The increment in resistance can be produced by the diffusion limi

The increment in resistance can be produced by the diffusion limitation of the Li ions through GDC-0449 the electrolyte among the wires (at high cycling rates) or by a continuous amorphization of Si upon cycling. The second effect is also known to occur in the PCI-32765 supplier shorter wires [10]. Nevertheless, in the case of longer wires, the increment in resistance at high cycling rates due to diffusion constraints

is more significant.The previous statements can be corroborated when observing the percentage of the capacity obtained galvanostatically during the galvanostatic/potentiostatic cycling. As can be observed in Figure  6, during the first four cycles, when the cycling rate is C/10, the lithiation capacity obtained galvanostatically (galvanostatic lithiation) is similar in anodes with wires of 70 and 130 μm. The current density of C/10 is moderate, giving enough time to the Li ions to diffuse; thus, the most of the lithium storage (80%) is obtained galvanostatically. On the other hand, when

the cycling rate is C/2, the Li diffusion is in its limit for the longer wires. With the diffusion limitation, the Li ions may be incorporated mainly at the wire Selleck CH5183284 tips, making the mean path for electrons and Li ions longer and, consequently, the mean electric resistance higher. As discussed before, when the resistance increases, the voltage limits are reached 5-Fluoracil order sooner, and the galvanostatic mode stops also sooner. That is why the percentage of

charge is much lower for longer wires after cycle 5. Additionally, the capacity decreases continuously because in every delithiation cycle, some charge remains in the wires, and there is always less space for lithiation every cycle. Figure 6 Curves of the percentage of the lithiation capacity obtained galvanostatically. The first four cycles were performed at a cycling rate of C/10 and the rest at C/2. The amount of Li used for the formation of the solid electrolyte interface (SEI), normalized to the weight of Si, also scales when scaling the size of the wires. The sum of the irreversible Li losses (difference between the lithiation and delithiation capacities) during the first four cycles amounts to 1,606 mAh/g for the short wires and 3,087 mAh/g for the long wires (1.92 times the value for short wires). The SEI forms mainly during these first cycles, being the losses minimal afterwards. Considering the active portion of the wires with lengths 70 and 130 μm, the scaling factor is 2, value very close to the value 1.92 of the proportion of SEI. Thus, one may say that the SEI scales with the length, but tests with other wire lengths are necessary to confirm the theory. For the moment, the reason of this scaling is not clear. The SEI is an important structural component of the anode, which may be a decisive factor for the mechanical stability of the anode.

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