Importance of nanostructure for high capacity negative electrode materials for Li-ion batteries

Sn–Co–C alloys are currently used as negative electrode materials for Li-ion batteries. A comparison between sputter deposited and mechanically alloyed Sn–Co–C materials has revealed a difference in the achieved specific capacity of materials prepared by the two methods. Only the sputtered materials reached the expected capacity even though both types of materials showed similar X-ray diffraction patterns. The structure of these materials has been described as being grains of amorphous CoSn embedded in a carbon matrix. Here, the sizes of the CoSn grains were determined using small angle neutron scattering measurements on various Sn30Co30C40 samples. Small grain sizes, on the order of 10 Å, were obtained for the sputtered samples while grain sizes between 55 and 100 Å were obtained for samples with the same composition but prepared by mechanical alloying methods. The inability of the mechanically prepared materials to achieve their theoretical capacity may be due to the larger size of the CoSn grains.