Single Electrode Entropy Change for LiCoO₂Electrodes

Abstract

Heat effects are known to be crucial for the performance of the Libattery. In addition to the irreversible effects (e.g. Joule heat), reversible heat effects may play a role. These effects, i.e. the Soret and Seebeck effects, are however scarcely investigated. We report the initial and stationary state thermoelectric potentials of a pouch cell having electrodes where Li is intercalated in CoO 2 . The electrolyte consists of 1.0 M lithium hexafluoro phosphate dissolved in a mixture of equal volumes of ethylene carbonate and diethyl carbonate. The two identical electrodes were thermostatted at different temperatures, the average being always 298 K and the potential was measured over several days. We observe two time-dependent phenomena with characteristic times of 4.5 and 21.3 hours. We explain them by thermal diffusion of salt and esters. The Seebeck coefficient varies from -2.8 mV/K in the initial state to 1.5 mV/K in the state characterized by partial Soret equilibrium. The time-variation of the signal is used to compute Seebeck coefficients, the contributions to the heats of transfer, and the reversible heat effect at the LiCoO 2 -and carbon electrodes. The electrode heat effects in a Li-battery is surprisingly asymmetric, given the small entropy of the total reaction. The LiCoO 2 electrode heat varies from 51 to -46 kJ/mol, while the carbon electrode will vary from -73 to 56 kJ/mol. Uncertainties in computations are within 15%. This may explain why the reaction entropy depends largely on the state of charge. It may also be important for thermal modeling of the battery.

Review—Reversible Heat Effects in Cells Relevant for Lithium-Ion Batteries

“…Simultaneously, they observed heating during cell discharge. This means that the LCO Peltier heat is positive for the anode reaction, as observed also by Richter et al 24 Maeda measured a cooling effect during lithium de-intercalation in graphite, 64 suggesting that also graphite has a positive Peltier heat for the anode reaction. For this to be possible, Li-metal as an anode must also have a positive Peltier heat (see Table IIb).…”

“…46 The transported entropy of lithium is large and positive. 16,24 It is interesting to see that the electrodes with the largest reversible heat effect in the half-cell, such as graphite around x = 0, may have the smallest Peltier heat. This can be explained by Eq.…”

“…18,21,22 A few reports of Seebeck coefficients can be found for cells with electrodes of materials relevant to the LIB. 16,18,[21][22][23][24] The primary motivation for these experiments were their relevance to thermogalvanic cells, however, and the purpose of that research was to contribute to waste heat energy harvesting. This is an interesting application, but the results obtained are equally useful for LIBs!…”

Probing the thermal effects of voltage hysteresis in anionic redox-based lithium-rich cathodes using isothermal calorimetry

“…Such a relatively high loading was needed to generate reliable amounts of heat flow from the coin cells. Owing to the high porosity of carefully designed electrodes, in addition to the relatively low C-rates studied herein, the liquid-phase transport limitations (and their associated entropy production 56,57 ) are expected to remain very low compared to those in the solid phase, as also suggested via previous modelling 57,58 investigations.…”

Non-isothermal Transport of Multi-phase Fluids in Porous Media. The Entropy Production

“…Transport in porous media take place, not only under pressure gradients. Temperature gradients will frequently follow from transport of mass, for instance in heterogeneous catalysis [10], in polymer electrolyte fuel cells, in batteries [9,11], or in capillaries in frozen soils during frost heave [12]. The number of this type of phenomena is enormous.…”