Hyper-Dendritic Nanoporous Zinc Foam Anodes

The low cost, significant reduction potential and relative safety of the zinc electrode is a common hope for a reductant in secondary batteries, but it is limited mainly to primary implementation due to shape change. In this work, we exploit such shape change for the benefit of static electrodes through the electrodeposition of hyper-dendritic nanoporous zinc foam. Electrodeposition of zinc foam resulted in nanoparticles formed on secondary dendrites in a three-dimensional network with a particle size distribution of 54.1–96.0 nm. The nanoporous zinc foam contributed to highly oriented crystals, high surface area and more rapid kinetics in contrast to conventional zinc in alkaline mediums. The anode material presented had a utilization of ~88% at full depth-of-discharge (DOD) at various rates indicating a superb rate capability. The rechargeability of Zn0/Zn2+ showed significant capacity retention over 100 cycles at a 40% DOD to ensure that the dendritic core structure was imperforated. The dendritic architecture was densified upon charge–discharge cycling and presented superior performance compared with bulk zinc electrodes. A synthetic method turns a problem with plate metal batteries into a path for greater stability and may lead to safer, cheaper batteries. Zinc is more abundant and easier to handle than lithium, but electrodes made from it suffer from shape-change effects that prevent stable cycling. Now, by electrodepositing nanoporous zinc foam onto a traditional current collector, Daniel Steingart from Princeton University in the USA and colleagues have exploited a problem with zinc electrodes — the formation of dendritic crystals that can short-circuit batteries. The team broke convention by deliberately conditioning their zinc electrodes at potentials far from equilibrium. This created a hyper-dendritic network foam that forms with 88% current efficiency and remains stable for over 100 recharge cycles — results superior to those of conventional batteries with non-porous zinc electrodes. The low cost, significant reduction potential and relative safety of the zinc electrode is a common hope for a reductant in secondary batteries, but it is limited mainly to primary implementation due to shape change. In this work, we exploit such shape change for the benefit of static electrodes through the electrodeposition of hyper-dendritic nanoporous zinc foam. Electrodeposition of zinc foam resulted in nanoparticles formed on secondary dendrites in a three-dimensional network with a particle size distribution of 54.1–96.0 nm. The nanoporous zinc foam contributed to highly oriented crystals, high surface area and more rapid kinetics in contrast to conventional zinc in alkaline mediums. The anode material presented had a utilization of ~88% at full depth-of-discharge (DOD) at various rates indicating a superb rate capability. The rechargeability of Zn0/Zn2+ showed significant capacity retention over 100 cycles at a 40% DOD to ensure that the dendritic core structure was imperforated. The dendritic architecture was densified upon charge–discharge cycling and presented superior performance compared with bulk zinc electrodes.

conductive, at the nanoscale. We find that the nanoporous zinc structure presents a modified activation energy for electrochemical cycling. Conductive paths throughout the internal structure result in a more uniform current distribution: reducing non-uniform concentration gradients and anode polarization losses 13 . We exploit dendrites, normally a limitation against rechargeable zinc electrodes, by initially operating at states far from equilibrium (cathodic overpotentials from ~200 to ~600 mV) and thereafter cycling towards equilibrium, presents a rethinking of the process of recharging batteries. The dendritic network upon recharging a battery at standard operating conditions may densify the architecture over cycles and suppress initiation of dendrites if the three-dimensional core is kept intact. Recent work has indicated that formed zinc structures with submicron features can undergo 50 charge-discharges cycles without failure 14 . Here, we describe a simple mechanism for making such structures, with the option to alter the surface kinetics as a function of process regime.
We investigated the electrodeposition of zinc with hydrogen at different overpotentials by characterizing its electrochemical and morphological properties. We studied the kinetics of the produced material via electrochemical techniques such as chronocoulometry and Butler-Volmer modeling. We characterized the morphology of zinc by scanning and transmission electron microscopy methods. We determined the particle size distribution and specific surface area of zinc foam through X-ray diffraction, differential scanning calorimetry and Brunauer-Emmet-Teller (BET) techniques. Finally, we investigated the reversibility of Zn 0 /Zn 2+ redox chemistry and discharge-rate-capability of the material for battery applications. The hyper-dendritic zinc electrode showed significant capacity retention over 100 cycles, and due to the electrochemical synthesis procedure can be reformed in situ if need be.

Materials and Methods
Experiments carried out in alkaline conditions where 8.9 M KOH pellets (Sigma-Aldrich) and 0.61 M ZnO (Sigma-Aldrich) were dissolved in deionized water and stirred overnight. The zinc foam was prepared electrochemically from flooded undivided cells containing 10 ml of electrolyte in a three-electrode setup that was controlled by a potentiostat (Keithley Model 2401 Low Voltage SourceMeter Instrument). The setup consisted of a 0.5 mm diameter platinum wire (Sigma-Aldrich) working electrode, an Hg/HgO reference electrode (Koslow Scientific Company) and a nickel mesh (Dexmet Corporation) that served as the counter electrode.
Electrodeposited zinc on a platinum wire was controlled with a defined charge passed of 100 coulombs (C), using a controlled-potential coulometry method and a surface area of 0.260 cm 2 .
For studies of the rechargeability of the zinc foam, the working electrode was prepared with a charge passed of 3000 C, and a conventional nickel electrode with excess capacity served as a counter electrode. The platinum electrode was initially activated and cleaned by dipping it in nitric acid (Sigma-Aldrich) for 1 hour at 60 °C, and then rinsed with DI water. The nickel electrode was prepared by mixing into a slurry consisting of 50 wt.% NiOOH (Sigma-Aldrich), 40 wt.% graphite (Timcal Timrex KS6) and 10 wt.% PTFE (Sigma-Aldrich), and rolled on to a flat surface using a rolling pin. The rolled electrode was then dried in a vacuum oven at 125°C for 2 hours. The assembly was then compressed in a uniaxial press using approximately 4 tons of force on a nickel mesh current collector. The full cell was sandwiched between two layers of non-woven cellulose membrane (Freudenberg), and topped up with either 10 ml electrolyte for flooded cells or 1 ml electrolyte for non-flooded cells. The uncompensated resistance, Ru, was evaluated for each setup in this work through a current interrupt method. The Ru values obtained were under 4 Ω for all methods and hence neglected, due to the absence of any significant effect on the measured potential. For in situ studies, the zinc was kept in solution. For ex situ studies, the sample was rinsed three times in 8.9 M KOH to ensure the dissolution of ZnO, neutralized with DI water five times, and then dried under vacuum for 1 hour at 110 °C. The samples were then prepared by grinding with mortar, pestle and small amounts of acetone until a fine powder was obtained. Purum bulk zinc powder (Sigma-Aldrich) was used as a standard comparison. The Tafel studies were carried out using a 0.5 mm thick zinc foil with a surface area of 0.260 cm 2 (Sigma-Aldrich) cleaned in 10 wt.% sulfuric acid (Sigma-Aldrich) and rinsed with DI water, as a standard.

Results and Discussion
Zinc metal is highly soluble in strong alkaline solutions and equilibrates with zincate ions, Zn(OH)4 2-, at pH values >12. The charge and discharge reaction of zinc electrodes, the precipitation of ZnO at supersaturated concentration of zincate ions, and hydrogen evolution are seen in (1), (2) and (3), respectively 15 : Zinc has an equilibrium potential that is about 0.4 V lower than the equilibrium potential of hydrogen evolution at pH 14 thus having a direct influence on the efficiency of electrodeposited zinc.
Electrodeposited zinc foam on platinum at 100 C charge passed and different potentials is included with the supplementary information. The metallic foam structure was achieved at potentials applied above -1.6 V and a substantial amount of hydrogen evolved with zinc foam was observed at -2.0 V. The figure in the supplementary section shows that the measured current increased more rapidly at higher applied potentials, despite being well beyond the limiting current for zinc in this electrolyte. The increase in current over time was most likely due to an increased surface: while reaction (3) is competitive with reaction (1), we found in all described experiments coulombic efficiencies of 84.9 ± 3.1%, indicating a similar fraction of current formed H2(g) at each test.
Electrodeposited zinc has been reported as mossy, epitaxial layers, boulder-like and dendritic morphologies at low to high overpotentials 7,9,[17][18][19] . The morphology of electrodeposited zinc is dependent on the intermediate product zincate. Precipitation of zincate at the surface interface generally initiates dendritic morphologies at limiting current densities due to non-uniform concentration gradients. Effort on controlling mass transport by convection was carried out by Ito et al. 7 where they quantified the change in morphology of deposited zinc. In this work, we mainly operated at limiting current densities with no convection control at high overpotentials.
Hexagonal crystals organized into fernlike dendrites with particles sized between 2 to 10 µm were achieved seen in Figure 1A to 1D. At extremely high cathodic overpotentials (>~500 mV, assuming an equilibrium potential of zinc at -1.38 V), oriented zinc pillars were formed at nanoscale, seen in Figure 1H and 1J, with a mean particle size of 73.05 ± 8.95 nm measured from 49 randomly selected zinc particles. At higher overpotentials, transport rate of zincate ions, Zn(OH)4 2will be limited at the surface-liquid interface, causing unpredictable growth of primary and secondary dendrites. The elemental composition of the samples in Figure 1 were analyzed with EDS. A small amount of the oxide was formed: 3.2, 3.8, 5.9, 6.5 and 8.2 wt.%, in increasingly negative potentials, respectively. The amount of potassium was below 0.6 wt.% for all measurements thus negligible.
This support our expectation that the majority of the deposited foam was metallic zinc.
The hyper-dendritic nanoporous zinc foam prepared at -2.0 V was further investigated under a TEM. The TEM image taken of hyper-dendritic nanoporous zinc foam is seen in Figure 2A.   (4) where I0 is the exchange current, η the activation overpotential, αc the cathodic charge transfer coefficient. Figure 4 shows the measured Tafel plots and Butler-Volmer fits at different potentials. Calculated kinetic parameters are given for the different potentials in Table 1. Osório et al. 31 presented the results of corrosion rates as a function of secondary arm spacing for zinc alloys. They concluded that a shorter secondary dendrite arm spacing resulted in an increased amount of hydrogen evolved at equilibrium states for zinc alloys. A more rapid corrosion rate with shorter dendrite arm spacing achieved at increasingly negative potentials was evident for the anode material in this work as well.   the pressure range of 0.05 < P/P0 < 0.3. In contrast to bulk zinc, which has a specific surface area of 0.5 -1 m 2 /g, the specific surface area of zinc foam was 10 to 25 times larger. Assuming the dendrites are composed of solid, spherical particles with smooth surface in narrow size distribution and a theoretical density of 7.14 g/cm 3 , a mean particle size diameter of 69.10 nm was calculated from (5) where ρ is the theoretical density in g/cm 3 and SBET is the specific surface area in m 2 /g. This is in agreement with what was observed in the TEM.
DSC profiles of prepared hyper-dendritic zinc foam and conventional bulk zinc are seen in where γsl is the solid-liquid interfacial energy in J/m 2 , Hf is the bulk heat of fusion in J/g, ρs is the density of the solid in g/m 3 , TMB is the bulk melting temperature in K and TM(d) is the nanoparticle melting temperature in K. All physical constants were taken from the literature and the melting points were measured from the onset of the endothermic phase transition 32,33 . found, indicating that pure hexagonal zinc crystals were formed. An increased relative intensity of the peak at 36.29° for the zinc foam (B) compared to the bulk zinc (A) indicates that zinc was preferentially deposited in highly oriented {002} direction. X-ray peak broadening analysis was carried out through the Scherrer equation, seen in (7), which estimates the mean crystallite size from the full width at half maximum of a peak: where d is the mean crystallite size in nm, K is a dimensionless shape factor (0.94), λ is the diffraction wavelength in nm, β is the full width at half maximum in radians and θ is the Bragg angle in radians. Assuming Gaussian peak profiles and spherical crystals with cubic symmetry,  Discharge-profiles as a function of rate of prepared zinc foam on platinum at -2.0 V and at ambient temperature with charge passed of 3000 C is seen in Figure 7. Flat discharge-profile plateaus at -1.375 to -1.1 V and specific capacities of 719.2 ± 12.1 mAh/g corresponding to a consistent coulombic efficiency of 87.7 ± 1.5% at discharge-rates from 2C to C/10 were measured. The discharge-profiles of zinc foam showed no significant decrease in discharge capacity at 100% depth-of-discharge (DOD) and at high rates. The decrease of the voltage plateau at high discharge-rates was directly correlated to higher ohmic resistance. The high utilization of the hyper-dendritic nanoporous zinc foam showed great rate capability and was comparable to results of conventional zinc anodes in a sponge form-factor 14,34 . passed of 3000 C controlled through chronocoulometry. The three-dimensional dendritic structure of nanoporous zinc on nickel mesh was confirmed under a SEM, seen on Figure 8D, and Wang et al. 17 results support that zinc deposits on face-centered cubic nickel substrates with no structural misfits. The specific capacity of the nickel counter electrode was roughly 5 times higher than the working electrode to ensure no capacity limitations. The prototype cells were cycled at rate of C/5 corresponding to a current of 166.67 mA, and to 40% DOD which is equivalent to a theoretical specific capacity of 328 mAh/g, to ensure that the core dendritic structure was intact. A 15 min rest between each charge and discharge step and a voltage cutoff during discharge at -1.35 V vs. Hg/HgO or 1.6 V vs. Ni(OH)2/NiOOH was used. Figure 8A shows the summarized cycling data of the specified electrodeposited zinc up to 100 cycles, and According to the kinetic study in this paper, the open circuit potential was lower for the foam architecture, thus the charge-discharge trend over cycles indicated a densification of the foam to bulk structures. This indicates that if a cell starts with metallic zinc that was formed further from equilibrium than the average cycling protocol demands, the zinc will restructure to form a more dense network as opposed to a more dendritic network. The discharge voltage profile for the flooded cell showed promising stability with less than ~1 % potential fade per cycle, a specific discharge capacity of 303.1 ± 24.9 mAh/g, and low resistive losses across the cell over time (0.4 -0.8 Ω). At the 100th cycle, a specific discharge capacity of 282.3 mAh/g was achieved corresponding to 86.1% of the initial capacity. For the non-flooded cells, the hyper-dendritic zinc was compared to bulk zinc, and presented superior capacity retention over 100 cycles.

Conclusions
We have introduced a method of synthesizing inexpensive three-dimensional zinc anodes in the form-factor of hyper-dendritic nanoporous foam, in situ. The zinc foam exhibits stable primary anode performance at high coulombic efficiency (87.7 ± 1.5%) and secondary anode performance beyond 100 cycles at 40% DOD. The superior performance is attributed to the three-dimensional dendritic matrix at nanoscale formed by branch growth on secondary dendrites. The branch formation resulted in zinc nanoparticles with a size distribution of 54.1 -96.0 nm. The achieved nanoparticles contributed to: more rapid kinetics in comparison to conventional bulk zinc of the redox chemistry, 10 to 25 times larger specific surface area than bulk zinc, a 2.8 °C decrease in melting point in comparison to bulk zinc and highly oriented zinc deposition. Most interesting is that by operating mesoscale isotropic three-dimensional network of dendrites far from equilibrium seems to densify the structure at standard battery operating conditions, as evidenced by the morphological change and the shift in open circuit potential from hyper-dendritic zinc foam as a function of cycle number. This is in contrast to most literature work with zinc which starts flat or packed which seeks to emulate the "equilibrium condition". If hyper-dendritic structures can be recreated-locally in operando, a battery with a zinc anode capable of indefinite cycle life may be possible. Perhaps, if dendritic behavior can be maximized and controlled locally, it may deter system limiting short circuits globally, as the system will move towards more dense structures as opposed to more branched structures.