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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...

Nickel-rich layered lithiated Ni−Co−Mn oxides (NCMs) are emerging as the most promising candidates for next-generation Li-ion battery cathodes. Progress, however, is hindered by an incomplete understanding of processes that lead to performance-limiting impedance growth and reduced cycling stability. These processes typically involve surface reconstruction and O 2 release at the cathode surface, both of which are difficult to monitor in the working cell. We demonstrate that online electrochemical mass spectrometry can be used to measure the gas release from NCMs of varying Ni content at practically relevant potentials and under operando electrochemical conditions. We find that for cathode potentials up to 4.3 V (vs Li + /Li) there is virtually no trade-off between Ni-mediated specific-charge enhancement and parasitic surface reactions. However, at potentials greater than 4.3 V, surface-reconstruction processes giving rise to substantial CO 2 and O 2 release occur, implying that surfacereconstructed layers a few nanometers thick may form already after the first charge. Ni content and the Ni/Co ratio are found to govern the onset, rate, and extent of these surface-reconstruction processes. These results provide novel insights into the role of Ni in governing the surface stability and performance of Li-ion layered oxides.