**2. Experimental method**

As shown in **Table 1**, voltage hold tests were conducted by applying overvoltages of 2.8, 2.9, 3.0, 3.2, and 3.5 V continuously for 1 week to commercially used cylindrical EDLCs with a rated voltage of 2.5 V. Using a Sanyo Electric Co., Ltd., incubator (MIR-254) and a charging/discharging tester (PS-97010) by PowerSystem Co., Ltd., three accelerated deterioration samples were produced for each voltage at a constant temperature of 25°C.


**Table 1.**

*Experimental conditions and sample properties.*
