Zinc-Sponge Battery Electrodes that Suppress Dendrites

Brandon  J. Hopkins; Megan  B. Sassin; Joseph  F. Parker; Jeffrey  W. Long; Debra  R. Rolison

doi:10.3791/61770

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Podsumowanie
Streszczenie
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Dyskusje
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Podsumowanie

The goal of the reported protocols is to create rechargeable zinc-sponge electrodes that suppress dendrites and shape change in zinc batteries, such as nickel–zinc or zinc–air.

Streszczenie

We report two methods to create zinc-sponge electrodes that suppress dendrite formation and shape change for rechargeable zinc batteries. Both methods are characterized by creating a paste made of zinc particles, organic porogen, and viscosity-enhancing agent that is heated under an inert gas and then air. During heating under the inert gas, the zinc particles anneal together, and the porogen decomposes; under air, the zinc fuses and residual organic burns out, yielding an open-cell metal foam or sponge. We tune the mechanical and electrochemical properties of the zinc sponges by varying zinc-to-porogen mass ratio, heating time under inert gas and air, and size and shape of the zinc and porogen particles. An advantage of the reported methods is their ability to finely tune zinc-sponge architecture. The selected size and shape of the zinc and porogen particles influence the morphology of the pore structure. A limitation is that resulting sponges have disordered pore structures that result in low mechanical strength at low volume fractions of zinc (<30%). Applications for these zinc-sponge electrodes include batteries for grid-storage, personal electronics, electric vehicles, and electric aviation. Users can expect zinc-sponge electrodes to cycle up to 40% depth of discharge at technologically relevant rates and areal capacities without the formation of separator-piercing dendrites.

Wprowadzenie

The purpose of the reported fabrication methods is to create zinc (Zn) sponge electrodes that suppress dendrite formation and shape change. Historically, these problems have limited the cycle life of Zn batteries. Zinc-sponge electrodes have resolved these issues, enabling Zn batteries with longer cycle lives¹^,²^,³^,⁴^,⁵^,⁶. The sponge structure suppresses dendrite formation and shape change because (1) the fused Zn framework electrically wires the entire volume of the sponge; (2) the pores hold zincate near the Zn-sponge surface; and (3) the sponge has a high surface area that decreases local current density below values identified to sprout dendrites in alkaline electrolytes⁷. However, if sponge surface area is too high, substantial corrosion occurs⁵. If the sponge pores are too large, the sponge will have a low volumetric capacity⁵. Also, if the sponge pores are too small, the Zn electrode will have insufficient electrolyte to access Zn during discharge, resulting in low power and capacity⁵^,⁶.

The rationale behind the reported fabrication methods is to create Zn sponges with appropriate sponge porosities and pore diameters. Experimentally, we find that Zn sponges with porosities from 50 to 70% and pore diameters near 10 µm cycle well in full-cell batteries and display low corrosion rates⁵. We note that existing methods to manufacture commercial metal foams fail to achieve similar morphologies on these length scales⁸, so the reported fabrication methods are needed.

The advantages of the methods reported here over alternatives are characterized by fine control of sponge features and by the ability to fabricate large, dense Zn sponges with technologically relevant areal-capacity values⁵^,⁶^,⁹^,¹⁰. Alternative methods to create Zn foams may be unable to create comparable 10 µm pores with sponge porosities near 50%. Such alternatives may, however, require less energy to fabricate because they avoid high-temperature processing steps. Alternative processes include the following strategies: cold sintering Zn particles¹¹, depositing Zn on three-dimensional host structures¹²^,¹³^,¹⁴^,¹⁵^,¹⁶^,¹⁷, cutting Zn foil into two-dimensional foams¹⁸, and creating Zn foams via spinodal decomposition¹⁹ or percolation dissolution²⁰.

The context of the reported methods in the wider body of the published literature is primarily established by work from Drillet et al.²¹. They adapted methods of fabricating porous ceramics to create one of the earliest reported three-dimensional, albeit fragile, Zn foams for batteries. These authors, however, failed to demonstrate rechargeability, likely because of the poor connectivity between the Zn particles. Prior to rechargeable Zn-sponge electrodes, the best alternative to a Zn foil electrode was a Zn-powder electrode, wherein Zn powder is mixed with a gel electrolyte. Zinc-powder electrodes are commercially used in primary alkaline batteries (Zn–MnO₂) but have poor rechargeability because Zn particles become passivated by Zn oxide (ZnO), which can increase local current density that spurs dendrite growth³^,²². We note that there are other dendrite-suppression strategies that do not involve foam or sponge architectures²³^,²⁴.

The reported Zn-sponge fabrication methods require a tube furnace, sources of air and nitrogen gas (N₂), and a fume hood. All steps can be performed at a lab desk without environmental control, but exhaust from the tube furnace during heat treatment should be piped to a fume hood. Resulting electrodes are appropriate for those interested in creating rechargeable Zn electrodes capable of high areal capacity (> 10 mAh cm_geo^–2)⁶.

The first reported fabrication method is an emulsion-based route to create Zn-sponge electrodes. The second, is an aqueous-based route. An advantage of the emulsion route is its ability to create Zn paste that, when dried, is easy to demold from a mold cavity. A disadvantage is its reliance on expensive materials. For the aqueous route, sponge preforms can be challenging to demold, but this process uses inexpensive and abundant materials.

Both methods involve mixing Zn particles with a porogen and viscosity-enhancing agent. The resulting mixture is heated under N₂ and then breathing air (not synthetic air). During heating under N₂, the Zn particles anneal and the porogen decomposes; under breathing air, the annealed Zn particles fuse and the porogen burns out. These processes yield metal foams or sponges. The mechanical and electrochemical properties of the Zn sponges can be tuned by varying Zn-to-porogen mass ratio, heating time under N₂ and air, and size and shape of the Zn and porogen particles.