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...

In this work we present a novel on-line electrochemical mass spectrometer design, which enables quantitative gas evolution analysis with a sealed battery design, applied to the study of the charging processes in a Li-O2 battery. Successive charge/discharge cycles were performed using Vulcan-carbon based positive electrodes in electrolytes composed of 0.2 M LiTFSI and two different solvents: i) propylene carbonate (PC), and, ii) bis(2-methoxyethyl) ether (diglyme). Results on the PC based electrolyte reveal a strong potential dependence of the evolved gaseous products which is maintained throughout subsequent cycles, consisting predominantly of O2 below 3.7 V and of predominantly CO2 above 3.7 V. The observed capacity fading is most likely caused by the gradual accumulation of discharge products which can only be oxidized at high anodic potentials. With diglyme electrolyte, the predominant gas during charging is O2. However, while the number of electrons/O2 closely corresponds to the oxidation of Li2O2 at the beginning of each charging cycle (2 e−/O2), it increases with potential and with the number of cycles, suggesting the gradual formation of other oxygen-containing discharge products which can only be oxidized at high potential with the parallel formation of CO2.

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High-energy Li1.17Ni0.19Co0.10Mn0.54O2 (HE-NCM) is a lithium-rich layered oxide with alternating Li- and transition-metal (TM) layers in which excess lithium ions replace transition metals in the host structure. HE-NCM offers a capacity roughly 50 mAh g–1 higher compared to that of conventional layered oxides but suffers from capacity loss and voltage fade upon cycling. Differential capacity plots (taken over 100 cycles) show that the origin of the fading phenomenon is a bulk issue rather than a surface degradation. Although previous studies indicate only minor changes in the bulk material, long duration in situ synchrotron X-ray powder diffraction measurements, in combination with difference Fourier analysis of the data, revealed an irreversible transition-metal motion within the host structure. The extensive work provides new insights into the fading mechanism of the material.

The use of 1-methyl-1-butylpyrrolidinium bis(trifluoromethylsulfonyl)imide (Pyr 14 TFSI) electrolyte in different Li-O 2 cell setups is here investigated. In a one-compartment Li-O 2 cell, the pyrrolidinium ion is reduced on metallic lithium, producing substantial amounts of alkenes and amines. To avoid this, a simple two-compartment cell is used, with propylene carbonate as anode electrolyte and a Li + -ion solid electrolyte as separator. Another explored option is the substitution of lithium in the one-compartment cell with lithiated LTO (LLTO). Unfortunately, the absence of an SEI leads to the reduction of O 2 at LLTO, making it not useful as counter electrode for Li-O 2 cell evaluation. All the configurations above are characterized by a first discharge specific capacity double than that obtained with unreactive electrolytes. The use of an edge-sealed two-compartment LLTO-Vulcan cell resulted in the usual discharge capacity of ≈200 mAh g −1 C at the first cycle, eliminating the effects of Pyr 14 TFSI reduction; nevertheless, the poor cyclability even in this cell design suggests that Pyr 14 TFSI might not have sufficient long-term stability against the attack of O 2•− during discharge or of oxygen species during charge. © The Author Ionic liquids have attracted much attention as electrolyte solutions for many electrochemical applications in the last decade, like electrochemical actuators and electrochromic windows 1 dye-sensitized solar cells 2 waste treatment, 3 supercapacitors 4 and lithium batteries. 5,6 This is due to their unique properties, like non-flammability and negligible vapor pressure, which would improve battery safety, and their low melting point and reasonably low viscosity, enabling their use as solvents even at room temperature. Another attractive property is their very high anodic electrochemical stability 7 that widens the potential window in which they can be employed in electrochemical applications. Even though their production and purification is more demanding than organic solvents due to their ionic character, they are easy to separate and recycle.All these properties made them attractive also as electrolyte solvents in the non-aqueous Li-O 2 secondary batteries, introduced by K.M. Abraham et al. in 1996. 8 Owing to the high specific capacity of the oxygen cathode of Li-O 2 batteries, their potential application for full-electric vehicles attracted much interest, promising to extend vehicle range. Their practical specific energy, including the weight of inactive electrode components and electrolyte, has been estimated in a recent review, 9 which claims a practically achievable specific energy of 1300 Wh kg −1 for Li-O 2 cells, which would be a roughly four-fold improvement over state-of-the-art Li-ion batteries. 10 It is important to note, however, that a more recent analysis of the battery-system level specific energy achievable with Li-O 2 vs. Li-ion batteries suggests that the specific energy improvement would be less than two-fold and the volumetric energy density would be infe...