Quantifying the Open-Circuit Voltage (OCV) Hysteresis in Ternary Li-Rich Mn-Ni Layered Oxides

Abstract

To achieve the high energy densities needed for automotive applications while keeping the cathode active material (CAM) cost as low as possible, lithium- and manganese-rich NCMs (LMR-NCMs), e. g., Li1.14(Ni0.26Co0.14Mn0.60)0.86O2, are considered as possible candidates. Furthermore, in the past years there was a tendency for replacing expensive and supply-limited cobalt from cathode active materials, resulting in cobalt-free LMR-NMs with the general composition x Li2MnO3 · (1-x) LiMO2 (M = Ni, Mn; 0.33 ≤ x ≤ 0.5) or alternatively expressed as Li1+δ[NiyMnz]1-δO2(y+z= 1, 0.14 ≤ δ ≤ 0.20).1 Even though these materials are able to reversibly provide capacities of ≈ 250 mAh/g, there are numerous challenges to overcome, like extensive gas evolution, capacity and voltage fading over extended charge/discharge cycling, and the substantial open-circuit voltage (OCV) hysteresis, which hamper the implementation of LMR-NCMs in the automotive sector.2–5 It was shown in previous publications that the OCV hysteresis, an intrinsic property of LMR-NCMs, significantly lowers the charge/discharge energy efficiency and thus increases heat generation.2, 6, 7 Furthermore, it was qualitatively shown that the voltage fading as well as the OCV hysteresis is dependent on x in x Li2MnO3 · (1-x) LiMO2 (M = Ni, Mn, Co).5, 6 In order to investigate the relation between composition and OCV hysteresis for Co-free overlithiated layered oxides, we synthesized CAMs with various compositions with the general formula x Li2MnO3 · (1-x) LiMO2 (M = Ni, Mn; 0.33 ≤ x ≤ 0.5), proving the presence of the layered structure of the materials with Rietveld refinements on the collected X-ray diffraction data. The OCV hysteresis of the synthesized and electrochemically activated LMR-NMs is determined from slow charge/discharge cycles with intermittent OCV holds. Based on these data, the OCV hysteresis of LMR-NMs can be correlated with their specific composition. References D. Andre, S.-J. Kim, P. Lamp, S. F. Lux, F. Maglia, O. Paschos and B. Stiaszny, J. Mater. Chem. A, 3(13), 6709–6732 (2015). L. Kraft, T. Zünd, D. Schreiner, R. Wilhelm, F. J. Günter, G. Reinhart, H. A. Gasteiger and A. Jossen, J. Electrochem. Soc., 168(2), 20537 (2021). J. R. Croy, M. Balasubramanian, K. G. Gallagher and A. K. Burrell, Accounts of chemical research, 48(11), 2813–2821 (2015). T. Teufl, B. Strehle, P. Müller, H. A. Gasteiger and M. A. Mendez, J. Electrochem. Soc., 165(11), A2718-A2731 (2018). J. R. Croy, K. G. Gallagher, M. Balasubramanian, B. R. Long and M. M. Thackeray, J. Electrochem. Soc., 161(3), A318-A325 (2014). F. Friedrich, S. Pieper and H. A. Gasteiger, J. Electrochem. Soc., 168(12), 120502 (2021). B. Strehle, T. Zünd, S. Sicolo, A. Kiessling, V. Baran and H. A. Gasteiger, J. Electrochem. Soc., 169(2), 20554 (2022). Acknowledgement This work is financially supported by the BASF SE Network on Electrochemistry and Battery Research.