthermore, galvanostatic intermittent titration technique (GITT) experiments also support this important regulation effect of Co doping on the ion transport dynamics of FeS2. The excellent ion transport dynamics induce a strong pseudo-capacitance behavior in both SIBs and LIBs, and their pseudo-capacitance contributions are more than 90% at 1.0 mV s-1. This work provides a new perspective to improve the alkali metal ion storage performance by optimizing the ion transport dynamics.

Identifying the Transfer Kinetics of Adsorbed Hydroxyl as a Descriptor of Alkaline Hydrogen Evolution Reaction.

The key descriptor that dominates the kinetics of the alkaline hydrogen evolution reaction (HER) has not yet been unequivocally identified. Herein, we focus on the adsorbed hydroxyl (OHad ) transfer process (OHad + e- ⇄ OH- ) and reveal its crucial role in promoting the overall kinetics of alkaline HER based on Ni/Co-modified MoSe2 model catalysts (Ni-MoSe2 and Co-MoSe2 ) that feature almost identical water dissociation and hydrogen adsorption energies, but evidently different activity trends in alkaline (Ni-MoSe2 ≫ Co-MoSe2 ) and acidic (Co-MoSe2 ≥ Ni-MoSe2 ) media. Experimental and theoretical calculation results demonstrate that tailoring MoSe2 with Ni not only optimizes the hydroxyl adsorption, but also promotes the desorption of OH- and the electron-involved conversion of OHad to OH- , all of which synergistically accelerate the kinetics of OHad + e- ⇄ OH- and thereby the overall kinetics of the alkaline HER.

Unraveling the Beneficial Microstructure Evolution in Pyrite for Boosted Lithium Storage Performance.

Pyrite FeS2 as a high-capacity electrode material for lithium-ion batteries (LIBs) is hindered by its unstable cycling performance owing to the large volume change and irreversible phase segregation from coarsening of Fe. Here, the beneficial microstructure evolution in MoS2 -modified FeS2 is unraveled during the cycling process; the microstructure evolution is responsible for its significantly boosted lithium storage performance, making it suitable for use as an anode for LIBs. Specifically, the FeS2 /MoS2 displays a long cycle life with a capacity retention of 116 % after 600 cycles at 0.5 A g-1 , which is the best among the reported FeS2 -based materials so far. A series of electrochemical tests and structural characterizations substantially revealed that the introduced MoS2 in FeS2 experiences an irreversible electrochemical reaction and thus the in situ formed metallic Mo could act as the conductive buffer layer to accelerate the dynamics of Li+ diffusion and electron transport. More importantly, it can guarantee the highly reversible conversion in lithiated FeS2 by preventing Fe coarsening. This work provides a fundamental understanding and an effective strategy towards the microstructure evolution for boosting lithium storage performances for other metal sulfide-based materials.

Toward Understanding the Enhanced Pseudocapacitive Storage in 3D SnS/MXene Architectures Enabled by Engineered Surface Reactions.

The optimization of three-dimensional (3D) MXene-based electrodes with desired electrochemical performances is highly demanded. Here, a precursor-guided strategy is reported for fabricating the 3D SnS/MXene architecture with tiny SnS nanocrystals (≈5 nm in size) covalently decorated on the wrinkled Ti3 C2 Tx nanosheets through Ti-S bonds (denoted as SnS/Ti3 C2 Tx -O). The formation of Ti-S bonds between SnS and Ti3 C2 Tx was confirmed by extended X-ray absorption fine structure (EXAFS). Rather than bulky SnS plates decorated on Ti3 C2 Tx (SnS/Ti3 C2 Tx -H) by one-step hydrothermal sulfidation followed by post annealing, this SnS/Ti3 C2 Tx -O presents size-dependent structural and dynamic properties. The as-formed 3D hierarchical structure can provide short ion-diffusion pathways and electron transport distances because of the more accessible surface sites. In addition, benefiting from the tiny SnS nanocrystals that can effectively improve Na+ diffusion and suppress structural variation upon charge/discharge processes, the as-obtained SnS/Ti3 C2 Tx -O can generate pseudocapacitance-dominated storage behavior enabled by engineered surface reactions. As predicted, this electrode exhibits an enhanced Na storage capacity of 565 mAh g-1 at 0.1 A g-1 after 75 cycles, outperforming SnS/Ti3 C2 Tx -H (336 mAh g-1 ), SnS (212 mAh g-1 ), and Ti3 C2 Tx (104 mAh g-1 ) electrodes.

A Hybrid Assembly of MXene with NH2 -Si Nanoparticles Boosting Lithium Storage Performance.

A facile hybrid assembly between Ti3 C2 Tx MXene nanosheets and (3-aminopropyl) triethoxylsilane-modified Si nanoparticles (NH2 -Si NPs) was developed to construct multilayer stacking of Ti3 C2 Tx nanosheets with NH2 -Si NPs assembling together (NH2 -Si/Ti3 C2 Tx ). NH2 -Si/Ti3 C2 Tx exhibits a significantly enhanced lithium storage performance compared to pristine Si, which is attributed to the robust crosslinking architecture and considerably improved electrical conductivity as well as shorter Li+ diffusion pathways. The optimized NH2 -Si/Ti3 C2 Tx anode with Ti3 C2 Tx : NH2 -Si mass ratio of 4 : 1 displays an enhanced capacity (864 mAh g-1 at 0.1 C) with robust capacity retention, which is significantly higher than those of NH2 -Si NPs and Ti3 C2 Tx anodes. Furthermore, this work demonstrates the important effect of the MXene-based electrode architecture on the electrochemical performance and can guide future work on designing high-performance Si/MXene hybrids for energy storage applications.

Constructing Conductive Bridge Arrays between Ti3C2Tx MXene Nanosheets for High-Performance Lithium-Ion Batteries and Highly Efficient Hydrogen Evolution.

Ti3C2Tx is a member of the MXene family with high potential for electrochemical applications, including lithium-ion batteries (LIBs) and hydrogen evolution reaction (HER). However, severe interlayer restacking not only causes a great loss of the active sites but also decreases the ionic diffusion channels, both of which significantly degrades the electrochemical performances of LIBs and HER. The common interlayer spacers could increase interlayer space but reduce the conductivity. Herein, we introduce in situ carbon nanotube (CNT) arrays between Ti3C2Tx MXene nanosheets (3D CNTs@Ti3C2Tx) as the conductive bridges for achieving a unique architecture with high conductivity, fast ion/mass transfer channels, and high exposure of the activity sites. In this architecture, 1D CNTs can not only be viewed as the interlayer spacer that prevents Ti3C2Tx MXene nanosheets from recombining but also connect with the neighbor Ti3C2Tx MXene nanosheets providing more ion/electron transport channels. Benefiting from this unique structure that could improve ion/electron transfer kinetics and capacitive contribution, 3D CNTs@Ti3C2Tx displays high specific capacity as an anode for LIBs (491 mA h g-1 at 320 mA g-1). Furthermore, 3D CNTs@Ti3C2Tx also exhibits excellent HER performance in alkaline medium (the overpotential is 93 mV at 10 mA cm-2) and excellent water splitting performance. This strategy that in situ construction of CNT arrays between the MXene nanosheets proves an effective method for the rational design of multifunctional energy storage/conversion materials.

Optimizing Dispersion, Exfoliation, Synthesis, and Device Fabrication of Inorganic Nanomaterials Using Hansen Solubility Parameters.

Hansen solubility parameters (HSPs) were established by Hansen in 1967 and predict miscibility between different material systems. So far, HSP theory works across polymers, crystalline bulk solids and nanomaterials and can be used to identify single solvents or, more likely, blends of solvents that deliver not only the initial solubility but also control it during reaction processes. This minireview summarizes the recent progress on HSP theory to optimize dispersion, exfoliation, synthesis, and device fabrication of inorganic nanomaterials. First, we briefly introduce HSP theory and determination of HSPs. Then, we discuss in detail the utilization of HSPs for inorganic nanomaterials, focusing on carbon nanomaterials, two-dimensional non-graphene nanomaterials, and metal oxide nanoparticles. Finally, challenges and perspectives of HSP theory in inorganic nanomaterials are reviewed.

Atomic Heterointerface-Induced Local Charge Distribution and Enhanced Water Adsorption Behavior in a Cobalt Phosphide Electrocatalyst for Self-Powered Highly Efficient Overall Water Splitting.

Developing economical and highly efficient noble metal-free electrocatalysts for overall water splitting is an essential precondition for renewable energy conversion. Herein, we highlight atomic heterointerface engineering in constructing highly efficient cobalt phosphide (CoP)/Co9S8 electrocatalysts for full water splitting. A CoP/Co9S8 hybrid was prepared for the first time by partial homogeneous transformation of in situ-formed Co9S8, in which the atomic heterointerface was formed between CoP and Co9S8. Systematic experiments and theoretical calculations confirm that the as-formed atomic heterointerface can induce local charge distribution in CoP/Co9S8, which can not only accelerate the charge transfer but also optimize the hydrogen adsorption energy of CoP in favor of the fast transformation of Hads into H2. Meanwhile, the Co9S8 component can also increase the water adsorption capability of CoP/Co9S8. Benefiting from these outstanding advantages, an alkaline electrolyzer based on CoP/Co9S8 as both electrodes achieves a low cell voltage of 1.6 V at an operating current density of 10 mA cm-2, and at the same time, it can also be self-powered by a home-assembled Zn-air battery employing the same CoP/Co9S8 as the air electrode for prospectively achieving renewable energy conversion. This work demonstrates the importance of heterostructure engineering in developing noble metal-free catalysts for high-performance water electrolysis.

Solubility-Parameter-Guided Solvent Selection to Initiate Ostwald Ripening for Interior Space-Tunable Structures with Architecture-Dependent Electrochemical Performance.

Despite significant advancement in preparing various hollow structures by Ostwald ripening, one common problem is the intractable uncontrollability of initiating Ostwald ripening due to the complexity of the reaction processes. Here, a new strategy on Hansen solubility parameter (HSP)-guided solvent selection to initiate Ostwald ripening is proposed. Based on this comprehensive principle for solvent optimization, N,N-dimethylformamide (DMF) was screened out, achieving accurate synthesis of interior space-tunable MoSe2 spherical structures (solid, core-shell, yolk-shell and hollow spheres). The resultant MoSe2 structures exhibit architecture-dependent electrochemical performances towards hydrogen evolution reaction and sodium-ion batteries. This pre-solvent selection strategy can effectively provide researchers great possibility in efficiently synthesizing various hollow structures. This work paves a new pathway for deeply understanding Ostwald ripening.

Rational Construction of Multivoids-Assembled Hybrid Nanospheres Based on VPO4 Encapsulated in Porous Carbon with Superior Lithium Storage Performance.

The design of a new nanostructured anode material with high tap density while still keeping the common advantages of the hollow structure is a great challenge for future lithium-ion batteries (LIBs). Here, multivoids-assembled hierarchically meso-macroporous nanospheres based on VPO4 encapsulated in porous carbon (MVHP-VPO4@C NSs) were designed and fabricated. This unique structure can evidently decrease the excessive interior space in hollow spheres or multishelled hollow spheres to gain high volumetric energy density and at the same time can alleviate the large mechanical strain during the cycling process. As expected, MVHP-VPO4@C NSs show good lithium storage behavior with gravimetric discharge capacity of 628 mAh g-1 after 100 cycles at a current density of 100 mA g-1. Furthermore, the full cell (LiFePO4 cathode//MVHP-VPO4@C NSs anode) also exhibits outstanding lithium storage performance. The insight obtained from this structure may provide guidance for the design of other electrode materials experiencing large volume variation during the lithiation-delithiation process.

Achieving Fully Reversible Conversion in MoO3 for Lithium Ion Batteries by Rational Introduction of CoMoO4.

Electrode materials based on conversion reactions with lithium ions generally show much higher energy density. One of the main challenges in the design of these electrode materials is to improve initial Coulombic efficiency and alleviate the volume changes during the lithiation-delithiation processes. Here, we achieve fully reversible conversion in MoO3 as an anode for lithium ion batteries by the hybridization of CoMoO4. The porous MoO3-CoMoO4 microspheres are constructed by homogeneously dispersed MoO3 and CoMoO4 subunits and their lithiation/delithiation processes were studied by ex situ TEM to reveal the mechanism of the reversible conversion reaction. Co nanoparticles are in situ formed from CoMoO4 during the lithiation process, which then act as the catalyst to guarantee the reversible decomposition of Li2O, thus effectively improving the reversible specific capacity and initial Coulombic efficiency. Moreover, the pores in MoO3-CoMoO4 microspheres also greatly enhance their mechanical strength and provide enough cavity to alleviate volume changes during repeated cycling. Such a design concept makes MoO3 to be a potential promising anode in practical applications. The full cell (LiFePO4 cathode/MoO3-CoMoO4 anode) displays a high capacity up to 155.7 mAh g-1 at 0.1 C and an initial Coulombic efficiency as high as 97.35%. This work provides impetus for further development in electrochemical charge storage devices.

Basil Seed Inspired Design for a Monodisperse Core-Shell Sn@C Hybrid Confined in a Carbon Matrix for Enhanced Lithium-Storage Performance.

Tin anode materials have attracted much attention owing to their high theoretical capacity, although rapid capacity fade is commonly observed mainly because of structural degradation resulting from volume expansion. Herein, we report a versatile strategy based on a basil seed inspired design for constructing a monodisperse core-shell Sn@C hybrid confined in a carbon matrix (Sn basil seeds). Analogous to the structure of basil seeds soaked in water, Sn basil seeds are used to tackle the volume expansion problem in lithium-ion batteries. Monodisperse Sn cores are encapsulated by a thick carbon layer, which thus lowers the electrolyte contact area. The obtained Sn basil seeds are closely packed to construct a framework that supplies fast electron transport and provides a reinforced mechanical backbone. As a consequence, an ensemble of this hybrid network shows significantly enhanced lithium-storage performance with a high capacity of 870 mAh g-1 at a current density of 0.4 A g-1 over 600 cycles. After the intense cycling, the Sn cores transform into ultrafine nanocrystals with sizes of 3-6 nm. The structural and morphological evolution of the Sn cores can reasonably explain the gradual increase in the capacity and the long-term cycling ability of our Sn basil seeds.

A three-dimensional porous MoP@C hybrid as a high-capacity, long-cycle life anode material for lithium-ion batteries.

Metal phosphides are great promising anode materials for lithium-ion batteries with a high gravimetric capacity. However, significant challenges such as low capacity, fast capacity fading and poor cycle stability must be addressed for their practical applications. Herein, we demonstrate a versatile strategy for the synthesis of a novel three-dimensional porous molybdenum phosphide@carbon hybrid (3D porous MoP@C hybrid) by a template sol-gel method followed by an annealing treatment. The resultant hybrid exhibits a 3D interconnected ordered porous structure with a relatively high surface area. Benefiting from its advantages of microstructure and composition, the 3D porous MoP@C hybrid displays excellent lithium storage performance as an anode material for lithium-ion batteries in terms of specific capacity, cycling stability and long-cycle life. It presents stable cycling performance with a high reversible capacity up to 1028 mA h g(-1) at a current density of 100 mA g(-1) after 100 cycles. By ex situ XRD, HRTEM, SAED and XPS analyses, the 3D porous MoP@C hybrid was found to follow the Li-intercalation reaction mechanism (MoP + xLi(+) + e(-)↔ LixMoP), which was further confirmed by ab initio calculations based on density functional theory.

Multidimensional Germanium-Based Materials as Anodes for Lithium-Ion Batteries.

Metallic germanium is an ideal anode for lithium-ion batteries (LIBs), owing to its high theoretical capacity (1624 mA h g(-1) ) and low operating voltage. Herein, we highlight recent advances in the development of Ge-based anodes in LIBs, although improvements in their coulombic efficiency (CE), capacity retention, and rate performance are still required. One of the major concerns facing the development of Ge anodes is the controlled formation of microstructures. In this Focus Review, we summarize Ge-based materials with different structural dimensions, that is, zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), three-dimensional (3D), and even monolithic and macroscale structures. Moreover, the design of Ge-based oxide materials, as an effective route for achieving higher Li-storage capacities and cycling performance, is also discussed. Finally, we briefly summarize new types of Ge-based materials, such as ternary germanium oxides, germanium sulfides, and germanium phosphides, and predict that they will bring about a reformation in the field of LIBs.

Structure Interlacing and Pore Engineering of Zn2GeO4 Nanofibers for Achieving High Capacity and Rate Capability as an Anode Material of Lithium Ion Batteries.

An interlaced Zn2GeO4 nanofiber network with continuous and interpenetrated mesoporous structure was prepared using a facile electrospinning method followed by a thermal treatment. The mesoporous structure in Zn2GeO4 nanofibers is directly in situ constructed by the decomposition of polyvinylpyrolidone (PVP), while the interlaced nanofiber network is achieved by the mutual fusion of the junctions between nanofibers in higher calcination temperatures. When used as an anode material in lithium ion batteries (LIBs), it exhibits superior lithium storage performance in terms of specific capacity, cycling stability, and rate capability. The pore engineering and the interlaced network structure are believed to be responsible for the excellent lithium storage performance. The pore structure allows for easy diffusion of electrolyte, shortens the pathway of Li(+) transport, and alleviates large volume variation during repeated Li(+) extraction/insertion. Moreover, the interlaced network structure can provide continuous electron/ion pathways and effectively accommodate the strain induced by the volume change during the electrochemical reaction, thus maintaining structural stability and mechanical integrity of electrode materials during lithiation/delithiation process. This strategy in current work offers a new perspective in designing high-performance electrodes for LIBs.

Germanium quantum dots embedded in N-doping graphene matrix with sponge-like architecture for enhanced performance in lithium-ion batteries.

Germanium quantum dots embedded in a nitrogen-doped graphene matrix with a sponge-like architecture (Ge/GN sponge) are prepared through a simple and scalable synthetic method, involving freeze drying to obtain the Ge(OH)4 /graphene oxide (GO) precursor and subsequent heat reduction treatment. Upon application as an anode for the lithium-ion battery (LIB), the Ge/GN sponge exhibits a high discharge capacity compared with previously reported N-doped graphene. The electrode with the as-synthesized Ge/GN sponge can deliver a capacity of 1258 mAh g(-1) even after 50 charge/discharge cycles. This improved electrochemical performance can be attributed to the pore memory effect and highly conductive N-doping GN matrix from the unique sponge-like structure.

Point decoration of silicon nanowires: an approach toward single-molecule electrical detection.

Probing interactions of biological systems at the molecular level is of great importance to fundamental biology, diagnosis, and drug discovery. A rational bioassay design of lithographically integrating individual point scattering sites into electrical circuits is capable of realizing real-time, label-free biodetection of influenza H1N1 viruses with single-molecule sensitivity and high selectivity by using silicon nanowires as local reporters in combination with microfluidics. This nanocircuit-based architecture is complementary to more conventional optical techniques, but has the advantages of no bleaching problems and no fluorescent labeling. These advantages offer a promising platform for exploring dynamics of stochastic processes in biological systems and gaining information from genomics to proteomics to improve accurate molecular and even point-of-care clinical diagnosis.