Highly Symmetrical Six‐Transition Metal Ring Units Promising High Air‐Stability of Layered Oxide Cathodes for Sodium‐Ion Batteries

Abstract

Layered oxides are the most prevalent cathodes for sodium-ion batteries (SIBs), but their poor air stability significantly limits their practical application owing to the rapid performance degradation of aged materials and the cost increase for material storage and transportation. Here, an effective strategy of constructing stable transition metal (TM) layers with a highly symmetrical six-TM ring is suggested to enhance structure stability, thus hindering ambient air corrosion. The density functional theory calculations reveal that the higher symmetry ensures a higher thermodynamic energy for H 2 O insertion into Na layer. The combined analyses of selected area electron diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis, and chemical titration indicate that the six-TM ring structure can effectively suppress the series of aging processes including water insertion, the spontaneous loss of lattice sodium, TM valence increment and residual alkali formation. Benefiting from the overall suppression of aging process, the strategy results in an excellent improvement in capacity retention after air exposure from 13.57% to 95.59%, and exhibits a good universality for both P2-and O3-cathodes, which are the two most common structures of Nabased layered oxides with different aging mechanism. These findings provide new insight to design high-performance cathodes for SIBs.

Boosting the Ultrastable High-Na-Content P2-type Layered Cathode Materials with Zero-Strain Cation Storage via a Lithium Dual-Site Substitution Approach

“…Sodium ion batteries (SIBs) have been regarded as a prospective alternative to lithium-ion batteries (LIBs) owing to the merits of sodium abundance in the Earth’s crust and the low price of sodium resources. − Exploiting long-life cathode materials with superior reliability plays an utmost role in the commercial application of SIBs. Among most candidates, layered oxides Na x TMO 2 (TM represents transition metals) have gained considerable attention due to high theoretical specific capacity, feasible synthesis, and environmental benignity. − Considering the coordination of the Na-ions [i.e., octahedral (O) or prismatic (P)] and the number of the O-ion layer or TMO 6 layer in a one-unit cell (typically, 1–3), Na x TMO 2 cathode materials are categorized into P2, P3, O2, and O3 phases. ,,, P2-type and O3-type layered oxides are better suited to functioning as cathode materials for SIBs.…”

O3-Type Na0.95Ni0.40Fe0.15Mn0.3Ti0.15O2 Cathode Materials with Enhanced Storage Stability for High-Energy Na-Ion Batteries

“…Among various sodium cathode candidates, layered transition metal oxides Na x TMO 2 (where TM refer to transition metal ion) have attracted much attention due to its advantages such as high specific capacity, simple preparation, and environmental friendliness. − Compared with P2- and P3-type layered oxides, the sufficient sodium content enables O3-type layered oxides to be advantageous over counterparts in full-cell applications. − However, their cycle life is still subject to undesired structural degradation caused by transition metal slab sliding upon charging to high voltages (>4 V), leading to fast capacity decay and poor cycling stability in NIBs. − In addition, most O3-type materials suffer from notable performance deterioration when storing in a humid environment, which undoubtedly increases their cost of transportation and preservation. ,,− For O3-NaNi 0.5 Mn 0.5 O 2 , when the charging cutoff voltage is greater than 4.1 V, its specific capacity is up to 180 mAh g –1 . , However, a series of complex phase transitions (O3–O′3–P3–P′3–P3′–O1) cause significant internal stress, leading to the collapse of layered structure, resulting in capacity decay and poor rate performance. ,, In addition, when O3-NaNi 0.5 Mn 0.5 O 2 was exposed to air for 2 h, the structure changed from O3 phase to O′3- and P3-Na 1– y Ni 0.5 Mn 0.5 O 2 , which also leads to the degradation in electrochemical performance. , Undoubtedly, further practical application of O3-type layered oxides for NIBs requires addressing both phase transition reversibility during deep desodiation and humid sensitivity when exposed to ambient air.…”

Local Construction of Mn-Based Layered Cathodes through Covalency Modulation for Sodium-Ion Batteries

“…5 Among different promising cathode materials, Mn-based layered oxides (Na x MnO 2 ) have attracted extensive attention owing to their high theoretical specific capacity, simple synthetic process, and low cost. 6 Na x MnO 2 materials are mainly divided into P2-and O3-type structures based on the oxygen layer stacking manners and Na + occupation sites. 7 Specifically, P2-type materials exhibit lower Na + migration energy barriers because of more open framework structures and larger trigonal prismatic sites.…”

“…Sodium-ion batteries (SIBs) have been considered as promising sustainable solutions for large-scale energy storage because of the abundance of low-cost sodium raw materials. − However, the shortage of suitable cathode materials largely hinders the practical application of SIBs . Among different promising cathode materials, Mn-based layered oxides (Na x MnO 2 ) have attracted extensive attention owing to their high theoretical specific capacity, simple synthetic process, and low cost . Na x MnO 2 materials are mainly divided into P2- and O3-type structures based on the oxygen layer stacking manners and Na + occupation sites .…”