Faculty of Chemical Technology, Poznan University of Technology, PL-60 965 Poznan, Poland.
Phys Chem Chem Phys. 2019 Jan 23;21(4):2115-2120. 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 (Δφi) 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 (LiMn2O4, LiFePO4 and LiCoO2) potential temperature coefficients dφi/dT, assuming that Soret and Thomson effects are negligible. The single cathode potential temperature coefficients, estimated in such a way, were positive (dφ(LiMn2O4)/dT = 0.86 mV K-1, dφ(LiFePO4)/dT = 0.86 mV K-1 and dφ(LiCoO2)/dT = 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 (dE/dT) were determined. In this case, all temperature coefficients of the cell voltage were negative (dE(Li|LiMn2O4)/dT = -0.20 mV K-1, dE(Li|LiFePO4)/dT = -0.08 mV K-1 and dE(Li|LiCoO2)/dT = -0.25 mV K-1). The temperature coefficient of the single metallic-lithium electrode, dφLi/dT, was calculated from the temperature coefficients dE/dT of isothermal cells consisting of the cathodes and a lithium counter-electrode and the dφi/dT values measured in non-isothermal cells: dE/dT = dφi/dT - dφLi/dT. The dφLi/dT value was 1.03 mV K-1. The measured difference in the dφ/dT values for metallic lithium and graphite (LiC6) anodes was small (dE/dT = dφ(C6Li)/dT - dφLi/dT = -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 dφLi/dT = 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 change, ΔSi, for reversible single electrode reactions were all positive (for different states of charge) and ranged between ca. 70 J mol-1 K-1 and 120 J mol-1 K-1.
通常情况下，锂离子电池电极（在恒温下）的电位是相对于金属锂来表示的，假设金属锂的电位为零。在电位温度系数的情况下，由于热力学第三定律，不能应用类似的假设。在这里，使用“忽略热扩散电位假设”来估计单个电极的电位温度系数。在非等温对称电池中测量了三种锂离子电池正极的开路电压（Δφi）随温度 T 的变化关系（两个电极的组成相同，但温度不同）。假设索雷特和汤姆逊效应可以忽略，根据所测量的值，解释为单个正极（LiMn2O4、LiFePO4 和 LiCoO2）的电位温度系数 dφi/dT。以这种方式估计的单个正极的电位温度系数为正（dφ(LiMn2O4)/dT = 0.86 mV K-1、dφ(LiFePO4)/dT = 0.86 mV K-1 和 dφ(LiCoO2)/dT = 0.83 mV K-1）。除了在非等温电池中的测量外，还确定了由这些正极和金属锂参比电极组成的等温电池的开路电压温度系数（dE/dT）。在这种情况下，电池电压的所有温度系数均为负值（dE(Li|LiMn2O4)/dT = -0.20 mV K-1、dE(Li|LiFePO4)/dT = -0.08 mV K-1 和 dE(Li|LiCoO2)/dT = -0.25 mV K-1）。从由正极和锂对电极组成的等温电池的开路电压温度系数 dE/dT 和在非等温电池中测量的 dφi/dT 值计算单个金属锂电极的电位温度系数 dφLi/dT：dE/dT = dφi/dT - dφLi/dT。dφLi/dT 值为 1.03 mV K-1。金属锂和石墨（LiC6）阳极的 dφ/dT 值之间的差异较小（dE/dT = dφ(C6Li)/dT - dφLi/dT = -0.08 mV K-1）。使用包含不同荷电状态（SOC）和金属锂对电极的不同电极的等温电池开路电压的文献数据，考虑到 dφLi/dT = 1.03 mV K-1，计算单个电极的性质。所有单个电极的温度系数在不同的 SOC 值下均为正值，范围在 1.69 mV K-1 至 0.84 mV K-1 之间。可逆单电极反应的熵变ΔSi值均为正值（对于不同的电荷状态），范围在约 70 J mol-1 K-1 至 120 J mol-1 K-1 之间。
