Single crystals are typically brittle, inelastic materials. Such mechanical responses limit their use in practical applications, particularly in flexible electronics and optical devices. Here we describe single crystals of a well-known coordination compound-copper(II) acetylacetonate-that are flexible enough to be reversibly tied into a knot. Mechanical measurements indicate that the crystals exhibit an elasticity similar to that of soft materials such as nylon, and thus display properties normally associated with both hard and soft matter. Using microfocused synchrotron radiation, we mapped the changes in crystal structure that occur on bending, and determined the mechanism that allows this flexibility with atomic precision. We show that, under strain, the molecules in the crystal reversibly rotate, and thus reorganize to allow the mechanical compression and expansion required for elasticity and still maintain the integrity of the crystal structure.

However, the limited lithium resource in earth is detrimental to the further application due to the possible increasing cost and unstable energy supply. [12][13][14][15] Therefore, there is an urgent demand for developing alternative energy storage devices with low cost while maintaining a comparable performance to LIBs. Among them, sodium-ion batteries (SIBs) have become the worldwide focus owing to abundant resources and low cost. [16][17][18][19] To develop high-performance SIBs, it remains challenging to discover/develop suitable electrode materials (especially cathode) to satisfy the requirement of long-term cycling stability and rate-capability.Owing to larger radius of Na + than that of Li + (0.98 vs 0.69 Å), various cathode materials with large open frameworks, including layered transitionmetal oxides [19][20][21][22][23][24][25] and polyanionic compounds, [2,13,18,[26][27][28][29][30][31][32][33] have been developed for (1) the improved sodium storage capacity, (2) the facilitated Na + diffusion in the lattice, and (3) the restricted structure degradation caused by Na + insertion/extraction. The NASICON (sodium (Na) super ion conductor) Na x M 2 (NO 4 ) 3 (M = transition metal, N = P 5+ , Si 4+ , S 6+ , and Mo 6+ ) structure with 3D large open framework allows for rapid and reversible ion diffusion in the lattice, which is now developed as electrode with promising electrochemical performance. [34] Among these, the Na 3 V 2 (PO 4 ) 3 (NVP) becomes a "shining star" with high sodium diffusion ability and remarkable high energy density (i.e., 400 Wh kg −1 ). [18] However, the high ionic diffusion ability of NVP is accompanied with poor electronic conductivity, [35] which results in the low utilization of active materials even at low rates. In order to obtain remarkable performance of NVP, the hurdles of poor rate capability and cycling stability need to be further addressed. Recently, carbon-coated active nanocrystals embedded in a porous carbon matrix, which demonstrated excellent rate performance and cycling stability for Li 3 V 2 (PO 4 ) 3 cathodes, [36,37] as well as for NVP cathodes. [38][39][40] In general, the porous carbon content is usually high, which may lead to the decrease of tap density and entire cell volumetric energy density. [41,42] Li and co-workers reported adaptive graphene gel films as a highly compact electrode with Na 3 V 2 (PO 4 ) 3 (NVP) is regarded as a promising cathode for advanced sodiumion batteries (SIBs) due to its high theoretical capacity and stable sodium (Na) super ion conductor (NASICON) structure. However, strongly impeded by its low electronic conductivity, the general NVP delivers undesirable rate capacity and fails to meet the demands for quick charge. Herein, a novel and facile synthesis of layer-by-layer NVP@reduced graphene oxide (rGO) nanocomposite is presented through modifying the surface charge of NVP gel precursor. The well-designed layered NVP@rGO with confined NVP nanocrystal in between rGO layers offers high electronic and ionic conductivity as well as sta...

Si has attracted enormous research and manufacturing attention as an anode material for lithium ion batteries (LIBs) because of its high specific capacity. The lack of a low cost and effective mechanism to prevent the pulverization of Si electrodes during the lithiation/ delithiation process has been a major barrier in the mass production of Si anodes. Naturally abundant gum arabic (GA), composed of polysaccharides and glycoproteins, is applied as a dualfunction binder to address this dilemma. Firstly, the hydroxyl groups of the polysaccharide in GA are crucial in ensuring strong binding to Si. Secondly, similar to the function of fiber in fiberreinforced concrete (FRC), the long chain glycoproteins provide further mechanical tolerance to dramatic volume expansion by Si nanoparticles. The resultant Si anodes present an outstanding capacity of ca. 2000 mAh/g at a 1 C rate and 1000 mAh/g at 2 C rate, respectively, throughout 500 cycles. Excellent long-term stability is demonstrated by the maintenance of 1000 mAh/g specific capacity at 1 C rate for over 1000 cycles. This low cost, naturally abundant and environmentally benign polymer is a promising binder for LIBs in the future.

Magnesium-ion batteries (MIBs) show great potential for large-scale energy storage because of the advantages of low cost and safety, but their application is severely hindered by the difficulty in finding desirable electrode materials. Herein, we develop a bilayer-structured vanadium oxide (Mg 0.3 V 2 O 5 $1.1H 2 O) with synergistic effect of Mg 2+ ions and lattice water as the cathode material for MIBs. The pre-intercalated Mg 2+ ions provide high electronic conductivity and excellent structural stability. The lattice water enables fast Mg 2+ ions mobility because of its charge shielding effect. As a result, the Mg 0.3 V 2 O 5 $1.1H 2 O exhibits excellent rate performance and an unprecedented cycling life with capacity retention of 80.0% after 10,000 cycles. In addition, the Mg 0.3 V 2 O 5 $1.1H 2 O exhibits good electrochemical performance in full MIBs. This scalable Mg 2+ host material is a promising candidate as a cathode for MIBs, and its high performance is expected to meet the requirements for large-scale storage applications.

With the enormous development of the electric vehicle market, fast charging battery technology is highly required. However, the slow kinetics and lithium plating under fast charging condition of traditional graphite anode hinder the fast charging capability of lithium‐ion batteries. To develop anode materials with rapid Li‐ions diffusion capability and fast reaction kinetics has received widely attentions. This review summarizes the current status in the exploration of fast charging anode materials, mainly including the critical challenge of achieving fast charging capability, the inherent structures and lithium storage mechanisms of various anode materials, as well as the recent progress to improve the rate performance involving morphology regulation, structure design, surface/interface modification, as well as forming multiphase systems. Finally, the challenges and future directions of developing fast charging Li‐ion batteries are highlighted.