They published their work on October. 3 in Energy Material Advances.
"Aqueous ZIBs hold great potential as competitive candidates for next-generation energy storage devices," said corresponding author Ying Bai, professor at School of Materials Science and Engineering in Beijing Institute of Technology. "Thanks to their high theoretical capacity, low cost, and high security."
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."
