Origin of the high specific capacity in sodium manganese hexacyanomanganate

Sodium manganese hexacyanomanganate, NaxMn[Mn(CN)6], is an electrochemically active Prussian blue analog (PBA) that has been studied experimentally as an electrode material in rechargeable sodium-ion batteries. It has a reversible specific capacity of 209 mAh/g, which is substantially higher than the theoretical specific capacity of 172 mAh/g expected for two reduction events conventional in the PBAs. It has been suggested the high specific capacity originates from this compound\u27s unique ability to insert a third sodium ion per formula unit. However, the plausibility of this mechanism has remained ambiguous. Here we use density-functional theory (DFT) with a hybrid functional to calculate the formation energies of various oxidation states and magnetic phases of the NaxMn[Mn(CN)6] system. We confirm that the compound Na3Mn(II)[Mn(I)(CN)6] is, indeed, thermodynamically stable. It contains manganese(I) and the sodium ions occupy the interfacial position of the lattice subcubes. We also provide strong evidence that the phase of the fully oxidized Mn[Mn(CN)6] compound is charge-disproportionated, containing manganese(II) and manganese(IV). We proceed to show that the presence of crystalline water increases the reduction potential of the system and that the hydrated compounds have theoretical crystal geometries and reduction potentials that closely match experiment. This work clarifies the charge-storage mechanism in a well-known but less-understood PBA

Origin of the High Specific Capacity in Sodium Manganese Hexacyanomanganate br

International audienceSodium manganese hexacyanomanganate, NaxMn[Mn(CN)6], is anelectrochemically active Prussian blue analogue (PBA) that has been studiedexperimentally as an electrode material in rechargeable sodium-ion batteries. It hasa reversible specific capacity of 209 mA h g-1, which is substantially higher than thetheoretical specific capacity of 172 mA h g-1expected for two reduction eventsconventional in PBAs. It has been suggested that the high specific capacityoriginates from this compound's unique ability to insert a third sodium ion performula unit. However, the plausibility of this mechanism has remainedambiguous. Here, we use density functional theory (DFT) with a hybridfunctional to calculate the formation energies of various oxidation states andmagnetic phases of the NaxMn[Mn(CN)6] system. We confirm that the compound Na3MnII[MnI(CN)6]is,indeed,thermodynamically stable. It contains manganese(I), and the sodium ions occupy the interfacial position of the lattice subcubes.We also provide strong evidence that the phase of the fully oxidized Mn[Mn(CN)6] compound is charge-disproportionated,containing manganese(II) and manganese(IV). We proceed to show that the presence of crystalline water increases the reductionpotential of the system and that the hydrated compounds have theoretical crystal geometries and reduction potentials that closelymatch the experiment. This work clarifies the charge-storage mechanism in a well-known but less-understood PB

Electronic structure and electron-transport properties of three metal hexacyanoferrates

Metal hexacyanometallates, or Prussian blue analogs (PBAs), are active materials in important electrochemical technologies, including next-generation sodium- and potassium- ion batteries. They have tunable properties including reduction potential, ionic conductivity, and color. However, little is known about their electronic conductivities. In this work, we use density-functional theory to model electronic structure and to explore the likely electron-conduction mechanism in three promising cathodes (manganese, iron, and cobalt hexacyanoferrate) in each of three oxidation states. First, we demonstrate that hybrid functionals reliably reproduce experimentally observed spin configurations and geometric phase changes. We confirm these materials are semiconductors or insu- lators with band gaps ranging from 1.90 eV up to 4.94 eV. We further identify that for most of the compounds the electronic band edges originate from carbon-coordinated- iron orbitals, suggesting that doping at the carbon-coordinated site may strongly affect carrier conductivity. Finally, we calculate charge-carrier effective masses, which we find are very heavy. This study is an important foundation for making electronic conductivity a tunable PBA material property

The Effect of Particle-Size Distribution on the Electrochemical Performance of a Red Phosphorus-Carbon Composite Anode for Sodium-Ion Batteries

Sodium-ion batteries will have an important role as a complement to lithium-ion in a future where lithium or cobalt, two critical elements for lithium-ion batteries, become scarce or prohibitively expensive. Red phosphorus (RP) is a promising candidate as an anode for sodium-ion batteries because of its low potential and high specific capacity. Its main disadvantage is its 490% volumetric expansion during sodiation. This leads to particle pulverization and substantial reduction of the cycle life. Furthermore, RP has an extremely low electronic conductivity of 10-14 S cm-1. Both issues have been previously addressed by ball milling RP with a carbon matrix. This decreases the RP particle size and also forms a more electronically conductive composite. However, it is challenging to determine the RP particle size independent of the size of the composite particles. Consequently, little is known about how much the RP particle size must be reduced to improve anode performance. Here we quantify the relationship between the RP particle-size distribution and its cycle life for the first time by separating the ball milling process into two steps. An initial wet ball milling is used to control the RP particle-size distribution, which is measured via dynamic light scattering. This is followed by a dry milling step to produce RP-graphite composites. We found that wet milling breaks apart the largest RP particles in the range of 2 to 10 µm decreases the Dv90 from 1.85 to 1.26 µm and significantly increases the cycle life of the RP. Furthermore, we determined that the length of time of the second milling step affects the uniformity of the carbon distribution in the composite. Photoelectron spectroscopy and transmission electron microscopy confirms the successful formation of a carbon coating, thus improving the performance of the resulting material. The RP with a Dv90 of 0.79 µm mixed with graphite for 48h delivered 1,354 mA h g-1 with high coulombic efficiency (>99%) and cyclability (88% capacity retention after 100 cycles). These results are an important step in the development of cyclable, high-capacity anodes for sodium-ion batteries

Filling Vacancies in a Prussian Blue Analogue Using Mechanochemical Post-Synthetic Modification

Mechanochemical grinding of polycrystalline powders of the Prussian blue analogue (PBA) Mn[Co(CN)$_{\textbf6}$]_{\textbf{2/3}}\boldsymbol\Box_{\textbf{1/3}}\cdot\boldmath xH$_{\textbf 2}$O and K$_{\textbf 3}$Co(CN)$_{\textbf 6}$ consumes the latter and chemically modifies the former. A combination of inductively-coupled plasma and X-ray powder diffraction measurements suggests the hexacyanometallate vacancy fraction in this modified PBA is reduced by approximately one third under the specific conditions we explore. We infer the mechanochemically-driven incorporation of [Co(CN)$_{\textbf 6}$]$^{\textbf 3-}$ ions onto the initially-vacant sites, coupled with intercalation of charge-balancing K$^+$ ions within the PBA framework cavities. Our results offer a new methodology for the synthesis of low vacancy PBAs, unlocking novel, high capacity PBA battery materials.</p

Filling vacancies in a Prussian blue analogue using mechanochemical post-synthetic modification

Mechanochemical grinding of polycrystalline powders of the Prussian blue analogue (PBA) Mn[Co(CN)6]2/3□1/3·xH2O and K3Co(CN)6 consumes the latter and chemically modifies the former. A combination of inductively-coupled plasma and X-ray powder diffraction measurements suggests the hexacyanometallate vacancy fraction in this modified PBA is reduced by approximately one third under the specific conditions we explore. We infer the mechanochemically-driven incorporation of [Co(CN)6]3− ions onto the initially-vacant sites, coupled with intercalation of charge-balancing K+ ions within the PBA framework cavities. Our results offer a new methodology for the synthesis of low-vacancy PBAs

Revealing the structural complexity of Prussian blue analogues: the case of K2Cu[Fe(CN)6]

We report the synthesis, crystal structure, thermal response, and electrochemical behaviour of the Prussian blue analogue (PBA) K2Cu[Fe(CN)6]. From a structural perspective, this is the most complex PBA yet characterised: its triclinic crystal structure results from an interplay of cooperative Jahn–Teller order, octahedral tilts, and a collective `slide\u27 distortion involving K-ion displacements. These different distortions give rise to two crystallographically-distinct K-ion channels with different mobilities. Variable-temperature X-ray powder diffraction measurements show that K-ion slides are the lowest-energy distortion mechanism at play, as they are the only distortion to be switched off with increasing temperature. Electrochemically, the material operates as a K-ion cathode with a high operating voltage, and an improved initial capacity relative to higher-vacancy PBA alternatives. On charging, K+ ions are selectively removed from a single K-ion channel type and the slide distortions are again switched on and off accordingly. We discuss the functional importance of various aspects of structural complexity in this system, placing our discussion in the context of other related PBAs

Electronic Structure and Electron-Transport Properties of Three Metal Hexacyanoferrates

Metal hexacyanometallates, or Prussian blue analogs (PBAs), are active materials in important electrochemical technologies, including next-generation sodium- and potassium-ion batteries. They have tunable properties, including reduction potential, ionic conductivity, and color. However, little is known about their electronic conductivities. In this work, we use density-functional theory to model the electronic structure and to explore the likely electron-conduction mechanism in three promising cathodes (manganese, iron, and cobalt hexacyanoferrate) in each of three oxidation states. First, we demonstrate that hybrid functionals reliably reproduce experimentally observed spin configurations and geometric phase changes. We confirm these materials are semiconductors or insulators with band gaps ranging from 1.90 eV up to 4.94 eV. We further identify that for most of the compounds, the electronic band edges originate from carbon-coordinated iron orbitals, suggesting that doping at the carbon-coordinated site may strongly affect carrier conductivity. Finally, we calculate charge-carrier effective masses, which we find are very heavy. This study is an important foundation for making electronic conductivity a tunable PBA material property

Effect of the Particle-Size Distribution on the Electrochemical Performance of a Red Phosphorus-Carbon Composite Anode for Sodium-Ion Batteries

Red phosphorus (RP) is a promising candidate as an anode for sodium-ion batteries because of its low potential and high specific capacity. It has two main disadvantages. First, it experiences 490% volumetric expansion during sodiation, which leads to particle pulverization and substantial reduction of the cycle life. Second, it has an extremely low electronic conductivity of 10(-14) S cm(-1). Both issues can be addressed by ball milling RP with a carbon matrix to form a composite of electronically conductive carbon and small RP particles, less susceptible to pulverization. Through this procedure, however, the resulting particle-size distribution of the RP particles is difficult to determine because of the presence of the carbon particles. Here, we quantify the relationship between the RP particle-size distribution and its cycle life for the first time by separating the ball-milling process into two steps. The RP is first wet-milled to reduce the particle size, and then the particle-size distribution is measured via dynamic light scattering. This is followed by a dry-milling step to produce RP-graphite composites. We found that wet milling breaks apart the largest RP particles in the range of 2-10 mu m, decreases the Dv90 from 1.85 to 1.26 mu m, and significantly increases the cycle life of the RP. Photoelectron spectroscopy and transmission electron microscopy confirm the successful formation of a carbon coating, with longer milling times leading to more uniform carbon coatings. The RP with a Dv90 of 0.79 mu m mixed with graphite for 48 h delivered 1354 mA h g(-1) with high coulombic efficiency (&gt;99%) and cyclability (88% capacity retention after 100 cycles). These results are an important step in the development of cyclable, high-capacity anodes for sodium-ion batteries

Paving the Way toward Highly Efficient, High-Energy Potassium-Ion Batteries with Ionic Liquid Electrolytes

Potassium-ion batteries (KIB) are a promising complementary technology to lithium-ion batteries because of the comparative abundance and affordability of potassium. Currently, the most promising KIB chemistry consists of a potassium manganese hexacyanoferrate (KMF) cathode, a Prussian blue analog, and a graphite anode (723 W h l–1 and 359 W h kg–1 at 3.6 V). No electrolyte has yet been formulated that is concurrently stable at the high operating potential of KMF (4.02 V vs K+/K) and compatible with K+ intercalation into graphite, currently the most critical hurdle to adoption. Here, we combine a KMF cathode and a graphite anode with a KFSI in Pyr1,3FSI ionic liquid electrolyte for the first time and show unprecedented performance. We use high-throughput techniques to optimize the KMF morphology for operation in this electrolyte system, achieving 119 mA h g–1 at 4 V vs K+/K and a Coulombic efficiency of >99.3%. In the same ionic liquid electrolyte, graphite shows excellent electrochemical performance and we demonstrate reversible cycling by operando X-ray diffraction. These results are a significant and essential step forward toward viable potassium-ion batteries