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1. Polymorphism in Weberite Na ₂ Fe ₂ F ₇ and its Effects on Electrochemical Properties as a Na-Ion Cathode

   https://doi.org/10.1021/acs.chemmater.3c00233  

   Foley, Emily E; Wu, Vincent C; Jin, Wen; Cui, Wei; Yoshida, Eric; Manche, Alexis; Clément, Raphaële J (May 2023, Chemistry of Materials)

   Weberite-type sodium transition metal fluorides (Na2M2+Mʹ3+F7) have emerged as potential high performance sodium intercalation cathodes, with predicted energy densities in the 600-800 Wh/kg range and fast Na-ion transport. One of the few weberites that has been electrochemically tested is Na2Fe2F7, yet inconsistencies in its reported structure and electrochemical properties have hampered the establishment of clear structure-property relationships. In this study, we reconcile structural characteristics and electrochemical behavior using a combined experimental-computational approach. First principles calculations reveal the inherent metastability of weberite-type phases, the close energetics of several Na2Fe2F7 weberite polymorphs, as well as their predicted (de)intercalation behavior. We find that as-prepared Na2Fe2F7 samples inevitably contain a mixture of polymorphs, with local probes such as solid-state nuclear magnetic resonance and Mössbauer spectroscopy providing unique insights into the distribution of Na and Fe local environments. Polymorphic Na2Fe2F7 exhibits a respectable initial capacity yet steady capacity fade, a consequence of the transformation of the Na2Fe2F7 weberite phases to the more stable perovskite-type NaFeF3 phase upon cycling, as revealed by ex situ synchrotron X-ray diffraction and solid-state NMR. Overall, these findings highlight the need for greater control over weberite polymorphism and phase stability through compositional tuning and synthesis optimization. 

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2. High Voltage Irreversibilities in NASICON Na _3+y V _2–y Mg _y (PO ₄ ) ₃ Cathodes

   https://doi.org/10.1021/acs.chemmater.5c01434  

   Gonzalez-Correa, Eliovardo; Mazumder, Madhulika; Kumar, Keshav; Ghosh, Subham; Senguttuvan, Premkumar; Clément, Raphaële J (August 2025, Chemistry of Materials)

   Na3+yV2-yMgy(PO4)3 (0 ≤ y ≤ 1) NASICON compounds, thanks to their relative simplicity and the presence of a single redox-active species, are of interest for understanding the function and limitations of more complex, mixed transition metal NASICON-type Na-ion cathodes. Partial Mg substitution for V in Na3+yV2-yMgy(PO4)3 leads to a gradual transition from a two-phase to a solid solution Na (de)intercalation mechanism during electrochemical cycling. When cycled over a narrow voltage window (3.8 – 2.75 V vs. Na+/Na0), these cathodes exhibit good structural and electrochemical reversibility, leading to a high capacity retention. Yet, when the upper cutoff voltage is increased from 3.8 V to 4.2 V, significant irreversibilities arise, accompanied by a notable evolution of the electrochemical profiles during the first cycle. We focus here on the y = 0.5 and 1 compounds and use a combination of electrochemical testing, advanced characterization, and first principles density functional theory calculations to investigate the redox and structural processes taking place at high potentials. In-situ and ex-situ X-ray diffraction, and 23Na and 31P solid-state NMR reveal bulk structural rearrangements, at least partly caused by Na extraction from Na(1) sites. For the y = 1 cathode, an irreversible phase transition leads to a change in the symmetry of the crystal structure from rhombohedral to monoclinic. For both compounds, substantial distortions of the VO6 octahedra and irreversible changes to the local structure are observed using X-ray absorption spectroscopy during high voltage cycling, but our 51V NMR results show no evidence for V migration. Instead, first principles nudged elastic band calculations suggest low energy migration pathways for Mg2+ into vacant face-sharing Na(1) sites at high states of charge. These findings are consistent with the significant evolution of the electrochemical profile during the first cycle, voltage hysteresis, and subsequent rapid capacity decay. More broadly, these results could suggest similar structural rearrangements (e.g., Mn migration) in related V- and Mn-containing NASICON cathodes, which also show poor reversibility when cycled up to high potentials. 

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3. The Role of FEC-Electrolyte Additive on the Chemo-Mechanical Stability of NaCrO ₂ Cathodes in Na-Ion Batteries

   https://doi.org/10.1149/MA2025-013319mtgabs  

   Teklie, Debash; Wable, Minal; Capraz, Omer Özgür (July 2025, ECS Meeting Abstracts)

   Na-ion batteries have taken more interest in recent years as an alternative battery chemistry to Li-ion batteries because of material abundance, cost, and sufficient volumetric energy density for large-scale energy storage applications. However, Na-ion batteries suffer from rapid capacity fade associated with chemo-mechanical instabilities such as the formation of resistive solid-electrolyte / cathode-electrolyte interphase (SEI/CEI) layers, irreversible phase formations, and particle fracture. The cathode materials are fragile, especially metal oxides, therefore Na-ion cathodes are more prone to mechanical deformations upon larger volumetric expansions/reductions during Na-ion intercalation. Electrolyte additives have been utilized to improve the electrochemical performance of Li-ion and Na-ion batteries by modifying the chemistry of the SEI layers. In situ stress measurements on Si anode in Li-ion batteries demonstrated the generation of less mechanical deformations in the electrode when cycled in the presence of FEC additives.¹However, there is not much known about the impact of electrolyte additives on the chemo-mechanical properties of CEI layers in Na-ion battery cathodes. Furthermore, the question still stands about how the electrolyte additives may impact the mechanical stability of the Na-ion cathodes. To address this gap, we systematically investigated the role of FEC additives on the electrochemical performance and associated chemo-mechanical instabilities in NaCrO2 cathodes. Experiments were performed in organic electrolytes with/without FEC additives. First, the talk will start with presenting the impact of FEC additives on the capacity retention and cyclic voltammeter profiles of NaCrO2 cathodes. Then, digital image correlation and multi-beam optical stress sensor techniques were employed to probe electrochemical strain and stress generation in the composite NaCrO2 cathodes during electrochemical cycling in organic electrolytes with/without FEC additives. Surface chemistry of the NaCrO2 cathodes after cycling was investigated with the FT-IR measurements. In summary, the talk will present contrast differences in the electrochemical and chemo-mechanical properties of NaCrO2 cathodes when cycled in the presence of the FEC additives. Acknowledgement: This work is supported by National Science Foundation (award number 2321405). Reference: 1) Tripathi et al 2023 J. Electrochem. Soc. 170 090544 

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4. In Operando Strain Evolution in Sodium Chromium Oxide Cathode for Na-Ion Batteries during Battery Cycling

   https://doi.org/10.1149/MA2023-015879mtgabs  

   Wable, Minal; Bal, Batuhan; Capraz, Omer_Ozgur Özgür (August 2023, ECS Meeting Abstracts)

   Layer-structured Na intercalation compounds such as NaxMO2 (M=Co, Mn, Cr) have attracted much attention as cathode materials for sodium-ion batteries due to their high volumetric and gravimetric energy densities. Among them, NaCrO2 with layered rock salt structure is one of the promising cathodes since NaCrO2 has a desirable flat and smooth charge/discharge voltage plateau.1 In addition, NaCrO2 has the highest thermal stability at charged state which makes it a potentially safer cathode material.2 The NaCrO2 exhibits a reversible capacity of 110 mAh g-1 with good cycling performance.3 However, the transition metal oxide (TMO) cathode materials in NIBs undergo severe chemo-mechanical deformations which leads to capacity fade and poor cycling and is the limiting factor of NIBs. The electrochemical characterization and examination of the electrode structure were the primary focus of several investigations. To improve the lifespan and performance of electrode materials for Na-ion batteries, it is vital to comprehend how Na ions impact the chemo-mechanical stability of the electrodes. In this talk, we will discuss the driving forces behind the structural and interfacial deformations on NaCrO2 cathodes. Digital image correlation measurements were conducted to probe strain evolution in the electrode during cycling. The free-standing composite NaCrO2 electrode was used for stain measurements in custom-cell assembly. The battery was cycled against Na metal in 1 M NaClO4 in PC. The first part of the study involves structural and interfacial deformations in the lower voltage range of 2.3 V to 3.5 V where x<0.5 in NaxCrO2. And the second part focuses on the structural and interfacial deformations in the voltage range of 2.3 V to 4.7 V where x>0.5 in NaxCrO2. In the preliminary studies, we observed that the initial insertion of Na ions leads to negative strain evolution (contraction) in the electrode, followed by expansions in the electrode at a higher state of discharge. Similar phenomena are also observed during charge cycles, where extraction of Na results in an initial contraction in the electrode, followed by expansion at a higher state of charge. Understanding the mechanisms behind chemo-mechanical deformations will allow to tune structure & material property for better electrochemical performance. 

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5. Enhanced charge storage of nanometric ζ-V ₂ O ₅ in Mg electrolytes

   https://doi.org/10.1039/d0nr05060a  

   Johnson, Ian D.; Nolis, Gene; Yin, Liang; Yoo, Hyun Deog; Parajuli, Prakash; Mukherjee, Arijita; Andrews, Justin L.; Lopez, Mario; Klie, Robert F.; Banerjee, Sarbajit; et al (November 2020, Nanoscale)

   null (Ed.)

   V 2 O 5 is of interest as a Mg intercalation electrode material for Mg batteries, both in its thermodynamically stable layered polymorph (α-V 2 O 5 ) and in its metastable tunnel structure (ζ-V 2 O 5 ). However, such oxide cathodes typically display poor Mg insertion/removal kinetics, with large voltage hysteresis. Herein, we report the synthesis and evaluation of nanosized ( ca . 100 nm) ζ-V 2 O 5 in Mg-ion cells, which displays significantly enhanced electrochemical kinetics compared to microsized ζ-V 2 O 5 . This effect results in a significant boost in stable discharge capacity (130 mA h g −1 ) compared to bulk ζ-V 2 O 5 (70 mA h g −1 ), with reduced voltage hysteresis (1.0 V compared to 1.4 V). This study reveals significant advancements in the use of ζ-V 2 O 5 for Mg-based energy storage and yields a better understanding of the kinetic limiting factors for reversible magnesiation reactions into such phases. 

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