Co-Substitution Enhances the Rate Capability and Stabilizes the Cyclic Performance of O3-Type Cathode NaNi0.45- xMn0.25Ti0.3Co xO2 for Sodium-Ion Storage at High Voltage

Stabilized O3-Type Layered Sodium Oxides with Enhanced Rate Performance and Cycling Stability by Dual-Site Ti4+ /K+ Substitution

High-capacity O3-type layered sodium oxides are considered one of the most promising cathode materials for the next generation of Na-ion batteries (NIBs). However, these cathodes usually suffer from low high-rate capacity and poor cycling stability due to structure deformation, native air sensitivity, and interfacial side reactions. Herein, a multi-site substituted strategy is employed to enhance the stability of O3-type NaNi0.5 Mn0.5 O2 . Simulations indicate that the Ti substitution decreases the charge density of Ni ions and improves the antioxidative capability of the material. In addition, the synergistic effect of K+ and Ti4+ significantly reduces the formation energy of Na+ vacancy and delivers an ultra-low lattice strain during the repeated Na+ extraction/insertion. In situ characterizations verify that the complicated phase transformation is mitigated during the charge/discharge process, resulting in greatly improved structure stability. The co-substituted cathode delivers a high-rate capacity of 97 mAh g-1 at 5 C and excellent capacity retention of 81% after 400 cycles at 0.5 C. The full cell paired with commercial hard carbon anode also exhibits high capacity and long cycling life. This dual-ion substitution strategy will provide a universal approach for the new rational design of high-capacity cathode materials for NIBs.

Dual Doping with Cation and Anion for Enhancing the Structural Stability of Sodium-Ion Layered Cathode

First-principles-based calculations were implemented to explore the ideal combination of cations and anions as dual dopants for enhancing the structural stability of the sodium-ion layered cathode for application in sodium...

Ultralow Volume Change of P2-Type Layered Oxide Cathode for Na-Ion Batteries with Controlled Phase Transition by Regulating Distribution of Na. Angew Chem Int Ed Engl 2021;60:20960-20969. [PMID: 34258863 DOI: 10.1002/anie.202108109]

Most P2-type layered oxides exhibit a large volume change when they are charged into high voltage, and it further leads to bad structural stability. In fact, high voltage is not the reason which causes the irreversible phase transition. There are two internal factors which affect structural evolution: the amount and distribution of Na ions retained in the lattice. Hereon, a series of layered oxides Na_2/3 Mn_x Ni_x-1/3 Co_4/3-2x O₂ (1/3≤x≤2/3) were synthesized. It is observed that different components have different structural evolutions during the charge/discharge processes, and further researches find that the distribution of Na ions in layers is the main factor. By controlling the distribution of Na ions, the phase transition process can be well controlled. As the referential component, P2-Na_2/3 Mn_1/2 Ni_1/6 Co_1/3 O₂ cathode with uniform distribution of Na ions is cycled at the voltage window of 1.5-4.5 V, which exhibits a volume change as low as 1.9 %. Such a low strain is beneficial for cycling stability. The current work provides a new and effective route to regulate the structural evolution of the promising P2-type layered cathode for sodium ion batteries.

Synergistic Effect of Polymorphs in Doped NaNi0.5Mn0.5O2 Cathode Material for Improving Electrochemical Performances in Na-Batteries

Layered NaNi0.5Mn0.5O2, employed as cathode materials in sodium ion batteries, is attracting interest due to its high working potential and high-capacity values, thanks to the big sodium amount hosted in the lattice. Many issues are, however, related to their use, particularly, the complex phase transitions occurring during sodium intercalation/deintercalation, detrimental for the structure stability, and the possible Mn dissolution into the electrolyte. In this paper, the doping with Ti, V, and Cu ions (10% atoms with respect to Ni/Mn amount) was used to stabilize different polymorphs or mixtures of them with the aim to improve the capacity values and cells cyclability. The phases were identified and quantified by means of X-ray powder diffraction with Rietveld structural refinements. Complex voltammograms with broad peaks, due to multiple structural transitions, were disclosed for most of the samples. Ti-doped sample has, in general, the best performances with the highest capacity values (120 mAh/g at C/10), however, at higher currents (1C), Cu-substituted sample also has stable and comparable capacity values.

High-Voltage Cathode α-Fe2O3 Nanoceramics for Rechargeable Sodium-Ion Batteries

Previously, α-Fe2O3 nanocrystals are recognized as anode materials owing to their high capacity and multiple properties. Now, this work provides high-voltage α-Fe2O3 nanoceramics cathodes fabricated by the solvothermal and calcination processes for sodium-ion batteries (SIBs). Then, their structure and electrical conductivity were investigated by the first-principles calculations. Also, the SIB with the α-Fe2O3 nanoceramics cathode exhibits a high initial charge-specific capacity of 692.5 mA h g-1 from 2.0 to 4.5 V at a current density of 25 mA g-1. After 800 cycles, the discharge capacity is still 201.8 mA h g-1, well exceeding the one associated with the present-state high-voltage SIB. Furthermore, the effect of the porous structure of the α-Fe2O3 nanoceramics on sodium ion transport and cyclability is investigated. This reveals that α-Fe2O3 nanoceramics will be a remarkably promising low-cost and pollution-free high-voltage cathode candidate for high-voltage SIBs.

Revealing the role of dopants in mitigating degradation phenomena in sodium-ion layered cathodes

Prevention of the degradation of sodium-based layered cathode materials is the key to developing high-performance and high-stability sodium-ion batteries. In this study, the working mechanism of Mg and Ti dopants in mitigating degradation was investigated through the use of first-principles calculations. More specifically, the effects of each dopant in suppressing the phase transition, lattice expansion and shrinkage, and possible oxygen generation during repeated charging and discharging processes were validated. The results showed that the pristine structure exhibits irreversible O3-P3 phase transition after 75% desodiation, while doping with Mg or Ti effectively delays this transition. In addition, the change in lattice parameters as well as in the volume during desodiation was investigated. It was found that both dopants reduce the magnitude of structural change, which potentially improves the structural stability. Furthermore, introducing the dopants increases the thickness of the Na diffusion channel, possibly leading to an enhanced rate capability. Finally, the oxygen atomic charge variation during charging indicated that doping can enhance the oxygen stability by reducing the initial charge of oxygen as well as its increase during desodiation.

A comprehensive study of the multiple effects of Y/Al substitution on O3-type NaNi0.33Mn0.33Fe0.33O2 with improved cycling stability and rate capability for Na-ion battery applications

O3-NaNi0.33Mn0.33Fe0.33O2 layered oxide has attracted increasing attention as one of the most promising materials for Na-ion battery applications due to air stability and environmental friendliness, but the complex phase transitions and inferior cycling stability are extremely challenging to overcome. Cation substitution has been widely used to stabilize crystal structures and improve electrochemical performance for SIBs. Based on past experimental results, it was discovered that the transition metal-oxygen bond energy of the introduced dopant is an important factor for optimizing electrochemical performance. In this study, we validated our hypothesis that yttrium (Y)-which possesses high bond energy for oxygen-is most likely to be an ideal doping element by conducting a comparative study of substituting Mn in O3-NaNi0.33Mn0.33Fe0.33O2 layered oxide with aluminum (Al) and Y through elemental doping. As hypothesized, the electrochemical properties of NaNi0.33Mn0.33Fe0.33O2 have increased markedly by introducing a small amount of Y and Al, and the Y-doped materials showed superior rate performance and cycling stability due to enhanced Na+ diffusion reaction kinetics and layered structure stability. Furthermore, the substitution of Y for Mn can improve thermal stability and alleviate phase transformations. The improvement mechanism of Y substitution can be attributed to a larger d-spacing and stronger metal-oxygen bond. These results suggest that structural modulation is an effective strategy to reinforce electrochemical properties of layered oxides and provides some guidance about designing promising electrode materials.

Atomic layer deposition of Al2O3 on P2-Na0.5Mn0.5Co0.5O2 as interfacial layer for high power sodium-ion batteries

Surface modification is one of the impressive and widely used technique to improve the electrochemical performance of sodium-ion batteries by modifying the electrode-electrolyte interface. Herein, we used the atomic layer deposition (ALD) to modify the surface of P2-Na_0.5Mn_0.5Co_0.5O₂ by sub-monolayer Al₂O₃ coating on the prefabricated electrodes. Phase purity is confirmed using various structural and morphological studies. The pristine electrode delivered an initial discharge capacity of 154 mAh g^-1 at 0.5C, and inferior rate performance of 23 mAh g^-1 at 40C rate. On the other hand, the interfacial modified cathode with 5 cycles of ALD coating delivers a high capacity of 174 and 45 mAh g^-1 at 0.5C and 40C rate, respectively. The Co^2+/3+ redox couple is utilized for the faradaic process with high reversibility along with suppressed P2-O2 phase transition. The presence of the Al₂O₃ layer acts as an artificial cathode electrolyte interface by suppressing the electrolyte oxidation at higher cutoff potentials. This is clearly validated by the reduced charge transfer resistance of surface modified electrodes after cycling at various current rates. Even at an elevated temperature condition (50 °C), interfacial layer significantly improves the safety of the cell and ensures the stability of the cathode.

Air-Stable and High-Voltage Layered P3-Type Cathode for Sodium-Ion Full Battery

The development of highly efficient and stable cathodes for sodium-ion batteries (SIBs) is strategically critical to achieving large-scale electrical energy storage. Creating air-stable and high-voltage layered cathodes for sodium-ion full batteries still remains a challenge. Herein, we describe a rational design and preparation of a stable P3-Na_2/3Ni_1/4Mg_1/12Mn_2/3O₂ cathode. The cathode displays a satisfactory working voltage of 3.6 V and excellent cyclic stability over 100 cycles at a 1 C rate without obvious capacity fading. The results of ex situ X-ray diffraction (XRD) demonstrate that the P3-type structure is well retained even when charged to 4.4 V. Furthermore, the structural characterization by XRD Rietveld refinement, scanning electron microscopy, and electrochemical testing certifies that the cathode maintains its structure commendably even when soaked in water for 12 h. In particular, the P3- Na_2/3Ni_1/4Mg_1/12Mn_2/3O₂∥hard carbon full battery exhibits a desired competitively high voltage of 3.45 V and an attractive energy density of up to 412.2 W h kg^-1 based on the cathode. The comprehensive results achieved by the specially designed strategy provide guidance toward the exploration of stable cathodes in the application of SIBs as modern energy-storage devices.

Electrode Materials for High-Performance Sodium-Ion Batteries

Sodium ion batteries (SIBs) are being billed as an economical and environmental alternative to lithium ion batteries (LIBs), especially for medium and large-scale stationery and grid storage. However, SIBs suffer from lower capacities, energy density and cycle life performance. Therefore, in order to be more efficient and feasible, novel high-performance electrodes for SIBs need to be developed and researched. This review aims to provide an exhaustive discussion about the state-of-the-art in novel high-performance anodes and cathodes being currently analyzed, and the variety of advantages they demonstrate in various critically important parameters, such as electronic conductivity, structural stability, cycle life, and reversibility.