A TEM study of morphological and structural degradation phenomena in LiFePO4-CB cathodes

TL;DR

This study uses TEM to investigate how morphological and structural degradation in LiFePO4-CB cathodes affects battery performance during storage and cycling, revealing SEI formation and loss of conductivity.

Contribution

It provides detailed TEM-based insights into degradation phenomena in LiFePO4-CB cathodes, highlighting effects on conductivity and capacity.

Findings

SEI layers form on LFP particles during storage and cycling

Loss of CB crystallinity and aggregation occur over time

Degradation reduces cathode conductivity and capacity

Abstract

LiFePO4-based cathodes suffer from various degradation mechanisms which influences the battery performance. In this paper morphological and structural degradation phenomena in laboratory cathodes made of LiFePO4 (LFP) mixed with carbon black (CB) in an 1 mol L-1 LiPF6 in EC:DMC (1:1 by weight) electrolyte are investigated by transmission electron microscopy (TEM) at various preparation, assembling, storage and cycling stages. High-resolution TEM (HRTEM) imaging shows that continuous SEI layers are formed on the LFP particles and that both storage and cycling affects the formation. Additionally loss of CB crystallinity, CB aggregation and agglomeration is observed. Charge-discharge curves and impedance spectra measured during cycling confirm that these degradation mechanisms reduce the cathode conductivity and capacity.