Metal Free Hybrid Capacitor Using Intercalation of 1-Butyl-3-Methylimidazolium Cation to Acetylene Black, Ketjen Black and Graphite
Authors: Chiku Masanobu , Tanimoto Yuuya , Higuchi Eiji , Inoue Hiroshi 
Published: February 2020

1. Introduction

Lithium ion capacitor (LIC) is one of the most promising hybrid capacitors (HCs) with the negative electrode for lithium ion battery (LIB) and activated carbon (AC) positive electrode. Recent progress of LIC realized its high energy density over 80 Wh kg-1 and LIC is already commercially available for several applications like backup power supply, load leveling for natural power sources, energy regeneration etc.
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Moreover, LIC has better cyclic performance and power density than LIB so high performance LIC is considered as next generation power sources for mobile electric devices and electric vehicles. However, the charging mechanism of the carbon negative electrode is insertion of Li+ ion and its potential is 0.01 V vs. Li/Li+ which is quite similar to the Li metal precipitating potential (0V vs. Li/Li+). It is well known that the precipitation of Li metal causes internal short circuiting and deadly explosion. To avoid such risk, Li free HC is one of the most promising idea. In this work, we realized the Li+ free HC with ionic liquid for safe and high energy density power sources. Recent study suggested the large cation like 1-Butyl-3-methylimidazolium (BMI+) can be inserted/deserted into carbon electrode. So we used the ionic liquid (1-Butyl-3-methylimidazolium tetrafluoroborate (BMIBF4)) as electrolyte and insertion/desertion of BMI+ into carbon electrode was used as negative electrode reaction. AC was used as positive electrode. Three different carbon materials (acetylene black (AB), ketjen black (KB) and graphite) were investigated as negative electrode materials.

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Experimental Mo sheet was used as current collector. AB, KB and graphite powder was dispersed into N-methyl-2-pyrrolidon with polyvinylidene difluoride which is used as binder. The slurry was sonicated for 15 min and casted onto Mo current collector. AC was mixed with polytetrafluoroethylene in mortar and cold pressed with 30 MPa for 5 min, which was used as positive electrode. BMIBF4 was dried under vacuum for 24 h at 85 °C. The homemade glass cell was used for HC. Ag wire was used as pseudo reference electrode.

3. Results and discussion

Fig. 1 shows the charge/discharge curves of HCs with the KB, AB and graphite negative electrodes. HCs with AB and graphite negative electrode was charged to 3.7 V but the cell voltage was limited below 3.2 V in the case of the HC with KB negative electrode because decomposition of the electrolyte was started below 3.5 V. Excepting the charge voltage, three HCs showed almost the same features.
Fig. 2 shows the changes in the potential of KB, AB and graphite negative electrode and AC positive electrode. Each negative electrode showed plateaus during charge/discharge process around -2 V vs Ag and -1 V vs Ag, respectively. On the other hand, AC positive electrode showed triangular charge/discharge curves which is typical for the electrochemical double layer capacitor. These phenomena suggests the charge/discharge process of negative electrode in HCs is insertion/desertion of BMI+ and the positive electrode worked as electrochemical double layer capacitor.
Fig. 3 shows the change in discharge capacitance with cycle number for the HCs. The discharge capacitance of HC with AB and graphite negative electrode rapidly decreased. However, the HC with KB electrode showed long life cycle durability over 1500 cycles. This might be due to the property of KB, which has shell like carbon layers and it prevented collapse of the negative electrode materials.

References

1) K. Naoi et. al. Energy Environ. Sci., 5, 9363 (2012)

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