Unique solvation structure induced by anionic Cl⁻ in aqueous zinc ion batteries

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Article information

Type: research article

Volume 10, Issue 9, 15 May 2024, e30592; https://doi.org/10.1016/j.heliyon.2024.e30592

Abstract

Aqueous zinc ion batteries (AZIBs) have garnered significant attention in large-scale static energy storage battery systems due to their low cost, high safety and environmental friendliness. However, it has some inherent problems during operation, such as the occurrence of side reactions (hydrogen evolution reaction, HER) and anode corrosion, formation of by-products and growth of metal dendrites. To analyze the mechanism of generation from aspect of the electrolyte solvation structure and make cell efficiency further improvements based on it, so we use DFT calculations to find the most stable solvation structure in AZIBs with ZnCl2 as the electrolyte and analyze it. We define the relative concentration Cr, and calculate different groups metal cation cluster structures such as [Zn(H2O)n]2+, [ZnCl(H2O)n]+, [ZnCl2(H2O)n] and [ZnCl3(H2O)n]‐ that exist at different Cr. We discuss the effect of different clusters formed due to the Cr variations on the battery performance in terms of three aspects: the structural conformation, the cluster characteristics (including the hydrogen bonding network, bond lengths, bond angles, as well as the electrostatic potential ESP) and the cluster performance (including the adsorption energy Ea, binding energy Eb, and desolvation energy Edes). The results shows that the electrolyte metal cation Zn2+ can be coordinated with up to six H2O molecules in first shell, and this metal cation solvation structure contributes to the occurrence and formation of side reactions and by-products, which reduces the battery efficiency. Increasing the electrolyte anion Cl− concentration by appropriately increasing the Cr helps to desolvate the metal cation cluster structure, which greatly improves the battery efficiency and suppresses the side reactions and by-products. Yet the improvement effect was not obviously further improved by further increasing the Cl− concentration.

Keywords: AZIBs; Solvation structure; Dissolving effect; Metal cation cluster; ESP

Research Background

Aqueous zinc ion batteries (AZIBs) are considered strong candidates for large-scale stationary energy storage due to their low cost, high safety, and environmental friendliness. However, their operation is accompanied by issues such as hydrogen evolution reaction (HER), anode corrosion, byproduct formation, and zinc dendrite growth. The authors attribute these issues to the solvation structure of Zn²⁺ ions in the electrolyte. Consequently, they systematically investigate the solvation clusters in ZnCl₂ electrolyte using density functional theory (DFT) and explore the influence of Cl⁻ concentration on this structure and battery performance.

Research Approach and Computational Methods
1、Define the dimensionless relative concentration Cr (≈ [Cl⁻]/[Zn²⁺]), covering the range of Cr from 0 to 3.
2、Construct and optimize four representative clusters:

  • [Zn(H₂O)ₙ]²⁺
  • [ZnCl(H₂O)ₙ]⁺
  • [ZnCl₂(H₂O)ₙ]
  • [ZnCl₃(H₂O)ₙ]⁻
    Calculate their most stable configurations.

3、Systematically compare clusters across three dimensions:

  • Structural configurations (coordination number, bond lengths, bond angles, hydrogen bond networks)
  • Cluster properties (electrostatic potential distribution ESP)
  • Cluster performance (adsorption energy Ea, binding energy Eb, desorption energy Edes)

Key Findings
1、Under low Cr (insufficient Cl⁻) conditions, the primary solvation shell of Zn²⁺ is surrounded by up to 6 water molecules, forming [Zn(H₂O)₆]²⁺. The water molecules in this structure are highly reactive, readily decomposing at the electrode surface to produce HER and generate byproducts, thereby reducing coulombic efficiency.
2、As Cr concentration increases, Cl⁻ gradually replaces water molecules:

  • Cr ≈ 1 → predominant [ZnCl(H₂O)₅]⁺
  • Cr ≈ 2 → predominant [ZnCl₂(H₂O)₄]
  • Cr ≥ 3 → [ZnCl₃(H₂O)₃]⁻ appears
    Cluster charge decreases (+2 → 0 → –1), water molecule count reduces, hydrogen bond network weakens, and desorption energy (Edes) significantly decreases.

3、Calculations indicate that increasing Cr from 0 to 2 reduces desorption energy by approximately 30–40%, effectively inhibiting water molecule decomposition and dendrite growth, while significantly enhancing battery cycling stability and coulombic efficiency.
4、Further increasing Cr > 2 leads to saturated performance improvements, demonstrating that “moderately increasing Cl⁻ concentration” achieves optimal modification effects without requiring excess.

Conclusion

• By regulating the Cl⁻ concentration in the electrolyte, the solvation structure of Zn²⁺ can be precisely controlled: Cl⁻ replaces water molecules in the first solvation shell, reducing water activity and weakening the hydrogen bond network, thereby suppressing side reactions and dendrite formation.

• This establishes a novel “solvation engineering” strategy for AZIBs electrolyte design—significantly enhancing battery performance solely by adjusting ZnCl₂ concentration without requiring expensive additives, balancing cost-effectiveness and scalability.