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• A core–shell structure polypyrrole coated Cu/Cu 2 O nanowires was synthesized. • Li-storage performance of Cu 2 O can be enhanced by introduction of Cu metal and Ppy. • The high capacity of 401 mAh g−1 is still delivered at 1 Ag−1 after 500 cycles. • A phase transformation is proved to be the main reasons for capacity increase. A core-shell structure polypyrrole coated Cu/Cu 2 O (Cu/Cu 2 O@Ppy) nanowires was synthesized by a facile hydrothermal method. Benefiting from the nanowire morphology, the confinement effect of Ppy shell and the modification of Cu metal, the Cu/Cu 2 O@Ppy as the anode of lithium ion battery exhibits an increased reversible capacity of 787 mAh g−1 under the current density of 0.1 A g−1 after 250 cycles. In addition, a good long-life rate capacity also can be obtained (401 mAh g−1 at 1 A g−1 after 500 cycles). More to the point, the increased capacity of Cu/Cu 2 O@Ppy nanowires is explained by the analysis of capacity differential curve, ex-situ X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. It was demonstrated that the constant formation of solid electrolyte interface film and the transformation of Cu to Cu 2 O lead to the increased capacity in the initial cycles. Furthermore, a phase transformation of host material from Cu 2 O to CuO is proved to be the main reasons for the subsequent cycles of capacity increase. This in-situ phase transformation of electrode material provides a novel perspective for the increased capacity phenomenon of metal compounds in lithium ion battery. [ABSTRACT FROM AUTHOR]

• Fe 2 O 3 "mulberry" architecture, quantum dots and bulk particles were synthesized. • Dominant pseudocapacitive feature of the assembled render fast ionic storage. • Less SEI on surface enables the assembled to realize high coulombic efficiency. Constructing pseudocapacitive materials to combine the battery-level energy density with the cycle life and power density of supercapacitors is a promising energy storage technique. Nanostructuring can greatly induce extrinsic pseudocapacitance, but excessive solid electrolyte interphase (SEI) as the activity surface increases usually results in a poor coulombic efficiency (CE) of battery. Here, we separately investigate the electrochemical properties of self-assembled Fe 2 O 3 "mulberry" architecture, quantum dots and bulk particles. Employed as an anode of lithium ion battery, the mulberry architecture retains a dominant pseudocapacitance behavior and high electrochemical activity. It delivers an initial capacity of 1383.8 mAh g−1, 22.1% higher than Fe 2 O 3 bulk in similar size, in which the pseudocapacitance contribution up to 80.6% at 0.4 mV s−1. Furthermore, the mulberry architecture decreases the activity surface contacted with electrolyte to enable a limited SEI, achieving a higher coulombic efficiency (initial CE: 80.1%, average CE: 98.9%) well above that of individual quantum dots. This work is expected to inspire the design of novel high-performance anode materials. [ABSTRACT FROM AUTHOR]