Optimal Electrolyte Compositions for Aqueous Zinc-Ion Batteries with MnO₂ Cathodes

Enhancing Performance, Stability, and Longevity through Superior Electrolyte Design

Key Takeaways

Mildly acidic ZnSO₄-based electrolytes with Mn²⁺ additives provide a balanced environment for Zn²⁺ ion transport and electrode stability.
Hydrogel and densified electrolytes incorporating additives like Bi₂O₃ and SrTiO₃ significantly improve ionic conductivity and suppress side reactions.
Innovative electrolyte formulations such as Zinc Triflate and solid polymer electrolytes offer enhanced safety and extended cycle life.

Introduction

Aqueous zinc-ion batteries (ZIBs) utilizing manganese dioxide (MnO₂) cathodes have garnered significant attention in the energy storage sector due to their inherent safety, low cost, and environmental sustainability. Central to the performance and longevity of these batteries is the choice of electrolyte, which plays a crucial role in facilitating ion transport, maintaining electrode stability, and mitigating side reactions. This comprehensive analysis delves into the most effective electrolyte compositions for aqueous Zn-MnO₂ batteries, integrating consensus insights from recent studies and advancements in the field.

Overview of Aqueous Zinc-Ion Batteries with MnO₂ Cathodes

Zinc-ion batteries operate on the reversible intercalation and deintercalation of Zn²⁺ ions between the anode and cathode during charging and discharging cycles. MnO₂ serves as an attractive cathode material due to its high theoretical capacity, abundant availability, and favorable redox properties. However, challenges such as MnO₂ dissolution, Zn dendrite formation, and side reactions necessitate the optimization of electrolyte formulations to enhance battery performance and durability.

Detailed Analysis of Effective Electrolytes

1. Mildly Acidic ZnSO₄-Based Electrolytes

The foundation of many high-performance aqueous ZIBs lies in mildly acidic zinc sulfate (ZnSO₄) solutions. Typically maintained at a pH around 4, these electrolytes provide an optimal environment for Zn²⁺ ion transport while maintaining the stability of both the Zn anode and MnO₂ cathode.

Composition: Aqueous ZnSO₄ solution, often in a concentration range of 1–2 M.
Advantages:
- Facilitates reversible insertion and extraction of Zn²⁺ ions in the MnO₂ cathode.
- Neutral to mildly acidic pH minimizes corrosion and enhances electrode stability.
- Cost-effective and environmentally benign, making it suitable for large-scale applications.
Enhancements:
- Incorporation of Mn²⁺ salts (e.g., MnSO₄) to suppress MnO₂ dissolution and improve cyclability.
- Addition of stabilizing agents like Bi₂O₃ to prevent the formation of inactive phases such as ZnMn₂O₄.

2. ZnSO₄ Mixed with MnSO₄ Additives

Enhancing the basic ZnSO₄ electrolyte with manganese sulfate (MnSO₄) additives has proven effective in addressing specific challenges associated with MnO₂ cathodes.

Composition: 2 M ZnSO₄ combined with 0.1–0.5 M MnSO₄.
Advantages:
- Mn²⁺ ions act as a supporting additive, mitigating the dissolution of MnO₂ during cycling.
- Improves the reversibility of MnO₂ by replenishing lost Mn²⁺ ions, thereby enhancing capacity retention.
- Reduces the formation of inactive ZnMn₂O₄ phases, which are detrimental to battery performance.
Limitations:
- While effective in suppressing Mn dissolution, it does not fully prevent Zn dendrite formation, which can impact cyclability.

3. Bismuth Oxide (Bi₂O₃)-Modified Electrolytes

The introduction of Bi₂O₃ additives into the ZnSO₄ electrolyte system has shown significant improvements in electrode stability and cycle life.

Composition: ZnSO₄ electrolyte with Bi₂O₃ additives blended into the MnO₂ cathode.
Advantages:
- Minimizes the formation of inactive ZnMn₂O₄ by stabilizing the cathode structure.
- Prolongs the cycle life of the battery, ensuring better capacity retention over extended use.
- Maintains the structural integrity of the MnO₂ cathode during repeated charge-discharge cycles.

4. Hydrogel Electrolytes

Hydrogel-based electrolytes offer a versatile platform for enhancing the mechanical and electrochemical properties of aqueous ZIBs.

Composition: Hydrogels such as sodium polyacrylate/polyacrylamide (SPI/PAAM) or polyacrylamide-based gels, often supplemented with additives like graphene oxide and ethylene glycol.
Advantages:
- High ionic conductivity coupled with excellent mechanical flexibility, making them suitable for flexible battery applications.
- Retains water molecules, which ensures stable performance even under anti-freezing conditions.
- Enhances compressibility and durability, reducing the likelihood of mechanical degradation during cycling.
- Suppresses dendrite formation and hydrogen evolution, thereby improving cyclability and safety.

5. Zinc Triflate (Zn(CF₃SO₃)₂) Electrolytes

Zinc triflate-based electrolytes represent an advanced alternative to traditional ZnSO₄ solutions, offering enhanced stability and performance.

Composition: Aqueous solution of Zinc Triflate (Zn(CF₃SO₃)₂).
Advantages:
- Bulky anions in Zinc Triflate help stabilize the Zn electrode/electrolyte interface, reducing side reactions.
- Improves the reactivity and stability of both Zn anode and MnO₂ cathode compared to ZnSO₄-based electrolytes.
- Offers better overall battery performance, including higher capacity retention and longer cycle life.
Limitations:
- Potentially higher cost compared to conventional ZnSO₄ electrolytes.

6. Densified Electrolytes with SrTiO₃ Additives

Incorporating SrTiO₃ nanoparticles into the electrolyte formulation significantly enhances the performance metrics of aqueous Zn-MnO₂ batteries.

Composition: Conventional aqueous ZnSO₄ electrolyte with added perovskite SrTiO₃ powder.
Advantages:
- Reduces water molecule activity, thereby minimizing side reactions such as hydrogen evolution and corrosion.
- Improves the Zn²⁺ transference number, ensuring more efficient ion transport.
- Enables high specific capacity and homogeneous zinc deposition, which are critical for battery longevity.
- Promotes ultra-long cycling stability, maintaining high specific capacities even after 500 cycles.

7. Solid Polymer Electrolytes (SPEs)

Solid polymer electrolytes offer a non-liquid alternative that enhances the safety and stability of aqueous ZIBs.

Composition: Polymers such as Nafion ionomer or other solid-state materials.
Advantages:
- Non-flammable and inherently safer than liquid electrolytes, reducing the risk of leakage and thermal runaway.
- Reduces capacity decay during prolonged charge-discharge cycles, enhancing overall battery lifespan.
- Provides a stable interface between the electrodes, minimizing side reactions and improving cyclability.

8. Alkaline and Hybrid Electrolytes

While traditionally less favored for rechargeable systems, alkaline and hybrid electrolyte formulations still hold specific advantages for certain applications.

Composition: Alkaline electrolytes such as potassium hydroxide (KOH) solutions, often in concentrations of 1–2 M.
Advantages:
- High solubility of zinc salts leads to elevated ionic conductivity.
- Suitable for primary Zn-MnO₂ systems and applications requiring high-rate discharge capabilities.
Limitations:
- Prone to polarization and side reactions in rechargeable designs, limiting long-term cyclability.
- Less compatible with MnO₂ polymorphs used in rechargeable applications.
Hybrid Electrolytes:
- Combine acidic and mildly alkaline conditions to balance Zn²⁺ intercalation with proton participation.
- Enhance energy density and prevent early-stage capacity degradation caused by over-oxidation or phase transitions.

Comparison of Electrolytes

Electrolyte Type	Composition	Advantages	Limitations
ZnSO₄-Based	1–2 M ZnSO₄, pH ~4	Cost-effective, good Zn²⁺ transport, enhanced stability with Mn²⁺ additives	Potential MnO₂ dissolution without additives
ZnSO₄ + MnSO₄	2 M ZnSO₄ + 0.1–0.5 M MnSO₄	Suppresses Mn dissolution, improves reversibility, reduces inactive phases	Does not fully prevent Zn dendrite formation
Bi₂O₃-Modified	ZnSO₄ + Bi₂O₃ additive	Minimizes ZnMn₂O₄ formation, prolongs cycle life, maintains cathode stability	Added complexity in electrolyte preparation
Hydrogel	SPI/PAAM or polyacrylamide-based gels + additives	High ionic conductivity, mechanical flexibility, suppresses side reactions	Potentially higher cost, complex fabrication
Zinc Triflate	Zn(CF₃SO₃)₂ aqueous solution	Enhanced electrode stability, better performance than ZnSO₄	Higher cost compared to ZnSO₄
Densified Electrolytes	ZnSO₄ + SrTiO₃ powder	Reduces water activity, improves Zn²⁺ transference, ultra-long cycling	Requires precise additive incorporation
Solid Polymer	Nafion ionomer or similar polymers	Non-flammable, reduces capacity decay, stable electrode interface	Potentially higher material costs
Alkaline	1–2 M KOH	High ionic conductivity, suitable for high-rate discharge	Prone to side reactions, less suitable for rechargeable systems
Hybrid Acid–Alkaline	Combination of acidic and mildly alkaline solutions	Balances Zn²⁺ intercalation and proton participation, enhances energy density	Complex electrolyte management, potential for phase instability

Key Considerations in Electrolyte Selection

pH Balance: Maintaining a mildly acidic environment is critical for balancing Zn²⁺ ion transport and minimizing MnO₂ dissolution. The electrolyte's pH directly influences the electrochemical reactions at the cathode and anode.
Additive Integration: Incorporating additives such as Mn²⁺ salts, Bi₂O₃, or SrTiO₃ can significantly enhance electrolyte performance by stabilizing electrode interfaces and suppressing unwanted side reactions.
Ionic Conductivity: High ionic conductivity is essential for efficient charge transport. Hydrogels and densified electrolytes often offer superior ionic conductivity compared to traditional aqueous solutions.
Water Activity Management: Controlling water activity within the electrolyte prevents side reactions like hydrogen evolution and corrosion, thereby enhancing battery longevity.
Mechanical Stability: Electrolyte formulations, especially hydrogels, must provide sufficient mechanical stability to accommodate volume changes during cycling without compromising structural integrity.
Safety and Environmental Impact: Electrolytes should be non-toxic, environmentally benign, and safe for large-scale applications, particularly for grid-scale energy storage solutions.
Cost and Scalability: The chosen electrolyte should balance performance enhancements with cost-effectiveness and scalability to ensure practical applicability in commercial battery systems.

Conclusion

The optimization of electrolyte compositions is paramount in enhancing the performance, stability, and longevity of aqueous zinc-ion batteries utilizing MnO₂ cathodes. Mildly acidic ZnSO₄-based electrolytes, especially when augmented with Mn²⁺ and other stabilizing additives, provide a robust foundation for reliable Zn²⁺ ion transport and electrode integrity. Advanced formulations, including hydrogel and densified electrolytes with additives like Bi₂O₃ and SrTiO₃, further elevate the performance metrics by enhancing ionic conductivity and suppressing detrimental side reactions. Additionally, innovative electrolyte alternatives such as Zinc Triflate and solid polymer electrolytes offer promising pathways for achieving safer and longer-lasting battery systems. Ultimately, the strategic selection and engineering of electrolyte compositions, tailored to specific application requirements, are crucial for the advancement and commercialization of high-performance aqueous Zn-MnO₂ batteries.