Abstract
Aluminum is a common dopant across oxide cathodes for improving the bulk and cathode-electrolyte interface (CEI) stability. Aluminum in the bulk is known to enhance structural and thermal stability, yet the exact influence of aluminum at the CEI remains unclear. To address this, we utilized a combination of X-ray photoelectron and absorption spectroscopy to identify aluminum surface environments and extent of transition metal reduction for Ni-rich LiNi0.8Co0.2−yAlyO2 (0%, 5%, or 20% Al) layered oxide cathodes tested at 4.75 V under thermal stress (60 °C). For these tests, we compared the conventional LiPF6 salt with the more thermally stable LiBF4 salt. The CEI layers are inherently different between these two electrolyte salts, particularly for the highest level of Al-doping (20%) where a thicker (thinner) CEI layer is found for LiPF6 (LiBF4). Focusing on the aluminum environment, we reveal the type of surface aluminum species are dependent on the electrolyte salt, as Al-O-F- and Al-F-like species form when using LiPF6 and LiBF4, respectively. In both cases, we find cathode-electrolyte reactions drive the formation of a protective Al-F-like barrier at the CEI in Al-doped oxide cathodes.
Highlights
Aluminum is a common dopant across oxide cathodes for improving the bulk and cathode-electrolyte interface (CEI) stability
The influence of Al-doping on the CEI layer was evaluated for Ni-rich layered oxides (LiM1−yAlyO2 where M is transition metal (TM) = Ni/Co) by comparing electrodes under two testing conditions: (1) charged to 4.75 V at room temperature (RT) with no constant voltage (CV) hold and (2) charged to 4.75 V at 60 °C with a 10 hr CV hold versus Li-metal
From a comprehensive study of Ni-rich LiNi0.8Co0.2−yAlyO2 electrodes held under thermal stress at 4.75 V, we have identified a clear influence of Al-doping on the formation of the CEI layer at high voltages/temperatures
Summary
Aluminum is a common dopant across oxide cathodes for improving the bulk and cathode-electrolyte interface (CEI) stability. We utilized a combination of X-ray photoelectron and absorption spectroscopy to identify aluminum surface environments and extent of transition metal reduction for Ni-rich LiNi0.8Co0.2−yAlyO2 (0%, 5%, or 20% Al) layered oxide cathodes tested at 4.75 V under thermal stress (60 °C) For these tests, we compared the conventional LiPF6 salt with the more thermally stable LiBF4 salt. A primary goal of these modifications is to overcome or limit problems that arise from the instability of the LiPF6 salt, which is known to decompose and form HF and POF320,21 at high voltages or under thermal stress These species are highly reactive, readily attack the layered oxide material, and play a key role in transition metal (TM) reduction and dissolution at the CEI9,22. Extension of this investigation to Al-doped and Al-free layered oxides can provide insight into the influence of Al-doping on the CEI layer formation and stability between these two electrolyte salts
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