Ion transport and phase transformation in thin film intercalation electrodes

Abstract

Thin film battery electrodes of the olivine structure LiFePO4 and the spinel phase LiMn2O4 are deposited through ion-beam sputtering. The intercalation kinetics is studied by cyclo-voltammetry using variation of the cycling rate over 4 to 5 orders of magnitude. The well-defined layer geometry allows a detailed quantitative analysis. It is shown that LiFePO4 clearly undergoes phase separation during intercalation, although the material is nano-confined and very high charging rates are applied. We present a modified Randles–Sevcik evaluation adapted to phase-separating systems. Both the charging current and the overpotential depend on the film thickness in a systematic way. The analysis yields evidence that the grain boundaries are important short circuit paths for fast transport. They increase the electrochemical active area with increasing layer thickness. Evidence is obtained that the grain boundaries in LiFePO4 have the character of an ion-conductor of vanishing electronic conductivity.

Numerical simulations of cyclic voltammetry for lithium-ion intercalation in nanosized systems: finiteness of diffusion versus electrode kinetics

“…This model was compared and fitted with multi-particle (bulk) experiments using LiMn 2 O 4 and LiCoO 2 cathodes in organic solvents and is used here, in the present work, to analyze the behavior of nanosystems. Planar and spherical geometries are approximations generally used to mimic lithium manganese oxide (LMO) thin films, porous electrodes and single particles [2,3], which are the most common types of electrode material configuration [4][5][6][7][8][9][10][11]. LMO nanorods, which can be compared with a cylindrical geometry, have also been used [12,13].…”

In-situ lithiation of lithium manganese oxide in a transmission electron microscope

“…Since those effects can't be taken into account for measurement of diffusion coefficients the experimental values span a large range from 10 × 10 −13 -10 × 10 −8 cm 2 /s at room temperature [119][120][121]. Where more detailed studies with knowledge of the microstructure showed PLD produced thin films to be on the lower end (10 × 10 −11 -10 × 10 −12 cm 2 /s decreasing with decreasing Li content) [122] as well as for electrodeposited and sintered thin films with 50 nm grain size (10 × 10 −11 -10 × 10 −9 cm 2 /s [123] and the Li diffusion coefficient significantly dropping for thick electrodes (10 × 10 −11 -10 × 10 −8 cm 2 /s for 304 -112 µm electrode thicknesses) [124].…”