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The use of fast surface redox storage (pseudocapacitive) mechanisms can enable devices that store much more energy than electrical double-layer capacitors (EDLCs) and, unlike batteries, can do so quite rapidly. Yet, few pseudocapacitive transition metal oxides can provide a high power capability due to their low intrinsic electronic and ionic conductivity. Here we demonstrate that two-dimensional transition metal carbides (MXenes) can operate at rates exceeding those of conventional EDLCs, but still provide higher volumetric and areal capacitance than carbon, electrically conducting polymers or transition metal oxides. We applied two distinct designs for MXene electrode architectures with improved ion accessibility to redox-active sites. A macroporous Ti 3 C 2 T x MXene film delivered up to 210 F g −1 at scan rates of 10 V s −1 , surpassing the best carbon supercapacitors known. In contrast, we show that MXene hydrogels are able to deliver volumetric capacitance of ∼1,500 F cm −3 reaching the previously unmatched volumetric performance of RuO 2 . Pseudocapacitors based on redox-active materials have relatively high energy density but suffer from low power capability. Here the authors report that two-dimensional transition metal carbides exhibit high gravimetric, volumetric and areal capacitance values at high charge/discharge rates.

It is generally accepted that solid–electrolyte interphase formed on the surface of lithium-battery electrodes play a key role in controlling their cycling performance. Although a large variety of surface-sensitive spectroscopies and microscopies were used for their characterization, the focus was on surface species nature rather than on the mechanical properties of the surface films. Here we report a highly sensitive method of gravimetric and viscoelastic probing of the formation of surface films on composite Li 4 Ti 5 O 12 electrode coupled with lithium ions intercalation into this electrode. Electrochemical quartz-crystal microbalance with dissipation monitoring measurements were performed with LiTFSI, LiPF 6 , and LiPF 6  + 2% vinylene carbonate solutions from which structural parameters of the surface films were returned by fitting to a multilayer viscoelastic model. Only a few fast cycles are required to qualify surface films on Li 4 Ti 5 O 12 anode improving in the sequence LiPF 6  < LiPF 6  + 2% vinylene carbonate << LiTFSI. The solid-electrolyte interphase formed on Li-battery electrodes strongly affects their cycling performance, however the mechanical properties of the surface films are not well-known. Here the authors report a sensitive gravimetric/viscoelastic method to probe surface film formation on composite electrodes, coupled with Li-ion intercalation.

Since their discovery in 2011, MXenes are extensively studied as materials for electrochemical energy storage systems. The high electric conductivity, 2D structure, enabling ions insertion, and excellent chemical stability make MXenes an attractive choice for energy storage applications. This review is focused on the utilization of MXenes in aqueous electrolyte solutions, as cost-effective and safe medium. This review describes the compatibility and electrochemical performance of MXenes as electrodes for charge storage devices. We present their composition with capacitive compounds or redox-active materials such as nano-carbons, metal-oxides, and conducting polymers. Other applications of MXenes as complementary components in energy devices including current collectors, separators, and binders are presented as well. Finally, we discuss the challenges and limitations of MXenes compounds and propose a perspective for their future development.

A primary atomic-scale effect accompanying Li-ion insertion into rechargeable battery electrodes is a significant intercalation-induced change of the unit cell volume of the crystalline material. This generates a variety of secondary multiscale dimensional changes and causes a deterioration in the energy storage performance stability. Although traditional in situ height-sensing techniques (atomic force microscopy or electrochemical dilatometry) are able to sense electrode thickness changes at a nanometre scale, they are much less informative concerning intercalation-induced changes of the porous electrode structure at a mesoscopic scale. Based on a electrochemical quartz-crystal microbalance with dissipation monitoring on multiple overtone orders, herein we introduce an in situ hydrodynamic spectroscopic method for porous electrode structure characterization. This new method will enable future developments and applications in the fields of battery and supercapacitor research, especially for diagnostics of viscoelastic properties of binders for composite electrodes and probing the micromechanical stability of their internal electrode porous structure and interfaces. Characterizing intercalation-induced changes in energy storage electrodes is challenging. A spectroscopic method based on the quartz-crystal microbalance can now simultaneously track the interfacial reliability and mechanical stability of battery electrodes.

Among extensively studied Li‐ion cathode materials, LiCoO2 (LCO) remains dominant for portable electronic applications. Although its theoretical capacity (274 mAh g−1) cannot be achieved in Li cells, high capacity (≤240 mAh g−1) can be obtained by raising the charging voltage up to 4.6 V. Unfortunately, charging Li‐LCO cells to high potentials induces surface and structural instabilities that result in rapid degradation of cells containing LCO cathodes. Yet, significant stabilization is achieved by surface coatings that promote formation of robust passivation films and prevent parasitic interactions between the electrolyte solutions and the cathodes particles. In the search for effective coatings, the authors propose RbAlF4 modified LCO particles. The coated LCO cathodes demonstrate enhanced capacity (>220 mAh g−1) and impressive retention of >80/77% after 500/300 cycles at 30/45 °C. A plausible mechanism that leads to the superior stability is proposed. Finally the authors demonstrate that the main reason for the degradation of 4.6 V cells is the instability of the anode side rather than the failure of the coated cathodes. Enhanced stability of 4.6V LiCoO2 electrodes is achieved through application surface coating consisting of Rb, Al, and F. This coating enables effective protection of the cathode's particles from parasitic attacks and promotes the formation of robust passivation layers (SEI). Using fluorinated electrolyte solution, a high capacity of 220 mAh g−1 and impressive long‐term stability (500 cycles with 80% capacity retention) are demonstrated.

Nanoporous layers are widely spread in nature and among artificial devices. However, complex characterization of extensively nanoporous thin films showing porosity-dependent softening lacks consistency and reliability when using different analytical techniques. We introduce herein, a facile and precise method of such complex characterization by multi-harmonic QCM-D (Quartz Crystal Microbalance with Dissipation Monitoring) measurements performed both in the air and liquids (Au-Zn alloy was used as a typical example). The porosity values determined by QCM-D in air and different liquids are entirely consistent with that obtained from parallel RBS (Rutherford Backscattering Spectroscopy) and GISAXS (Grazing-Incidence Small-Angle Scattering) characterizations. This ensures precise quantification of the nanolayer porosity simultaneously with tracking their viscoelastic properties in liquids, significantly increasing sensitivity of the viscoelastic detection (viscoelastic contrast principle). Our approach is in high demand for quantifying potential-induced changes in nanoporous layers of complex architectures fabricated for various electrocatalytic energy storage and analytical devices. Thin porous layers are largely used, but a reliable method to quantify their porosity is missing. Here the authors demonstrate a method, based on quartz crystal microbalance measurements with dissipation monitoring, for accurate assessment of porosity and mechanical properties in thin porous films.

One of the biggest hurdles to realise high-performance Li-metal batteries is the instability of Li metal towards all relevant electrolytes. Now, an approach is reported to improve Li cells’ stability by upshifting the Li electrodes’ potential to reduce their voltage gap with the electrolyte electrochemical stability windows.

Among extensively studied Li‐ion cathode materials, LiCoO 2 (LCO) remains dominant for portable electronic applications. Although its theoretical capacity (274 mAh g −1 ) cannot be achieved in Li cells, high capacity (≤240 mAh g −1 ) can be obtained by raising the charging voltage up to 4.6 V. Unfortunately, charging Li‐LCO cells to high potentials induces surface and structural instabilities that result in rapid degradation of cells containing LCO cathodes. Yet, significant stabilization is achieved by surface coatings that promote formation of robust passivation films and prevent parasitic interactions between the electrolyte solutions and the cathodes particles. In the search for effective coatings, the authors propose RbAlF 4 modified LCO particles. The coated LCO cathodes demonstrate enhanced capacity (>220 mAh g −1 ) and impressive retention of >80/77% after 500/300 cycles at 30/45 °C. A plausible mechanism that leads to the superior stability is proposed. Finally the authors demonstrate that the main reason for the degradation of 4.6 V cells is the instability of the anode side rather than the failure of the coated cathodes.