Kinetic Analysis of Protected Li0.5C6 Anode for 4 V-Class Aqueous Hybrid Capacitor Using Electrochemical Impedance Spectroscopy
Authors: Sato Yusuke , Makino Sho , Hideshima Sho , Mochizuki Dai , Sugimoto Wataru 
Published: February 2020

Electrochemical capacitors have higher power density and longer cyclability than commercial rechargeable batteries such as lithium-ion batteries (LIBs), however, the energy density is comparatively lower. In order to overcome this disadvantage, we have developed a 4 V-class aqueous hybrid capacitor, which consists of a capacitive or pseudo-capacitive cathode (e.g. activated carbon, RuO2 or MnO2) immersed in aqueous electrolyte combined with a water-stable protected pre-lithiated graphite (Li
x
C6) anode. The Li
x
C6 anode is protected from the aqueous electrolyte (catholyte) with a water-stable lithium-conducting glass ceramic membrane (LTAP). A polymer or gel electrolyte doped with lithium salt and ionic liquid is used as a buffer layer to prevent contact of the anode with LTAP.

A half-lithiated graphite anode (protected Li0.5C6 anode) was prepared by controlled galvanostatic lithiation. One of the complications of the protected Li0.5C6 anode is the high resistance of the ceramic membrane and the multiple interfaces. In an attempt to decrease the resistance of the protected Li0.5C6 anode, we deconvoluted the various resistance components of the protected Li0.5C6 anode by electrochemical impedance spectroscopy with two types of LTAP purchased from Ohara Glass (LICGC AG-01 and LICGC SP-01) with varying thickness. The protected anode was prepared using pre-lithiation in lithium-ion containing aqueous electrolyte (aqueous lithiation). The aqueous lithiation for the protected anode is more economical and environmental-friendly than lithiation using a separate pre-doping cell.

Two semi-circles and a Warburg impedance are observed in the impedance spectra of the protected Li0.5C6 anode (counter electrode: Pt) in 1.0 M Li2SO4 aqueous electrolyte, which can be divided into four resistances; bulk resistance (R
bulk), the grain boundary resistance of LTAP (R
grain), the interfacial resistance (R
inteface), and charge transfer resistance (R
c-t) from the high frequency region. The sum of R
inteface and R
c-t of the anode prepared with LICGC SP-01 was one-third of that with AG-01, reflecting the three times higher ionic conductivity of SP-01. By using thinner LTAP, both R
grain and (R
inteface + R
c-t) decreased. The apparent activation energy (E
a) which gives an indication of the activation barrier of lithium-ion transfer of the protected Li0.5C6 anode, was calculated based on measurement conducted at different temperatures (10, 20, 25, 30, 40, 50, 60, and 70 ℃) of protected Li0.5C6 anode prepared with SP-01 (t= 150 and 45 μm). The apparent activation energy of the deconvoluted resistances was derived from electrochemical impedance spectroscopy data (Figure 1 (a)). The apparent activation energy of the anode prepared using thicker SP-01 (t= 150 μm) calculated from R
bulk was E
a-bulk= 10.3 ± 2.2 kJ mol-1, which was four to five times smaller than the E
a-grain (43.1 ± 10.0 kJ mol-1) and the E
a-interface+(c-t) (49.5 ± 1.0 kJ mol-1) (Figure 1 (b)), while that prepared using thinner SP-01 (t= 45 μm) calculated 13.6 ± 2.2, 29.4 ± 4.3, and 41.5 ± 2.5 kJ mol-1, respectively. The results suggest that the activation barrier of lithium-ion transfer is attributed mainly to grain boundary and interfacial resistance regardless of the thickness of LTAP. The lower E
a-grain for the thinner SP-01 (t= 45 μm) originates from less grain boundaries.

The change in the individual resistance was evaluated after prolonger cycles with ‘re-lithiation’ of the protected Li0.5C6 anode using aqueous lithiation method. The performance of the protected Li0.5C6 anode deteriorates after a few thousand cycles, which is accompanied by a positive shift of the anode potential. After re-lithiation the anode potential is regained and (R
inteface + R
c-t) drastically decreased.

Acknowledgment: This work was supported in part by the Advanced Low Carbon Technology Research and Development Program of the Japan Science and Technology Agency (JST ALCA, JPMJAL1008) and Chubu Electric Power Co., Inc.

Figure 1