Biopolymer-Based Protective Layer for Stable and Highly Reversible Zinc Metal Anodes
Authors: Jang Jooyoung , Jo Changshin 
Published: February 2024

Zinc (Zn) metal has attracted considerable attention because of its natural abundance and stability in aqueous environments compared with lithium/sodium metal anodes. Moreover, Zn metal as anode showed a high theoretical capacity (820 mAh g−1), high energy per volume (5855 mA cm−3), and low operational potential (–0.78 V vs SHE) in electrochemical systems. However, Zn metal suffers from dendrite growth and poor plating/stripping reversibility, resulting from inhomogeneous Zn ion flux and contamination by generally used glass fiber membranes (GFs) as separators. Although studies to inhibit dendritic Zn growth have been conducted, dendritic Zn remains still a major problem during long-term cycling under practical rate and capacity conditions. In this context, it is essential to investigate Zn deposition behavior from initial nucleation to Zn growth morphologies after long-term cycling.

Among the problematic factors pointed out by researchers, the irregular pore distribution of GFs causes uneven Zn ion flux. Moreover, the detached fibers after cycling contaminate the Zn anode surface, resulting in nonuniform Zn deposition and trapping of Zn metal in the separator. To stabilize Zn metal anodes, introducing a protective layer can be an effective strategy to construct ion channels with abundant functional groups for high affinity with Zn metaland protect from direct contact with GFs. Organic polymers are suitable candidates for construction of protective layer which tolerates the volume changes of Zn metal and exhibit good chemical compatibility with Zn metal.

Polysaccharides are favorable for use as gel polymers, separators, and binders owing to their cost-effectiveness and good electrochemical properties. Besides, they contain various oxygen-containing groups, resulting in relatively high ionic conductivity. However, it is not sufficient to fabricate membranes for the stabilization of Zn metal using polysaccharide alone because of insufficient ion conductive properties. This problem can be solved by blending polymers with high ionic conductivities such as poly(ethylene oxide) (PEO), poly(vinylidene difluoride), and poly(vinyl chloride). Notably, PEO has mainly been used in gel polymer electrolytes. The hydroxyl groups of PEO can facilitate the transport of metal ions and ether linkages can enhance the affinity with cations, resulting high ionic conductivity.

In this study, we designed a functional protective layer by blending polysaccharide and PEO and investigated Zn deposition behavior from the formation of initial nucleation to Zn growth process after long-term cycling. The protective layer, fabricated by a one-pot method, featured highly dispersed blending polymers by phase separation and contained uniformly distributed oxygen-containing functional groups. Moreover, we demonstrated that the protective layer induced the formation of regular Zn nucleation sites and smooth Zn deposition after long-term cycling under the realistic conditions required for high-energy metal battery systems. Using the Zn||Cu and Zn||Zn cells stabilized by the protective layer under low capacity (0.1 mAh cm−2), we confirmed that homogeneous initial Zn deposition patterns were induced by ex situ SEM images of Cu and Zn electrodes. Subsequently, smooth and dense Zn growth was confirmed under fast Zn plating/stripping conditions, indicating that protective layer can induce basal plane (002) dominant deposition to mitigate dendrite formation and protect physically and chemically from GFs.

Based on the morphological Zn deposition behavior, we successfully fabricated a functional protective layer and significantly enhanced the electrochemical performance of Zn metal battery systems. The Zn||Cu cells with a functional protective layer showed significantly increased long-term cycling above 3850 cycles (more than 160 days) at high rate and capacity condition (2 mA cm−2 and 1 mAh cm−2). In addition, the durability of protective layer was confirmed by a high-rate test (at 10 mA cm−2). These results are consistent with those of the Zn||Zn symmetric cell tests under harsh conditions of 5 mA cm−2 and 5 mAh cm−2. Furthermore, full cell with the protective layer using vanadium disulfides as the cathode exhibited enhanced cyclability over 1000 cycles with 87 % capacity retention compared to the bare full cell that short-circuited at the 340th cycle.

Our work demonstrated that there are two important factors affecting the durability of Zn anodes: (i) Zn metal surface should be protected by a smooth and dense layer with high affinity to Zn metal and ions and (ii) physical and chemical contamination from GFs.

Figure 1