Extraction of layerwise conductivities in carbon-enhanced, multilayered LiFePO4 cathodes

We performed experimental studies to determine electronic properties of multilayered LiFePO4 cathodes in order to quantify reductions in LiFePO4 matrix resistivity and/or contact resistances between matrices and current collectors by addition of carbon black and graphite. In order to extract these layerwise and interlayer properties, we extended the Schumann-Gardner approach to analysis of a four-point probe experiment and solved the resulting coupled nonlinear equations numerically. We studied five cathodes with varying amounts (3-12 wt %) and types (carbon black, graphite) of conductive additives. LiFePO4 particles within the electrodes were precoated with carbon before mixing with additives and binder. Experimental results showed reductions of similar to 62% in electrical resistivities of LiFePO4 matrix with addition of carbon black from 3 to 10 wt %; addition of graphite additives produced only small reductions. For concentrations above 6 wt % of conductive additives, homogeneous electronic resistivities were observed. Contact resistances at interfaces between LiFePO4 matrix and carbon coating of current collector and between carbon coating and current collector were similar in all cases, indicating consistency in manufacturing. Future work will focus on combining models for capacitive loss with models for conductive properties, along with experimental verifications. (c) 2005 The Electrochemical Society.