2, Stanford Synchrotron Radiation Lightsource, Menlo Park, California, United States
Anodes that undergo alloying reactions with lithium and sodium are attractive substitutes for graphite in LIBs and SIBs for their high capacities. However, these high-capacity alloy type anodes suffer from short lifetimes associated with severe volume changes upon lithiation and sodiation. It is well-recognized that nanoporous architectures can help accommodate these large volume changes, but this mechanism is not well understood. To improve the lifetime of high-capacity nanoporous anode materials, a fundamental understanding of the structural changes upon cycling is necessary. We utilize nanoporous tin (NP-Sn) and antimony tin (NP-SbSn), synthesized by the selective etching, as a platform to study the deleterious effects of volume change in these alloying-type anodes under transmission X-ray microscopy (TXM).
TXM utilizing hard X-ray’s enables imaging of thick samples during battery operation with a large field of view. Imaging of this type is more representative of the actual electrode environment. The resolution of the final image at the Stanford Synchrotron Research Laboratory beam line 6-2 is ~30 nm, and is well matched to the morphology of our NP-Sn and NP-SbSn. Operando TXM shows that NP-Sn and NP-SbSn expand by 40 and 60% during lithiation, which is significantly less than in bulk Sn (130%). More importantly, the pore system in NP-SbSn stayed completely intact while some degree of fracturing was observed in NP-Sn, suggesting that NP-SbSn is more mechanically stable. This agrees with our observation that NP-SbSn shows better stability than NP-Sn when cycled with sodium. Specifically, we found that NP-Sn and NP-SbSn show comparable performance when cycled with Li+ (650 mAhg-1 initial capacity with 98% capacity retention after 200 cycles for NP-Sn and 560 mAhg-1 initial capacity with 90% capacity retention over 200 cycles for NP-SbSn). By contrast, NP-SbSn demonstrates much better cycle stability than NP-Sn when cycled with Na+ (430 mAhg-1 initial capacity with 85% capacity retention after 100 cycles for NP-SbSn compared to 550mAhg-1 initial capacity with only 50% capacity retention after 90 cycles for NP-Sn. It is known that intermetallics reduce the strain of these alloying anodes during ion intercalation by spreading out the intercalation voltages. It appears from our operando TXM studies that in porous materials, intermetallics further help preserve the pore system during cycling, allowing for longer cycle life with high-strain intercalant such as Na+. Overall, these nanoporous metals appear to be promising solutions to combat volume expansion due to their well-maintained structures and open pores throughout cycling, allowing for good electrolyte penetration and uniform lithiation and sodiation of these electrode materials.