A sulfur-rich copolymer@carbon nanotubes hybrid cathode is introduced for lithium-sulfur batteries produced by combining the physical and chemical confinement of polysulfides. The binderfree and metal-current-collector-free cathode of dual confinement enables an efficient pathway for the fabrication of high-performance sulfur copolymer carbon matrix electrodes for lithium-sulfur batteries.

MXenes, as an emerging family of conductive two-dimensional materials, hold promise for late-model electrode materials in Li-ion batteries. A primary challenge hindering the development of MXenes as electrode materials is that a complete understanding of the intrinsic storage mechanism underlying the charge/discharge behavior remains elusive. This article presents two key discoveries: first, the characteristics of the Ti 3 C 2 T x structure can be modified systematically by calcination in various atmospheres, and second, these structural changes greatly affect Li-ion storage behavior, which reveals the mechanism for lithium storage in Ti 3 C 2 T x MXene. Specifically, via ammonization, the interlayer spacing gets dilated and uniform, giving rise to only one redox couple. In stark contrast, there are two well-recognized redox couples corresponding to two interlayer spacings in pristine Ti 3 C 2 T x MXene, in which Li-ion (de)intercalation occurs between interlayers in a sequential manner as evidenced by ex situ X-ray diffraction (XRD). Notably, the XRD diffraction peaks shift hardly in the whole range of charge/discharge voltage, indicating a zero-strain feature upon Li-ion (de)intercalation. Moreover, the diffusion-controlled contribution percentage to capacity inversely depends on the scan rate. The understanding suggests a new design principle of the MXene anode: reduced lateral size to shorten the diffusion path and dilated interlayer spacing.

As a major branch of hybrid perovskites, two-dimensional (2D) hybrid double perovskites are expected to be ideal systems for exploring novel ferroelectric properties, because they can accommodate a variety of organic cations and allow diverse combinations of different metal elements. However, no 2D hybrid double perovskite ferroelectric has been reported since the discovery of halide double perovskites in the 1930s. Based on trivalent rare-earth ions and chiral organic cations, we have designed a new family of 2D rare-earth double perovskite ferroelectrics, A4MIMIII(NO3)8, where A is the organic cation, MI is the alkaline metal or ammonium ion, and MIII is the rare-earth ion. This is the first time that ferroelectricity is realized in 2D hybrid double perovskite systems. These ferroelectrics have achieved high-temperature ferroelectricity and photoluminescent properties. By varying the rare-earth ion, variable photoluminescent properties can be achieved. The results reveal that the 2D rare-earth double perovskite systems provide a promising platform for achieving multifunctional ferroelectricity.

Developing nano-or atom-scale Pt-based electrocatalysts for hydrogen evolution reaction (HER) is of considerable importance to mitigate the issues associated with low abundance of Pt. Here, a protocol for constructing a hierarchical Pt-MXene-single-walled carbon nanotubes' (SWCNTs) heterostructure for HER catalysts is presented. In the heterostructure, highly active nano/atom-scale metallic Pt is immobilized on Ti 3 C 2 T x MXene flakes (MXene@Pt) that are connected with conductive SWCNTs' network. The hierarchical heterostructure is constructed by filtrating a mixed colloidal suspension containing MXene@Pt and SWCNTs. Taking the advantages of the hydrophilicity and reducibility of MXene, the MXene@Pt colloidal suspension is prepared by spontaneously reducing Pt cations into metallic Pt without additional reductants or post-treatments. The so-fabricated hierarchical HER catalysts, in the form of membrane, show high stability during 800 h operation, a high volume current density of up to 230 mA cm −3 at −50 mV versus reversible hydrogen electrode (RHE) and a low overpotential of −62 mV versus RHE at the current density of −10 mA cm −2. This solution-processed strategy offers a simple, efficient, yet scalable approach to construct stable and efficient HER catalysts. Given the properties and the structure-activity relationships of the hierarchical Pt-MXene-SWCNTs' heterostructure, other MXenes probably show greater promise in HER electrocatalysis.

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Piezoelectric materials are technologically important, and the most used are perovskite ferroelectrics. In recent years, more and more emerging areas have put forward new requirements for piezoelectric materials, such as light weight, low acoustic impedance, good flexibility, and biocompatibility. In this context, hybrid organic–inorganic perovskite ferroelectrics have emerged as promising supplements, because they combine attractive features of inorganic and organic materials. Among them, hybrid double-metal perovskites have recently been found to exhibit excellent ferroelectricity. However, their potential as piezoelectric materials has not been exploited. Here, we describe large piezoelectric response in hybrid rare-earth double perovskite relaxor ferroelectrics (RM3HQ)2­RbLa­(NO3)6 and (RM3HQ)2­NH4La­(NO3)6 (RM3HQ = R-N-methyl-3-hydroxyl­quinuclidinium). They are simultaneously ferroelectric and ferroelastic crystals, with the R3 ferroelectric phase and P213 paraelectric phase. We found that ferroelectric polar microdomains and paraelectric nonpolar regions coexist in a wide temperature range through variable-temperature piezoresponse force microscopy images. The two-phase coexistence reveals low energy barriers of transitions between the two phases and between the polar microdomains with different polarization directions. These lead to the easy polarization rotation of the polar microdomains upon applying a stress and, accordingly, the large piezoelectric response up to 106 pC N–1 for (RM3HQ)2­RbLa­(NO3)6. This finding represents a significant step toward novel applications of piezoelectric materials based on lead-free hybrid perovskites.

MXenes have emerged as promising high-volumetric-capacitance supercapacitor electrode materials, whereas their voltage windows are not wide. This disadvantage prevents MXenes from being made into aqueous symmetric supercapacitors with high energy density. To attain high energy density, constructing asymmetric supercapacitors is a reliable design choice. Here, we propose a strategy to achieve high energy density of hydrogen ion aqueous-based hybrid supercapacitors by integrating a negative electrode of Ti3C2 T x MXene and a positive electrode of redox-active hydroquinone (HQ)/carbon nanotubes. The two electrodes are separated by a Nafion film that is proton permeable in H2SO4 electrolyte. Upon charging/discharging, hydrogen ions shuttle back and forth between the cathode and anode for charge compensation. The proton-induced high capacitance of MXene and HQ, along with complementary working voltage windows, simultaneously enhance the electrochemical performance of the device. Specifically, the hybrid supercapacitors operate in a 1.6 V voltage window and deliver a high energy density of 62 Wh kg–1, which substantially exceeds those of the state-of-the-art aqueous asymmetric supercapacitors reported so far. Additionally, the device exhibits excellent cycling stability and the all-solid-state planar hybrid supercapacitor displays exceptional flexibility and integration for bipolar cells to boost the capacitance and voltage output. These encouraging results provide the possibility of designing high-energy-density noble-metal-free asymmetric supercapacitors for practical applications.

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