Adding a small amount of aluminum chloride (AlCl₃) can improve PBA cathode cycling stability by chemically consuming residual water and helping form a protective surface layer. This reduces the insertion of bulky hydrated sodium species, limits lattice deformation during Na⁺ cycling, and shields the PBA/electrolyte interface from HF-related corrosion. The result is slower structural and interfacial degradation, leading to better capacity retention over extended cycling.
Core takeaway: AlCl₃ acts as both a water-scavenging Lewis acid and an interfacial stabilizer. By reducing coordinated-water-driven lattice strain and promoting an aluminum-containing passivation layer, it protects PBA cathodes from the coupled structural and electrolyte-related failure mechanisms that cause capacity fade.
Why PBA Cathodes Lose Stability During Cycling
Residual water occupies the PBA framework
Prussian Blue Analogues contain open three-dimensional channels that enable rapid sodium-ion transport. However, synthesis-related vacancies and interstitial sites can retain coordinated or lattice water.
This water is not electrochemically harmless. It can interact with sodium ions to form hydrated species such as Na(OH₂)⁺, increasing the effective size of the ion entering or leaving the PBA framework.
Hydrated sodium ions increase mechanical stress
The insertion and extraction of larger hydrated sodium species can cause greater local expansion and contraction than bare Na⁺ transport. Repeated deformation places stress on the cyano-bridged lattice and can progressively damage the electrode structure.
This degradation may appear as capacity loss, poorer coulombic efficiency, and increased polarization during long-term cycling.
Water can worsen electrolyte-side reactions
Residual water can also promote electrolyte decomposition and contribute to the formation of corrosive species. In electrolytes containing fluorinated salts such as NaPF₆, these reactions can generate hydrofluoric acid (HF).
HF attack can corrode the PBA surface, destabilize transition-metal sites, and accelerate interfacial degradation.
How AlCl₃ Addresses These Failure Mechanisms
AlCl₃ scavenges residual coordinated water
AlCl₃ is a strong Lewis acid, meaning that aluminum can accept electron density from oxygen-containing species. In the electrolyte environment, it can interact with water associated with hydrated sodium complexes and chemically reduce the amount of mobile or reactive water.
The practical effect is a lower tendency for hydrated sodium species to participate in repeated intercalation and deintercalation.
Reduced hydration limits lattice deformation
With less coordinated water available, sodium-ion transfer is less likely to impose the same degree of steric and mechanical stress on the PBA framework. This helps preserve the open lattice and its sodium-ion transport pathways.
The additive therefore addresses a root cause of capacity fade rather than merely improving the initial electrode-electrolyte contact.
An aluminum-containing passivation layer forms
The primary mechanism described for AlCl₃ addition includes formation of a hydrated aluminum oxide layer, represented as Al₂O₃·3H₂O, on the PBA surface.
This layer acts as a protective barrier between the active cathode and the electrolyte. It can suppress direct chemical attack while still permitting sodium-ion transport if the film remains sufficiently thin and uniform.
The surface is better protected from HF
By shielding the PBA surface, the passivation layer reduces exposure of the active material to HF and other corrosive electrolyte-decomposition products. This helps preserve the transition-metal–cyanide framework and limits progressive interfacial damage.
The overall benefit is therefore coupled: less water-driven lattice strain plus less electrolyte-driven surface corrosion.
Why the Effect Improves Capacity Retention
Structural and chemical protection reinforce each other
PBA degradation is rarely caused by a single process. Hydration, lattice deformation, electrolyte decomposition, and surface corrosion can accelerate one another.
AlCl₃ can interrupt this feedback cycle by reducing the water-related mechanical burden while simultaneously passivating the cathode surface.
The open framework remains more usable
PBA cathodes rely on open channels for fast sodium-ion movement. Preserving these channels and the surrounding lattice helps maintain access to redox-active sites during repeated cycling.
This supports more stable reversible capacity, particularly during extended charge-discharge testing.
The additive works at the interface
The improvement is not simply a bulk-material effect. AlCl₃ primarily modifies the electrolyte–cathode interface, where hydrated ions, solvent decomposition products, and corrosive species interact with the PBA surface.
That is why electrolyte formulation and additive concentration must be evaluated together with cathode composition and cell assembly conditions.
How AlCl₃ Fits With Other PBA Stabilization Strategies
Electrolyte-film-forming additives address a related problem
Fluoroethylene carbonate, or FEC, can promote formation of an inorganic, NaF-rich passivating film. This is a different but complementary mechanism from AlCl₃’s water-scavenging and aluminum-containing surface protection.
The two approaches should not be assumed to be interchangeable. Their compatibility depends on electrolyte composition, additive concentration, voltage range, and the specific PBA chemistry.
Low-defect materials reduce the water reservoir
Vacancies are important because they can trap coordination water and weaken structural stability. Synthesizing sodium-rich, low-defect PBA materials can therefore reduce the amount of water that AlCl₃ must manage.
An additive is most effective when it complements, rather than compensates for, poor control of the underlying PBA structure.
Zero-strain compositions reduce mechanical demand
Introducing electrochemically inert elements such as nickel or zinc can reduce volume change in selected PBA frameworks. This lowers the mechanical stress caused by sodium-ion cycling.
Combining a mechanically stable PBA with a controlled electrolyte interface may provide greater durability than relying on either strategy alone.
Understanding the Trade-offs
Excessive AlCl₃ can be counterproductive
A Lewis-acid additive is chemically reactive, so its concentration must be optimized. Too much additive may increase electrolyte viscosity, alter ion coordination and desolvation, or produce an overly resistive surface film.
The reported example of approximately 1 wt% should be treated as a formulation starting point, not a universal optimum.
Passivation must remain ionically accessible
A protective layer is beneficial only if it suppresses parasitic reactions without blocking Na⁺ transport. An excessively thick or poorly controlled aluminum-containing film could increase interfacial impedance and reduce rate capability.
Electrochemical impedance, polarization, and rate testing are therefore important alongside capacity-retention measurements.
The mechanism requires direct verification
Improved cycling alone does not prove that water scavenging or Al₂O₃·3H₂O formation is responsible. Water content, surface chemistry, impedance evolution, and post-cycling morphology should be examined where possible.
This is particularly important because electrolyte salt, solvent, electrode loading, and cell sealing can independently influence apparent cycling stability.
Laboratory reproducibility matters
Small differences in residual moisture, electrolyte volume, electrode compaction, and crimp-seal quality can obscure the effect of a chemical additive. Controlled cell assembly and reliable battery testing systems are essential for separating the additive’s interfacial contribution from cell-to-cell variation.
How to Apply This to Your Testing
AlCl₃ should be evaluated as part of a controlled electrolyte–cathode interface study rather than as an isolated formulation change.
- If your primary focus is long-term capacity retention: Compare a baseline electrolyte with an approximately 1 wt% AlCl₃ formulation under identical loading, voltage limits, formation protocol, and cycling conditions.
- If your primary focus is understanding the mechanism: Measure electrolyte water content and use surface or structural characterization to test for reduced hydration effects and formation of an aluminum-containing passivation layer.
- If your primary focus is high-rate performance: Check impedance and rate capability, because an excessively resistive passivation film could improve durability while limiting sodium-ion kinetics.
- If your primary focus is reproducible laboratory data: Use controlled-environment cell assembly, consistent electrode compaction, and calibrated battery testers to isolate the additive’s effect from manufacturing variability.
The most reliable interpretation is that AlCl₃ improves PBA cycling stability by controlling both hydrated-ion-induced lattice strain and corrosive cathode–electrolyte reactions.
Summary Table:
| Mechanism | Effect on PBA Cathode |
|---|---|
| Water scavenging | Reduces hydrated Na+ insertion, minimizing lattice strain |
| Al-containing passivation layer | Shields cathode from HF corrosion, preserving structure |
| Lower water content | Limits electrolyte decomposition, enhancing interfacial stability |
| Protective film | Maintains Na+ transport while blocking parasitic reactions |
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