Enhanced rate performance and cycling stability of a CoCO3–polypyrrole composite for lithium ion battery anodes

Abstract

A CoCO₃–polypyrrole composite (CC–PPy) for lithium ion battery anodes was prepared by first synthesizing urchin-like CoCO₃ microspheres (CC) via a hydrothermal route and further modifying them with a PPy coating. The resulting CC–PPy exhibits excellent cycling stability, outstanding rate performance and a great recovery capability compared to CC, delivering a reversible capacity of 1070.7, 811.2, 737.6, 518.7, 504.5 and 559 mA h g⁻¹ after 100 cycles at 0.1, 1, 2, 3, 4 and 5 C, respectively, and a recovery capacity of up to 1787 mA h g⁻¹ after 500 cycles from 1 to 5 C. A more comprehensive lithium storage mechanism of CoCO₃ has been proposed to support the experimental data, which includes two-step conversion reactions with a total theoretical value of 7 Li per CoCO₃. The ‘first-order’ reaction involves reduction of CoCO₃ to metallic Co and the formation of Li₂CO₃, and the second reaction involves the further reduction of Li₂CO₃ to Li_xC₂ (x = 0, 1, 2), along with the formation of Li₂O. The lithiation and delithiation processes of CC and CC–PPy have been compared based on their potential profiles and CV curves, which show clear two-order character. The kinetic factors for the superior performance of CC–PPy are analyzed based on the Nyquist plots. Furthermore, the transition from CoCO₃ to Li₂CO₃ to Li₂O and its reversibility is confirmed by ex situ IR spectra recorded at the different discharge–charge states of CC–PPy.