New Insights into the Performance Degradation of Fe-Based Layered Oxides in Sodium-Ion Batteries: Instability of Fe3+/Fe4+ Redox in α-NaFeO2,Chemistry of Materials

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New Insights into the Performance Degradation of Fe-Based Layered Oxides in Sodium-Ion Batteries: Instability of Fe3+/Fe4+ Redox in α-NaFeO2
Chemistry of Materials ( IF 7.2 ) Pub Date : 2015-09-11 00:00:00 , DOI: 10.1021/acs.chemmater.5b02918
Eungje Lee , Dennis E. Brown ₁ , Esen E. Alp , Yang Ren , Jun Lu , Jung-Je Woo , Christopher S. Johnson

Affiliation

The emergence of sodium-ion batteries (SIBs) employing cathodes based on earth abundant sodium and iron is expected to be ideal for large-scale electrical energy storage systems, for which the cost factor is of primary importance. However, these iron-based layered oxides still show unsatisfactory cycle performance, and the redox of the fleeting Fe³⁺/Fe⁴⁺ couple needs to be better understood. In this study, we examine the quasi-reversibility of the layered α-NaFeO₂ cathode in sodium-ion cells. A NaFeO₂ powder sample that has the O3-type layered structure was synthesized via a solid-state synthesis method. The changes in Fe oxidation states and crystallographic structures were examined during the electrochemical sodium cycling of the NaFeO₂ electrodes. Ex situ Mössbauer spectroscopy analysis revealed the chemical instability of Fe⁴⁺ in a battery cell environment: more than 20% of Fe⁴⁺ species that was generated in the desodiated Na_1–xFeO₂ electrode was spontaneously reduced back to Fe³⁺ states during open circuit storage of the charged cell. The in situ synchrotron X-ray diffraction further revealed the nonequilibrium phase transition behavior of the NaFeO₂ cathode. A new layered phase (denoted as O″3) was observed in the course of sodium deintercalation, and an asymmetric structural behavior during cycling was identified. These findings explain the quasi-reversibility of α-NaFeO₂ in the sodium cell and provide guidance for the future development of iron-based cathode materials for sodium-ion batteries.

中文翻译：

新的洞察铁基层状氧化物的钠离子电池性能下降：铁的不稳定³⁺ / Fe的⁴⁺在氧化还原α-NaFeO ₂

预计采用基于富含钠和铁的阴极的钠离子电池（SIB）的出现对于大型电能存储系统是理想的选择，对于这些系统而言，成本因素至关重要。然而，这些铁基层状氧化物仍显示出不令人满意的循环性能，并且需要更好地理解短暂的Fe ³⁺ / Fe ⁴⁺对的氧化还原。在这项研究中，我们检查层状α-NaFeO的准可逆性₂中钠离子电池阴极。具有O3的NaFeO ₂粉末样品通过固态合成方法合成了α-型层状结构。在NaFeO ₂电极的电化学钠循环过程中检查了Fe氧化态和晶体结构的变化。异位Mössbauer光谱分析揭示了电池单元环境中Fe ⁴⁺的化学不稳定性：在经过去离子处理的Na _1–_x FeO ₂电极中生成的超过20％的Fe ⁴⁺物种自发还原为Fe ³⁺态在带电电池的开路存储期间。原位同步加速器X射线衍射进一步揭示了NaFeO ₂的非平衡相变行为阴极。一种新的层状相（表示为○“3中脱嵌钠的过程中，观察到），并且在循环过程中的非对称结构的行为被确定。这些发现解释α-NaFeO的准可逆性₂在钠电池和用于铁基的阴极材料为钠离子电池的未来发展提供指导。

更新日期：2015-09-11

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