Introduction
Lithium-ion capacitor (LIC) is a novel asymmetric capacitor possessing the combined electric double layer capacitors (EDLC) as well as lithium-ion batteries (LIB) and having characteristics of high power and high durability. Since activated carbon as a cathode material does not contain lithium-ion in it, lithium pre-doping process is required to manufacture LIC. Some LICs are mass-produced by several companies, but their lithium pre-doping methods are mainly the one using lithium metal. Lithium metal is not only potentially flammable, but also expensive. Furthermore, this method needs perforated current collector and is time consuming due to the diffusion of lithium-ion.

Therefore, we aimed to develop a simple and cost-effective new lithium pre-doping method for mass production of LIC. For this purpose, we focused on lithium carbonate (Li2CO3) as an industrially available and inexpensive lithium source. Li2CO3 also has a high theoretical capacity of 725 mAh/g and is stable even in an atmospheric environment. The concept of this new lithium pre-doping is mixing carbonate salt as a sacrificial salt in a cathode and apply high voltage to decompose it. Only the lithium-ion remain in the cell and the rest is expelled outside of the cell as gas.

Results and Discussion
First, we made two types of cathodes, one with and one without Li2CO3 to confirm the concept. A three-electrode cell was assembled using the cathode, a hard carbon (HC) anode, a lithium metal as a reference electrode, a polyethylene microporous membrane as a separator, and an electrolyte (1.0M LiPF6 in PC). CCCV charging was conducted to the three-electrode cell, and decomposition of Li2CO3 in the cathode and intercalation of lithium-ion into HC were observed.

In order to elucidate the decomposition mechanism of Li2CO3, we analyzed the gas generated by GC/MS and the compounds formed on the surface of cathode by 1H-NMR and IC/MS after first charging. From these results, the decomposition mechanism of Li2CO3, in other words, the lithium pre-doping mechanism using Li2CO3 was clarified.

We applied this lithium pre-doping method to LIC full cell and achieved high power and high durability.