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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 LiNi xMn zCo1− 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. [ABSTRACT FROM AUTHOR]

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 possible mechanisms for the occurring Al dissolution behavior at elevated potentials. Either, an accelerated aluminum dissolution process, or a continuous passivation/LiPF6-decomposition process. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The practically available specific energy of Li ion batteries (LIB) is highly depending on the used specific charge/discharge current, since the respective overpotentials of each electrode affect the two vital specific energy parameters, specific capacity and voltage. Focusing on the positive composite electrode as the specific energy bottleneck, the overall nature of the overpotential is discussed for the LiNi1/3Co1/3Mn1/3O2 (NCM) active material. It is shown that the characteristic overpotentials during charge (delithiation) and discharge (lithiation) is state of charge (SOC) and depth of discharge (DOD) dependent, respectively. It was demonstrated that the discharge characteristics are intertwined with the previous charge conditions, particularly with the charging time and the specific charge capacity. Increasing both in parallel can even lead to a deterioration of the subsequent specific discharge capacity. Furthermore, Li+ transport pathways within the NCM composite electrode are discussed and their influence on the observed overpotential evaluated. Changes of the overpotential are found to be mainly associated with changes within the NCM crystal structure, which is experimentally supported by the correlation of the SOC dependent overpotential with the XRD determined c-axis lattice parameter. Consequently, the Li+ transport within the active material is mostly responsible for limiting the practically available specific energy. [ABSTRACT FROM AUTHOR]

Diverse LiPF 6 hydrolysis products evolve during lithium-ion battery cell operation at elevated operation temperatures and high operation voltages. However, their impact on the formation and stability of the electrode/electrolyte interfaces is not yet investigated and understood. In this work, literature-known hydrolysis products of LiPF 6 dimethyl fluorophosphate (DMFP) and diethyl fluorophosphate (DEFP) were synthesized and characterized. The use of DMFP and DEFP as electrolyte additive in 1 M LiPF 6 in EC:EMC (1:1, by wt) was investigated in LiNi 1/3 Mn 1/3 Co 1/3 O 2 /Li half cells. When charged to a cutoff potential of 4.6 V vs Li/Li + , the additive containing cells showed improved cycling stability, increased Coulombic efficiencies, and prolonged shelf life. Furthermore, low amounts (1 wt % in this study) of the aforementioned additives did not show any negative effect on the cycling stability of graphite/Li half cells. DMFP and DEFP are susceptible to oxidation and contribute to the formation of an effective cathode/electrolyte interphase as confirmed by means of electrochemical stability window determination, and X-ray photoelectron spectroscopy characterization of pristine and cycled electrodes, and they are supported by computational calculations.

Thanks to its high operating voltage, the LiNi 0.5 Mn 1.5 O 4 (LNMO) spinel represents a promising next-generation cathode material candidate for Lithium ion batteries. However, LNMO-based full-cells with organic carbonate solvent electrolytes suffer from severe capacity fading issues, associated with electrolyte decomposition and concurrent degradative reactions at the electrode/electrolyte interface, especially at elevated temperatures. As promising alternatives, two selected LiTFSI/pyrrolidinium bis(trifluoromethane-sulfonyl)imide room temperature ionic liquid (RTIL) based electrolytes with inherent thermal stability were investigated in this work. Linear sweep voltammetry (LSV) profiles of the investigated LiTFSI/RTIL electrolytes display much higher oxidative stability compared to the state-of-the-art LiPF 6 /organic carbonate based electrolyte at elevated temperatures. Cycling performance of the LNMO/Li 4 Ti 5 O 12 (LTO) full-cells with LiTFSI/RTIL electrolytes reveals remarkable improvements with respect to capacity retention and Coulombic efficiency. Scanning electron microscopy (SEM) images and X-ray diffraction (XRD) patterns indicate maintained pristine morphology and structure of LNMO particles after 50 cycles at 0.5C. The investigated LiTFSI/RTIL based electrolytes outperform the LiPF 6 /organic carbonate-based electrolyte in terms of cycling performance in LNMO/LTO full-cells at elevated temperatures.