Deep Dive into Positive Electrode Materials for Sodium-Ion Batteries

Positive electrode materials are a cornerstone of sodium-ion batteries, significantly influencing key performance metrics such as energy density, cycle life, and rate capability. Currently, the predominant cathode material technologies for sodium-ion batteries include transition metal oxides, polyanionic compounds, and Prussian blue analogues.

Transition metal oxides, particularly layered oxides represented by NaxMO2 (where M is a transition metal or dopant), have garnered significant attention due to their simple preparation, ease of scaling, high energy density, and excellent rate performance.

The primary synthesis methods for layered oxide cathode materials are solution-based and solid-state approaches. While solution-based methods share similarities with lithium-ion battery cathode production processes and can leverage existing production lines, solid-state methods offer advantages in terms of simplicity and lower costs.

Despite their promise, layered oxides face challenges such as irreversible phase transitions and poor stability in humid environments. To address these issues, Shandong Huana New Energy has introduced a novel strategy involving functional metal doping to enhance crystal structure stability and mitigate phase transitions associated with large sodium ion radii. Additionally, by controlling factors such as alkalinity, pH, and free sodium content, the company has effectively mitigated the challenges posed by the hygroscopic nature of layered oxides and their reactivity with water, oxygen, or carbon dioxide.

Tunnel-structured oxides, such as Na0.44MnO2, feature unique S-shaped channels that endow them with exceptional rate capability and stability. Their remarkable inertness in air and water makes them promising candidates for aqueous sodium-ion batteries. However, their low reversible capacity has significantly hindered their widespread commercialization.

Polyanionic compounds, with the general formula NaxMy(XaOb)Zw, consist of three-dimensional frameworks formed by the strong covalent bonding of polyanionic polyhedra and transition metal polyhedra. These materials exhibit exceptional electrochemical stability, with minimal volume change during sodium ion insertion and extraction, enabling cycle lives of up to 10,000 cycles. However, their inherent low electronic conductivity often necessitates the use of carbon coating and doping strategies to enhance performance.

Prussian blue analogs (PBAs), with the general formula NaxM1[M2(CN)6]1‐y·?y·nH2O, are transition metal cyanometallate coordination polymers featuring open three-dimensional frameworks that facilitate sodium ion intercalation and deintercalation. PBAs offer high theoretical capacities and can be synthesized at relatively low temperatures. However, challenges such as poor rate performance, limited cycle life, low Coulombic efficiency (≤90%), and the presence of crystal water and release of cyanide at elevated temperatures hinder their practical application.

In comparison, layered oxide cathode materials are generally considered more suitable for commercialization due to their superior overall performance and more mature manufacturing processes.