Precise Electrochemical Analysis of Carbon-Based Negative Electrode
Authors: Dosho Daiki , Matsumoto Shinji , Mukofukasawa Hayate , Kunimi Takao , Kobayashi Yoshikazu , Seki Shiro 
Published: December 2024

Lithium-ion batteries (LIBs) are required to have high energy density, high power density, and long life properties. The development of materials for active materials, which are directly involved in the redox reaction of electrodes, is important for improving the performance of storage batteries. Therefore, precise electrochemical properties of active materials should be understood. However, applied electrodes used for general performance evaluation of storage batteries are mainly composed of active material, conductivity additive, and binder, and it is difficult to evaluate only electrochemical properties of the active material. Therefore, we investigated the single-particle electrochemical measurements (SPEM), which enables electrochemical measurement of a single active material particle by using a probe with a Pt wire covered by glass. On the other hand, diluted electrode method[1], which can be relatively easily achieved by replacing some of the active material in to the applied electrode with electrochemically inactive Al2O3. In this study, HCS[2] particles, in which multiple Si nanoparticles are constrained by non-graphitizable carbon (HC) as electrode active materials to physically suppress the volume changes according to the redox reactions, and HC fabricated by similar conditions as the case of HCS, were analyzed and evaluated to compare and investigate electrochemical measurements using SPEM and diluted electrode methods.

To prepare the probe for the measurement, the Pt wire (f 20 mm) was inserted into a glass capillary, heated and coated. To ensure sufficient contact between the active material particles and the probe, the tip of the probe was polished. All electrochemical measurements by the SPEM were performed in a glove box with an Ar inert atmosphere. The open cell was used for the measurements consisted of a Li metal foil (counter electrode/reference electrode) on to a stainless steel cup, a glass separator covered with active material particles, and dropped of electrolyte (1.0 mol kg-1 EC-LiFSA). Electrochemical measurements were performed by the fabricated probe into contact with a single active material particle and observed it from above using side with a digital microscope. A dilute applied electrode sheet containing active material , conductivity aid, binder, Al2O3 was prepared. A slurry of HCS and HC, acetylene black (AB), and binder in a mass ratio of 62.5:17.5:20 was applied onto the Cu foil current collector. After drying the prepared electrode sheet in a thermostatic oven for 24 hours, the sheet was punched out at f 16 mm in diameter to prepare a 2032-type coin cell. The diluted electrodes were prepared with varying the weight ratio of x HCS: (1 - x ) Al2O3(x = 1, 0.5, 0.1), and constant current charge-discharge tests were performed, respectively.

Fig. 1(a) and (b) show the constant-current charge-discharge profiles of the applied electrodes fabricated with active material percentages of 100 and 10 wt%, respectively. The obtained charge-discharge profiles showed a gradual potential changes around 1.5 - 0.2V. This voltage changes can be attributed to the structural disorder of HC in the HCS. Fig. 2 shows the constant-current charge-discharge profiles using a single HCS particle. The electric capacity in the single particle was calculated from the volume density assuming the particles to be true spheres. The obtained charge-discharge profile exhibits sufficient capacity, suggesting that feasible single-particle measurement of HCS single particles. Moreover, multiple potential changes similar to the charge-discharge behavior of graphite (C6) were observed around 1.5 - 0.2 V. This result might be suggested to the single particle measurement corresponds to the observation of a local area, unlike the applied electrode, where comprehensive information on multiple active materials (104 order grains/cm2) is obtained. A stepwise reaction might be proceed layer by layer, similar to C6. In addition, Fig. 1(b) shows a different charge-discharge profile compared to Fig. 2, suggesting that the environmental conditions at the 10 wt% diluted electrode are not similar to those of the result of single-particle measurement. Of course is further investigation in the active material fraction is necessary. In the presentation, We will report the results of constant-current charge-discharge tests of composite electrodes with active material weight percentages of 100, 50, 10, and 1 wt%, constant-current charge-discharge tests using HCS and HC single particles, and constant-potential tests.

[1] T. Sawahashi et. al., RSC Adv.,
13, 21667-21672 (2023).

[2] U. Tsunoda et. al., Mater. Adv.,
4, 4436-4443 (2023).

Figure 1