High-voltage electrodes based on, for example, LiNi0.5 Mn1.5 04 (LNMO) active material require oxidative stability of inactive materials up to 4.95 V vs. Li|Li+ . Referring to literature, they are frequently supposed to be unstable, though conclusions are still controversial and clearly depend on the used investigation method. For example, the galvanostatic method, as a common method in battery research, points to the opposite, thus to a stability of the inactive materials, which can be derived from, for example, the high decomposition plateau at 5.56 V vs. Li|Li+ and stable performance of the LNMO charge/discharge cycling. This work aims to unravel this apparent contradiction of the galvanostatic method with the literature by a thorough investigation of possible trace oxidation reactions in cumulative manner, that is, over many charge/discharge cycles. Indeed, the cumulated irreversible specific capacity amounts to ≈10 mAh g-1 during the initial 50 charge/discharge cycles, which is determined by imitating extreme LNMO high-voltage conditions using electrodes solely consisting of inactive materials. This can explain the ambiguities in stability interpretations of the galvanostatic method and the literature, as the respective irreversible specific capacity is obviously too low for distinct detection in conventional galvanostatic approaches and can be only detected at extreme high-voltage conditions. In this regard, the technique of chronoamperometry is shown to be an effective and proper complementary tool for electrochemical stability research in a qualitative and quantitative manner.

Lithium-bis(hexafluorobutan-2,3-diol)-borate (R1S), synthesized as a novel electrolyte component in lithium-ion battery (LIB) cells, was evaluated and introduced as a bifunctional interphase additive for both, anodes and cathodes with the purpose of improving the cell performance during high voltage operation. For the graphite based anode it was demonstrated, that the additive could diminish the amount of parasitic capacity required for the formation of an effective solid electrolyte interphase (SEI), which could be concluded from improved Coulombic efficiency data. In parallel, improved capacity retention on the cathode attributed to an altered cathode electrolyte interphase (CEI) by the same additive was observed. Contrary to the CEI formed by the benchmark electrolyte formulation, the interphase of the additive containing electrolyte formulation is significantly influenced in an electrochemical manner (oxidation reactions), as derived from potentiodynamic measurements. As a result, the additive-tuned CEI was found smoother and less resistive compared to the additive-free counterpart, which was demonstrated by electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) of the harvested LiNi1/3Co1/3Mn1/3O2 (NMC111) electrodes after charge/discharge cycling. The simultaneous improvement of the interphases on cathode and anode by the additive could finally enhance the overall performance of a NMC111/graphite cell. (c) 2018 The Electrochemical Society.

Acetonitrile-based electrolytes for lithium

The further development of lithium ion batteries: operating at high voltages requires basic understanding of the occurring capacity fade mechanisms. In this work, the overall specific capacity loss with regard to reversible and irreversible processes for LiNi1/3Co1/3Mn1/3O2 (NCM111)/Li half cells, cycled at a charge cutoff potential of 4.6 V vs Li/Li+, has been investigated in detail. By means of total X-ray fluorescence (TXRF) technique it was shown that specific capacity losses associated with the amount of dissolved transition metals are negligible, implying a still intact NCM111 active material after 53 cycles. It was demonstrated that the specific capacity fade during cycling at constant specific currents can be mainly attributed to the increase of the delithiation (charge) hindrance, whereas lithiation (discharge) hindrance is only present after a specific current increase, leading to, apparent specific capacity losses, and to decreased Coulombic efficiencies. This could be proven by the determination of the NCM lithiation degree in the discharged state with inductively coupled plasma optical emission spectroscopy (ICP-OES). Moreover, by, decreasing the kinetic hindrance in the NCM material, it was shown that most of the observed specific capacity losses after 53 cycles are reversible: The influence of the active material and the cathode electrolyte interphase (CEI) on the specific capacity fade has been discussed. The results of the X-ray photoelectron spectroscopy (XPS) studies revealed that the CEI thickness is predominately dependent on the applied temperature (thermal-chemical origin) rather than the applied electrode potential (electrochemical origin). Finally, the absence of a fade in specific capacity for LiNi0.5Mn1.5O4 (LNMO) at an even higher charge cutoff potential of 4.95 V vs Li/Li+ points to a strong active material dependence than solely to the impact of electrolyte decomposition and CEI formation.

Previous studies have shown that electrolyte additives based on metals and semimetals (LiBOB, Mg (TFSI)(2), Al(TFSI)(3)) as well as additives containing trimethylsiloxyl (TMS) groups as ligands can have positive impact on the cycling performance of lithium ion battery cells due to solid electrolyte interphase (SEI) and/or cathode electrolyte interphase (CEI) film forming properties and/or scavenging properties towards acidic impurities. In this study, both active functionalities (metal core and trialkylsiloxy based ligands) were combined into one using Al, Ti and B as metal cores combined with TMS ligands (M(TMS)(x)). All investigated additives M(TMS) x were able to improve the cycling performance regarding Coulombic efficiency, energy efficiency and capacity retention of LiNi1/3Co1/3Mn1/3O2 (NCM111)/Li half-cells and NCM111/graphite full-cells at high potentials (> 4.3 V vs. Li/Li+). The formed CEI was studied by means of electrochemical impedance spectroscopy, scanning electron microscopy and X-ray photoelectron spectroscopy. The obtained results indicate that the investigated additives are either actively incorporated into the formed CEI layer (in case of Al, Ti as metal core) or interacting with decomposition products (in case of B as metal core) resulting in lower charge-transfer impedance and hence improved long-term cycling behavior. (C) 2017 Elsevier Ltd. All rights reserved.

The kinetic stabilization of the cathode/electrolyte interface is essential to enhance cycle life and safety of lithium-ion batteries at high voltage application. The addition of only 2 wt.% Mg powder to the cathode slurry were found to significantly increase the cycle life of LiNi1/3Co1/3Mn1/3O2/Li half cells upon cycling to 4.6 V vs. Li/Li+, in terms of higher capacity retention, less parasitic reactions and less self-discharge. The presence of dissolved Mg2+ cations in the electrolyte during formation of the cell lead to an increased hydrolysis of the conducting salt LiPF6 and the formation of stable and effective LiPFxOy/Mg(PFxOy)2 species on the charged cathode surface.

Allylboronic acid pinacol ester (ABPE) was investigated as shutdown overcharge additive to increase the intrinsic safety of lithium ion cells during operation at elevated charge cutoff potentials up to 4.5 V vs. Li/Li+. It was demonstrated that the additive had no negative influence on the cycling performance of LiNi1/3Co1/3Mn1/3O2 (NMC-111)/graphite full cells operated in a standard operation voltage range between 2.5 V and 4.2 V. Electrochemical impedance spectroscopy was used to study the influence of the electrolyte additive on the impedance before and after overcharge and concomitant cell shutdown. Thereby, an immense increase in the charge transfer resistance after electrode shutdown was observed, thus, hinting to the formation of a lithium ion-insulating layer on the positive electrode surface. To further elucidate the working principle of the shutdown additive, surface investigations by means of scanning electron microscopy and X-ray photoelectron spectroscopy were carried out. The existence of a surface layer consisting mainly of polymeric species was proven. The presented results open up a new family of compounds for overcharge protection. (C) 2016 The Electrochemical Society. All rights reserved.

In a lithium ion battery, balancing of active materials is an essential requirement with respect to safety and cycle life. However, capacity oversizing of negative electrodes is associated with decrease of specific energy/energy density. In this work, the required trade-off between maximized specific energy and minimized risk of lithium plating is thoroughly investigated by evaluating underlying potential/voltage curves. The adjustment of targeted state of charge (SOC) for both, positive and the negative electrode, can be achieved by intentional selection of only two parameters: negative/positive electrode active mass ratio and charge cutoff voltage. For investigation and controlling reasons, specific charge capacity reveals to be a simple but effective tool to indirectly predict electrode potentials. While cell kinetics/overvoltage are influenced by both electrodes, specific capacity losses are affected by a single electrode. The latter only correlate with negative electrode's BET surface area as long as specific capacity losses of negative electrodes are higher compared to positive electrodes. Based on these insights, a more systematic performance and safety optimized handling of the trade-off between specific energy and safety risk can be realized. (C) 2017 The Electrochemical Society. All rights reserved.

The inability of imide salts to form a sufficiently effective passivation layer on aluminum current collectors is one of the main obstacles that limit their broad application in electrochemical energy-storage systems. However, under certain circumstances, the use of electrolytes with imide electrolyte salts in combination with the aluminum current collector is possible. In this contribution, the stability of the aluminum current collector in electrolytes containing either lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) or lithium fluorosulfonyl-(trifluoromethanesulfonyl) imide (LiFTFSI) as conductive salt was investigated by electrochemical techniques, that is, cyclic voltammetry (CV) and chronocoulometry (CC) in either room-temperature ionic liquids or in ethyl methyl sulfone. In particular, the influence of the solvent, operating temperature, and thickness of the native oxide layer of aluminum on the pit formation at the aluminum current collector surface was studied by means of scanning electron microscopy. In general, a more pronounced aluminum dissolution and pit formation was found at elevated temperatures as well as in solvents with a high dielectric constant. An enhanced thickness of the native aluminum oxide layer increases the oxidative stability versus dissolution. Furthermore, we found a different reaction rate depending on dwell time at the upper cut-off potential for aluminum dissolution in TFSI- and FTFSI-based electrolytes during the CC measurements; the use of LiFTFSI facilitated the dissolution of aluminum compared to LiTFSI. Overall, the mechanism of anodic aluminum dissolution is based on: i) the attack of the Al2 O3 surface by acidic species and ii) the dissolution of bare aluminum into the electrolyte, which, in turn, is influenced by the electrolyte's dielectric constant.

Increasing the specific energy of a lithium ion battery and maintaining its cycle life is a predominant goal and major challenge for electrochemical energy storage applications. Focusing on the positive electrode as the specific energy bottleneck, cycle life characteristics of promising layered oxide type active materials (LiMO2) has been thoroughly investigated. Comparing the variety of LiMO2 compositions, it could be shown that the "Ni-rich" (Ni >= 60% for M in LiMO2) electrodes expectably revealed best performance compromises between specific energy and cycle life at 20 degrees C, but only LiNi0.6Mn0.2Co0.2O2 (NMC622) could also maintain sufficient cycle performance at elevated temperatures. Focusing on NMC622, it could be demonstrated that the applied electrochemical conditions ( charge capacity, delithiation amount) in the formation cycles significantly influence the subsequent cycling performance. Moreover, the insignificant transition metal dissolution, demonstrated by means of total X-ray fluorescence (TXRF) technique, and unchanged lithiation degree in the discharged state, determined by the measurement of the Li+ content by means of the inductively coupled plasma optical emission spectroscopy (ICP-OES) technique, pointed to a delithiation (charge) hindrance capacity fade mechanism. Considering these insights, thoughtful modifications of the electrochemical charge conditions could significantly prolong the cycle life. (C) 2017 Elsevier B.V. All rights reserved.

Three phosphorus containing molecules, tris(2,2,3,3,3-pentafluoropropyl) phosphate (5F-TPrP), tris (1,1,1,3,3,3-hexafluoropropan-2-yl) phosphate (HFiP) and tris(1,1,1,3,3,3-hexafluoropropan-2-y1) phosphite (THFPP), were investigated as high voltage and flame retardant electrolyte additives for lithium ion batteries. The effect of the oxidation state of the phosphorus atom as well as the influence of branched vs. linear 'fluorinated propyl groups were investigated regarding cycling performance and flammability of the resulting electrolyte. In the case of a high voltage battery application, all three investigated molecules showed an improvement regarding the cycling performance in NCM111/Li half -cells. Post mortem analysis of the NCM111 electrodes via SEM and XPS indicates that the different groups of two phosphates (5F-TPrP vs. HFiP) have an impact on the thickness, morphology and composition of the cathode electrolyte interphase (CEI). If the electrolyte formulation contains the linear side group (5F-TPrP), the thickness of the CEI increases, whereas for the branched group (HFiP) it decreases compared to the CEI formed in 1 M LiPF6 EC:DEC (1 :1) used as reference electrolyte. Furthermore, addition of at least 20 wt.% of 5F-TPrP to the reference electrolyte formulation resulted in a non-flammable electrolyte formulation. (C) 2017 Elsevier B.V. All rights reserved.

To improve the intrinsic safety of lithium ion batteries (LIBs) by preventing cells from a thermal runaway, we studied two carbene adduct electrolyte additives. The recently synthesized compounds (1,3-dimethylimidazolidin-2-mu m-trifluoroborate (NHC-BF3) and 1,3-dimethylimidazolidin-2-mu m-tetra-fluorotrifluoromethylphosphate (NHC-PF4CF3)) were investigated on LiNi1/3Co1/3Mn1/3O2 (NMC111) electrodes in Li metal and Li-ion cell setups as overcharge protection shutdown additives in 1M LiPF6 in EC:DEC (3:7, by wt.) electrolyte. By varying the NHC-ligand (-BF3,-PF6,-PF4CF3) in the molecule, the shutdown potential of the investigated carbene adduct electrolyte additives can be tailored for specific applications with different cut-off potentials. NHC-BF3 was identified as a promising candidate for the application with NMC111 electrodes up to 4.4 V vs. Li/Li+, whereas the carbene adduct NHC-PF(4)CF3 is ideal for the high-voltage application with the NMC-based electrode up to 4.6 V vs. Li/Li+. Next to electrochemical investigations in NMC111/Li and NMC111/graphite cells, Atomic Force Microscopy (AFM) and X-Ray Photoelectron Spectroscopy (XPS) were performed to verify the presence of a decomposition layer on the cathode, responsible for the shutdown effect. Furthermore, it has been proven that the investigated electrolyte additives have no influence on the cell performance under normal conditions in both, Li metal and Li-ion cell setups. (C) 2017 Elsevier B.V. All rights reserved.

Here, we report on methyl 3-cyanopropanoate (MCP) in combination with the conductive salt lithium bis(trifluoromethane)sulfonyl imide (LiTFSI) as a safe single-solvent electrolyte for lithium-ion batteries (LIBs). To investigate the extent of anodic aluminum dissolution, an innovative electrochemical technique was introduced. Long-term full-cell [LiNi1/3Mn1/3Co1/3O2 (NMC111)/graphite] cycling results confirm the applicability of the LiTFSI/MCP-based electrolyte with state-of-the-art LIB active materials.

The long-term influence of the most commonly used conducting salt in electrolyte formulations, lithium hexafluorophosphate, on the aluminum current collector stability in high voltage lithium ion batteries was investigated. By means of different surface sensitive techniques (scanning electron microscopy, atomic force microscopy and X-ray photoelectron spectroscopy), after 1003 simulated charge/discharge cycles, anodic aluminum dissolution was found to take place at elevated potential (4.95 V vs. Li/Li+) but only to a minor extent. Pitting of the Al collector could be assessed in the nanometer range. Furthermore, it could be revealed that local pit formation is related to local "native" grooves on the aluminum foil, which develop during the production process of the aluminum foil. The obtained results were evaluated and compared to a reference electrolyte containing the alternative conducting salt lithium bis(trifluoromethanesulfonyl) imide. Our findings imply two possiblemechanisms for the occurring Al dissolution behavior at elevated potentials. Either, an accelerated aluminum dissolution process, or a continuous passivation/LiPF6-decomposition process. (C) 2017 The Electrochemical Society. All rights reserved.

Increasing specific energy of lithium ion battery cells (LIBs) and their cycle life requires deeper understanding of complex processes taking place during the cell operation, This work focuses on the electrode potential development and the interactions between negative and positive electrode in a quasi LIB full cell by applying over-discharge conditions. By analysis of the potential profiles, a characteristic potential plateau at approximate to 3.56 V vs. Li/Li+ was detected at the graphite negative electrode, which can be assigned to the Cu oxidation process of the negative electrode current collector. Also at the positive electrode, a time shifted potential plateau was observed, which could be attributed to a competitive reaction between conventional discharge (lithiation) and parasitic Cu reduction (plating) on the positive electrode surface. The proposed mechanism involving the presence of elemental Cu on the positive electrode surface was confirmed by SEM-EDX mapping experiments. The relevance of Cu dissolution and deposition as well as possible solution approaches are discussed. (C) 2017 Elsevier B.V. All rights reserved.

Increasing the operation voltage of electrochemical energy storage devices is a viable measure to realize higher specific energies and energy densities. A sufficient oxidative stability of electrolytes is the predominant requirement for successful high voltage applicability. The common method to investigate oxidative stability of LIB electrolytes is related to determination of the electrochemical stability window (ESW), on e.g. Pt or LiMn2O4 electrodes. However, the transferability of the obtained results to practical systems is questionable for several reasons. In this work, we evaluated the validity of the potentiodynamic based ESW method by comparing the obtained data with the results of galvanostatic based techniques, applied on commercial positive electrodes. We demonstrated that the oxidative stabilities, determined by the two techniques, are in good accordance with each other. However, the investigation of electrolytes being incompatible to Li metal, renders conventional ESW measurements useless when metallic Li is used as counter - and reference electrode in the ESW setup. For this reason, we introduced an alternative setup based on Li4Ti5O12 full cells. On the example of a butyronitrile-based electrolyte, we finally demonstrated that this electrolyte is not only reductively but also oxidatively less stable than common LiPF6/organic carbonate based electrolytes.

The required boost in the specific energy of lithium-ion battery (LIB) cells can only be achieved by increasing the cell voltage and/or the specific capacities of the electrodes. In the latter regard, the positive electrode constitutes the specific energy bottleneck. Lithium transition-metal oxides (LiMO2) such as LiNixMnzCo1-x-zO2 (NMC) are regarded as the most suitable positive electrode materials for next-generation high-specific-energy LIBs. In this work, the electrochemically induced structural stability limits as well as the associated reversible specific energies and specific energy efficiencies were assessed by means of constant current charge/discharge experiments for the most popular and promising LiMO2 compositions. The electrochemically induced structural stability of the positive host material was not determined by the applied charge cut-off potential, but rather by the amount of extracted Li+ ions. In this regard, the electrochemically induced structural stability order of selected LiMO2 compositions was modified by assessing the structural stability as a function of the Li+-ion extraction ratio. With respect to application, relevant requirements (e.g., specific energy, specific energy efficiency, temperature-dependent structural stability, kinetics) revealed that NMC532 and NMC622 showed the best compromise among the various LiMO2 compositions, revealing significant insight into the structure-property relationship.