In recent years, the development of supercapacitors based on polymer electrolytes has made significant progress in order to obtain improved energy densities and additional features such as stretchability and wearability [
49,
50]. Moreover, polymer electrolytes loaded with highly-concentrated salt solutions are considered to be ideal electrolytes but have been rarely reported [
51]. Polyacrylamide (PAM) has a property of ultra-absorbing as a polymer, owing to the osmotic pressure difference caused by ionic groups inside and outside the gel network [
52]. We find that LiNO
3 has a higher solubility of 6.9 g/10 mL than that of Li
2SO
4·H
2O (3.5 g/10 mL) at 22 ℃ which can be expected to effectively improve both ionic conductivity and electrochemical performance. Herein, we develop highly-concentrated LiNO
3-loaded polymer electrolytes with a wide voltage window in supercapacitor device, of which PAM is selected as the main polymer. For previous studies of Li
2SO
4-PAM polymer electrolytes endowed the activated carbon electrode with a stable 2.2 V performance, we conduct a full-scale tentative experiment for 2.1 V to 2.4 V voltage windows for symmetric supercapacitors [
53]. Apart from the approximately rectangular shapes of CV curves corresponds to distinctive characteristic of EDLC, the 2.3 V based device exhibits an undistorted maximum voltage (
Figs. 6a and
c). By calculating the CV curves, capacitances at 10 mV/s in the descending sequence are 224 F/g (2.3 V) > 211 F/g (2.2 V) > 205 F/g (2.1 V) > 203 F/g (2.4 V) respectively, exceeding the capacitances exhibited by many neutral electrolytes [
54,
55]. At the comparative GCD test, within a 2.1–2.4 V voltage range, the charge–discharge plots are highly symmetrical, testifying a reversible ion adsorption/desorption procedure at the surface of CQA10 carbon. (
Figs. 6b and
d). Besides, the measured ionic conductivity of LiNO
3-PAM electrolyte has also been directly increased to 5.05 mS/cm [
53]. A superb capacitance of 229 F/g achieves under a scan rate of 5 mV/s, together with maintained value of 168 F/g under a high scan rate of 200 mV/s (74.3% capacitance retention), displaying a good rate capability (
Fig. 6c). In the EIS test, due to the high concentration conditions will induce the formation of extensive aggregation, the
Rs and
Rct values are slightly high of 3.6 and 4.2 Ω, respectively (Fig. S11 in Supporting information) [
56]. Benefiting from the greatly extended potential windows, LiNO
3-PAM polymer electrolyte and 1 mol/L TEMABF
4/PC organic electrolyte based cells offer boosted energy densities of 41.1 and 61.3 Wh/kg, corresponding to the volumetric energy densities of 22.8 and 34.1 Wh/L (
Fig. 6e and Fig. S12 in Supporting information) [
21,
57]. Besides, the Ragone plot further reveals considerable aqueous device energy densities of 13.1 Wh/kg and 13.7 Wh/kg at ~246 W/kg under KOH/H
2SO
4 electrolytes. In addition, after 10,000 successive cycles at 10 A/g, the discharge capacity of CQA10 in LiNO
3-PAM exhibits limited loss to 71.1% with 99.0% coulombic efficiency retaining (
Fig. 6f). With operating temperature ranging from 0 ℃ to 80 ℃, the assembled device has stably lighted up a light-emitting diode (LED, 2 V/150 mA) for about 10 min, revealing the broad temperature applicability (
Fig. 6g) [
58,
59]. Overall, the CQA10-loaded devices achieve distinguished specific capacitance values comparing with relevant literatures (Table S3 in Supporting information). Considering the relationship between energy density and the square of voltage window, the development of LiNO
3-PAM electrolyte to improve the working voltage window marks a significant step towards high energy storage equipment [
60,
61].