Regulated Zinc Tetrafluoroborate Salt-Based Electrolyte Enables Highly Reversible Zinc Ion Batteries

Abstract

Rechargeable zinc-ion batteries (ZIBs) are seen as an attractive option for renewable energy storage systems due to their abundance, eco-friendliness, cost effectiveness, low electrochemical potential, and large specific capacity (820 mAh g^-1 and 5,855 mAh cm^-3). The performance of ZIBs relies on the use of a suitable electrolyte consisting of Zn salts and solvents, capable of maintaining contact with the anode and cathode of the battery while providing a fast charge transfer mechanism. The proper selection of Zn salts and solvents plays a critical role in the performance and stability of the battery, as their interaction directly determines the reversibility of the zinc anode, cathode insertion/extraction behavior, and parasitic side reactions (i.e., dendrite formation, hydrogen evolution reaction [HER], zinc anode corrosion, and passivation). In zinc battery systems, various zinc salts, including Zn(ClO₄)₂, ZnSO₄⋅7H₂O, ZnCl₂, and Zn(OTf)₂, have been widely used in aqueous or non-aqueous electrolytes. The low-cost Zn(BF₄)₂⋅xH₂O is rarely used in ZIBs, although it contains a key component for forming a favorable solid electrolyte interphase (SEI). This is because aqueous electrolytes of inorganic zinc salts based on Zn(BF₄)₂⋅xH₂O exhibit high acidity at pH < 1, resulting in severe corrosion of metallic zinc electrodes and thermal spontaneous HER. At the same time, the uneven and arbitrarily aggregated SEI leads to the uneven distribution of Zn²⁺ ion flux and electric field, resulting in rampant dendrite growth. The key to solving these problems is thus to regulate the interfacial chemistry and solvation structure through electrolyte engineering and construct a uniform and stable SEI. A highly efficient SEI can simultaneously achieve high stability and high utilization of zinc anodes during cycling, which is essential for ultimately building a rechargeable ZIB with high energy density and reliability.

In our first work, we employed a hydrophilic organic solvent, vinylene carbonate (VC) and hydrated Zn(BF₄)₂⋅4H₂O salt for a hydrous organic electrolyte, denoted as ZnBF-VC. With the ZnBF-VC electrolyte, the VC molecules preferentially adsorb on the Zn surface to block water molecules and zinc metal. At the same time, hydrophilic VC molecules can combine with the Zn²⁺ and bound H₂O of Zn(BF₄)₂⋅4H₂O to form a new Zn²⁺-solvation sheath of [Zn(H₂O)₂(VC)₄]²⁺. This not only increases the transfer number and electrochemical stability window (ESW) of Zn²⁺ but also promotes formation of an organic/inorganic-hybrid SEI rich in ZnF₂ and ZnCO₃. As a result, a dense, dendrite-free, and corrosion-free Zn electrodeposition is achieved, protecting the electrode and inhibiting further decomposition of the electrolyte. The ZnǀǀZn cells assembled by the organic electrolyte were cycled at 0.5 mA cm^-2 for more than 2,200 h, and the ZnǀǀCu cells maintained an excellent Coulombic efficiency (CE) of ~99.7% at 550 cycles at 1 mA cm^-2. Additionally, the ZnBF-VC electrolyte reduced the dissolution of MnHCF, maintaining a high retention rate of 85.3% over 1,300 cycles at 0.4 A g^-1.

Considering the high utilization rate, excellent adjustability, and strong electrochemical stability of eutectic electrolytes, we designed a hybrid eutectic electrolyte composed of acetamide, Zn(BF₄)₂⋅4.4H₂O, and water (1ace-1H₂O). Acetamide enters the solvation sheath of Zn²⁺ to form a structure of Zn(H₂O)₃(acetamide)(BF₄)₂ with reduced water content, which is conducive to inhibiting the HER, widening the electrochemical window to 2.1 V, and forming an organic/inorganic SEI rich in ZnCO₃/ZnF₂. Benefiting from the above advantages, the Zn||Zn cells cycled more than 1,300 hours at 1 mA cm^-2, and the Zn||Cu operated over 1,800 cycles with an average Coulomb efficiency of ~99.8%, demonstrating high dendrite-free performance and reversibility. Notably, the distinct amide group (-C=O, -NH₂) offers sufficient sites for interaction with water molecules to create hydrogen bond donors and acceptors, obliterating the initial network of hydrogen bonds between water molecules and drastically lowering the electrolyte's freezing point. As a result, the Zn||PANI full cell can operate in a broad temperature range of 60 ℃ to –40 ℃, and the capacity at –40 ℃ reaches ~54.2% of the capacity at room temperature.

In addition, most traditional liquid electrolytes may have leakage and corrosion problems during cycling and are accompanied by irreversible by-products, hindering their practical application. Because of their high safety and good mechanical stability, gel electrolytes can effectively avoid the inherent defects of liquid electrolytes. Here, we developed an asymmetric gel electrolyte composed of a PVDF-based organic electrolyte and a polyacrylamide (PAM) hydrogel electrolyte (HGE) to meet the requirements of the cathode and anode at the same time. The PVDF-based gel electrolyte migrates toward the zinc anode, enabling highly reversible galvanizing/stripping without dendrites and corrosion, and the PAM HGE performs H⁺ and Zn²⁺ insertion/extraction on the cathode side with high performance. Hence, Zn||Zn cells using organic solid electrolyte at a current density of 0.5 mA cm^-2 can cycle stably for 1,000 hours, while Zn||Cu cells using organic solid electrolyte still show significantly improved zinc plating/stripping CE of ~99.7% after 1,000 cycles. Additionally, the Zn||MnO₂ full cells using this asymmetric gel electrolyte continue demonstrating outstanding cycle stability, with a capacity retention rate of ~82.3%, after 1,500 cycles. Furthermore, these asymmetric electrolyte-based flexible-pouch ZIBs demonstrated steady specific capacity under a variety of folding conditions.

In summary, this thesis meticulously designs a variety of novel electrolyte types based on Zn(BF₄)₂⋅xH₂O to enhance the stability and durability of ZIBs. Additionally, we conducted a thorough study to better understand the underlying mechanisms. These findings are expected to have a significant impact on promoting the development and commercialization of ZIBs.

Date of Award	8 May 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Juan Antonio ZAPIEN (Supervisor)