Aqueous ion batteries have attracted recent attention due to their low cost, high safety, and environmental friendliness, making them a promising alternative to conventional lithium-ion batteries. Among the various electrode materials explored for aqueous ion batteries, Prussian blue analogues (PBAs), with the general formula A₂T[M(CN)₆] (where A = Li, Na, K; T = Fe, Co, Ni, Mn, Cu; and M = Fe, Mn, Co), emerge as a highly promising class of materials. PBAs are characterized by their large open frameworks, abundant ion insertion sites, and ease of synthesis, which contribute to their superior electrochemical performance. This review systematically summarizes the structural properties, synthesis strategies, and applications of PBAs in aqueous lithium-ion, sodium-ion, potassium–ion, and ammonium-ion batteries, while addressing the key challenges and future directions for their development.

The crystal structure of PBAs, determined by X-ray diffraction, consists of a face-centered cubic (FCC) framework, where Fe³⁺ and M²⁺ ions are octahedrally coordinated to the carbon and nitrogen atoms of cyanide groups, respectively. This structure creates a three-dimensional porous network capable of accommodating various guest ions. PBAs, which are derived via substituting Fe with other transition metals (i.e., Cr, V, Mn, Co, Ni, and Cu), retain the same structural framework, but exhibit enhanced electrochemical properties due to the presence of dual redox-active centers (i.e., Fe³⁺/Fe²⁺ and M³⁺/M²⁺). These redox pairs enable high theoretical capacities and reaction potentials, making PBAs suitable for high-performance battery applications.

However, PBAs often suffer from structural defects, such as Fe(CN)₆ vacancies, which can compromise their electrochemical stability. These vacancies, typically formed during rapid precipitation, can lead to a structural collapse during cycling, particularly in systems involving multi-electron reactions. To mitigate these issues, researchers have developed some strategies to synthesize low–defect PBAs with an improved structural integrity and cycling stability.

PBAs can be synthesized by various methods, i.e., co-precipitation, hydrothermal synthesis, and single-precursor approaches. Co-precipitation as one of the most common methods involves the reaction of Fe³⁺ salts with [Fe²⁺(CN)₆]^4– in aqueous solutions, resulting in the formation of PB or its analogues. Hydrothermal synthesis involves the production of well-crystallized PBAs with controlled morphologies at high temperatures and pressures. The single-precursor method, which relies on the slow release of Fe³⁺ or Fe²⁺ ions from [Fe(CN)₆]^4– or [Fe(CN)₆]^3– precursors, offers a route to highly mono-dispersive PBA nanoparticles.

Despite the widespread use of lithium-ion batteries, their application in aqueous systems is limited due to the poor compatibility of PBAs with hydrated Li⁺ ions. The small ionic radius of Li⁺ leads to a structural instability in PBAs, thus having a rapid capacity decay. However, recent studies have explored the use of salt-in-water electrolytes to improve the reversibility of Li⁺ insertion in PBAs, offering a potential pathway for their application in aqueous lithium-ion batteries.

Aqueous sodium-ion batteries with their low cost and natural abundance of sodium emerge as a viable alternative to lithium–ion batteries. PBAs, particularly those based on Fe, Mn, and Co, have the excellent performance in sodium-ion batteries due to their ability to accommodate larger Na⁺ ions. For instance, FeFe–PBAs and MnFe–PBAs exhibit high specific capacities and long cycle lives, making them attractive candidates for grid-scale energy storage. However, the presence of structural vacancies remains a challenge as they can lead to capacity fading during cycling.

Aqueous potassium-ion batteries face some challenges due to the large ionic radius of K⁺, which can cause significant volume changes during cycling. PBAs with their large interstitial spaces are well-suited for K⁺ insertion and demonstrate high reaction potentials and excellent cycling stability. For instance, K₂NiFe(CN)₆ has a remarkable performance with a high capacity retention over thousands of cycles. In addition, mixed-ion systems, such as Na⁺/K⁺ hybrid batteries, are also developed to leverage the advantages of both ions, further enhancing the performance of PBAs in potassium-ion batteries.

Aqueous ammonium-ion batteries, though less explored, offer unique advantages due to the large ionic radius of NH₄⁺, which matches well with the interstitial spaces in PBAs. This compatibility results in high reaction potentials and stable cycling. Recent studies have demonstrated that PBAs such as CuFe-PBAs can achieve high specific capacities and excellent cycle life in NH₄⁺batteries, particularly when paired with Zn anodes. However, the development of full-cell configurations remains a challenge due to the limited availability of NH₄⁺-compatible anodes.

Aqueous proton batteries, though less common, have shown a potential due to the unique transport mechanism of protons in PBAs. Protons can rapidly diffuse through the hydrogen-bonded water networks within the PBA structure, enabling a high-rate performance. PBAs such as Na_o·₄(VO)₃[Fe(CN)₆]₂·12H₂O demonstrate high capacities and excellent cycling stability in acidic electrolytes, making them promising candidates for high-power applications.

Despite the development of PBAs for aqueous ion batteries, several challenges still remain. The presence of structural vacancies and water in the PBA framework can lead to a capacity fading and a structural instability during cycling. Future research should focus on understanding the mechanisms of vacancy formation and developing strategies to eliminate these defects. In addition, the optimization of electrolyte composition and the enhancement of electronic conductivity in PBAs are also critical for improving their performance in practical applications.

In conclusion, PBAs represent a versatile and promising class of materials for aqueous ion batteries, offering high capacities, long cycle lives, and superior rate capabilities. PBAs have a potential to play a key role in the development of next-generation energy storage systems via addressing the existing challenges and further exploring their electrochemical properties.

Doping with impurity defects or sustained multi-terminal external electric fields can enhance performance of artificial optoelectronic synapses based on two-dimensional materials. But doping causes varying degrees of damage to the original lattice structure, while external fields would increase additional power consumption. Here, we demonstrate an effective surface charge transfer doping approach sensitive to air that facilitates the fabrication of reconfigurable MoS₂/MoTe₂ devices. MoS₂/MoTe₂ undergoes electron transfer with surface-adsorbed O₂/H₂O, resulting in varying degrees of p-type doping that affects the Schottky barrier and the built-in electric field strength at the PN junction. The doping level can be reconstructed by a brief gate bias resulting in controllable photocurrent. Due to the conduction of the reverse PN junction, the low dark current and high photoelectric response result in an extremely low power consumption per detectable spike (0.73 pJ), and stability is maintained during an 80,000 s reconstruction process. Notably, hardware-level self-noise reduction is achieved through feature-based long/short-term memory, and recognition accuracy on the processed Modified National Institute of Standards and Technology (MNIST) dataset improved 39%. The unique photo-electro co-modulation strategy paves a promising path for future development of artificial vision systems.

The exceptional resistive switching characteristics and neuromorphic computational potential of memristors are crucial for advancing information processing in both traditional and non-traditional computing paradigms. However, the non-ideal resistive switching behavior of conventional oxide-based memristors hardly meets the performance requirements for neuromorphic computing applications. Besides, the two-terminal memristors are restricted by their configuration limitations toward multi-field/multi-functional modulation. Herein, this article presents a 2D GaSe/MoS₂ heterojunction thin-film transistor with four-terminal (4-T) tuning capability and flexible programming/erasing operations for non-volatile storage. The heterojunction transistor demonstrates an exceptional resistance switching ratio exceeding 10⁷, an ultra-wide modulation range of 10–10⁶, highly reliable stability, and cyclic durability. The in situ Kelvin probe force microscope and dynamic characterization reveal the conduction mediated by defect-induced space charge limitations, as well as the tuning filling process of trap states within the channel by dual-gate terminals. This device functions as a 4-T artificial synapse, capable of achieving basic optoelectronic synaptic operations. The self-denoising and pattern recognition capabilities exhibited by artificial neural networks based on this device serve as excellent examples for developing efficient and energy-saving neuromorphic computing architectures.

Ether electrolytes for potassium-ion batteries exhibit a broader electrochemical window and greater applicability, yet most of them are high-concentration electrolytes with elevated cost. In this study, we propose the use of a weakly solvating cyclic ether electrolyte with tetrahydropyran (THP) as the solvent. This approach induces the formation of a thin and dense inorganic-rich solid electrolyte interphase (SEI) film, which is accompanied by a decrease in the activation energy of electrode interfacial reactions due to the weak ligand binding of THP with K⁺. Density functional theory (DFT) simulations also corroborate the hypothesis that K⁺ has a lower binding energy with THP. During potassium storage process, the phenomenon of solvent co-intercalation of graphite does not occur, which greatly reduces the destruction of the graphite structure and enables a superior electrochemical performance and enhanced cycling stability at a lower concentration (2 M). At a current density of 0.2 C (55.8 mA·g^–1), the battery can be stably cycled for 800 cycles (approximately 8 months) with a specific capacity of 171.8 mAh·g^–1. This study provides a new ether-based electrolyte for potassium ion batteries and effectively reduces the electrolyte cost, which is expected to inspire further development of energy storage batteries.

Industrially prepared artificial graphite (AG) is attractive for potassium-ion batteries (PIBs), but its rate performance is poor and the production process is energy intensive, so developing an efficient strategy to produce novel graphite with low energy consumption and high performance is economically important. Herein, a nanostructured graphite composed of multi-walled carbon nanotubes (MWCNTs) and graphite shells was prepared by one-pot method through low-temperature pyrolysis of iron-based metal-organic framework (MOF) and carbon source. The high graphitization degree of nanostructured graphite makes the initial Coulombic efficiency (ICE) exceed 80%, and the three-dimensional (3D) conductive network ensures a specific capacity of 234 mAh·g⁻¹ after 1000 cycles at a high current density of 500 mA·g⁻¹. In addition, the typical graphite potassium storage mechanism is also demonstrated by in situ X-ray diffraction (XRD) and in situ Raman spectroscopy, and its practicality is also proved by the voltage of the full cells. This work provides a feasible way to optimize the practical production process of AG and expand its application in energy storage.

Carbon-based electrodes of potassium-ion batteries are of great research interest ascribed to their low cost and environmentally friendly distinctions. However, traditional carbon materials usually exhibit weak mechanical properties and incomplete crosslinking, resulting in poor stability and electrochemical performance. Herein, we report a new strategy for modifying reduced graphene oxide into a uniform few-layer structure through a sol–gel method combined with acid etching treatment. The obtained chemical cross-linking and mechanically reinforced carbon network constructed by graphene (CNCG) demonstrates excellent electrochemical and mechanical properties. Adopted as a free-standing anode (~ 7 mg·cm⁻²) for potassium ion battery, the as-achieved CNCG delivers a high reversible specific capacity of 317.7 mAh·g⁻¹ at a current density of 50 mA·g⁻¹ and admirable cycle stability (208.4 mAh·g⁻¹ at 50 mA·g⁻¹ after 500 cycles). The highly reversible structural stability and fully cross-linked properties during potassiation are revealed by ex-situ characterization. This work provides new ideas for the synthesis of new carbon materials and the development of high-performance electrodes.

Aqueous rechargeable zinc ion batteries (ARZIBs) have received unprecedented attention owing to the low cost and high-safety merits. However, their further development and application are hindered by the issues of electrodes such as cathode dissolution, zinc anode dendrite, passivation, as well as sluggish reaction kinetics. Designing heterostructure electrodes is a powerful method to improve the electrochemical performance of electrodes by grafting the advantages of functional materials onto the active materials. In this review, various modified heterostructure electrodes with optimized electrochemical performance and wider applications are introduced. Moreover, the synergistic effect between active materials and functional materials are also in-depth analyzed. The specific modification methods are divided into interphase modification (electrode-electrolyte interphase and electrode-current collector interphase) and structure optimization. Finally, the conclusion and future perspective on the optimization mechanism of functional materials, and the cost issue of practical heterostructure electrodes in ARZIBs are also proposed. It is expected that this review can promote the further development of ARZIBs towards practical utility.