Hierarchical Mn3O4/Graphene Microflowers Fabricated via a Selective Dissolution Strategy for Alkali-Metal-Ion Storage

Carbon nanoparticle-entrapped macroporous Mn3O4 microsphere anodes with improved cycling stability for Li-ion batteries

Manganese oxide (Mn₃O₄) has garnered substantial attention as a low-cost, environment-friendly anode material. It undergoes a conversion reaction involving the formation of Li₂O and metallic Mn to provide high-energy Li-ion batteries. However, its low electrical conductivity and significant volume change reduce its capacity during the initial lithiation/delithiation, hindering its practical application. To improve the cycle performance, we propose a new composite structure wherein we entrap carbon nanoparticles in macroporous Mn₃O₄ microspheres with a unique maze-like porous interior. We fabricate the Mn₃O₄/C composites using a scalable two-step process involving the thermal decomposition of MnCO₃ in water vapor and mixing in a carbon-dispersed solution. The fabricated Mn₃O₄/C composites with varying carbon contents exhibit a high maximum discharge capacity retention of 86% after 50 cycles, compared to the 18% given by bare Mn₃O₄. The entrapped carbon nanoparticles improve the cycle performance both electrochemically and physically. The microstructure of the composite particles and the fabrication process developed in this study will help improve the performance of other conversion-type anode materials that suffer from cycle degradation, including inexpensive transition metal oxides and sulfides.

Polymorphic Effects on Electrochemical Performance of Conversion-Based MnO2 Anode Materials for Next-Generation Li Batteries

In this study, four different MnO₂ polymorphs are synthesized with a controlled morphology of hollow porous structures to systematically investigate the influences of polymorphs in conversion-based material. As the structure of these materials transforms into nanosized metal and maintains an extremely low-crystalline phase during cell operation, the effects of polymorphs are overlooked as compared to the case of insertion-based materials. Thus, differences in the ion storage behaviors among various MnO₂ polymorphs are not well identified. Herein, the structural changes, charge storage reaction, and electrochemical performance of the different MnO₂ polymorphs are investigated in detail. The experimental results demonstrate that the charge storage reactions, as part of which spinel-phased MnO₂ formation is observed after lithiation and delithiation instead of recovery of the original phases, are similar for all the samples. However, the electrochemical performance varies depending on the initial crystal structure. Among the four polymorphs, the spinel-type λ-MnO₂ delivers the highest reversible capacity of ≈1270 mAh g^-1 . The structural similarity between the cycled and pristine states of λ-MnO₂ induces faster kinetics, resulting in the better electrochemical performance. These findings suggest that polymorphs are another important factor to consider when designing high-performance materials for next-generation rechargeable batteries.

Structural Engineering and Coupling of Two-Dimensional Transition Metal Compounds for Micro-Supercapacitor Electrodes

The development of portable, wearable, and miniaturized integrated electronics has significantly promoted the immense desire for planar micro-supercapacitors (MSCs) among the extremely competitive energy storage devices. However, their energy density is still insufficient owing to the low electrochemical performance of conventional electrode materials. Compared with their bulk counterparts, the large specific surface area and fast ion transport with efficient intercalation of two-dimensional (2D) transition metal compounds have spurred the research platforms for their exploitation in the creation of high-performance MSCs. This Outlook presents a systematic summary of cutting-edge research on atomically thin, layered structures of transition metal dichalcogenides, MXenes, and transition metal oxides/hydroxides. Special emphasis is given to the rapid and durable storage of ions, benefiting from the low ion diffusion barriers of host interlayer spaces. Moreover, various strategies have been described to circumvent the structural damage due to the volume change and simultaneously evincing remarkable electronic properties.

Novel Hoberman Sphere Design for Interlaced Mn3O4@CNT Architecture with Atomic Layer Deposition-Coated TiO2 Overlayer as Advanced Anodes in Li-Ion Battery

The Hoberman sphere is a stable and stretchable spatial structure with a unique design concept, which can be taken as the ideal prototype of the internal mechanical/conductive skeleton for the anode with large volume change. Herein, Mn₃O₄ nanoparticles are interlaced with a Hoberman sphere-like interconnected carbon nanotube (CNT) network via a facile self-assembly strategy in which Mn₃O₄ can "locally expand" in the CNT network, limit the volume expansion to the interior space, and maintain a stable outer surface of the hybrid particle. Furthermore, an ultrathin uniform ALD-coated TiO₂ shell is adopted to stabilize the solid electrolyte interphase (SEI), provide high electron conductivity and lithium ion (Li⁺) diffusivity with lithiated Li_xTiO₂, and enhance the reaction kinetics of the Mn₃O₄ by an "electron-density enhancement effect". With this design, the Mn₃O₄@CNT/TiO₂ exhibits a high capacity of 1064 mAh g^-1 at 0.1 A g^-1, a stable cycling stability over 200 cycles, a superior rate capability, and a commercial-level areal capacity of 4.9 mAh cm^-2. In this way, a novel electrode design strategy is achieved by the Hoberman sphere-like CNT design along with the in situ porous formation, which can not only achieve a high-performance anode for LIBs but also can be widely adapted in a variety of advanced electrode materials for alkali metal ion batteries.

Facile fabrication of a vanadium nitride/carbon fiber composite for half/full sodium-ion and potassium-ion batteries with long-term cycling performance

Vanadium-based composite anodes have been designed for applications in alkali metal ion batteries, including lithium-ion batteries (LIBs), sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs). However, the problems of inferior long-term cycling stability caused by the large volume change and dissolution of vanadium-based active materials during cycles and slow diffusion for large radii of Na⁺ and K⁺ still limit their underlying capability and need to be addressed. In the present work, we initially designed and fabricated a vanadium nitride/carbon fiber (VN/CNF) composite via a facile electrospinning method followed by the ammonization process. The obtained VN/CNF composite anode exhibited excellent half/full sodium and potassium storage performance. When used as an anode material for SIBs, it delivered a high capacity of 403 mA h g^-1 at 0.1 A g^-1 after 100 cycles and as large as 237 mA h g^-1 at 2 A g^-1 even after 4000 cycles with negligible capacity fading. More importantly, the VN/CNFs//Na₃V₂(PO₄)₃ full cell by coupling the VN/CNF composite anode with the Na₃V₂(PO₄)₃ (NVP) cathode also exhibited a desirable capacity of 257 mA h g^-1 at 500 mA g^-1 after 50 cycles. Besides, when further evaluated as an anode for PIBs, the VN/CNF composite anode achieved a large capacity of 266 mA h g^-1 after 200 cycles at 0.1 A g^-1 and maintained a stable capacity of 152 mA h g^-1 at 1 A g^-1 even after 1000 cycles, showing significant long-term cycling stability. This is one of the best performances of vanadium-based anode materials for SIBs and PIBs reported so far.

A Mn3O4 nanospheres@rGO architecture with capacitive effects on high potassium storage capability

A two dimensional (2D) Mn₃O₄@rGO architecture has been investigated as an anode material for potassium-ion secondary batteries. Herein, we report the synthesis of a Mn₃O₄@rGO nanocomposite and its potassium storage properties. The strong synergistic interaction between high surface area reduced graphene oxide (rGO) sheets and Mn₃O₄ nanospheres not only enhances the potassium storage capacity but also improves the reaction kinetics by offering an increased electrode/electrolyte contact area and consequently reduces the ion/electron transport resistance. Spherical Mn₃O₄ nanospheres with a size of 30-60 nm anchored on the surface of rGO sheets deliver a high potassium storage capacity of 802 mA h g^-1 at a current density of 0.1 A g^-1 along with superior rate capability even at 10 A g^-1 (delivers 95 mA h g^-1) and cycling stability. A reversible potassium storage capacity of 635 mA h g^-1 is retained (90%) after 500 cycles even at a high current density of 0.5 A g^-1. Moreover, the spherical Mn₃O₄@rGO architecture not only offers facile potassium ion diffusion into the bulk but also contributes surface K⁺ ion storage. The obtained results demonstrate that the 2D spherical Mn₃O₄@rGO nanocomposite is a promising anode architecture for high performance KIBs.