Solutions for Dendrite Growth of Electrodeposited Zinc.

Dendritic growth is ubiquitous in metallurgy, electroplating, rechargeable zinc-air batteries, and other secondary batteries, seriously affecting the service life of zinc electrode. However, the dendrite growth of electrodeposited zinc at large charging currents remains unresolved. Here, inhibition of dendrite growth of electrodeposited zinc is summarized by means of electrolyte modification and additives, electrode reformation and architecture optimization, and synergetic coupling of multiphysics. Moreover, the mechanism of dendrite growth is investigated on the basis of ion transport, electrochemical reaction, and electrocrystallization, demonstrating that the dendritic morphology can only be partly suppressed but not completely cured by means of ion diffusion and activation control. The partially conductive and partially insulating structure is a feasible measure to avert the negative effects of dendrite growth at large currents, which can extend the cycle life of zinc-based secondary batteries and increase the battery capacity.

Introduction

Zinc electrodeposition widely exists in the fields of metallurgy, galvanization, and secondary batteries.[1] Moreover, fast charge is of great importance to enhance the electrodeposition efficiency. Unfortunately, dendritic growth of electrodeposited zinc quickly arises at large currents, giving rise to shape change, short circuit of electrodes, potential safety hazards, and poor cycle life of metal batteries.[2]

The process of zinc electrodeposition comprises ion transport, electrochemical reaction, and electrocrystallization,[3] and the depositing morphology is the equilibrium result of thermodynamics and kinetics. Consequently, the morphological change of depositing zinc should be jointly determined by ion diffusion, electrochemical reaction kinetics, an atomic bond, crystal nucleation, and growth. The driving force of ion transfer in the electrolyte mainly comes from the concentration gradient, electric field, and liquid convection. The rate of electrodeposited zinc depends on the overpotential and current density abiding by the Butler–Volmer equation.[4] The uneven distribution of local current density at the electrode surface will inevitably lead to dendrite growth of electrodeposited zinc. Subsequently, the dendritic morphology will lead to uneven potential distribution, in turn facilitating dendrite growth. During crystallization, the adsorbed atom results in sediment surface heterogeneity like screw dislocation because of small binding energy itself, which is generated either on the outer Helmholtz plane or directly in the lattice vacancy of the electrode surface. In addition, multiple electrochemical plating and stripping cycles can progressively amplify the heterogeneous morphology of the zinc electrode, bringing about dendritic growth.

Inhibition of Dendrite Growth

To suppress dendrite growth during electrodeposition, researchers take effective measures,[5−7] as shown in Figure , including (i) electrolyte reformation and additives, (ii) electrode architecture and alloy design, (iii) electrode–electrolyte interfacial engineering, and (iv) synergistic coupling of multiple fields. The additives were added to the electrolyte for reduction of local current density at the electrode surface,[8−11] but active materials would be contaminated with these impurities, bringing about battery capacity loss. Neutral electrolyte, ionic liquid, and solid-state electrolyte can be employed as alleviating dendritic growth,[12−14] while the conductivity of these electrolytes is lower than that of alkaline electrolytes. A graphene layer was deposited on stainless steel for epitaxial electrodeposition of metal,[15] or doping of metal electrodes was used for promoting the preferential growth and reducing the surface energy of the electrode.[16] The three-dimensional architecture of the porous electrode worked by increasing the active area of the reaction,[17,18] which can minimize dendritic growth but increase hydrogen evolution rate. The use of pulse charging to prolong ion migration time or electrolyte flowing to enhance ion diffusibility,[19] which can reduce the ion concentration gradient, stemmed from an electrochemical reaction but increased cost. Besides, dendritic morphology of electrodeposited metal can be controlled by the external magnetic field induced directional movement of gas bubbles.[20]

Figure 1

Mechanism of dendrite growth of electrodeposited metal and solutions.

Guiding Dendrite Growth

Dendrite formation can be suppressed to some extent based on the above ion diffusion or activation control, but in fact, zinc dendrites continue to grow at large currents. In this case, the dendrites need to be inhibited by means of physical isolation such as an insulating separator.[21] The original aim of the separator is to prevent physical contact of the electrodes while allowing ionic transfer; regrettably, the dendrites would always puncture the separator in spite of dendritic inhibition, and part of the deposits falls off from the main stem, forming dead zinc. Moreover, electrochemical stability and electric resistance are challenges for insulating separators. A separator coated with functionalized nanocarbon was developed not to suppress but to allow dendritic growth along a certain direction.[22] A partially conductive separator is composed of a layer of insulating membrane and a layer of conductive porous material, which is placed between the cathode and anode. The direction of dendrite growth would be switched once it comes into contact with the conductive part of the separator; thus, the depositing layer becomes denser, which can increase the electric capacity of secondary batteries. When the power is directly connected to the conductive part of the separator, the electrodepositing direction of zinc is opposed to the electric field.[23] This structure would not cause short circuits even at the large charging currents. More importantly, the depositing morphology would be more uniform because of the potential of the depositing layer being lower than that of the conductive layer, as shown in Figure .

Figure 2

Dendrite growth of depositing zinc. (a) Dendrite growth causing short circuit of the batteries, (b) dendrite growth puncturing a separator, (c) partially conductive separator guiding dendrite growth, and (d) insulator encapsulating anode-reversing dendrite growth.

Conclusions

Dendritic morphology of electrodeposited zinc is discrete, disperse, and weak in stickiness, leading to the reduced capacity as well as the short circuit of the batteries. Dendrite growth during electrodeposition comes from rotating screw dislocation of the depositing atoms, which is subjected to the limited diffusion of ions and the blocked transfer of electrons. The dendritic morphology can be suppressed by way of physiochemical modification of the electrode or the electrolyte and synergetic coupling of multiphysics. Another way is that dendrite growth is not inhibited but guided by means of a partially conductive structure.