Abstract
In this work we study the structural degradation of a laboratory Li-ion battery LiFePO4/Carbon Black (LFP/CB) cathode by various electron microscopy techniques including low kV Focused Ion Beam (FIB)/Scanning Electron Microscopy (SEM) 3D tomography. Several changes are observed in FIB/SEM images of fresh and degraded cathodes, including cracks in the LFP particles, secondary disconnected particles, and agglomeration of CB. Low voltage (1 kV) SEM images show that the CB agglomerates have a different brightness than the fresh CB, due to charging effects. This suggests that the electronic conductivity of the CB agglomerates is low compared to that of the fresh CB particles. HRTEM analysis shows that fresh CB particles are quasi crystalline, whereas the LFP/CB interface in the degraded electrode shows amorphous carbon surrounding the LFP particles. The presence of the amorphous carbon is known to impede the electronic conductivity and thereby decreasing percolation in the cathode and reducing the electrode capacity.
Highlights
Lithium-ion batteries find widespread use in many electricity storage applications, from portable devices to electric vehicles (EV), because of their high energy density and design flexibility [1e3].R
It is well known that mechanical stress related to expansion/contraction of the LFP particles during charging/discharging cycles leads to the formation of micro-cracks inside the LFP particles [6e8], so electronic conductivity and homogeneous dispersion of carbon black (CB) play an important role for long term performance and durability [9,10]
In the images recorded at 1 kV with the E-T detector (Fig. 2 a,c) it is possible to distinguish three different phases: the grains with the brightest contrast correspond to LFP particles, the almost black regions correspond to CB and the large gray areas in-between correspond to pores filled with silicon resin
Summary
Lithium-ion batteries find widespread use in many electricity storage applications, from portable devices to electric vehicles (EV), because of their high energy density and design flexibility [1e3].R. Limited lifetime is still a challenge for several Lithium-ion battery materials. LiFePO4 (LFP) is an interesting material for lithium-ion battery porous cathodes because of its long durability and inherent safety [4,5]. It is well known that mechanical stress related to expansion/contraction of the LFP particles during charging/discharging cycles leads to the formation of micro-cracks inside the LFP particles [6e8], so electronic conductivity and homogeneous dispersion of CB play an important role for long term performance and durability [9,10]. The formation of cracks in the LFP grains leads to disconnected secondary particles, resulting in an increased ionic resistivity and a capacity drop of the electrode. Agglomeration of the CB particles decreases electronic percolation, i.e. increasing the electric resistivity in the CB network from the current collector to the LFP particles [10]
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