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

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 LixC2 (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.