Promising Three-Dimensional Flowerlike CuWO₄ Photoanode Modified with CdS and FeOOH for Efficient Photoelectrochemical Water Splitting

This paper describes a novel promising film based on the flowerlike CuWO₄ structure, and applied to photoelectrochemical (PEC) water splitting as a photoanode first. The growth mechanism and microstructure of CuWO₄ are discussed in detail. The PEC measurements indicate that flowerlike CuWO₄ exhibited a photocurrent density of 0.58 mA/cm² at 0.8 V versus RHE. When coupled with CdS and FeOOH layers, the triple CuWO₄/CdS/FeOOH photoanode exhibited further improved PEC performance with a higher photocurrent density of about 2.05 mA/cm² at 0.8 V versus RHE and excellent operation stability. The remarkable PEC performance stems from several crucial factors: (i) ideal band gap; (ii) improved light absorption; (iii) efficient charge–hole pair separation and collection

Controllably Designed “Vice-Electrode” Interlayers Harvesting High Performance Lithium Sulfur Batteries

An interlayer has been regarded as a promising mediator to prolong the life span of lithium sulfur batteries because its excellent absorbability to soluble polysulfide efficiently hinders the shuttle effect. Herein, we designed various interlayers and understand the working mechanism of an interlayer for lithium sulfur batteries in detail. It was found that the electrochemical performance of a S electrode for an interlayer located in cathode side is superior to the pristine one without interlayers. Surprisingly, the performance of the S electrode for an interlayer located in anode side is poorer than that of pristine one. For comparison, glass fibers were also studied as a nonconductive interlayer for lithium sulfur batteries. Unlike the two interlayers above, these nonconductive interlayer did displays significant capacity fading because polysulfides were adsorbed onto insulated interlayer. Thus, the nonconductive interlayer function as a “dead zone” upon cycling. Based on our findings, it was for the first time proposed that a controllably optimized interlayer, with electrical conductivity as well as the absorbability of polysulfides, may function as a “vice-electrode” of the anode or cathode upon cycling. Therefore, the cathodic conductive interlayer can enhance lithium sulfur battery performance, and the anodic conductive interlayer may be helpful for the rational design of 3D networks for the protection of lithium metal

Graphene Nanoribbons Derived from the Unzipping of Carbon Nanotubes: Controlled Synthesis and Superior Lithium Storage Performance

Graphene nanoribbons (GNRs) from chemical unzipping of carbon nanotubes (CNTs) have been reported to be a suitable candidate for lithium ion battery materials, but very few of them focused on controlling GNRs with different unzipping levels. Here we present a study of GNRs with controlled unzipping level and the prevailing factors that affect the lithium storage performance at early and final unzipping level; besides, the effect of thermal reduction has been investigated. On the basis of Raman and BET surface area tests, we found that the unzipping of CNTs starts with surface etching and then proceeds to partial and full unzipping and finally fragmentation and aggregation. Galvanostatic charge–discharge reveals that defect increase is mainly responsible for the capacity enhancement at the early unzipping level; surface area drop is associated with the capacity fade at the final unzipping level. Surface functional groups can result in low electrical conductivity and therefore cause capacity drop within several cycles. The GNRs with controlled unzipping level display different electrochemical behaviors and thus can provide rational choices for researchers who are searching for desired functions using GNRs as additives in lithium ion batteries

SnO₂/Reduced Graphene Oxide Interlayer Mitigating the Shuttle Effect of Li–S Batteries

The short cycle life of lithium–sulfur batteries (LSBs) plagues its practical application. In this study, a uniform SnO₂/reduced graphene oxide (denoted as SnO₂/rGO) composite is successfully designed onto the commercial polypropylene separator for use of interlayer of LSBs to decrease the charge-transfer resistance and trap the soluble lithium polysulfides (LPSs). As a result, the assembled devices using the separator modified with the functional interlayer (SnO₂/rGO) exhibit improved cycle performance; for instance, over 200 cycles at 1C, the discharge capacity of the cells reaches 734 mAh g^–1. The cells also display high rate capability, with the average discharge capacity of 541.9 mAh g^–1 at 5C. Additionally, the mechanism of anchoring behavior of the SnO₂/rGO interlayer was systematically investigated using density functional theory calculations. The results demonstrate that the improved performance is related to the ability of SnO₂/rGO to effectively absorb S₈ cluster and LPS. The strong Li–O/Sn–S/O–S bonds and tight chemical adsorption between LPS and SnO₂ mitigate the shuttle effect of LSBs. This study demonstrates that engineering the functional interlayer of metal oxide and carbon materials in LSBs may be an easy way to improve their rate capacity and cycling life

Metal–Organic Frameworks-Derived Co₂P@N-C@rGO with Dual Protection Layers for Improved Sodium Storage

The Co₂P nanoparticles hybridized with unique N-doping carbon matrices have been successfully designed employing ZIF-67 as the precursor via a facile two-step procedure. The Co₂P nanostructures are shielded with reduced graphene oxide (rGO) to enhance electrical conductivity and mitigate volume expansion/shrinkage during sodium storage. As anode materials for sodium-ion batteries (SIBs), the novel architectures of Co₂P@N-C@rGO exhibited excellent sodium storage performance with a high reversible capacity of 225 mA h g^–1 at 50 mA g^–1 after 100 cycles. Our study demonstrates the significant potential of Co₂P@N-C@rGO as anode materials for SIBs

Promising Dual-Doped Graphene Aerogel/SnS₂ Nanocrystal Building High Performance Sodium Ion Batteries

We report the effort in designing layered SnS₂ nanocrystals decorated on nitrogen and sulfur dual-doped graphene aerogels (SnS₂@N,S-GA) as anode material of SIBs. The optimized mass loading of SnS₂ along with the addition of nitrogen and sulfur on the surface of GAs results in enhanced electrochemical performance of SnS₂@N,S-GA composite. In particular, the introduction of nitrogen and sulfur heteroatoms could provide more active sites and good accessibility for Na ions. Moreover, the incorporation of the stable SnS₂ crystal structure within the anode results in the superior discharge capacity of 527 mAh g^–1 under a current density of 20 mA g^–1 upon 50 cycles. It maintains 340 mAh g^–1 even the current density is increased to 800 mA g^–1. Aiming to further systematically study mechanism of composite with improved SIB performance, we construct the corresponding models based on experimental data and conduct first-principles calculations. The calculated results indicate the sulfur atoms doped in GAs show a strong bridging effect with the SnS₂ nanocrystals, contributing to build robust architecture for electrode. Simultaneously, heteroatom dual doping of GAs shows the imperative function for improved electrical conductivity. Herein, first-principles calculations present a theoretical explanation for outstanding cycling properties of SnS₂@N,S-GA composite

Superior Cathode Performance of Nitrogen-Doped Graphene Frameworks for Lithium Ion Batteries

Development of alternative cathode materials is of highly desirable for sustainable and cost-efficient lithium-ion batteries (LIBs) in energy storage fields. In this study, for the first time, we report tunable nitrogen-doped graphene with active functional groups for cathode utilization of LIBs. When employed as cathode materials, the functionalized graphene frameworks with a nitrogen content of 9.26 at% retain a reversible capacity of 344 mAh g^–1 after 200 cycles at a current density of 50 mA g^–1. More surprisingly, when conducted at a high current density of 1 A g^–1, this cathode delivers a high reversible capacity of 146 mAh g^–1 after 1000 cycles. Our current research demonstrates the effective significance of nitrogen doping on enhancing cathode performance of functionalized graphene for LIBs

Electrochemical Changes in Lithium-Battery Electrodes Studied Using ⁷Li NMR and Enhanced ¹³C NMR of Graphene and Graphitic Carbons

An anode composed of tin-core, graphitic-carbon-shell nanoparticles distributed on graphene nanosheets, Sn@C-GNs, is studied during the lithiation process. ⁷Li NMR provides an accurate measure of the stepwise reduction of metallic Sn to lithium–tin alloys and reduction of the graphitic carbon. The metallic nanoparticle cores are observed to form ordered, crystalline phases at each step of the lithiation process. The ⁷Li 2D experiments presented provide insight into the proximity of the various phases, reflecting the mechanism of the electrochemical reaction. In particular, a sequential model of nanoparticle lithiation, rather than a simultaneous process, is suggested. Movement of lithium ions between two elements of the nanostructured Sn@C-GNs material, the metallic core and carbon shell, is also observed. Conventional ¹³C solid-state NMR, SSNMR, experiments on <5 mg of active material from electrochemical cells were found to be impossible, but signal enhancements (up to 18-fold) via the use of extended echo trains in conjunction with magic-angle spinning enabled NMR characterization of the carbon. We demonstrate that the ¹³C data is extremely sensitive to the added electron density when the graphitic carbon is reduced. We also investigate ex situ carbon electrodes from cycled Li–O₂ cells, where we find no evidence of charge sharing between the electrochemically active species and the graphitic carbon in the ¹³C NMR spectroscopy

Observation of Surface/Defect States of SnO₂ Nanowires on Different Substrates from X-ray Excited Optical Luminescence

SnO₂ nanowires (NWs) have been successfully synthesized on two different substrates (stainless steel (SS) and copper) via a facile hydrothermal process. SnO₂ NWs with varying degrees of crystallinity are obtained on different substrates. The growth mechanisms are also deducted by observing the morphology revolution at various reaction times. Furthermore, the electronic structures and optical properties have been investigated by X-ray absorption near edge structure (XANES) and X-ray excited optical luminescence (XEOL) measurements. The yellow-green luminescence from SnO₂ NWs is originated from the intrinsic surface states. Compared with SnO₂ NWs on copper, a near infrared (NIR) luminescence is observed for SnO₂ NWs on SS, which resulted from poor crystallinity and an abundance of defect/surface states

Atomic Layer Deposition of Lithium Tantalate Solid-State Electrolytes

3D all-solid-state microbatteries are promising onboard power systems for autonomous devices. The fabrication of 3D microbatteries needs deposition of active materials, especially solid-state electrolytes, as conformal and pinhole free thin films in 3D architectures, which has proven very difficult for conventional deposition techniques, such as chemical vapor deposition and physical vapor deposition. Herein, we report an alternative technique, atomic layer deposition (ALD), for achieving ideal solid-state electrolyte thin films for 3D microbatteries. Lithium tantalate solid-state electrolytes, with well-controlled film composition and film thickness, were grown by ALD at 225 °C using subcycle combination of 1 × Li₂O + <i>n</i> × Ta₂O₅ (1 ≤ <i>n</i> ≤ 10). The film composition was tunable by varying Ta₂O₅ subcycles (<i>n</i>), whereas the film thickness displayed a linear relationship with ALD cycle number, due to the self-limiting nature of the ALD process. Furthermore, the lithium tantalate thin films showed excellent uniformity and conformity in 3D anodic aluminum oxide template. Moreover, impedance testing showed that the lithium tantalate thin film exhibited a lithium ion conductivity of 2 × 10^–8 S/cm at 299 K. The lithium tantalate thin films by ALD, featured with well-controlled film thickness and composition, excellent step coverage, and moderate ionic conductivity at room temperature, would be promising solid-state electrolytes for 3D microbatteries