Dual-ion batteries represent an emerging energy storage technology that leverages the simultaneous intercalation of both anions and cations into their respective electrodes. Unlike conventional batteries that rely on single-ion shuttles, these systems require electrolyte formulations capable of facilitating stable and efficient transport of multiple ionic species. The electrolyte must maintain electrochemical stability across wide voltage windows while minimizing decomposition and gas evolution. This article examines the key components and considerations in designing electrolytes for dual-ion batteries, focusing on solvent selection, ionic liquid integration, additive engineering, and operational challenges.

Organic solvents form the foundation of dual-ion battery electrolytes due to their ability to dissolve various salts while providing adequate ionic conductivity. Carbonate-based solvents such as ethylene carbonate and dimethyl carbonate are commonly used because of their wide electrochemical stability windows and compatibility with graphite-based cathodes. These solvents typically operate effectively within a voltage range of 3.0 to 5.0 volts, which aligns with the requirements for anion intercalation at the cathode. Ether-based solvents like 1,2-dimethoxyethane offer lower viscosity and improved ion mobility but often exhibit narrower stability windows, limiting their use in high-voltage applications. The choice of solvent significantly impacts the solvation structure of both anions and cations, influencing their transport numbers and intercalation kinetics.

Ionic liquids have gained attention as alternative electrolytes due to their non-flammability, negligible vapor pressure, and wide thermal stability ranges. Imidazolium and pyrrolidinium-based ionic liquids with bis(trifluoromethanesulfonyl)imide anions demonstrate particularly high anodic stability, making them suitable for dual-ion systems. These liquids provide a stable medium for both small cations like lithium and bulky anions such as hexafluorophosphate or tetrafluoroborate. However, their high viscosity often results in lower ionic conductivity compared to organic solvents, necessitating optimization of operating temperatures or blending with conventional solvents. The concentration of ionic liquids in these blends must be carefully controlled to balance viscosity and electrochemical performance.

Additives play a critical role in enhancing electrolyte functionality by addressing specific challenges in dual-ion systems. Fluoroethylene carbonate is widely incorporated to improve solid-electrolyte interphase formation on anode surfaces, preventing excessive electrolyte reduction. For cathode protection, additives like lithium difluoro(oxalato)borate stabilize the electrode-electrolyte interface during anion intercalation. Specialized additives also mitigate gas generation, a common issue in dual-ion batteries caused by electrolyte decomposition at high voltages. Tris(trimethylsilyl) phosphite has shown effectiveness in scavenging reactive species that lead to gas formation while simultaneously improving cycle life.

Salt concentration represents another critical parameter in electrolyte formulation. Conventional lithium-ion batteries typically use salt concentrations around 1 molar, but dual-ion systems often benefit from higher concentrations ranging from 3 to 5 molar. These concentrated electrolytes reduce solvent activity, minimizing parasitic reactions at high voltages while promoting the formation of anion-rich solvation structures. However, excessively high concentrations increase viscosity and cost, requiring careful optimization. Mixed-salt systems incorporating lithium and sodium salts have demonstrated improved ion transport by altering the solvation environment for both cations and anions.

The electrochemical stability window of the electrolyte must exceed the operational voltage range of the battery to prevent decomposition. For dual-ion batteries with graphite cathodes, electrolytes must remain stable up to at least 5 volts versus lithium to accommodate anion intercalation. Linear carbonates tend to oxidize at lower potentials, making them unsuitable unless blended with more stable components. Sulfone-based solvents like ethyl methyl sulfone provide superior anodic stability but often require heating due to high melting points. The stability window is also influenced by salt selection, with lithium bis(oxalato)borate offering better high-voltage performance than traditional lithium hexafluorophosphate.

Compatibility with electrode materials extends beyond electrochemical stability to include interfacial reactions. Aluminum current collectors commonly used in dual-ion battery cathodes are susceptible to corrosion in certain ionic liquids and at elevated voltages. Incorporating corrosion inhibitors or using alternative collector materials becomes necessary in these cases. At the anode side, electrolytes must form stable interphases with various materials including graphite, hard carbon, or lithium metal if present. The interphase composition and morphology directly affect cation transport efficiency and cycling stability.

Electrolyte decomposition poses a persistent challenge in dual-ion batteries due to the high operating voltages required for anion intercalation. Decomposition products can accumulate at electrode surfaces, increasing impedance and reducing capacity over time. Continuous electrolyte consumption also raises safety concerns and shortens battery lifespan. Strategies to mitigate decomposition include using purified solvents with minimal water content, incorporating sacrificial additives that form protective layers, and operating within carefully controlled voltage limits. Gas generation from electrolyte breakdown presents additional complications, particularly in sealed battery systems where pressure buildup can occur. In-situ gas analysis techniques have identified carbon dioxide and ethylene as common byproducts of carbonate solvent decomposition at high voltages.

Temperature sensitivity affects electrolyte performance across the operational range of dual-ion batteries. Low temperatures exacerbate viscosity issues, particularly in ionic liquid-based systems, while high temperatures accelerate decomposition reactions. Thermal management becomes essential for maintaining optimal ionic conductivity and preventing thermal runaway. Some advanced formulations incorporate temperature-responsive components that automatically adjust viscosity or reactivity based on environmental conditions.

Future developments in dual-ion battery electrolytes will likely focus on improving sustainability without compromising performance. Bio-derived solvents and less toxic ionic liquids are under investigation as potential replacements for conventional materials. Solid-state electrolytes could address many stability and safety concerns but currently face challenges in achieving sufficient dual-ion conductivity. Computational modeling approaches are increasingly employed to screen potential electrolyte components and predict their behavior under operating conditions, accelerating the development of optimized formulations.

The complexity of dual-ion battery electrolytes arises from the need to satisfy multiple competing requirements simultaneously. A successful formulation must enable rapid transport of chemically dissimilar ions, maintain stability across extreme potentials, and resist degradation over hundreds of cycles. While significant progress has been made in understanding these systems, further research is needed to overcome existing limitations and unlock the full potential of dual-ion battery technology. The interplay between solvent chemistry, salt selection, and additive engineering will continue to dictate advancements in this field, with each component requiring precise optimization for specific battery configurations and applications.