Size Effect in SnO₂/Al₂O₃ Core/Shell Nanowires after Battery Cycling

Abstract

Full utilization of the high storage capacity of conversion electrode materials as tin oxide (SnO2) in lithium‐ion batteries is hindered by the high volumetric expansion due to the high lithium storability which can lead to major cell damage and consequent safety issues. To overcome this issue, two promising approaches, nanostructures and buffer layers, are combined and evaluated. SnO2 nanowires (NWs) are coated with an aluminum oxide (Al2O3) buffer layer to investigate the combination SnO2–Al2O3. Strong differences in the crystallinity after cycling between the SnO2/Al2O3 core/shell NW‐based heterostructure and uncoated SnO2 NWs based on detailed structural analysis are shown via transmission electron microscopy (TEM) and determination of the elemental distribution of tin, oxygen, lithium, and aluminum via energy‐dispersive X‐Ray spectroscopy and energy‐filtered TEM in the as‐prepared and postmortem nanostructures. The core/shell NWs exhibit two different states after charge/discharge cycling, amorphous or crystalline, depending on the NW diameter; for the uncoated SnO2 NWs, only an amorphous postmortem structure is found. Additionally, differences in the elemental distribution for the amorphous and crystalline postmortem SnO2/Al2O3 core/shell NWs, especially for tin, are measured. Consequently, the structures and effects of the Al2O3 coating on the lithiation behavior of SnO2 NW‐based heterostructures are discussed.

High-Resolution Nanoanalytical Insights into Particle Formation in SnO2/ZnO Core/Shell Nanowire Lithium-Ion Battery Anodes

“…The SEM images of as-prepared and postmortem SnO 2 NWs are included in Figure a,b: from the comparison of the as-prepared and postmortem uncoated SnO 2 NWs, we found that the NW morphology is maintained after cycling (Figure a,b). This is consistent with literature results of charge/discharge cycled SnO 2 NWs from ref , which showed that the wire-like structure was maintained although the crystalline structure became amorphized. However, the coated structures in Figure c–f showed more dramatic structural changes, although the initial wire-like structure can be still seen but an irregular surface has formed.…”

“…The observed material response of SnO 2 /ZnO core/shell NWs on the charge/discharge cycling in LIBs is unlike the material response of uncoated SnO 2 NWs. For these, we recently showed by TEM that the general wire-shaped structure was maintained, although the wire became amorphous with large Sn-rich areas within the NW structure . This observation agreed with the expected charge response of SnO 2 , i.e., the formation of Li 4.4 Sn surrounded by a Li 2 O matrix in the fully charged state .…”

“…However, the theoretical total capacity is not reached due to the use of a carbon-based substrate, which can also store Li . Since the storage capacity of the carbon-based substrates is lower than the storage capacities for SnO 2 and ZnO but the total electrode weight including the carbon-based substrates goes into the total anode weight, the overall measured capacities are lower than the theoretical capacities for SnO 2 and ZnO.…”

“…After 10 cycles at 50 mA/g, the charge/discharge currents were increased incrementally to 400 mA/g (Figure d). Since the carbon-based substrate can also store Li, the total anode weight was used (SnO 2 /ZnO core/shell NWs + carbon felt) for the normalization of the currents. The first discharge (lithiation of the working electrode) cycle at 50 mA/g took 8:41 h and the complete charge/discharge cycle took 14:22 h. A noticeable capacity loss is visible between the first and the second cycle because of the irreversible and reversible steps of the lithiation reactions of SnO 2 and ZnO , normalZ normaln normalO + 2 normalL normali + + 2 normale − ↔ normalZ normaln + normalL normali 2 normalO x normalL normali + + normalZ normaln + x normale − ↔ normalL normali x normalZ normaln goodbreak0em2em⁣ ( x ≤ 1 ) normalS normaln normalO 2 + 4 normalL normali + + 4 normale − → normalS normaln + 2 normalL normali 2 normalO y normalL normali + + normalS normaln + y normale −<...…”

Defect Location in Laterally Aligned Tin Oxide Nanowires: The Role of Growth Direction, Interface Dimensionality, and Catalyst

“…It is important to note that the normal vector of the growth front does not have to be parallel to the NW axis but can be tilted to it . For SnO 2 NWs, the growth front of the {301} SnO 2 was reported already in the literature. ,, The projection of the normal vector of this plane onto the (011̅) SnO 2 plane, which is the SnO 2 plane at the SnO 2 –Al 2 O 3 interface, is parallel to the NW axis along [011] SnO 2 . , Hence, we would expect the same growth front for the analyzed laterally aligned SnO 2 NWs on r-plane sapphire. The following boundary conditions for the abrupt formation of a (complete) SnO 2 lattice plane during the plane-wise VLS growth of laterally aligned NWs can be defined with the above findings: SnO 2 –Au (l) interface: energy minimization SnO 2 NW (two-dimensional SnO 2 –SnO 2 homoepitaxy) SnO 2 facet edges (edges of growth front) 3 facet edges: no restrictions (NW surface and sidewalls) 1 facet edge: optimum lattice match at SnO 2 –Al 2 O 3 edge (one-dimensional SnO 2 –Al 2 O 3 heteroepitaxy) …”