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

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 15 min. With this improved method, the dissolution of TMs in LIBs for positive electrode materials such as LiNi_0.5 Mn_1.5 O₄ (LNMO) or LiNi_x Co_y Mn_z O₂ (NCM, x + y + z = 1) can be studied in future in more detail.

Accessing copper oxidation states of dissolved negative electrode current collectors in lithium ion batteries

A novel capillary electrophoresis (CE) method with ultraviolet-visible spectroscopy (UV-Vis) detection for the investigation of dissolved Cu⁺ and Cu²⁺ in lithium ion battery (LIB) electrolytes was developed. This method is of relevance, as the current collector at the anode of LIBs may dissolve under certain operation conditions. In order to preserve the actual oxidation states of dissolved copper in the electrolytes and to prevent any precipitation during sample preparation and CE measurements, neocuproine (NC) and ethylenediamine tetraacetic (EDTA) were effectively applied as complexing agents for both ionic copper species. However, precipitation and loss of the Cu⁺ -NC-complex for quantification occurred in presence of the commonly applied conducting salt lithium hexafluorophosphate (LiPF₆ ). Therefore, acetonitrile (ACN) was added to the sample in order to suppress this precipitation. Dissolution experiments with copper-based negative electrode current collectors in a LIB electrolyte were conducted at 60°C under non-oxidizing atmosphere. First findings regarding the copper species via CE revealed dissolved Cu⁺ and mainly Cu²⁺ . However, primarily Cu⁺ dissolved from the passivating oxide layer (Cu₂ O and CuO) of the current collector due to the formation of acidic electrolyte decomposition products. Due to the instability of Cu⁺ in the electrolyte a further disproportionation reaction to Cu⁰ and Cu²⁺ occurred. The results show the high potential of this CE method for prospective investigations regarding the current collector stability in new battery electrode formulations and correlations of dissolution events with dissolution mechanisms and battery cell operation conditions.

Investigating the oxidation state of Fe from LiFePO4 -based lithium ion battery cathodes via capillary electrophoresis

A capillary electrophoresis (CE) method with ultraviolet/visible (UV-Vis) spectroscopy for iron speciation in lithium ion battery (LIB) electrolytes was developed. The complexation of Fe²⁺ with 1,10-phenantroline (o-phen) and of Fe³⁺ with ethylenediamine tetraacetic acid (EDTA) revealed effective stabilization of both iron species during sample preparation and CE measurements. For the investigation of small electrolyte volumes from LIB cells, a sample buffer with optimal sample pH was developed to inhibit precipitation of Fe³⁺ during complexation of Fe²⁺ with o-phen. However, the presence of the conducting salt lithium hexafluorophosphate (LiPF₆ ) in the electrolyte led to the precipitation of the complex [Fe(o-phen)₃ ](PF₆ )₂ . Addition of acetonitrile (ACN) to the sample successfully re-dissolved this Fe²⁺ -complex to retain the quantification of both species. Further optimization of the method successfully prevented the oxidation of dissolved Fe²⁺ with ambient oxygen during sample preparation, by previously stabilizing the sample with HCl or by working under counterflow of argon. Following dissolution experiments with the positive electrode material LiFePO₄ (LFP) in LIB electrolytes under dry room conditions at 20°C and 60°C mainly revealed iron dissolution at elevated temperatures due to the formation of acidic electrolyte decomposition products. Despite the primary oxidation state of iron in LFP of +2, both iron species were detected in the electrolytes that derive from oxidation of dissolved Fe²⁺ by remaining molecular oxygen in the sample vials during the dissolution experiments.

Mn2+ or Mn3+ ? Investigating transition metal dissolution of manganese species in lithium ion battery electrolytes by capillary electrophoresis

A new CE method with ultraviolet-visible detection was developed in this study to investigate manganese dissolution in lithium ion battery electrolytes. The aqueous running buffer based on diphosphate showed excellent stabilization of labile Mn³⁺ , even under electrophoretic conditions. The method was optimized regarding the concentration of diphosphate and modifier to obtain suitable signals for quantification. Additionally, the finally obtained method was applied on carbonate-based electrolytes samples. Dissolution experiments of the cathode material LiNi_0.5 Mn_1.5 O₄ (lithium nickel manganese oxide [LNMO]) in aqueous diphosphate buffer at defined pH were performed to investigate the effect of a transition metal-ion-scavenger on the oxidation state of dissolved manganese. Quantification of both Mn species revealed the formation of mainly Mn³⁺ , which can be attributed to a comproportionation reaction of dissolved and complexed Mn²⁺ with Mn⁴⁺ at the surface of the LNMO structure. It was also shown that the formation of Mn³⁺ increased with lower pH. In contrast, dissolution experiments of LNMO in carbonate-based electrolytes containing LIPF₆ showed only dissolution of Mn²⁺ .