Electrochemical Li-Ion Intercalation in Octacyanotungstate-Bridged Coordination Polymer with Evidence of Three Magnetic Regimes

Discovery of novel compounds capable of electrochemical ion intercalation is a primary step toward development of advanced electrochemical devices such as batteries. Although cyano-bridged coordination polymers including Prussian blue analogues have been intensively investigated as ion intercalation materials, the solid-state electrochemistry of the octacyanotungstate-bridged coordination polymer has not been investigated. Here, we demonstrate that an octacyanotungstate-bridged coordination polymer Tb­(H₂O)₅[W­(CN)₈] operates as a Li⁺-ion intercalation electrode material. The detailed magnetic measurements reveal that the tunable amount of intercalated Li⁺ ion in the solid-state redox reaction between paramagnetic [W^V(CN)₈]^3– and diamagnetic [W^IV(CN)₈]^4– in the framework enables the electrochemical control of different magnetic regimes. While the initial ferromagnetic long-range ordering is irreversibly lost upon lithium insertion, electrochemical switching between paramagnetic and short-range ordering regimes can be achieved

Bimetallic Cyanide-Bridged Coordination Polymers as Lithium Ion Cathode Materials: Core@Shell Nanoparticles with Enhanced Cyclability

Prussian blue analogues (PBAs) have recently been proposed as electrode materials for low-cost, long-cycle-life, and high-power batteries. However, high-capacity bimetallic examples show poor cycle stability due to surface instabilities of the reduced states. The present work demonstrates that, relative to single-component materials, higher capacity and longer cycle stability are achieved when using Prussian blue analogue core@shell particle heterostructures as the cathode material for Li-ion storage. Particle heterostructures with a size dispersion centered at 210 nm composed of a high-capacity K_0.1Cu­[Fe­(CN)₆]_0.7·3.8H₂O (CuFe-PBA) core and lower capacity but highly stable shell of K_0.1Ni­[Fe­(CN)₆]_0.7·4.1H₂O have been prepared and characterized. The heterostructures lead to the coexistence of both high capacity and long cycle stability because the shell protects the otherwise reactive surface of the highly reduced state of the CuFe-PBA core. Furthermore, interfacial coupling to the shell suppresses a known structural phase transition in the CuFe-PBA core, providing further evidence of synergy between the core and shell. The structure and chemical state of the heterostructure during electrochemical cycling have been monitored with ex situ X-ray diffraction and X-ray absorption experiments and compared to the behavior of the individual components

Charge Storage Mechanism of RuO₂/Water Interfaces

Capacitive energy storage at the electrochemical double layer formed on a particle surface can enable efficient devices that deliver high power and exhibit excellent reversibility. However, even with state of the art nanocarbons with highly controlled morphology to maximize the specific surface area, the available energy density remains far below that of existing rechargeable batteries. Utilizing nanoparticles of transition metal oxides is a viable option to alleviate the conflict between energy and power densities by accommodating additional electrons around the surface transition metal sites, called “pseudocapacitance”. However, an understanding of pseudocapacitive surfaces has been limited due to a lack of suitable analysis methodology. Here, we focus on the RuO₂/water interface and elaborate on a reaction scheme including charge transfer into related surface orbitals using density functional theory calculations based on interfacial structures determined under a given electrode potential at a fixed pH of 0. The extensive contributions of the surface oxygen atoms and their surface-site dependence are revealed through the Ru–O orbital hybridization and O–H bond breaking/formation, largely deviating from the general explanation based only on the nominal valence states (penta-, tetra-, or trivalent) of Ru atoms

Fabrication of a Cyanide-Bridged Coordination Polymer Electrode for Enhanced Electrochemical Ion Storage Ability

Host frameworks with the ability to store guest ions are very important in a wide range of applications including electrode materials for Li-ion batteries. In this report, we demonstrate that the ion storage ability of the cyanide-bridged coordination polymer (Prussian blue analogue, PBA) can be enhanced by suppressing vacancy formation. K-ions in the vacancy-suppressed PBA framework K_1.72Mn­[Mn­(CN)₆]_0.93·□_0.07·0.65H₂O (□: a [Mn­(CN)₆]^4– defect) were electrochemically extracted. The open circuit voltages (OCVs) during K-ion extraction exhibited two specific plateaus at 3.0 and 3.7 V vs Li/Li⁺. Ex situ X-ray diffraction and IR spectroscopy revealed drastic structural and electronic changes during K-ion extraction. Furthermore, after K-ion extraction, the vacancy-suppressed PBA framework was applied to the cathode material for a Li-ion battery. The charge/discharge experiments revealed that the framework can accommodate a large amount of Li-ions

Distinguishing between High- and Low-Spin States for Divalent Mn in Mn-Based Prussian Blue Analogue by High-Resolution Soft X‑ray Emission Spectroscopy

We combine Mn <i>L</i>_2,3-edge X-ray absorption, high resolution Mn 2p–3d–2p resonant X-ray emission, and configuration–interaction full-multiplet (CIFM) calculation to analyze the electronic structure of Mn-based Prussian blue analogue. We clarified the Mn 3d energy diagram for the Mn²⁺ low-spin state separately from that of the Mn²⁺ high-spin state by tuning the excitation energy for the X-ray emission measurement. The obtained X-ray emission spectra are generally reproduced by the CIFM calculation for the Mn²⁺ low spin state having a stronger ligand-to-metal charge-transfer effect between Mn <i>t</i>_2g and CN π orbitals than the Mn²⁺ high spin state. The d–d-excitation peak nearest to the elastic scattering was ascribed to the Mn²⁺ LS state by the CIFM calculation, indicating that the Mn²⁺ LS state with a hole on the <i>t</i>_2g orbital locates near the Fermi level

Precise Electrochemical Control of Ferromagnetism in a Cyanide-Bridged Bimetallic Coordination Polymer

Magnetic coordination polymers can exhibit controllable magnetism by introducing responsiveness to external stimuli. This report describes the precise control of magnetism of a cyanide-bridged bimetallic coordination polymer (Prussian blue analogue: PBA) through use of an electrochemical quantitative Li ion titration technique, i.e., the galvanostatic intermittent titration technique (GITT). K_0.2Ni­[Fe­(CN)₆]_0.7·4.7H₂O (NiFe-PBA) shows Li ion insertion/extraction reversibly accompanied with reversible Fe³⁺/Fe²⁺ reduction/oxidation. When Li ion is inserted quantitatively into NiFe-PBA, the ferromagnetic transition temperature <i>T</i>_C gradually decreases due to reduction of paramagnetic Fe³⁺ to diamagnetic Fe²⁺, and the ferromagnetic transition is completely suppressed for Li_0.6(NiFe-PBA). On the other hand, <i>T</i>_C increases continuously as Li ion is extracted due to oxidation of diamagnetic Fe²⁺ to paramagnetic Fe³⁺, and the ferromagnetic transition is nearly recovered for Li₀(NiFe-PBA). Furthermore, the plots of <i>T</i>_C as a function of the amount of inserted/extracted Li ion <i>x</i> are well consistent with the theoretical values calculated by the molecular-field approximation

Configuration-Interaction Full-Multiplet Calculation to Analyze the Electronic Structure of a Cyano-Bridged Coordination Polymer Electrode

To understand the electronic-structure changes of electrode materials during the charge/discharge processes is one of the most important fundamental aspects to improve the battery performance. Soft X-ray absorption spectroscopy (XAS) was used to study a bimetallic NiFe Prussian blue analogue electrode. XA spectra were obtained during the charge/discharge and were analyzed by the configuration-interaction full-multiplet (CIFM) calculation, in which the strong charge transfer due to the σ/π-donation and back-donation of cyanide was taken into account. The CIFM calculation revealed that the metal-to-ligand charge transfer (MLCT) played an important role in the electronic state of Ni–N bond. The Fe³⁺–C bond in the charged state is dominated by both the MLCT and ligand-to-metal charge transfer (LMCT), whereas only the MLCT strongly affects the Fe²⁺–C bond in the discharged state

Redox Potential Paradox in Na_<i>x</i>MO₂ for Sodium-Ion Battery Cathodes

Raising the operating potential of the cathode materials in sodium-ion batteries is a crucial challenge if they are to outperform state-of-the-art lithium-ion batteries. Although the layered transition metal oxides, NaMO₂ (M: transition metal), are the most promising cathode materials owing to their high theoretical capacity with much more stable nature than Li_1–<i>x</i>MO₂ system, factors influencing the redox potential have not yet been fully understood. Here, we identify redox potential paradox, <i>E</i>(Ni³⁺/Ni²⁺) > <i>E</i>(Ni⁴⁺/Ni³⁺), in an identical structural framework, namely, NaTi⁴⁺_0.5Ni²⁺_0.5O₂ and NaFe³⁺_0.5Ni³⁺_0.5O², which is induced by transition of the oxides from Mott–Hubbard to negative charge-transfer regimes. The origin of the unusually low <i>E</i>(Ni⁴⁺/Ni³⁺) is the surprisingly large contribution (over 80%) of oxygen orbital to the redox reaction, of which the primary effect on the electrochemical property is demonstrated for the first time, providing a firm platform to design better cathodes for advanced sodium-ion batteries

Reversible Solid State Redox of an Octacyanometallate-Bridged Coordination Polymer by Electrochemical Ion Insertion/Extraction

Coordination polymers have significant potential for new functionality paradigms due to the intrinsic tunability of both their electronic and structural properties. In particular, octacyanometallate-bridged coordination polymers have the extended structural and magnetic diversity to achieve novel functionalities. We demonstrate that [Mn­(H₂O)]­[Mn­(HCOO)_2/3(H₂O)_2/3]_3/4­[Mo­(CN)₈]·H₂O can exhibit electrochemical alkali-ion insertion/extraction with high durability. The high durability is explained by the small lattice change of less than 1% during the reaction, as evidenced by <i>ex situ</i> X-ray diffraction analysis. The <i>ex situ</i> X-ray absorption spectroscopy revealed reversible redox of the octacyanometallate. Furthermore, the solid state redox of the paramagnetic [Mo^V(CN)₈]^3‑/diamagnetic­[Mo^IV(CN)₈]^4‑ couple realizes magnetic switching