Low-Voltage FIB/SEM Tomography for 3D Microstructure Evolution of LiFePO₄/C Electrode

Abstract

This work presents an investigation of the degradation mechanisms that occur in LiFePO 4 /C battery electrodes during charge/discharge cycling. Impedance spectra were measured on a fresh electrode and an electrode aged by cycling. The spectra were modeled with an equivalent circuit which indicates that both the ionic and electronic pathways in the electrode were negatively affected by the cycling. Focused Ion Beam/Scanning Electron Microscopy (FIB/SEM) tomography of both electrodes shows that cycling causes agglomerations of Carbon black (CB). In addition to this, Lowvoltage FIB/SEM revealed non-conductive CB in the aged electrode.

Complementary analyses of aging in a commercial LiFePO4/graphite 26650 cell

“…The combination of low-voltage SEM and FIB/SEM have recently been used for 3D electron percolation analysis of a CB additive in a laboratory LiFePO 4 /CB positive electrode [17], [30].…”

“…Figure 9 shows cross-sectional In-lens images recorded at 1 kV after FIB milling of the Gr electrodes. The low accelerating voltage enables a detection of charging effects on carbonaceous materials [17], [30]. Dark gray particles are graphite grains which are observed to dissipate electron charging (induced by the SEM electrode beam) to the ground.…”

“…The most important degradation process seems to be the formation of a CEI layer [11]- [13], [17] at the electrode/electrolyte interface. The layer is possibly a mixture of carbon and electrolyte decomposition products [11]- [13], [17], [26], [30], [55] and is expected to partially block the electrolyte passage thereby increasing the ionic resistance. This increases the polarization of the aged electrodes during charge/discharge cycling, as shown by GCPL results in previous section and confirmed by the increased ohmic resistance R E in the impedance spectra of A26650.…”

“…The low-voltage analysis was previously used to detect charging effect in the CB network of a laboratory LFP electrode to study the changes in the electron dissipation capability in the CB network [17], [30]. Similar to the cathode sample from F26650 the cathode samples from A26650 reveal no significant charging effects.…”

“…The morphology changes in the electrodes due to cycling are characterized by a palette of complementary analysis techniques such as X-ray diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Impedance spectroscopy and low-kV FIB/SEM tomography. The latter technique was developed to observe the electron percolation in SOFC anode Ni-networks [29] and has later been used to study the electron percolation in a laboratory LiFePO 4 /CB electrode [17], [30]. Here we use it to examine the electron percolation in the M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT commercial graphite electrodes, in order to identify disconnected particles in the aged anode.…”