Toward stable aqueous Zn-ion batteries

Bai explained that among various anode materials, the direct adoption of Zn anode could lead to highly competitive gravimetric and volumetric capacities (820 mAh g-1 and 5855 mAh cm-3) as well as suitable redox potential (-0.762 V versus standard hydrogen). Although metallic Zn anode has such advantages, it is still suffering from many challenges, including Zn dendrite generation and side reactions (hydrogen evolution, Zn corrosion and passivation).

These issues bring about unstable cycling performance with low Coulombic efficiency (CE), capacity fading, and even short-circuit when the dendrite punctures the separator, which limit the further development of ZIBs and impede their practical applications. Accordingly, it is crucial to develop high-performance metallic Zn anode.

To modify metallic Zn electrodes, Bai said a great number of strategies have been proposed, such as anode structure design, alloying with other metals, electrolyte optimization, and building artificial protective layer. Owing to effective performance enhancement, possibility in large-scale manufacturing, tunability and designability of physical and chemical properties, constructing protective coating has arisen widespread interest.

Bai and her team overviewed the advances of constructing artificial layer to stabilize Zn anode according to the protection mechanisms. First, porous materials like nano-CaCO3, Kaolin, Zn-based montmorillonite, and MOF-based materials can regulate Zn2+ flux for stable cycling by the selective channels and pores.

While the carbon-based materials, including reduced graphene oxide (rGO), porous carbon, and mesoporous hollow carbon, improve the cyclability of ZIBs via increasing the Zn deposition sites, regulating electric field distribution and reducing local current density. Apart from the inorganic protective layers, polyamide with unique hydrogen-bonding network and coordination with metal ions, polymer Glue, ZnF2 and S/N-rich organic compounds etc were also developed to stabilize Zn anode and prolong the lifespans, but the introduced polymer components may result in increased polarization and nucleation overpotential. Therefore, according to Bai, there is still great room for developing functional protective layers for stable Zn anode.

"Taking account of Zn dendrites are caused by non-uniform metal deposition, involving uneven electric fields and Zn2+ ion flux," Bai said. "Combining two mechanisms and constructing a NaTi2(PO4)3 with carbon coating (NTP-C) protective layer onto the surface of Zn metal anode can regulate Zn deposition behavior for stable aqueous ZIBs.

Since the carbon coated NTP particles are imbedded in the carbon matrix, the protective layer can not only increase the Zn deposition sites for homogeneous nucleation, and uniform electric field for reduced local current density, but also regulate the distribution of Zn2+ flux simultaneously. In such way, the generation of Zn dendrites and side reactions can be suppressed."

"NTP-C protective layer provides an isolation function between water and Zn and NTP-C@Zn exhibits the highest corrosion potential (-0.987?V vs. Ag/AgCl) compared with bare Zn and NTP@Zn electrodes." Bai said. "The artificial layer can suppress corrosion and protect Zn anode from side reactions to achieve stable cycling among all the three kinds of electrodes."

"By combining the advantages of NTP and carbon, NTP-C achieves dual Zn deposition layers and further prolongs the lifespan of symmetrical cell to more than 600?h.". Bai said. "The NTP-C protective layer provides a low nucleation overpotential and plating/stripping polarizations in symmetrical cell as well. The NTP-C@Zn symmetrical cell exhibits a stable cycling performance even at a harsh condition (100 mA cm-2), which enables its application in high-load electrodes and supercapacitors."

"Furthermore, the NTP-C coated Zn electrode displays a short nucleation and 2D diffusion period, followed by a 3D diffusion process represented by a stable current density." Bai said. "The constrained 2D diffusion brings about a unaggregated interface, suggesting the adsorbed Zn2+ ions appear to be locally reduced to Zn0 with constrained 2D behavior, which is due to the increased energy barrier for Zn2+ lateral migration and the abundant nucleation sites of the introduced NTP-C coating."

This work combines two protective mechanisms and offers a new inspiration for the practical applications of Zn metal anode in ZIBs. Bai said, such strategy could help to develop suitable metallic anode for commercialization.

"Although great progress has been achieved, the development of Zn anodes for ZIBs is still facing massive challenges, such as investigating the electrochemical performance at large deposition areal capacity and high degree of discharge." Bai said. "In practical application, Zn anode is often matched to a cathode with a high mass loading of active materials, and the capacity ratio between theses electrodes in the battery cell need to be carefully adjusted to achieve a high energy density, long cycle life battery cell. In general, there are still practical issues that need to be paid more attention to investigate."

Other contributors include Jingjing Yang, Ran Zhao, Yahui Wang, and Chuan Wu, School of Materials Science and Engineering, Beijing Institute of Technology.

Research Report:Regulating Uniform Zn Deposition via Hybrid Artificial Layer for Stable Aqueous Zn-Ion Batteries