Dendritic Growth in Battery Anodes: On the Hidden Role of Crystallographic Anisotropy at Propagating Electrochemical Interfaces

Abstract Dendritic growth is a major cause of battery failure, occurring under diffusion-limited conditions regardless of material crystallinity. We developed an operando visualization setup─complemented by post-mortem analysis using microscopy and synchrotron scattering─to study electrochemical dendrite growth of metals with spatiotemporal resolution. Using Zn electro-crystallization in a series of ZnCl2 electrolytes as a model system, we found that dendritic growth is most severe in both dilute (0.01–0.5 m) and highly concentrated (10–30 m) systems, as predicted by the dimensionless second Damköhler number, Da,II = J0/Jlim. However, we discovered unexpectedly that in highly concentrated electrolytes, dendrites propagate via Continuous Growth (Dendrite Mode I: CG)─initiating from only a few dominant nuclei and propagating anisotropically in one primary direction, with parallel arms suggesting Zn’s six-fold hexagonal lattice symmetry. Advancing along this preferred direction at rates up to ∼0.1 mm s–1 toward the counter electrode, these dendrites pose a critical challenge, making highly concentrated, water-in-salt electrolytes particularly susceptible to fatal diffusion-limited dendrite growth. We hypothesize that Mode I: CG arises from Zn’s high surface anisotropy, dictated by its low-symmetry hexagonal lattice; once nucleated, Zn crystals preferentially propagate in the ⟨100⟩ direction rather than forming new independent nuclei. In stark contrast, Cu─a high-symmetry cubic crystal─exhibits Dendrite Mode II: IN, characterized by Independent Nucleation throughout growth, with no apparent orientational correlation or dominance by a few dendrites. This finding highlights a hidden role of crystal anisotropy in electrochemical dendrite formation, unaccounted for in classical Da,II analyses. We further hypothesize that introducing a polymeric “capping” agent that lowers the energy barrier for independent nucleation may shift dendrite growth from Mode I: CG to Mode II: IN. Indeed, we found that the capping agent (e.g., PEG-400) disrupts rapid anisotropic dendritic propagation and promotes numerous smaller structures characteristic of Mode II: IN─surprisingly, without significantly affecting key electrochemical properties such as reaction kinetics J0 and ionic conductivity σ. The transition from Mode I: CG to Mode II: IN enhances the performance of Zn anodes by a factor of 10 in practical coin cells under fast-charging conditions (50 mA cm–2 and 5 mAh cm–2). The results highlight the overlooked role of crystal anisotropy in electrochemical diffusion-limited dendrite growth. Task-specific interventions are required for mitigating low-symmetry metals’ propensity for Mode I: CG, where rapid dendrite advancement occurs, in concentrated electrolytes─such as the emerging water-in-salt systems─under fast-charging conditions.