Single Particle Measurement Technique Using Tweezers-Type Probe for Insertion Materials

To evaluate the electrochemical properties of active materials of lithium-ion batteries, those composite electrodes including binder and conductive carbon are generally prepared and tested as coin-type and pouch-type cells. However, the use of composite electrode sometimes makes it difficult to investigate the intrinsic electrochemical properties of active material because the electrochemical response of composite electrode is strongly influenced by its porosity, thickness and composition. In order to eliminate such secondary factors from the electrochemical evaluation of active material, we have focused on single particle measurement, in which a single particle of active material can be measured ^[1]. So far, we have used a needle-type probe (current corrector) to measure the electrochemical properties of various single particles such as LiFePO₄ ^[2] and graphite ^[3]. However, the needle-type probe is sometimes difficult to apply to the active materials with large volume changes during charge and discharge. In order to solve this problem, we have developed a tweezers-type probe (Fig. 1). In this study, this tweezers-type probe was applied to evaluate the intrinsic electrochemical properties of Si-based anode materials with large volume changes.

In single particle measurement, an optical microscope was set in a globe box filled with dry Ar gas, and a measurement cell was set on this stage. A single particle of target active material was pinched with a tweezers-type probe under microscope observation, and then its electrochemical measurement was carried out. A mixed solvent of ethylene carbonate (EC) and propylene carbonate (PC) (1:1 in volume) containing 1 mol dm^-3 LiPF₆ was used as an electrolyte solution.

Fig. 2 shows charge-discharge curves of a Si particle measured by using a tweezers-type probe. Typical charge and discharge curves of Si with potential plateaus at around 0.2 V and 0.4 V were observed ^[4]. When using a needle probe, it was difficult to maintain the contact with Si particle, resulting in poor reproducibility. In contrast, even repeating cycles, the contact between the tweezers-type probe and Si particle could be maintained by using the tweezers-type probe. This result clearly shows the advantage of tweezers-type probe on evaluation of intrinsic electrochemical properties of advanced active materials with large volume changes.

Reference

[1] Kaoru Dokko, Natsuko Nakata, Kiyoshi Kanamura, Journal of Power Sources 189 (2009) 783-785.

[2] Hirokazu Munakata, Bunpei Takemura, Takamitsu Saito, Kiyoshi Kanamura, Journal of Power Sources 217 (2012) 444 – 448.

[3] Kaoru Dokko, Natsuko Nakata, Yushi Suzuki, Kiyoshi Kanamura, J. Phys. Chem. C 114 (2010) 8646-8650.

[4] S. D. Beattie, D. Larcher, M. Morcrette, B. Simon and J.-M. Tarascon, J. Electrochem. Soc. 115 (2008) A158-A163.