No abstract

A high battery temperature has been shown to be critical for lithium-ion batteries in terms of performance, degradation, and safety. Therefore, a precise knowledge of heat sources and sinks in the battery is essential. We have developed a thermal model for lithium-ion batteries, a model that includes terms not included before, namely, Peltier and Dufour heat effects. The model is derived using non-equilibrium thermodynamics for heterogeneous systems, the only theory which is able to describe in a systematic manner the coupling of heat, mass, and charge transport. The idea of this theory is to deal with surfaces as two-dimensional layers. All electrochemical processes in these layers are defined using excess variables, implying, for instance, that the surface has its own temperature. We show how the Peltier and Dufour heats affect a single cell and may produce an internal temperature rise of 8.5 K in a battery stack with 80 modules. The heat fluxes leaving the cell are also functions of these reversible heat effects. Most of the energy that is dissipated as heat occurs in the electrode surfaces and the electrolyte-filled separator. The analysis shows that better knowledge of experimental data on surface resistances, transport coefficients, and Dufour and Peltier heats is essential for further progress in thermal modeling of this important class of systems.

We review measurements of reversible heat effects in lithium-ion batteries, i.e. entropy changes and Seebeck coefficients of cells with relevant electrodes. We show how to compute the Peltier heat of battery electrodes from Seebeck coefficients. The Seebeck coefficient depends on the heat of transfer (Soret effect), which is found from the difference of initial and stationary state values of the Seebeck coefficient. We apply non-equilibrium thermodynamics theory and obtain initial Peltier heats not reported before. For the oxidation of lithium metal we propose the value 34 ± 2 kJ mol −1 when the electrolyte contains 1 M LiPF 6 , while the value is 29 ± 1 kJ mol −1 when the electrolyte contains 1 M LiClO 4 . The positive values imply that the electrode cools when it serves as an anode. For oxidation of lithium under stationary state conditions, the stationary state Peltier heat is ≈120 kJ mol −1 . A large reversible heating effect can then be expected for the single electrode; much larger than expected from the full-cell entropy change. These values have a bearing on thermal modelling of batteries. Peltier heats for anodic reactions are presented in tables available for such modelling. We discuss the need for measurements and point at opportunities.

Transport properties are essential for the understanding and modeling of electrochemical cells, in particular complex systems like lithium-ion batteries. In this study, we demonstrate how a certain degree of freedom in the choice of variables allows us to efficiently determine a complete set of transport properties. We apply the entropy production invariance condition to different sets of electrolyte variables and obtain a general set of formulas. We demonstrate the application of these formulas to an electrolyte typical for lithium-ion batteries, 1M lithium hexafluoro-phosphate in a 1:1 wt. % mixture of ethylene and diethyl carbonates. While simplifications can be introduced, they provide inadequate predictions of conductivity and transport numbers, and we argue that a full matrix of Onsager coefficients is needed for adequate property predictions. Our findings highlight the importance of a complete set of transport coefficients for accurate modeling of complex electrochemical systems and the need for careful consideration of the choice of variables used to determine these properties.

Thermogalvanic cells convert waste heat directly to electric work. There is an abundance of waste heat in the world and the opportunity represented by thermogalvanic cells may be underused. We discuss theoretical tools that can help us understand and therefore improve the cell performance. One theory is able to describe all aspects of the conversion, nonequilibrium thermodynamics. We recommend to use the theory with operationally defined, independent variables, as others have done before. These describe well-defined experiments. Three invariance criteria can be used as a basis: of local electroneutrality, of entropy production invariance, and of emf's independence of the frame of reference. Alternative formalisms are using different sets of variables, ionic or neutral components. We show that the heat flux is not the same in the two formalisms and derive a new relationship between the heat fluxes. The heat flux enters the definition of the Peltier coefficient and is essential for the understanding of the Peltier heat at the electrode interfaces and of the Seebeck coefficient of the cell. The Soret effect can occur independent of any Seebeck-effect, but the Seebeck effect will be affected by the presence of a Soret effect. Common misunderstandings are pointed out. Peltier coefficients are needed for interpretation and design of measurements.