Sebastian Buechele

Sodium manganese hexacyanoferrate Na₂Mn[Fe(CN)₆] (NaMnHCF) is a promising cathode material for sodium‐ion batteries, owing to its voltage profile similar to that of lithium iron phosphate (LFP) and its use of abundant, inexpensive resources. This study presents full cell cycling data for NaMnHCF against hard carbon (HC) anodes with various common carbonate‐based electrolytes across different voltage windows. Post‐mortem analyses indicate that, in addition to NaMnHCF degradation, Na⁺‐ion inventory loss significantly contributes to capacity decline during cycling. Surprisingly, an ICP‐OES analysis of the post‐mortem anodes show that the correct electrolyte choice can entirely prevent the commonly cited manganese dissolution of NaMnHCF during cycling. This work also highlights methods for characterizing and processing NaMnHCF and the broader Prussian White family of materials, helping to introduce these materials to a wider audience. Finally, a comparison between NaMnHCF/HC and LFP/graphite is provided, examining both cost and electrochemical performance.

Among the various cathode active materials for Na-ion batteries Prussian Blue Analogs (PBA) are a promising candidate. From a cost and sustainability perspective, Na₂Mn[Fe(CN)]₆ (NaMnHCF) is of particular interest due to its high potential and superior electrochemical performance.^[1] However, it suffers from poor cycling stability which is believed to be partly due to structural distortions caused by the Jahn-Teller effect of oxidized Mn³⁺ and dissolution of Mn in the electrolyte.^[1,2] An alternative PBA like Na₂Fe[Fe(CN)]₆ (NaFeHCF) does not have these issues, but operates at a lower average potential of about 3.15 V versus Na⁺/Na during discharge (versus 3.45 V for NaMnHCF).^[3] Therefore, in this study, the systematic replacement of Mn is investigated to find an optimum of high potential and good cycling stability.^[4] As an example, Figure 1 shows the voltage curves for the first C/20 cycle in sodium half cells with Na₂Mn[Fe(CN)]₆ (green) or Na₂Mn_0.8Fe_0.2[Fe(CN)]₆ (blue). The material with the higher Fe content clearly shows a second, lower plateau and an overall lower potential. The materials were compared in terms of their electrochemical properties and characterized by SEM, XRD, ICP-OES and TGA. References Song, J., Wang, L., Lu, Y., Liu, J., Guo, B., Xiao, P., ... & Goodenough, J. B. (2015). Journal of the American Chemical Society, 137(7), 2658-2664. Xu, C., Ma, Y., Zhao, J., Zhang, P., Chen, Z., Yang, C., ... & Hu, Y. S. (2023). Angewandte Chemie International Edition, 62(13), e202217761. Wang, L., Song, J., Qiao, R., Wray, L. A., Hossain, M. A., Chuang, Y. D., ... & Goodenough, J. B. (2015). Journal of the American Chemical Society, 137(7), 2548-2554. Chen, X., Xia, Y., Fang, X., Zhang, K., Xiong, Y., & Jian, Z. (2022). Journal of Materials Science, 57(29), 14015-14025. Figure 1

Sodium‐ion batteries are an emerging technology that is still at an early stage of development. The electrode processing for anode and cathode is expected to be similar to lithium‐ion batteries (drop‐in technology), yet a detailed comparison is not published. There are ongoing questions about the influence of the active materials on processing parameters such as slurry viscosity, coating thicknesses, drying times, and behavior during fast drying. Herein, the expected drying time for the same areal capacity of anodes (graphite vs. hard carbon) and cathodes (lithium iron phosphate vs. Prussian blue analogs) are compared based on respective specific capacities reported in the literature. Estimates are made for the materials’ impact on production speed or dryer length. Within the experimental part, water‐based slurries of the same composition are mixed using different active materials according to identical procedure and the viscosity is compared. When drying at a constant drying rate (0.75 g m⁻² s⁻¹), lithium iron phosphate electrodes with different areal capacities (1–3 mAh cm⁻²) are shown to have the highest adhesion. For high drying rates (3 g m⁻² s⁻¹) at constant areal capacity, especially the investigated electrodes based on hard carbon show that no binder migration occurs.

Dimethyl terephthalate (DMT) is a redox shuttle molecule that leads to unwanted self-discharge of lithium-ion cells. It can be created in situ as a breakdown product of polyethylene terephthalate (PET), which is a surprisingly common polymer for the adhesive tapes found in commercial cells. This study investigates the shuttling mechanism and electrochemical stability of DMT, as well as its impact on the performance of LFP/graphite pouch cells with LiFSI and LiPF₆ conducting salts. Cyclic voltammetry shows that DMT has a redox potential of 1.5 V vs Li⁺/Li and is redox active in the full voltage range of LFP/graphite cells. Ultra-high precision coulometry and open-circuit storage experiments show that DMT lowers the coulombic efficiency, increases the charge endpoint capacity slippage, and dramatically accelerates the reversible self-discharge of LFP/graphite pouch cells. Gas chromatography-mass spectrometry shows that DMT is stable over weeks in cells with LiPF₆, but only for several days in cells with LiFSI. A well-insulating solid-electrolyte interphase layer derived from vinylene carbonate can prevent DMT from shuttling. However, VC can be consumed, and passivation layers can deteriorate in aged cells, so the best way to prevent DMT-induced self-discharge of lithium-ion batteries is to eliminate PET components.

Prussian White is a promising active material for the positive electrode of sodium-ion batteries as it is comprised of Na, Mn, Fe, C, and N and thus offers high sustainability and low cost. However, exposure of Prussian White to moisture results in chemical changes due to the formation of surface contaminants, as well as structural changes due to the absorption of water into the bulk crystal structure. Here we report an analysis of the formation rate of surface contaminants and bulk water absorption by weight tracking, infrared spectroscopy, and X-ray diffraction over extended periods of storage in high relative humidity air for fully sodiated Na_1.8Mn_0.8Fe_0.2[Fe(CN)₆]_0.9 and partially sodiated Na_1.3Mn_0.8Fe_0.2[Fe(CN)₆]_0.9. Fully sodiated Prussian White gains almost 20% in mass due to the formation of interstitial water during 20 h of storage in 100% relative humidity at 25 °C. Surface hydroxides and carbonates are found after storage and a structural change from the rhombohedral to a monoclinic crystal structure is observed. It is found that vacuum drying of Prussian White powder or electrodes at 150 °C can remove the majority of interstitial water and restore the rhombohedral crystal structure, but not remove surface contaminants. Prussian White immersed in water during aqueous electrode processing also shows interstitial water and a monoclinic crystal structure, but no surface contaminants. This suggests that aqueous electrode processing of Prussian White is feasible when effective drying strategies are employed. Indeed, Prussian White electrodes made from H₂O-based slurries with CMC/NaPAA binder vacuum-dried at 150 °C show higher specific capacity and similar capacity retention in full cells as Prussian White electrode made from NMP-based slurries with PVDF binder.

Unwanted parasitic reactions in lithium-ion cells lead to self-discharge and inefficiency, especially at high temperatures. To understand the nature of those reactions this study investigates the open circuit storage losses of LFP/graphite and NMC811/graphite pouch cells with common alkyl carbonate electrolytes. The cells perform a storage test at 40 °C with a 500 h open circuit period after formation at temperatures between 40 °C and 70 °C. Cells formed at elevated temperature showed a high reversible storage loss that could be assigned to a redox shuttle generated in the electrolyte during formation. A voltage hold after formation can reduce the shuttle-induced self-discharge as indicated by significantly lower reversible storage losses, the absence of shuttling currents in cyclic voltammetry and improved metrics in ultra-high precision cycling. The addition of two weight percent vinylene carbonate can prevent redox shuttle generation and leads to almost zero reversible self-discharge.

Unwanted self-discharge of LFP/AG and NMC811/AG cells can be caused by in situ generation of a redox shuttle molecule after formation at elevated temperature with common alkyl carbonate electrolyte. This study investigates the redox shuttle generation for several electrolyte additives, e.g., vinylene carbonate and lithium difluorophosphate, by measuring the additive reduction onset potential, first cycle inefficiency and gas evolution during formation at temperatures between 25 and 70 °C. After formation, electrolyte is extracted from pouch cells for visual inspection and quantification of redox shuttle activity in coin cells by cyclic voltammetry. The redox shuttle molecule is identified by GC-MS and NMR as dimethyl terephthalate. It is generated in the absence of an effective SEI-forming additive, according to a proposed formation mechanism that requires residual water in the electrolyte, catalytic quantities of lithium methoxide generated at the negative electrode and, surprisingly, polyethylene terephthalate tape within the cell.