Mitigating Jahn-Teller Effect in Layered Cathode Material Via Interstitial Doping for High-Performance Sodium-Ion Batteries

Enabling Rapid and Stable Sodium Storage via a P2-Type Layered Cathode with High-Voltage Zero-Phase Transition Behavior

Currently, a major target in the development of Na-ion batteries is the concurrent attainment of high-rate capacity and long cycling stability. Herein, an advanced Na-ion battery with high-rate capability and long cycle stability based on Li/Ti co-doped P2-type Na0.67Mn0.67Ni0.33O2, a host material with high-voltage zero-phase transition behavior and fast Na+ migration/conductivity during dynamic de-embedding process, is constructed. Experimental results and theoretical calculations reveal that the two-element doping strategy promotes a mutually reinforcing effect, which greatly facilitates the transfer capability of Na+. The cation Ti4+ doping is a dominant high voltage, significantly elevating the operation voltage to 4.4 V. Meanwhile, doping Li+ shows the function in charge transfer, improving the rate performance and prolonging cycling lifespan. Consequently, the designed P2-Na0.75Mn0.54Ni0.27Li0.14Ti0.05O2 cathode material exhibits discharge capacities of 129, 104, and 85 mAh g- 1 under high voltage of 4.4 V at 1, 10, and 20 C, respectively. More importantly, the full-cell delivers a high initial capacity of 198 mAh g-1 at 0.1 C (17.3 mA g-1) and a capacity retention of 73% at 5 C (865 mA g-1) after 1000 cycles, which is seldom witnessed in previous reports, emphasizing their potential applications in advanced energy storage.

Doping Engineering in Electrode Material for Boosting the Performance of Sodium Ion Batteries

In recent years, sodium ion batteries (SIBs) emerged as promising alternative candidates for lithium ion batteries (LIBs) due to the high abundance and low cost of sodium resources. However, their commercialization has been hindered by inherent limitations, such as low energy density and poor cycling stability. To address these issues, doping methodology is one of the most promising approaches to boosting the structural and electrochemical properties of SIB electrodes. This review provides a comprehensive overview of recent advancements in doping strategies, focusing on the improvement of the performance of SIBs. Various dopants including s- and p-block elements, transition metals, oxides, carbonaceous materials, and many more dopants are discussed in terms of their effects on enhancing the electrochemical properties of SIBs. Furthermore, the mechanisms responsible for the improvement in the performance of doped SIBs materials are also discussed. It also highlights the importance of doping sites in the crystal lattice, which also play a crucial role in doping in optimizing electrode structure, enhancing ion diffusion kinetics, and stabilizing electrode/electrolyte interfaces. The review ends by looking at the recent studies in simultaneous multiple heteroatom doping, offering valuable perspectives for a high performance SIB. This study provides valuable insight into the researchers and battery industries striving for advancements in energy storage technologies.

Cation Configuration and Structural Degradation of Layered Transition Metal Oxides in Sodium-Ion Batteries

Given the pressing depletion of lithium resources, sodium-ion batteries (SIBs) stand out as a cost-effective alternative for energy storage solutions in the near future. Layered transition metal oxides (LTMOs) emerge as the leading cathode materials for SIBs due to their superior specific capacities and abundant raw materials. Nonetheless, achieving long-term stability in LTMOs for SIBs remains a challenge due to the inevitable structural degradation during charge-discharge cycles. The complexity and diversity of cation configurations/superstructures within the transition metal layers (TMO2) further complicate the understanding for newcomers. Therefore, it is critical to summarize and discuss the factors leading to structural degradation and the available strategies for enhancing LTMOs' stability. In this review, the cationic configurations of TMO2 layers are introduced from a crystallographic perspective. It then identifies and examines four key factors responsible for structural decay, alongside the impacts of various modification strategies. Finally, more effective and practical research approaches for investigating LTMOs have been proposed. The work aims to enhance the comprehension of the structural deterioration of LTMOs and facilitate a substantial improvement in their cycle life and energy density.

Activation of MnO6 Units via an Interfacial Electric Field: Electron Injection into Mn t2g for Rapid and Stable Sodium Ion Storage in CeO2/MnOx

Manganese-based oxides (MnOx) suffer from sluggish charge diffusion kinetics and poor cycling stability in sodium ion storage. Herein, an interfacial electric field (IEF) in CeO2/MnOx is constructed to obtain high electronic/ionic conductivity and structural stability of MnOx. The as-designed CeO2/MnOx exhibits a remarkable capacity of 397 F g-1 and favorable cyclic stability with 92.13% capacity retention after 10,000 cycles. Soft X-ray absorption spectroscopy and partial density of states results reveal that the electrons are substantially injected into the Mn t2g orbitals driven by the formed IEF. Correspondingly, the MnO6 units in MnOx are effectively activated, endowing the CeO2/MnOx with fast charge transfer kinetics and high sodium ion storage capacity. Moreover, In situRaman verifies a remarkably increased structural stability of CeO2/MnOx, which is attributed to the enhanced Mn─O bond strength and efficiently stabilized MnO6 units. Mechanism studies show that the downshift of Mn 3d-band center dramatically increases the Mn 3d-O 2p orbitals overlap, thus inhibiting the Jahn-Teller (J-T) distortion of MnOx during sodium ion insertion/extraction. This work develops an advanced strategy to achieve both fast and sustainable sodium ion storage in metal oxides-based energy materials.

Internal Vanadium Doping and External Modification Design of P2-Type Layered Mn-Based Oxides as Competitive Cathodes toward Sodium-Ion Batteries

P2-type layered manganese-based oxides have attracted considerable interest as economical, cathode materials with high energy density for sodium-ion batteries (SIBs). Despite their potential, these materials still face challenges related to sluggish kinetics and structural instability. In this study, a composite cathode material, Na0.67Ni0.23Mn0.67V0.1O2@Na3V2O2(PO4)2F was developed by surface-coating P2-type Na0.67Ni0.23Mn0.67V0.1O2 with a thin layer of Na3V2O2(PO4)2F to enhance both the electrochemical sodium storage and material air stability. The optimized Na0.67Ni0.23Mn0.67V0.1O2@5wt %Na3V2O2(PO4)2F exhibited a high discharge capacity of 176 mA h g-1 within the 1.5-4.1 V range at a low current density of 17 mA g-1. At an increased current density of 850 mA g-1 within the same voltage window, it still delivered a substantial initial discharge capacity of 112 mAh g-1. These findings validate the significant enhancement of ion diffusion capabilities and rate performance in the P2-type Na0.67Ni0.33Mn0.67O2 material conferred by the composite cathode.

Developing an abnormal high-Na-content P2-type layered oxide cathode with near-zero-strain for high-performance sodium-ion batteries

Layered transition metal oxides (NaxTMO2) possess attractive features such as large specific capacity, high ionic conductivity, and a scalable synthesis process, making them a promising cathode candidate for sodium-ion batteries (SIBs). However, NaxTMO2 suffer from multiple phase transitions and Na+/vacancy ordering upon Na+ insertion/extraction, which is detrimental to their electrochemical performance. Herein, we developed a novel cathode material that exhibits an abnormal P2-type structure at a stoichiometric content of Na up to 1. The cathode material delivers a reversible capacity of 108 mA h g-1 at 0.2C and 97 mA h g-1 at 2C, retaining a capacity retention of 76.15% after 200 cycles within 2.0-4.3 V. In situ diffraction studies demonstrated that this material exhibits an absolute solid-solution reaction with a low volume change of 0.8% during cycling. This near-zero-strain characteristic enables a highly stabilized crystal structure for Na+ storage, contributing to a significant improvement in battery performance. Overall, this work presents a simple yet effective approach to realizing high Na content in P2-type layered oxides, offering new opportunities for high-performance SIB cathode materials.

Slow-Released Cationic Redox Activity Promoted Stable Anionic Redox and Suppressed Jahn-Teller Distortion in Layered Sodium Manganese Oxides

Manganese-based layered oxides are considered promising cathodes for sodium ion batteries due to their high capacity and low-cost manganese and sodium resources. Triggering the anionic redox reaction (ARR) can exceed the capacity limitation determined by conventional cationic redox. However, the unstable ARR charge compensation and Jahn-Teller distortion of Mn3+ ions readily result in structural degradation and rapid capacity fade. Here, we report a P2-type Na0.8Li0.2Mn0.7Cu0.1O2 cathode that shows a capacity retention of 84.5% at 200 mA/g after 200 cycles. Combining in situ X-ray diffraction and multi other ex situ characterizations, we reveal that the enhanced cycling stability is ascribed to a slow release of cationic redox activity which can well suppress the Jahn-Teller distortion and favor the ARR reversibility. Furthermore, density-functional theory calculations demonstrate that the inhibited interlayer migration and reduced band gap facilitate the stability and kinetic behavior of ARR. These findings provide a perspective for designing high-energy-density cathode materials with ARR activity.

Sustainable, low Ni-containing Mg-doped layered oxides as cathodes for sodium-ion batteries

The supply of battery-grade nickel to produce positive electrodes of sodium-ion batteries may soon become insufficient. For this reason, it is crucial to find new electrode materials that minimize its use or even fully remove this element from synthesis. We have prepared a Na0.67Mg0.05FexNiyMnzO2 (0 ≤ x ≤ 0.2; y = 0.05, 0.15; 0.6 ≤ z ≤ 0.9) series with low Ni and Fe contents by a single and easily scalable sol-gel method. This procedure yields high-purity and crystalline samples as evidenced by structural, morphological, and spectroscopic studies, including X-ray diffraction, electron microscopy, and X-ray photoelectron spectroscopy. The electrochemical tests showed an exceptional performance for the F0N05 sample with the lowest (Ni + Fe) contents, at 5 C (ca. 100 mA h g-1), and good capacity retention after 100 cycles. This excellent behaviour was also evidenced when cycling at -15 °C. These results were confirmed by electrochemical techniques, such as cyclic voltammetry and impedance spectroscopy, that evidenced a fast exchange of sodium ions due to a significant capacitive contribution and high apparent diffusion coefficients. Post-mortem analysis of the F0N05 electrodes by XRD showed the reversible insertion and the absence of detrimental P2-O2 and P2-P2' transitions, while XPS spectra demonstrated the reversible redox activity of both transition metals and oxygen.

Single-Crystal Growth of P2-Type Layered Oxides with Increased Exposure of {010} Planes for High-Performance Sodium-Ion Batteries

An increase in the size of single-crystal particles can effectively reduce the interfacial side reactions of layered oxides for sodium-ion batteries at high voltages but may result in sluggish Na+ transport. Herein, single-crystal Na0.66Ni0.26Zn0.07Mn0.67O2 with increased proportions of {010} planes is synthesized by adding low-cost NaCl as the molten salt. With the assistance of a NaCl molten salt, the median diameter (D50) of single-crystal Na0.66Ni0.26Zn0.07Mn0.67O2 increases to 10.46 μm relative to that of the comparison sample without NaCl (6.57 μm). Electrolyte decomposition on the surface of single-crystal Na0.66Ni0.26Zn0.07Mn0.67O2 is considerably suppressed, owing to a decrease in the specific surface area. Moreover, the increased exposure of {010} planes is favorable for improving the Na+ transport kinetics of single-crystal particles. Therefore, at 100 mA g-1, single-crystal Na0.66Ni0.26Zn0.07Mn0.67O2 exhibits a high-capacity retention of 96.6% after 100 cycles, which is considerably greater than that of the comparison sample (86.8%). Moreover, the rate performance of single-crystal Na0.66Ni0.26Zn0.07Mn0.67O2 (average discharge capacity of 81.2 mAh g-1) is superior to that of the comparison sample (average discharge capacity of 61.2 mAh g-1) at 2000 mA g-1. This work provides a new approach for promoting the single-crystal growth of layered oxides for highly stable interfaces at high voltages without compromising Na+ transport kinetics.