Spontaneous and field-induced crystallographic reorientation of metal electrodeposits at battery anodes

Abstract

The propensity of metal anodes of contemporary interest (e.g., Li, Al, Na, and Zn) to form non-planar, dendritic morphologies during battery charging is a fundamental barrier to achievement of full reversibility. We experimentally investigate the origins of dendritic electrodeposition of Zn, Cu, and Li in a three-electrode electrochemical cell bounded at one end by a rotating disc electrode. We find that the classical picture of ion depletion–induced growth of dendrites is valid in dilute electrolytes but is essentially irrelevant in the concentrated (≥1 M) electrolytes typically used in rechargeable batteries. Using Zn as an example, we find that ion depletion at the mass transport limit may be overcome by spontaneous reorientation of Zn crystallites from orientations parallel to the electrode surface to dominantly homeotropic orientations, which appear to facilitate contact with cations outside the depletion layer. This chemotaxis-like process causes obvious texturing and increases the porosity of metal electrodeposits.

Controlling electrochemical growth of metallic zinc electrodes: Toward affordable rechargeable energy storage systems

“…This predicts that Zn electrodeposits are (002) textured at small overpotential but (101) textured at large overpotential, which is consistent with experimental observation (Fig. 5D) (28).…”

“…3B) that can be quite different from classical expectations (47). A second consequence of this chemotaxis-like electrodeposit growth is the increase in deposit porosity and the transition in crystal texturing-Horizontally oriented Zn plates observed at a underlimiting current density are tilted up above J lim , which can be captured by x-ray diffraction (see the section discussing crystallography for details) (28). When a rotating disk electrode (RDE) is applied to artificially limit the diffusion layer thickness δ O to a small value of <10 m, the reorientation from horizontal to vertical is completely suppressed, and the strong (002) texturing of Zn is preserved throughout the deposition.…”

“…Aligning the c axis parallel to the magnetic field therefore minimizes the magnetization energy. We note that the deposition was performed at current densities close to the diffusion limit, meaning that the magnetohydrodynamic convection induced by the high magnetic field may also play a role in determining the texture, as shown in the RDE study as discussed previously (28). Future studies could focus on further exploring the Zn plating/stripping efficiency achieved under a high magnetic field and the possible effect of such magnetic field on the overall operation of a battery.…”

“…An artificial convective flow that constrains the diffusion layer thickness can preserve the (002) texturing in prolonged deposition (Fig. 5D) (28). This flow-assisted configuration has been adopted in scaled-up alkaline Zn batteries (~100 Wh) and demonstrated the capability of enhancing the plating/ stripping efficiency of Zn metal at elevated current densities (30,148).…”

“…2E points outward from the electrode surface into the bulk electrolyte as a manifestation of the chemotaxis-like growth, which will be discussed later. The significance of this categorization becomes clear when one examines the plating/stripping reversibility for each of the morphologies: horizontally aligned plate-like (over 99%) > nonaligned plate-like (80 to 90%) ≈ randomly oriented moss/wire-like (80% to 90%) > vertically oriented dendrites (<50%) (28)(29)(30).…”