Four symmetric compounds deriving from the alkyl chain containing tripropyl phosphate were designed and synthesized by varying substitution of fluorine in the side chains and added as co-solvents to yield 10%, 20% and 30% concentrated electrolyte formulations. The formulated electrolytes were physicochemically and electrochemically characterized and compared to a state-of-the-art organic carbonate-based electrolyte in regard to the flammability as well as any occurring trade-off in cycling performance. The addition of phosphates resulted in superior flammability behavior of the electrolyte as the flammability could be severely reduced with increased concentration of the phosphates. As the addition of such phosphate compounds to the electrolyte usually comes with a trade-off in cycling performance, electrochemical behavior was thoroughly investigated regarding ionic conductivity, anodic stability limit and cycling stability in lithium metal and lithium ion cells. The influence of the varying fluorine content as well as position of the substituted fluorine was determined and discussed. The tripropyl phosphate derivatives showed very promising cycling results hand in hand with a significant improvement achieved regarding the flammability of the electrolyte. (C) 2018 The Electrochemical Society.

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.

Using flame retardants (FRs) in lithium ion battery (LIB) electrolytes is usually a tradeoff between electrochemical performance and electrolyte flammability. Fluorinated FRs are a promising class of FRs which are currently under investigation. During this work, three FRs originating from triethyl phosphate with varying degree of fluorination were investigated regarding their electrochemical stability on cathode (LiNi0.33Co0.33Mn0.33O2, NCM) and anode (graphite) in half cells. During long-term cycling, changes in performance were observed. Especially on the anode side the FR addition showed a decrease in performance in comparison to the standard electrolyte (DEC/EC 1:1, 1M LiPF6). The electrolytes containing the three FRs were extracted from the cells and analyzed regarding their changes in composition and structural degradation. The decomposition products were investigated by gas chromatography (GC) with electron impact (EI) ionization and mass selective (MS) detection. To obtain more information with regard to the identification of unknown decomposition products further GC‐MS experiments with positive chemical ionization (PCI) and negative chemical ionization (NCI) were performed. Twelve different volatile organic decomposition products were identified. These decomposition products can be subdivided regarding their basic structure. Ether based, carbonate based and phosphate based fluorinated and non-fluorinated decomposition products were identified. Furthermore, possible formation pathways for all groups of decomposition products were postulated taking existing literature into account.

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.

Electrolyte solutions containing lithium-cyclo-difluoromethane-1,1-bis(sulfonyl)imide (LiDMSI) as an additive were specifically designed for measurements in lithium plating-stripping model experiments on copper electrodes. LiDMSI was implemented into two different electrolyte solutions. The first electrolyte setup consisted of a 1 M solution of LiTFSI in PC as an electrolyte which is known to show a comparably limited performance for reversible Li deposition. The second setup was comprised 1 M LiAsF6 in 1,3-dioxolane as the base electrolyte which depicts a well-tested performance for lithium deposition–dissolution. The addition of LiDMSI yielded significantly improved results in regard to Coulombic efficiencies and cycling stability in both electrolyte compositions. Furthermore, it negated the formation of high surface area, e.g., dendritic lithium, which depicts the main source for the limited safety of rechargeable lithium metal batteries. In the case of the PC-based electrolyte system, the LiDMSI-containing electrolyte illustrates a slightly lowered over-potential on the copper substrate, while for the dioxolane-based setup the over-potentials were almost completely equal. In order to compare the morphologies of the lithium deposits, SEM images were utilized.

Electrolyte solutions, containing the lithium sulfonyl methide salts lithium-tris(trifluoromethanesulfonyl)methide (LiTFSM) and lithium-[bis(trifluoromethylsulfonyl)-pentafluoroethylsulfonyl]methide (LiPFSM) dissolved in organic carbonate solvents, were electrochemically investigated in Li/graphite, Li/LiNi1/3Co1/3Mn1/3O2 (NCM) half-cells and compared to the LiPF6 based electrolyte with regard to their ionic conductivity, electrochemical stability, thermal stability at 60 degrees C and the anodic dissolution behavior vs. Al. While the investigated salts show almost the same performance in Li/graphite half-cells compared to the LiPF6 containing electrolyte, the methide salts show very promising results in Li/NCM half-cells, which depict superior capacity retention as well as higher Coulombic efficiencies compared to the LiPF6 containing electrolyte. Taking the limited anodic stability as well as the occurring anodic dissolution into account, both salts but especially LiTFSM indicate the applicability in lithium ion battery electrolytes for cells with a cutoff potential up to 4.3 V vs. Li/Li+. (C) The Author(s) 2015. Published by ECS. All rights reserved.

Lithium-cyclo-difluoromethane-1,1-bis(sulfonyl)imide (LiDMSI) was evaluated as an electrolyte additive in lithium-ion batteries for improved high voltage applications. Cycling the cathode at high potentials leads to the electrochemical oxidation of the salt to form a cathode electrolyte interphase (CEI) layer on the cathode surface. With the addition of 2 wt% of LiDMSI to the 1 M LiPF6 in 1 : 1 (by wt) EC : DEC electrolyte, the capacity retention and the Coulombic efficiency in LiNi1/3Co1/3Mn1/3O2/Li-half-cells as well as in LiNi1/3Co1/3Mn1/3O2/graphite-full-cells were improved. The cycling results point out the less over-potential and resistance at the cathode/electrolyte interface. These improvements are studied by SEM, EIS and XPS techniques.

Electrolyte solutions have vital function in lithium-ion batteries. Due to their modular composition, there is a broad variety of electrolyte component combinations. In this work, we present electrochemical results on newly investigated electrolyte solution components. The standard electrolyte salt in commercial batteries, LIPF6, was replaced by new imide and sulfonate anion based salts, with enhanced stability. The use of propylene carbonate was enabled by the application of new SEI forming electrolyte additives. Electrolyte solvents, such as adiponitrile and γ-butyrolactone were investigated in combination with LiBF4 as electrolyte salt. In order to evaluate these materials, various electrochemical techniques like galvanostatic cycling, conductivity and electrochemical stability window detection, cyclic voltammetry, etc. were applied. Furthermore, the electrode/electrolyte interfaces and interphases were studied via spectroscopic and spectrometric techniques.

Lithium-ion batteries (LIBs) have surpassed any other battery technology for consumer electronics and mobile devices in the last decades mostly due to the high energy density and the lack of memory effect. Following the invention of the concept, a lot of effort has been put into improving the energy, the power as well as the safety of the LIBs. [1] Besides other aspects, the flammability of carbonates, frequently used as electrolyte components in most batteries, is one of the major drawbacks. There are different approaches to overcome this problem. One approach is to replace the liquid electrolytes with, for example ionic liquids, gel polymer electrolytes or even solid electrolytes. [2]. In another approach flame retardants (FRs) are added to suppress the flammability of the electrolyte to the point at which it becomes non-flammable for example phosphorous containing compounds. [3] Xu et al. tested different phosphate containing compounds with varying concentrations.[4, 5] This work displays a comparison study on five phosphorous containing FRs. The main goal was to gain deeper insight into the correlation between the fluorine content of the phosphate and the flammability of the electrolyte mixture. Special focus of the study was laid upon the fluorine content of the terminal carbon of the side chains. Electrolyte compositions with different concentrations of the FRs were prepared using standard electrolyte components with 1 M Lithium hexafluorophosphate as conducting salt and a mix of carbonates as solvent. Since the goal was to improve the flammability of the electrolyte the crucial investigation is depicted by the self-extinguishing time (SET), which is a common way in literature to display the flammability of a certain liquid. To test the possible use of the mixtures in LIBs constant current cycling experiments were performed in half-cells on anode material (T44 graphite) as well as on cathode material (LiNi 1/3 Mn 1/3 Co 1/3 O 2 (NMC)). Physical properties like the flashpoint, the viscosity and the ionic conductivity completed the measurements. Figure 1 displays the flammability of the tested electrolytes and the content in % is plotted vs the self-extinguishing time (SET) in s/g. It was shown that the tested phosphates have potential for application as components in non-flammable electrolytes, which yields in an increase in safety without a big trade-off in performance. [1] J. Wen, Y. Wen, C. Yu, J. Chen, Wen, Materials express, 2 (2012) 197-212. [2] J.B. Goodenough, Y. Kim, Chem Mater, 22 (2010) 587-603. [3] A. Granzow, Accounts Chem Res, 11 (1978) 177-183. [4] K. Xu, J Electrochem Soc, 150 (2003) A161. [5] K. Xu, M.S. Ding, S.S. Zhang, J.L. Allen, T.R. Jow, J Electrochem Soc, 149 (2002) A622-A626.

Electrolyte solutions, containing the lithium salts lithium-cyclo-difluoromethane-1,1-bis(sulfonyl)imide (abbreviated as LiDMSI) and lithium-cyclo-hexafluoropropane-1,1-bis(sulfonyl)imide (LiHPSI) dissolved in organic carbonate solvents, were electrochemically investigated on graphite and LiNi1/3Mn1/3Co1/3O2 (NMC) electrodes and compared to the electrolyte salt LiPF6 with regard to conductivity, the electrochemical stability window, the anodic dissolution behavior vs. aluminum as well as the thermal stability behavior at 60°C. XPS studies were carried out to investigate the influence of the salt on the composition and the thickness of the solid electrolyte interphase (SEI). Constant current cycling experiments proved the potential applicability of the investigated salts for lithium ion batteries.

Electrolyte solutions containing the di-lithium cation salts Li2CF(SO2NSO2CF3)(2) (abbreviated bisimide), Li-2(SO2NSO2CF3)(SO3) (imide-sulfonate), Li2CF2(SO2NSO2CF3)(SO2C(SO2CF3)(2) (imide-methanide) dissolved in carbonate solvents were electrochemically investigated on graphite and LiFePO4 electrodes and compared to the electrolyte salt LiPF6. An electrochemical characterization containing the conductivity, the electrochemical stability window, the anodic dissolution behavior of aluminum as well as the thermal behavior at 60 degrees C was performed for these electrolyte solutions. Constant current cycling experiments proved the potential applicability of the investigated salts for use in lithium ion batteries. The order of the salts in regard to their cycling performance reaches from methanide-imide showing the best results over bisimide to imide-sulfonate which is the worst among the investigated salts. (C) 2013 The Electrochemical Society. [DOI: 10.1149/2.013304jes] All rights reserved.