(Invited) Kinetic Analysis of Water-Stable Protected Graphite Electrode for High Rate Aqueous Lithium Ion Capacitors
Authors: Sugimoto Wataru , Sato Yusuke , Saito Ryota , Takimoto Daisuke , Hideshima Sho 
Published: December 2020

The so-called water-stable protected Li anode is a composite negative electrode that is compatible with aqueous electrochemistry and has been successfully used for aqueous Li-O2 batteries.1,2 The multi-layered composite electrode is composed of a water stable lithium-ion conducting glass ceramic (e.g. LTAP, Li1+x+y
Ti2-x
Al
x
P3-y
Si
y
O12), a Li electrode and a polymer electrolyte buffer layer to insulate the solid electrolyte from the Li electrode. We have applied the protected Li anode to aqueous hybrid supercapacitors, extending the cell voltage to 4 V and allowing the use of pseudocapacitive oxide electrodes such as RuO2 and MnO2.3,4 Later, protected Li anodes were expanded to pre-doped carbon (protected Li
x
C6 electrode), paving the way to the first aqueous lithium-ion capacitor.5

Despite the many advantageous of the aqueous lithium ion capacitor with pseudocapacitve positive electrodes, the protected anode suffers from large resistance due to its multi-component structure, leading to insufficient power density and energy efficiency. Improved protected negative electrodes were prepared by studying the effect of different buffer electrolytes, salts and ionic liquid additives, negative electrodes, binders, and solid electrolytes. The best performing protected anode so far is one incorporating a 50 μm thick LTAP with an alginate gel buffer layer and a pre-doped graphite electrode (Li
x
C6 | Alg-LiFSI/P13FSI | LTAP50). Overall, a 95% reduction in total resistance compared to our initial cell (Li | PEO-LiTFSI | LTAP150) was achieved.6

Protected Li
x
C6 negative electrodes with different LTAP thickness at various pre-lithiation ratio (x=0.1, 0.5, and 0.9) were characterized by electrochemical impedance spectroscopy (EIS) between 10 to 70°C. The total resistance and apparent activation energies E
app calculated from EIS were more or less independent on pre-lithiation ratio. At low temperature (10°C), R
grain was the main contributor to the total resistance, while at high temperature (70°C), R
bulk became the major source of resistance. The E
app, bulk was ~10 kJ/mol, which is 1/3 to 1/4 of E
app, grain and E
app, interface+ct, which have an E
app~40 kJ/mol. The results from the EIS analysis implies that it is important to reduce R
bulk to improve the high temperature performance, while for low temperature performance, R
grain needs to be reduced. R
bulk is the bulk resistance from the electrolytes (1.0 M Li2SO4 and Alg-LiFSI/P13FSI) and the intrinsic resistance of LTAP. R
grain mainly comes from the grain boundary resistance of LTAP. Overall, the general message is that the solid electrolyte is at present the determining factor of the aqueous lithium-ion capacitor and development of a thin water stable solid electrolyte is important for improving the power density and energy efficiency of the 4 V- aqueous hybrid capacitor.

This work was partially supported by an Advanced Low Carbon Technology Research Development Program (JST-ALCA, JPMJAL1008).

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