What nano can do for energy storage.

A CSNano has been attracting a large number of submissions onmaterials for electrical energy storage and publishing several in each recent issues (read two examples from the May 2014 issue). The need for more efficient storage of electrical energy at all scales, from solar and wind farms to wearable electronics like Google Glass, requires development of devices offering the high energy densities of batteries along with the long cycle lives, high powers, and short charging times of supercapacitors. This has led to a dramatic expansion of research activities at the energy frontier, but why do we need “nano” in energy storage? Indeed, there are many reasons: (1) Short diffusion distances make it possible to charge batteries faster or to draw high current in a short time (increase the power). (2) Large expansion that leads to failure and short cycle lives of microparticles can be accommodated;silicon use in Li-ion battery anodes is a good example, as nanoparticles can survive cycling. Thus, we can use materials capable of larger energy storage or increase the lifetime of currently used materials. (3) Transport of multivalent ions in bulk materials is slow, and use of nanomaterials can enable practical Mgor Al-ion batteries, capable of storing much more energy than the currently used Li-ion batteries. This list is hardly exhaustive, but it is already clear that nanostructuring of existing materials (e.g., metal oxides or silicon) and development of new nanomaterials (e.g., twodimensional, 2D, carbides, and carbonitrides;MXenes ) and hybrid nanomaterials/ nanostructures (e.g., nanoparticles on graphene) are seen as pathways to new solutions for electrical energy storage. Nanoscale design of the structure and chemistry of electrode materials may enable us to develop a new generation of devices that approach the theoretical limit for electrochemical storage and deliver electrical energy rapidly and efficiently. On the fundamental side, understanding nanoscale processes in energy storagematerials is essential to uncover the underlying mechanisms. With this knowledge, new concepts can be formulated that will be developed into revolutionary new electrical energy storage devices and technologies. A large variety of processes that occur during charge storage may be confusing for chemists and material scientists that are new to the field. In battery materials, conversion from one state to another occurs at a constant potential until the phase transformation is completed. Charge storage in supercapacitors typically includes ion adsorption in the electric double-layer capacitor (EDLC) or fastelectrochemical reactionat thesurface of the activematerial (pseudocapacitors). Rapidly available surface charge without diffusion limitation is the origin of their high power density, and the absence of bulk phase transformation leads to high reversibility and long life (up to 1,000,000 cycles). However, since the charge is confined to the surface, the energy densities of EDLCs are less than those of batteries (three-dimensional, 3D, chemical storage). Classical (2D) double-layer charge storage is possible on theplanar surfaces of graphene aerogels or convex outer surfaces of carbon nanotubes or onions. In commercially used microporous activated carbons, ions are confined (electrosorbed) in pores. This process is significantly different than the conventional double layer and is rather similar to gas adsorption or adsorption of organicmolecules fromwater by activated carbons, but driven by an electrical rather than a chemical potential. Nanomaterials and hybrid nanomaterials may enable us to build energy storage devices with the energy densities of the best batteries but with the high power, fast charging, and Nanoscale design of the structure and chemistry of electrode materials may enable us to develop a new generation of devices that approach the theoretical limit for electrochemical storage and deliver electrical energy rapidly and efficiently.