A new frontier in energy storage is emerging with the development of long-life aqueous zinc batteries, largely attributed to the innovative use of dual-salt electrolytes. This breakthrough addresses critical limitations that have historically hampered the widespread adoption of zinc-ion battery technology, paving the way for safer, more sustainable, and cost-effective energy solutions, particularly for large-scale applications like grid storage.
The Promise and Problems of Aqueous Zinc Batteries
Aqueous zinc-ion batteries (AZIBs) have long been considered a promising alternative to lithium-ion batteries due to their inherent safety, low cost, and environmental friendliness. Zinc is abundant, non-toxic, and stable in water, making it an ideal candidate for large-scale energy storage systems. However, their practical application has been severely limited by several key challenges:
- Dendrite Formation: During repeated charging and discharging cycles, zinc metal tends to deposit unevenly on the anode, forming needle-like structures called dendrites. These dendrites can pierce the battery separator, leading to short circuits, reduced cycle life, and safety concerns.
- Corrosion and Side Reactions: The water-based electrolyte can lead to undesirable side reactions, including hydrogen evolution (water dissociation) and corrosion of the zinc anode. These reactions consume the electrolyte, generate insulating byproducts, and further degrade battery performance and capacity.
- Limited Cycle Life: The combined effect of dendrite growth, corrosion, and other parasitic reactions results in rapid capacity decay and a significantly shortened lifespan for AZIBs compared to their lithium-ion counterparts.
Overcoming these obstacles is crucial for unlocking the full potential of aqueous zinc batteries for grid storage and other applications where safety and cost are paramount.
The Dual-Salt Electrolyte Solution
Recent advancements have demonstrated that specially designed dual-salt electrolytes can effectively mitigate these issues, significantly extending the lifespan and improving the performance of aqueous zinc batteries. This innovative approach centers on manipulating the solvation structure of zinc ions and the overall electrolyte environment.
How Dual-Salt Electrolytes Work
A dual-salt electrolyte incorporates two different salts into the aqueous solution, often at high concentrations, to create a more stable and efficient operating environment for the zinc anode. The key mechanisms include:
- Suppressing Water Activity: In highly concentrated dual-salt electrolytes, the water molecules are preferentially coordinated with the salt ions, leading to “low water activity.” This reduced availability of free water molecules effectively suppresses undesirable side reactions like hydrogen evolution and corrosion, which are typically triggered by water dissociation.
- Regulating Zinc Ion Solvation: The presence of a second salt, particularly with specific anions, can alter how zinc ions interact with the electrolyte. For instance, some studies show that anions from the co-salt can enter the inner solvation structure of Zn²⁺, accelerating the desolvation kinetics and promoting more uniform zinc deposition. This controlled deposition is critical for preventing dendrite formation.
- Forming Stable Solid Electrolyte Interphases (SEIs): Dual-salt, highly concentrated electrolytes can facilitate the formation of a stable, anion-derived solid electrolyte interphase (SEI) layer on the zinc anode surface. This protective layer acts as a barrier, shielding the zinc from direct contact with the electrolyte and further inhibiting corrosion and dendrite growth, leading to enhanced cycling stability and Coulombic efficiency.
- Improving Ion Transport: Optimized dual-salt electrolytes can also improve the ionic conductivity and facilitate faster and more reversible ion transport, which is essential for good battery performance, especially at high current densities.
Examples of Dual-Salt Electrolyte Success
Various research efforts have highlighted the effectiveness of dual-salt electrolytes:
- One study demonstrated a dual-salt highly-concentrated electrolyte (15 m ZnCl₂ + 10 m NH₄NH₂SO₃) that achieved a record-long cycling life of 2200 hours in a symmetric zinc cell, showing almost 100% capacity retention after 4000 cycles in a full cell.
- Another design using a dual salt/dual solvent electrolyte (Zn(BF₄)₂/Zn(Ac)₂ in water/TEGDME) achieved an impressive zinc anode reversibility with an average Coulombic efficiency of 99.80% for 150 cycles at an ultrahigh depth of discharge of 60%. This system utilizes an “inner co-salt and outer co-solvent” synergistic effect to inhibit water activity and accelerate desolvation kinetics.
- Research utilizing a highly concentrated salt electrolyte with dual salts (1 m Zn(OTf)₂ + 20 m LiTFSI) reported excellent capacity retention of 92% after 300 cycles with an average Coulombic efficiency of 99.62%, significantly outperforming low concentration electrolytes.
The Impact and Future of Long-Life Aqueous Zinc Batteries
The development of long-life aqueous zinc batteries, powered by advanced dual-salt electrolytes, represents a significant stride toward a more sustainable energy future.
Key Advantages
- Enhanced Safety: Aqueous electrolytes eliminate the flammability risks associated with organic electrolytes in lithium-ion batteries.
- Lower Cost: Zinc is significantly more abundant and cheaper than lithium, translating to lower manufacturing costs for batteries.
- Environmental Friendliness: Zinc is non-toxic and environmentally benign, making these batteries a greener choice.
- Extended Lifespan: With the suppression of dendrites and corrosion, these batteries can achieve thousands of charge-discharge cycles, making them viable for long-term energy storage.
Promising Applications
These improved aqueous zinc batteries are ideally suited for:
- Grid-Scale Energy Storage: Their safety, low cost, and long cycle life make them excellent candidates for storing renewable energy from solar and wind farms, helping to stabilize the electrical grid.
- Stationary Storage: For homes and businesses seeking reliable backup power or off-grid solutions.
- Developing Regions: Their affordability and readily available materials can make energy storage more accessible in developing countries.
While lithium-ion batteries remain the preferred choice for mobile applications requiring high energy density and compact size, long-life aqueous zinc batteries present a compelling alternative for large-scale, stationary energy storage. This ongoing research in electrolyte engineering and anode protection is crucial for fully realizing the potential of this robust and sustainable battery technology.
