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As an emerging electrochemical energy storage device, potassium-ion batteries (PIBs) have drawn growing interest due to the resource-abundance and low cost of potassium. Graphite-based materials, as the most common anodes for commercial Li-ion batteries, have a very low capacity when used an anode for Na-ion batteries, but they show reasonable capacities as anodes for PIBs. The practical application of graphitic materials in PIBs suffers from poor cyclability, however, due to the large interlayer expansion/ shrinkage caused by the intercalation/deintercalation of potassium ions. Here, a highly graphitic carbon nanocage (CNC) is reported as a PIBs anode, which exhibits excellent cyclability and superior depotassiation capacity of 175 mAh g-1at 35 C. The potassium storage mechanism in CNC is revealed by cyclic voltammetry as due to redox reactions (intercalation/deintercalation) and double-layer capacitance (surface adsorption/desorption). The present results give new insights into structural design for graphitic anode materials in PIBs and understanding the double-layer capacitance effect in alkali metal ion batteries.

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Goldschmidt-Pauling contraction of the H-O polar-covalent bond elongates and polarizes the other noncovalent part of the hydrogen bond (O:H-O), that is, the O:H van der Waals bond, significantly, through the Coulomb repulsion between the electron pairs of adjacent oxygen (O-O). This process enlarges and stiffens those H2O molecules having fewer than four neighbors such as molecular clusters, hydration shells, and the surface skins of water and ice. The shortening of the H-O bond raises the local density of bonding electrons, which in turn polarizes the lone pairs of electrons on oxygen. The stiffening of the shortened H-O bond increases the magnitude of the O1s binding energy shift, causes the blue shift of the H-O phonon frequencies, and elevates the melting point of molecular clusters and ultrathin films of water, which gives rise to their elastic, hydrophobic, highly-polarized, ice-like, and low-density behavior at room temperature.

We explore in-depth the interfacial interaction between Fe 3 O 4 nanoparticles and graphene nanosheets as well as its impact on the electrochemical performance of Fe 3 O 4 /graphene anode materials for lithium-ion batteries. Fe 3 O 4 /graphene hybrid materials are prepared by direct pyrolysis of Fe(NO 3 ) 3 ?9H 2 O on graphene sheets. The interfacial interaction between Fe 3 O 4 and graphene nanosheets is investigated in detail by thermogravimetric and differential scanning calorimetry analysis, Raman spectrum, X-ray photoelectron energy spectrum and Fourier transform infrared spectroscopy. It was found that Fe 3 O 4 nanoparticles disperse homogeneously on graphene sheets, and form strong covalent bond interactions (Fe-O-C bond) with graphene basal plane. The strong covalent links ensure the high specific capacity and long-period cyclic stability of Fe 3 O 4 /graphene hybrid electrodes for lithium-ion batteries at high current density. The capacity keeps as high as 796 mAhg 21 after 200 cycles without any fading in comparison with the first reversible capacity at the current density of 500 mAg 21 (ca. 0.6 C). At 1 Ag 21 (ca. 1.3 C), the reversible capacity attains ca. 550 mAhg 21 and 97% of initial capacity is maintained after 300 cycles. This work reveals an important factor affecting the high-rate and cyclic stability of metal oxide anode, and provides an effective way to the design of new anode materials for lithium-ion batteries. Recently, many efforts have also been made to prepare graphenebased nanocomposites with metal oxides including SnO 2 , 11,12