Impact of Trifluoromethylation of Adiponitrile on Aluminum Dissolution Behavior in Dinitrile-Based Electrolytes

Aluminum dissolution behavior of adiponitrile (ADN) and its trifluoromethylated derivative 3-(trifluoromethyl)adiponitrile (ADN-CF3) as single or co-solvent with propylene carbonate (PC) was determined in electrolytes with lithium bis(trifluoromethylsulfonyl) imide (LiTFSI) as conducting salt via selected electrochemical, spectroscopic and physicochemical methods. ADN-CF3 is introduced as a promising electrolyte solvent affording reduced aluminum dissolution in the presence of imide salts. In cases where neither electrolyte components nor decomposition products thereof enable the formation of protective surface layers on aluminum current collectors, both the viscosity and relative permittivity of the solvents could be identified as key parameters for reducing aluminum dissolution. High viscosities reduce the mobility of involved species yielding increased complex formation of Li+ and TFSI− ions or solvent molecules, hindering a reaction of TFSI− anions with the passivating aluminum oxide surface to Al(TFSI)x. Low relative permittivity yields lesser ionic dissociation of the lithium salt and lower solubility of Al(TFSI)x species in viscous electrolytes. Hence, reduced aluminum dissolution was observed by substituting electrolyte solvents from PC to ADN to ADN-CF3. The obtained results significantly contribute to better understanding of anodic aluminum dissolution behavior, while encouraging future design of advanced electrolytes with high viscosities and low-permittivity solvents that possess high oxidative stabilities

Impact of single vs. blended functional electrolyte additives on interphase formation and overall lithium ion battery performance

Two functional high-voltage additives, namely 2-(2,2,3,3,3-pentafluoropropoxy)-1,3,2-dioxaphospholane (PFPOEPi) and 1-methyl-3,5-bis(trifluoromethyl)-1H-pyrazole (MBTFMP) were combined as functional additive mixture in organic carbonate–based electrolyte formulation for high-voltage lithium battery application. Their impact on the overall performance in NMC111 cathode-based cells was compared with the single-additive–containing electrolyte counterpart. The obtained results point to similar cycling performance of the additive mixture containing electrolyte formulation compared with the MBTFMP-containing cells, whereas the single PFPOEPi-containing cells displayed the best cycling performance in NMC111||graphite cells. With regard to the cathode electrolyte interphase (CEI), characterized and analyzed by means of scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), both the MBTFMP and the PFPOEPi functional additives decompose on the NMC111 surface in single-additive–containing electrolyte formulations. However, the thickness of the CEI formed in the additive mixture–containing electrolyte formulation is determined by the MBTFMP additive, whereas the PFPOEPi additive impacts a change in the composition of the CEI. Furthermore, the MBTFMP additive decomposes prior to the PFPOEPi and, therefore, dominates the cycling performance of NMC111||graphite cells containing functional additive mixture–based electrolyte. This systematic approach allows us to understand the synergistic impact of each functional additive in an electrolyte formulation containing an additive mixture and helps to identify the right additive combination for advanced electrolyte formulation as well as to elucidate whether the single-additive or the additive mixture approach is more effective for the development of advanced functional electrolytes for lithium-based cell chemistries

The Role of Electrolyte Additives on the Interfacial Chemistry and Thermal Reactivity of Si-Anode-Based Li-Ion Battery

Silicon (Si) has gained huge attention as an anode material for next-generation high-capacity lithium-ion batteries (LIBs). However, despite its overwhelming beneficial features, its large-scale commercialization is hampered due to unavoidable challenges such as colossal volume change during (de)alloying, inherent low electronic and ionic conductivities, low Coulombic efficiency, unstable/dynamic solid electrolyte interphase (SEI), electrolyte drying and so forth. Among other strategies, the use of a fraction dose of chemical additives is hailed as the most effective, economic and scalable approach to realize Si-anode-based LIBs. Functional additives can modify the nature and chemical composition of the SEI, which in turn dictates the obtainable capacity, rate capability, Coulombic/energy efficiency, safety, and so forth of the battery system. Thus, we report a systematic and comparative investigation of various electrolyte additives, namely tetraethoxysilane (TEOS), (2-cyanoethyl)triethoxysilane (TEOSCN), vinylene carbonate (VC), fluoroethylene carbonate (FEC), and a blend of TEOSCN, VC, and FEC (i.e., VC/FEC/TEOSCN) using electrochemical analysis, X-ray photoelectron spectroscopy, density functional theory calculation, and differential scanning calorimetry. The ternary mixture (FEC/VC/TEOSCN) results in a thinner SEI layer consisting of high shear modulus SEI-building species (mainly LiF). It also provides much improved thermal stability amid all tested additives, showing its potentiality to enable high capacity and safer Si-based anode LIBs. Thus, nitrile-functionalized silanes are highly promising electrolyte additives to boost the electrochemical performance and safety-induced risks of Si-based anode LIBs, emanating from the formation of a robust SEI layer

Methyl-group functionalization of pyrazole-based additives for advanced lithium ion battery electrolytes

A novel methylated pyrazole derivative, namely 1-methyl-3,5-bis(trifluoromethyl)-1H-pyrazole (MBTFMP) wassynthesized for the first time and comprehensively characterized for high voltage application in lithium ionbatteries (LIBs). The MBTFMP reactivity and performance was compared to the known 3,5-bis(trifluoromethyl)-1H-pyrazole (BTFMP) functional additive via cyclic voltammetry (CV), constant current cycling as well as postmortem analysis techniques on the graphite and LiNi1/3Mn1/3Co1/3O2 (NMC111) electrodes, such as scanningelectron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS). By means of quantum chemistry (QC)calculations, reductive and oxidative stabilities of MBTFMP and BTFMP functional molecules and their reactivitywith the cathode surface were determined. Both reduction and oxidation of BTFMP molecule was coupled withthe intermolecular H-transfer that narrowed BTFMP containing electrolyte electrochemical stability windowcompared to MBTFMP functional additive. The obtained results demonstrate the benefits of hydrogen atomsubstitution of BTFMP by a methyl-group at the nitrogen atom that resulted in significant improvement of theNMC111||graphite cell cycling performance. This work reveals that with a smart selection of the substitutiongroup and its position in the molecule, functional additives can be tailored in respect of vital physicochemicalproperties relevant for the high voltage LIB application

Tetrahydrothiophene 1-oxide as highly effective co-solvent for propylene carbonate-based electrolytes

Propylene carbonate (PC) together with cyclic sulfur compounds such as tetrahydrothiophene 1-oxide (THT1oxide) as co-solvent and lithium hexafluorophosphate (LiPF6) as conducting salt are introduced as new aprotic liquid electrolytes for lithium-ion batteries. Starting with the single solvent electrolyte LiPF6 in PC, by addition of THT1oxide, the ion transport properties even at temperatures down to −20 °C are improved by the different solvation behavior of Li+ ions due to the high Li+ ion affinity of the sulfinyl (-S=O) group and by the resulting decrease of the Li+ ion complex size. Electrolytes that contain Li+ ion complexes with both PC and THT1oxide molecules in the solvation shell are able to form protective interphase layers on graphite and NCM111 (LiNi1/3Co1/3Mn1/3O2) electrodes that are both permeable for Li+ ions while ensuring good electronic insulation, thus enabling stable cycling in lithium-ion cells with only minor capacity fading. THT1oxide/PC-based electrolytes afford better long-term as well as low temperature cycling behavior compared to established state-of-the-art (SOTA) organic carbonate-based electrolytes. The obtained results allow for the design of new co-solvents for PC and comparable cyclic organic carbonates, and provide a non-toxic and cheap alternative to crown ethers without affecting the Li+ ion transference/transport numbers

Influence of the Fluorination Degree of Organophosphates on Flammability and Electrochemical Performance in Lithium Ion Batteries

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

A propylene carbonate based gel polymer electrolyte for extended cycle life and improved safety performance of lithium ion batteries

A poly(vinylidene difluoride-co-hexafluoropropylene) (PVdF-HFP)-based gel polymer electrolyte (GPE) containing propylene carbonate (PC)-based liquid electrolyte was developed to enhance the safety performance of LiNi0.5Mn0.3Co0.2O2/graphite (NMC532/graphite) lithium ion batteries. The PC-based liquid electrolyte (PEV-LE) consists of 1 mol L−1 LiPF6 as lithium salt, PC as the main solvent and ethylene sulfite (ES, 2% by weight) as well as vinylene carbonate (VC, 2% by weight) as solid electrolyte interphase (SEI) forming additives. Electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS) revealed that the combination of ES and VC additives facilitates the formation of effective interphases at the respective electrolyte/electrode interfaces, thus contributing to a remarkable cycle life of NMC532/graphite cell comprising PEV-GPE. Flash point measurements and differential scanning calorimetry (DSC) confirmed significantly improved safety performance of PEV compared to the state-of-the-art electrolyte. PEV-GPE is a promising alternative to state-of-the-art electrolyte as it shows extended cycle life and enhanced thermal stability in NMC532/graphite lithium ion cells

Influence of the Fluorination Degree of Organophosphates on Flammability and Electrochemical Performance in Lithium Ion Batteries: Studies on Fluorinated Compounds Deriving from Triethyl Phosphate

Three symmetric substances originating from triethyl phosphate were specifically synthesized with varying degree of fluorination at the side chain. Different concentrations of each phosphate were evaluated as co-solvent with regard to their flammability and the electrochemical cycling performance. With higher degree of fluorination and a higher amount of the phosphate in the electrolyte, the self-extinguishing time (SET), a value to determine and compare the flammability of electrolytes, could be significantly lowered to yield a non-flammable electrolyte mixture. A specifically designed SET device is introduced, which offers more accurate results due to lowered standard deviations by minimizing random and systematic errors. As the application of phosphates as co-solvents results in a trade-off in cycling performance, a thorough determination in regard to the ionic conductivity, the anodic oxidation stability and the compatibility with anode and cathode material was carried out in half- and full-cells. The manuscript strives to establish a deeper understanding of the influence that the utilization of phosphates as co-solvents entail with special focus on the fluorination degree. It could be shown that the partially fluorinated phosphate offers the best cycling results and therefore the lowest trade-off in performance, while a severe improvement in SET could be achieved compared to the reference electrolyte