The global energy transition, driven by the imperative to mitigate climate change and secure sustainable power sources, has placed unprecedented demands on advanced energy storage technologies. While lithium-ion batteries have dominated portable electronics and electric vehicles, concerns over lithium’s geographical concentration, supply chain volatility, and cost have spurred intense research into alternative chemistries. The sodium-ion battery emerges as a prime candidate for large-scale stationary storage due to the abundance, low cost, and even global distribution of sodium resources. Its working principle parallels that of its lithium counterpart, yet the larger ionic radius of Na+ (1.02 Å vs. Li+‘s 0.76 Å) imposes distinct requirements on host structures, particularly for the cathode.
Among the various cathode families for sodium-ion battery systems—including layered oxides and polyanionic compounds—Prussian Blue Analogues (PBAs) stand out. Their general formula is NaxMA[MB(CN)6]y□1-y·mH2O, where MA and MB are transition metals, □ represents [MB(CN)6] vacancies, and mH2O is water in zeolitic and/or coordinated sites. PBAs possess an open, face-centered cubic framework with large interstitial sites and wide channels, facilitating rapid and reversible Na+ (de)intercalation. They offer high theoretical capacity (often ~170 mAh g-1 for two redox-active centers) and potentially low-cost synthesis from aqueous solutions.
However, the practical deployment of PBA cathodes in sodium-ion battery technology is hindered by intrinsic challenges: prevalent vacancies and high crystallinity water content leading to structural instability, low electronic conductivity, and capacity fading during cycling. Addressing these issues has been the focus of substantial research and development efforts worldwide. Patent literature serves as a critical indicator of technological progress, commercial strategy, and the competitive landscape. This analysis, based on patent data from 2014 to early 2025, aims to delineate the evolution, key innovators, and core technological pathways in the development of Prussian Blue-based cathodes for sodium-ion battery applications.
Global Patent Filing Trends and Technology Lifecycle
The trajectory of patent filings for PBA cathodes closely mirrors the technological and commercial confidence in sodium-ion battery systems. An analysis of annual application numbers reveals distinct phases of development, as summarized in the table below.
| Phase | Timeframe | Patent Activity | Characteristics & Driving Factors |
|---|---|---|---|
| Nascent Stage | 2014-2015 | Negligible to very low | Basic academic research; no clear industrial pull; technology in concept validation. |
| Fluctuating Exploration | 2016-2020 | Low, with significant year-on-year volatility | Increased R&D entry; focus on fundamental material science hurdles (defects, water); industrial adoption hesitant due to performance gaps. |
| Accelerated Growth | 2021-2023 | Rapid and sustained increase, peaking in 2023 | Breakthroughs in material control; announcement of pilot lines and commercial ventures (e.g., CATL’s AB battery system); strong policy and market push for energy storage. |
| Consolidation & Focus | 2024 (Projected) | Potential moderation or continued growth | Shift from fundamental material patents to application-oriented, manufacturing process, and cell integration IP. |
The period of Fluctuating Exploration (2016-2020) is particularly telling. The volatility in annual filings suggests a period of trial and error, where various synthesis and modification strategies were being tested. The noticeable dip around 2020 likely corresponds to a recognition of persistent bottlenecks—such as achieving simultaneously low water content, high Na content, and good cycling stability—which tempered initial enthusiasm. The subsequent surge from 2021 onward marks a pivotal turn, indicating that key technical barriers were being overcome sufficiently to justify significant industrial investment and patent protection strategies, accelerating the path towards sodium-ion battery commercialization.
Key Innovators and the Collaborative Ecosystem
The innovation landscape for PBA cathodes is characterized by a dynamic interplay between leading battery manufacturers, academic institutions, and specialized start-ups. A leading Chinese battery giant, Contemporary Amperex Technology Co., Limited (CATL), stands as the most prolific patent filer, with over 120 patent families identified in this domain. Its sustained and high-volume filing activity, especially from 2021-2023, underscores a strategic commitment to securing a dominant IP position for sodium-ion battery technology, with PBAs as a core cathode candidate.
Academic and research institutions have been the bedrock of fundamental innovation. Universities like Zhejiang University and Jiangxi University of Science and Technology, along with research academies such as those under the Chinese Academy of Sciences, have contributed extensively to early-stage material discovery, doping strategies, and synthesis methodologies. Their patents often focus on novel chemical approaches and deep material characterization.
Specialized technology companies and start-ups, such as Hanxing Technology and Huzhou Chaona, play a crucial role in bridging the gap between laboratory proof-of-concept and industrial-scale manufacturing. Their patent portfolios increasingly emphasize scalable synthesis routes, cost-effective raw material processing, and electrode engineering specifically for sodium-ion battery packs. This creates a collaborative “industry-university-research” ecosystem where foundational knowledge is translated into commercially viable solutions, speeding up the overall development cycle for sodium-ion battery systems.
Core Technological Pathways and Patent Analysis
The patent literature reveals three predominant, often interlinked, strategies to enhance the performance of PBA cathodes for sodium-ion battery applications: 1) Synthesis optimization for low-defect, low-water materials; 2) Elemental doping; and 3) Surface coating/engineering.
1. Synthesis Optimization: Targeting the Root Causes
The primary objective here is to minimize the two major intrinsic flaws: [Fe(CN)6] vacancies (□) and coordinated/zeolitic water (mH2O). Vacancies destabilize the framework and reduce active sites, while water reacts with electrolytes, consumes active Na+, and gasifies causing swelling. Patents disclose sophisticated control over crystallization kinetics.
A core principle is slow and controlled crystallization. This involves precise regulation of reactant addition rates (e.g., controlled dripping), concentration, temperature, and stirring dynamics during the co-precipitation reaction:
$$ M^{2+}_{(aq)} + [Fe^{II}(CN)_{6}]^{4-}_{(aq)} \rightarrow M[Fe(CN)_{6}]_{(s)} $$
Slower supersaturation leads to fewer nucleation sites and allows ions to integrate more orderly into the growing lattice, reducing vacancy formation. For example, patents describe using chelating agents (e.g., EDTA, citrate) to temporarily bind M2+ ions, effectively slowing their release and availability for reaction, thereby promoting the growth of highly crystalline, low-defect particles.
Post-synthesis treatments are equally critical. A prominent method is ion exchange, where a pre-formed potassium-rich PBA (“Prussian White”) is treated in a sodium salt solution (e.g., NaCl in organic solvents) to replace K+ with Na+. This bypasses the direct synthesis of sodium-rich PBAs, which often leads to high water content, and can yield materials with very low hydration levels. Another approach is defect repair, where a pre-synthesized, vacancy-bearing PBA is treated with a solution containing [Fe(CN)6]4- anions, allowing them to fill vacant sites in a solid-state or surface-mediated reaction.
Innovative synthesis environments are also patented. One notable method employs a “water-in-salt” micro-reactor system, where the aqueous reaction medium is confined within a high-concentration salt matrix, drastically reducing the activity and mobility of free water and leading to products with exceptionally low water content. Electrochemical synthesis methods have also been disclosed, where the transition metal cation is oxidatively released from an anode at a controlled current, providing a steady, slow supply for reaction with the cyanometallate anion, enabling exquisite control over precipitation rates and defect density.
2. Elemental Doping: Engineering the Framework and Electronic Structure
Doping involves the partial substitution of the framework metal ions (MA and/or MB) with other elements to tailor properties. The goals are multifold: stabilize the crystal structure against phase transitions or distortion during Na+ extraction/insertion; suppress detrimental effects like the Jahn-Teller distortion of Mn3+; improve electronic conductivity; and modulate operating voltage.
Patent strategies can be categorized:
- Single/Multi-Element Doping: Introducing elements like Ni, Co, Cu, Zn, or Ti into Mn- or Fe-based PBAs. Ni or Co doping, for instance, can enhance structural stability and electronic conductivity. A patent might claim a material with formula Na2Mn1-x-yFexNiy[Fe(CN)6] where Ni doping specifically improves cycle life.
- Gradient Doping: Creating a concentration gradient of the doping element from the particle core to the surface. A common design is a Mn-rich core for capacity, coated with a Ni- or Co-rich surface layer that is more stable and conductive, protecting the bulk from electrolyte attack and improving overall kinetics.
- High-Entropy Doping: A recent, advanced strategy involves incorporating multiple (often four or more) different transition metals in near-equimolar ratios into the MA site. The resulting configurational entropy stabilizes the single-phase crystal structure. Patents describe materials like Na2Mn0.2Fe0.2Co0.2Ni0.2Cu0.2[Fe(CN)6], which demonstrate remarkable cycling stability due to the collective “cocktail effect” of the dopants suppressing individual detrimental phenomena.
The doping effect can be conceptualized by its impact on the crystal field stabilization energy and the band structure. Introducing a dopant D modifies the local electronic environment:
$$ \Delta E_{\text{CFSE}} \text{ and } \rho(e^-) = f(M_{\text{host}}, D, \text{concentration}) $$
leading to altered Na+ diffusion barriers and electronic conductivity, directly benefiting sodium-ion battery performance metrics.
3. Surface Coating and Engineering: Shielding and Connecting
Surface modifications address issues of interfacial instability, poor conductivity, and hygroscopicity. Coating creates a physical barrier between the reactive PBA surface and the electrolyte/moisture.
Patent-protected coating materials and methods include:
- Carbon Coatings: Using precursors like glucose, polymers, or graphene oxide that are pyrolyzed to form a conductive carbon layer. This enhances electron transport across particles and can provide some hydrophobicity.
- Oxide/Fluoride Coatings: Thin, conformal layers of metal oxides (e.g., Al2O3, TiO2, ZrO2) or fluorides (e.g., AlF3) deposited via atomic layer deposition (ALD), sol-gel, or in-situ reactions. These coatings are excellent at scavenging HF from the electrolyte and preventing transition metal dissolution.
- Conductive Polymer Coatings: Polymers like polyaniline (PANI) or polypyrrole (PPy) are electrochemically or chemically polymerized on the PBA surface. They improve conductivity and can flexibly accommodate volume changes.
- Core-Shell PBA Structures: An innovative approach involves creating a particle with a sodium-rich PBA core and a thin, sodium-poor or sodium-free PBA shell. The shell, having fewer Na+ ions to exchange with ambient moisture, acts as a naturally hydrophobic and stable barrier, protecting the high-capacity core. This is a unique material-level solution to the hygroscopicity problem intrinsic to sodium-ion battery PBA cathodes.
The coating process is often designed to be integral to the synthesis or a simple subsequent step, ensuring scalability. The effectiveness of a coating (C) can be modeled as a function of its thickness (d), conductivity (σ), and chemical stability (S):
$$ \text{Performance Enhancement} \propto \frac{\sigma_C \cdot S_C}{d_C} $$
Patents seek to optimize this balance for the sodium-ion battery environment.
| Technology Pathway | Primary Objective | Key Mechanisms | Exemplary Patent Approaches |
|---|---|---|---|
| Synthesis Optimization | Reduce [Fe(CN)6] vacancies & H2O content | Control crystallization kinetics; Post-synthesis ion exchange/repair | Slow drip co-precipitation; Chelation-assisted synthesis; “Water-in-salt” microreactor; Electrochemical synthesis. |
| Elemental Doping | Stabilize structure, improve conductivity, modulate voltage | Lattice parameter tuning; Electronic structure modification; Suppression of Jahn-Teller effect | Single (Ni, Co) and multi-element doping; Core-shell gradient doping; High-entropy multi-metal doping. |
| Surface Coating | Improve interfacial stability, conductivity, and moisture resistance | Physical barrier formation; Enhanced electron percolation; HF scavenging | Carbon coating (pyrolysis); Metal oxide/fluoride ALD coating; Conductive polymer wrapping; Na-poor PBA shell. |
Conclusion and Strategic Recommendations
The patent analysis confirms that Prussian Blue Analogues remain at the forefront of cathode development for the emerging sodium-ion battery industry. The technology has evolved from fundamental material discovery to focused engineering solutions addressing specific commercialization hurdles. The collaborative innovation ecosystem, combining corporate R&D power with academic ingenuity, has been instrumental in this progress. Key advancements in controlled synthesis, intelligent doping, and multifunctional coating have significantly improved the viability of PBA cathodes.
Looking ahead, to solidify the position of PBAs in the future sodium-ion battery market, several strategic foci are recommended:
- Pursue Ultra-Low-Cost Manufacturing Processes: Future patents should increasingly target synthesis routes that minimize energy consumption, use low-purity or alternative feedstocks, and enable high-yield, continuous production. The cost per kilowatt-hour (kWh) of the final sodium-ion battery is the ultimate metric, and cathode material cost is a major lever.
- Deepen the Understanding of Multi-Element Interaction Mechanisms: While high-entropy and gradient doping show promise, predictive modeling and advanced in-situ characterization are needed to move from empirical discovery to rational design. Patents covering materials based on such deep mechanistic understanding will be more robust and valuable.
- Develop “Smart” Multi-Functional Coatings: Next-generation coatings should be designed to perform multiple roles simultaneously: superior ion/electron conduction, self-healing upon micro-crack formation, and adaptive porosity for optimal electrolyte wetting. Integrating coating steps seamlessly into the particle synthesis flow will be crucial for scalability.
- Embrace System-Level Integration IP: As material properties mature, patenting activity will likely shift towards cell design (e.g., electrolyte formulations optimized for PBAs, electrode architectures), state-of-health algorithms, and recycling methods specifically tailored for sodium-ion battery systems with PBA cathodes.
The journey of Prussian Blue from a pigment to a cornerstone of modern energy storage is emblematic of the innovation driving the sodium-ion battery sector. The rich tapestry of patents analyzed here not only maps the technical achievements but also charts a course for the continued refinement and successful deployment of this promising technology in the global sustainable energy landscape.