**2. Experimental**

#### *2.1. Preparation of G*

Graphite oxide (GO) was synthesized by the modified Hummers' method using graphite powder [56]. A graphite oxidation procedure was executed before the synthesis of GO [57,58]. 4 g of graphite powder were added into a solution composed of 2 g of potassium persulfate (K2S2O8), 2 g of phosphorus pentoxide (P2O5), and 30 mL of conc. sulfuric acid (H2SO4). The mixture solution was heated to 80 ◦C under continuous stirring for 6 h. When it was cooled down to room temperature, rinse with deionized (DI) water was performed repeatedly by centrifugation until the neutral pH level was achieved. Afterward, 4 g of the pre-oxidized graphite powder were added into 100 mL of conc. H2SO4 solution in an ice bath. Then 12 g of potassium permanganate (KMnO4) were slowly added at 35 ◦C. The stirring was continued for 2 h until the color of the mixture turned to dark brown. Subsequently, a solution containing 200 mL of DI water and 40 mL of hydrogen peroxide (H2O2, 30 vol% in water) was added slowly while a violent chemical reaction occurred. A yellow-brown intermediate was produced when the reaction was completed, which was then put in a dilute aqueous hydrochloric acid (HCl) solution to remove metal ions. After ultrasonication for 1 h, rinse with DI water was repeatedly performed by centrifugation until the neutral pH level was achieved to obtain the GO powder.

20 mg of GO were added in 100 mL of DI water to prepare the GO solution. After ultrasonication for 2 h to make better dispersion of GO, the suspension was transferred to an autoclave, which was placed in a furnace for the hydrothermal process at 200 ◦C for 2 h. When cooling down to room temperature, the product was collected by filtration, and then dried at 80 ◦C for 12 h. After grinding, the powder of G was acquired [59].

#### *2.2. Preparation of NG Composites*

55 mg of G were mixed with 8.6 mL of ammonia hydroxide solution (28 vol%~30 vol%) in 70 mL of DI water. After ultrasonication for 2 h to make better dispersion of G, the suspension was transferred to an autoclave, which was placed in a furnace for the hydrothermal process at 140 ◦C for 6 h. When cooling down to room temperature, the product was repeatedly rinsed by DI water until the neutral pH level was achieved, and then collected by centrifugation. After dried at 80 ◦C for 12 h and grinding, the NG powder was obtained [52].

#### *2.3. Preparation of NG/MnO2 (NGM) Composites*

55 mg of G were mixed with 8.6 mL of ammonia hydroxide solution (28 vol%~30 vol%) in 70 mL of DI water. After ultrasonication for 2 h to make better dispersion of G, the suspension was transferred

to an autoclave, which was placed in a furnace for the hydrothermal process at 140 ◦C for 6 h. When cooling down to the room temperature, five different weights of potassium permanganate (KMnO4) were added, respectively, to prepare the mixtures containing 8.9 mM, 17.8 mM, 26.7 mM, 35.6 mM, and 44.5 mM of KMnO4 solutions. After ultrasonication for 30 min, every mixture was transferred back to the autoclave, which was then placed in the furnace for another hydrothermal process at 160 ◦C for 2 h. When cooling down to room temperature, every mixture was taken out and repeatedly rinsed by DI water until the neutral pH level was achieved, and then collected by centrifugation. After dried at 80 ◦C for 12 h and grinding, five NGM composites with various contents of Mn were obtained [52]. They were named as x-NGM, in which x was 1, 2, 3, 4, and 5, respectively, to represent the five KMnO4 concentrations as mentioned above used during the preparation processes.

#### *2.4. Fabrication of Electrodes*

100 mg of G, NG and x-NGM composites were mixed with 12.5 mg of carbon black in 2 mL of absolute ethanol, respectively. After ultrasonication for 10 min to make better dispersion, 0.5 g of ethyl cellulose and 1 mL of terpineol were added to the three kinds of suspensions. Ultrasonication for another 10 min was performed to obtain more even mixing. The subsequent stirring for 10 min was to evaporate some ethanol, to achieve an appropriate consistency of the G, NG and x-NGM slurries for fabricating the electrodes of ASSCs.

A polyimide (PI) tape with the dimension of 3.5 cm × 2.5 cm was attached and stuck to a graphite paper to obtain a PI/graphite flexible substrate. On the other hand, a square hole with the length of 1.6 cm was made in the center of transparency, which was placed upper the flexible substrate and fixed by the 3M tape. The transparency was closely attached to the substrate, and the effective area was thus defined. Afterward, an appropriate amount of the G, NG and x-NGM slurries was uniformly coated within the square on the flexible substrate by the doctor-blade method. After standing at the room temperature overnight, the transparency was removed to acquire the electrodes, which were then calcined at 200 ◦C for 1 h to eliminate organics. Three different mass loadings were used for coating active materials on the PI/graphite flexible substrates. The resulting electrodes were named as Gy, NGy, and x-NGMy, in which y was 1, 2, and 3, to represent the mass loadings of 1 mg, 2 mg, and 3 mg, respectively.
