MnO@Mn2O3 nanocrystals embedded in biochar networks for sustainable anode of lithium-ion batteries

Abstract

MnO is an alternative for commercial graphite as anode material for lithium-ion batteries (LIBs), if the issue of unstable performance caused by volume changes, poor conductivity, and changes in valence state of Mn is addressed. In this study, a series of nano MnO/biochar-containing composite materials were prepared with Mn(NO3)2, urea and pomelo peel as raw materials, via simple hydrothermal treatment and calcination, as anode materials for LIBs, where the biochar was used as inexpensive conductive medium and volume buffer. The electrochemical performance of the composite materials shows complicated correlation with the biochar contents, which associate with different carbonization temperatures. Elaborate characterizations with XRD, SEM, TEM, and Mn 2p, Mn 3s XPS spectra were conducted and jointly analyzed, which revealed that the surface of MnO nanocrystals undergoes different degrees of oxidation. The thickness of the surface amorphous Mn2O3 layer on the MnO nanocrystals influences the long-term electrochemical stability more significantly than the circumstances of biochar encapsulation, which was proved through cyclic voltammetry (CV) measurements. A composite material with moderate biochar content and thin layer of Mn2O3 coating on MnO nanocrystals will show more stable electrochemical performance. Based on this principle, one regulated composite material (MnOx/BC-650) gives an ultra-stable capacity of 446.9 mAh∙g−1 at 500 mA∙g−1 after 200 cycles. This study provides new insight for both revealing the true structure of manganese oxides and developing long-term stable MnO-containing anode materials for LIBs.