Single Particle Measurement for Volume Change Behavior of a Silicon Single Particle

Lithium-ion batteries (LIBs) with high gravimetric and volumetric energy density are very important key devices for the establishment of the sustainable energy system which is consisted of solar cells, wind power generations, smart grid, and batteries. In addition, the LIBs are extensively expected for a power supply of plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs). Much higher energy density is needed to increase the mileage per charge. Current LIBs use the composite electrodes which are composed of active materials, binders, conductive agents, and current collector. The charging and discharging performance of the composite electrode must be affected by the “composite” state (kinds of materials, mixing ratio, thickness, and tap density). In addition to that, the materials which have large volume change during the charging and discharging are strongly affected by the state of the composite electrode. It is difficult to distinguish the electrochemical characteristics of only active materials from that of the composite electrode.

However, the intrinsic properties of active materials are very important to evaluate the electrode performance and understand the mechanism of the charging and discharging reaction. In this study, the single particle measurement technique was used to study the electrochemical lithiation of silicon single particle. This technique is very useful to measure the intrinsic properties of single particle of the active materials in the actual liquid electrolyte. Our group has reported that some positive electrode active materials have very good rate characteristics [1,2]. The microelectrode with 20 mm diameter is used as the micro probe to contact and apply the electrochemical technique to the one active material particle. The electrodeposited Cu covers the tip of the micro probe because the Cu is stable against the electrode potential of the lithiation of silicon. Our previous study has reported that this technique is applicable to measure the volume expansion of silicon single particle during the lithiation [3]. Figure 1 shows the drastic volume expansion behavior of a silicon single particle during the lithiation. A silicon particle contacted with micro-probe was expanding with the lithiation. The apparent volume expansion ratio is larger than the theoretical expectation. So far, we estimate that the reasons are the aggregation state of the silicon particle, the emergence of amorphous phase of the lithiated silicon and anisotropic properties of volume change. In order to discuss the reason of larger volume expansion and volume change mechanism of the silicon particle more precisely, micro-tweezers system was used to move the silicon particle to SEM observation and Raman spectroscopy measurement after the 1st charging by the single particle measurement technique. Some researchers use an in-situ TEM technique in order to understand the mechanism of silicon lithiation and delithiation [4]. This excellent technique can observe the silicon volume change behavior accompanied with the lithiation and delithiation with very high magnification, but it needs high vacuum condition. The advantage of our technique is the electrochemical condition is very near to the actual LIBs. Therefore, it is important to combine their knowledge and our results to find the volume change mechanism of silicon electrode during the lithiation and delithiation. In this presentation, we would like to discuss the comparison.

Furthermore, high capacity active materials which have volume change during charging and discharging need the way to restrict or accommodate the volume change for the usage to practical battery electrodes. From the viewpoint of such approach, the binder effect is very important. A small piece of composite electrode with 20~40 mm was picked up to measure the electrochemical characteristics in the single particle measurement system. Comparing with the pristine silicon particle measurement results, we would like to discuss the binder effect to volume change behavior.

References

(1) K. Dokko, N. Nakata, K. Kanamura, J. Power Sources 189, 783 (2009)

(2) H. Munakata, B. Takemura, T. Saito, K. Kanamura, J. Power Sources 217, 444 (2012)

(3) K. Nishikawa, H. Munakata, K. Kanamura, J. Power Sources 243,630 (2013)

[4]. M. T. McDowell, S. W. Lee, J. T. Harris, B. A. Korgel, C. Wang, W. D. Nix, and Y. Cui, Nano Lett., 13, 758 (2013)

Figure 1. Volume expansion behavior of silicon single particle during the first lithiation