Approaches to Control Air Stability, Capacity Retention, Gas Generation, and Impedance Growth of Layered Oxide Positive Electrode Materials for Sodium-Ion Batteries

Sodium-ion batteries made of earth-abundant elements could potentially become affordable, scalable, and sustainable energy storage solutions. Among different sodium-ion positive electrode chemistries, layered transition metal oxides are of special interest due to their promising energy densities. Specific energy densities close to LiFePO4-based lithium-ion batteries have been demonstrated. [1] Promising capacity retention has been demonstrated as well, with >4,000 cycles to 80% at 30°C. [1] However, the poor air-stability of layered sodium-ion positive electrode materials poses challenges in materials storage and processing in both laboratory and industry scales. Moreover, sodium layered oxides can go through complex phase transitions during electrochemical sodium intercalation, which can result in poor capacity retention unless mitigated by effective materials design. Other challenges are gas generation, impedance growth and sodium plating during cycling, which must all be addressed to make viable layered oxide-based sodium-ion batteries for large-scale storage of renewable energy from wind and solar. In this contribution, we first explore compositional modification as an approach to mitigate structural degradation in sodium layered oxides during air exposure and sodium intercalation. A combination of substituent elements is utilized to dramatically improve air stability as evidenced by X-ray diffraction (XRD) and scanning electron microscopy (SEM) experiments. X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDS) are employed to understand the underlying mechanism that leads to improved air stability. Furthermore, coin cell results suggest that compositional modification enhances the capacity retention without sacrificing reversible capacity. [2] Secondly, we explore the performance of layered oxide positive electrodes in machine-made 250 mAh pouch cells with hard carbon negative electrodes. The capacity retention and impedance growth are studied in 40°C cycling tests with various electrolyte additives. High cycling stability with >99% capacity retention after 200 cycles can be attained with simple electrolyte additives. However, specialized additives need to be selected to suppress gas generation and impedance growth. Symmetric cell impedance spectroscopy is used to reveal whether impedance growth stems from the positive or negative electrode. [3] Gas generation is assessed by in-situ volume measurements using the Archimedes principle. [4] Interestingly, sodium-ion cells show distinct differences to lithium-ion batteries in terms of their gas generation during the formation cycle. Moreover, sodium plating can have a surprising effect on the gas generation during charge/discharge cycling. Using ultra-high precision coulometry, the critical conditions for sodium plating and its effect on cell performance are determined. In-situ volume measurements reveal a surprising mechanism that explains the interesting relationship between gas generation and cell voltage in sodium-ion batteries. [5] References [1] J. Barker, 242nd ECS Meeting, Atlanta (2022). [2] L. Zhang et al., manuscript in preparation. [3] H. Hijazi, Z. Ye, et al., manuscript in preparation. [4] C. Aiken et al., J. Electrochem. Soc. 161, A1548–A1554 (2014). [5] E. Alter et al., manuscript in preparation. Figure 1. Discharge capacity (a), discharge capacity normalized to cycle 5 (b), and voltage polarization vs cycle number (c) for Na-ion pouch cells cycling at C/5 and 40°C with various electrolyte additives in 1M NaPF6 EC:DEC (1:1 w/w). Figure 1

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