Effect of Li on the Uniformity of Zn Recharge Reaction in the Anodic ZnO Passivation Layer
Authors: Komiya Ayumu , Wang Tanyanyu , Kunimoto Masahiro , Asano Tsuyoshi , Nishikitani Yoshinori , Homma Takayuki 
Published: December 2024

Alkaline rechargeable Zn batteries are regarded as next-generation energy storage systems owing to their cost-effectiveness, safety, and high theoretical capacity. Nevertheless, their application is limited because of the degradation of Zn anodes during charge-discharge cycles. The most notable example of the degradation phenomena is dendrite formation during charge. Under conditions with high current density, current is concentrated at the protruding points of the electrode surface due to significant growth of the diffusion layer. This induces dendritic structures to grow perpendicular to the substrate, which causes short circuits in the worst case. Several methods, such as adding additives to the electrolyte and modifying the surface of electrodes via alloying or coating, have been proposed to suppress dendrites. In particular, additives have been widely investigated to achieve a cost-effective and simple battery manufacturing process. Their effects are often explained in terms of adsorption behavior on pristine Zn surfaces. In practical cases, however, surface states of Zn are more complicated due to the ZnO formation during discharge. Previous studies have suggested that the passivation layer on metal anodes (e.g. Al and Li) has a significant influence on dendritic growth behavior.[1, 2] Therefore, in this study, we investigate the additive effect on the dendritic growth of Zn in the presence of the ZnO passivation layer formed after discharge.
In particular, we focus on the additive effect of Li+. Li+ induces crystalline defects in ZnO by inhibiting the crystallization process.[3] Since the electrical properties of ZnO are greatly affected by crystalline defects, Li+ is a qualified additive to focus on in this study, which investigates the influence of the ZnO passivation layer. In addition, Li+ is an interesting target to investigate its additive effects during recharge since it has been identified by our group as an effective additive to inhibit undesirable property changes such as mossy growth and passivation.[3, 4]
Fig. 1 shows the surface morphology of the Zn recharged in the presence of varying concentrations of Li+. Without Li+, the Zn electrodeposits contain leaf-like dendrites. In contrast, 0.1 M Li+ addition leads to finer morphology with fewer dendrites. Conversely, with 2 M Li+ addition, the rough deposits containing more and larger dendrites are observed. Notably, the Zn deposition morphology at the initial charge was independent of the concentration of Li+. Therefore, Zn deposition morphology during recharge was most likely affected by the property difference of the ZnO passivation layer formed during discharge. Moreover, the potential profile of Zn during recharge suggested that the conductivity of the ZnO passivation layer was dependent on the concentration of Li+: it increased with 0.1 M Li+ addition and decreased with 2 M Li+ addition. The conductivity changes were attributed to the carrier density change due to crystalline defect formation. These results suggest that the ZnO passivation layer with high conductivity is ideal for suppressing the dendritic growth of Zn during recharge. Furthermore, controlling crystalline defects with additives is an effective strategy for forming a conductive ZnO passivation layer.

References:

[1] H. Chen et al., ACS Appl. Mater. Interfaces, 9, 22628 (2017)
[2] Y. Gu et al., Nat. Commun., 9, 1339 (2018)
[3] T. Otani et al., Electrochim. Acta, 305, 90 (2019)
[4] Y. Onabuta et al., J. Electrochem. Soc., 169, 092504 (2022)

Figure 1