Real-Time Measurement of Expansion and Contraction of Silicon Electrodes in Lithium-Ion Batteries

Silicon has been the focus of many research studies as the next generation high-capacity anode material for lithium-ion batteries. However, the mechanical stability of the material remains a bottleneck to the commercialization of the material. Much work has been devoted to make nanostructured silicon composites to suppress the volume expansion. Yet, the amount of dimensional change on an electrode has not been directly monitored. Here we used an in-situ electrochemical dilatometer to quantify the thickness change of silicon electrodes during charge and discharge in order to develop techniques to stabilize the electrode.

Bulk silicon particles with a size of 10-20mm from Sigma Aldrich was used to make the electrodes. We found that the degree of electrode thickness increase depends on the electrode composition (amount of carbon black and binder in the electrode). An electrode with 10wt% polyvinylidene fluoride binder gives as much as 600% increase in thickness after initial discharge. This means that an electrode that is initially 20mm thick is expanded to as much as 140mm thick after lithiation. The amount of thickness change is less when 20% binder is used. Fig. 1 shows the change in electrode thickness with respect to cumulative capacity with 20% carbon black and 20% carboxymethyl cellulose during the first three cycles. As expected, an increase in electrode thickness is observed during lithiation (discharge), and a decrease in thickness during delithiation (charge). The increase and decrease in electrode thickness are however not linear, and a model is devised to explain the mechanical expansion and contraction behaviors in a composite electrode. In brief, the electrode undergoes three stages of expansion during initial lithiation. At the beginning of discharge, the thickness change is small, as the composite electrode contains space between the particles that can accommodate the volume expansion (Stage I). Beyond a certain point, the particles impinge on each other and the volume expansion of the particles lead to an overall increase in the film thickness (Stage II). The amount of increment during stage II is similar to the theoretical increase in volume (dotted line in Fig. 1) for alloying Li with Si. The binder then losses the ability to hold the particles together, leading to an accelerated increase in thickness at the end of lithiation (Stage III). During delithiation, the contraction behavior is different from that during expansion (Stage IV), as seen from the dilatometer results. This is partly because the particle can contract in all three directions, as opposed to one direction during lithiation. The accuracy of the measurement from the dilatometer is verified by cross-sectional SEM test.

After understanding the mechanism during charge and discharge, electrodes with different binders were then tested to compare the role of the binder on cycle stability. Electrodes with polyimide (PI) binder show better cycling stability than those with polyvinylidene fluoride and carboxymethyl cellulose, which is attributed to the ability of the PI binder to hold the particles together after expansion. Our results show that binder breakdown is one of the main causes of electrode degradation for Si electrodes. More experimental details and results will be shown during the presentation. With further optimization, even bulk silicon particle has the potential to be used for commercial applications.