Effect of vinylene carbonate on SEI formation on LiMn2O4 in carbonate-based electrolytes.

Spinel LiMn2O4 (LMO) is a well-known cathode material for lithium-ion batteries. In order to elucidate the molecular mechanism of the solid electrolyte interface (SEI) formation and the effect of an additive, vinylene carbonate (VC), we systematically studied the spontaneous and electrochemical reactions of solvents and a salt (LiPF6) in electrolytes with LMO in the absence and presence of VC. X-ray photoelectron spectroscopy (XPS) results of the LMO surfaces after soaking in the electrolyte solutions showed that the carbonate solvents as well as VC spontaneously decomposed on the LMO surfaces to form new compounds, such as alcohols, ethers, and carboxylates. The ratio of the produced LiF to MnF2 was similar for both with and without VC. Considering these spontaneously formed initial SEI components, we then investigated the variation of the SEI compositions during the initial electrochemical process until 3.8 V vs. Li+/Li. The role of the additive was studied and found that the electrochemical reaction of VC produced more organic compounds and led to an increase in the LiF/MnF2 ratio of the SEI layer. Based on the hard and soft acid and base theory, we proposed the mechanisms of the SEI formation via spontaneous and electrochemical reactions on the LMO thin film cathode with and without VC.

Crossover from localized to itinerant states in hydrocarbon Mott insulators.

Crossover from an itinerant state to an isolated electronic state in electron-doped polycyclic aromatic hydrocarbon (PAH) was studied for the two smallest zigzag-type molecules of naphthalene (NN) and anthracene (AN) by focusing on their 1 : 1 stoichiometry, A1(NN) and A1(AN), with alkali metals (A = K and Rb). The competition between on-site Coulombic repulsion energy (U) and bandwidth (W) was argued in terms of their magnetic and electrical properties upon lattice expansion, when A varies from K, with a smaller ionic radius, to Rb, with a larger ionic radius. The temperature-dependence of magnetic susceptibility shows a pronounced hump associated with antiferromagnetic (AFM) interactions for Rb1(NN), being similar to those of K1(NN) and K1(AN) in the earlier report. On the other hand, Rb1(AN) intriguingly exhibits paramagnetic susceptibility, observed in a nearly localized electron system, being apart from an highly correlated Mott insulating state. Crystal structural analyses of the X-ray diffraction profiles show a small difference in lattice parameters of the ab plane among K1(NN), K1(AN), and Rb1(NN), whereas Rb1(AN) exhibits a significantly larger value than those of the others, being indicative of greatly modified interaction energies. The different magnetic properties observed in Rb1(AN) are interpreted from its modified intermolecular distance. The possibility of an emergent metallic state of K1(AN) under high pressure has also been described, referring to electrical transport measured under high pressure.

Tuning of the ground state in electron doped anthracene.

High quality bulk samples of anthracene (AN) doped with potassium (K) in 1 : 1 and 2 : 1 stoichiometries were successfully prepared by a method involving a room temperature solid-state mechanical diffusion process prior to intercalation reactions during heat treatment, and their physical properties were studied using both magnetic and optical measurements. The transfer of almost one electron from K to AN in K1(AN) was confirmed by SQUID and ESR measurements. A pronounced magnetic hump centered at 150 K associated with antiferromagnetic interactions was observed, which can most likely be interpreted in terms of on-site Coulomb repulsions of the Mott insulating states. Optical spectra of K1(AN) clearly showed the insulating states, as well as the electron occupation of the LUMO-derived band of AN. Our results demonstrated tuning of the ground state of a typical bulk hydrocarbon by alkali metal intercalation.