Method development for the investigation of Mn2+/3+ , Cu2+ , Co2+ , and Ni2+ with capillary electrophoresis hyphenated to inductively coupled plasma-mass spectrometry.

Abstract

The lifetime of lithium ion batteries (LIBs) decreases under continuous cycling due to various degradation processes, such as dissolution of transition metals (TMs) from the electrodes. Therefore, suitable methods to analyze the oxidation states of TMs are mandatory to better understand the dissolution mechanisms of TMs from positive and negative electrodes (LIBs). To investigate the dissolution of Mn2+ and Mn3+ in electrolytes of LIBs, a previously implemented capillary electrophoresis (CE) method with UV/Vis spectroscopy detection was further developed with the aim of higher sensitivities and additional detection of other dissolved divalent TMs such as Co2+ , Ni2+ , and Cu2+ . Therefore, inductively coupled plasma-mass spectrometry was applied instead of UV/Vis for detection. This also allows the use of Ga3+ instead of the previously used Cu2+ as an internal standard, which solves the limitation of this method for cycled LIBs due to copper dissolution from the copper-based current collector. The CE buffer based on sodium diphosphate as complexing agent for the stabilization of Mn3+ and cetyltrimethylammonium bromide as dynamic capillary wall modifier was optimized in terms of concentrations and pH. Finally, both manganese species and Co2+ , Ni2+ , and Cu2+ could be analyzed within 15min. With this improved method, the dissolution of TMs in LIBs for positive electrode materials such as LiNi0.5 Mn1.5 O4 (LNMO) or LiNix Coy Mnz O2 (NCM, x+y+z=1) can be studied in future in more detail.