MnSn2 and MnSn2–TiO2 nanostructured anode materials for lithium-ion batteries

The high theoretical lithium storage capacity of Sn makes it an enticing anode material for Li-ion batteries (LIBs); however, its large volumetric expansion during Li–Sn alloying must be addressed. Combining Sn with metals that are electrochemically inactive to lithium leads to intermetallics that can alleviate volumetric expansion issues and still enable high capacity. Here, we present the cycling behavior of a nanostructured MnSn2 intermetallic used in LIBs. Nanostructured MnSn2 is synthesized by reducing Sn and Mn salts using a hot injection method. The resulting MnSn2 is characterized by x-ray diffraction and transmission electron microscopy and then is investigated as an anode for LIBs. The MnSn2 electrode delivers a stable capacity of 514 mAh g−1 after 100 cycles at a C/10 current rate with a Coulombic efficiency >99%. Unlike other Sn-intermetallic anodes, an activation overpotential peak near 0.9 V versus Li is present from the second lithiation and in subsequent cycles. We hypothesize that this effect is likely due to electrolyte reactions with segregated Mn from MnSn2. To prevent these undesirable Mn reactions with the electrolyte, a 5 nm TiO2 protection layer is applied onto the MnSn2 electrode surface via atomic layer deposition. The TiO2-coated MnSn2 electrodes do not exhibit the activation overpotential peak. The protection layer also increases the capacity to 612 mAh g−1 after 100 cycles at a C/10 current rate with a Coulombic efficiency >99%. This higher capacity is achieved by suppressing the parasitic reaction of Mn with the electrolyte, as is supported by x-ray photoelectron spectroscopy analysis.