Decomposition of LiPF₆in High Energy Lithium-Ion Batteries Studied with Online Electrochemical Mass Spectrometry

Abstract

The chemical and electrochemical instabilities of LiPF 6 in carbonate electrolytes for Li-ion batteries were studied with online electrochemical mass spectrometry (OEMS). Decomposition of carbonate electrolytes based on LiPF 6 eventually results in the formation of POF 3 , which is readily detected and followed in situ during operation of Li-rich HE-NCM-based Li-ion cells. Electrode potentials above ∼4.2 V leads to carbonate solvent oxidation and presumably the formation of ROH species, which subsequently hydrolyze the LiPF 6 salt and initiate a thermally activated autocatalytic electrolyte decomposition cycle involving POF 3 as a reactive intermediate. Activation of the Li 2 MnO 3 domains of the Li-rich cathode contributes along with electrolyte and separator impurities to further POF 3 generation. Electrode potentials below ∼2.5 V vs. Li + /Li impede POF 3 formation and presumably also further electrolyte decomposition by scavenging reactive intermediate species. As a result, much less POF 3 gas was detected upon the 2 nd charge when using Li metal counter electrode, contrary to delithiated LiFePO 4 . In situ OEMS confirm that the parasitic reactions involving LiPF 6 constitute an intricate reaction scheme, but more importantly, provide further evidence about what the components of this scheme are and how these may interact with each other. Rechargeable Li-ion batteries are nowadays extensively used to power electronics and are entering the transportation sector by powering electric vehicles (EV). A wide range of both negative (e.g. graphite) and positive electrode materials (e.g. layered cobalt oxides, spinel-type manganese oxides, and olivine-type iron phosphates) have been thoroughly investigated and are now in widespread use in commercial batteries. The specific energy of Li-ion batteries is limited mainly by the positive electrode materials, having typical practical specific charges of ∼150 mAh/g and average operating potentials of ∼3.8 V vs. Li + /Li, which significantly inhibits the introduction of Li-ion batteries as power source in new applications. In recent years, the layered Li-rich cobalt-nickel-manganese oxides xLi 2 MnO 3 (1−x)(LiMO 2 ) (x ∼ 0.5, M = Co, Ni, Mn), hereafter called HE-NCM, have been shown to exhibit a high and reversible specific charge (∼250 mAh/g) and a competitive average operating potential (∼3.75 V vs. Li + /Li). [1][2][3][4] The origin of such a high specific charge is not yet completely understood, as the exact structure of the HE-NCM materials is highly dependent on the synthesis conditions and models coming from structural characterization are still under debate. Several reports have shown the presence of so-called Li 2 MnO 3 domains in the compound 5,6 whereas other groups demonstrated the monophasic character of their materials.7 However, during the first charge, a long potential plateau at ∼4.5 V vs. Li + /Li, not observed for conventional layered oxides, results from the delithiation process of the Li 2 MnO 3 domains accompanied by oxygen extraction. The extracted oxy...

The Role of Oxygen Release from Li- and Mn-Rich Layered Oxides during the First Cycles Investigated by On-Line Electrochemical Mass Spectrometry

“…Neither O 2 nor CO 2 are expected to be reduced at the LFP potential. 57 Its potential was monitored in a T-cell with a Li reference-electrode to Table I. Estimation of the molar fraction and thickness of the spinel-like surface layer for Model A (lattice oxygen-derived CO 2 only above ≈4.6 V) and Model B (CO 2 evolved prior to O 2 evolution at ≈4.6 V also due to the reaction with lattice oxygen), based on the gas evolution for the HE-NCM/Li cell data in Figure 3 (black lines).…”

Oxygen Release and Surface Degradation of Li- and Mn-Rich Layered Oxides in Variation of the Li2MnO3Content

“…19,20 At first sight, this seemed consistent with the mass spectrometrically detected O 2 and CO 2 evolution starting during the activation plateau, which was interpreted to indicate the release of Li 2 O from the bulk of the material during activation and accompanying structural changes within the bulk material. 15,17,18,23,24 However, the exact quantification of the released oxygen by Strehle et al and Luo et al revealed that the amount of released oxygen is an order of magnitude too low to be consistent with the assumed electrochemical oxygen release 25,26 and also that the O 2 evolution does not start until right after the activation plateau (only <10 μmol O2 /g AM during the plateau, but >100 μmol O2 /g AM following thereafter; shown by Strehle et al). 25 Both observations suggested that the observed oxygen release cannot be ascribed to a loss of oxygen from the bulk of the material, but that the oxygen is only being released from the near-surface Thus, more recent studies propose that bulk and surface of these overlithiated materials show distinctly different properties, rationalized by a bulk-shell model.…”

A New Method for Determining the Concentration of Electrolyte Components in Lithium-Ion Cells, Using Fourier Transform Infrared Spectroscopy and Machine Learning

“…[7][8][9][10][11][12][13][14][15][16][17][18][19][20] These instruments are costly, and these methods require the preparation and measurement of many calibration solutions. Often the columns or detectors used in chromatography experiments cannot be exposed to the high temperature decomposition products of LiPF 6 , so these experiments often focus on the organic portions of the electrolyte, after the water-soluble portions of the electrolyte have been removed.…”