Highly reversible zinc metal anode enabled by strong Brønsted acid and hydrophobic interfacial chemistry

Grid-scale energy storage is crucial for renewable energy integration. Rechargeable aqueous zinc (Zn) batteries (RAZBs) are promising due to Zn's abundance, low cost, high theoretical capacity, and low redox potential. However, RAZBs face challenges due to the Zn anode's reactivity in aqueous electrolytes. Spontaneous reactions with water cause hydrogen evolution reactions (HER) and the formation of alkaline byproducts like Zn₄SO₄(OH)ₓH₂O, Zn(OH)₂, or ZnO. These byproducts lead to poor Coulombic efficiency (CE), dendrite growth, and capacity degradation of cathode materials. Corrosion persists throughout the battery's lifespan, impacting both anode and cathode. Existing electrolyte designs focus on suppressing water reactivity, but this alone is insufficient to address the accumulation of alkaline byproducts. This paper proposes a direct solution by introducing a strong Brønsted acid, HTFSI, to eliminate alkaline byproducts and enable uniform Zn deposition.

Several electrolyte design strategies have been explored to reduce water reactivity towards Zn metal. High-concentration aqueous electrolytes minimize free water molecules, while localized high-concentration aqueous electrolytes (LHCEs) with non-solvating diluents improve rate capability and reduce cost. Fluorinated interphases enhance anion interfacial chemistry, and organic co-solvents replace active water molecules. However, concerns about safety with flammable solvents remain. Recent work with aqueous ZnCl₂ electrolytes incorporating LiCl or additional chloride salts improves efficiency and mitigates HER. While these studies focus on suppressing water reactivity, there's a lack of strategies to tackle the deposited alkaline byproducts, which is the key focus of this research. The authors hypothesize that introducing acidic species could directly and effectively solve this problem.

The researchers added HTFSI, a strong Brønsted acid, to a conventional aqueous electrolyte (1 m ZnSO₄). The effects on Zn metal self-corrosion were investigated through in situ pH monitoring of Zn||Zn cells, SEM, EDS, and XRD analyses of Zn foil surfaces after soaking in different electrolytes. Electrochemical performance was evaluated in Zn||Cu and Zn||Zn cells by measuring CE, voltage profiles, and cycling stability at various current densities and areal capacities. The electrochemical performance of Zn||ZnV₆O₁₃ full cells was also assessed, including rate capability, self-discharge tests, and long-term cycling stability. The morphology of deposited Zn was characterized by SEM and EDS mapping. In situ Raman spectroscopy studied cathode surface changes in real-time. XPS and GIXRD were employed to analyze the chemical composition and structure of the Zn electrode surface at different depths. Ab initio molecular dynamics (AIMD) simulations were used to understand the distribution of species at the interface. The synthesis of ZnVO involved dissolving V2O5 in water and hydrogen peroxide, followed by adding Zn(CH3COO)2·2H2O and heating in a Teflon vessel. Electrochemical measurements included cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), chronoamperometry (CA), and Tafel plots. Ion conductivity was measured in a micro-electrochemical cell using EIS. Zn||Zn and Zn||Cu cells were assembled using 2032-type coin cells. Zn||ZVO batteries were assembled using a cathode of ZVO, Super C65, and PTFE on Ti mesh.

The addition of HTFSI significantly improved the reversibility of Zn anodes. In Zn||Cu cells, the CE reached 99.7% at 1 mA cm⁻² and >99.8% under higher current density and areal capacity (4 mA cm⁻², 2 mAh cm⁻²). Zn||Zn cells exhibited stable cycling for over 2200 h at 4 mA cm⁻² and 4 mAh cm⁻². Zn||ZnV₆O₁₃ full cells demonstrated high capacity retention (76.8%) after 2000 cycles. The improved performance is attributed to two main factors. First, the strong Brønsted acid nature of HTFSI prevents the accumulation of insoluble alkaline byproducts (like ZSH) on the Zn surface. Second, the hydrophobic TFSI anions enrich at the Zn electrolyte interface, creating a water-deficient microenvironment that promotes the formation of a ZnS-rich protective interphase. This interphase inhibits self-corrosion and HER, enabling uniform Zn deposition. The in situ Raman and XPS results show that this ZnS-rich layer is formed at the electrode-electrolyte interface and this is consistent with the AIMD simulations which show a water-deficient region is formed at this interface. Tests with other acids (H₂SO₄, HCl, H₃PO₄, HOTf) confirmed the unique effectiveness of HTFSI, highlighting the synergistic effect between H⁺ and TFSI⁻. The TFSI⁻ anion alone cannot provide reversible Zn cycling. The superior performance of HTFSI compared to other acids is attributed to the unique properties of the TFSI anion.

The findings directly address the long-standing challenge of Zn anode reversibility in aqueous batteries. The use of HTFSI as an electrolyte additive offers a simple yet effective solution to the problem of alkaline byproduct accumulation, a significant factor limiting the performance and lifespan of RAZBs. The results demonstrate the potential of modulating the corrosion pathways of Zn metal to achieve high reversibility and long-term cycling stability. The synergistic effect of the strong Brønsted acidity and hydrophobicity of HTFSI is crucial for creating a stable and protective interface on the Zn anode, which is essential for high-performance aqueous Zn-ion batteries. This work challenges the conventional wisdom that acidic additives are detrimental to Zn anodes. The success highlights the significance of electrolyte design in optimizing the interfacial chemistry for high performance.

This research demonstrates that a strong Brønsted acid with hydrophobic moieties, such as HTFSI, can significantly improve the reversibility and stability of Zn anodes in aqueous batteries. The improved performance stems from the inhibition of alkaline byproduct formation and the formation of a ZnS-rich protective interphase. This study opens new avenues for developing highly efficient and long-lasting aqueous Zn-ion batteries. Future research could explore other strong Brønsted acids with tailored hydrophobic properties to further enhance Zn anode performance and investigate the applicability of this strategy to other aqueous battery systems.

The study primarily focused on ZnSO₄-based electrolytes. Further investigation is needed to assess the effectiveness of HTFSI in other aqueous electrolyte systems. The long-term stability of the ZnS-rich interphase under various operating conditions also needs further evaluation. The long-term stability under different temperatures also needs to be investigated.