Energy production and storage technologies have attracted a great deal of attention for day-to-day applications. In recent decades, advances in lithium-ion battery (LIB) technology have improved living conditions around the globe. LIBs are used in most mobile electronic devices as well as in zero-emission electronic vehicles. However, there are increasing concerns regarding load leveling of renewable energy sources and the smart grid as well as the sustainability of lithium sources due to their limited availability and consequent expected price increase. Therefore, whether LIBs alone can satisfy the rising demand for small- and/or mid-to-large-format energy storage applications remains unclear. To mitigate these issues, recent research has focused on alternative energy storage systems. Sodium-ion batteries (SIBs) are considered as the best candidate power sources because sodium is widely available and exhibits similar chemistry to that of LIBs; therefore, SIBs are promising next-generation alternatives. Recently, sodiated layer transition metal oxides, phosphates and organic compounds have been introduced as cathode materials for SIBs. Simultaneously, recent developments have been facilitated by the use of select carbonaceous materials, transition metal oxides (or sulfides), and intermetallic and organic compounds as anodes for SIBs. Apart from electrode materials, suitable electrolytes, additives, and binders are equally important for the development of practical SIBs. Despite developments in electrode materials and other components, there remain several challenges, including cell design and electrode balancing, in the application of sodium ion cells. In this article, we summarize and discuss current research on materials and propose future directions for SIBs. This will provide important insights into scientific and practical issues in the development of SIBs.

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Sodium-ion batteries: present and future

Grid-scale energy storage systems (ESSs) that can connect to sustainable energy resources have received great attention in an effort to satisfy ever-growing energy demands. Although recent advances in Li-ion battery (LIB) technology have increased the energy density to a level applicable to grid-scale ESSs, the high cost of Li and transition metals have led to a search for lower-cost battery system alternatives. Based on the abundance and accessibility of Na and its similar electrochemistry to the well-established LIB technology, Na-ion batteries (NIBs) have attracted significant attention as an ideal candidate for grid-scale ESSs. Since research on NIB chemistry resurged in 2010, various positive and negative electrode materials have been synthesized and evaluated for NIBs. Nonetheless, studies on NIB chemistry are still in their infancy compared with LIB technology, and further improvements are required in terms of energy, power density, and electrochemical stability for commercialization. Most recent progress on electrode materials for NIBs, including the discovery of new electrode materials and their Na storage mechanisms, is briefly reviewed. In addition, efforts to enhance the electrochemical properties of NIB electrode materials as well as the challenges and perspectives involving these materials are discussed.

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Recent Progress in Electrode Materials for Sodium-Ion Batteries

Currently, there is increasing interest in developing ‘beyond lithium’ battery technologies to augment, or in certain situations replace, lithium ion batteries (LIBs). Room temperature sodium ion batteries (NIBs) offer an attractive combination of low cost and plentiful constituents and a wide range of phases, structures and stoichiometries available for optimisation. Sodium layered oxides are considered to be promising candidates as cathode materials, due to their flexibility and versatility, as well as their intrinsically fast structural diffusion of Na ions which leads to enhanced rate capability. In particular, sodium manganese based layered oxides (generally NaxMn1−y−zMyTMzO2, where TM represents one or more transition metals, and M consists of one or more non-transition metals) are a key family of materials, in part due to the relatively low cost and environmentally friendly nature of the manganese, and consequently are worthy of a detailed investigation. Examination of these systems, particularly in terms of stoichiometry and phase, has shown that significant advances have been made recently, both in terms of understanding the mechanisms behind electrochemical performance, and in terms of refining these to produce improved materials. The goal of this review is to present the current developments in sodium manganese based layered oxides (particularly with respect to electrochemical performance, physical properties and manganese content), to discuss the current state of this field of research and to draw conclusions regarding where future investigations may be most fruitfully directed.

High performance manganese-based layered oxide cathodes: overcoming the challenges of sodium ion batteries

Recently, the attention to sodium-ion batteries has been refocused on large-scale energy storage applications, due to sodium's low cost and infinite abundance. Sodium is one of the most abundant elements on earth and exhibits chemical properties similar to lithium. Owing to their superior sodium storage capability especially for excellent safety and stability, Ti-based compounds have been extensively investigated as both cathode and anode materials. Herein we outline the current cathodes and anodes, and emphasize the critical roles of titanium in developing advanced electrodes for sodium-ion batteries. The latest advances and progress in the exploration of Ti-based compounds with various different frameworks such as NASICON, tunnel, MXenes, spinel, and layered structures are systematically reviewed. The straightforward linking of the structure–function–property relationship for Ti-based compounds, especially for layered Na/Ti-containing oxides, is summarized and analyzed. The titanium element plays a critical role in both positive and negative electrodes, i.e., supplying the charge transfer and high safety for anodes and greatly enhancing the structural and cycling stability for cathodes. Based on the bi-functional roles of titanium, a new concept of symmetric Na-ion cells employing layered Na/Ti-containing oxides as bipolar electrodes is proposed and realized. Symmetric Na-ion cells already offered a high voltage and withstood long time charge–discharge processes, demonstrating the practicality beyond the proof of concept. The participation of titanium in sodium-based electrode materials will greatly promote the development of room-temperature sodium-ion batteries towards stationary energy storage.

Recent advances in titanium-based electrode materials for stationary sodium-ion batteries

The Li-excess oxide compound is one of the most promising positive electrode materials for next generation batteries exhibiting high capacities of >300 mA h g−1 due to the unconventional participation of the oxygen anion redox in the charge compensation mechanism. However, its synthesis has been proven to be highly sensitive to varying conditions and parameters where nanoscale phase separation may occur that affects the overall battery performance and life. In addition, several thermodynamic and kinetic drawbacks including large first cycle irreversible capacity, poor rate capability, voltage fading, and surface structural transformation need to be addressed in order to reach commercialization. This review will focus on the recent progress and performance trends over the years and provide several guidelines and design considerations based on the library of work done on this particular class of materials.

Performance and design considerations for lithium excess layered oxide positive electrode materials for lithium ion batteries

Synthesis of metal ion substituted P2-Na2/3Ni1/3Mn2/3O2 cathode material with enhanced performance for Na ion batteries

Enhanced electrochemical performance of Ti substituted P2-Na2/3Ni1/4Mn3/4O2 cathode material for sodium ion batteries

Improvement of cycling performance in Ti substituted 0.5Li2MnO3–0.5LiNi0.5Mn0.5O2 through suppressing metal dissolution

A new route for the synthesis of Li2MnO3 based cathode material with enhanced first cycle efficiency and cycleability for lithium ion batteries

Synthesis of Li-excess layered cathode material with enhanced reversible capacity for Lithium ion batteries through the optimization of precursor synthesis method

Shinji Yamamoto

Papers

Synthesis of metal ion substituted P2-Na2/3Ni1/3Mn2/3O2 cathode material with enhanced performance for Na ion batteries

Enhanced electrochemical performance of Ti substituted P2-Na2/3Ni1/4Mn3/4O2 cathode material for sodium ion batteries

Improvement of cycling performance in Ti substituted 0.5Li2MnO3–0.5LiNi0.5Mn0.5O2 through suppressing metal dissolution

A new route for the synthesis of Li2MnO3 based cathode material with enhanced first cycle efficiency and cycleability for lithium ion batteries

Synthesis of Li-excess layered cathode material with enhanced reversible capacity for Lithium ion batteries through the optimization of precursor synthesis method