Aprotic Li-O 2 cells have attracted considerable research interest due to its outstandingly high theoretical specific capacity. However, published discharge capacities vary considerably among different researchers despite only minor differences in the tested cell components. Some research groups observe low discharge capacities and formation of passivating layers of Li 2 O 2 on the electronically conducting cathode support, while other groups report large capacities and toroidal Li 2 O 2 crystals as discharge product. In this study we show that these differences may be due to water and protons, both possible impurities in Li-O 2 cells, having a large effect on discharge capacity and Li 2 O 2 morphology. As evidenced by XRD, FTIR and UV-visible analysis, Li 2 O 2 is still the main discharge product in Li-O 2 cells containing water, and moreover the Li 2 O 2 yield increases with the concentration of water in the electrolyte. On-line electrochemical mass spectrometry was employed to understand the differences in the discharge-charge behavior due to the addition of water and protons. While water seems to get oxidized at high potentials during charge, protons are consumed at the beginning of the discharge leading to a variety of reactive oxygen species and thus to degradation of cell components.

In the present work, the extent and the role of oxygen release during the first charge of lithium-and manganese-rich Li 1.17 [Ni 0.22 Co 0.12 Mn 0.66 ] 0.83 O 2 (also referred to as HE-NCM) was investigated with on-line electrochemical mass spectrometry (OEMS). HE-NCM shows a unique voltage plateau at around 4.5 V in the first charge, which is often attributed to a decomposition reaction of the Li-rich component Li 2 MnO 3 . For this so-called "activation", it has been hypothesized that the electrochemically inactive Li 2 MnO 3 would convert into MnO 2 while lattice oxide ions are oxidized and released as O 2 (or even CO 2 ) from the host structure. However, qualitative and quantitative examination of the O 2 and CO 2 evolution during the first charge shows that the onset of both reactions is above the 4.5 V voltage plateau and that the amount of released oxygen is an order of magnitude too low to be consistent with the commonly assumed Li 2 MnO 3 activation. Instead, the amount of released oxygen can be correlated to a structural rearrangement of the active material which occurs at the end of the first charge. In this process, oxygen depletion from the HE-NCM host structure leads to the formation of a spinel-like phase. This phase transformation is restricted to the near-surface region of the HE-NCM particles due to the poor mobility of oxide ions within the bulk. From the evolved amount of O 2 and CO 2 , the thickness of the spinel-like surface layer was estimated to be on the order of ≈2-3 nm, in excellent agreement with previously reported (S)TEM data. Since the discovery of the positive electrode material LiCoO 2 and its commercialization in the Li-ion technology by Sony in 1991, 1 analogous layered oxides (LiMeO 2 , Me = Ni, Co, Mn, Al, etc.) were studied, aiming at higher intrinsic specific capacity, energy, stability, and lower costs.2-7 Among others, Li[Ni 1/3 Co 1/3 Mn 1/3 ]O 2 (NCM-111) showed very interesting performances in terms of specific capacity and stability. 8,9 Recently, materials characterized by an increase of exploitable Li + charge drew a lot of attention. 10,11 These so-called Lirich compounds result from the substitution of part of the transition metal ions by Li + , in a structural arrangement closely related to the layered structure. 11-14Li 2 MnO 3 (or Li[Li 1/3 Mn 2/3 ]O 2 ) is the simplest structure in this category and crystallizes in the monoclinic system (space group C2/m), while the common LiMeO 2 -based layered structures crystallize in the hexagonal system (space group R-3m). 11,13,14 The two structures are very close to each other despite this difference in symmetry, related simply to the Li + ordering in the transition metal sites. This similarity is evident in the structure of the Li-rich NCM Li 1+x Me 1-x O 2 (Me = Ni, Co, Mn), also referred to as high-energy NCM (HE-NCM), where the hexagonal symmetry of the layered structure is broken by the superstructure of Li + in the transition metal sites, shown by the superlattice reflections in the diffractograms. 15,16 This s...

The instability of currently used electrolyte solutions and of the carbon support during charge-discharge in non-aqueous lithium-oxygen cells can lead to discharge products other than the desired Li2O2, such as Li2CO3, which is believed to reduce cycle-life. Similarly, discharge in an O2 atmosphere which contains H2O and CO2 impurities would lead to LiOH and Li2CO3 discharge products. In this work we therefore investigate the rechargeability of model cathodes pre-filled with four possible Li-air cell discharge products, namely Li2O2, Li2CO3, LiOH, and Li2O. Using Online Electrochemical Mass Spectrometry (OEMS), we determined the charge voltages and the gases evolved upon charge of pre-filled electrodes, thus determining the reversibility of the formation/electrooxidation reactions. We show that Li2O2 is the only reversible discharge product in ether-based electrolyte solutions, and that the formation of Li2CO3, LiOH, or Li2O is either irreversible and/or reacts with the electrolyte solution or the carbon during its oxidation.

The performance degradation of graphite/LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC) lithium ion cells, charged and discharged up to 300 cycles at different operating conditions of temperature and upper cutoff potential (4.2V/25 • C, 4.2V/60 • C, 4.6V/25 • C) was investigated. A combination of electrochemical methods with X-ray diffraction (XRD) both in situ and ex situ as well as neutron induced PromptGamma-Activation-Analysis (PGAA) allowed us to elucidate the main failure mechanisms of the investigated lithium ion cells. In situ XRD investigations of the NMC material revealed that the first cycle irreversible capacity is the cause of slow lithium diffusion kinetics. In full-cells, however, this "lost" lithium ions can be used to build up the SEI of the graphite electrode during the initial formation cycle. A new systematic approach to correlate the lithium content in NMC with its lattice parameters (c, a) allows a convenient quantification of the loss of active lithium in aged cells by determining the c/a ratio of harvested NMC cathodes in the discharged state using ex situ XRD. Besides loss of active lithium, transition metal dissolution/deposition on graphite and growth of cell impedance strongly effect cell aging, especially at elevated temperatures and high upper cutoff potentials. Besides their current use in portable power electronics, lithium ion batteries have recently been used for battery electric vehicles (BEV) and are envisioned for large-scale energy storage. For the latter applications, life times of >10 years are required so that it is essential to understand and quantify the mechanisms that contribute to battery failure. Among the commercially available lithium-ion battery chemistries, 1,2 the graphite/LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC) system is one of the materials currently envisioned for automotive applications. 3 This cathode material demonstrates high capacity, good structural stability due to its small volume changes (<2%) during Li insertion and extraction, and high thermal stability in the charged state. [4][5][6] In addition, this material could theoretically be operated with high charge cutoff potentials up to 5.0 V, as its bulk structure is claimed to be stabilized by the presence of Mn 4+ , 7 even though other authors suggest that irreversible structural changes occur at these very high potentials and at high temperature.8 Due to its sloped potential profile, the capacity and also the average cell voltage increase with increasing charging potential. 7,9 Despite the improved safety and cycling performance of NMC material, operating NMC based cells (full-cells or half-cells) at elevated temperatures or at high charge potential leads to poor cycle life. [10][11][12][13] During cycling of graphite/NMC full-cells, transition metal dissolution from the NMC material is found to be a crucial factor controlling capacity fade. 11,12 In one of these studies, Zheng et al. demonstrated that upper cutoff potentials of >4.3 V lead to transition metal dissolution from NMC and thus compromise cycling performa...

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