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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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This content will become publicly available on August 26, 2026
High Voltage Irreversibilities in NASICON Na 3+y V 2–y Mg y (PO 4 ) 3 Cathodes
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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- Award ID(s):
- 2141754
- PAR ID:
- 10682777
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- Chemistry of Materials
- Volume:
- 37
- Issue:
- 16
- ISSN:
- 0897-4756
- Page Range / eLocation ID:
- 6323 to 6334
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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