In situ TEM study of lithiation into a PPy coated α-MnO2/graphene foam freestanding electrode

Abstract

Even with the many desirable properties, natural abundance and low cost of α-MnO₂, its application as the anode in lithium-ion batteries has been limited because of its low intrinsic electrical conductivity and large volume expansion occurring during charge/discharge cycles. In this work, a ternary composite electrode consisting of MnO₂-polypyrrole (PPy) core–shell arrays is grown on graphene foam (GF) to address the above critical issues. The freestanding MnO₂–PPy/GF electrode exhibits a high reversible capacity of 945 mA h g⁻¹ at 0.1 A g⁻¹ after 150 cycles with a coulombic efficiency of over 98%, far better than 550 mA h g⁻¹ for the uncoated counterpart. An in situ TEM examination reveals several functional features of the PPy coating that ameliorate the MnO₂ conversion reaction kinetics, and thus the electrochemical performance of the electrode. The PPy coated MnO₂ nanowires have a lithiation speed three times faster than that of the uncoated MnO₂ along with improved electronic conduction and a stable structure against volume expansion. Such a rational design of an electroactive core and a highly conductive polymer shell on a GF conductive substrate offers a potential solution to developing novel MnO₂-based electrodes with enhanced electrochemical performance.