Thermal properties of lithium-ion battery and components

Experimental thermal property data of the Sony US-18650 lithium-ion battery and components are presented, as well as thermal property measuring techniques. The properties in question are specific heat capacity (C-p), thermal diffusivity (alpha), and thermal conductivity (k), in the presence and absence of electrolyte [1 M LiPF6 in ethylene carbonate-dimethyl carbonate (EC:DMC, 1:1 wt %)]: The hear capacity of the battery, C-p, is 0.96 +/- 0.02 J g(-1) K-1 at an open-circuit voltage (OCV) of 2.75 V, and 1.04 +/- 0.02 J g(-1) K-1 at 3.75 V. The thermal conductivity, k, was calculated from k = alpha rho C-p where alpha was measured by a xenon-flash technique. In the absence of electrolyte, k increases with OCV, for both the negative electrode (NE) and the positive electrode (PE). For the NE, the increase is 26% as the OCV increases from 2.75 to 3.75 V, whereas for the PE the increase is only 5 to 6%. The dependence of both C-p and k on OCV is explained qualitatively by considering the effect of lithiation and delithiation on the electron carrier density, which leads to n-type semiconduction in the graphitic NE material, but a change from semiconducting to metallic character in LixCoO2 PE material. The overall effect is an increase of C-p and k with OCV. For k this dependence is eliminated by electrolyte addition, which, however, greatly increases the effective k of the layered battery components by lowering the thermal contact resistance. For both NE and PE, the in-plane k value (measured along layers) is nearly one order of magnitude higher than the cross-plane k. This is ascribed mostly to the high thermal conductivity of the current collectors and to a lesser extent to the orientation of particles in the layers of electrodes. (C) 1999 The Electrochemical Society. S0013-4651(98)06-095-9. All rights reserved.