Metal–Organic Framework Hybrid‐Assisted Formation of Co3O4/Co‐Fe Oxide Double‐Shelled Nanoboxes for Enhanced Oxygen Evolution

Rational design of complex metal–organic framework (MOF) hybrid precursors offers a great opportunity to construct various functional nanostructures. Here, a novel MOF‐hybrid‐assisted strategy to synthesize Co₃O₄/Co‐Fe oxide double‐shelled nanoboxes is reported. In the first step, zeolitic imidazolate framework‐67 (ZIF‐67, a Co‐based MOF)/Co‐Fe Prussian blue analogue (PBA) yolk–shell nanocubes are formed via a facile anion‐exchange reaction between ZIF‐67 nanocube precursors and [Fe(CN)₆]^3? ions at room temperature. Subsequently, an annealing treatment is applied to prepare Co₃O₄/Co‐Fe oxide double‐shelled nanoboxes. Owing to the structural and compositional benefits, the as‐derived Co₃O₄/Co‐Fe oxide double‐shelled nanoboxes exhibit enhanced electrocatalytic performance for oxygen evolution reaction in alkaline solution.     

Core–Shell Nitrogen‐Doped Carbon Hollow Spheres/Co3O4 Nanosheets as Advanced Electrode for High‐Performance Supercapacitor

Co₃O₄/nitrogen‐doped carbon hollow spheres (Co₃O₄/NHCSs) with hierarchical structures are synthesized by virtue of a hydrothermal method and subsequent calcination treatment. NHCSs, as a hard template, can aid the generation of Co₃O₄ nanosheets on its surface; while SiO₂ spheres, as a sacrificed‐template, can be dissolved in the process. The prepared Co₃O₄/NHCS composites are investigated as the electrode active material. This composite exhibits an enhanced performance than Co₃O₄ itself. A higher specific capacitance of 581 F g^?1 at 1 A g^?1 and a higher rate performance of 91.6% retention at 20 A g^?1 are achieved, better than Co₃O₄ nanorods (318 F g^?1 at 1 A g^?1 and 67.1% retention at 20 A g^?1). In addition, the composite is employed as a positive electrode to fabricate an asymmetric supercapacitor. The device can deliver a high energy density of 34.5 Wh kg^?1 at the power density of 753 W kg^?1 and display a desirable cycling stability. All of these attractive results make the unique hierarchical Co₃O₄/NHCS core–shell structure a promising electrode material for high‐performance supercapacitors.     

Rattle‐Type Fe3O4@SiO2 Hollow Mesoporous Spheres as Carriers for Drug Delivery

Rattle‐type Fe₃O₄@SiO₂ hollow mesoporous spheres with different particle sizes, different mesoporous shell thicknesses, and different levels of Fe₃O₄ content are prepared by using carbon spheres as templates. The effects of particle size and concentration of Fe₃O₄@SiO₂ hollow mesoporous spheres on cell uptake and their in vitro cytotoxicity to HeLa cells are evaluated. The spheres exhibit relatively fast cell uptake. Concentrations of up to 150 µg mL^?1 show no cytotoxicity, whereas a concentration of 200 µg mL^?1 shows a small amount of cytotoxicity after 48 h of incubation. Doxorubicin hydrochloride (DOX), an anticancer drug, is loaded into the Fe₃O₄@SiO₂ hollow mesoporous spheres, and the DOX‐loaded spheres exhibit a somewhat higher cytotoxicity than free DOX. These results indicate the potential of Fe₃O₄@SiO₂ hollow mesoporous spheres for drug loading and delivery into cancer cells to induce cell death.     

Yolk–Shelled C@Fe3O4 Nanoboxes as Efficient Sulfur Hosts for High‐Performance Lithium–Sulfur Batteries

Owing to the high theoretical specific capacity (1675 mA h g^?1) and low cost, lithium–sulfur (Li–S) batteries offer advantages for next‐generation energy storage. However, the polysulfide dissolution and low electronic conductivity of sulfur cathodes limit the practical application of Li–S batteries. To address such issues, well‐designed yolk–shelled carbon@Fe₃O₄ (YSC@Fe₃O₄) nanoboxes as highly efficient sulfur hosts for Li–S batteries are reported here. With both physical entrapment by carbon shells and strong chemical interaction with Fe₃O₄ cores, this unique architecture immobilizes the active material and inhibits diffusion of the polysulfide intermediates. Moreover, due to their high conductivity, the carbon shells and the polar Fe₃O₄ cores facilitate fast electron/ion transport and promote continuous reactivation of the active material during the charge/discharge process, resulting in improved electrochemical utilization and reversibility. With these merits, the S/YSC@Fe₃O₄ cathodes support high sulfur content (80 wt%) and loading (5.5 mg cm^?2) and deliver high specific capacity, excellent rate capacity, and long cycling stability. This work provides a new perspective to design a carbon/metal‐oxide‐based yolk–shelled framework as a high sulfur‐loading host for advanced Li–S batteries with superior electrochemical properties.     

Insight into Ca‐Substitution Effects on O3‐Type NaNi1/3Fe1/3Mn1/3O2 Cathode Materials for Sodium‐Ion Batteries Application

O3‐type NaNi_1/3Fe_1/3Mn_1/3O₂ (NaNFM) is well investigated as a promising cathode material for sodium‐ion batteries (SIBs), but the cycling stability of NaNFM still needs to be improved by using novel electrolytes or optimizing their structure with the substitution of different elements sites. To enlarge the alkali‐layer distance inside the layer structure of NaNFM may benefit Na⁺ diffusion. Herein, the effect of Ca‐substitution is reported in Na sites on the structural and electrochemical properties of Na_1?xCa_x/2NFM (x = 0, 0.05, 0.1). X‐ray diffraction (XRD) patterns of the prepared Na_1?xCa_x/2NFM samples show single α‐NaFeO₂ type phase with slightly increased alkali‐layer distance as Ca content increases. The cycling stabilities of Ca‐substituted samples are remarkably improved. The Na_0.9Ca_0.05Ni_1/3Fe_1/3Mn_1/3O₂ (Na_0.9Ca_0.05NFM) cathode delivers a capacity of 116.3 mAh g^?1 with capacity retention of 92% after 200 cycles at 1C rate. In operando XRD indicates a reversible structural evolution through an O3‐P3‐P3‐O3 sequence of Na_0.9Ca_0.05NFM cathode during cycling. Compared to NaNMF, the Na_0.9Ca_0.05NFM cathode shows a wider voltage range in pure P3 phase state during the charge/discharge process and exhibits better structure recoverability after cycling. The superior cycling stability of Na_0.9Ca_0.05NFM makes it a promising material for practical applications in sodium‐ion batteries.     

Na2Ti3O7@N‐Doped Carbon Hollow Spheres for Sodium‐Ion Batteries with Excellent Rate Performance

Uniform Na₂Ti₃O₇ hollow spheres assembled from N‐doped carbon‐coated ultrathin nanosheets are synthesized. A unique multilayer structure of nanosheets is presumed to significantly reduce energy consumption during the diffusion process of sodium ions, while the carbon‐coated structure can increase the overall conductivity. The as‐prepared sample used as an anode in sodium‐ion batteries exhibits the best rate performance ever reported for Na₂Ti₃O₇, delivering more than 60 mAh g^?1 after 1000 continuous cycles at the high rate of 50 C, which was achieved due to its unique structure.     

SnS2/Co3S4 Hollow Nanocubes Anchored on S‐Doped Graphene for Ultrafast and Stable Na‐Ion Storage

SnS₂ has been widely studied as an anode material for sodium‐ion batteries (SIBs) based on the high theoretical capacity and layered structure. Unfortunately, rapid capacity decay associated with volume variation during cycling limits practical application. Herein, SnS₂/Co₃S₄ hollow nanocubes anchored on S‐doped graphene are synthesized for the first time via coprecipitation and hydrothermal methods. When applied as the anode for SIBs, the sample delivers a distinguished charge specific capacity of 1141.8 mAh g^?1 and there is no significant capacity decay (0.1 A g^?1 for 50 cycles). When the rate is increased to 0.5 A g^?1, it presents 845.7 mAh g^?1 after cycling 100 times. Furthermore, the composite also exhibits an ultrafast sodium storage capability where 392.9 mAh g^?1 can be obtained at 10 A g^?1 and the charging time is less than 3 min. The outstanding electrochemical properties can be ascribed to the enhancement of conductivity for the addition of S‐doped graphene and the existence of p–n junctions in the SnS₂/Co₃S₄ heterostructure. Moreover, the presence of mesopores between nanosheets can alleviate volume expansion during cycling as well as being beneficial for the migration of Na⁺.     

Origin of Fracture‐Resistance to Large Volume Change in Cu‐Substituted Co3O4 Electrodes

The electrode materials conducive to conversion reactions undergo large volume change in cycles which restrict their further development. It has been demonstrated that incorporation of a third element into metal oxides can improve the cycling stability while the mechanism remains unknown. Here, an in situ and ex situ electron microscopy investigation of structural evolutions of Cu‐substituted Co₃O₄ supplemented by first‐principles calculations is reported to reveal the mechanism. An interconnected framework of ultrathin metallic copper formed provides a high conductivity backbone and cohesive support to accommodate the volume change and has a cube‐on‐cube orientation relationship with Li₂O. In charge, a portion of Cu metal is oxidized to CuO, which maintains a cube‐on‐cube orientation relationship with Cu. The Co metal and oxides remain as nanoclusters (less than 5 nm) thus active in subsequent cycles. This adaptive architecture accommodates the formation of Li₂O in the discharge cycle and underpins the catalytic activity of Li₂O decomposition in the charge cycle.     

Rational Design of Na(Li1/3Mn2/3)O2 Operated by Anionic Redox Reactions for Advanced Sodium‐Ion Batteries

In an effort to develop high‐energy‐density cathodes for sodium‐ion batteries (SIBs), low‐cost, high capacity Na(Li_1/3Mn_2/3)O₂ is discovered, which utilizes the labile O 2p‐electron for charge compensation during the intercalation process, inspired by Li₂MnO₃ redox reactions. Na(Li_1/3Mn_2/3)O₂ is systematically designed by first‐principles calculations considering the Li/Na mixing enthalpy based on the site preference of Na in the Li sites of Li₂MnO₃. Using the anionic redox reaction (O^2?/O^?), this Mn‐oxide is predicted to show high redox potentials (≈4.2 V vs Na/Na⁺) with high charge capacity (190 mAh g^?1). Predicted cathode performance is validated by experimental synthesis, characterization, and cyclic performance studies. Through a fundamental understanding of the redox reaction mechanism in Li₂MnO₃, Na(Li_1/3Mn_2/3)O₂ is designed as an example of a new class of promising cathode materials, Na(Li_1/3M_2/3)O₂ (M: transition metals featuring stabilized M⁴⁺), for further advances in SIBs.     

Countering the Segregation of Transition‐Metal Ions in LiMn1/3Co1/3Ni1/3O2 Cathode for Ultralong Life and High‐Energy Li‐Ion Batteries

High‐voltage layered lithium transition‐metal oxides are very promising cathodes for high‐energy Li‐ion batteries. However, these materials often suffer from a fast degradation of cycling stability due to structural evolutions. It seriously impedes the large‐scale application of layered lithium transition‐metal oxides. In this work, an ultralong life LiMn_1/3Co_1/3Ni_1/3O₂ microspherical cathode is prepared by constructing an Mn‐rich surface. Its capacity retention ratio at 700 mA g^?1 is as large as 92.9% after 600 cycles. The energy dispersive X‐ray maps of electrodes after numerous cycles demonstrate that the ultralong life of the as‐prepared cathode is attributed to the mitigation of TM‐ions segregation. Additionally, it is discovered that layered lithium transition‐metal oxide cathodes with an Mn‐rich surface can mitigate the segregation of TM ions and the corrosion of active materials. This study provides a new strategy to counter the segregation of TM ions in layered lithium transition‐metal oxides and will help to the design and development of high‐energy cathodes with ultralong life.     

Composition and Architecture Design of Double‐Shelled Co0.85Se1−xSx@Carbon/Graphene Hollow Polyhedron with Superior Alkali (Li,Na, K)‐Ion Storage

The exploration of materials with reversible and stable electrochemical performance is crucial in energy storage, which can (de) intercalate all the alkali‐metal ions (Li⁺, Na⁺, and K⁺). Although transition‐metal chalcogenides are investigated continually, the design and controllable preparation of hierarchical nanostructure and subtle composite withstable properties are still great challenges. Herein, component‐optimal Co_0.85Se_1?xS_x nanoparticles are fabricated by in situ sulfidization of metal organic framework, which are wrapped by the S‐doped graphene, constructing a hollow polyhedron framework with double carbon shells (CoSSe@C/G). Benefiting from the synergistic effect of composition regulation and architecture design by S‐substitution, the electrochemical kinetic is enhanced by the boosted electrochemistry‐active sites, and the volume variation is mitigated by the designed structure, resulting in the advanced alkali‐ion storage performance. Thus, it delivers an outstanding reversible capacity of 636.2 mAh g^?1 at 2 A g^?1 after 1400 cycles for Li‐ion batteries. Remarkably, satisfactory initial charge capacities of 548.1 and 532.9 mAh g^?1 at 0.1 A g^?1 can be obtained for Na‐ion and K‐ion batteries, respectively. The prominent performance combined with the theory calculation confirms that the synergistic strategy can improve the alkali‐ion transportation and structure stability, providing an instructive guide for designing high‐performance anode materials for universal alkali‐ion storage.     

Boosting the electrochemical performance of LiNi0.8Co0.1Mn0.1O2 cathode materials with Zn3(PO4)2 surface coating

Ni-rich layered oxides have been demonstrated to be promising cathode materials for high-energy–density batteries. However, most of them suffer from sluggish kinetics and structural instability, severely impeding their practical applications. Herein, the surface of LiNi_0.8Co_0.1Mn_0.1O₂ cathode is modified with ionic, conducting zinc phosphate (Zn₃(PO₄)₂) nanolayers. The nanolayers are autogenously formed from the reaction of NH₄H₂PO₄ with ZnO assisted with citric acid. The as-prepared 3 wt% Zn₃(PO₄)₂ coated sample exhibits a first discharge capacity of 203.4 mAh g^-1and excellent capacity retention for 100 cycles. The surface Zn₃(PO₄)₂ nanolayers positively impact the cell performance by scavenging HF and H₂O in the electrolyte, leading to less formation of byproducts on the surface of the cathodes, which lowers the cell resistance and polarization voltage. Our study provides a simple and efficient strategy to design and optimize promising layer-structural cathodes for LIBs.