Impedance studies on cathodes in Li-ion cells

Li-ion batteries have generated widespread interest as batteries of choice for a number of applications, because of their attractive energy and power densities. However, the performance at sub-ambient temperatures is very limited due to the increase in cell impedance at sub-ambient temperatures. AC impedance measurements seem to indicate that the ohmic resistance-which includes electrolyte resistance, electrodes bulk resistance, current collector and tab resistances of the cells increases only marginally at sub-ambient temperatures. Most of the increase in cell impedance comes from the electrode/electrolyte interfacial impedance. Sandia National Laboratories and others have shown that the increase in impedance comes mostly from the cathode electrolyte interface and not from the anode electrolyte interface. The question then is “where exactly does the increase in cathode impedance come from?” Whether it comes from the interfacial resistance (charge transfer+the solid-electrolyte-interphase resistance) or from diffusion of Li⁺ inside the cathode. To answer the question, we are investigating the impedance characteristics of the LiCoO₂ and other metal oxide cathodes at different temperatures and cell voltage. The data seem to indicate that the interfacial impedance of the cathode/electrolyte interface mostly accounts for the cell impedance