Si swarf is an industrial waste generated during slicing Si ingot to produce Si wafers for solar cells. The weight of Si swarf is almost the same as that of the Si wafers. Si nanopowder possesses flake-like shape consisting of two largely different sizes, i.e., several hundred nm and less than 30 nm.
Si swarf was milled in acetone using the ball milling method. Fabricated Si nanopowder was annealed at 1000°C under hydrogen atmosphere, followed by annealing at 1000°C under ethylene atmosphere to coat Si nanopowder with a ∼10 nm carbon layer. C-coated Si nanopowder, Ketjen Black, carboxymethylcellulose ammonium and poly(vinyl alcohol) were mixed with the weight ratio of 50:25:20:5. A working electrode was fabricated by coating copper foil with this mixture, and dried at 120°C under vacuum. A Si working electrode, a lithium foil as a counter electrode, and a polyethylene separator were packed in a coin cell (CR 2032-type) with 1 M LiPF6 in a mixture of ethylene carbonate and diethyl carbonate (1:1 by volume). The cells were cycled in the cell voltage range between 0.01 and 1.5 V. The lithiation and delithiation current densities were set at 180 mA/g for the 1st~5th cycles and at 1800 mA/g for the subsequent 6th~300th cycles.
Addition of 10wt% FEC to the electrolytic solution gave the delithiation capacity of ~1600 mAh/g at the 100th cycles because of formation of a thin solid electrolyte interphase which prevented cracking of Si by suppressing the Si volume change [3]. Without FEC, the delithiation capacity decreased from the 40th cycle. C-coating at the C/Si weight ratio of ~0.1 greatly suppressed crack formation because Si was lithiated homogeneously by formation of electric passes and frameworks with high density of active materials are formed [3]. Without C-coating, the delithiation capacity decreased to 200 mAh/g at the 20th cycle.
Limitation of the delithiation capacity at 1500 mAh/g after deep lithiation at 0.01 V gave the delithiation capacity of 1480 mAh/g at the 300th cycle. On the other hand, the delithiation capacity at the 300th cycle decreased to 950 and 860 mAh/g for no capacity limitation and limitation of the lithiation capacity at 1500 mAh/g after deep delithiation at 1.5 V, respectively [4].
Lithiation and delithiation curves can be divided to 9 regions from their slopes. These regions can be categorized to 5 regions by the reaction mechanism as reported previously [4].
The overvoltage at each region boundary for the limitation of the delithiation capacity was less than that for the limitation of the lithiation capacity. This result indicates better interparticle contacts for the limitation of the delithiation capacity than that for the limitation of the lithiation capacity. From the reaction mechanism reported previously [5-9], it is known that an c-Li15Si4 layer is formed during a few cycles on the Si surface and is present through during the lithiation and delithiation cycles. Because this phase is more stable than a-Li15Si4, the delethiation proceeds from the inner a-LixSi (0<x3.75) phase, which causes only slight change of the Si size for the limitation of the delithiation capacity case. However, for the limitation of the lithiation capacity, only a-LixSi (0<x<3.75) and a-Si are present, leading to pealing-off of Si due to the large Si volume change. For no capacity limitation, the overvoltage during delithiation was close to that for the limitation of the delithiation capacity, while the overvoltage during lithiation was close to that for the limitation of the lithiation capacity. This result indicates that a part of the interparticle contacts is restored during deep lithiation and destroyed during deep delithiation.
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