Peltier Heats of LiFePO4 electrodes from a Thermoelectric Cell

Abstract

Temperature is known to affect ageing and performance of lithium ion batteries [1]. We need to improve the understanding of the heat sources in the electrode compartments in order to accurately model internal temperature profiles. Reports of reversible heat effects local to electrode surfaces in lithium-ion batteries are therefore important, but are scarce in literature [2]. The reversible heat production at a battery electrode interface is investigated in this experimental work, measuring the Peltier heat of the electrode LiFePO4 electrode. The Peltier heat at an electrode can be found by measuring the change in electric potential across the cell with two identical electrodes when a temperature difference is applied [3]. We present the Seebeck coefficient and Peltier heat of a symmetric pouch cell consisting of two commercial LiFePO4 electrodes and a 1M LiPF6 and Ethylene Carbonate (EC) /Diethyl Carbonate (DEC) (50:50 w%) electrolyte-soaked separator. The electrodes have been thermostatted to the wanted temperatures T and T+ ΔT, respectively. Al (s,T) | LiFePO4 (s,T) | EC, DEC, LiPF6 | LiFePO4 (s,T + ΔT) | Al (s,T+ ΔT) We show that the thermoelectric effect is time-dependent due to a build up of concentration gradients within the cell, and the establishment of not only one, but two Soret equilibria. The initial Seebeck coefficient gives the Peltier heat of one electrode for a homogeneous electrolyte in a battery [4]. We derive equations that will describe the approach of the cell to steady state, and use this to estimate the initial value. The consequences of the Peltier heat of LiFePO4 for battery discharging and charging will be discussed. Acknowledgment The authors are grateful to the Research Council of Norway through its Centres of Excellence funding scheme, project number 262644, PoreLab. We acknowledge ENERSENSE for financial support. The group of authors also acknowledge the Research Council of Norway (RCN) project no. 228739, for funding through the project "Life and Safety for Li-ion batteries in Maritime conditions (SafeLiLife)".