Correlating Gas Evolution and Oxygen Release to the Electrochemical Full-Cell Performance of Lithium-Rich Layered Oxides
Authors: Teufl Tobias , Strehle Benjamin , Pritzl Daniel , Gasteiger Hubert A. , Mendez Manuel 
Published: February 2020

Lithium- and manganese-rich layered oxides, also referred to as high energy NCM (HE-NCM), are a promising class of materials for applications in future Li-ion batteries. Due to their high specific discharge capacity (~250 mAh/g) and the high operating voltage, the energy density of next-generation batteries could be increased using these materials [1]. However, these materials exhibit several problems that hinder their commercialization such as low first cycle coulombic efficiency, poor rate capabilities, as well as a significant voltage-hysteresis between charge and discharge accompanied by a rapid voltage decay [2]. So far it is well known that the first cycle activation is a crucial step and strongly influences the further cycling performance of the material [3].
It has been shown by on-Line electrochemical mass spectrometry (OEMS) that during initial cycling oxygen is released from the cathode active material, leading to a chemical surface degradation, forming a spinel-type surface layer [4, 5]. A similar mechanism has been recently reported for Ni-rich NCM materials, where it has been proposed that the release of singlet oxygen might lead to rapid reactions with the electrolyte, leading to rapid capacity fading [6].
In this study, we want to examine possible effects causing the fast capacity decay of Graphite//HE-NCM full-cells and propose strategies how to overcome these issues. Therefore, we will first examine the gas evolution by OEMS in order to obtain information about electrolyte and LiPF6 decomposition. In a further step, we will then correlate the gas-evolution to the electrochemical performance using coin and pouch type full-cells. From these experiments, we aim to get a profound understanding about the influence of different effects onto cell-fading. Figure 1 shows the gas evolution during the first two cycles for HE-NCM with the composition x Li2MnO3 • 1-x LiMO2 (M = Ni, Co, Mn). It can be seen that the oxygen release also induces CO2 evolution, initiating at the same potential.
In this study the role of critical parameters as temperature, oxygen release, as well as material and electrolyte composition and the effects on the electrochemical performance will be discussed. Therefore, we will show OEMS experiments and correlate the gas evolution to distinct degradation mechanisms leading to capacity fading of these materials.

Literature:

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Acknowledgement:

The authors want to thank BASF SE for their financial support of this work.

Figure 1: Gas evolution of HE-NCM during the first two cycles, the upper panel shows the potential curves, the middle panel the oxygen release and the lower panel the CO2 evolution. Data were measured between 2.0 and 4.8 V using a Li-CE, the first charge was carried out at C/10, afterwards the cell was cycled with C/5.

Figure 1