Vladimir Oleshko1

1, National Institute of Standards and Technology, Gaithersburg, Maryland, United States

With ever increasing interest to prospective nanostructured materials for electrical energy storage (EES), the fact that strong correlations exist between the volume plasmon energy (Ep) and other intrinsic solid state parameters (valence electron density, cohesive energy, elastic moduli, thermal and electrical conductivities, etc.), open opportunities to probe and tailor properties of candidate materials in order to provide the ultimate performance and fully utilize their unique technological advantages. We discuss here relationships between the gravimetric capacity (Cg), elastic moduli, cohesive energy density and Ep of some prospective EES materials composed of Group I-IV elements. Periodic trends and factors influencing physico-mechanical properties, Cg and energy density of EES materials also will be considered. We show that the origin of connections between the material properties and the Ep lies in the nature of electron-ion interactions and in the essentially exponential decay of electron density with interatomic distance. For EES materials with metallic and preferentially covalent bonding, this is established by the universal binding energy relation (UBER), which describes the shape of the binding energy curve in a wide range of situations varying from cohesion in bulk solids to bimetallic adhesion, chemisorption on metal surfaces and bonding. In the UBER, the bulk modulus, Bm, is related to Ecoh and the equilibrium Wigner-Seitz (WS) atomic radius, rwse, via the dimensionless anharmonicity parameter:
η = rwse/l, as Bm = (1/12π)rwse-3Ecohη2 = (4π2e0m/9e2h2Nve) Ecohη2(Ep2-Eg2) (1),
where l is the characteristic length describing the width of the binding energy curve or the range over which strong forces act (this sets the range of the Hooke’s-law region), Nve is the number of valence electrons per atom, and Eg is the band gap. From Eqn. (1), BmEcoh/VwseEp2–Eg2, where Vwse is the volume of the Wigner-Seitz cell at equilibrium. Bm, and Ecoh/Vwse and can be linearized in a log-log scale as a function of Ep2-Eg2 and described by an universal equation of the type Pm = A(Ep2-Eg2)B, where Pm = Cg, Bm, or Ecoh/Vwse, and A and B are lsf structure-dependent parameters. For Bm, and Ecoh/Vwse, scaling can be deduced from Eqn. (1) since the valence electron density governs variations of both Bm and Ecoh/Vwse with Ep and contribution from η only slightly reduces the exponential factor. Universality and scaling in Ep-property relationships indicate that Ep is an invaluable parameter to investigate the properties of engineering EES materials. We will describe how to employ these relations in order to determine and map nanoscale physico-mechanical properties of EES materials in situ using spatially-resolved low-loss EELS and STEM spectroscopic imaging and illustrate some applications with selected examples, including nanophase silicon and Si-Li alloys and conductive carbons utilized in composite electrodes for high-performance rechargeable batteries.