Towards High-Energy and Anti-Self-Discharge Zn-Ion Hybrid Supercapacitors with New Understanding of the Electrochemistry

Highlights A surface engineering strategy was proposed to design hierarchically porous structure on fibrous carbon cathodes with O/N heteroatom functional groups. High-energy and anti-self-discharge Zn-ion hybrid supercapacitors (ZHSs) were realized. ZHS electrochemistry was investigated and new insights were provided. Supplementary Information The online version contains supplementary material available at 10.1007/s40820-021-00625-3.


S1 Experimental Section
Specific capacity (C, mAh g -1 ), energy density (E, Wh kg -1 ) and power density (P, W kg -1 ) were determined by GCD tests, and their calculation formulas are as follows. (S2) t E 3600 P  = (S3) in which I is current density (unit: A g -1 ), t is charge or discharge time (unit: s) and U (unit: V) is voltage during a charge or discharge process. Note that, since zinc foils serve as active materials and current collectors at the same time in ZHSs, their mass is excessive. Therefore, capacity, energy density and power density of the ZHSs were calculated based on the weight of cathode materials. Fig. S1 (a) SEM image and (b) TEM image of CF; (c) (d) SEM images of MPCF. CF has a relatively smooth surface without porous carbon layer like that in HPCF (Fig. 1e). Crosssectional SEM image of MPCF in (d) clearly shows that there are many deep grooves on the surface of the MPCF sample [S1].    Capacity of CF cathode is only ~0.6 mAh g -1 , which means that it almost does not possess electrochemical activity in ZHSs.

Fig. S7
CV curves at 2 mV s -1 of HPCF carbon electrodes and zinc electrodes in threeelectrode systems, in which HPCF or metallic zinc serves as working electrode, stainless steel foil serves as counter electrode and Ag/AgCl electrode serves as reference electrode.

Fig. S9
Fitting data of EIS spectra of MPCF and HPCF cathodes in Fig. 3g. Inset is corresponding equivalent circuit model. Rs contains electrolyte resistance and contact resistance between electrodes and current collectors, and Rct is charge-transfer resistance at electrode/electrolyte interface. Rs of MPCF and HPCF cathodes is very small (1.6 and 3.3 Ω), resulting from good electrical conductivity of these fibrous carbon cathodes. Rct of MPCF and HPCF cathodes is 295.7 and 151.8 Ω, respectively. The much smaller Rct value of HPCF cathode demonstrates its fast kinetics during electrochemical reactions.

Fig. S10
Cycling behavior at 20 A/g of HPCF cathode-based ZHS with 2 M ZnSO4 aqueous electrolyte. Capacity retention is 93% over 6000 charge/discharge cycles. Insets are the first and last 5 cycles of GCD curves. (e) discharge to 0.2 V, then charge to 1.8 V, and finally discharge to 1.0 V; (f) discharge to 0.2 V, then charge to 1.8 V, and finally discharge to 0.2 V

Fig. S16
Illustration of self-discharge behavior test. An electrochemical device such as ZHS, Zn-ion battery or supercapacitor was charged/discharged through GCD technique at 0.1 A g -1 for 5 cycles and then maintained at an expected voltage using constant voltage charging technique for 30 min. This is to make internal environment of the electrochemical device reaches a relatively stable state.

Fig. S17
GCD curves at 0.1 A g -1 of (a) AC//AC symmetric supercapacitor, (b) AC//Zn ZHS, and (c) V10O24· 12H2O//Zn Zn-ion battery. 2 M ZnSO4 aqueous electrolyte is used in the three electrochemical energy storage systems. AC materials with a high specific surface area (e.g., 1923 m 2 g -1 for the used AC material herein) are representative electrode materials for symmetric supercapacitors and ZHSs, [S2] and vanadium oxides such as V10O24· 12H2O are high-performance cathode materials for Zn-ion batteries [S3]. Therefore, AC//AC symmetric supercapacitor, AC//Zn ZHS and V10O24· 12H2O//Zn Zn-ion battery are chosen in this work and their self-discharge behaviors are compared with that of our fibrous carbon cathode-based ZHSs. Specific capacity of the AC//AC symmetric supercapacitor, AC//Zn ZHS and V10O24· 12H2O//Zn Zn-ion battery is 6, 121, and 300 mAh g -1 , respectively. More information about the synthesis, physicochemical characteristics and electrochemical performance of the used AC and V10O24· 12H2O materials herein can be found in our previous work [S2, S3].

Fig. S18
Illustration of electric field and its effect on anions insides ZHSs. Movement of anions on cathode surface is restricted by coulombic force.

Fig. S19
Self-discharge behaviors of HPCF cathode-based ZHS with different initial voltages of 1.4, 1.6 and 1.8 V -- [S11] MXene-rGO ZnSO4 (aq) 0.2-1.6 50 (0.4 A g -1 ) 34.9 0.28 [S12] a Previously reported cathodes are composed of active materials, conductive additives, binder and current collectors, but their capacity, energy density and power density are calculated only based on active materials. Our fibrous carbon cathodes are free-standing electrodes. Therefore, if the whole mass of above cathodes were used to calculate capacity, energy