Impact of Oxygen Depleted Surface Layers Created through Aqueous Buffer-Washing on the Electrochemistry of Overlithiated Materials
Authors: Kipfer Tim , Hartmann Louis , Gasteiger Hubert Andreas 
Published: November 2022

To achieve sufficiently high energy densities needed for automotive applications while simultaneously keeping the raw material cost as low as possible, lithium- and manganese-rich NCMs (LMR-NCM), like Li1.14(Ni0.26Co0.14Mn0.60)0.86O2, are promising candidates.1 However, such overlithiated materials still suffer from significant capacity and voltage fading over their cycle life, which is caused amongst other factors by structural changes and the release of large amounts of highly reactive lattice oxygen in the initial cycles.2–4 The released oxygen triggers a cascade of detrimental side reactions which lead to the formation of protic species, the dissolution of the active material, and the loss of cyclable lithium.2, 5–7 Therefore, the release of lattice oxygen from the cathode active material (CAM) must be hindered in order to successfully implement the class of LMR-NCMs in lithium-ion batteries.
As the released lattice oxygen stems from the near-surface region of the CAM,3 a surface-treatment with the goal of forming an O-depleted, passivating surface layer might be a reasonable approach to improve the cycling stability of LMR-NCMs. To achieve this, an aqueous buffer-washing, as proposed by Hartmann et al., seems to be a promising approach that results in an O-depleted surface layer via a post-treatment step, which significantly reduces the amount of oxygen release during the cell formation cycles and, in turn, drastically increases the capacity retention.7, 8

To prepare an O-depleted surface layer, we exchange lithium in the near-surface region with protons by washing the material under mild conditions in an aqueous buffer (pH ~ 7-8), by which a defined amount of protons can be exchanged into the material while avoiding transition metal dissolution at high pH.8, 9 By subsequently annealing the materials at different temperatures between 120 °C and 450 °C, we are able to create O-depleted surface layers by the release of water from the CAM. As seen in our earlier publication, the thickness of the O-depleted layer is highly tunable by adjusting the buffer concentration.8

To further investigate how this post-treatment procedure impacts the electrochemical properties of LMR-NCM, we prepared buffer-washed materials with nominally either 2 nm, 4 nm, or 9 nm thick O-depleted surface layers, the formation of which can be followed by TGA-MS. The impedance of electrodes prepared with these materials will be determined as a function of state-of-charge using a µ-reference electrode, showing that the LMR-NCM impedance is strongly affected by the thickness of the O-depleted surface layer and by the temperature at which it was formed. For thick surface layers, XRD analysis provides insight into the structure of the formed surface layers as a function of annealing temperature.
Furthermore, we will show not only a better cycling performance of the buffer-treated material, but also better rate-performance of cells built with LMR-NCM with a stabilized O-depleted surface layer.

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Acknowledgement

This work is financially supported by the BASF SE Network on Electrochemistry and Battery Research.