The effect of interface heterogeneity on zinc metal anode cyclability

Zinc metal batteries (ZMBs) are promising candidates for low-cost, intrinsically safe, and environmentally friendly energy storage systems. However, the anode is plagued with problems such as the parasitic hydrogen evolution reaction, surface passivation, corrosion, and a rough metal electrode morphology that is prone to short circuits. One strategy to overcome these issues is understanding surface processes to facilitate more homogeneous electrodeposition of zinc by guiding the alignment of electrodeposited zinc. Using Scanning Electrochemical Microscopy (SECM), the charge transport rate on zinc metal anodes was mapped, demonstrating that manipulating electrolyte concentration can influence zinc electrodeposition and solid electrolyte interphase (SEI) formation in ZMBs. Using XPS and Raman spectroscopy, it is demonstrated that an SEI is formed on zinc electrodes at neutral pH, composed primarily of a Zn4(OH)6SO4·xH2O species, its formation being attributed to local pH increases at the interface. This work shows that more extended high-rate cycling can be achieved using a 1 M ZnSO4 electrolyte and that these systems have a reduced tendency for soft shorts. The improved cyclability in 1 M ZnSO4 was attributed to a more homogeneous and conductive interface formed, rather than the bulk electrolyte properties. This experimental methodology for studying metal battery electrodes is transferable to lithium metal and anode-free batteries, and other sustainable battery chemistries such as sodium, magnesium, and calcium.


Scanning Electrochemical Microscopy (SECM)
Ferrocenemethanol was chosen as a redox mediator for this system because of its stability in aqueous solvents. 1Figure S1 shows a cyclic voltammogram (CV) of 1 mM FcMeOH in 0.01 M ZnSO4.At an ultramicroelectrode (UME), a recorded CV has a sigmoid-type shape, rather than the classic 'duck shape' recorded at larger electrodes.At large disk electrodes, mass transport towards the surface mostly occurs perpendicular to the surface, which is known as planar diffusion.Thus, as the voltage is increased or decreased, a diffusion limitation is reached, causing the current to decrease, and the usual 'duck shape' CV to be recorded.Mass transport to UMEs occurs by hemispherical diffusion, meaning that diffusion of redox species to the surface comes from all directions.Consequently, no mass transport limitation is seen unless extremely high sweep rates are used.Thus, a sigmoidal steady-state voltammogram is recorded, where the plateaus seen are the kinetic limitation of electron transfer. 2For the FcMeoH/FcMeOH + redox couple, a probe voltage of +0.5 V vs Ag/AgCl was used to drive the oxidation of the mediator.
The voltage traces of zinc plating and stripping prior to the SECM measurements are depicted in Figures S2-S21.In a symmetric coin cell, the voltage trace reflects the sum of the response of both electrodes.In the SECM, when the WE is plated, the CE is stripped, and vice versa; however, it is expected that the electrolysis processes will be more dominant on the counter electrode since the counter electrode (CE -Pt) is prone to hydrogen evolution.Hence, the voltage trace mainly reflects the stripping process on the WE.The plating voltage traces (a) typically show a decrease in the potential and then a plateau at approximately -0.9V vs. Ag/AgCl.The initial drop is due to the nucleation overpotential that adds to the reduction potential and the overpotential contributed by the bulk resistance of the electrolyte.Since the time interval between the collected points is longer than the peak time, the nucleation peaks were not recorded.(b) As expected, no nucleation peaks were recorded during stripping since we estimate that zinc plating on the Pt-CE was minimal.The slight overpotential decrease during the stripping is attributable to the reduction of the porosity of the electrode.The overpotential for the processes in 1 M ZnSO4 is slightly higher than in 2 M; this could be due to the higher conductivity of the 2 M electrolyte and the lower zinc reduction potential in the more concentrated electrolyte.Probe approach curve measurements were repeated, including measurements for plating 2-5 to investigate how the kinetics of electron transfer change in the initial cycling of the cell.Positive feedback was observed for both bare zinc and plating 1, and the feedback response transitioned to negative feedback for plating 2-5.This indicates that the interface becomes more insulating following the first plating.

Galvanostatic cycling and impedance spectroscopy measurements.
Figure S24: Left: A Voltage trace (E vs. t) of successive galvanostatic cycles at 0.5 mA h cm −2 of zinc in a symmetric cell with 2 M ZnSO4 aqueous electrolyte and a GF separator.This is equivalent to constant current density 0.5mA cm -2 at 1 C (plating and stripping 0.5mAh cm -2 , which is equivalent to 1.2 mg cm -2 zinc).Right: impedance modulus at 0.1Hz (black), 1kHz (blue).High-rate cycling (11 mA cm -2 ) GF cells -repeat data                    Figure S54 depicts the cycling of Mn2O3 cathode vs. zinc metal.Since Mn2O3 requires activation before delivering a stable capacity, the Mn2O3 electrodes are initially cycled in the voltage range of 0.8-1.8V for 15 cycles at 25 mA g -1 , and then the cell is cycled at a larger voltage range.Hence, the performance of the cell should be compared for the following cycles.The cell with the 2 M electrolyte delivers a slightly higher capacity for initial cycles than the cell with the 1 M electrolyte.However, the capacity drops after 20 cycles, and 1 M ZnSO4 electrolyte shows better capacity retention at the end of 80 cycles.The voltage profiles are similar in both the electrolytes except for the first cycle after activation.

Figure S2 :
Figure S2: Repeat data for probe approach curves for plating one through five.

Figure S3 :
Figure S3: Repeat probe approach curve measurement for 2 M ZnSO4 plating ten.

Figure S35 :
Figure S35: (a) 2 M ZnSO4 on a Celgard 3501 separator.(b) 2 M ZnSO4 on a GF separator.(c) 1 M ZnSO4 on a Celgard 3501 separator.(d) 1 M ZnSO4 on a GF separator.An experimental test whereby a drop of electrolyte was added to both Celgard 3501 (polymer) and GF separators was performed.Both electrolytes formed a drop on top of the polymer

Figure S38 :
Figure S38: (a) An overpotential trace (overpotential vs t) of successive galvanostatic cycling of a total of 1.25 mAh cm-2 of Zn at 10 mA cm-2 in zinc symmetric cell with 1 M ZnSO4 aqueous electrolyte, GF separator, at 10 °C (left Y axe) and the total impedance at 10 Hz, measured during OCV (right Y axe).The galvanostatic impedance spectra (GEIS) were recorded every 1.25 hours, between 500 -10 Hz with 100 µA amplitude.The singlespectrum measurement time was nine seconds.A drop in the overpotential (a spike in the overpotential trace) was observed when the GEIS measurements were taken at stages II and IV (b-f).

Figure S39 :Figure S40 :
Figure S39: (a) An overpotential trace (overpotential vs t) of successive galvanostatic cycling of a total of 1.25 mAh cm-2 of Zn at 10 mA cm-2 in zinc symmetric cell with 2 M ZnSO4 aqueous electrolyte, GF separator, at 10 °C (left Y axe) and the total impedance at 10 Hz, measured during OCV (right Y axe).The galvanostatic impedance spectra (GEIS) were recorded every 1.25 hours, between 500 -10 Hz with 100 µA amplitude.The singlespectrum measurement time was nine seconds.A drop in the overpotential (a spike in the overpotential trace) was observed when the GEIS measurements were taken at stages II and IV (b-f).

Figure S47 :Figure S48 :
Figure S47: SEM Image of a zinc electrode soaked in 2 M ZnSO4 for one hour with a view field of 100 µm view field.SEM image was taken with an excitation voltage of 5 kV in resolution mode.

Figure S50 :
Figure S50: SEM image after tenth plating step in 2 M ZnSO4 (100 µm view field).SEM image was taken with an excitation voltage of 5 kV in resolution mode.

Figure S51 :
Figure S51: SEM image after first plating step in 1 M ZnSO4 (100 µm view field).SEM image was taken with an excitation voltage of 5 kV in resolution mode.

Figure S52 :
Figure S52: SEM image after fifth plating step in 1 M ZnSO4 (100 µm view field).SEM image was taken with an excitation voltage of 5 kV in resolution mode.

Figure S53 :
Figure S53: SEM image after tenth plating step in 1 M ZnSO4 (100 µm view field).SEM image was taken with an excitation voltage of 5 kV in resolution mode.

Table S1 :Atomic content ratios in the XPS analysed samples.
Figure S41: Reference Raman Spectrum for ZnO.