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

Sodium-ion batteries (SIBs) are considered as promising alternatives to lithium-ion batteries owing to the abundant sodium resources. However, the limited energy density, moderate cycling life, and immature manufacture technology of SIBs are the major challenges hindering their practical application. Recently, numerous efforts are devoted to developing novel electrode materials with high specific capacities and long durability. In comparison with carbonaceous materials (e.g., hard carbon), partial Group IVA and VA elements, such as Sn, Sb, and P, possess high theoretical specific capacities for sodium storage based on the alloying reaction mechanism, demonstrating great potential for high-energy SIBs. In this review, the recent research progress of alloy-type anodes and their compounds for sodium storage is summarized. Specific efforts to enhance the electrochemical performance of the alloy-based anode materials are discussed, and the challenges and perspectives regarding these anode materials are proposed.

Alloy-Based Anode Materials toward Advanced Sodium-Ion Batteries

Phosphorus is a nonmetal with several allotropes, from the highly reactive white phosphorus to the thermodynamically stable black phosphorus (BP) with a puckered orthorhombic layered structure. The bulk form of BP was first synthesized in 1914, but received little attention until it was rediscovered in 2014 as a member of the new wave of 2D layered nanomaterials. BP can be exfoliated to a single sheet that acts as a semiconductor with a tunable direct band gap, a high carrier mobility at room temperature, and an in-plane anisotropy. The development of BP applications is hampered by surface degradation, thus efforts to achieve effective BP passivation are ongoing, such as its integration in van der Waals heterostructures. BP has been tested as a novel nanomaterial in batteries, transistors, sensors, and photonics. This Review begins with the origin of the BP story, following the path from a bulk material to modern few/single layers. The physical and chemical properties are summarized, and the state-of-the-art of BP applications highlighted.

Black Phosphorus Rediscovered: From Bulk Material to Monolayers

Metal-free elemental photocatalysts for hydrogen (H2 ) evolution are more advantageous than the traditional metal-based inorganic photocatalysts since the nonmetal elements are generally cheaper, more earth-abundant, and environmentally friendly. Black phosphorus (BP) has been attracting increasing attention in recent years based on its anisotropic 2D layered structure with tunable bandgap in the range of 0.3-2.0 eV; however, the application of BP for photocatalytic H2 evolution has been scarcely reported experimentally although being theoretically predicted. Herein, for the first time, the visible light photocatalytic H2 evolution of BP nanosheets prepared via a facile solid-state mechanochemical method by ball-milling bulk BP is reported. Without using any noble metal cocatalyst, the visible light photocatalytic hydrogen evolution rate of BP nanosheets reaches 512 µmol h-1 g-1 , which is ≈18 times higher than that of the bulk BP, and is comparable or even higher than that of graphitic carbon nitrides (g-C3 N4 ).

Black Phosphorus Revisited: A Missing Metal-Free Elemental Photocatalyst for Visible Light Hydrogen Evolution.

High specific surface area carbon (HSSAC) is a class of promising high-capacity anode materials for sodium-ion batteries (SIBs). A critical bottleneck of the HSSAC anode, however, is the ultra-low initial coulombic efficiency (ICE) in commonly used ester-based electrolytes. This phenomenon further prohibits improving the specific capacity, long-term stability and rate capability of HSSAC anodes. This work reports the largely enhanced anode performance of several different HSSAC anodes in ether-based electrolytes. Very importantly, with the reduced graphene oxide (rGO) anode as one example, the ICE can be as high as 74.6% accompanied by a large reversible specific capacity of 509 mA h g−1 after 100 cycles at a current density of 0.1 A g−1. 75.2% of the capacity was retained after 1000 cycles at 1 A g−1. Even at a high current density of 5 A g−1, the specific capacity of the rGO anode can be obtained at 196 mA h g−1. This extraordinary performance is ascribed to the stable, thin, compact, uniform and ion conducting solid electrolyte interphase (SEI) formed in an ether-based electrolyte. Fortunately, this SEI-modifying strategy is generic and is independent of the specific microstructures of HSSAC anodes, indicating a promising avenue for manipulating the SEI on HSSAC anodes through utilizing ether solvents to enable achievement of high ICE for large-capacity HSSAC anodes for practical applications.

Achieving superb sodium storage performance on carbon anodes through an ether-derived solid electrolyte interphase

For a nonaqueous sodium-ion battery (NIB), phosphorus materials have been studied as the highest-capacity negative electrodes. However, the large volume change of phosphorus upon cycling at low voltage causes the formation of new active surfaces and potentially results in electrolyte decomposition at the active surface, which remains one of the major limiting factors for the long cycling life of batteries. In this present study, powerful surface characterization techniques are combined for investigation on the electrode/electrolyte interface of the black phosphorus electrodes with polyacrylate binder to understand the formation of a solid electrolyte interphase (SEI) in alkyl carbonate ester and its evolution during cycling. The hard X-ray photoelectron spectroscopy (HAXPES) analysis suggests that SEI (passive film) consists of mainly inorganic species, which originate from decomposition of electrolyte solvents and additives. The thicker surface layer is formed during cycling in the additive-free electrol...

Xue Fang Yu

Papers

Black Phosphorus as a High-Capacity, High-Capability Negative Electrode for Sodium-Ion Batteries: Investigation of the Electrode/Electrolyte Interface