Changes in the partial molar entropy of lithium- and manganese-rich layered transition metal oxides (LMR-NCM) are investigated using a recently established electrochemical measuring protocol, in which the open-circuit voltage (OCV) of a cell is recorded during linear variation of the cell temperature. With this method, the entropy changes of LMR-NCM in half-cells were precisely determined, revealing a path dependence of the entropy during charge and discharge as a function of state of charge, which vanished as a function of OCV. This observation is in line with other hysteresis phenomena observed for LMR-NCM, of which the OCV hysteresis is the most striking one. For a systematic investigation of the entropy changes in LMR-NCM, measurements were conducted during the first activation cycle and in a subsequent cycle. In addition, two LMR-NCM materials with different degrees of overlithiation were contrasted. Contributions from configurational and vibrational entropy are discussed. Our results suggest that the entropy profile during activation exhibits features from the configurational entropy, while during subsequent cycling the vibrational entropy dominates the entropy curve.

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For Li-ion battery development, Li- and Mn-rich layered oxide cathodes (LMRNCM, Li1+x[NiCoMn]1-xO2 with typically 0.1 < x < 0.2), are currently under development. Their exceptionally high gravimetric capacity is, however, accompanied by a significant voltage hysteresis, which complicates the SOC management and reduces the round-trip energy efficiency of the cell.1 In particular, a significant part of the voltage hysteresis is still present during open circuit conditions (OCV) being independent of the applied current and thus a material-specific property (see Figure 1a). The hysteresis is not only expressed in the voltage profile but also in other parameters, such as a path dependent cathode resistance2 and a hysteresis of the LMRNCM lattice parameters3. To investigate the OCV hysteresis, potentiometric entropy measurements were conducted to calculate the changes in the partial molar free entropy of the Li (de-)intercalation during charge and discharge, and from this, the reversible heat Qrev. The entropy is accessible via the temperature dependence of the OCV. To minimize errors from temperature history4 and relaxation5, a linear temperature variation method was established based on the work by Liebmann et al. 6 Using this method, entropy changes within the LMRNCM were measured as a function of state of charge (SOC) to reveal a path dependence between charge and discharge (see Figure 1b). This path dependence vanishes when the entropy is correlated to the OCV of the respective SOC indicating that the structural changes within the LMRNCM are rather a function of OCV than SOC, as can be seen in Figure 1c. This is in agreement with a previous study by Strehle et al.3 where a similar behavior was found for the lattice parameters of LMRNCM by diffraction methods. However, the herein conducted measurements did not reveal the entropy as a cause of the path-dependence (or hysteresis) but rather demonstrated it to be another indication of this phenomenon. Ultimately, the reversible heat Qrev will be compared to the energy loss correlating to the OCV hysteresis indicating that the latter heat term is significantly larger and independent from the entropy changes within the LMRNCM. Figure 1: (a) Voltage hysteresis and (b,c) changes in the partial molar free entropy for LMRNCM. A LMRNCM/Li Swagelok T-cell (equipped with a Li reference electrode) was cycled at C/10 at 25°C between 2.0-4.7 V. For the OCV curve, intermediate OCV phases of 1h were applied every ≈10% SOC (a). The entropy was measured in various T-cells via linear temperature variation after relaxation of the OCV at different SOCs during charge and discharge and is shown as a function of SOC (b) and OCV (c) with trendlines as guide for the eye. Acknowledgements: We want to acknowledge BASF SE for the support within the frame of its scientific network on electrochemistry. References: J. R. Croy, K. G. Gallagher, M. Balasubramanian, Z. Chen, Y. Ren, D. Kim, S.-H. Kang, D. W. Dees, and M. Thackeray, J. Phys. Chem. C, 117, 6525 (2013). T. Teufl, D. Pritzl, S. Solchenbach, H. A. Gasteiger, and M. A. Mendez, J. Electrochem. Soc., 166, A1275 (2019). B. Strehle, T. Zünd, A. Kießling, F. Friedrich, V. Baran, and H. A. Gasteiger, Manuscript in preparation. I. Zilberman, A. Rheinfeld, and A. Jossen, J. Power Sources, 395, 179 (2018). P. J. Osswald, J. Garche, A. Jossen, and H. E. Hoster, Electrochim. Acta, 177, 270 (2015). T. Liebmann and C. Heubner, J. Solid State Electrochem., 32, 245 (2019). Figure 1

For Li-ion battery development, Li- and Mn-rich layered oxide cathodes (HE-NCM, Li1+x[M]1-xO2 with M = Ni, Mn, Co and typically 0.1 < x < 0.2) are currently under development. Their exceptionally high gravimetric capacity is, however, accompanied by a significant voltage hysteresis which reduces the round-trip energy efficiency of the cell.1 In particular, a significant part of the voltage hysteresis is still present during measurements at open circuit voltage (OCV hysteresis, see Figure 1a) and hence leads to an energy loss, which is independent of the applied current. To investigate this OCV hysteresis, potentiometric entropy measurements were conducted in half-cells to calculate the overall entropy changes ΔS of Li (de-)intercalation during charge and discharge, and from this, the reversible heat Qrev. ΔS is accessible via the temperature dependence of the OCV. To minimize errors from temperature history2 and relaxation3, a linear temperature variation method was established based on the work by Liebmann et al. 4 Using this method, entropy changes of Li/Mn-rich NCM half-cells were measured as a function of state of charge (SOC), revealing a path dependency of ΔS between charge and discharge, which can only be ascribed to entropy changes of the cathode active material (see Figure 1b). This path dependency vanishes when the entropy is correlated to the OCV of the respective Mn-rich NCM SOC, indicating that the structural changes within the cathode are rather a function of OCV than SOC, as can be seen in Figure 1c. This is in agreement with a previous study by Strehle et al.,5 where a similar behavior was found for the hysteresis in the evolution of the Mn-rich NCM lattice parameters between the charge and the discharge direction. However, the measurements conducted in the present study did not reveal the entropy as a cause of the path-dependency (or hysteresis), but rather demonstrated it to be another indication of this phenomenon. Ultimately, the reversible heat Qrev will be compared to the energy loss correlating to the OCV hysteresis, QOCV, indicating that the latter heat term is significantly larger and independent of the entropy changes within the Mn-rich NCM. Acknowledgements: We want to acknowledge BASF SE for the support within the frame of its scientific network on electrochemistry. References: 1. J. R. Croy, K. G. Gallagher, M. Balasubramanian, Z. Chen, Y. Ren, D. Kim, S.-H. Kang, D. W. Dees, and M. Thackeray, J. Phys. Chem. C, 117, 6525 (2013). 2. I. Zilberman, A. Rheinfeld, and A. Jossen, J. Power Sources, 395, 179 (2018). 3. P. J. Osswald, J. Garche, A. Jossen, and H. E. Hoster, Electrochim. Acta, 177, 270 (2015). 4. T. Liebmann and C. Heubner, J. Solid State Electrochem., 32, 245 (2019). 5. B. Strehle, T. Zünd, A. Kießling, R. Wilhelm, F. Friedrich, V. Baran, and H. A. Gasteiger, Manuscript in preparation. Figure 1: Voltage hysteresis and entropy changes in Li/HENCM half-cells. A Swagelok® T-cell with HENCM against a metallic Li anode (equipped with a Li reference electrode) was cycled at C/10 at 25°C between 2.0-4.7 V. For the OCV curve, intermediate OCV phases of 1h were applied (a). The entropy was measured in various T-cells via linear temperature variation after relaxation of the OCV at different SOCs during charge and discharge, and the data are shown in absolute values as a function of SOC (b) and OCV (c). Figure 1