Li-ion battery commercialized by Sony in 1991 has the highest energy-density among practical rechargeable batteries and is widely used in electronic devices, electric vehicles, and stationary energy storage system in the world. Moreover, the battery market is rapidly growing in the world and further fast-growing is expected. With expansion of the demand and applications, price of lithium and cobalt resources is increasing. We are, therefore, motivated to study Na- and K-ion batteries for stationary energy storage system because of much abundant Na and K resources and the wide distribution in the world. In this account, we review developments of Na- and K-ion batteries with mainly introducing our previous and present researches in comparison to that of Li-ion battery.

Towards K-Ion and Na-Ion Batteries as "Beyond Li-Ion".

The increasing demand for replacing conventional fossil fuels with clean energy or economical and sustainable energy storage drives better battery research today. Sodium-ion batteries (SIBs) are considered as a promising alternative for grid-scale storage applications due to their similar “rocking-chair” sodium storage mechanism to lithium-ion batteries, the natural abundance, and the low cost of Na resources. Searching for appropriate electrode materials with acceptable electrochemical performance is the key point for development of SIBs. Layered transition metal oxides represent one of the most fascinating electrode materials owing to their superior specific capacity, environmental benignity, and facile synthesis. However, three major challenges (irreversible phase transition, storage instability, and insufficient battery performance) are known for cathodes in SIBs. Herein, a comprehensive review on the latest advances and progresses in the exploration of layered oxides for SIBs is presented, and a detailed and deep understanding of the relationship of phase transition, air stability, and electrochemical performance in layered oxide cathodes is provided in terms of refining the structure–function–property relationship to design improved battery materials. Layered oxides will be a competitive and attractive choice as cathodes for SIBs in next-generation energy storage devices.

Layered Oxide Cathodes for Sodium-Ion Batteries: Phase Transition, Air Stability, and Performance

The exponential increase in research focused on two-dimensional (2D) metal oxides has offered an unprecedented opportunity for their use in energy conversion and storage devices, especially for promising next-generation rechargeable batteries, such as lithium-ion batteries (LIBs) and sodium-ion batteries (NIBs), as well as some post-lithium batteries, including lithium–sulfur batteries, lithium–air batteries, etc. The introduction of well-designed 2D metal oxide nanomaterials into next-generation rechargeable batteries has significantly enhanced the performance of these energy-storage devices by providing higher chemically active interfaces, shortened ion-diffusion lengths, and improved in-plane carrier-/charge-transport kinetics, which have greatly promoted the development of nanotechnology and the practical application of rechargeable batteries. Here, the recent progress in the application of 2D metal oxide nanomaterials in a series of rechargeable LIBs, NIBs, and other post lithium-ion batteries is reviewed relatively comprehensively. Current opportunities and future challenges for the application of 2D nanomaterials in energy-storage devices to achieve high energy density, high power density, stable cyclability, etc. are summarized and outlined. It is believed that the integration of 2D metal oxide nanomaterials in these clean energy devices offers great opportunities to address challenges driven by increasing global energy demands.

Two-Dimensional Metal Oxide Nanomaterials for Next-Generation Rechargeable Batteries.

Sodium-ion batteries (SIBs) have been considered as potential candidates for stationary energy storage because of the low cost and wide availability of Na sources. O3-type layered oxides have been considered as one of the most promising cathodes for SIBs. However, they commonly show inevitable complicated phase transitions and sluggish kinetics, incurring rapid capacity decline and poor rate capability. Here, a series of sodium-sufficient O3-type NaNi0.5 Mn0.5-x Ti x O2 (0 ≤ x ≤ 0.5) cathodes for SIBs is reported and the mechanisms behind their excellent electrochemical performance are studied in comparison to those of their respective end-members. The combined analysis of in situ X-ray diffraction, ex situ X-ray absorption spectroscopy, and scanning transmission electron microscopy for NaNi0.5 Mn0.2 Ti0.3 O2 reveals that the O3-type phase transforms reversibly into a P3-type phase upon Na+ deintercalation/intercalation. The substitution of Ti for Mn enlarges interslab distance and could restrain the unfavorable and irreversible multiphase transformation in the high voltage regions that is usually observed in O3-type NaNi0.5 Mn0.5 O2 , resulting in improved Na cell performance. This integration of macroscale and atomicscale engineering strategy might open up the modulation of the chemical and physical properties in layered oxides and grasp new insight into the optimal design of high-performance cathode materials for SIBs.

Ti‐Substituted NaNi0.5Mn0.5‐xTixO2 Cathodes with Reversible O3−P3 Phase Transition for High‐Performance Sodium‐Ion Batteries

With the unprecedentedly increasing demand for renewable and clean energy sources, the sodium-ion battery (SIB) is emerging as an alternative or complementary energy storage candidate to the present commercial lithium-ion battery due to the abundance and low cost of sodium resources. Layered transition metal oxides and Prussian blue analogs are reviewed in terms of their commercial potential as cathode materials for SIBs. The recent progress in research on their half cells and full cells for the ultimate application in SIBs are summarized. In addition, their electrochemical performance, suitability for scaling up, cost, and environmental concerns are compared in detail with a brief outlook on future prospects. It is anticipated that this review will inspire further development of layered transition metal oxides and Prussian blue analogs for SIBs, especially for their emerging commercialization.

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The Cathode Choice for Commercialization of Sodium-Ion Batteries: Layered Transition Metal Oxides versus Prussian Blue Analogs

Na-ion batteries have become promising candidates for large-scale energy-storage systems because of the abundant Na resources and they have attracted considerable academic interest because of their unique behavior, such as their electrochemical activity for the Fe3+/Fe4+ redox couple. The high-rate performance derived from the low Lewis-acidity of the Na+ ions is another advantage of Na-ion batteries and has been demonstrated in NaFe1/2Co1/2O2 solutions. Here, a solid solution of NaFeO2-NaCoO2 is synthesized and the mechanisms behind their excellent electrochemical performance are studied in comparison to those of their respective end-members. The combined analysis of operando X-ray diffraction, ex situ X-ray absorption spectroscopy, and density functional theory (DFT) calculations for Na1– x Fe1/2Co1/2O2 reveals that the O3-type phase transforms into a P3-type phase coupled with Na+/vacancy ordering, which has not been observed in O3-type NaFeO2. The substitution of Co for Fe stabilizes the P3-type phase formed by sodium extraction and could suppress the irreversible structural change that is usually observed in O3-type NaFeO2, resulting in a better cycle retention and higher rate performance. Although no ordering of the transition metal ions is seen in the neutron diffraction experiments, as supported by Monte-Carlo simulations, the formation of a superlattice originating from the Na+/vacancy ordering is found by synchrotron X-ray diffraction for Na0.5Fe1/2Co1/2O2, which may involve a potential step in the charge/discharge profiles.

Understanding the Structural Evolution and Redox Mechanism of a NaFeO2–NaCoO2 Solid Solution for Sodium-Ion Batteries

High‐entropy layered oxide materials containing various metals that exhibit smooth voltage curves and excellent electrochemical performances have attracted attention in the development of positive electrode materials for sodium‐ion batteries. However, a smooth voltage curve can be obtained by suppression of the Na+‐vacancy ordering, and therefore, transition metal slabs do not need to be more multi‐element than necessary. Here, the Na+‐vacancy ordering is found to be disturbed by dual substitution of TiIV for MnIV and ZnII for NiII in P2‐Na2/3[Ni1/3Mn2/3]O2. Dual‐substituted Na2/3[Ni1/4Mn1/2Ti1/6Zn1/12]O2 demonstrates almost non‐step voltage curves with a reversible capacity of 114 mAh g−1 and less structural changes with a high crystalline structure maintained during charging and discharging. Synchrotron X‐ray, neutron, and electron diffraction measurements reveal that dual‐substitution with TiIV and ZnII uniquely promotes in‐plane NiII–MnIV ordering, which is quite different from the disordered mixing in conventional multiple metal substitution.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/adma.202300714

Takuya Asari

Papers

Understanding the Structural Evolution and Redox Mechanism of a NaFeO2–NaCoO2 Solid Solution for Sodium-Ion Batteries

Impact of Ti and Zn Dual‐Substitution in P2 Type Na2/3Ni1/3Mn2/3O2 on Ni–Mn and Na‐Vacancy Ordering and Electrochemical Properties