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cost, environmental friendliness, and universal availability. [6][7][8][9][10][11][12][13][14][15][16][17][18] To improve the performance and broaden the applications of TENGs, numerous efforts have been made with focus on both enhancement of the surface charge density and development of new structures/modes. [ 2,4,7,9,13,[19][20][21][22][23][24][25] So far, the electric power output of TENGs has been improved up to ≈500 W m −2 by sophisticated design of device structure. [ 18,[26][27][28] Demonstrated TENGs have been working in four modes, that is, the contact-separation or contact mode in short, sliding mode, single-electrode mode, and free-standing triboelectric-layer mode. The performance of the TENGs in contact-mode is better than that of TENGs in the sliding mode, because a much higher maximum displacement of TENGs in the sliding mode is required than that of TENGs working in the contact-mode, in order to achieve the same level of open circuit voltage. [ 29 ] The TENGs working in the contact-mode are particularly interesting for applications like foot-mounted wearable electronic systems because of the short vertical displacement involved and ease of integration.The fundamental principle of TENGs is based on the coupled effects of triboelectrifi cation and electrostatic induction. A theoretical model has been proposed for the TENGs working in the contact mode by Wang and co-workers. By utilizing the derived governing equation, the real-time output characteristics and the relationship between the optimum external resistance and TENG para meters were numerically simulated. [ 30 ] However, the model was not validated by corresponding experiments with measured real-time output characteristics. Furthermore, it is known that the real-time output characteristics would be affected by the materials, device structural parameters, operation conditions, and equivalent resistance. Assumptions made on simple profi les of working velocity may infl uence the model's validity in real situations. For instance, during real human walking, the previous wearing trials of an intelligent footwear system illustrated that the velocity profi le was complex and varied signifi cantly with time for a foot-mounted device. [ 31,32 ] Therefore, it is highly desirable to develop a theoretical model that is fully verifi ed by corresponding experiments. Such an experimentally verifi ed model is essential for designing and optimizing TENGs for intended applications.In this paper, based on conservation of charges and zerovoltage for close-loop circuits, a theoretical model is presented Harvesting mechanical energy from human activities by triboelectric nanogenerators (TENGs) is an effective approach for sustainable, maintenance-free, and green power source for wireless, portable, and wearable electronics. A theoretical model for contact-mode triboelectric nanogenerators based on the principles of charge conservation and zero loop-voltage is illustrated. Explicit expressions for the output current, voltage, and power are presented for the TENGs ...

Transition metal sulfides are deemed as attractive anode materials for potassium-ion batteries (KIBs) due to their high theoretical capacities based on conversion and alloying reaction. However, the main challenges are the low electronic conductivity, huge volume expansion, and consequent formation of unstable solid electrolyte interphase (SEI) upon potassiation/depotassiation. Herein, zinc sulfide dendrites deeply nested in the tertiary hierarchical structure through a solvothermalpyrolysis process are designed as an anode material for KIBs. The tertiary hierarchical structure is composed of the primary ultrafine ZnS nanorods, the secondary carbon nanosphere, and the tertiary carbon-encapsulated ZnS subunits nanosphere structure. The architectural design of this material provides a stable diffusion path and enhances effective conductivity from the interior to exterior for both K + ions and electrons, buffers the volume expansion, and constructs a stable SEI during cycling. A stable specific capacity of 330 mAh g −1 is achieved after 100 cycles at the current density of 50 mA g −1 and 208 mAh g −1 at 500 mA g −1 over 300 cycles. Using density functional theory calculations, we discover the interactions between ZnS and carbon interface can effectively decrease the K + ions diffusion barrier and therefore promote the reversibility of K + ions storage.

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The unique open ZnSe/C nanocages composed of sub-10 nm nanoparticles can dramatically reduce diffusion-induced stresses and enhance the structural integrity with multi-hierarchy stress-buffered effect, resulting in excellent cycle stability.

This paper presents an investigation of novel contact-mode hybrid nanogenerators comprising cascaded piezoelectric and triboelectric units. For the first time, a theoretical analysis of the contact-mode hybrid generator is presented to describe the relationships among transfer charges, voltage, current and average output power in terms of materials properties, hybrid generator structural parameters, harvesting and operational conditions. New hybrid generators with much enhanced piezoelectricity are fabricated via a simple, room-temperature, cost-effective route by using nonwoven fabrics made from electrospun Polyvinyledenedifluoride-trifluoroethylene (PVDF-TrFE)/Ag nanowire nanofibers and Polydimethylsiloxane (PDMS) composites with graphite nanoparticles. The results provide a powerful tool for synthesis and selection of materials, design and optimization of the configuration and operation of such kind of hybrid generators as well as determination of the value of external capacitor.