Asymmetric Sandwich Membranes Containing Micron-Size Silicon As Anode Material for High Capacity Lithium-Ion Batteries
Authors: Byrd Ian , Webber Theron , Wu Ji 
Published: February 2020

Silicon is deemed as one of the most promising anode materials for next-generation high capacity lithium ion batteries to power electric vehicles and store intermittent energy sources. However, the large volume change and poor mechanical strength of silicon can cause electrode pulverization, loose contact with conductive additives and thus poor cycling performance. Although there is exciting advancement in cyclability using silicon nanomaterials (nanowires, nanotubes, nano-thin films, and nanoparticles), the cycling performance of silicon micron powders is far away satisfying. Herein, we reported the synthesis of novel asymmetric sandwich membranes containing micron-sized silicon powders (≈1 µm) to accommodate the large volume expansion (≈300%) of silicon during the lithiation and de-lithiation processes. These silicon membranes were prepared using a facile phase-inversion method and systematically characterized using Scanning Electron Microscopy, Transmission Electron Microscopy, Surface Area Analyzer, Thermogravimetric Analyzer, Raman Spectroscopy, and Powder X-Ray Diffraction. Raman spectra shows the characteristic peak of crystalline silicon at a 510 cm-1, as well as broad G and D peaks centered at 1580 cm-1 and 1375 cm-1, respectively. X-Ray Diffraction shows peaks typical of cubic phase silicon. Thermogravimetric analysis confirm that the content of silicon in these sandwich asymmetric membranes is ~35%-45%, depending on whether silicon membranes were coated with carbonaceous asymmetric membranes or not. Scanning Electron Microscopy clearly shows the asymmetric membrane structures with silicon particles embedded within two layers of carbonaceous membranes. The size and size distribution of silicon micron powders are obtained using Transmission Electron Microscopy. As fabricated membranes were glued directly onto copper current collectors and assembled into half-cell lithium ion batteries. An overall capacity of 610 mAh g-1 can be maintained for 100 cycles with an 88% retention rate, applying a current density of 510 mAh g-1. The Coulombic efficiency is around 99.8% on average. It is notable that such a stable cycling performance has rarely been reported for electrodes made from micron-sized silicon particles. The improved cycling performance is believed to be attributed to the unique network of nanopores and micropores where silicon particles are embedded within. By coating silicon asymmetric membranes with one layer or two layers of carbonaceous asymmetric membranes, a stable solid electrolyte interphase can be maintained, leading to a much more stable cycling performance than single-layer membranes alone.  In comparison, the overall capacity of pure Si micron-particles showed an initially high capacity of 970 mAh g-1 and then rapidly degraded to 10 mAh g-1 in as few as 30 Cycles. Carbonaceous asymmetric membranes that don’t contain silicon particles demonstrated a high cycling stability with a 98% retention rate after 100 cycles. However their specific capacity is quite low (230 mAh g-1). Single-layer silicon membranes without sandwich structures have a high initial capacity of 936 mAh g-1, but suffer from a 38% capacity loss after 100 cycles. Finally, it shall be pointed out that these membranes can be produced in large scale because the overall process is fully compatible with the conventional roll-to-roll membrane technology.

Figure 1