The global transition towards sustainable energy systems has created an unprecedented demand for efficient, safe, and cost-effective electrochemical energy storage technologies. While lithium-ion batteries (LIBs) currently dominate this landscape, concerns regarding the scarcity and geopolitical concentration of lithium resources, coupled with rising costs, have spurred intense research into alternative chemistries. Among these, sodium-ion batteries (SIBs) present a compelling candidate due to the natural abundance, wide geographical distribution, and low cost of sodium. Furthermore, SIBs often exhibit superior low-temperature performance and enhanced safety profiles compared to their lithium counterparts, making them particularly attractive for large-scale grid storage and other cost- and reliability-sensitive applications.
A critical component in the development of high-performance sodium-ion battery systems is the cathode material. Prussian blue analogues (PBAs), with the general formula AxM[Fe(CN)6]y·zH2O (where A is an alkali metal like Na, and M is a transition metal like Fe, Mn, Ni, etc.), have emerged as highly promising cathode candidates for sodium-ion battery applications. Their open framework structure enables facile sodium ion diffusion, leading to potentially high rate capabilities. Moreover, they offer a high theoretical specific capacity (up to ~170 mAh g-1 depending on the structure) and can be synthesized from abundant, low-cost precursors, aligning perfectly with the economic drivers for sodium-ion battery adoption.
However, a significant challenge inherent to PBAs synthesized via conventional aqueous co-precipitation methods is the incorporation of water molecules within their crystal lattice. This water exists in two primary forms: coordinated water (bound to the Fe sites) and interstitial water (occupying the large cavities of the framework). The presence of this water, especially in high amounts, is detrimental to the performance and longevity of a sodium-ion battery. During electrochemical cycling, water can leach into the electrolyte, participating in parasitic side reactions at the electrodes, accelerating electrolyte decomposition, causing gas evolution, and leading to rapid capacity fade and poor Coulombic efficiency. It also reduces the effective volume available for sodium storage. Therefore, developing effective strategies to minimize the water content in PBA cathodes is paramount for realizing their full potential in practical sodium-ion battery devices.
Concurrently, enhancing the safety of batteries is a non-negotiable requirement. Conventional sodium-ion battery configurations employ flammable liquid organic electrolytes, which pose inherent risks of leakage and thermal runaway. Replacing these with solid-state electrolytes (SSEs) is a definitive path toward safer batteries. However, typical inorganic SSEs often suffer from low ionic conductivity at room temperature, high interfacial resistance with electrodes, and mechanical rigidity. Quasi-solid-state electrolytes (QSEs), which incorporate a liquid electrolyte within a solid polymer matrix, offer an excellent compromise. They retain relatively high ionic conductivity, maintain good interfacial contact with electrodes due to their viscoelastic nature, and significantly improve safety by eliminating electrolyte leakage and reducing flammability. Developing compatible QSEs for sodium-ion battery systems is thus a crucial research direction.
This article details a comprehensive study addressing these key challenges in sodium-ion battery technology. We report a simple yet highly effective thermal treatment strategy to drastically reduce the crystalline water content in iron-based Prussian blue (NaxFe[Fe(CN)6]) cathode materials. We then employ in-situ diagnostic techniques to unravel the sodium storage mechanism and identify sources of irreversible capacity loss. To mitigate this loss, we introduce a sodium compensation strategy using a pre-sodiation additive. Finally, we integrate this optimized, low-water-content PBA cathode with a hard carbon anode using a high-performance poly(ethylene glycol) diacrylate (PEGDA)-based quasi-solid-state electrolyte, constructing a durable and safe quasi-solid-state sodium-ion battery full cell. The synergistic approach of cathode engineering, mechanism understanding, and electrolyte innovation provides a viable pathway for advancing practical sodium-ion battery technology.
Material Synthesis and Electrolyte Preparation
Synthesis of Prussian Blue Cathodes: The iron-based Prussian blue material was synthesized via a controlled co-precipitation method. Briefly, an aqueous solution containing ferrous sulfate (FeSO4·7H2O) and sodium citrate (as a chelating agent to control crystallization) was slowly added to an aqueous solution of sodium ferrocyanide (Na4Fe(CN)6) under constant stirring. The mixture was maintained at a constant temperature for 6 hours to allow complete reaction and particle growth. The resulting precipitate was collected, washed thoroughly with deionized water and ethanol to remove impurities, and then vacuum-dried at 120°C for 24 hours. This product, denoted as Hw-PBAs (High-water PBAs), served as the precursor. To obtain the low-water-content material, the Hw-PBAs powder was subjected to a mild thermal treatment in an argon atmosphere. It was heated to 270°C at a slow ramp rate of 0.5 °C min-1, held for 2 hours, and then allowed to cool naturally. The final product is denoted as Lw-PBAs (Low-water PBAs).
Preparation of Sodium Compensation Additive: Commercial sodium oxalate (Na2C2O4) was recrystallized from an aqueous solution to reduce its particle size from hundreds of micrometers to the micrometer scale, thereby enhancing its electrochemical reactivity as a sacrificial sodium source.
Fabrication of Quasi-Solid-State Electrolyte (QSE): The polymer-based QSE was prepared via in-situ thermal polymerization. The precursor solution was formulated by mixing a liquid electrolyte (1.0 M NaClO4 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 by volume) with 5 wt% fluoroethylene carbonate (FEC) additive) with poly(ethylene glycol) diacrylate (PEGDA) monomer. The mass ratio of PEGDA to the liquid electrolyte was 7:93. Azobisisobutyronitrile (AIBN, 5 wt% relative to PEGDA) was added as a thermal initiator. This homogeneous solution could be directly injected into a battery cell and subsequently polymerized at 60°C for 10 hours to form a stable, gel-like quasi-solid-state electrolyte membrane.
Electrode Fabrication and Cell Assembly:
For liquid electrolyte half-cells (vs. Na/Na+), the cathode slurry was prepared by mixing the active material (Hw-PBAs or Lw-PBAs), conductive carbon (Keifenblack), and polyvinylidene fluoride (PVDF) binder in a weight ratio of 8:1:1 using N-methyl-2-pyrrolidone (NMP) as solvent. The slurry was coated onto carbon-coated aluminum foil and dried.
For liquid electrolyte full cells, a hard carbon (HC) anode was prepared similarly (HC:KB:PVDF = 8:1:1) on copper foil. The positive-to-negative capacity (N/P) ratio was controlled between 1.1 and 1.2.
For the quasi-solid-state full cell, the cathode slurry included the Lw-PBAs active material, the recrystallized Na2C2O4 sodium compensation additive, KB, and PVDF binder in a weight ratio of 6.4:1.6:1.0:1.0. The QSE precursor solution was injected into the assembled cell (cathode | separator | anode) before the final sealing and polymerization step.
All cell assembly procedures were conducted in an argon-filled glovebox with water and oxygen levels below 0.1 ppm.
Characterization of Low-Water-Content Prussian Blue
Thermogravimetric analysis (TGA) provided direct evidence of the effectiveness of the thermal treatment in removing water. The TGA curve for the precursor Hw-PBAs showed three distinct weight loss stages in an argon atmosphere. The results are summarized below:
| Material | Weight Loss (RT-120°C) Adsorbed H2O | Weight Loss (120-200°C) Interstitial H2O | Weight Loss (200-270°C) Coordinated H2O | Total H2O Loss (RT-270°C) |
|---|---|---|---|---|
| Hw-PBAs | 3.10% | 6.10% | 6.89% | 16.09% |
| Lw-PBAs | ~0.44% | ~0.74% | ~0.00% | ~1.18% |
The data reveals that the thermal treatment at 270°C successfully removed nearly all coordinated water and the majority of interstitial water, reducing the total water content in the material from over 16% to just about 1.2%. This represents a reduction of more than 92% in the volatile content below 270°C, confirming the synthesis of a truly low-water-content PBA material crucial for stable sodium-ion battery operation.
X-ray diffraction (XRD) patterns indicated a structural evolution upon dehydration. The Hw-PBAs exhibited a typical cubic structure. After thermal treatment, the Lw-PBAs pattern showed peak shifts and the emergence of new reflections, indicating a transformation towards a rhombohedral (trigonal) phase, which is a common, more sodium-rich, and often higher-capacity phase for PBAs. The removal of water molecules from the lattice contracts the unit cell and allows for this structural rearrangement. Inductively coupled plasma optical emission spectrometry (ICP-OES) and TGA results were combined to estimate the chemical formula of Lw-PBAs as Na1.91Fe[Fe(CN)6]·~3.2H2O, confirming a high sodium content and low residual water.
X-ray photoelectron spectroscopy (XPS) analysis provided insights into the chemical state changes. The Fe 2p spectrum of Lw-PBAs showed a significant increase in the ratio of Fe3+ to Fe2+ compared to Hw-PBAs. This is logically explained by the dehydration process: as [Na(H2O)]+ species are removed from the lattice, charge compensation occurs through the oxidation of some Fe2+ to Fe3+. The O 1s spectrum of Lw-PBAs showed the disappearance of the peak associated with coordinated water, consistent with TGA, and an enhanced signal from metal-oxygen bonds (likely surface Fe-O from minimal surface oxidation). Scanning electron microscopy (SEM) confirmed that the thermal treatment did not drastically alter the particle morphology; both materials consisted of micron-sized cubic particles, with Lw-PBAs exhibiting a slightly rougher surface texture.
Electrochemical Performance in Half-Cell and Full-Cell Configurations
The electrochemical superiority of the Lw-PBAs cathode was first evaluated in sodium half-cells (vs. Na metal). Figure 1a compares the cycling stability of Hw-PBAs and Lw-PBAs at a current density of 100 mA g-1 within a voltage window of 2.0-3.8 V. The Lw-PBAs electrode delivered an initial discharge capacity of approximately 103 mAh g-1 and maintained a capacity of 91 mAh g-1 after 340 cycles, corresponding to an excellent capacity retention of 88%. In stark contrast, the Hw-PBAs electrode suffered from continuous capacity decay, retaining only 73% of its initial capacity after the same number of cycles. This dramatic improvement underscores the critical role of water removal in stabilizing the Prussian blue framework for long-term cycling in a sodium-ion battery.
The galvanostatic charge-discharge profiles of Lw-PBAs (Figure 1b) displayed two distinct plateaus at around 3.2 V and 2.9 V, corresponding to the redox couples of low-spin FeII/FeIII (bonded to carbon) and high-spin FeII/FeIII (bonded to nitrogen), respectively. The clear plateau at 3.2 V indicates highly reversible redox activity of the C-coordinated iron, which is facilitated by the more open and stable dehydrated structure. The rate capability test (Figure 1c) further highlighted the advantages of Lw-PBAs. It delivered specific capacities of 126, 112, 110, 108, and 107 mAh g-1 at current densities of 10, 50, 100, 200, and 500 mA g-1, respectively. When the current density was returned to 10 mA g-1, a capacity of 125 mAh g-1 was recovered, demonstrating remarkable structural resilience and fast sodium ion kinetics—key attributes for a high-power sodium-ion battery.
Next, a full sodium-ion battery was assembled by pairing the Lw-PBAs cathode with a hard carbon (HC) anode using a conventional liquid electrolyte (denoted as Lw-PBAs|LE|HC). The cell operated between 1.5 and 3.8 V. While showing promising cycling stability, this initial full cell exhibited a low initial Coulombic efficiency (ICE) of only 67.3%. This irreversible capacity loss stems from two main factors: (i) the formation of a solid electrolyte interphase (SEI) on the hard carbon anode, which consumes sodium ions irreversibly, and (ii) irreversible structural changes in the cathode during the first cycle, as identified by in-situ analysis discussed later. To compensate for this active sodium loss and boost the energy density of the sodium-ion battery, we incorporated the recrystallized sodium oxalate (Na2C2O4) as a pre-sodiation additive into the cathode.
The electrochemical activity of the micron-sized Na2C2O4 was confirmed in a half-cell, where it decomposed during the first charge, releasing a high specific capacity of ~407 mAh g-1 (theoretical: 400 mAh g-1) in the voltage range of 2.0-4.2 V. This decomposition reaction, presumably forming CO2 and Na2CO3 or related species, provides a source of sodium ions to replenish those lost to SEI formation and cathode irreversibility. Incorporating 20 wt% of this additive into the Lw-PBAs cathode significantly enhanced the full-cell performance. The initial discharge capacity surged to 158 mAh g-1 (based on Lw-PBAs mass), a 92.7% increase compared to the cell without the additive. The cell also demonstrated good rate capability and exceptional long-term cycling stability at a high current density of 500 mA g-1, retaining 64 mAh g-1 after 1400 cycles.
In-Situ Mechanistic Insights into Sodium Storage
To understand the electrochemical behavior at a fundamental level, we employed in-situ characterization techniques on the operating sodium-ion battery full cell (Lw-PBAs|LE|HC).
In-Situ XRD on Lw-PBAs Cathode: During the first charge process, as the voltage increased past 3.2 V, the separate (110) and (104) diffraction peaks of the rhombohedral Lw-PBAs began to merge into a single broad peak. This signifies a phase transition from the initial rhombohedral phase to a cubic phase as sodium ions are extracted. Crucially, during the subsequent discharge, this merged peak did not split back into the two distinct peaks. This irreversibility in the phase transition is a primary contributor to the initial capacity loss observed in the PBA cathode and highlights a key structural consideration for cathode design in sodium-ion battery systems.
In-Situ Raman and XRD on Hard Carbon Anode: The storage mechanism in the hard carbon anode was also probed. In-situ XRD showed no shift in the (002) graphitic peak and no appearance of metallic sodium peaks, ruling out conventional intercalation or plating mechanisms. In-situ Raman spectroscopy provided clearer insight. The hard carbon exhibited characteristic D-band (~1350 cm-1, disorder) and G-band (~1594 cm-1, graphitic) peaks. The intensity ratio ID/IG, which correlates with defect concentration, was monitored. During discharge (sodium insertion), the ID/IG ratio decreased, indicating that sodium ions were being adsorbed onto the defect sites and pores within the disordered carbon structure. This adsorption/filling mechanism is responsible for the majority of the capacity in this type of hard carbon anode for sodium-ion batteries. The initial drop in ID/IG during the first discharge did not fully reverse upon charge, confirming the irreversible sodium trapping associated with SEI formation and pore filling, which accounts for the anode’s contribution to the low ICE.
The irreversible losses from both electrodes can be quantitatively described. The overall initial Coulombic efficiency (ICE) of a full cell is given by:
$$ICE_{cell} = \frac{Q_{discharge}}{Q_{charge}} \times 100\%$$
The charge capacity $Q_{charge}$ includes both reversible sodium extraction from the cathode and irreversible losses. These losses can be partitioned into cathode irreversibility ($Q_{irr,c}$) and anode irreversibility ($Q_{irr,a}$), primarily from SEI formation. A sodium compensation additive like Na2C2O4 provides an extra source of sodium ions ($Q_{comp}$) during the first charge, effectively offsetting these losses:
$$Q_{charge} = Q_{rev,c} + Q_{irr,c} + Q_{irr,a} – Q_{comp}$$
Where $Q_{rev,c}$ is the reversible capacity of the cathode. By supplying $Q_{comp}$, the apparent $ICE_{cell}$ is dramatically improved, as evidenced by the experimental results.
Performance of the Quasi-Solid-State Sodium-Ion Full Battery
The final and most significant step was integrating all optimized components into a quasi-solid-state configuration. The PEGDA-based QSE was first characterized. Electrochemical impedance spectroscopy (EIS) yielded a high room-temperature ionic conductivity of 3.51 mS cm-1, which is comparable to many liquid electrolytes and sufficient for effective battery operation. Linear sweep voltammetry (LSV) revealed an electrochemical stability window extending beyond 4.9 V vs. Na/Na+, indicating excellent anodic stability against oxidation by the high-voltage PBA cathode.
The complete quasi-solid-state sodium-ion battery, denoted as Lw-PBAs|GPE|HC (where GPE is the gel polymer electrolyte), was assembled. Its electrochemical performance is summarized in the table below:
| Performance Metric | Result | Condition / Current Density |
|---|---|---|
| Initial Discharge Capacity | 147.8 mAh g-1 | 100 mA g-1 |
| Rate Capability | 105, 94, 82, 70, 58 mAh g-1 | 20, 50, 100, 200, 500 mA g-1 |
| Cycling Stability | >200 stable cycles | 100 mA g-1 |
| Coulombic Efficiency | ~100% (during cycling) | 100 mA g-1 |
The quasi-solid-state sodium-ion battery demonstrated very good performance across all metrics. The high initial capacity benefited directly from the sodium compensation additive. The respectable rate capability, despite the use of a polymer electrolyte, attests to the fast kinetics enabled by the low-water-content PBA cathode and the high ionic conductivity of the QSE. Most importantly, the cell cycled stably for over 200 cycles with near-perfect Coulombic efficiency, showcasing the combined benefits of a stable cathode, effective sodium compensation, and a compatible, safe quasi-solid-state electrolyte. This configuration successfully addresses the core challenges of water-induced degradation and safety, marking a significant step towards practical quasi-solid-state sodium-ion battery technology.
Conclusion and Perspectives
This study presents a holistic strategy for developing high-performance quasi-solid-state sodium-ion batteries based on Prussian blue analogue cathodes. The cornerstone of this approach is a simple thermal treatment that effectively removes over 90% of the crystalline water from the PBA material. This dehydration process transforms the material’s structure and chemical state, leading to dramatically improved cycling stability (88% retention after 340 cycles vs. 73% for the hydrated sample) and enhanced rate performance in a sodium-ion battery.
Through in-situ characterization, we elucidated the mechanistic details of sodium storage: an irreversible phase transition in the PBA cathode and a defect-adsorption mechanism in the hard carbon anode, both contributing to initial irreversible capacity loss. The innovative use of Na2C2O4 as an integrated sodium compensation agent successfully mitigated this loss, boosting the initial energy output of the full cell by 92.7%.
Finally, by employing a PEGDA-based quasi-solid-state electrolyte with high ionic conductivity and a wide voltage window, we constructed a safe and leak-free sodium-ion battery prototype. This quasi-solid-state sodium-ion battery delivered promising specific capacities across a range of current densities and sustained stable operation for hundreds of cycles.
This work underscores that managing crystalline water is not merely an option but a necessity for unlocking the potential of PBAs in reliable sodium-ion batteries. Furthermore, it demonstrates a viable integration pathway combining cathode engineering, interface understanding, and polymer electrolyte science. Future work may focus on scaling up the synthesis of low-water PBAs, optimizing the polymer electrolyte composition for wider temperature operation, and exploring other pre-sodiation techniques. The findings contribute valuable insights and a practical framework for the ongoing development of safe, low-cost, and high-performance energy storage systems based on sodium-ion battery chemistry.