MnSn₂and MnSn₂-TiO₂nanostructured 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 MnSn₂intermetallic used in LIBs. Nanostructured MnSn₂is synthesized by reducing Sn and Mn salts using a hot injection method. The resulting MnSn₂is characterized by x-ray diffraction and transmission electron microscopy and then is investigated as an anode for LIBs. The MnSn₂electrode delivers a stable capacity of 514 mAh g^-1after 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 TiO₂protection layer is applied onto the MnSn₂electrode surface via atomic layer deposition. The TiO₂-coated MnSn₂electrodes do not exhibit the activation overpotential peak. The protection layer also increases the capacity to 612 mAh g^-1after 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.