Facilitating Phase Evolution for a High-Energy-Efficiency, Low-Cost O3-Type NaxCu0.18Fe0.3Mn0.52O2 Sodium Ion Battery Cathode

Sodium layered oxide cathodes: properties, practicality and prospects

Rechargeable sodium-ion batteries (SIBs) have emerged as an advanced electrochemical energy storage technology with potential to alleviate the dependence on lithium resources. Similar to Li-ion batteries, the cathode materials play a decisive role in the cost and energy output of SIBs. Among various cathode materials, Na layered transition-metal (TM) oxides have become an appealing choice owing to their facile synthesis, high Na storage capacity/voltage that are suitable for use in high-energy SIBs, and high adaptivity to the large-scale manufacture of Li layered oxide analogues. However, going from the lab to the market, the practical use of Na layered oxide cathodes is limited by the ambiguous understanding of the fundamental structure-performance correlation of cathode materials and lack of customized material design strategies to meet the diverse demands in practical storage applications. In this review, we attempt to clarify the fundamental misunderstandings by elaborating the correlations between the electron configuration of the critical capacity-contributing elements (e.g., TM cations and oxygen anion) in oxides and their influence on the Na (de)intercalation (electro)chemistry and storage properties of the cathode. Subsequently, we discuss the issues that hinder the practical use of layered oxide cathodes, their origins and the corresponding strategies to address their issues and accelerate the target-oriented research and development of cathode materials. Finally, we discuss several new Na layered cathode materials that show prospects for next-generation SIBs, including layered oxides with anion redox and high entropy and highlight the use of layered oxides as cathodes for solid-state SIBs with higher energy and safety. In summary, we aim to offer insights into the rational design of high-performance Na layered oxide cathode materials towards the practical realization of sustainable electrochemical energy storage at a low cost.

Fast Na+ Kinetics and Suppressed Voltage Hysteresis Enabled by a High-Entropy Strategy for Sodium Oxide Cathodes

O3-type layered transition metal cathodes are promising energy storage materials due to their sufficient sodium reservoir. However, sluggish sodium ions kinetics and large voltage hysteresis, which are generally associated with Na+ diffusion properties and electrochemical phase transition reversibility, drastically minimize energy density, reduce energy efficiency, and hinder further commercialization of sodium-ion batteries (SIBs). Here, this work proposes a high-entropy tailoring strategy through manipulating the electronic local environment within transition metal slabs to circumvent these issues. Experimental analysis combined with theoretical calculations verify that high-entropy metal ion mixing contributes to the improved reversibility of redox reaction and O3-P3-O3 phase transition behaviors as well as the enhanced Na+ diffusivity. Consequently, the designed O3-Na0.9Ni0.2Fe0.2Co0.2Mn0.2Ti0.15Cu0.05O2 material with high-entropy characteristic could display a negligible voltage hysteresis (<0.09 V), impressive rate capability (98.6 mAh g-1 at 10 C) and long-term cycling stability (79.4% capacity retention over 2000 cycles at 5 C). This work provides insightful guidance in mitigating the voltage hysteresis and facilitating Na+ diffusion of layered oxide cathode materials to realize high-rate and high-energy SIBs.

Progress and perspectives on iron-based electrode materials for alkali metal-ion batteries: a critical review

Iron-based materials with significant physicochemical properties, including high theoretical capacity, low cost and mechanical and thermal stability, have attracted research attention as electrode materials for alkali metal-ion batteries (AMIBs). However, practical implementation of some iron-based materials is impeded by their poor conductivity, large volume change, and irreversible phase transition during electrochemical reactions. In this review we critically assess advances in the chemical synthesis and structural design, together with modification strategies, of iron-based compounds for AMIBs, to obviate these issues. We assess and categorize structural and compositional regulation and its effects on the working mechanisms and electrochemical performances of AMIBs. We establish insight into their applications and determine practical challenges in their development. We provide perspectives on future directions and likely outcomes. We conclude that for boosted electrochemical performance there is a need for better design of structures and compositions to increase ionic/electronic conductivity and the contact area between active materials and electrolytes and to obviate the large volume change and low conductivity. Findings will be of interest and benefit to researchers and manufacturers for sustainable development of advanced rechargeable ion batteries using iron-based electrode materials.

Structure and Performance of NaxMn0.85Al0.1Fe0.05O2 (0.7 ≤ x ≤ 1.0) Composite Materials for Sodium-Ion Batteries

P2 and O3 structures are two important sodium manganese oxide phases for sodium-ion batteries; however, encounter Na-deficient and poor rate performance, respectively. Herein, a systematic study of Na_xMn_0.85Al_0.1Fe_0.05O₂ (0.7 ≤ x ≤ 1.0) materials is performed by employing solid-state NMR, X-ray diffraction, and electrochemical analysis, to provide an in-depth understanding on the structure and the correlated performance for the rational design. The interlayer spacing of α-NaMnO₂ broadens, and the content of distorted O3 structures (α- and β-NaMnO₂) increases with raising Na content. It is exhibited that the NaMn_0.85Al_0.1Fe_0.05O₂ composite material presents better rate and cycling performance than P2-type Na_0.7Mn_0.85Al_0.1Fe_0.05O₂, delivering a capacity of 87 mAh g^-1 at 5 C. Significantly, the determinants of performance are further discussed, which reveal that diffusion coefficient is probably not the decisive factor restricts the rate performance of O3 and composite materials. The phase transition relaxation and the interfacial charge transfer resistance should be seriously addressed for further improvement.

O3-Type Na2/3Ni1/3Ti2/3O2 Layered Oxide as a Stable and High-Rate Anode Material for Sodium Storage

Sodium-ion batteries (SIBs) are currently the most promising candidates for large-scale energy storage devices owing to their low cost and abundant resources. Titanium-based layered oxides have attracted widespread attention as promising anode materials due to delivering a safe potential of about 0.7 V (vs Na⁺/Na) and a small volume contraction during cycles; P2-type Ti-based layered oxides are typically reported, due to the challenging synthesis of the O3-type counterpart resulting from the high percentage of unstable Ti³⁺. Herein, we report an anomalous O3-Na_2/3Ni_1/3Ti_2/3O₂ layered oxide as an ultrastable and high-rate anode material for SIBs. The anode material delivers a reversible capacity of 112 mA h g^-1 after 300 cycles at 0.1 C, a good capacity retention rate of 91% after 1400 cycles at 2 C, and, in particular, a capacity of 52 mA h g^-1 even at a high rate of 20 C (1780 mA g^-1). Furthermore, the in situ X-ray diffraction monitoring reveals no phase transitions and almost zero strain both underlie the good long-cycle stability. The measured high apparent Na⁺ diffusion coefficient (2.06 × 10^-10 cm² s^-1) and the low migration energy barrier (0.59 eV) from density functional theory calculations are responsible for the superior rate capability. Our results promise advanced high-performance O3-type Ti-based layered oxides as promising anode materials toward application for SIBs.

Copper-Stabilized P'2-Type Layered Manganese Oxide Cathodes for High-Performance Sodium-Ion Batteries

Layered sodium manganese oxides are promising low-cost and high-capacity cathode materials for commercialization of sodium-ion batteries (SIBs). P'2-type Na_0.67MnO₂ with an orthorhombic structure has been considered as a significant candidate for SIBs. However, the Jahn-Teller distortion and undesired phase transitions will lead to poor structural stability and unsatisfactory cycling performance. Herein, a systematic investigation on partially copper-doped P'2-type Na_0.67Cu_xMn_1-xO₂ (x = 0, 0.05, 0.1, and 0.2) series as cathodes for SIBs reveals the relationship between doping concentrations and Na storage properties. With proper copper content, P'2 Na_0.67Cu_0.1Mn_0.9O₂ exhibits a suppressed Jahn-Teller effect as well as relatively less phase transitions, which can deliver a high specific capacity of 222.7 mA h g^-1 at 10 mA g^-1 within 1.5-4.2 V, with a capacity retention of 76% at 1 A g^-1 after 300 cycles. The electrochemical mechanism is systematically investigated via in situ X-ray diffraction observations and density functional theory calculations, which provide fundamental guidelines for developing high-performance cathodes for SIBs.