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It is a significant challenge to design a dense high-sulfur-loaded cathode and meanwhile to acquire fast sulfur redox kinetics and suppress the heavy shuttling in the lean electrolyte, thus to acquire a high volumetric energy density without sacrificing gravimetric performance for realistic Li–S batteries (LSBs). Herein, we develop a cation-doping strategy to tailor the electronic structure and catalytic activity of MoSe2 that in situ hybridized with conductive Ti3C2T x MXene, thus obtaining a Co-MoSe2/MXene bifunctional catalyst as a high-efficient sulfur host. Combining a smart design of the dense sulfur structure, the as-fabricated highly dense S/Co-MoSe2/MXene monolith cathode (density: 1.88 g cm–3, conductivity: 230 S m–1) achieves a high reversible specific capacity of 1454 mAh g–1 and an ultrahigh volumetric energy density of 3659 Wh L–1 at a routine electrolyte and a high areal capacity of ∼8.0 mAh cm–2 under an extremely lean electrolyte of 3.5 μL mgs –1 at 0.1 C. Experimental and DFT theoretical results uncover that introducing Co element into the MoSe2 plane can form a shorter Co–Se bond, impel the Mo 3d band to approach the Fermi level, and provide strong interactions between polysulfides and Co-MoSe2, thereby enhancing its intrinsic electronic conductivity and catalytic activity for fast redox kinetics and uniform Li2S nucleation in a dense high-sulfur-loaded cathode. This deep work provides a good strategy for constructing high-volumetric-energy-density, high-areal-capacity LSBs with lean electrolytes.

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Iron oxide with different crystal phases (α- and γ-Fe2O3) has been applied to electrode coatings and been demonstrated to ultrasensitive and selective electrochemical sensing toward heavy metal ions (e.g., Pb(II)). A range of Pb(II) contents in micromoles (0.1 to 1.0 μM) at α-Fe2O3 nanoflowers with a sensitivity of 137.23 μA μM(-1) cm(-2) and nanomoles (from 0.1 to 1.0 nM) at γ-Fe2O3 nanoflowers with a sensitivity of 197.82 μA nM(-1) cm(-2) have been investigated. Furthermore, an extended X-ray absorption fine structure (EXAFS) technique was applied to characterize the difference of local structural environment of the adsorbed Pb(II) on the surface of α- and γ-Fe2O3. The results first showed that α- and γ-Fe2O3 had diverse interaction between Pb(II) and iron (hydro)oxides, which were consistent with the difference of electrochemical performance. Determining the responses of Cu(II) and Hg(II) as the most appropriate choice for comparison, the stripping voltammetric quantification of Pb(II) with high sensitivity and selectivity at γ-Fe2O3 nanoflower has been demonstrated. This work reveals that the stripping performances of a nanomodifier have to be directly connected with its intrinsic surface atom arrangement.

Tin-based composites hold promise as anodes for high-capacity lithium/sodium-ion batteries (LIBs/SIBs); however, it is necessary to use carbon coated nanosized tin to solve the issues related to large volume changes during electrochemical cycling, thus leading to the low volumetric capacity for tin-based composites due to their low packing density. Herein, we design a highly dense graphene-encapsulated nitrogen-doped carbon@Sn (HD N–C@Sn/G) compact monolith with Sn nanoparticles double-encapsulated by N–C and graphene, which exhibits a high density of 2.6 g cm–3 and a high conductivity of 212 S m–1. The as-obtained HD N–C@Sn/G monolith anode exhibits ultrahigh and durable volumetric lithium/sodium storage. Specifically, it delivers a high volumetric capacity of 2692 mAh cm–3 after 100 cycles at 0.1 A g–1 and an ultralong cycling stability exceeding 1500 cycles at 1.0 A g–1 with only 0.019% capacity decay per cycle in lithium-ion batteries. Besides, in situ TEM and ex situ SEM have revealed that the unique double-encapsulated structure effectively mitigates drastic volume variation of the tin nanoparticles during electrode cycling. Furthermore, the full cell using HD N–C@Sn/G as an anode and LiCoO2 as a cathode displays a superior cycling stability. This work provides a new avenue and deep insight into the design of high-volumetric-capacity alloy-based anodes with ultralong cycle life.

The development of cost-effective and versatile sensing system for simultaneous and rapid quantitation of multiple targets is highly demanded for environmental surveillance, food safety inspection, home healthcare, etc. This work reports on (1) paper-based microarrays relying on fluorescence turn-off of carbon nanodots (CDs) for analyte recognition and (2) a stand-alone smartphone-based portable reader (SBR) installed with a customdesigned APP (SBR-App), which can accurately and reproducibly acquire fluorescence change from the microarray chip, automatically report the results, generate and share the reports via wireless network. Simultaneous detection of Hg 2+ , Pb 2+ , and Cu 2+ in the Pearl River water samples was achieved with the reported sensing system. End-user operation is limited to pipet samples to the microarray chip, insert the chip to the SBR, and open the SBR-App to acquire an image 5 min after sample introduction. There is no requirement for complicated sample pre-treatment and expensive equipment except for a smartphone. This versatile and costeffective smartphone-based sensing system featured with reliability and simplicity is ideally suited for user-and eco-friendly point-ofneed detection in resource-constrained environments.

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High-theoretical-capacity silicon anodes hold promise in lithium-ion batteries (LIBs). Nevertheless, their huge volume expansion (∼300%) and poor conductivity show the need for the simultaneous introduction of low-density conductive carbon and nanosized Si to conquer the above issues, yet they result in low volumetric performance. Herein, we develop an integration strategy of a dually encapsulated Si structure and dense structural engineering to fabricate a threedimensional (3D) highly dense Ti 3 C 2 T x MXene and graphene dual-encapsulated Si monolith architecture (HD-Si@Ti 3 C 2 T x @ G). Because of its high density (1.6 g cm −3 ), high conductivity (151 S m −1 ), and 3D dense dual-encapsulated Si architecture, the resultant HD-Si@Ti 3 C 2 T x @G monolith anode displays an ultrahigh volumetric capacity of 5206 mAh cm −3 (gravimetric capacity: 2892 mAh g −1 ) at 0.1 A g −1 and a superior long lifespan of 800 cycles at 1.0 A g −1 . Notably, the thick and dense monolithic anode presents a large areal capacity of 17.9 mAh cm −2 . In-situ TEM and ex-situ SEM techniques, and systematic kinetics and structural stability analysis during cycling demonstrate that such superior volumetric and areal performances stem from its dual-encapsulated Si architecture by the 3D conductive and elastic networks of MXene and graphene, which can provide fast electron and ion transfer, effective volume buffer, and good electrolyte permeability even with a thick electrode, whereas the dense structure results in a large volumetric performance. This work offers a simple and feasible strategy to greatly improve the volumetric and areal capacity of alloy-based anodes for large-scale applications via integrating a dual-encapsulated strategy and dense-structure engineering.