Room-temperature stationary sodium-ion batteries have attracted great attention particularly in large-scale electric energy storage applications for renewable energy and smart grid because of the huge abundant sodium resources and low cost. In this article, a variety of electrode materials including cathodes and anodes as well as electrolytes for room-temperature stationary sodium-ion batteries are briefly reviewed. We compare the difference in storage behavior between Na and Li in their analogous electrodes and summarize the sodium storage mechanisms in the available electrode materials. This review also includes some new results from our group and our thoughts on developing new materials. Some perspectives and directions on designing better materials for practical applications are pointed out based on knowledge from the literature and our experience. Through this extensive literature review, the search for suitable electrode and electrolyte materials for stationary sodium-ion batteries is still challenging. However, after intensive research efforts, we believe that low-cost, long-life and room-temperature sodium-ion batteries would be promising for applications in large-scale energy storage system in the near future.

Room-temperature stationary sodium-ion batteries for large-scale electric energy storage

The room temperature Na-ion secondary battery has been under focus lately due to its feasibility to compete against the already well-established Li-ion secondary battery. Although there are many obstacles to overcome before the Na-ion battery becomes commercially available, recent research discoveries corroborate that some of the cathode materials for the Na-ion battery have indeed indisputable advantages over its Li-ion counterparts. In this publication, a comprehensive review of layered oxides (NaTMO2, TM = Ti, V, Cr, Mn, Fe, Co, Ni, and a mixture of 2 or 3 elements) as a viable Na-ion battery cathode is presented. Single TM systems are well characterized not only for their electrochemical performance but also for their structural transitions during the cycle. Binary TM systems are investigated in order to address issues regarding low reversible capacity, capacity retention, operating voltage, and structural stability. As a consequence, some materials already have reached an energy density of 520 mW h g−1, which is comparable to that of LiFePO4. Furthermore, some ternary TM systems retained more than 72% of their capacity along with over 99.7% Coulombic efficiency for 275 cycles. The goal of this review is to present the development of Na layered oxide materials in the past as well as the state of the art today in order to emphasize the compatibility and durability of layered oxides as powerful candidates for Na-ion battery cathode materials.

A comprehensive review of sodium layered oxides: powerful cathodes for Na-ion batteries

The ever-growing demand for advanced rechargeable lithium-ion batteries in portable electronics and electric vehicles has spurred intensive research efforts over the past decade. The key to sustaining the progress in Li-ion batteries lies in the quest for safe, low-cost positive electrode (cathode) materials with desirable energy and power capabilities. One approach to boost the energy and power densities of batteries is to increase the output voltage while maintaining a high capacity, fast charge–discharge rate, and long service life. This review gives an account of the various emerging high-voltage positive electrode materials that have the potential to satisfy these requirements either in the short or long term, including nickel-rich layered oxides, lithium-rich layered oxides, high-voltage spinel oxides, and high-voltage polyanionic compounds. The key barriers and the corresponding strategies for the practical viability of these cathode materials are discussed along with the optimization of electrolytes and other cell components, with a particular emphasis on recent advances in the literature. A concise perspective with respect to plausible strategies for future developments in the field is also provided.

High-voltage positive electrode materials for lithium-ion batteries

This work presents an up-to-date information on Na-based battery materials. On the one hand, it explores the feasibility of two novel energy storage systems: Na-aqueous batteries and Na–O2 technology. On the other hand, it summarises new advances on non-aqueous Na-ion systems. Although all of them can be placed under the umbrella of Na-based systems, aqueous and oxygen-based batteries are arising technologies with increasing significance in energy storage research, while non-aqueous sodium-ion technology has become one of the most important research lines in this field. These systems meet different requirements of energy storage: Na-aqueous batteries will have a determining role as a low cost and safer technology; Na–O2 systems can be the key technology to overcome the need for high energy density storage devices; and non-aqueous Na-ion batteries have application in the field of stationary energy storage.

Update on Na-based battery materials. A growing research path

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

Stable Na+ ion storage cathodes with adequate reversible capacity are now greatly needed for enabling Na-ion battery technology for large scale and low cost electric storage applications. In light of the superior Li+ ion storage performance of layered oxides, pure P2-phase Na0.67[Mn0.65Ni0.15Co0.2]O2 microflakes are synthesized by a simple sol–gel method and tested as a Na+ ion storage cathode. These layered microflakes exhibit a considerably high reversible capacity of 141 mA h g−1 and a slow capacity decay to 125 mA h g−1 after 50 cycles, showing much better cyclability than previous NaMnO2 compounds. To further enhance the structural and cycling stability, we partially substituted Co3+ by Al3+ ions in the transition-metal layer to synthesize Na0.67[Mn0.65Ni0.15Co0.15Al0.05]O2. As expected, the Al-substituted material demonstrates a greatly improved cycling stability with a 95.4% capacity retention over 50 cycles, possibly serving as a high capacity and stable cathode for Na-ion battery applications.

Synthesis and electrochemical behaviors of layered Na0.67[Mn0.65Co0.2Ni0.15]O2 microflakes as a stable cathode material for sodium-ion batteries

Na-stabilized Li1.2−xNax[Co0.13Ni0.13Mn0.54]O2 is synthesized by introducing larger Na ions into the Li slabs of the layered material through a simple polymer-pyrolysis method. The structural and morphological characterization reveals that the Na doping leads to a more ordered structure with regular cubic morphology and enlarged Li layer spacing. Electrochemical experiments show that the Na-doped Li1.17Na0.03[Co0.13Ni0.13Mn0.54]O2 electrode can deliver larger reversible discharge capacity (307 mA h g−1), higher initial coulombic efficiency (87%), greatly improved rate capability (139 mA h g−1 at 8 C) and cycling stability (89% capacity retention after 100 cycles) in comparison with the undoped Li1.2[Co0.13Ni0.13Mn0.54]O2 electrode. The superior electrochemical performance of the Na-doped material is due to the enhancement of the structural stability and the enlargement of the Li slab space of the layered material, which facilitate the stabilization of the host lattice and allow rapid diffusion of Li ions in the bulk lattice.

Enhanced high-rate capability and cycling stability of Na-stabilized layered Li1.2[Co0.13Ni0.13Mn0.54]O2 cathode material

Layered Li[Li0.2Co0.13Ni0.13Mn0.54]O2 nanoparticles were synthesized by a simple polymer-pyrolysis method and then coated with 3 wt% Al2O3 to form a ∼4 nm thick protective skin. The Al2O3-coated Li[Li0.2Co0.13Ni0.13Mn0.54]O2 electrode demonstrates a high initial coulombic efficiency of 96.1%, a large reversible capacity of ∼311 mAh g−1, and a good cyclability with 83.8% capacity retention after 70 cycles. Particularly, this material can deliver a quite high capacity of ∼239 mAh g−1 at a high rate of 400 mA g−1. This superior electrochemical performance results from the well-crystallized nanocores and effective surface modification of the material. The former provides a short diffusion path and fast transport channels for lithium ion insertion/extraction reactions and the latter restrains the elimination of oxide ion vacancies and metal ion rearrangement during charge–discharge cycling. Due to their simplicity and applicability, the synthetic method along with the surface modification technique is easily adopted to make high performance xLi2MnO3·(1 − x)LiMO2 materials for practical battery applications.

Improved electrochemical performances of nanocrystalline Li[Li0.2Mn0.54Ni0.13Co0.13]O2 cathode material for Li-ion batteries

Mg-doped Li[Li0.2–2xMgxCo0.13Ni0.13Mn0.54]O2 is synthesized by introducing Mg ions into the transition-metal (TM) layer of this layered compound for substituting Li ions through a simple polymer-pyrolysis method. The structural and morphological characterization reveals that the doped Mg ions are uniformly distributed in the bulk lattice, showing an insignificant impact on the layered structure. Electrochemical experiments reveal that, at a Mg doping of 4%, the Li[Li0.16Mg0.04Co0.13Ni0.13Mn0.54]O2 electrode can deliver a larger initial reversible capacity of 272 mAh g–1, an improved rate capability with 114 mAh g–1 at 8 C, and an excellent cycling stability with 93.3% capacity retention after 300 cycles. The superior electrochemical performances of the Mg-doped material are possibly due to the enhancement of the structural stability by substitution of Li by Mg in the TM layer, which effectively suppresses the cation mixing arrangement, leading to the alleviation of the phase change during lithium-ion inse...

Magnesium-Doped Li1.2[Co0.13Ni0.13Mn0.54]O2 for Lithium-Ion Battery Cathode with Enhanced Cycling Stability and Rate Capability

The effect of the cutoff voltages on the working voltage decay and cyclability of the lithium-rich manganese-based layered cathode (LRMO) was investigated by electrochemical measurements, electrochemical impedance spectroscopy, ex situ X-ray diffraction, transmission electron microscopy, and energy dispersive spectroscopy line scan technologies. It was found that both lower (2.0 V) and upper (4.8 V) cutoff voltages cause severe voltage decay with cycling due to formation of the spinel phase and migration of the transition metals inside the particles. Appropriate cutoff voltage between 2.8 and 4.4 V can effectively inhibit structural variation as the electrode demonstrates 92% capacity retention and indiscernible working voltage decay over 430 cycles. The results also show that phase transformation not only on high charge voltage but also on low discharge voltage should be addressed to obtain highly stable LRMO materials.

Wei He

Papers

Synthesis and electrochemical behaviors of layered Na0.67[Mn0.65Co0.2Ni0.15]O2 microflakes as a stable cathode material for sodium-ion batteries

Enhanced high-rate capability and cycling stability of Na-stabilized layered Li1.2[Co0.13Ni0.13Mn0.54]O2 cathode material

Improved electrochemical performances of nanocrystalline Li[Li0.2Mn0.54Ni0.13Co0.13]O2 cathode material for Li-ion batteries

Magnesium-Doped Li1.2[Co0.13Ni0.13Mn0.54]O2 for Lithium-Ion Battery Cathode with Enhanced Cycling Stability and Rate Capability

Understanding Voltage Decay in Lithium-Rich Manganese-Based Layered Cathode Materials by Limiting Cutoff Voltage