Temperature coefficients of Li-ion battery single electrode potentials and related entropy changes - revisited

• Published in: Physical chemistry chemical physics : PCCP 2019-01, Vol.21 (4), p.2115-212
• Main Authors: Swiderska-Mocek, A, Rudnicka, E, Lewandowski, A
• Format: Article
• Language: English
• Subjects: Cathodes; Coefficients; Electrode potentials; Electrodes; Entropy; Lithium compounds; Lithium manganese oxides; Lithium-ion batteries; Mathematical analysis; Open circuit voltage; Temperature; Temperature dependence; Thermal diffusion
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c8cp06638h

Usually the potentials of Li-ion battery electrodes (at constant temperature) are expressed against metallic lithium, assuming that it equals zero. In the case of potential temperature coefficients, and hence entropies, no similar assumption can be applied, as it is against the third principle of thermodynamics. Here, single electrode potential temperature coefficients were estimated using a 'negligible thermal diffusion potential assumption'. The open circuit voltage (Δ i ) dependence on temperature T , for three Li-ion battery cathodes, was measured in non-isothermal symmetrical cells (both electrodes had the same composition but were kept in different temperatures). The measured values were interpreted as single cathode (LiMn 2 O 4 , LiFePO 4 and LiCoO 2 ) potential temperature coefficients d i /d T , assuming that Soret and Thomson effects are negligible. The single cathode potential temperature coefficients, estimated in such a way, were positive (d (LiMn 2 O 4 )/d T = 0.86 mV K −1 , d (LiFePO 4 )/d T = 0.86 mV K −1 and d (LiCoO 2 )/d T = 0.83 mV K −1 ). In addition to the measurements in non-isothermal cells, the temperature coefficients of the open circuit voltage of isothermal cells consisting of these cathodes and a metallic lithium reference (d E /d T ) were determined. In this case, all temperature coefficients of the cell voltage were negative (d E (Li|LiMn 2 O 4 )/d T = −0.20 mV K −1 , d E (Li|LiFePO 4 )/d T = −0.08 mV K −1 and d E (Li|LiCoO 2 )/d T = −0.25 mV K −1 ). The temperature coefficient of the single metallic-lithium electrode, d Li /d T , was calculated from the temperature coefficients d E /d T of isothermal cells consisting of the cathodes and a lithium counter-electrode and the d i /d T values measured in non-isothermal cells: d E /d T = d i /d T − d Li /d T . The d Li /d T value was 1.03 mV K −1 . The measured difference in the d /d T values for metallic lithium and graphite (LiC 6 ) anodes was small (d E /d T = d (C 6 Li)/d T − d Li /d T = −0.08 mV K −1 ). Literature data on the temperature coefficients of the isothermal cell open circuit voltage containing different electrodes at different states of charge (SOC) and metallic-lithium counter electrodes were used for the calculation of single electrode properties, taking into account that d Li /d T = 1.03 mV K −1 . The temperature coefficients of all single electrodes were positive for different SOC values and ranged between 1.69 mV K −1 and 0.84 mV K −1 . The values of entropy c