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

Even with the many desirable properties, natural abundance and low cost of -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.