in-Situ Measurement of the Thickness Change of Dense Si Electrodes in Lithium-Ion Batteries Using Electrochemical Dilatometry

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 electrode remains a bottleneck to the commercialization of the material. Many studies were devoted to nanostructured silicon composites with voids to accommodate the volume expansion [1]. Yet, full capability of silicon cannot be utilized because of the low volumetric energy density of these nanostructures. To increase the volumetric energy density compared to graphite, dense silicon electrodes are needed. Volume expansion within the electrode becomes an important factor affecting its cycle stability. In reality, how much is the volume expansion? What is the mechanism during the charge and discharge? Few studies have addressed these issues. In this work, we employed electrochemical dilatometry to measure the thickness change of Si electrodes during charge and discharge to understand the behavior with time. We demonstrate that the amount of electrode expansion and contraction is non-linear with the amount of lithium, which is explained by a model. We identify binder breakdown as a one of the causes of capacity degradation. Better reversibility in thickness change is achieved by using a more flexible binder such as polyimide, resulting in better cycle stability.

Bulk silicon particles with a size of 10-20 μm from Sigma Aldrich was used to make the electrodes. The powder was mixed with acetylene black (Alfa Aesar) and binder to make an electrode. Electrode composition and type of binder were varied to study their effects on electrode volume change and capacity during charge-discharge. Typical electrode thickness is between 20-25 μm with 60wt% active material and a packing density of about 1.2 g cm^-3. This corresponds to an electrode loading of about 1.5 mg silicon cm^-2. The electrodes were assembled in an electrochemical dilatometer (ECD-1 from EL-Cell) with Li metal as counter electrodes for the thickness measurement. Similar device has been used by others to measure thickness change of various electrodes [2,3].   

Fig. 1 shows the change in thickness with respect to cumulative capacity of a Si electrode with 20% carbon black and 20% carboxymethyl cellulose during the first three cycles. As expected, an increase in electrode thickness is observed during lithiation (increase in capacity), and a decrease in thickness during delithiation (decrease in capacity). The increase and decrease in electrode thickness are however not linear. A three-stage expansion model is used to describe the observation. At the beginning of lithiation (stage I), the electrode thickness change is small, as the composite electrode contains space between the particles that can accommodate the volume expansion. 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, indicating that it is due to structural change within the particle. Further incorporation of lithium into the electrode leads to an accelerated increase in thickness (stage III). The onset of stage III expansion depends on the type of binder used in the electrode, which suggests that it is affected by the ability of the binder to hold the particles together. During delithiation (stage IV), the contraction behavior is different from that during expansion. This is partly because the particle can contract in all three directions, as opposed to one direction during lithiation. 

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. The 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.

Acknowledgement

This research is sponsored by the GRF/ECS Scheme (21202014) managed by the Research Grants Council, the Government of the Hong Kong SAR.   

References

1. Wu, H. & Cui, Y. Nano Today 7, 414-420 (2012).

2. Hantel, M. M. et al. J. Electrochem. Soc. 159, A1897-A1903 (2012).

3. Kim, J. S. et al. J. Power Sources 244, 521-526 (2013).