O3-NaFe(1/3-x)Ni1/3Mn1/3AlxO2 Cathodes with Improved Air Stability for Na-Ion Batteries

Charge Tuning and Anchor Effect Achieving Stable High-Voltage Layered Metal Oxides for Sodium-Ion Battery

Layered metal oxides (LMOs) that can stably operate at high voltage are vital to developing high-energy sodium-ion batteries (SIBs). However, the irreversible oxygen redox reaction of LMOs at the high voltage region (i.e., >4.0 V vs. Na+/Na) will cause serious oxygen evolution and structural instability, and therefore sharp capacity fading. Herein, we report the charge tuning and anchor effect of La doping to enhance the cycling stability of layered NaNi1/3Fe1/3Mn1/3O2 (NFM) under extreme conditions (e.g., high voltage and temperature). The La doping introduces additional negative charges to the oxygen ligands of transition metals (TM) and facilitates charge transfer, thereby reducing oxygen evolution at 4.1 V and enhancing the kinetics of the redox process. Moreover, the as-formed LaO6 octahedra serve as stable anchors for the TM layers, preventing the formation of lattice nanocracks. As a result, the La-modified NFM exhibits a high capacity of 173.4 mAh g-1 at 0.1C (1C = 150 mA g-1) and capacity retention of 71.21% at 2.0-4.2 V after 500 cycles at 1C, which are higher than those of NFM. Additionally, the full cell achieves a high energy density of 438.78 Wh kg-1 (based on cathode mass), with a retention of 70.2% over 400 cycles, implying the great application potential of this concept.

Crystal structure modulation enabling fast charging and stable layered sodium oxide cathodes

Layered oxide cathodes show great promise for commercial applications due to their low cost, high specific capacity, and energy density. However, their rapid capacity decay and slow kinetics primarily caused by harmful phase transitions and a high energy barrier for Na+ diffusion result in inferior battery performance. Herein, we modulate the crystal structure of layered oxide cathodes by replacing the Fe3+ site with Al3+, which strengthens the transition metal layers and enlarges the Na translation layer owing to the smaller ion radius of Al3+ and the stronger bonding energy of Al-O. This restrains the Jahn-Teller effect owing to transition metal dissolution and improves the electrochemical kinetics. Consequently, the modified cathodes exhibited an excellent high-rate performance of 111 mA h g-1 at a high rate of 5.0C and an unexpectedly long cycling life with a 73.88% capacity retention rate after 500 cycles at 5.0C, whereas the bare cathode exhibited a rate performance of 97.3 mA h g-1 with a low capacity retention rate of 48.42% after 500 cycles at 5.0C. This study provides valuable insights into tuning the crystal structure for designing fast charging and highly stable O3-type cathodes.

A trace lithium occupation strategy enhances the structural stability of a layered NaNi0.4Fe0.2Mn0.4O2 cathode enabling stable cycling

Trace lithium ions are embedded into sodium-ion sites through Li+/Na+ exchange to enhance the structural stability and mitigate interface erosion of layered NaNi0.4Fe0.2Mn0.4O2. The harmful O'3 phase transition in L-NFM was alleviated effectively by Li+ sites during the cycling process.

Suppressed P3-O3' Phase Transition and Enhanced Na+ Diffusion Kinetics of O3-Type Layered Oxide Cathode via Multivariate Doping

O3-NaNi1/3Fe1/3Mn1/3O2 has attracted much attention as a cathode for sodium-ion batteries, because of its low cost and high sodium-ion storage capacity. However, its slow Na+ diffusion kinetics and harmful P3-O3' phase transition with severe bulk strain at high voltage leads to poor rate capability and fast capacity fading. Herein, we propose a multivariate doping strategy with Cu, Mg, and Ti ions to solve the above problems of the O3-NaNi1/3Fe1/3Mn1/3O2 cathode. The O3-Na(Ni1/3Fe1/3Mn1/3)0.9Cu0.03Mg0.02Ti0.05O2 (NFMCMT) cathode exhibits enlarged Na+ diffusion channels and a strengthened layered structure, which improves the Na+ diffusion kinetics, suppresses the harmful P3-O3' phase transition at high voltages, and inhibits the intragranular fatigue cracks, leading to enhanced rate capability and cycling performance. As a result, the NFMCMT exhibits outstanding performance in the 2-4.1 V voltage window, delivers a discharge capacity of 151.2 mAh g-1 with the 81.5% capacity retention for 100 cycles at 0.1 C, and 83.1% capacity retention for 300 cycles at 5 C. Especially in the rate performance, the NFMCMT delivers a 115.6 mAh g-1 and 100.1 mAh g-1 discharge capacity even at 5 and 10 C. This work provides an effective multivariate doping strategy for development of high-performance layered oxide cathodes for sodium-ion batteries.

A Na-Free Surface Enables "Three-In-One" Enhancement of Structural, Interfacial and Air Stability for Sodium-Ion Battery Cathodes

O3-type layered oxides are regarded as one of the most promising cathode materials for sodium-ion batteries. However, the multistep phase transitions, severe electrode/electrolyte parasitic reactions, and moisture sensitivity are challenging for their practical application because of the highly active Na+. Here, a Na-free layer is built on the surface of NaNi1/3Mn1/3Fe1/3O2 (NMF111) via a leaching treatment and the subsequent surface reconstruction. Accordingly, both the structural degradation from bulk to surface and the overgrowth of the solid electrolyte interface (SEI) are greatly ameliorated, which results in the improved capacity retention of modified NMF111 from 58.3% to 89.6% after 400 cycles at 1 C. Besides, the Na-free surface with rock-salt structure prevents the H+/Na+ exchange and then enables good reversibility and low polarization of the optimal NMF111 when exposed to wet air (50% RH) for 4 days. This work opens a new avenue for the comprehensive cyclability improvement of layered oxides via surface reconstruction.

An Active Strategy to Reduce Residual Alkali for High-Performance Layered Oxide Cathode Materials of Sodium-Ion Batteries

Residual alkali is one of the biggest challenges for the commercialization of sodium-based layered transition metal oxide cathode materials since it can even inevitably appear during the production process. Herein, taking O3-type Na0.9Ni0.25Mn0.4Fe0.2Mg0.1Ti0.05O2 as an example, an active strategy is proposed to reduce residual alkali by slowing the cooling rate, which can be achieved in one-step preparation method. It is suggested that slow cooling can significantly enhance the internal uniformity of the material, facilitating the reintegration of Na+ into the bulk material during the calcination cooling phase, therefore substantially reducing residual alkali. The strategy can remarkably suppress the slurry gelation and gas evolution and enhance the structural stability. Compared to naturally cooled cathode materials, the capacity retention of the slowly cooled electrode material increases from 76.2% to 85.7% after 300 cycles at 1 C. This work offers a versatile approach to the development of advanced cathode materials toward practical applications.

High-Entropy Layered Oxide Cathode Enabling High-Rate for Solid-State Sodium-Ion Batteries

Na-ion O3-type layered oxides are prospective cathodes for Na-ion batteries due to high energy density and low-cost. Nevertheless, such cathodes usually suffer from phase transitions, sluggish kinetics and air instability, making it difficult to achieve high performance solid-state sodium-ion batteries. Herein, the high-entropy design and Li doping strategy alleviate lattice stress and enhance ionic conductivity, achieving high-rate performance, air stability and electrochemically thermal stability for Na0.95Li0.06Ni0.25Cu0.05Fe0.15Mn0.49O2. This cathode delivers a high reversible capacity (141 mAh g-1 at 0.2C), excellent rate capability (111 mAh g-1 at 8C, 85 mAh g-1 even at 20C), and long-term stability (over 85% capacity retention after 1000 cycles), which is attributed to a rapid and reversible O3-P3 phase transition in regions of low voltage and suppresses phase transition. Moreover, the compound remains unchanged over seven days and keeps thermal stability until 279 ℃. Remarkably, the polymer solid-state sodium battery assembled by this cathode provides a capacity of 92 mAh g-1 at 5C and keeps retention of 96% after 400 cycles. This strategy inspires more rational designs and could be applied to a series of O3 cathodes to improve the performance of solid-state Na-ion batteries.

Multifunctional Effect of Fe Substitution in Na Layered Cathode Materials for Enhanced Storage Stability

Developing stable cathode materials that are resistant to storage degradation is essential for practical development and industrial processing of Na-ion batteries as many sodium layered oxide materials are susceptible to hygroscopicity and instability upon exposure to ambient air. Among the various layered compounds, Fe-substituted O3-type Na(Ni1/2Mn1/2)1-xFexO2 materials have emerged as a promising option for high-performance and low-cost cathodes. While previous reports have noted the decent air-storage stability of these materials, the role and origin of Fe substitution in improving storage stability remain unclear. In this study, we investigate the air-resistant effect of Fe substitution in O3-Na(Ni1/2Mn1/2)1-xFexO2 cathode materials by performing systematic surface and structural characterizations. We find that the improved storage stability can be attributed to the multifunctional effect of Fe substitution, which forms a surface protective layer containing an Fe-incorporated spinel phase and decreases the thermodynamical driving force for bulk chemical sodium extraction. With these mechanisms, Fe-containing cathodes can suppress the cascades of cathode degradation processes and better retain the electrochemical performance after air storage.

Understanding the Aging Mechanism of Na-Based Layered Oxide Cathodes with Different Stacking Structures

Manganese-based layered oxides are one of the most promising cathodes for Na-ion batteries, but the prospect of their practical application is challenged by high sensitivity to ambient air. The stacking structure of materials is critical to the aging mechanism between layered oxides and air, but there remains a lack of systematic study. Herein, comprehensive research on model materials P-type Na0.50MnO2 and O-type Na0.85MnO2 reveals that the O-phase displays a much higher dynamic affinity toward moisture air compared to P-type compounds. For air-exposed O-type material, Na+ ions are extracted from the crystal lattice to form alkaline species at the surface in contact with air, accompanying by the increase of the valence state of transition metals. The series of undesired reactions result in an increase of interfacial resistance and huge capacity loss. Comparatively, the insertion of H2O into the Na layer is the main reaction during air-exposure of P-type material, and the inserted H2O can be extracted by high-temperature treatment. The H2O de/insertion process not only causes no performance degradation but also can enlarge the interlayer distance. With these understandings, we further propose a washing-resintering strategy to recover the performance of aged O-type materials and an aging strategy to build high-performance P-type materials.