Improved Mass Loading of Si Electrodes in Li Ion Batteries with Si Swarf/Graphite Sheet Composites

Abstract

The theoretical capacity of Si anodes (3578 mAh/g) is ~10 times as high as that of graphite anodes (372 mAh/g). Large volume changes of Si anodes during lithiation and delithiation result in peeling-off of Si and decreases in lithiation and delithiation capacities.Flake-shaped Si swarf is dispersed and wrapped by graphite sheets by sonication in N-methylpyrrolidone (NMP) and filtering the solution. The lateral size and thickness of Si swarf are in the range of 3 nm~2 μm and 2~40 nm, respectively. Graphite sheets are fabricated by sonication of expanded graphite in NMP. The lateral size and thickness of graphite sheets are in the range of 0.5~13 μm and 1~13 nm, respectively.Si/graphite sheet composites, C-coated Si swarf fabricated under ethylene atmosphere or pristine Si swarf with the theoretical capacities of 3044 mAh/g, 3287 mAh/g or 3578 mAh/g, respectively, is mixed with Ketjen black, cross-linked poly(acrylic acid) (CLPA) and poly(vinyl alcohol) (PVA) with the weight ratio of 60:20:10:10. CR2032-type coin cells are assembled with a Si electrode, a Li foil, a polyethylene separator, and 1 M LiPF6 EC/DEC (1:1 vol) electrolyte with 10 wt% fluoroethylene carbonate (FEC). The cells are cycled in the cell voltage range between 0.01 and 1.5 V during 300 cycles.The delithiation capacity of the Si/graphite sheet composite electrodes at the 300th cycle increases 14%, compared to the C-coated Si electrode. The decrease in the delithiation capacity is suppressed by graphite sheets after 180 cycles.TEM and cross section SEM images of the Si/graphite sheet composite electrodes show dispersed Si nanopowder between ultrathin graphite sheets even after the 300th cycle. In the case of the C-coated Si electrode, Si fuses after the 300th cycle.The delithiation limitation at 1200 mAh/g after deep lithiation improves the cyclability of the Si electrodes. The Si/graphite sheet composite electrode keeps the delithiation capacity of 1200 mAh/g until the 901th cycle, while the C-coated Si electrodes until the 660th cycle.The electrochemical impedance spectroscopy indicates low internal resistances for the Si/graphite sheet composite electrode. The series resistances for Si/graphite sheet composite electrode after the 6th and 300th cycles are a half and two-thirds of those for the C-coated Si electrode, respectively. The charge transfer resistance after the 300th cycle for the Si/graphite sheet composite electrode is two-thirds of that for the C-coated Si electrodes.The theoretical areal capacity is increased from 2 mAh/cm2 to 6 and 10 mA/cm2. Lithiation periods at the constant voltage of 0.01 V are 60 min for the thick Si/GS electrode without delithiation limitation and the C-coated Si electrode with delithiation limitation, 75 min for the C-coated Si electrode without delithiation limitation, and 90 min for the Si/graphite electrodes with delithiation limitation. The Si/graphite sheet composite electrodes show higher delithiation capacities than those of the C-coated Si electrodes. The delithiation capacity of the Si/graphite sheet composite electrode at the theoretical capacity of 6 mAh/cm2 decreases from 4.9 mAh/cm2 to 1.6 mAh/cm2 during 100 cycles. Delithiation limitation at 1200 mAh/g keeps the delithiation capacity of 4 mA/cm2 at the current density of 5 mA/cm2 and the theoretical capacity of 10 mAh/cm2 for more than 75 cycles.J. Electrochem. Soc. 168 (2021) 020521.