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The crucial role of mechanical stress in voltage hysteresis of lithium ion batteries in charge-discharge cycles is investigated theoretically and experimentally. A modified Butler-Volmer equation of electrochemical kinetics is proposed to account for the influence of mechanical stresses on electrochemical reactions in lithium ion battery electrodes. It is found that the compressive stress in the surface layer of active materials impedes lithium intercalation, and therefore, an extra electrical overpotential is needed to overcome the reaction barrier induced by the stress. The theoretical formulation has produced a linear dependence of the height of voltage hysteresis on the hydrostatic stress difference between lithiation and delithiation, under both open-circuit conditions and galvanostatic operation. Predictions of the electrical overpotential from theoretical equations agree well with the experimental data for thin film silicon electrodes.

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In this paper, a leaf-like porous CuO-graphene nanostructure is synthesized by a hydrothermal method.The as-prepared composite is characterized using XRD, Raman, SEM, TEM and nitrogen adsorptiondesorption. The growth mechanism is discussed by monitoring the early growth stages. It is shown that the CuO nanoleaves are formed through oriented attachment of tiny Cu(OH) 2 nanowires.Electrochemical characterization demonstrates that the leaf-like CuO-graphene are capable of delivering specific capacitances of 331.9 and 305 F g À1 at current densities of 0.6 and 2 A g À1 , respectively. A capacity retention of 95.1% can be maintained after 1000 continuous charge-discharge cycles, which may be attributed to the improvement of electrical contact by graphene and mechanical stability by the layer-by-layer structure. The method provides a facile and straightforward approach to synthesize CuO nanosheets on graphene and may be readily extended to the preparation of other classes of hybrids based on graphene sheets for technological applications. Recently, graphene nanosheets (GNS) based on transition metal oxides 2,3,22 have been studied and are expected to show improved capacitance owing their enhanced electronic conductivity, due to graphene materials possessing rapid electron transfer, high mechanical strength, high elasticity, and