Reversible heat effects of lithium metal- and porous lithium iron phosphate electrodes

Abstract

We report Peltier heats and reaction entropy of cells with lithium metal- and near fully lithiated lithium iron phosphate (LFP) electrodes. The Peltier heats were measured through the Seebeck coefficient of thermogalvanic cells. The value obtained for the Peltier heat of Li-metal, -32 ± 3 kJ/mol at 298 K, supports previously reported values. When close to being fully lithiated, LFP goes through a phase transition from a two-phase mixture to a solid solution. The value of Peltier heats obtained for LFP vary from -15 ± 1 to -72 ± 9 kJ/mol. The variation is explained by large entropy changes of lithium iron phosphate, near the phase transition. The cell entropy difference of lithium iron phosphate against lithium metal varied from -64 ± 3 to +50 ± 20 J/K mol. The negative Peltier heats means that the electrodes generates heat when acting as an anode, which leads to a temperature rise in the electrode compartment, and absorbs heat when acting as a cathode. The local reversible heat effect is equal to or larger in magnitude than the net reversible heat effect. The time-dependence of the Seebeck coefficient, the Soret effect, was found to differ between the cells with planar Li-metal electrodes to porous LFP.