Conversion-reaction cathodes can potentially double the energy density of current Li-ion batteries. However, the poor cycling stability, low energy efficiency, and low power density of conversion-reaction cathodes limit their applications for Li-ion batteries. Herein, we report a revolutionary advance in a conversion-reaction cathode by developing a core-shell FeOF@PEDOT nanorods, in which partial substitution of fluorine with oxygen in FeF3 substantially enhance the reaction kinetics and reduce the potential hysteresis, while conformal nanolayer PEDOT coating provides a roubst fast electronic connection and prevents the side reactions. The FeOF@PEDOT nanorods deliver a capacity of 560 mA h g(-1) at 10 mA g(-1) with an energy density of >1100 W h kg(-1), which is more than two times higher than the theoretical energy density of LiCoO2. The FeOF@PEDOT nanorods can maintain a capacity of ~430 mA h g(-1) at 50 mA g(-1) (840 W h kg(-1)) for over 150 cycles with capacity decay rate of only 0.04% per cycle, which is 2 orders of magnitude lower than the capacity decay rate ever reported among all conversion-reaction cathodes. Detailed characterizations were conducted to identify the structure and mechanism responsible for these significant improvements that could translate into a Li-ion cell with a 2× increase in energy density.

Sodium layered-oxide cathodes show promise as an alternative to lithium layered oxides for grid-scale energy storage applications. Among the many sodium layered-oxide chemistries, O3-type structures with a mixture of nickel, manganese, and iron are of interest due to their high theoretical energy density. The main challenges of these materials include poor cycle life and air stability due to surface reactivity in both electrolytes and ambient moisture. Surface modifications can mitigate both challenges by protecting the underlying cathode material. Phosphate coatings are of particular interest due to their low cost and high conductivity, but they have not yet been explored for O3 sodium layered-oxide materials. Furthermore, the relative benefits of different coating methods and phosphate groups are not well-understood. Herein, the electrochemical performance and air stability of two coating methods are compared, and their unique challenges with an O3 sodium layered oxide are explored. Unlike lithium layered oxides or P2-type sodium layered oxides, O3 sodium layered oxides suffer from irreversible extraction of sodium during the coating process resulting in metal oxide formation. This sodium extraction causes reduced capacity and conductivity at high coating contents and highlights the unique challenges of O3-type sodium layered oxides. Despite these challenges, a 1% (NaPO 3 ) n coating achieves significant improvement over the uncoated material, corresponding to a 14% gain in capacity retention after 100 cycles and only a loss of 30 mA h g −1 after 9 days in humid air, compared to 80 mA h g −1 lost in the uncoated sample.

Interest in sodium-ion battery cathode materials, particularly for grid-scale energystorage applications, has considerably increased in the last few years, sparking a push toward industrially practical research. In addition to more full-cell and pouch-cell research, there has also been more study on the use of an industrial synthesis technique for producing sodium-based cathode materials. Hydroxide coprecipitation is the industry standard for synthesizing lithiumbased layered-oxide cathode materials because it is scalable and produces materials with highly tunable particle morphology with low impurity. The tuning of particle morphology allows for improved energy density and reduced surface reactivity, leading to better stability in air and electrolyte. Despite these benefits, the ability to synthesize sodium-based layered-oxide cathode materials with beneficial particle morphology is considerably more challenging than it is for lithium-based layered oxides due to the complexity of numerous interconnected variables. We herein provide a review of the hydroxide coprecipitation method for both sodium-and lithium-based cathode materials, highlighting its benefits and the need for further studies on utilizing coprecipitated sodium-based layered-oxide materials. We then perform an in-depth analysis utilizing a combination of literature, fundamental chemistry, and experimental data to discuss the challenges of hydroxide coprecipitation and provide a framework for overcoming these challenges.

Lithium‐ion batteries are ubiquitous in modern society, and their importance is rapidly increasing with the popularization of electric vehicles (EVs). Consumer electronics and EVs greatly benefit from lithium‐ion batteries (LIBs) despite their high cost and limited materials abundance. However, large‐scale applications, such as grid‐scale energy storage require an alternative solution. Sodium‐ion batteries (SIBs) have gained increasing attention within the last decade as an alternative solution to grid‐scale storage as they utilize several cheaper and more abundant materials compared with LIBs. The most likely candidate for SIB industrialization will use a layered‐oxide cathode, allowing comparisons to be drawn to the industrialization of lithium layered oxide cathodes. A notable difference between sodium and lithium layered oxides is the broader range of viable metals that reversibly insert sodium, and an even larger set of possible metal combinations. To predict the optimal compositions for SIB commercialization, herein, the fundamental crystal‐chemical and electrochemical properties of each 3d transition‐metal ion is examined and the history of LIB industrialization is discussed for further insight.

Increased interest in alternatives to lithium-ion batteries has led to promising work on sodium-ion batteries, particularly with layered oxide cathode materials. For practical applications, however, their lower energy density and cycle life relative to lithium-ion layered oxide cathodes make them an inadequate alternative. A greater utilization of sodium ions with charging voltages >4 V versus Na would increase the energy density, but higher cutoff voltages cause decreased cycling stability. To date, very little is known on the capacity fade mechanisms of layered oxide cathodes operating in the high-voltage regime. We herein report, for the first time, the effects of extended high-voltage cycling in O3-type Na(Ni 0.3 Fe 0.4 Mn 0.3 )O 2 . By analyzing the extended cycling performance in conjunction with X-ray diffraction, galvanostatic intermittent titration technique, electrochemical impedance spectroscopy, and X-ray photoelectron spectroscopy, the interconnected mechanisms of capacity fade are elucidated. An irreversible loss of the high-voltage (OP2) phase transition above 4 V due to iron migration causes rapid capacity fade during the initial stage of cell operation. After the disappearance of the OP2 phase, electrolyte decomposition and structural degradation continue to occur, leading to a significant impedance growth and faster capacity fade than cells cycled at 4 V. This study provides valuable insight into the fundamental limitation of O3 layered oxide cathodes and offers guidelines for future materials modification.