Monitoring the Species Formed at High Cathode Potentials at Ni-Rich Cathodes in Lithium-Ion Batteries Using a Novel Dual Working Electrode
Authors: Reinschluessel Leonhard Johannes , Reuter Lennart , Gasteiger Hubert Andreas 
Published: December 2024

The pursuit to optimize cathode active materials (CAMs) stands as a paramount endeavor within academia and industry to push for higher energy densities in lithium-ion batteries (LiBs). Predominately, aims have focused on the increase of the Nickel content of CAMs to achieve Ni-rich (> 80%) layered transition metal oxides such as NCAs (LiNixCoyAlzO2, where x+y+z=1), owing to their higher achievable capacity at given cut-off voltages. With the increase in Ni-content, however, the structural stability of these CAMs is significantly decreased at high upper cut-off potentials, resulting in capacity fading.[1] The reduced structural stability is accompanied by the release of reactive oxygen species, occurring at ~80% delithiation.[2,3] The released oxygen can trigger a cascade of follow-up reactions, such as the chemical oxidation of electrolyte solvents, conduction salt decomposition, and transition metal (TM) dissolution.[4,5] Charging to even higher voltages (> 4.7 V vs. Li+/Li at 25°C) induces the so called electrochemical oxidation of the electrolyte, which as a result of its reactive reaction products, can induce further CAM degradation.[6-8]

These reactions significantly impede the lifetime of batteries, and hence require in depth understanding of their underlying mechanisms to establish improved cell chemistries which enable high voltage operation of cells. On-line electrochemical mass spectrometry (OEMS) for the detection of gas evolution, or synchrotron operando X-ray absorption spectroscopy (XAS) for the detection of TM-dissolution delivered valuable insights in the mechanistic understanding.[9] However, as these methods are very time and cost-intensive, a simplified cell design based on a dual working electrode setup was developed by our group, which enables the investigation of parasitic reactions without the use of complex auxiliary devices. In its first proof-of-concept application, it enabled the quantification of oxygen release via the measurement of reductive currents at an sense carbon electrode.[9]

In the present study, we apply a three-electrode setup for the qualitative analysis of parasitic species formed when Ni-rich cathodes are charged to high cell voltages in LiBs. As in our previous study, the working electrode (WE) is based on LiNi0.80Co0.15Al0.05O2 (NCA; BASF TODA Battery Materials LLC, Japan) and the sense electrode (SE) is a carbon black based electrode (Vulcan-type carbon, XC-72, Tanaka, Japan), both coated on a stainless-steel mesh. As counter electrode (CE), a lithium iron phosphate (LFP) based electrode, delithiated to a degree of 90% and capacitively oversized was used, providing a constant CE potential (at ~3.43 V vs. Li+/Li). This setup allows us to cycle the NCA WE within a desired potential window, while holding the carbon SE at a constant voltage against the LFP CE. Hence, the SE can be used to reduce the species formed at the NCA WE, with the reduction current being a measure of their quantity. The experiments were conducted in a LP47 electrolyte (1 M LiPF6 in EC:DEC (3:7)).
We apply the developed cell setup to study the formation of O2, H2O, H+, HF, and dissolved transition metal species (TMz+) while the NCA WE is charged to 5.0 V vs. Li+/Li via an intermittent CV charging protocol at temperatures of 10, 25, and 45°C by holding the sense electrode at potentials where the parasitic reactions of interest can be probed. The hereby derived reductive currents are compared to temperature dependent gassing traces determined via OEMS, as well as the TM dissolution behavior measured via operando XAS.

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Acknowledgements:

The authors gratefully acknowledge funding from the German Federal Ministry of Education and Research (BMBF) through the project Aqua operaXX (grant number 03XP0328B) and German Federal Ministry of Economic Affairs and Climate Action through the project CAESAR (grant number 03EI3046F).