In this critical review, metal oxides-based materials for electrochemical supercapacitor (ES) electrodes are reviewed in detail together with a brief review of carbon materials and conducting polymers. Their advantages, disadvantages, and performance in ES electrodes are discussed through extensive analysis of the literature, and new trends in material development are also reviewed. Two important future research directions are indicated and summarized, based on results published in the literature: the development of composite and nanostructured ES materials to overcome the major challenge posed by the low energy density of ES (476 references).

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A review of electrode materials for electrochemical supercapacitors

Batteries are a key technology in modern society. They are used to power electric and hybrid electric vehicles and to store wind and solar energy in smart grids. Electrochemical devices with high energy and power densities can currently be powered only by batteries with organic liquid electrolytes. However, such batteries require relatively stringent safety precautions, making large-scale systems very complicated and expensive. The application of solid electrolytes is currently limited because they attain practically useful conductivities (10(-2) S cm(-1)) only at 50-80 °C, which is one order of magnitude lower than those of organic liquid electrolytes. Here, we report a lithium superionic conductor, Li(10)GeP(2)S(12) that has a new three-dimensional framework structure. It exhibits an extremely high lithium ionic conductivity of 12 mS cm(-1) at room temperature. This represents the highest conductivity achieved in a solid electrolyte, exceeding even those of liquid organic electrolytes. This new solid-state battery electrolyte has many advantages in terms of device fabrication (facile shaping, patterning and integration), stability (non-volatile), safety (non-explosive) and excellent electrochemical properties (high conductivity and wide potential window).

A lithium superionic conductor

Lithium (Li)-ion batteries (LIB) have governed the current worldwide rechargeable battery market due to their outstanding energy and power capability. In particular, the LIB's role in enabling electric vehicles (EVs) has been highlighted to replace the current oil-driven vehicles in order to reduce the usage of oil resources and generation of CO2 gases. Unlike Li, sodium is one of the more abundant elements on Earth and exhibits similar chemical properties to Li, indicating that Na chemistry could be applied to a similar battery system. In the 1970s-80s, both Na-ion and Li-ion electrodes were investigated, but the higher energy density of Li-ion cells made them more applicable to small, portable electronic devices, and research efforts for rechargeable batteries have been mainly concentrated on LIB since then. Recently, research interest in Na-ion batteries (NIB) has been resurrected, driven by new applications with requirements different from those in portable electronics, and to address the concern on Li abundance. In this article, both negative and positive electrode materials in NIB are briefly reviewed. While the voltage is generally lower and the volume change upon Na removal or insertion is larger for Na-intercalation electrodes, compared to their Li equivalents, the power capability can vary depending on the crystal structures. It is concluded that cost-effective NIB can partially replace LIB, but requires further investigation and improvement.

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Electrode Materials for Rechargeable Sodium-Ion Batteries: Potential Alternatives to Current Lithium-Ion Batteries

Solid-state electrolytes are attracting increasing interest for electrochemical energy storage technologies. In this Review, we provide a background overview and discuss the state of the art, ion-transport mechanisms and fundamental properties of solid-state electrolyte materials of interest for energy storage applications. We focus on recent advances in various classes of battery chemistries and systems that are enabled by solid electrolytes, including all-solid-state lithium-ion batteries and emerging solid-electrolyte lithium batteries that feature cathodes with liquid or gaseous active materials (for example, lithium–air, lithium–sulfur and lithium–bromine systems). A low-cost, safe, aqueous electrochemical energy storage concept with a ‘mediator-ion’ solid electrolyte is also discussed. Advanced battery systems based on solid electrolytes would revitalize the rechargeable battery field because of their safety, excellent stability, long cycle lives and low cost. However, great effort will be needed to implement solid-electrolyte batteries as viable energy storage systems. In this context, we discuss the main issues that must be addressed, such as achieving acceptable ionic conductivity, electrochemical stability and mechanical properties of the solid electrolytes, as well as a compatible electrolyte/electrode interface. This Review details recent advances in battery chemistries and systems enabled by solid electrolytes, including all-solid-state lithium-ion, lithium–air, lithium–sulfur and lithium–bromine batteries, as well as an aqueous battery concept with a mediator-ion solid electrolyte.

Lithium battery chemistries enabled by solid-state electrolytes

Layered double hydroxides (LDHs) are a class of ionic lamellar compounds made up of positively charged brucite-like layers with an interlayer region containing charge compensating anions and solvation molecules. Delamination of LDHs is an interesting route for producing positively charged thin platelets with a thickness of a few atomic layers, which can be used as nanocomposites for polymers or as building units for making new designed organic-inorganic or inorganic-inorganic nanomaterials. The synthesis of nanosized LDH platelets can be generally classified into two approaches, bottom-up and top-down. It requires modification of the LDH interlamellar environment and then selection of an appropriate solvent system. In DDS intercalated LDHs, the aliphatic tails of the DDS- anions exhibit a high degree of interdigitation in order to maximize guest-guest dispersive interactions. Bellezza reported that the LDH colloids can also been obtained by employing a reverse microemulsion approach.

Recent advances in the synthesis and application of layered double hydroxide (LDH) nanosheets.

Rechargeable lithium batteries are widely used in portable equipment today. However, there have always been safety issues arising from their combustible organic electrolytes. These issues are becoming more serious with the increasing size of batteries for use in electric vehicle (EV) or load-leveling applications. Nonflammable solid electrolytes would be the ultimate solution to the safety issue. Despite their high safety, the energy densities and power densities of solid systems have been too low for their practical use. We have succeeded in increasing their energy densities to levels comparable to those of liquid ones. However, power densities, or high-rate capabilities, remain poor. In this communication, we report that a buffer film with a thickness of only several nanometers interposed between the electrode and electrolyte materials improves the high-rate capability of solid-state lithium batteries. Low ionic conductivities of solid electrolytes have been the reason for the poor high-rate capability of solid-state lithium batteries. Although the conductivities of recently discovered solid electrolytes (> 10 S cm) are slightly lower than those observed for liquid electrolytes, Li ionic conduction in the solid electrolytes has become as fast as that of liquid electrolytes, by taking into account the fact that the transport number of ions in inorganic electrolytes is unity. However, the high-rate capability of solid systems, including solid electrolytes, remains inferior. This fact strongly suggests that the rate-controlling step is not in the bulk of the solid electrolytes, but at the interface between the electrode and the electrolyte materials. Ionic conduction at interfaces between different kinds of ionic conductors, or heterojunctions, is characterized by the term “nanoionics”; a frontier study was done for a LiI– Al2O3 composite, [7] and a sophisticated example was presented by Sata et al. In the latter, two kinds of F ion conductors, BaF2 and CaF2, were brought into contact with each other. Part of the F ions then transferred from one side to the other to reach an equilibrium, which produced vacancies in the former and interstitial ions in the latter, both of which contributed to ionic conduction at the interface and enhanced the ionic conduction. Similar nanoionic phenomena should take place at the interface between the electrode and the electrolyte materials, forming a space-charge layer. Because the compositions and structures of solid electrolytes have been well tailored to achieve high ionic conductivities, the ionic conductivity of the space-charge layer, where the compositions deviate from the optima, should be lower than that of the bulk, increasing the interfacial resistance. For instance, the Li4GeS4–Li3PS4 (thio-LISICON) system used in the present study has a high ionic conductivity of the order of 10 S cm at its optimum composition. However, variation in the composition reduces it to 10 S cm. The detrimental increase in the interfacial resistance would be prominent in bulk-type or non-thin-film solid-state lithium batteries. Sulfide electrolytes should be used in such batteries, because the ionic conductivities of oxide solid electrolytes, for example, are so low that they are available only in thin-film systems. On the other hand, the cathode should be an oxide, such as LiCoO2, because of its high electrode potential. [10,11]

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Enhancement of the High-Rate Capability of Solid-State Lithium Batteries by Nanoscale Interfacial Modification

LiNbO3-coated LiCoO2 as cathode material for all solid-state lithium secondary batteries

Layered MnO2 Nanobelts: Hydrothermal Synthesis and Electrochemical Measurements

Interfacial modification for high-power solid-state lithium batteries

All-solid lithium secondary batteries have excellent reliability including safety, but they are generally low in energy density or output density when compared with liquid electrolyte batteries. Specifically disclosed is an all-solid lithium battery using a lithium ion conductive solid electrolyte as the electrolyte, which battery is characterized in that the lithium ion conductive solid electrolyte is mainly composed of a sulfide and the surface of a positive electrode active material is covered with a lithium ion conductive oxide. The effect of the present invention is particularly remarkable when the positive electrode active material has a potential of not less than 3V during operation of the all-solid lithium battery, namely when the positive electrode active material undergoes an oxidation-reduction reaction at a potential of not less than 3V.

Lianqi Zhang

Papers

Enhancement of the High-Rate Capability of Solid-State Lithium Batteries by Nanoscale Interfacial Modification

LiNbO3-coated LiCoO2 as cathode material for all solid-state lithium secondary batteries

Layered MnO2 Nanobelts: Hydrothermal Synthesis and Electrochemical Measurements

Interfacial modification for high-power solid-state lithium batteries

All-solid lithium battery