Electrode Stabilization Strategies for Aqueous Zinc Batteries


Student thesis: Doctoral Thesis

View graph of relations



Awarding Institution
Award date28 Jul 2020


Aqueous zinc batteries (ZBs), established onto the stripping/plating reaction at the anode side and the ion intercalation or surface redox behaviors at the cathode side, are being intensively studied because of the high energy density of metallic zinc anode and the intrinsic safety performance. However, ZBs still suffer seriously from the instability issues at both electrodes. On the one hand, Zn metal anode encounters severe dendrite issue and thus dramatically shortens ZBs’ service lifespan, which are drawing much attentions while the challenges such as developing active dendrite protection strategies remain unresolved. On the other hand, cathode materials are also difficult to maintain stable in the aspects of crystal structure and chemical environment during the repeated ion storing/releasing processes, resulting in the gradual/sharp decay of battery capacity. Therefore, it is urgent to develop efficient electrode stabilization strategies to comprehensively improve the cycling lifespan and capacity retention of aqueous ZBs.

The dendritic issue in aqueous zinc-ion batteries (ZBs) using neutral/mild electrolytes has remained an intensive controversy for a long time: some researchers assert that dendrites severely exist while others claim great cycling stability without any protection. We clarified this issue by investigating charge/discharge-condition-dependent formation of Zn dendrites. Apparent lifespan degradation (120 to 1.2 h) was observed with increased current densities and areal capacities due to the formation of Zn dendrites (edge size: 0.69-4.37 μm). Therefore, at small current densities or loading mass, Zn dendrites might not be an issue, while the large conditions may rapidly ruin batteries. Based on this discovery, a first-in-class electrohealing methodology was also developed to eliminate already formed dendrites, generating extremely prolonged lifespans by 516% at 10 mA cm-2. Morphological analysis revealed that vertically aligned Zn dendrites with sharp tips gradually become passivated and finally generate a smooth surface. This developed electrohealing strategy may promote research on stabilizing metal anodes in various batteries evolving from passive prevention to active elimination, rescuing in-service batteries in situ to achieve elongated lifetime.

Despite multiple progresses in prolonging battery lifespans, however, rare effort was devoted to stabilizing Zn anode at commercial-grade cathode loading mass while demonstrations were only done with a laboratory level (≤2 mg cm-2). By the way, new dilemmas such as change of proton-storage behavior and interface pulverization emerged in turn. In this thesis, hydrogen-substituted graphdiyne (HsGDY) with sub-ångström level ion tunnels and robust chemical stability was designed as an artificial interface layer to address these issues. This strategy prolonged the symmetric cell lifespan to >2400 h (100 days), which is 37-times larger than the one without protection (63 h). Dual-fields simulation uncovered that HsGDY could redistribute Zn2+ concentration field through spatially forcing Zn2+ to deviate from the irregular electric field. Targeting at practical use, the as-assembled full batteries delivered a super-long lifespan of 50,000 cycles and maintained stable even at a commercial-grade cathode loading mass of up to 22.95 mg cm-2. This HsGDY-stabilized methodology represents a great progress in Zn anode protection and demonstrates enormous potential in ZBs.

At the cathode side, prussian blue analogue (PBA)-type metal hexacyanoferrates are considered as promising Zn2+ containers for ZBs. However, these PBA-type cathodes, such as cyanogroup iron hexacyanoferrate (FeHCF), suffer from ephemeral lifespan (≤1000 cycles) due to the instable coordination states. This is because the redox active sites of multivalent iron (Fe(III/II)) can only be very limited activated and thus utilized, which is attributed to the spatial resistance caused by the compact cooperation interaction between Fe and the surrounded cyanogroup, and the inferior conductivity. In this thesis, it was found that high-voltage scanning strategy can effectively activate the C-coordinated Fe in FeHCF cathode in ZBs. Thanks to this activation strategy, the Zn-FeHCF hybrid-ion battery achieves a record-breaking cycling performance of 5000 (82% capacity retention) and 10,000 cycles (73% capacity retention), respectively, together with a superior rate capability of maintaining 53.2% capacity at superhigh current density of 8 A g-1 (≈97 C). The reversible distortion and recovery of the crystalline structure caused by the (de)insertion of zinc and lithium ions is revealed. This stabilization strategy of activating low-spin Fe represents a substantial advance on PBA-type cathodes for ZBs.

In summary, electrode stabilization strategies ranging from in-situ electrochemical mediation to interfacial direction are developed in this thesis to eliminate the serious anodic dendrite issue and the cathodic degradation. It is believed that the studies and strategies presented in this thesis can pave the way for the design of high-performance ZBs with both long lifespan and high energy density.