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

Abstract

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.

One-Pot Size-Controlled Synthesis of Tin- and Antimony-Based Intermetallic Nanoparticles

“…XRD patterns corresponding to these samples showing MnSn 2 phase purity are shown in Figure S8. We have previously demonstrated that for MnSn 2 nanoparticles, brief air exposure (5 min) results in oxidation and segregation of Mn to the surface, forming a β–Sn core and mixed Mn–Sn oxide shell . Such behavior suggests an instability of MnSn 2 toward phase separation under ambient conditionsthis is unusual among intermetallic materials, which typically have enhanced compositional stability due to the relaxed free energy arising from atomic ordering.…”

“…For MnSn 2 nanoparticles, adding W­(CO) 6 to the reaction solution and increasing Sn­[N­(SiMe 3 ) 2 ] 2 concentration resulting in a decrease of mean nanoparticle size to 22 ± 3 nm (Figure D). In our initial report describing MnSn 2 nanoparticle synthesis, the same reaction without W­(CO) 6 resulted in much larger nanoparticles, with a mean size of 76 ± 16 nmthis further supports the role of W­(CO) 6 as a nucleating agent . Otherwise, reducing the amount of OLA used from 0.3 to 0.2 mmol, lowering the OLA/Sn 2+ ratio, resulting in mean nanoparticle size increasing to 48 ± 4 nm (Figure E).…”

“…In our initial report describing MnSn 2 nanoparticle synthesis, the same reaction without W(CO) 6 resulted in much larger nanoparticles, with a mean size of 76 ± 16 nm�this further supports the role of W(CO) 6 as a nucleating agent. 53 Otherwise, reducing the amount of OLA used from 0.3 to 0.2 mmol, lowering the OLA/Sn 2+ ratio, resulting in mean nanoparticle size increasing to 48 ± 4 nm (Figure 2E). Increasing the Sn precursor concentration, further reducing the OLA/Sn ratio, resulting in a further size increase to 55 ± 9 nm (Figure 2F).…”

“…This novel approach may allow access to d–p intermetallic compounds not yet synthesized at the nanoscale. Recently, our group has synthesized MnSn 2 nanoparticles in the solution phase for the first time using this approach and demonstrated their high capacity and stability as LIB anodes …”

“…Recently, our group has synthesized MnSn 2 nanoparticles in the solution phase for the first time using this approach and demonstrated their high capacity and stability as LIB anodes. 53 Here, we present the synthesis of several transition metal distannide (MSn 2 ) and antimonide (MSb x ) intermetallic nanoparticles, using amidinate compounds as the transition metal precursors. We identify reaction variables which provide both size control and phase selectivity of the nanoparticles.…”