Moisture-Driven Degradation Pathways in Prussian White Cathode Material for Sodium-Ion Batteries

The high-theoretical-capacity (∼170 mAh/g) Prussian white (PW), Na$_{x}$Fe[Fe(CN)$_{6}$]$_{y}$·nH$_{2}$O, is one of the most promising candidates for Na-ion batteries on the cusp of commercialization. However, it has limitations such as high variability of reported stable practical capacity and cycling stability. A key factor that has been identified to affect the performance of PW is water content in the structure. However, the impact of airborne moisture exposure on the electrochemical performance of PW and the chemical mechanisms leading to performance decay have not yet been explored. Herein, we for the first time systematically studied the influence of humidity on the structural and electrochemical properties of monoclinic hydrated (M-PW) and rhombohedral dehydrated (R-PW) Prussian white. It is identified that moisture-driven capacity fading proceeds via two steps, first by sodium from the bulk material reacting with moisture at the surface to form sodium hydroxide and partial oxidation of Fe$^{2+}$ to Fe$^{3+}$. The sodium hydroxide creates a basic environment at the surface of the PW particles, leading to decomposition to Na$_{4}$[Fe(CN)$_{6}$] and iron oxides. Although the first process leads to loss of capacity, which can be reversed, the second stage of degradation is irreversible. Over time, both processes lead to the formation of a passivating surface layer, which prevents both reversible and irreversible capacity losses. This study thus presents a significant step toward understanding the large performance variations presented in the literature for PW. From this study, strategies aimed at limiting moisture-driven degradation can be designed and their efficacy assessed

Synthesis and Structural Characterization of Ba7Li11Bi10 and AE4(Li,Tr)7Pn6 (AE = Sr, Ba, Eu; Tr = Ga, In; Pn = Sb, Bi)

Reported are the synthesis and crystal structure of Ba7Li11Bi10, a new ternary compound crystallizing in its own type with the monoclinic space group C2/m (a = 18.407(3) Å, b = 5.0258(9) Å, and c = 18.353(3) Å; β = 104.43(1)°; Pearson symbol mS56), and those of the structurally related quaternary phases Ba4(Li1−xGax)7Sb6, Ba4(Li1−xInx)7Sb6, Ba4(Li1−xInx)7Bi6, and Eu4(Li1−xInx)7Bi6 (crystallizing in the Eu4Li7Bi6 structure type with the same monoclinic space group C2/m (a = 18.4045(13)–17.642(4) Å, b = 5.012(4)–4.8297(10) Å, and c = 13.2792(10)–12.850(3) Å, β = 126.80(1)–125.85(1)°; Pearson symbol mS34). All studied compounds are identified among the products of the high-temperature reactions of the corresponding elements. Both types of crystal structures are based on corner- and edge-linked Li-centered Sb4 (or Bi4) tetrahedra, Sb6 (or Bi6) octahedra, and Sb2 or Bi2 dumbbells. Given the similarities between the two structures, it might be proposed that they represent the simplest members of a potentially large homologous series described with the general formulae (BaLi3Sb2)n(Ba3Li4Sb4)m or (BaLi3Bi2)n(Ba3Li4Bi4)m, where the more complicated “7-11-10” phase is the member with n = 2 and m = 1, while the “4-7-6” one is the intergrowth of the two components in an equal ratio. The computed electronic band structures of Ba7Li11Bi10 and idealized Ba4Li7Bi6 (a model for Ba4(Li1−xInx)7Bi6) are also discussed

Investigation of Valence Mixing in Sodium-Ion Battery Cathode Material Prussian White by Mossbauer Spectroscopy

Prussian white (PW), Na2Fe [Fe(CN)(6)], is a highly attractive cathode material for sustainable sodium-ion batteries due to its high theoretical capacity of similar to 170 mAhg(-1) and low-cost synthesis. However, there exists significant variability in the reported electrochemical performance. This variability originates from compositional flexibility possible for all Prussian blue analogs (PBAs) and is exasperated by the difficulty of accurately quantifying the specific composition of PW. This work presents a means of accurately quantifying the vacancy content, valence distribution, and, consequently, the overall composition of PW via Mossbauer spectroscopy. PW cathode material with three different sodium contents was investigated at 295 and 90 K. The observation of only two iron environments for the fully sodiated compound indicated the absence of [Fe(CN)(6)](4-) vacancies. Due to intervalence charge transfer between iron centers at 295 K, accurate determination of valences was not possible. However, by observing the trend of spectral intensities and center shift for the nitrogen-bound and carbon-bound iron, respectively, at 90 K, valence mixing between the iron sites could be quantified. By accounting for valence mixing, the sum of iron valences agreed with the sodium content determined from elemental analysis. Without an agreement between the total valence sum and the determined composition, there exists uncertainty around the accuracy of the elemental analysis and vacancy content determination. Thus, this study offers one more stepping stone toward a more rigorous characterization of composition in PW, which will enable further optimization of properties for battery applications. More broadly, the approach is valuable for characterizing iron-based PBAs in applications where precise composition, valence determination, and control are desired

A thermogravimetric study of thermal dehydration of copper hexacyanoferrate by means of model-free kinetic analysis

The kinetics of thermal dehydration of K2x/3Cu[Fe(CN)6]2/3·nH2O was studied using thermogravimetry for x = 0.0 and 1.0. Data from both non-isothermal and isothermal measurements was used for model-free kinetic analysis by the Friedman and KAS methods. The water content was determined to be n = 2.9 – 3.9, with an additional ~10% of water, likely surface adsorbed, that leaves very fast when samples are exposed to a dry atmosphere. The determined activation energies are 19 kJ (mol H2O)-1 for x = 0.0 and 16 kJ (mol H2O)-1 for x = 1.0. The dehydration is adequately described as a diffusion controlled single step reaction following the D3 Jander model. The determined dehydration enthalpy is, 11 kJ (mol H2O)-1 for x = 0.0 and 27 kJ (mol H2O)-1 for x = 1.0, relative to that of water. The increase with increasing x is evidence for that the H2O molecules form bonds to the incorporated K+ ions

Influence of sodium content on the thermal behavior of low vacancy Prussian white cathode material

Rechargeable sodium-ion batteries are the most attractive substitutes for lithium-ion batteries in large-scale energy storage devices due to wide spread reserves and low-cost of sodium resources and the similarities between sodium and lithium chemistry. However, finding a suitable cathode material is still a hurdle to be overcome. To date, Prussian white (PW), Na$_x$Fe[Fe(CN)$_6$]$_y$·nH$_2$O has stood out as one of the most promising Na-host materials due to its low cost, facile synthesis and competitive electrochemical capacity. Despite this, there are concerns that this material will thermally decompose at relatively low temperatures to form cyanogen gas, which is a safety hazard. Thus, low vacancy Na$_x$Fe[Fe(CN)$_6$]$_y$·nH$_2$O (x = 1.5, 1, 0.5 and 0) has been synthesized, and the influence of x on its thermal behavior systematically investigated. It is demonstrated that the thermal decomposition temperature, water content and moisture sensitivity of the samples strongly depend on the sodium content. The sample with x = 1.5 is found to be the most thermally stable and has the highest water content under the same experimental conditions. In addition, the sodium-rich samples (x = 1.5, 1 and 0.5) have higher surface water than the sodium-deficient one (x = 0). The local structure for this sample is also very different to the sodium-rich ones. Our findings offer new insights into the profound implications of proper material handling and safer operating conditions for practical Na-ion batteries and may be extended to analogous systems

Structural-electrochemical relations in the aqueous copper hexacyanoferrate-zinc system examined by synchrotron X-ray diffraction

The storage process of Zn2+ in the Prussian blue analogue (PBA) copper hexacyanoferrate (Cu[Fe(CN)6]2/3-nH2O - CuHCF) framework structure in a context of rechargeable aqueous batteries is examined by means of in operando synchrotron X-ray diffraction. Via sequential unit-cell parameter refinements of time-resolved diffraction data, it is revealed that the step-profile of the cell output voltage curves during repeated electrochemical insertion and removal of Zn2+ in the CuHCF host structure is associated with a non-linear contraction and expansion of the unit-cell in the range 0.36 < x < 1.32 for Znx/3Cu[Fe(CN)6]2/3-nH2O. For a high insertion cation content there is no apparent change in the unit-cell contraction. Furthermore, a structural analysiswith respect to the occupancies of possible Zn2+ sites suggests that the Fe(CN)6 vacancies within the CuHCF framework play an important role in the structural-electrochemical behavior of this particular system. More specifically, it is observed that Zn2+ swaps position during electrochemical cycling, hopping between cavity sites to vacant ferricyanide sites

Neutron Diffraction and EXAFS Studies of K_2<i>x</i>/3Cu[Fe(CN)₆]_2/3·<i>n</i>H₂O

The crystal structure of copper hexacyanoferrate (CuHCF), K_2<i>x/</i>3Cu­[Fe­(CN)₆]_2/3·<i>n</i>H₂O, with nominal compositions <i>x</i> = 0.0 and <i>x</i> = 1.0 was studied by neutron powder diffraction (NPD) and extended X-ray absorption fine structure (EXAFS) spectroscopy. The compound crystallizes in space group <i>Fm</i>3̅<i>m</i>, with <i>a</i> = 10.1036(11) Å and <i>a</i> = 10.0588(5) Å for <i>x</i> = 0.0 and <i>x</i> = 1.0, respectively. Difference Fourier maps for <i>x</i> = 0.0 show that the coordinated water molecules are positioned at a site 192l close to vacant N positions in the −Fe–C–N–Cu– framework, while additional zeolitic water molecules are distributed over three sites (8c, 32f, and 48g) in the −Fe–C–N–Cu– framework cavities. The refined water content for <i>x</i> = 0.0 is 16.8(8) per unit cell, in agreement with the ideal 16 (<i>n</i> = 4). For <i>x</i> = 1.0, the refinement suggests that 2.6 K atoms per unit cell (<i>x</i> = 0.98) are distributed only over the sites 8c and 32f in the cavities, and 13.9(7) water per unit cell are distributed over all the four positions. The EXAFS data for Fe, Cu, and K K-edges are in agreement with the NPD data, supporting a structure model with a linear −Fe–C–N–Cu– framework and K⁺ ions in the cavities