Generating Oxygen Vacancies in MnO Hexagonal Sheets for Ultralong Life Lithium Storage with High Capacity

Design of Gradient Ti Reconstituted Fe2O3 Anodes with Enhanced Lithium Affinity Modulated Electronic Structures: First-Principles Calculations and Experiment Verification

High-performance conversion transition metal oxides are strong candidates for advanced anode materials for lithium-ion batteries. However, the poor intrinsic conductivity and the large volume changes during battery operation are important constraints to its practical application. The heterogeneous atom doping strategy is an important way to modulate the electronic structure and surface states of the host materials. Herein, theoretical calculations reveal that heteroatomic Ti doping and its ionic or electronic compensation mechanisms can well modulate the electronic structure of Fe₂O₃ and change the surface Li-ion affinity. A Ti concentration gradient modification strategy for Fe₂O₃ is proposed to construct high-performance electrode materials. As a Li-ion battery anode, Ti concentration gradient-doped Fe₂O₃ achieves excellent long-cycle stability, with a reversible capacity of 1001.9 mAh g^-1 at 1 A g^-1 for 1200 cycles, and even maintains a reversible specific capacity compared to the theoretical capacity of commercial graphite electrodes at 2 A g^-1 for 2000 cycles. This combination of theoretical calculations and experiments offers ways to intelligently design and develop alkali metal ion batteries.

Nanocrystal Conversion Chemistry within Slit-like 2D Nanogap for High-Rate Cyclic Stability of Lithium-Ion Battery Anodes

Nanoscale optimization of late transition-metal oxides for fixing the reversible lithiation/delithiation mechanism with an in-depth mechanistic understanding of nanocrystal (NC) conversion chemistry is important for furthering next-generation Li-ion battery (LIB) technologies. Herein, 1 nm-thin Ni₃CoO_x (1 nm-NCO) nanosheets synthesized through isomorphic transformation of NiCo layered double hydroxides within a two-dimensional (2D)-SiO₂ envelope are chosen. The interconversion of metal/metal-oxide NCs under redox-switching thermal treatment, while retaining reversibility, inspired the accomplishment of identical consequences under the harsh operational conditions of LIB redox cycles by application of the thin-NCO-defined 2D nanospace. During charge/discharge cycles, 1 nm-NCO covered with an in situ formed solid-electrolyte-interphase layer enables fully reversible interconversion between the reactive NC redox pairs, as evidenced by detailed morphological and electrochemical analyses, thus providing high-rate capability with a specific capacity of 61.2% at 5.0 C relative to 0.2 C, outstanding cycle stability delivering a reversible capacity of 1169 mAh g^-1, and 913 mAh g^-1 with high average Coulombic efficiency (>99.2%) at 3.0 and 5.0 C for 1000 cycles, respectively, which has not been achieved with other transition-metal oxides. Such a nanospace-confinement effect on sustainability of reactive NCs to follow-up a highly reversible conversion reaction at fast charging in LIBs is operative within a slit-like ultrathin 2D nanogap from 1 nm-NCO only, as a relatively thicker 7 nm-NCO anode, with accompanying larger space available, has evidenced poor reversibility of NCs and inadequate cyclic stability under potential high-power density LIB application.

Multifunctional Cr Substitution Modulates Electrochemical Activity of Mn1-xCrxO for High-Performance Lithium-Ion Battery Anodes

Metal oxides are a promising candidate for lithium-ion battery (LIB) anodes due to their high theoretical capacity and long cycle life but also face inherent poor conductivity and volume variation, making them difficult to promote the application. The cation substitution strategy is an important means to facilitate improved rate and cycling performance. However, the effect of cation substitution on electrochemical activity is multivariate and complex, and a comprehensive and systematic analysis is essential for understanding the relationship between components and properties. Herein, the aliovalent heterogeneous Cr-substituted MnO was used as a model to systematically investigate the effects of Cr substitution on the crystal structure, electron distribution, defect construction, and electrochemical reaction processes. Theoretical calculations and experimental results reveal that Cr substitution can effectively modulate the electronic structure, build a built-in electric field, generate cationic defects, and catalyze the electrochemical reaction process, thereby improving the electrode kinetics and electrochemical activity of active materials. When the optimized Mn_0.94Cr_0.06O was used as the anode for LIBs, a reversible capacity of 1547.3 mAh g^-1 was obtained after 450 cycles at a current density of 1 C (1 C = 756 mA g^-1 for half-cells), and a reversible capacity of up to 1126.2 mAh g^-1 could be maintained even after 700 cycles at a current density of 2 C. The assembled Mn_0.94Cr_0.06O//LiCoO₂ full cell further confirms the scalability of the heterogeneous atom substitution strategy.

Iron vacancies engineering of FexC@NC hybrids toward enhanced lithium-ion storage properties

Defect engineering have profound influence on the energy storage properties of electrode hybrids by adjusting their intrinsic electronic characteristics. For iron carbide based materials, however, the effect of defect (especially cation vacancies) toward their electrochemical performance are still unclear. Herein, the feasible and scalable synthesis of Fe_xC@NC with 3D honeycomb-like carbon architecture and abundant Fe vacancies via template etching is reported. Such structure enable outstanding lithium-ion storage properties owing to hierarchical pores, improved intrinsic electrochemical activity, as well as the introduction of more active sites. As a result, the Fe_xC@NC-2 presents a high reversible specific capacity of 1079 mAh g^-1after 1000 cycles. Moreover, an excellent cycling stability can be achieved via maintaining a high-capacity retention (689 mAh g^-1, 98.4%) over 1000 cycles at 5 A g^-1. This study provides a feasible strategy for developing high-performance hybrids with hierarchical pore and rich defects structures.

Synthesis of MnO-Sn cubes embedding in nitrogen-doped carbon nanofibers with high lithium-ion storage performance

In this paper, a carbon nanofiber (CNF) hybrid nanomaterial composed of MnO-Sn cubes embedding in nitrogen-doped CNF (MnO-Sn@CNF) is synthesized through electrospinning and post-thermal reduction processes. It exhibits good electrochemical lithium-ion storage performance as the anode, such as high reversible capacity, outstanding cycle performance (754 mAh g^-1at 1 A g^-1after 1000 cycles), and good rate capability (447 mAh g^-1at 5 A g^-1). The excellent electrochemical properties are derived from a unique nanostructure design. MnO-Sn@CNF has a three-dimensional conductive network with a stable core-shell structure, which improves the electrical conductivity and mechanical stability of the materials. In addition, the mesopores on the surface of carbon fibers can shorten the diffusion distance of lithium ions and promote the combination of active sites of the material with lithium ions. The internal MnO and Sn form a heterostructure, which enhances the stability of the physical structure of the electrode material. This material design method provides a reference strategy for the development of high-performance lithium-ion batteries anode.

In Situ-Grown Heterostructured Co3S4/CNTs/C Nanocomposites with a Bridged Structure for High-Performance Supercapacitors

As one of the most competitive candidates for energy storage devices, supercapacitors have attracted extensive research interest due to their incomparable power density and ultralong cycling stability. However, the large surface area required for charge storage is an irreconcilable contradiction with the requirement of energy density. Therefore, a high energy density is a major challenge for supercapacitors. To solve the contradiction, Co₃S₄/CNTs/C with a bridged structure is designed, where CNTs generated in situ serve as a bridge to connect a porous carbon matrix and a Co₃S₄ nanoparticle, and Co₃S₄ nanoparticles are anchored on the topmost of CNTs. The porous carbon and Co₃S₄ are used for electrochemical double-layer capacitors and pseudocapacitors, respectively. This bridged structure can efficiently utilize the surface of Co₃S₄ nanoparticles to increase the overall energy storage capacity and provide more electrochemically active sites for charge storage and delivery. The materials show an energy density of 41.3 Wh kg^-1 at 691.9 W kg^-1 power density and a retaining energy density of 33.1 Wh kg^-1 at a high power density of 3199.9 W kg^-1 in an asymmetrical supercapacitor. The synthetic technique provides a simple method to obtain heterostructured nanocomposites with a high energy density by maximizing the effect of pseudocapacitor electrode active materials.

C/MnO@void@C with Triple Balances for Superior Microwave Absorption Performance

It is very promising and challenging to construct a yolk-shell structure with highly efficient microwave absorption (MA) performance through a simple fabrication process. Here, a novel C/MnO@void@C (MCC) yolk-shell structure has been successfully synthesized by one-step calcination without additional processing. The as-obtained MCC composites with tunable crystallinity degrees and hollowness can be obtained by treatment at various temperatures. The MCC composites treated at 700 °C (MCC-700) show an impressive MA performance, and the optimal reflection loss of -53.2 dB and an effective absorption bandwidth of 5.4 GHz can be obtained. This excellent performance results from multiple balance mechanisms. First, the regulated permittivity of MCC-700 due to proper crystallinity and hollowness is beneficial for the balance between dielectric loss (tan δ_ε) and impedance match (Z_im). Second, the optimal balance between the increasing polarization range and decreasing polarization intensity can be achieved, which is favorable for the improvement of the MA performance. Third, the multicore yolk-shell structure of MCC-700 is conducive to multiple scattering and continuous energy dissipation. Thus, our new findings provide a rational way for the utilization of yolk-shell structural manganese-based materials.

Oxygen Vacancies Boosting Lithium-Ion Diffusion Kinetics of Lithium Germanate for High-Performance Lithium Storage

Oxygen vacancies play a positive role in optimizing the physical and chemical properties of metal oxides. In this work, we demonstrated oxygen vacancy-promoted enhancement of Li-ion diffusion kinetics in Li₂GeO₃ nanoparticle-encapsulated carbon nanofibers (denoted as Li₂GeO_3-x/C) and accordingly boosted lithium storage. The introduction of the oxygen vacancies in Li₂GeO_3-x/C can enhance electronic conductivity and evidently decrease activation energy of Li-ion transport, thus resulting in evidently accelerated Li-ion diffusion kinetics during the lithiation/delithiation process. Thus, the Li₂GeO_3-x/C nanofibers exhibit an exceptionally large discharge capacity of 1460.5 mA h g^-1 at 0.1 A g^-1, high initial Coulombic efficiency of 81.3%, and excellent rate capability. This facile and efficient strategy could provide a reference for injecting the oxygen vacancies into other metal oxides for high-performance anode materials.

Insights into the Role of Dual-Interfacial Sites in Cu/ZrO2 Catalysts in 5-HMF Hydrogenolysis with Isopropanol

In this work, we synthesized a series of Cu/ZrO₂ catalysts with tunable V_o-Cu⁰ (oxygen vacancy adjacent to Cu metal) and V_Zr-Cu^δ+ (zirconium vacancy adjacent to electron-deficient Cu species) dual-interface sites and investigated the role of the dual-interface sites in the 5-hydroxymethylfurfural (5-HMF) hydrogenolysis reaction with isopropanol as the hydrogen source. By combining a series of in situ infrared characterization and catalytic performance analysis, it is identified that V_o-Cu⁰ interface sites were responsible for activating isopropanol dehydrogenation and C═O dissociation of 5-HMF, while the V_Zr-Cu^δ+ interface sites were responsible for the dehydroxylation of an intermediate product 5-methyl-2-furfuryl alcohol (5-MFA). Specifically, C-OH was first deprotonated on the V_Zr at the V_Zr-Cu^δ+ interface site to reduce the activation energy of 5-MFA dehydroxylation and then adjacent Cu^δ+ promoted the dissociation of the C-O bond by enhancing the adsorption energy while elongating the C-O bond, as confirmed by the density functional theory calculations. Because the dual-interface sites provided separate sites for activating intermediate products and reactants, the coupling reaction caused by competitive adsorption is thus well avoided. Therefore, the optimized Cu/ZrO₂ catalyst with the most V_Zr-Cu^δ+ and moderate V_o-Cu⁰ sites exhibited 98.4% of 2,5-dimethylfuran yield under the conditions of 180 °C and self-vapor pressure.

Oxygen Vacancy Enhanced Two-Dimensional Lithium Titanate for Ultrafast and Long-Life Bifunctional Lithium Storage

Boosting sufficient Li⁺ ion mobility in Li₄Ti₅O₁₂ (LTO) is crucial for high-rate performance lithium storage. Here, an ultrafast charge storage oxygen vacancy two-dimensional (2D) LTO nanosheet was successfully fabricated through a one-pot hydrothermal method. The selectively doped Al³⁺ into octahedron Li⁺/Ti⁴⁺ 16d sites not only provide bulk oxygen vacancy and appropriate distorted TiO₆ octahedra to facilitate Li⁺ ions diffusion, but also serve as a "pillar" to stabilize the Ti-O framework. The oxygen vacancy lowers Li⁺ ion diffusion energy barrier. Moreover, the 2D structure provides open diffusion channels for fast Li⁺ ion transport. As a result, the sample shows excellent electrochemical performance for bifunctional lithium storage. As a lithium-ion battery anode, the capacity retention reaches 112.8 mA h g^-1 after 5000 cycles at 40 C with a fading rate of 0.288% per 100 cycles. Meanwhile, as a lithium-ion capacitor anode, it exhibits an excellent rate capacity of 120 mA h g^-1 after 5000 cycles at 500 C with nearly 100% Coulombic efficiency. The produced LTO shows much higher rate capacity and longer lifetime than the reported LTO. Density functional theory calculations also demonstrate that oxygen vacancy can facilitate Li⁺ ion diffusion kinetics. The relationship between oxygen vacancy content and Li⁺ ions diffusion energy barrier in LTO is quantified. This work pioneers a defect engineering strategy for synthesized high-performance electrode materials.

Metal-Organic Framework-Derived Hierarchical MnO/Co with Oxygen Vacancies toward Elevated-Temperature Li-Ion Battery

Transition metal oxides for high-temperature lithium-ion batteries have captivated orchestrated efforts for next-generation high-energy-density anodes. However, due to inherent low tap density, poor conductivity, and structural instability, their poor cyclability capacity and rate performance at elevated temperatures hinder further implementation. Oxygen vacancies (O_v) engineered by manipulating the active sites and electrical conductivity is a promising method for superior lithium storage. Herein, hierarchical MnO/Co nanoparticle-embedded N-doped carbon nanotube (CNT)-assembled carbonaceous micropolyhedrons (O_v-MnO/Co NCPs) are constructed by a "4S" self-assembly, self-template, self-adaptive, and self-catalytic metal-organic framework template method with in situ oxygen vacancies introduced. Impressively, the internal nanoparticles with metallic Co and the external N-doped carbonaceous matrix entangled by fluffy self-generated CNTs synchronously constructed hierarchical micro/nano-secondary hybrids, facilitating highly compacted density, staggered conductive network, multidirectional diffusion pathways, and accelerated electrochemical kinetics. Experimental and density functional theory investigations systematically manifested that the O_v alongside the local built-in electric field within the crystal lattice induced the boosted electrical conductivity, additional active sites, and alleviated structural expansion, further achieving the exceptional diffusivity coefficient and pseudocapacitive capacity. Benefiting from the integrated structural and compositional optimization, the O_v-MnO/Co NCPs achieved distinguished "3C" performance with superior ultralong cyclability (a volumetric capacity of 1713.5 mAh cm^-3 at 1 A g^-1 up to 1000 cycles), good rate capacity (a well-maintained capacity of 670.2 mAh g^-1 even at 10 A g^-1), and considerable high-temperature capability at 60 °C.

Recent progress of defect chemistry on 2D materials for advanced battery anodes

The rational design of anode materials plays a significant factor in harnessing energy storage. With an in-depth insight into the relationships and mechanisms that underlie the charge and discharge process of two-dimensional (2D) anode materials. The efficiency of rechargeable batteries has significantly been improved through the implementation of defect chemistry on anode materials. This mini review highlights the recent progress achieved in defect chemistry on 2D materials for advanced rechargeable battery electrodes, including vacancies, chemical functionalization, grain boundary, Stone Wales defects, holes and cracks, folding and wrinkling, layered von der Waals (vdW) heterostructure in 2D materials. The defect chemistry on 2D materials provides numerous features such as a more active adsorption sites, great adsorption energy, better ions-diffusion and therefore higher ion storage, which enhances the efficiency of the battery electrode.

Superfine MnO2 Nanowires with Rich Defects Toward Boosted Zinc Ion Storage Performance

The core challenge of MnO₂ as the cathode material of zinc-ion batteries remains to be their poor electrochemical kinetics and stability. Herein, MnO₂ superfine nanowires (∼10 nm) with rich crystal defects (oxygen vacancies and cavities) are demonstrated to possess high efficient zinc-ion storage capability. Experimental and theoretical studies demonstrate that the defects facilitate the adsorption and diffusion of hydrogen/zinc for fast ion transportation and the build of a local electric field for improved electron migration. In addition, the superfine nanostructure could provide sufficient active sites and short diffusion pathways for further promotion of capacity and reaction kinetics of MnO₂. Remarkably, the defect-enriched MnO₂ nanowires manifest an energy density as high as 406 W h kg^-1 and an excellent durability over 1000 cycles without noticeable capacity degradation. Mechanistic analysis substantiates a reversible coinsertion/extraction process of H⁺ and Zn²⁺ with a simultaneous deposition/dissolution of zinc sulfate hydroxide hydrate nanoflakes. This work could enrich the fundamental understanding of defect engineering and nanostructuring on the development of advanced MnO₂ materials toward high-performance zinc-ion batteries.

Constructing the Efficient Ion Diffusion Pathway by Introducing Oxygen Defects in Mn2O3 for High-Performance Aqueous Zinc-Ion Batteries

Mn-based cathodes are admittedly the most promising candidate to achieve the practical applications of aqueous zinc-ion batteries because of the high operating voltage and economic benefit. However, the design of Mn-based cathodes still remains challenging because of the vulnerable chemical architecture and strong electrostatic interaction that lead to the inferior reaction kinetics and rapid capacity decay. These intrinsic drawbacks need to be fundamentally addressed by rationally decorating the crystal structure. Herein, an oxygen-defective Mn-based cathode (Oc_u-Mn₂O₃) is designed via a doping low-valence Cu-ion strategy. The oxygen defect can modify the internal electric field of the material and enhance the substantial electrostatic stability by compensating for the nonzero dipole moment. With the merits of oxygen deficiency, the Oc_u-Mn₂O₃ electrode exhibits the significant diffusion coefficient in the range from 1 × 10^-6 to 1 × 10^-8, and good rate performance. In addition, the Oc_u-Mn₂O₃ maintains the highly reversible cyclic stability with the capacity retention of 88% over 600 cycles. The charge storage mechanism is explored as well, illustrating that the oxygen defects can improve the electrochemical active sites of H⁺ insertion, achieving a better charge-storage capacity than Mn₂O₃.

Biomass-Derived P/N-Co-Doped Carbon Nanosheets Encapsulate Cu3P Nanoparticles as High-Performance Anode Materials for Sodium-Ion Batteries

Biomass-derived approaches have been accepted as a practical way for the design of transitional metal phosphides confined by carbon matrix (TMPs@C) as energy storage materials. Herein, we successfully synthesize P/N-co-doped carbon nanosheets encapsulating Cu₃P nanoparticles (Cu₃P@P/N-C) by a feasible aqueous reaction followed by a phosphorization procedure using sodium alginate as the biomass carbon source. Cu-alginate hydrogel balls can be squeezed into two-dimensional (2D) nanosheets through a freeze–drying process. Then, Cu₃P@P/N-C was obtained after the phosphorization procedure. This rationally designed structure not only improved the kinetics of ion/electron transportation but also buffered the volume expansion of Cu₃P nanoparticles during the continuous charge and discharge processes. In addition, the 2D P/N co-doped carbon nanosheets can also serve as a conductive matrix, which can enhance the electronic conductivity of the whole electrode as well as provide rapid channels for electron/ion diffusion. Thus, when applied as anode materials for sodium-ion batteries, it exhibited remarkable cycling stability and rate performance. Prominently, Cu₃P@P/N-C demonstrated an outstanding reversible capacity of 209.3 mAh g⁻¹ at 1 A g⁻¹ after 1,000 cycles. Besides, it still maintained a superior specific capacity of 118.2 mAh g⁻¹ after 2,000 cycles, even at a high current density of 5 A g⁻¹.

Inner-Stress-Optimized High-Density Fe3O4 Dots Embedded in Graphitic Carbon Layers with Enhanced Lithium Storage

The volume variation of electrode materials will lead to poor cyclability of lithium-ion batteries during the lithiation/delithiation process. Instead, inner-stress fragmentation is creatively used to change carbon-layer-capped Fe₃O₄ particles ∼30 nm in diameter into high-density Fe₃O₄ dots ∼4 nm in size embedded in ultrathin carbon layers. The optimized structure shows a remarkable 45.2% enhancement of lithium storage from 804.7 (the 10th cycle) to 1168.7 mA h g^-1 (the 250th cycle) at 500 mA g^-1, even retaining 1239.5 mA h g^-1 after another 550 cycles. The electrochemical measurements reveal the enhanced capacitive behavior of the high-density Fe₃O₄ dots@C layers, which have more extra active sites for the insertion/extraction of Li⁺ ions, confirmed by the differential capacity plots, leading to remarkably increased specific capacity during cycling. The restructured electrode also shows a superior rate capacity and excellent cycling stability (938.7 and 815.4 mA h g^-1 over 2000 cycles at 1000 and 2000 mA g^-1, respectively). X-ray photoelectron spectroscopy and transmission electron microscopy characterizations show that the optimized structure has stable structural and componential stability even at large rates. This work presents an MOF-guided synthesis of high-density Fe₃O₄-dots' anode material optimized by inner-stress fragmentation, showing a feasible route to design high-efficiency electrode materials.

Oxygen Defects in β-MnO2 Enabling High-Performance Rechargeable Aqueous Zinc/Manganese Dioxide Battery

Rechargeable aqueous Zn/manganese dioxide (Zn/MnO2) batteries are attractive energy storage technology owing to their merits of low cost, high safety, and environmental friendliness. However, the β-MnO2 cathode is still plagued by the sluggish ion insertion kinetics due to the relatively narrow tunneled pathway. Furthermore, the energy storage mechanism is under debate as well. Here, β-MnO2 cathode with enhanced ion insertion kinetics is introduced by the efficient oxygen defect engineering strategy. Density functional theory computations show that the β-MnO2 host structure is more likely for H+ insertion rather than Zn2+, and the introduction of oxygen defects will facilitate the insertion of H+ into β-MnO2. This theoretical conjecture is confirmed by the capacity of 302 mA h g-1 and capacity retention of 94% after 300 cycles in the assembled aqueous Zn/β-MnO2 cell. These results highlight the potentials of defect engineering as a strategy of improving the electrochemical performance of β-MnO2 in aqueous rechargeable batteries.

Improvement of Hard Carbon Electrode Performance by Manipulating SEI Formation at High Charging Rates

There is a growing demand for high-rate rechargeable batteries for powering electric vehicles and portable electronics. Here, we demonstrate a strategy for improving electrode performance by controlling the formation of solid electrolyte interphase (SEI). A composite electrode consisting of hard carbon (HC) and carbon nanotubes (CNTs) was used to study the formation of the SEI at different charging rates in an electrolyte consisting of 1 M NaClO₄ in a mixed solvent with ethylene carbonate (EC) and propylene carbonate (PC), as well as fluoroethylene carbonate (FEC) additive. The half-cell method was used to form the SEI at different charging rates (e.g., 1, 10, and 100 A/g). Symmetric capacitor cells were employed to study ion transport properties through the SEI. It was found that the SEI is a primary factor responsible for limiting the capacity of the composite anode material in conventional ester-based electrolytes. The electrode with the SEI formed at 100 A/g exhibited the lowest impedance and delivered nearly twice the capacity of the electrode with the SEI formed at 1 A/g. This significant difference is due to a thin SEI formed at the fast charging rate, as has been observed with ether-based electrolytes. An identical decay rate (0.11 mA h/g per cycle) was observed on the electrodes with SEIs formed at different charging rates in an ester electrolyte. No chemical difference among the three SEI layers was found. However, morphological differences of the SEI layers were observed. This difference is believed to account for the different electrochemical behaviors of the electrodes. This work shows that high charging rates can result in the formation of an optimal SEI layer, contradicting the widely accepted practice of using low charging rates during the SEI formation in alkali-ion batteries.