Structural Modulation of Cu-Mn-Fe Prussian Blue Analogs for Practical Sodium Ion Cylinder Cells

High-performance, cost-effective cathodes are essential for grid-scale sodium-ion batteries (SIBs). Prussian blue analogs (PBAs) have shown great potential as SIB cathodes, but achieving both high capacity and long lifespan remains challenging. In this study, a series of low-cost ternary PBAs synthesized through structural regulation is presented to simultaneously achieve high capacity, stable cycling performance, and broad temperature adaptability. Among them, CuHCF-3 demonstrates a specific capacity of 132.4 mAh g-1 with 73.3% capacity retention over 1000 cycles. In-depth analyses, using in situ techniques and density functional theory calculations, reveal a highly reversible three-phase transition (monoclinic ↔ cubic ↔ tetragonal) in Na1.96Cu0.45Mn0.55[Fe(CN)6]0.91·□0.09·2.14H2O (CuHCF-3), which is driven by synergistic interactions between Mn and Cu. Mn enhances conductivity, increases the operating voltage, and introduces additional redox centers, while Cu mitigates the Jahn-Teller distortions associated with Mn and buffers volume changes during cycling. This structural synergy results in excellent temperature stability across a wide temperature range (-20 to 55 °C). 18650-type cylindrical cells based on CuHCF-3 with high loading density achieve 73.54% capacity retention over 850 cycles. This study offers valuable insights for designing durable, high-capacity electrode materials for SIB energy storage applications.

Selective cesium extraction from highly saline solution using hybrid capacitive deionization with zinc-doped manganese hexacyanoferrate electrode

Recently, hybrid capacitive deionization (HCDI) has garnered significant attention for its potential in the selective extraction of cesium (Cs) from radioactive wastewater and salt lakes, which is crucial for resolving the supply-demand imbalance of cesium resources and eliminating radioactive contamination. However, developing HCDI electrodes capable of effectively separating and extracting Cs remains a significant challenge. In this work, we proposed an innovative strategy involving the doping of inactive metal ions to develop zinc-doped manganese hexacyanoferrate (ZMFC) as an HCDI cathode. This approach leveraged the synergistic effects of ion sieving of three-dimensional lattice and zinc doping to regulate electrochemical activity, enabling the selective electrochemical extraction of Cs+ ions from highly saline solutions. The optimized ZMFC-0.1 electrodes exhibited a maximum electrosorption capacity of 299.9 mg g-1 in a 400 mg L-1 Cs+ ion solution and maintained a high-capacity retention rate of 80.1 % after 50 consecutive absorption-desorption cycles in a 200 mg L-1 Cs+ ion solution. Moreover, ZMFC-0.1 showed a high selectivity coefficient of 27.4 at a Cs+/Na+ molar ratio of 1:40. Density functional theory (DFT) simulation revealed that manganese hexacyanoferrate (MnHCF) retains high stability and electrochemical activity following zinc atom doping. In-situ X-ray diffraction and theoretical calculations provided deeper insights into the mechanisms underlying reversible capacitive adsorption and selective Cs+ ion separation. The effectiveness of cesium selective extraction from brine in Xizang suggests that the ZMFC-0.1 electrodes hold significant promise for cesium recovery from salt lakes. This work presents a promising strategy for the electrochemical extraction of cesium from radioactive wastewater and salt lakes.

High-Entropy Layered Oxide Cathode Materials with Moderated Interlayer Spacing and Enhanced Kinetics for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) with low cost and environmentally friendly features have recently attracted significant attention for renewable energy storage. Sodium layer oxides stand out as a type of promising cathode material for SIBs owing to their high capacity, good rate performance, and high compatibility for manufacturing. However, the poor cycling stability of layer oxide cathodes due to structure distortion greatly impacts their practical applications. Herein, a high entropy doped Cu, Fe, and Mn-based layered oxide (HE-CFMO), Na0.95Li0.05Mg0.05Cu0.20Fe0.22Mn0.35Ti0.13O2 for high-performance SIBs, is designed. The HE-CFMO cathode possesses high-entropy transition metal (TM) layers with a homogeneous stress distribution, providing a moderated interlayer spacing to maintain the structure stability and enhance Na+ ion diffusion. In addition, Li doping in TM layers increases the Mn valence state, which effectively suppresses John-Teller effect, thus stabilizing the layered structure during cycling. Furthermore, the use of nontoxic and low-cost raw materials benefits future commercialization and reduces the risk of environmental pollution. As a result, the HE-CFMO cathode exhibits a super cycling performance with a 95% capacity retention after 300 cycles. This work provides a promising strategy to improve the structure stability and reaction kinetics of cathode materials for SIBs.

Prussian White/Reduced Graphene Oxide Composite as Cathode Material to Enhance the Electrochemical Performance of Sodium-Ion Battery

Prussian white (PW) is considered a promising cathode material for sodium-ion batteries. However, challenges, such as lattice defects and poor conductivity limit its application. Herein, the composite materials of manganese-iron based Prussian white and reduced graphene oxide (PW/rGO) were synthesized via a one-step in situ synthesis method with sodium citrate, which was employed both as a chelating agent to control the reaction rate during the coprecipitation process of PW synthesis and as a reducing agent for GO. The low precipitation speed helps minimize lattice defects, while rGO enhances electrical conductivity. Furthermore, the one-step in situ synthesis method is simpler and more efficient than the traditional synthesis method. Compared with pure PW, the PW/rGO composites exhibit significantly improved electrochemical properties. Cycling performance tests indicated that the PW/rGO-10 sample exhibited the highest initial discharge capacity and the best cyclic stability. The PW/rGO-10 has an initial discharge capacity of 128 mAh g-1 at 0.1 C (1 C = 170 mA g-1), and retains 49.53% capacity retention after 100 cycles, while the PW only delivers 112 mAh g-1 with a capacity retention of 17.79% after 100 cycles. Moreover, PW/rGO-10 also shows better rate performance and higher sodium ion diffusion coefficient (DNa+) than the PW sample. Therefore, the incorporation of rGO not only enhances the electrical conductivity but also promotes the rapid diffusion of sodium ions, effectively improving the electrochemical performance of the composite as a cathode material for sodium-ion batteries.

Boosting Manganese Selenide Anode for Superior Sodium-Ion Storage via Triggering α → β Phase Transition

Sodium-ion batteries (SIBs) have been extensively studied owing to the abundance and low-price of Na resources. However, the infeasibility of graphite and silicon electrodes in sodium-ion storage makes it urgent to develop high-performance anode materials. Herein, α-MnSe nanorods derived from δ-MnO2 (δ-α-MnSe) are constructed as anodes for SIBs. It is verified that α-MnSe will be transferred into β-MnSe after the initial Na-ion insertion/extraction, and δ-α-MnSe undergoes typical conversion mechanism using a Mn-ion for charge compensation in the subsequent charge-discharge process. First-principles calculations support that Na-ion migration in defect-free α-MnSe can drive the lattice distortion to phase transition (alpha → beta) in thermodynamics and dynamics. The formed β-MnSe with robust lattice structure and small Na-ion diffusion barrier boosts great structure stability and electrochemical kinetics. Hence, the δ-α-MnSe electrode contributes excellent rate capability and superior cyclic stability with long lifespan over 1000 cycles and low decay rate of 0.0267% per cycle. Na-ion full batteries with a high energy density of 281.2 Wh·kg-1 and outstanding cyclability demonstrate the applicability of δ-α-MnSe anode.

Surface Engineering Stabilizes Rhombohedral Sodium Manganese Hexacyanoferrates for High-Energy Na-Ion Batteries

The rhombohedral sodium manganese hexacyanoferrate (MnHCF) only containing cheap Fe and Mn metals was regarded as a scalable, low-cost, and high-energy cathode material for Na-ion batteries. However, the unexpected Jahn-teller effect and significant phase transformation would cause Mn dissolution and anisotropic volume change, thus leading to capacity loss and structural instability. Here we report a simple room-temperature route to construct a magical Co_x B skin on the surface of MnHCF. Benefited from the complete coverage and the buffer effect of Co_x B layer, the modified MnHCF cathode exhibits suppressed Mn dissolution and reduced intergranular cracks inside particles, thereby demonstrating thousands-cycle level cycling lifespan. By comparing two key parameters in the real energy world, i.e., cost per kilowatt-hours and cost per cycle-life, our developed Co_x B coated MnHCF cathode demonstrates more competitive application potential than the benchmarking LiFePO₄ for Li-ion batteries.

Anode-free sodium metal batteries as rising stars for lithium-ion alternatives

With the impact of the COVID-19 lockdown, global supply chain crisis, and Russo-Ukrainian war, an energy-intensive society with sustainable, secure, affordable, and recyclable rechargeable batteries is increasingly out of reach. As demand soars, recent prototypes have shown that anode-free configurations, especially anode-free sodium metal batteries, offer realistic alternatives that are better than lithium-ion batteries in terms of energy density, cost, carbon footprint, and sustainability. This Perspective explores the current state of research on improving the performance of anode-free Na metal batteries from five key fields, as well as the impact on upstream industries compared to commercial batteries.

Influence of Vacancies in Manganese Hexacyanoferrate Cathode for Organic Na-Ion Batteries: A Structural Perspective

Manganese hexacyanoferrates (MnHCF) are promising positive electrode materials for non-aqueous batteries, including Na-ion batteries, due to their large specific capacity (>130 mAh g^-1 ), high discharge potential and sustainability. Typically, the electrochemical reaction of MnHCF associates with phase and structural changes, due to the Jahn-Teller (JT) distortion of Mn sites upon the charge process. To understand the effect of the MnHCF structure on its electrochemical performance, two MnHCF materials with different vacancies content are investigated herein. The electrochemical results show that the sample with lower vacancy content (4 %) exhibits relatively higher capacity retention of 99.1 % and 92.6 % at 2^nd and 10^th cycles, respectively, with respect to 97.4 % and 79.3 % in sample with higher vacancy content (11 %). Ex-situ X-ray absorption spectroscopy (XAS) and ex situ X-ray diffraction (XRD) characterization results show that a weaker cooperative JT-distortion effect and relatively smaller crystal structure modification occurred for the material with lower vacancies, which explains the better electrochemical performance in cycled electrodes.

High Pseudocapacitance-Driven CoC2 O4 Electrodes Exhibiting Superior Electrochemical Kinetics and Reversible Capacities for Lithium-Ion and Lithium-Sulfur Batteries

In this study, cuboid-like anhydrous CoC₂ O₄ particles (CoC₂ O₄ -HK) are synthesized through a potassium citrate-assisted hydrothermal method, which possess well-crystallized structure for fast Li⁺ transportation and efficient Li⁺ intercalation pseudocapacitive behaviors. When being used in lithium-ion batteries, the as-prepared CoC₂ O₄ -HK delivers a high reversible capacity (≈1360 mAh g^-1 at 0.1 A g^-1 ), good rate capability (≈650 mAh g^-1 at 5 A g^-1 ) and outstanding cycling stability (835 mAh g^-1 after 1000 cycles at 1 A g^-1 ). Characterizations illustrate that the Li⁺ -intercalation pseudocapacitance dominates the charge storage of CoC₂ O₄ -HK electrode, together with the reversible reaction of CoC₂ O₄ +2Li⁺ +2e^- →Co+Li₂ C₂ O₄ on discharging and charging. In addition, CoC₂ O₄ -HK particles are also used together with carbon-sulfur composite materials as the electrocatalysts for lithium-sulfur (Li-S) battery, which displays a gratifying sulfur electrochemistry with a high reversibility of 1021.5 mAh g^-1 at 2 C and a low decay rate of 0.079% per cycle after 500 cycles. The density functional theory (DFT) calculations show that CoC₂ O₄ /C can regulate the adsorption-activation of reaction intermediates and therefore boost the catalytic conversion of polysulfides. Therefore, this work presents a new prospect of applying CoC₂ O₄ as the high-performance electrode materials for rechargeable Li-ion and Li-S batteries.

Structural complexity in Prussian blue analogues

We survey the most important kinds of structural complexity in Prussian blue analogues, their implications for materials function, and how they might be controlled through judicious choice of composition. We focus on six particular aspects: octahedral tilts, A-site 'slides', Jahn-Teller distortions, A-site species and occupancy, hexacyanometallate vacancies, and framework hydration. The promising K-ion cathode material K_xMn[Fe(CN)₆]_y serves as a recurrent example that illustrates many of these different types of complexity. Our article concludes with a discussion of how the interplay of various distortion mechanisms might be exploited to optimise the performance of this and other related systems, so as to aid in the design of next-generation PBA materials.