Molecular Dynamics Simulations of Li Insertion in a Nanocrystalline V 2 O 5 Thin Film Cathode
The behavior of lithium ion diffusion from an electrolyte into a polycrystalline layered cathode has been studied using molecular dynamics computer simulations. Lithium silicate glass was the model solid electrolyte whilethe cathode was a nanocrystalline vanadia with amorphous V 2 O 5 intergranular films (IGF) between the crystals. Nanosized V 2 O 5 crystals were aligned with their (001) planes parallel to electrolyte/cathode interface, rotated 90° from each other around this interface's normal in order to present two different orientations between the crystal planes for lithium intercalation via the amorphous vanadia IGF. A series of nanocrystalline vanadia cathodes with different IGF thicknesses was simulated to examine the effects of the IGF thickness on lithium transport into the cathodes. Results showed preferential diffusion of Li from the electrolyte into the amorphous vanadia IGF, with some of those Li diffusing into the crystalline V 2 O 5 from the IGF. Results also showed easier lithium diffusion from the IGF into the V 2 O 5 crystal along the (010) direction than along the (100) direction. Additionally, an optimum IGF thickness of 2.5-3.0 nm is suggested as being neither too thick to decrease the capacity of the cathode nor too thin to impede the transport of lithium from glassy electrolyte into the cathode.
Crystalline vanadium pentoxide with hierarchical mesopores and its capacitive behavior.
Crystalline vanadium pentoxide with hierarchical mesopores was synthesized by using a CTAB/BMIC cotemplate (CTAB = cetyltrimethylammonium bromide, BMIC = 1-butyl-3-methylimidazolium chloride). The material was fully characterized by SEM, TEM, N2 adsorption-desorption, XRD, XPS, and CV methods. By elaborate adjustment of the template proportions, the distribution and size of the hierarchical pores were tuned successfully. CTAB cationic surfactant contributed more to the larger mesopores, whereas BMIC ionic liquid was beneficial in forming the smaller nanopores. The vanadium-containing anions combined with CTA+ micelles and BMI+ rings through electrostatic interactions. The CTA(+)-O(VO)O(-)-BMI(+) entities built up an orderly array, which finally formed the hierarchical mesoporous framework during thermal treatment. The mesoporous vanadium pentoxide directed by the cotemplate of CTAB/BMIC = 1:1 showed many orderly crystalline structures and demonstrated a large capacitance (225 F g(-1)); it is thus a promising material for electrochemical capacitors. Two alternative solutions to the disappearance of capacitance due to insertion of K+ are proposed in view of possible future applications.
Frumkin intercalation isotherm — a tool for the description of lithium insertion into host materials: a review
Abstract This paper offers an integrated approach toward the elucidation of electroanalytical behavior of various Li-insertion anodes and cathodes. Specifically, the paper describes the application of a Frumkin adsorption/intercalation type isotherm as a tool for a quantitative description of electrochemical Li intercalation/deintercalation into various Li-insertion anodes and cathodes. Common and distinguished properties of such an intercalation process in a host material and adsorption of the same species on a metallic interface are compared and discussed, taking into account a pronounced difference in the corresponding potential profiles across the cells and possible kinetical rate-determining steps. The application of an equation combining the Frumkin isotherm with the conventional Butler–Volmer kinetics is exemplified by electrochemical Li insertion/deinsertion to/from a thin graphite and Li x CoO 2 electrodes. Four major electroanalytical techniques, SSCV, PITT, GITT and EIS, are frequently used for the determination of chemical diffusion characteristics of Li-ions. For these techniques, we have defined their characteristic time-invariant functions ( Iν −1/2 , It 1/2 , d E /d t 1/2 and A w , respectively) to present the diffusion time constant τ as a combination of one of these functions with the differential intercalation capacitance, C int . Such a presentation allows the above characteristic functions to be connected, and demonstrates the equivalence in application of the three differential techniques, PITT, GITT and EIS, for obtaining kinetic data. This conclusion was experimentally verified for a thin graphite anode and a Li x Ni 0.8 Co 0.2 O 2 cathode. This verification may have an important implication for the judicious utilization of GITT in distinguishing between possible rate-determining steps of a first-order phase transition. A common feature on the experimental plots of the chemical diffusion coefficient of Li + , D vs. the potential observed for a variety of Li-insertion compounds, appears in the form of a deep minima corresponding to the peak values of the differential intercalation capacitance. Taking into account a limitation due to Li transfer across the particle/solution interface and the intercalation process described by a Frumkin intercalation isotherm, log  D vs. E curves could be simulated for Li insertion electrodes. We can explain the deviation of the experimental current vs. time plots during a PITT of Li insertion electrodes from the theoretical plots in the long-time domain, in terms of the distribution in their particle size.