Sodium-ion batteries (SIBs) have attracted much scientific interest for use in large-scale energy storage systems because sodium is cheaper than lithium. However, the large radius of Na+ and barriers to Na+ transport result in sluggish kinetics and complicated structural distortion, leading to unsatisfactory rate capability and poor cycling stability. It therefore is essential to develop an electrode with enhanced kinetics and a stable structure during cycling to improve SIB performance. Among the various layered oxide cathodes, those with a spinel-like structure could play an important role in boosting electron transport because of their excellent intrinsic conductivity, including by coordinating with Na+ insertion/extraction. Moreover, thanks to the inherent high stability of the spinel-like phase, it could function as a stabilizer for host cathode structures. This review summarizes recent advances in spinel engineering on layered oxide cathodes to boost Na+ transport kinetics and provide structural stability to achieve high-performance SIBs, focusing particularly on post-spinel structures, layered oxide integrated spinel-like structures, and spinel transition. The insights proposed in this review will be useful for guiding rational structural engineering and design to drive the development of new materials and chemistries in Na-based electrode materials.

Spinel/Post-spinel engineering on layered oxide cathodes for sodium-ion batteries

Impossible voltage plateau regulation for the cathode materials with fixed active elemental center is a pressing issue hindering the development of Na‐superionic‐conductor (NASICON)‐type Na3V2(PO4)2F3 (NVPF) cathodes in sodium‐ion batteries (SIBs). Herein, a high‐entropy substitution strategy, to alter the detailed crystal structure of NVPF without changing the central active V atom, is pioneeringly utilized, achieving simultaneous electronic conductivity enhancement and diffusion barrier reduction for Na+, according to theoretical calculations. The as‐prepared carbon‐free high‐entropy Na3V1.9(Ca,Mg,Al,Cr,Mn)0.1(PO4)2F3 (HE‐NVPF) cathode can deliver higher mean voltage of 3.81 V and more advantageous energy density up to 445.5 Wh kg−1, which is attributed by the diverse transition‐metal elemental substitution in high‐entropy crystalline. More importantly, high‐entropy introduction can help realize disordered rearrangement of Na+ at Na(2) active sites, thereby to refrain from unfavorable discharging behaviors at low‐voltage region, further lifting up the mean working voltage to realize a full Na‐ion storage at the high voltage plateau. Coupling with a hard carbon (HC) anode, HE‐NVPF//HC SIB full cells can deliver high specific energy density of 326.8 Wh kg−1 at 5 C with the power density of 2178.9 W kg−1. This route means the unlikely potential regulation in NASICON‐type crystal with unchangeable active center becomes possible, inspiring new ideas on elevating the mean working voltage for SIB cathodes.

An Advanced High‐Entropy Fluorophosphate Cathode for Sodium‐Ion Batteries with Increased Working Voltage and Energy Density

Sanitary tissue is utilized as the precursor to construct a hard carbon microbelt paper (HCMB), which can be employed as a high-initial-coulombic-efficiency and low-discharge-platform K + -storage anode for 4.5 V hybrid capacitors. 

Tissue-derived carbon microbelt paper: a high-initial-coulombic-efficiency and low-discharge-platform K<sup>+</sup>-storage anode for 4.5 V hybrid capacitors

High-ionicity fluorophosphate lattice via aliovalent substitution as advanced cathode materials in sodium-ion batteries

Advances in metal phosphides for sodium-ion batteries

Covalent organic frameworks (COF) possess a robust and porous crystalline structure, making them an appealing candidate for energy storage. Herein, we report an exfoliated polyimide COF composite (P-COF@SWCNT) prepared by an in situ condensation of anhydride and amine on the single-walled carbon nanotubes as advanced anode for potassium-ion batteries (PIBs). Numerous active sites exposed on the exfoliated frameworks and the various open pathways promote the highly efficient ion diffusion in the P-COF@SWCNT while preventing irreversible dissolution in the electrolyte. During the charging/discharging process, K+ is engaged in the carbonyls of imide group and naphthalene rings through the enolization and π-K+ effect, which is demonstrated by the DFT calculation and XPS, ex-situ FTIR, Raman. As a result, the prepared P-COF@SWCNT anode enables an incredibly high reversible specific capacity of 438 mA h g-1 at 0.05 A g-1 and extended stability. The structural advantage of P-COF@SWCNT enables more insights into the design and versatility of COF as an electrode. 

Covalent Organic Framework with Highly Accessible Carbonyls and π‐Cation Effect for Advanced Potassium‐Ion Batteries

Summary The development of high-performance cathodes for sodium-ion batteries (SIBs) is one key issue required for the success of large-scale energy storage systems. In addition to performance degradation during cycling, electrode materials are usually hampered by instability when stored in various environments severely restricting their practical application. Here, we report a stable cathode material that shows no obvious capacity attenuation in various storage conditions. The cathode is composed of regular and homogeneous Na3V2(PO4)2O2F0.99Cl0.01 (NVPFCl) microcubes, with electrochemical properties, including high specific capacity (128.2 mA h g-1 at 0.1 C), excellent rate capability (79.8 mA h g-1 at 20 C), long-term cycle life, and all-climate performance. Studies using in/ex situ characterization and electrode kinetics are implemented to reveal the possible reasons for the stability and electrochemical activity of NVPFCl. This work may provide a pathway for the development of all-climate and storage-stable SIB cathode materials.

Air/water/temperature-stable cathode for all-climate sodium-ion batteries

Flexible power sources featuring high-performance, prominent flexibility and raised safety have received mounting attention in the area of wearable electronic devices. However, many great challenges remain to be overcome, notably the design and fabrication of flexible electrodes with excellent electrochemical performance and matching them with safe and reliable electrolytes. Herein, a facile approach for preparing flexible electrodes, which employs carbon cloth derived from commercial cotton cloth as the substrate of cathode and a flexible anode, is proposed and investigated. The promising cathode (NVPOF@FCC) with high conductivity and outstanding flexibility is prepared by efficiently coating Na3V2(PO4)2O2F (NVPOF) on flexible carbon cloth (FCC), which exhibits remarkable electrochemical performance and the significantly improved reaction kinetics. More importantly, a novel flexible quasi-solid-state sodium-ion full battery (QSFB) is feasibly assembled by sandwiching a P(VDF-HFP)-NaClO4 gel-polymer electrolyte film between the advanced NVPOF@FCC cathode and FCC anode. And the QSFBs are further evaluated in flexible pouch cells, which not only demonstrates excellent energy-storage performance in aspect of great cycling stability and high-rate capability, but also impressive flexibility and safety. This work offers a feasible and effective strategy for the design of flexible electrodes, paving the way for the progression of practical and sustainable flexible batteries.

Xiao-Tong Wang

Papers

An Advanced High‐Entropy Fluorophosphate Cathode for Sodium‐Ion Batteries with Increased Working Voltage and Energy Density

High-ionicity fluorophosphate lattice via aliovalent substitution as advanced cathode materials in sodium-ion batteries

Covalent Organic Framework with Highly Accessible Carbonyls and π‐Cation Effect for Advanced Potassium‐Ion Batteries

Air/water/temperature-stable cathode for all-climate sodium-ion batteries

Flexible quasi-solid-state sodium-ion full battery with ultralong cycle life, high energy density and high-rate capability