|Reversible discharge capacities (normalized by the content of active material) for anodes with (top) 30 wt % Sn/SnO2 NCs and (bottom) a high mass content (63.75%) of an active Sn-based material. Credit: ACS, Kravchyk et al. Click to enlarge.|
Researchers from ETH Zürich have synthesized highly monodisperse colloidal Sn and Sn/SnO2 (tin/tin dioxide) nanocrystals for use as an anode material in Li-ion batteries. Testing showed that 10 nm Sn/SnO2 nanocrystals enable high cycling stability, in contrast to commercial 100–150 nm powders of Sn and SnO2. In particular, reversible Li-storage capacities above 700 mAh g–1 were obtained after 100 cycles of deep charging (0.005–2 V) at a relatively high current of 1000 mAh g–1. A paper on their work appears in the Journal of the American Chemical Society.
Tin-based materials (like silicon-based materials) offer specific capacity values much higher than conventional graphite. However, these classes of materials suffer from large volume expansion during lithiation, which generates enormous mechanical stress and pulverizes the electrode during the charge/discharge cycles, resulting in rapid capacity fade.
In this work, we were driven by two goals: (i) to develop a robust synthesis of monodisperse Sn nanocrystals (NCs) and (ii) to study the size-dependent cycling performance of nanocrystalline Sn-based LIB anodes. For the latter, finely tunable morphologies and optimal surface chemistries in the sub-50 nm size range are considered crucial for achieving high charge/discharge cycling stabilities in the next-generation high-capacity anode materials based on Li alloys with Si, Sn, and Ge.
Furthermore, the potential use of Sn and the corresponding oxide SnO2 in a monodisperse colloidal state is broader by far, including solution-deposited transparent conductive oxides for electronics, photovoltaics, and sensors as well as quantum dots (sub-5 nm α-Sn) or low-temperature catalysts for growing nanowires. In addition, quantum-size effects in Sn nano-particles (NPs) have been demonstrated to modify their superconducting properties at cryogenic temperatures.—Kravchyk et al.
To address the volumetric change problem, the ETH team produced tin nanocrystals and embedded a large number of them in a porous, conductive permeable carbon matrix. Much like how a sponge can suck up water and release it again, an electrode constructed in this way can absorb lithium ions while charging and release them when discharging.
During the development of the nanomaterial, the issue of the ideal size for the nanocrystals arose, which also carries the challenge of producing uniform crystals. By influencing the time and temperature of the growth phase, the scientists were able to control the size of the crystals.
The trick here was to separate the two basic steps in the formation of the crystals—the formation of as small as a crystal nucleus as possible on the one hand and its subsequent growth on the other. We are the first to produce such small tin crystals with such precision.Maksym Kovalenko, project leader
The size of the nanocrystals did not affect the storage capacity during the initial charging and discharging cycle. After a few charging and discharging cycles, however, differences caused by the crystal size became apparent: batteries with ten-nanometer crystals in the electrodes were able to store considerably more energy than ones with twice the diameter. The scientists assume that the smaller crystals perform better because they can absorb and release lithium ions more effectively.
The researchers are now investigating the remaining challenges of producing optimum tin electrodes. These include the choice of the best possible carbon matrix and binding agent for the electrodes, and the electrodes’ ideal microscopic structure. Moreover, an optimal and stable electrolyte liquid in which the lithium ions can travel back and forth between the two poles in the battery also needs to be selected.
Ultimately, the production costs are also an issue, which the researchers are looking to reduce by testing which cost-effective base materials are suitable for electrode production. The aim is to prepare batteries with an increased energy storage capacity and lifespan for the market, in collaboration with a Swiss industrial partner.
Kravchyk K, Protesescu L, Bodnarchuk MI, Krumeich F, Yarema M, Walter M, Guntlin C, Kovalenko MV (2013) Monodisperse and Inorganically Capped Sn and Sn/SnO2 Nanocrystals for High-Performance Li-Ion Battery Anodes. Journal of the American Chemical Society doi: 10.1021/ja312604r
Jiajun Chen (2013) Recent Progress in Advanced Materials for Lithium Ion Batteries. Materials 6, 156-183; doi: 10.3390/ma6010156