In conventional batteries, energy storage typically relies on the movement of a single ionic species, either cations or anions, between electrodes. However, a newer class of energy storage systems, known as dual-ion batteries (DIBs), leverages the simultaneous participation of both cations and anions to enhance charge storage capacity. Among these, zinc-graphite dual-ion batteries have emerged as a promising candidate due to the natural abundance of zinc, its low redox potential, and the high theoretical capacity of graphite for anion intercalation. Unlike aluminum-graphite systems, which rely on Al3+ intercalation, zinc-graphite DIBs utilize Zn2+ plating/stripping at the anode and anion intercalation at the graphite cathode, offering distinct electrochemical behavior and optimization pathways.

The operational mechanism of a zinc-graphite dual-ion battery involves two key processes occurring in tandem during charge and discharge. During charging, Zn2+ cations from the electrolyte are reduced and deposited onto the zinc anode, while anions such as SO42− or ClO4− are intercalated into the graphite cathode. Upon discharge, the reverse occurs: Zn2+ dissolves back into the electrolyte, and anions de-intercalate from the graphite layers. This dual-ion mechanism allows for higher energy densities compared to conventional single-ion systems, as both electrodes contribute actively to charge storage. The voltage profile of such a battery typically exhibits distinct plateaus corresponding to the redox reactions at each electrode. The zinc anode operates at a potential near -0.76 V vs. SHE (standard hydrogen electrode), while the graphite cathode’s intercalation potential varies depending on the anion species, often ranging between 4.0–5.0 V vs. Li+/Li for sulfate-based electrolytes. The resulting full cell voltage is the difference between these two potentials, typically yielding 1.5–2.5 V depending on electrolyte composition and electrode materials.

A critical factor influencing the performance of zinc-graphite DIBs is the choice of electrolyte. Unlike lithium-ion systems, where organic solvents dominate, aqueous electrolytes are often preferred for zinc-based batteries due to their high ionic conductivity, low cost, and non-flammability. However, aqueous systems present challenges such as hydrogen evolution at the anode and limited electrochemical stability windows. Recent research has focused on optimizing electrolyte formulations to mitigate these issues. For instance, highly concentrated "water-in-salt" electrolytes, such as 30 mol/kg ZnCl2 or Zn(CF3SO3)2, have been shown to suppress water activity, thereby widening the electrochemical stability window and reducing parasitic reactions. Another approach involves hybrid aqueous/non-aqueous electrolytes, where additives like dimethyl sulfoxide (DMSO) or acetonitrile are introduced to modify solvation structures and improve Zn2+ deposition uniformity. The anion species in the electrolyte also play a crucial role in determining the graphite cathode’s intercalation behavior. Sulfate (SO42−) and perchlorate (ClO4−) anions are commonly studied due to their favorable intercalation potentials and reversible kinetics. Recent work has demonstrated that larger anions like bis(trifluoromethanesulfonyl)imide (TFSI−) can enable higher capacities but may suffer from slower diffusion kinetics due to their size.

Voltage profiles in zinc-graphite DIBs exhibit characteristic features that reflect the underlying electrochemical processes. During charging, the cell voltage rises sharply as anions begin intercalating into the graphite cathode, followed by a plateau corresponding to the bulk intercalation process. Simultaneously, the zinc anode undergoes deposition, which contributes to the overall voltage profile. The discharge curve mirrors this behavior, with a gradual voltage drop as anions de-intercalate and zinc dissolves. Unlike lithium-ion batteries, where phase transitions in electrode materials can create multiple plateaus, zinc-graphite DIBs often show smoother profiles due to the continuous nature of anion intercalation in graphite. However, the exact shape of the voltage curve can vary significantly with electrolyte composition. For example, electrolytes with high Zn2+ transference numbers may exhibit more pronounced polarization at the anode, leading to a steeper voltage rise during charging. Similarly, the choice of anion affects the graphite’s intercalation overpotentials, which can influence the flatness of the voltage plateau.

Research into electrolyte optimization has revealed several key trends for improving zinc-graphite DIB performance. One major focus has been on reducing dendrite formation at the zinc anode, which can lead to short circuits and capacity fade. Strategies such as electrolyte additives (e.g., polyethylene glycol or boric acid) have been shown to promote uniform zinc deposition by modifying the electrode-electrolyte interface. Another area of investigation is the suppression of hydrogen evolution, which competes with zinc deposition in aqueous systems. By adjusting pH levels or incorporating hydrophobic ionic liquids, researchers have achieved higher coulombic efficiencies for zinc plating/stripping. On the cathode side, efforts have centered on enhancing anion intercalation kinetics and stability. For instance, the use of expanded graphite or graphene-based materials can mitigate the structural stress caused by large anion intercalation, thereby improving cycle life. Additionally, pre-treatment of graphite with oxidizing agents has been found to increase the density of active sites for anion storage.

The interplay between electrolyte chemistry and electrode materials also impacts long-term cycling stability. In aqueous electrolytes, graphite cathodes can suffer from oxidative degradation at high potentials, leading to capacity fade over time. To address this, researchers have explored the use of protective coatings or alternative carbon materials with higher oxidation resistance. Similarly, zinc anodes may experience shape change or passivation due to uneven dissolution and re-deposition during cycling. Electrolyte additives that form stable solid-electrolyte interphases (SEIs) on zinc have shown promise in mitigating these issues. Recent studies have reported cycling lifetimes exceeding 500 cycles with capacity retention above 80% for optimized zinc-graphite DIBs, though further improvements are needed to compete with commercial lithium-ion systems.

Another important consideration is the rate capability of zinc-graphite DIBs, which is influenced by ion transport kinetics in both the electrolyte and electrodes. While zinc deposition/dissolution is generally fast, anion intercalation into graphite can be rate-limiting due to the large size of the anions and the need for solvation shell rearrangement. Electrolytes with low viscosity and high ionic conductivity, such as those based on ZnSO4 or Zn(TFSI)2, have demonstrated improved rate performance. Additionally, electrode engineering strategies like porous graphite structures or reduced graphene oxide composites can enhance mass transport and reduce diffusion limitations.

Future research directions for zinc-graphite dual-ion batteries are likely to focus on further optimizing the electrolyte-electrode interface, exploring new anion hosts beyond graphite, and scaling up cell designs for practical applications. While significant progress has been made in understanding the fundamental electrochemistry of these systems, challenges remain in achieving energy densities and cycle lives comparable to state-of-the-art lithium-ion batteries. Nevertheless, the unique advantages of zinc-graphite DIBs—including low cost, safety, and environmental friendliness—make them a compelling option for grid storage and other large-scale applications where energy density is not the primary constraint.

In summary, zinc-graphite dual-ion batteries represent a promising alternative to conventional battery chemistries by leveraging the simultaneous storage of cations and anions. Their voltage profiles and performance characteristics are heavily influenced by electrolyte composition, with ongoing research focused on improving stability, kinetics, and cycling longevity. While distinct from aluminum-graphite systems in their operational mechanisms, they share the broader potential of dual-ion batteries to push the boundaries of energy storage technology.