Iron-based materials strategies
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tion There is a pressing need to use renewable energy (solar, wind, geothermal) as well as to provide efficient storage and supply of mobile energy to enable the application of consumer electronics batteries to longer-range commuter electric vehicles. In the energy-storage sector, electrochemical energy storage based on the reversible conversion of chemical energy and electrical energy, both having a common carrier (i.e., electrons), has dominated for over two centuries. Li-ion batteries, which were commercialized by Sony in 1991, have far outperformed other technologies in terms of energy density with adequate rate capability, longevity, and large-scale commercial applications. Thus, Li-ion batteries hold the promise and burden to sustain our energy-demanding society. For illustrating the path toward rechargeable batteries with ultimate sustainability, we first spotlight the principle of present Li-ion battery systems (see the Introductory article in this issue). As illustrated in Figure 1, there are three main components: a positive electrode (also termed as cathode), a negative electrode (anode), and an electrolyte. Such batteries have employed low-voltage carbonaceous materials without Li as anodes and LiCoO2 as cathodes acting as a source of Li. This kind of Li-ion battery possesses a working voltage exceeding 3.6 V and gravimetric energy densities between 120 and 150 Wh/kg, which is two or three times that of typical Ni-Cd batteries. During the charge process, the chemical
reactions occurring at both electrodes can be expressed as follows: (Cathode) LiCoO 2 → Li1− x CoO2 + xLi + + xe −
(1)
(Anode)C6 + xLi+ + xe − → Li x C6 .
(2)
During the discharge process, the reactions proceed in the reverse direction. In the following sections, achievements and problems in elemental strategy will be discussed toward (simultaneous) replacing Co with Fe and Li with Na.
Why Fe in rechargeable batteries? Building batteries is a multifaceted engineering riddle that attempts to garner maximum energy/power density without compromising cost and safety. As a lightweight center of reversible redox reaction in the cathode material, 3d transition metals (Mn, Fe, Co, Ni) offer higher electronegativity using electrons that bind more strongly to the nuclear charge, whereby they are preferable because they can generate higher voltage to gain energy density. Mn, Co, and Ni have been the dominant elements in cathode materials in the form of LixMO2 (M = Co, Ni) and LixMn2O4 for commercial batteries until recently.1,2 However, keeping an eye on natural abundance (see Table I) and material cost, Fe is gaining ground. Being the fourth most abundant element widely distributed in the
Atsuo Yamada, University of Tokyo, Japan; [email protected] DOI: 10.1557/mrs.2014.89
© 2014 Materials Research Society
MRS BULLETIN • VOLUME 39 • MAY 2014 • www.mrs.org/bulletin
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IRON-BASED MATERIALS STRATEGIES
Figure 1. Concept of future rechargeable batteries with more abundant elements toward sustainable grid storage. In a lithium-ion bat
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