Supporting Information

Activate I0/I+ redox in an aqueous I2-Zn battery to achieve high voltage plateau

Xinliang Li1, Mian Li2*, Zhaodong Huang1, Guojin Liang1, Ze Chen1, Qi Yang1, Qing

Huang2, Chunyi Zhi1,3,4*

1Department of Materials Science and Engineering, City University of Hong Kong, 83

Tat Chee Avenue, Kowloon, Hong Kong, 999077, China

*E-mail: cy.zhi@cityu.edu.hk2Engineering Laboratory of Advanced Energy Materials, Ningbo Institute of Materials

Technology& Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang, 315201,

China

*E-mail: limian@nimte.ac.cn3Centre for Functional Photonics, City University of Hong Kong, Kowloon, Hong

Kong, 999077, China

4Center for Advanced Nuclear Safety and Sustainable Development, City University of

Hong Kong, Kowloon, Hong Kong, 999077, China

Electronic Supplementary Material (ESI) for Energy & Environmental Science.This journal is © The Royal Society of Chemistry 2020

mailto:cy.zhi@cityu.edu.hk

Experimental section

Halogenated Ti3C2I2 MXene synthesis. Ti3C2I2 MXene powder was synthesized by a

novel molten etching approach. The precursor Ti3AlC2 MAX ceramic powders were

purchased from Jilin 11 technology Co., Ltd (400 mesh), which was synthesized using

TiC, Ti, and Al precursors by a sintering process at around 1300 oC in an inert

atmosphere. First, Ti3AlC2 MAX ceramic powders were mixed with CuI2 etchant

according to a molar ratio of 1:6 and suffered the ball-milling treatment for 2h under

the N2 protection. After that, the mixture underwent a heat-treatment at 700oC for 7h in

an alumina crucible under the Ar protection. The whole etching reaction can be

formulated as:

3 2 3 2 2 3Ti AlC +5CuI Ti C I +5Cu + AlI

After that, NH4Cl/NH3·H2O solution was employed to remove the by-products,

including CuI2, CuI salts, obeying the following formula:

22 4 3 3 4 2

1 2 2 ( )2Cu + O NH NH Cu NH H O

The NH4Cl/NH3·H2O solution was obtained by mixing a 2M NH4Cl solution and a 2M

NH3·H2O solution with the volume ratio of 1:1. After being dried in a vacuum oven at

40oC for 48h, the Ti3C2I2 MXene powder was obtained.

Ti3C2I2 MXene cathode and electrolytes preparation. Typically, Ti3C2I2 MXene

cathode was prepared by homogeneously mixing Ti3C2I2 MXene powder with Super-P

powder and polyvinylidene fluoride (PVDF; Aladdin) binder in N-Methylpyrrolidone

(NMP; AR grade, Aladdin) solvent at a weight ratio of 7:2:1. Then, the slurry was

poured onto the flexible carbon cloth substance, followed by drying at 60 ºC for 24 h

under vacuum condition. The loading mass of each electrode was estimated to 1-1.5

mg. The capacity was calculated based on the Ti3C2I2 MXene mass or pure internal

iodine mass, which were named separately in the manuscript. The aqueous electrolyte

was prepared by dissolving 2 mol kg-1 ZnCl2 (Aladdin) and 1 mol kg-1 KCl (Aladdin)

or 2 mol kg-1 ZnSO4 in water, and vigorously stirring for 12 h at room temperature.

Other reference electrolytes including 2M ZnSO4 + 1M KCl, 2M ZnSO4 + 1M NaCl,

2M ZnSO4 + 1M NH4Cl, 2M ZnSO4 + 1M KF, 2M ZnSO4 + 1M NaF, and 2M ZnSO4

+ 1M NH4F were prepared by the same process.

Materials Characterization. In-situ Raman spectra were recorded with a WITec

alpha300 access with a laser of 532 nm wavelength. For this measurement, a

Ti3C2I2//Zn full cell was assembled in the quartz tube and connected to a galvanostatic

battery test system. X-ray diffractometer equipment (XRD; Bruker, D2 Avance) was

performed to identify the phase composition. Scanning electron microscopy (SEM; S-

4700, Hitachi) was employed to characterize the morphology and microstructure.

Transmission electron microscopy was conducted on a JEOL-2001F field emission. X-

ray photoelectron spectroscopy (XPS; ESCALAB 250) was used to reveal the surface

compositions evolution. UV-vis absorption spectra were collected by measuring the

reacted electrolyte, which was obtained by dipping the electrode films and separators

at different voltage states into the corresponding electrolytes, using a Cary 50 UV-vis

spectrophotometer (Varian) while employing the Zn(I3)2 electrolyte as the reference.

Electrochemical measurements. The electrochemical performance of the Ti3C2I2//Zn

battery was explored by assembling the CR2032 coin-type full cell in the open-air

environment, employing Ti3C2I2 as the cathode, Zn foil as the anode, 2M ZnCl2 + 1M

KCl or 2M ZnSO4 solution as the electrolyte. The cyclic voltammetry (CV) profiles

were collected using the electrochemical workstation (CHI 760D). The full cells were

cycled galvanostatically on a battery measurement device (LAND CT2001A) at room

temperature. For in-situ Raman measurement, a homemade test device with a

transparent window was employed as the cell case (Figure S12), and other conditions

remained the same.

Figure S1. SEM image of the Ti3AlC2 MAX ceramic.

Figure S2. EDX spectrum of the as-prepared Ti3C2I2 MXene.

Figure S3. SEM image of the as-prepared Ti3C2I2 MXene cathode.

Figure S4. (a) GCD curve of Ti3C2I2//ZnSO4 + KF//Zn battery at 0.5 A g-1 (b) GCD

curve of Ti3C2I2//ZnSO4 + KCl//Zn battery at 0.5 A g-1.

Figure S5. CV curve of the as-prepared Ti3C2OF//ZnSO4 + KF//Zn battery.

Figure S6. The calculated b values of the four redox peaks in CV curves in Figure 2c.

Figure S7. The prolonged cycling performance of the symmetric Zn//ZnCl2 + KCl//Zn

battery cycled at 1 mA cm-2 with a cut-off capacity of 0.5 mAh cm-2. Inset diagram

indicates the selected stripping/plating curves.

The redox of the Zn anode in such electrolyte are explored. Figure S7 shows the long-

term cycling performance of the symmetric Zn//ZnCl2 + KCl//Zn battery at the current

density of 1 mA cm-2 with a cut-off capacity of 0.5 mAh cm-2. An obvious self-

optimizing behavior is identified, that is, the polarization potential decreases as the

stripping/plating progresses, which represents the gradual decrease in nucleation energy

that promotes the uniform deposition of Zn metal.1, 2 In detail, the overpotential

decreases from the initial 18 to 15 mV after 24 cycles and remains stable in the

subsequent process.

Figure S8. Voltage profiles of Zn galvanostatic stripping/plating of the asymmetric

Cu//ZnCl2 + KCl//Zn battery cycled at 1 mA cm-2. Inset diagram indicates the

corresponding curve of CE values.

As shown in Figure S8, GCD curves of asymmetric Cu//ZnCl2 + KCl//Zn battery vs.

Cu cathode performed at 0.5 mA cm-2 with a cut-off potential of 0.6 V uncloses that the

low redox overpotential is limited to 44.7 mV. Besides, the corresponding Coulombic

efficiency (CE) curves also confirm this, in which after only the first two cycles of

activation, the CE value quickly reaches 95% without any deterioration in the following

cycles.

Figure S9. CV profile of Zn stripping/plating behavior of the asymmetric Cu//ZnCl2

+ KCl//Zn battery at 1 mV s-1.

CV measurement of Cu//ZnCl2 + KCl//Zn battery vs. Cu cathode is also conducted to

offer deep insight into the Zn plating/stripping chemistry. As exhibited in Figure S9,

at 1 mV s-1, the onset potentials of the Zn plating and stripping are located at -0.03 V

and -0.01 V, respectively. Their negligible potential difference and integrated current

responses indicate the rapid reaction kinetics and high reversibility.3 As noted above,

the stable and reversible Zn anode chemistry in the ZnCl2 + KCl electrolyte is clarified.

Figure S10. The long-term cyclability of Ti3C2I2//ZnSO4//Zn battery at 0.5 A g-1.

Figure S11. (a) GCD curve of Ti3C2I2//ZnCl2 + KCl//Zn battery at 0.5 A g-1 with

marked regions indicating the energy and capacity contributions from the plateaus of

I0/I+ and I-/I0 redox couples, respectively. (b) The calculated energy density and

capacity contributions from the two plateaus.

Figure S12. The homemade test device for in-situ Raman measurement.

Figure S13. Raman spectrum of the pure 2M ZnCl2 + 1M KCl electrolyte.

Figure S14. In-situ Raman spectra of the Ti3C2I2 cathodes at different states with the voltage interval of 0.05 V.

Figure S15. UV-vis spectra of the ZnCl2 + KCl electrolyte at different voltage states

within 1.4 V.

References

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