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

Quantifying Dissolved Transition Metals in Battery Electrolyte Solutions with NMR Paramagnetic Relaxation Enhancement

Transition metal dissolution is an important contributor to capacity fade in lithium-ion cells. NMR relaxation rates are proportional to the concentration of paramagnetic species, making them suitable to quantify dissolved transition metals in battery electrolytes. In this work, ⁷Li, ³¹P, ¹⁹F, and ¹H longitudinal and transverse relaxation rates were measured to study LiPF₆ electrolyte solutions containing Ni²⁺, Mn²⁺, Co²⁺, or Cu²⁺ salts and Mn dissolved from LiMn₂O₄. Sensitivities were found to vary by nuclide and by transition metal. ¹⁹F (PF₆^-) and ¹H (solvent) measurements were more sensitive than ⁷Li and ³¹P measurements due to the higher likelihood that the observed species are in closer proximity to the metal center. Mn²⁺ induced the greatest relaxation enhancement, yielding a limit of detection of ∼0.005 mM for ¹⁹F and ¹H measurements. Relaxometric analysis of a sample containing Mn dissolved from LiMn₂O₄ at ∼20 °C showed good sensitivity and accuracy (suggesting dissolution of Mn²⁺), but analysis of a sample stored at 60 °C showed that the relaxometric quantification is less accurate for heat-degraded LiPF₆ electrolytes. This is attributed to degradation processes causing changes to the metal solvation shell (changing the fractions of PF₆^-, EC, and EMC coordinated to Mn²⁺), such that calibration measurements performed with pristine electrolyte solutions are not applicable to degraded solutions-a potential complication for efforts to quantify metal dissolution during operando NMR studies of batteries employing widely-used LiPF₆ electrolytes. Ex situ nondestructive quantification of transition metals in lithium-ion battery electrolytes is shown to be possible by NMR relaxometry; further, the method's sensitivity to the metal solvation shell also suggests potential use in assessing the coordination spheres of dissolved transition metals.

Solution NMR of Battery Electrolytes: Assessing and Mitigating Spectral Broadening Caused by Transition Metal Dissolution

NMR spectroscopy is a powerful tool that is commonly used to assess the degradation of lithium-ion battery electrolyte solutions. However, dissolution of paramagnetic Ni²⁺ and Mn²⁺ ions from cathode materials may affect the NMR spectra of the electrolyte solution, with the unpaired electron spins in these paramagnetic solutes inducing rapid nuclear relaxation and spectral broadening (and often peak shifts). This work establishes how dissolved Ni²⁺ and Mn²⁺ in LiPF₆ electrolyte solutions affect ¹H, ¹⁹F, and ³¹P NMR spectra of pristine and degraded electrolyte solutions, including whether the peaks from degradation species are at risk of being lost and whether the spectral broadening can be mitigated. Mn²⁺ is shown to cause far greater peak broadening than Ni²⁺, with the effect of Mn²⁺ observable at just 10 μM. Generally, ¹⁹F peaks from PF₆ ^- degradation species are most affected by the presence of the paramagnetic metals, followed by ³¹P and ¹H peaks. Surprisingly, when NMR solvents are added to acquire the spectra, the degree of broadening is heavily solvent-dependent, following the trend of solvent donor number (increased broadening with lower solvent donicity). Severe spectral broadening is shown to occur whether Mn is introduced via the salt Mn(TFSI)₂ or is dissolved from LiMn₂O₄. We show that the weak ¹⁹F and ³¹P peaks in spectra of electrolyte samples containing micromolar levels of dissolved Mn²⁺ are broadened to an extent that they are no longer visible, but this broadening can be minimized by diluting electrolyte samples with a suitably coordinating NMR solvent. Li₃PO₄ addition to the sample is also shown to return ¹⁹F and ³¹P spectral resolution by precipitating Mn²⁺ out of electrolyte samples, although this method consumes any HF in the electrolyte solution.

Determining the oxidation states of dissolved transition metals in battery electrolytes from solution NMR spectra

Dissolved transition metal ions can induce peak shifts in the NMR spectra of degraded battery electrolytes. Here, we exploit this staightforward, accessible method to calculate magnetic moments for dissolved Ni²⁺, Mn²⁺, Co²⁺, and Cu²⁺; subsequent analysis of dissolution from LiMn₂O₄, LiNiO₂, and LiNi_0.5Mn_1.5O₄ shows that the dissolved metals are exclusively divalent.

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.