What are the comparative roles of nickel, iron, and cobalt substitution in tuning the performance of manganese-based Prussian Blue Analogue cathodes for sodium-ion batteries?

Iron substitution generally favors capacity, nickel favors structural resilience and fast cycling, and cobalt provides a balance of conductivity, stability, and manganese redox activity. In manganese-based Prussian Blue Analogue (PBA) cathodes, partial substitution changes metal–cyanide bond strength, crystal-field effects, electronic structure, and lattice distortion. The best choice depends on whether the priority is maximizing reversible sodium storage, preserving the framework during repeated cycling, or improving kinetics.

The central design trade-off is straightforward: Fe preserves or adds redox-active capacity, Ni suppresses manganese-driven lattice distortion, and Co improves electronic transport while helping activate manganese reduction. Substitution level and site occupancy determine whether these benefits outweigh capacity dilution or added material cost.

Why Substitution Changes MnHCF Performance

The open PBA framework creates both opportunity and risk

Manganese-based PBAs contain a rigid three-dimensional framework with large channels for sodium-ion transport. This architecture supports rapid insertion and extraction, but sodium cycling can still generate local stress when manganese changes oxidation state.

The most important instability is associated with cooperative Jahn-Teller distortion during manganese redox reactions. Repeated distortion can broaden the lattice-parameter distribution, increase polarization, and accelerate capacity loss.

Metal substitution changes more than composition

Replacing part of the manganese or iron sublattice modifies the surrounding crystal field and the strength of metal–cyanide bonds. It can therefore change redox potentials, electronic conductivity, sodium-ion kinetics, phase-transition behavior, and the magnitude of lattice deformation.

Substitution should consequently be treated as a structure-and-electronic-design strategy, not simply as a way to add another redox element.

Iron Substitution: The Capacity-Oriented Strategy

Fe preserves active redox couples

Among the three substitutions, iron is the most effective for maximizing overall discharge capacity when it maintains or increases the number of electrochemically active redox centers. Both transition-metal sites can contribute to sodium storage when their redox couples are accessible.

This is why Fe-containing PBAs can approach the high theoretical capacity associated with two sodium-ion insertions per formula unit. For comparison, frameworks with only one electrochemically active transition-metal site have substantially lower theoretical capacity.

Fe supports high active-site utilization

Iron substitution can provide additional accessible redox reactions while retaining the open PBA structure. The resulting capacity advantage is especially relevant when the material is designed to use both the manganese and iron sublattices effectively.

However, nominal composition alone does not guarantee this result. Vacancies, coordinated water, oxidation state, particle morphology, and the voltage window all determine how much of the theoretical redox capacity becomes reversible capacity.

The main limitation is structural stress

Iron is not primarily a lattice-buffering substitution in MnHCF. If manganese remains strongly involved in redox, Jahn-Teller distortion and associated phase changes may still limit rate capability and long-term retention.

Fe substitution is therefore the strongest choice when capacity is the first constraint, but it may require additional structural stabilization when high-rate operation or very long cycle life is required.

Nickel Substitution: The Structural-Buffering Strategy

Ni suppresses manganese distortion

Nickel substitution is valuable because it can reduce the cooperative Jahn-Teller distortion associated with manganese redox, particularly manganese oxidation-state changes involving Jahn-Teller-active configurations. This makes the framework less susceptible to repeated local deformation.

Nickel can function as a relatively stable structural component rather than contributing the same level of reversible capacity as a fully redox-active Mn or Fe site. Its benefit is therefore often expressed through improved durability rather than through a direct capacity increase.

Ni reduces lattice variation during sodium cycling

A key effect of Ni substitution is a reduction in the variance of the lattice constant, often described using Δa. Smaller lattice-parameter fluctuations indicate that sodium insertion and extraction impose less mechanical strain on the framework.

This buffering can reduce defect generation, limit particle cracking, and help preserve the electrode/electrolyte interface. A low-strain framework also reduces the likelihood that interphase damage will distort measurements of the intrinsic cathode kinetics.

Ni improves rate capability and cycling stability

By stabilizing the framework, Ni substitution can maintain sodium-ion diffusion pathways and electrical contact over repeated cycling. The result is typically improved rate capability, lower degradation under repeated sodium insertion and extraction, and better capacity retention.

Reported Ni-doped MnFe-PBA systems illustrate this trade-off: substantial discharge capacity can be retained while achieving strong long-term cycling performance. The exact benefit remains dependent on defect concentration, substitution level, particle size, and electrode density.

The main limitation is capacity dilution

If Ni replaces a redox-active site without providing an equivalent reversible redox contribution, the material may lose some theoretical capacity. Excessive Ni can therefore improve structural stability at the expense of gravimetric energy storage.

Ni is best understood as a stability and kinetics modifier. It is most useful when manganese distortion, rapid capacity decay, or high-rate polarization is the dominant failure mode.

Cobalt Substitution: The Conductivity-and-Activation Strategy

Co improves electronic transport

Cobalt substitution can enhance electronic conductivity through changes in the electronic structure and metal–cyanide bonding network. Better electronic transport lowers charge-transfer limitations and can reduce polarization during rapid cycling.

This role differs from Ni. Ni primarily buffers structural changes, whereas Co more directly targets the electronic and kinetic limitations of the cathode.

Co helps activate manganese reduction

Co substitution can also promote or activate manganese reduction reactions. This can increase the fraction of manganese-related redox chemistry that is accessible within the selected operating voltage range.

The benefit is not simply additional nominal manganese content. Co changes the local chemical environment, which can alter redox energetics and make otherwise limited manganese reactions more reversible or kinetically accessible.

Co contributes to structural stability

Cobalt can inhibit harmful lattice deformation while improving conductivity. In this respect, it offers a compromise between the capacity-focused role of Fe and the distortion-suppression role of Ni.

Its effectiveness depends strongly on the substitution site and oxidation-state distribution. A formulation that improves conductivity but introduces unfavorable phase behavior may not deliver better practical performance.

The main limitation is cost and composition sensitivity

Cobalt is generally less attractive than manganese or iron from a cost and resource perspective. Excessive substitution can also reduce the fraction of other useful redox centers or produce undesirable changes in phase behavior.

Co is therefore most compelling when electronic conductivity, rate performance, and access to manganese redox are limiting the material.

Comparing the Three Substitutions

Capacity

Fe has the clearest capacity advantage because it helps preserve a framework with multiple redox-active transition-metal sites. Ni may reduce capacity if it replaces an active site, while Co can improve usable capacity indirectly by activating manganese reactions and lowering polarization.

The relevant metric is not only theoretical capacity. Practical capacity depends on reversible redox participation, defect chemistry, sodium occupancy, and the ability of the electrode to access those reactions at the applied current.

Rate capability

Ni improves rate performance mainly by reducing lattice distortion and preserving diffusion pathways during cycling. Co contributes through improved electronic conductivity and more accessible manganese redox.

Fe alone can support high capacity but may require structural or compositional stabilization to match the high-rate performance of a well-designed Ni- or Co-substituted material.

Cycling stability

Ni is generally the strongest choice for suppressing redox-induced lattice damage and maintaining a low-strain framework. Co also improves stability by limiting deformation and improving electronic transport.

Fe can provide durable performance when the framework is well controlled, but its primary advantage is active redox capacity rather than mechanical buffering.

Energy density

Fe is usually favored for maximizing gravimetric capacity. Ni can improve practical energy retention over long cycling, even if it lowers initial theoretical capacity, while Co may improve delivered energy by reducing polarization and increasing the usable fraction of manganese redox.

The optimal energy-density formulation therefore depends on whether the application values initial capacity, sustained capacity, or high-power discharge.

Understanding the Trade-offs

More substitution is not automatically better

Each substituted element changes several properties simultaneously. Increasing Ni may suppress distortion but dilute redox-active sites; increasing Co may improve conductivity but raise cost; increasing Fe may improve capacity while leaving manganese-related structural instability insufficiently controlled.

The useful design variable is therefore the substitution window, not the maximum possible dopant concentration.

Theoretical capacity can mislead

PBAs can theoretically accommodate up to two sodium ions per formula unit when both transition-metal sites are redox active. That limit should not be used as a direct prediction of assembled-cell capacity.

Framework vacancies, coordinated water, inactive sites, incomplete sodium access, side reactions, and voltage cutoffs can all reduce the reversible capacity measured in practice.

Defect chemistry must be controlled

PBA performance is highly sensitive to vacancies and water within the hexacyanoferrate framework. These features affect electronic connectivity, sodium diffusion, lattice strain, and the stability of the electrode/electrolyte interface.

A nominally optimized Ni, Fe, or Co composition can still perform poorly if synthesis produces nonuniform substitution or excessive defects.

Electrode processing can obscure material effects

Poorly mixed slurries, nonuniform coating, inadequate pressing, or inconsistent active-material loading can distort rate and cycling comparisons. Differences attributed to elemental substitution may instead arise from changes in electrode porosity, contact resistance, or tap density.

Reliable screening requires consistent powder processing, slurry preparation, coating, drying, electrode pressing, and cell assembly across all compositions.

How to Apply This to Your Project

Composition should be selected according to the failure mechanism that limits the intended cell rather than according to a single headline capacity value.

• If your primary focus is maximum reversible capacity: Favor Fe-containing compositions that retain redox activity at both transition-metal sites, while monitoring manganese-driven distortion and defect chemistry.
• If your primary focus is rate capability and long cycle life: Favor Ni substitution that reduces Jahn-Teller distortion, minimizes lattice-constant variation, and preserves sodium-ion transport pathways.
• If your primary focus is conductivity and accessible manganese redox: Evaluate Co substitution for its ability to improve electronic transport, stabilize the framework, and activate manganese reduction reactions.
• If your primary focus is balanced performance: Screen mixed-substitution formulations, such as Fe-rich compositions with controlled Ni or Co incorporation, and compare capacity, polarization, lattice evolution, and retention under identical electrode-processing conditions.

The most reliable MnHCF design balances redox-active Fe, distortion-suppressing Ni, and conductivity-enhancing, manganese-activating Co against the required capacity, power, lifetime, and cost.

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