Simulating Synaptic Behaviors through Frequency Modulation in a Capacitor–Memristor Circuit

Memristors, known for their adjustable and non-volatile resistance, offer a promising avenue for emulating synapses. However, achieving pulse frequency-dependent synaptic plasticity in memristors or memristive systems necessitates further exploration. In this study, we present a novel approach to modulate the conductance of a memristor in a capacitor–memristor circuit by finely tuning the frequency of input pulses. Our experimental results demonstrate that these phenomena align with the long-term depression (LTD) and long-term potentiation (LTP) observed in synapses, which are induced by the frequency of action potentials. Additionally, we successfully implement a Hebbian-like learning mechanism in a simple circuit that connects a pair of memristors to a capacitor, resulting in observed associative memory formation and forgetting processes. Our findings highlight the potential of capacitor–memristor circuits in faithfully replicating the frequency-dependent behavior of synapses, thereby offering a valuable contribution to the development of brain-inspired neural networks

Electrically driven cation exchange for in situ fabrication of individual nanostructures.

Cation exchange (CE) has been recognized as a particularly powerful tool for the synthesis of heterogeneous nanocrystals. At present, CE can be divided into two categories, namely ion solvation-driven CE reaction and thermally activated CE reaction. Here we report an electrically driven CE reaction to prepare individual nanostructures inside a transmission electron microscope. During the process, Cd is eliminated due to Ohmic heating, whereas Cu+ migrates into the crystal driven by the electrical field force. Contrast experiments reveal that the feasibility of electrically driven CE is determined by the structural similarity of the sulfur sublattices between the initial and final phases, and the standard electrode potentials of the active electrodes. Our experimental results demonstrate a strategy for the selective growth of individual nanocrystals and provide crucial insights into understanding of the microscopic pathways leading to the formation of heterogeneous structures

Self-Assembled Framework Formed During Lithiation of SnS2 Nanoplates Revealed by in Situ Electron Microscopy.

Lithium-ion batteries (LIBs) commercially dominate portable energy storage and have been extended to hybrid/electric vehicles by utilizing electrode materials with enhanced energy density. However, the energy density and cycling life of LIBs must extend beyond the current reach of commercial electrodes to meet the performance requirements for transportation applications. Carbon-based anodes, serving as the main negative electrodes in LIBs, have an intrinsic capacity limitation due to the intercalation mechanism. Some nanostructured carbon materials offer very interesting reversible capacities and can be considered as future anode materials. However, their fabrication processes are often complicated and expensive. Theoretically, using a lithium metal anode is the best way of delivering high energy density due to its largest theoretical capacity of more than 3800 mAh g-1; however, lithium metal is highly reactive with liquid electrolytes. Alternative anodes are being explored, including other lithium-reactive metals, such as Si, Ge, Zn, V, and so forth. These metals react reversibly with a large amount of Li per formula unit to form lithium-metal alloys, rendering these materials promising candidates for next-generation LIBs with high energy density. Though, most of these pure metallic anodes experience large volume changes during lithiation and delithiation processes that often results in cracking of the anode material and a loss electrical contact between the particles. Nanosized metal sulfides were recently found to possess better cycling stability and larger reversible capacities over pure metals. Further improvements and developments of metal sulfide-based anodes rely on a fundamental understanding of their electrochemical cycling mechanisms. Not only must the specific electrochemical reactions be correctly identified, but also the microstructural evolution upon electrochemical cycling, which often dictates the cyclability and stability of nanomaterials in batteries, must be clearly understood. Probing these dynamic evolution processes, i.e. the lithiation reactions and morphology evolutions, are often challenging. It requires both high-resolution chemical analysis and microstructural identification. In situ transmission electron microscopy (TEM) coupled with electron energy loss spectroscopy (EELS) has recently been raised as one of the most powerful techniques for monitoring electrochemical processes in anode materials for LIBs. In this work, we focus on elucidating the origin of the structural stability of SnS2 during electrochemical cycling by revealing the microstructural evolution of SnS2 upon lithiation using in situ TEM. Crystalline SnS2 was observed to undergo a two-step reaction after the initial lithium intercalation: (1) irreversible formation of metallic tin and amorphous lithium sulfide and (2) reversible transformation of metallic tin to Li-Sn alloys, which is determined to be the rate-determining step. More interestingly, it was discovered that a self-assembled composite framework formed during the irreversible conversion reaction, which has not been previously reported. Crystalline Sn nanoparticles are well arranged within an amorphous Li2S "matrix" in this self-assembled framework. This nanoscale framework confines the locations of individual Sn nanoparticles and prevents particle agglomeration during the subsequent cycling processes, therefore providing desired structural tolerance and warranting a sufficientelectron pathway. Our results not only explain the outstanding cycling stability of SnS2 over metallic tin anodes, but also provide important mechanistic insights into the design of high-performance electrodes for next-generation LIBs through the integration of a unique nanoframework

Highly enhanced performance of spongy graphene as an oil sorbent

This work demonstrates a brand-new spongy graphene with a highly enhanced performance as an oil sorbent. The absorption capacity of the new spongy graphene to chloroform reaches 616 times of its own weight, which is approximately 8 times higher than that in previous reports. The absorption capacity towards other organic chemicals is also greatly improved

Large-scale chemical vapor deposition synthesis of graphene nanoribbions/carbon nanotubes composite for enhanced membrane capacitive deionization

The composite comprised of graphene and carbon nanotubes (CNTs) exhibited significantly enhanced electro-chemical performance due both to the improved dispersion and inhibition of restacking of graphene and CNTs. In this work, graphene nanoribbons (GNRs)/CNTs composite (GNRs/CNTs) was synthesized on gram-scale by chemical vapor deposition. Under optimal growth conditions, the yield of GNRs/CNTs as high as 26 g per gram catalyst could be achieved in 30 min growth time. The morphology and quality of the as-synthesized composite was verified by using SEM, TEM and Raman spectroscopy. The electrochemical properties of GNRs/CNTs was evaluated using cyclic voltammetry (CV) and galvanostatic charge-discharge techniques. GNRs/CNTs exhibited specific capacitance of 242.3 F/g at 0.5 A g(-1), which was over 4 times of that of CNTs. The GNRs/ CNTs based electrodes exhibited excellent cycling stability at 1 A g(-1) for over 4000 cycles, which can be attributed to the excellent electrical conductivity and the unique structure. When employed as electrode for membrane capacitive desalination, the desalination capacity of 16.46 mg g(-1) has been achieved under 1.2 V with 500 mg L-1 NaCl solution as feeding water

Dealloying-constructed hierarchical nanoporous bismuth-antimony anode for potassium ion batteries

Bi-Sb alloys are appealing anode materials for potassium ion batteries (PIBs) but challenged by their enormous volumetric variation during operation. Herein, a facile one-step dealloying protocol was devised and utilized to prepare the Bi-Sb alloys that manifest an exotic bicontinuous hierarchical nanoporous (np) microstructure ideal for volume-change mitigation and K+ transport percolation. The growth mechanism fostering the peculiar morphology of the np-(Bi,Sb) alloys was investigated and clarified via operando X-ray (XRD) and ex-situ scanning electron microscopy (SEM). In particular, the np-Bi6Sb2 electrode, optimized for comprehensive electrochemical performance, achieves decent reversible capacities and a superior lifespan, as benchmarked with the monometallic references and other Bi-Sb alloy electrodes. The (de)potassiation mechanism of the np-(Bi,Sb) alloys was studied by operando XRD and further rationalized by density functional theory (DFT) calculations, whereby a homogeneous (segregation-free) and robust two-step electrochemically-driven phase transformations’ catenation of (Bi,Sb) ↔ K(Bi,Sb)2 ↔ K3(Bi,Sb) was reliably established to substantiate the outstanding reversibility of the np-(Bi,Sb) anodes in PIBs

Achieving synergetic anion-cation redox chemistry in freestanding amorphous vanadium oxysulfide cathodes toward ultrafast and stable aqueous zinc-ion batteries

Flexible aqueous zinc-ion batteries (AZIBs) with high safety and low cost hold great promise for potential applications in wearable electronics, but the strong electrostatic interaction between Zn2+ and crystalline structures, and the traditional cathodes with single cationic redox center remain stumbling blocks to developing high-performance AZIBs. Herein, freestanding amorphous vanadium oxysulfide (AVSO) cathodes with abundant defects and auxiliary anionic redox centers are developed via in situ anodic oxidation strategy. The well-designed amorphous AVSO cathodes demonstrate numerous Zn2+ isotropic pathways and rapid reaction kinetics, performing a high reversible capacity of 538.7 mAhg-1 and high-rate capability (237.8 mAhg-1@40Ag-1). Experimental results and theoretical simulations reveal that vanadium cations serve as the main redox centers while sulfur anions in AVSO cathode as the supporting redox centers to compensate local electron-transfer ability of active sites. Significantly, the amorphous structure with sulfur chemistry can tolerate volumetric change upon Zn2+/H+ insertion and weaken electrostatic interaction between Zn2+ and host materials. Consequently, the AVSO composites display alleviated structural degradation and exceptional long-term cyclability (89.8% retention after 20 000 cycles at 40 Ag-1). This work can be generally extended to various freestanding amorphous cathode materials of multiple redox reactions, inspiring development of designing ultrafast and long-life wearable AZIBs.R.P. and F.C. contributed equally to this work. This work was supported by the National Key R&D Program of China (No. 2020YFB2007402), the Natural Science Foundation of Jiangsu Province (No. BK20220288), and the National Natural Science Foundation of China (No. 12174050 and 12234005), and the Fundamental Research Funds for the Central Universities

bipolar resistance switching in multiferroic bifeo3 polycrystalline films

Bismuth ferrite (BiFeO3) is an attractive multiferroic material. However, its application as a ferroelectric material in electronic devices is limited by large leakage current. In this letter, a bipolar resistance switching phenomenon was observed in BiFeO3 polycrystalline films prepared by sol-gel method. The resistance of a fresh film is in an intermediate state and can be transformed into either high-resistive or low-resistive states by applying an external voltage across the film. The I-V curves of the fresh film are nearly centrosymmetric before resistance switching. The resistance ratios between high- and low-resistance states are in the range of 2-3 orders of magnitude. The conductive atomic force microscopy measurements reveal that the grain boundaries are more conductive under a bias voltage of 10 V, and more resistive under 3 V than the grain interior. It was thought that grain boundaries and oxygen vacancies played a key role in the formation of bipolar resistance switching