LiPF6 Salt Research Articles

Towards Quantifying Capacity Losses Due to Solid Electrolyte Interphase Evolution in Silicon Thin Film Batteries

To better understand failure mechanisms in lithium-ion batteries (LIBs), it is imperative to quantify the capacity fading contributions from active material loss and solid electrolyte interphase (SEI) growth. However, these two contributions are indistinguishable in traditional electrochemical measurements. In this study, we used a model system in combination with (1) precision measurements of the overall Coulombic efficiency via electrochemical experiments and (2) X-ray reflectivity (XRR) measurements of the active material losses. The model system consisted of a 515 Å thick amorphous silicon (a-Si) thin film on silicon carbide in a half-cell geometry using a carbonate electrolyte with LiPF6 salt. Schematics of the cells used in operando XRR and precision electrochemistry measurements are shown in Fig. (a) and (b) respectively.Specifically, we used surface/interface sensitive operando XRR to monitor the a-Si/LixSi thickness during (de)lithiation in order to quantify the capacity fading due to active material loss. Combined with electrochemical analysis, we quantified the capacity loss originating in SEI growth during each cycle (Fig. (c)). We have found that SEI continues to grow with cycling, and the SEI thickness is dependent on the time spent at low potentials (Fig. (d)). The continued SEI growth results in increasing overpotentials due to increased SEI resistance, which leads to a lower extent of lithiation when the cut-off voltage is reached. Finally, we extracted a proportionality constant for SEI growth following a parabolic growth law. Our results are in fair agreement with the capacity losses observed in full cell systems.We envision that the proposed methodology can be used as a qualitative measure of the ion transport properties of the SEI. The model-based approach allows for quantitative determination of lithium ion loss mechanisms in LIBs by separately tracking lithium ions within the active materials and will aid in developing ways to reduce parasitic losses. Figure 1

Spatially and Time-Resolved Investigation of Lithium Plating on a Graphite Electrode during Fast Charging Using Operando Neutron Depth Profiling (NDP)

Apart from increasing the battery size of electric vehicles (EVs), another approach to reduce range anxiety and to achieve mass market penetration is to increase the charging power and thus significantly lower charging times. To date, commercial lithium-ion battery (LIB) cells are almost exclusively based on graphite as anode material.[1] Here, owing to the low potential of the graphite electrode during charge, the charging rate must be limited in order to avoid lithium plating, which can thermodynamically occur in regions of the electrode where the potential drops below 0 V vs. Li+/Li. Plated lithium is very reactive, leading to irreversible reactions with the electrolyte and therefore strongly reduces lifetime and safety of a battery.[2] Studying lithium plating in a battery cell is very challenging since post-mortem diagnostics do not reflect the electrode state during operation, since plated lithium will reintercalate into the active graphite material on a timescale of minutes at room temperature.[3] , [4] Hence, non-destructive operando techniques[5] , [6] are necessary to elucidate and follow lithium plating mechanisms in an operating cell.In this study, we present data collected via Neutron Depth Profiling (NDP) during lithium plating induced by fast charging of a graphite electrode at a rate of 2 C. NDP is a lithium-sensitive, non-destructive technique utilizing the neutron capture reaction of 6Li atoms to quantitatively measure the lithium distribution over the thickness of an electrode by probing the energy loss of charged particles formed by the neutron capture reaction.[7] We employ a recently developed operando NDP cell that shows a cycling performance comparable to a commercial cell even at high charging rates, and at the same time allows charged particles to exit the cell through a thin Kapton® window.[8] We were able to resolve the lithium depth distribution changes during the open circuit voltage (OCV) phase after a constant current charge with a time resolution of 2 min. This exceptionally high time resolution was achieved by combining the high cold neutron flux of 2.7×109 ncm-2s-1 at the NIST Center for Neutron Research (Washington D.C., USA) with a 6Li enriched electrolyte and a 6LiFePO4 cathode, which enhances the signal by an additional factor of 12. In order to achieve lithium plating already at a rate of 2 C, the LiPF6 salt concentration in the EC/EMC = 3/7 (w/w) electrolyte was reduced to 0.3 M.Figure 1 shows the NDP signal intensity as a function of the energy of the detected tritium particles, which translates into a depth profile: signals at an energy of ≈2400 keV represent lithium at the interface between the graphite electrode and the Cu current collector, and signals at decreasing energies represent lithium from increasing depth into the graphite electrode, with signals at an energy of ≈800 keV deriving from the graphite anode / separator interface. It is clearly visible that lithium plating starts towards the separator facing side of the graphite anode, caused by the lithium-ion concentration gradient forming over the thickness of the electrode during fast charging (dark blue profile); during the subsequent OCV phase, lithium re-distributes over the anode thickness, whereby this equilibration process is observed to be completed within ≈24 min.[1] C. Pillot, “The Rechargeable Battery Market and Main Trends 2014-2025”, 32nd International Battery Seminar and Exhibit, 3/9/2015, Presentation.[2] J. C. Burns, D. A. Stevens, J. R. Dahn, J. Electrochem. Soc. 2015, 162, A959.[3] Z. Y. Tang, J. J. Xue, C. Y. Liu, X. G. Zhuang, Acta Phys. - Chim. Sin. 2001, 17, 388.[4] M. Park, X. Zhang, M. Chung, G. B. Less, A. M. Sastry, J. Power Sources 2010, 195, 7904.[5] J. Wandt, P. Jakes, J. Granwehr, R. A. Eichel, H. A. Gasteiger, Mater. Today 2018, 21, 231.[6] R. Bhattacharyya, B. Key, H. Chen, A. S. Best, A. F. Hollenkamp, C. P. Grey, Nat. Mater. 2010, 9, 504.[7] M. Trunk, M. Wetjen, L. Werner, R. Gernhäuser, B. Märkisch, Z. Révay, H. A. Gasteiger, R. Gilles, Mater. Charact. 2018, 146, 127.[8] F. Linsenmann et al., A Lithium-Ion Battery Cell Design for Operando Neutron Depth Profiling, to be submitted. Figure 1

(Invited) Insight into Aqueous and Non-Aqueous Electrolyte Structure, Transport and Interfacial Properties from Molecular Modeling

Insight into Aqueous and Non-Aqueous Electrolyte Structure, Transport and Interfacial Properties from Molecular Modeling A molecular scale insight into ion transport and decomposition is important for understanding deficiencies of the currently used aqueous and non-aqueous electrolytes. In this presentation I will summarize progress made towards improving molecular scale understanding of the structure and electrochemistry for a wide range of aqueous and non-aqueous electrolytes. Modeling of non-aqueous electrolytes will focus on the competitive solvent and salt reduction at the passivated electrochemical interfaces using Born Oppenheimer Molecular Dynamics (BOMD) simulations using DFT functionals. These BOMD simulations included critical factors needed to realistically represent electrolyte reactivity at electrodes such as explicit description of the substrate – electrolyte interactions; accurate representation of electrolyte structure, ion pairing and aggregation near an electrode; and collection of sufficient statistics from multiple unique simulations that were initiated with differing initial configurations. For example, 20 BOMD simulations starting from different initial conditions using 2.0M LiPF6 in THF tetrahydrofuran/2-methyl tetrahydrofuran showed no solvent decomposition nor HF formation while only LiF formation was observed as a result of LiPF6 salt decomposition.1 The most frequently observed reduction events included a PF6 − coordinated to Li+ cations from the electrolyte and LiF surface that lead to anion defluorination and formation of 3LiF and PF3 gas. The solvent separated LiPF6 and did not actively participate in reduction. When surface defects in LiF were present near a high population of PF6 − the anions there was a preference for the LiPF6 reduction and repair the SEI without ether solvent decomposition. Interestingly, a number of fast diffusion events for F- from the electrolyte | LiF interface to the LiF-lithium metal interface was observed that would be expected to occur during Li stripping indicating that F- re-arrangement in the thin LiF passivation films should be also considered. 1 Electrolyte reduction at the passivated interfaces from these simulations will be contrasted with other solvents ranging from ethers with mixed salts or carbonates and results from the representative quantum chemistry (QC) calculations performed on the small model electrolyte clusters to estimate oxidation and reduction.In the second part of the presentation the non-reactive molecular dynamics (MD) using APPLE&P polarizable force field to examine bulk and interfacial properties of the lithium and zinc aqueous electrolytes and electrochemical interfaces with the focus on the transport mechanism and relative contribution to charge flux from the anions and cations.2 Combined together information from these modeling scales suggests numerous strategies for stabilizing the electrolyte – electrode interfaces for numerous aggressive high energy density cathodes combined coupled with graphite and metal anodes. References Chen, J.; Li, Q.; Pollard, T. P.; Fan, X.; Borodin, O.; Wang, C., Electrolyte design for Li metal-free Li batteries. Materials Today 2020.Borodin, O.; Self, J.; Persson, K. A.; Wang, C.; Xu, K., Uncharted Waters: Super-Concentrated Electrolytes. Joule 2020, 4 (1), 69-100.

Full Cell Understanding of Solvents in Battery Systems

Lithium ion batteries are complex systems with multiple components that can be optimized to obtain desired performance capability. These components include electrolyte salts, solvents, and additives, as well as electrode active materials and coatings. Rational optimization of battery components requires an understanding of the fundamental science driving the observed performance. Silatronix® has developed a suite of analytical tools and methods using a combination of full format batteries, lab R&D cells, and model systems to construct a complete understanding of the battery from the cell level performance to the fundamental chemistry. This process includes the recovery and analysis of all parts of a full format pouch cell, including liquid electrolyte, generated gas, and electrode surface layers (Fig. 1).Using this process, Silatronix® has evaluated several battery components, including organosilicon solvents for Li-ion electrolytes, to acquire a comprehensive picture of how these components provide observed performance benefits. For example, OS3®, a novel organosilicon chemistry that merges a silane with a Li+ coordinating nitrile functionality, has provided multiple performance benefits in various advanced Li-ion chemistries. Key experimental results include improved high temperature cycling stability in full format graphite/NMC622 pouch cells with 1%, 2%, or 3% OS3® (Fig. 2a), and using a model system of electrolyte in NMR tubes, we show that enhanced thermal stability of the bulk electrolyte results from HF scavenging and LiPF6 salt stabilization when 2-16% OS3® is present (Fig. 2b). Reduction of gas generation during high temperature testing in pouch cells with a commercial Si-based anode is also shown with 2% OS3® (Fig. 2c), and gas product identification and quantification is determined by GC-TCD for mechanistic analyses. In addition, OS materials change the reactivity of other electrolyte components, particularly at the electrolyte/electrode interfaces that govern critical performance attributes in Li-ion cells such as gas generation and interfacial impedance. These key performance benefits serve to enable several next generation battery chemistries, including a strong synergy with Si-based anodes. Figure 1

PERFORMANCE STUDY OF LiBOB/LiTFSI ELECTROLYTE SALT IN THE ALL-SOLID-STATE LITHIUM-ION BATTERY

PERFORMANCE STUDY OF LiBOB/LiTFSI ELECTROLYTE SALT IN THE ALL-SOLIDSTATE LITHIUM-ION BATTERY. Solid polymer electrolyte is developed mainly to provide safer lithiumion battery upon high temperature operation. In this research, we employ LiBOB and LiTFSI electrolyte salt in various concentration to replace commercially used LiPF6 salt. Solution cast method was performed to produce polymer electrolyte membrane. PVdF-HFP was chosen as polymer matrix due to high dielectric constant, and compatibility to wide array of electrode materials as well as electrolyte salts. Higher amount of electrolyte salts contributes to thicker membrane and hence higher current output of the lithium-ion battery half-cells. SEM, FT-IR spectroscopy, and cyclic voltammetry measurement was conducted to evaluate li-ion battery cell performance. Between the two electrolyte salts used in this experiment, LiTFSI salt exhibited better performance compared to LiBOB.

Design aspects of electrolytes for fast charge of Li‐ion batteries

AbstractThe electrolytes of Li‐ion batteries consist mainly of a LiPF6 salt dissolved in a carbonate‐based solvent mixture. Such electrolytes cannot support fast charge without detrimental impacts on performance and lifetime. Fast charge aggravates parasitic reactions of the electrolyte solvents and structural degradation of the lithium layered transition metal oxide cathode materials. This leads to not only the depletion of electrolyte solvents but also the loss of cyclable Li+ ions, accompanied by impedance growth and volumetric swelling of the battery. In this perspective, the design aspects of the electrolytes for fast charge of Li‐ion batteries are discussed and proposed.image

Fire-Preventing LiPF6 and Ethylene Carbonate-Based Organic Liquid Electrolyte System for Safer and Outperforming Lithium-Ion Batteries.

Battery safety is an ever-increasing significance to guarantee consumer's safety. Reducing or preventing the risk of battery fire and explosion is a must for battery manufacturers. Major reason for the occurrence of fire in commercial lithium-ion batteries is the flammability of conventional organic liquid electrolyte, which is typically composed of 1 M LiPF6 salt and ethylene carbonate (EC)-based organic solvents. Herein, we report the designed 1 M LiPF6 and EC-based nonflammable electrolyte including methyl(2,2,2-trifluoroethyl)carbonate, which breaks the conventional perception that EC-based liquid electrolyte is always flammable. The designed electrolyte also provides high anodic stability beyond the conventional charge cut-off voltage of 4.2 V. A graphite∥LiNi0.6Co0.2Mn0.2O2 lithium-ion full cell with our designed EC-based nonflammable electrolyte with a small fraction of vinylene carbonate additive under an aggressive condition of 4.5 V charge cut-off voltage, 0.5C rate, and 45 °C exhibits increased capacity, reduced interfacial resistance, and improved performance and rate capability. A basic understanding of how a high-voltage cathode-electrolyte interface and anode-electrolyte interface are stabilized and how failure modes are mitigated by fire-preventing electrolyte is discussed.

Highly Concentrated Electrolyte towards Enhanced Energy Density and Cycling Life of Dual‐Ion Battery

AbstractDual‐ion batteries (DIBs) have attracted much attention owing to their low cost, high voltage, and environmental friendliness. As the source of active ions during the charging/discharging process, the electrolyte plays a critical role in the performance of DIBs, including capacity, energy density, and cycling life. However, most used electrolyte systems based on the LiPF6 salt demonstrate unsatisfactory performance in DIBs. We have successfully developed a 7.5 mol kg−1 lithium bis(fluorosulfonyl)imide (LiFSI) in a carbonate electrolyte system. Compared with diluted electrolytes, this highly concentrated electrolyte exhibits several advantages: 1) enhanced intercalation capacity and cycling stability of the graphite cathode, 2) optimized structural stability of the Al anode, and 3) significantly increased battery energy density. A proof‐of‐concept DIB based on this concentrated electrolyte exhibits a discharge capacity of 94.0 mAh g−1 at 200 mA g−1 and 96.8 % capacity retention after 500 cycles. By counting both the electrode materials and electrolyte, the energy density of this DIB reaches up to ≈180 Wh kg−1, which is among the best performances of DIBs reported to date.

Enhanced electrochemical performance of lithia/Li2RuO3 cathode by adding tris(trimethylsilyl)borate as electrolyte additive

In this study, we used tris(trimethylsilyl)borate (TMSB) as an electrolyte additive and analysed its effect on the electrochemical performance of lithia-based (Lithia/Li2RuO3) cathodes. Our investigation revealed that the addition of TMSB modified the interfacial reactions between a lithia-based cathode and an electrolyte composed of the carbonate solvents and the LiPF6 salt. The decomposition of the LiPF6 salt and the formation of Li2CO3 and -CO2 was successfully reduced through the use of TMSB as an electrolyte additive. It is inferred that the protective layer derived from the TMSB suppressed the undesirable side reaction associated with the electrolyte and superoxides (O- or O0.5-) formed in the cathode structure during the charging process. This led to the reduction of superoxide loss through side reactions, which contributed to the increased available capacity of the Lithia/Li2RuO3 cathode with the addition of TMSB. The suppression of undesirable side reactions also decreased the thickness of the interfacial layer, reducing the impedance value of the cells and stabilising the cyclic performance of the lithia-based cathode. This confirmed that the addition of TMSB was an effective approach for the improvement of the electrochemical performance of cells containing lithia-based cathodes.

Electrochemical Reactivity and Passivation of Silicon Thin-Film Electrodes in Organic Carbonate Electrolytes.

This work focuses on the mechanisms of interfacial processes at the surface of amorphous silicon thin-film electrodes in organic carbonate electrolytes to unveil the origins of the inherent nonpassivating behavior of silicon anodes in Li-ion batteries. Attenuated total reflection Fourier-transform infrared spectroscopy, X-ray absorption spectroscopy, and infrared near-field scanning optical microscopy were used to investigate the formation, evolution, and chemical composition of the surface layer formed on Si upon cycling. We found that the chemical composition and thickness of the solid/electrolyte interphase (SEI) layer continuously change during the charging/discharging cycles. This SEI layer "breathing" effect is directly related to the formation of lithium ethylene dicarbonate (LiEDC) and LiPF6 salt decomposition products during silicon lithiation and their subsequent disappearance upon delithiation. The detected appearance and disappearance of LiEDC and LiPF6 decomposition compounds in the SEI layer are directly linked with the observed interfacial instability and poor passivating behavior of the silicon anode.

Electrochemical and Surface Chemistry Analysis of Lithium Lanthanum Zirconium Tantalum Oxide (LLZTO)/Liquid Electrolyte (LE) Interfaces

A stable solid electrolyte/liquid electrolyte (SE/LE) interface is essential to enable hybrid electrolytes, where the SE protects Li metal anodes and the LE improves SE/cathode interfacial kinetics. However, previous reports have shown that the promising SE Lithium Lanthanum Zirconium Tantalum Oxide (LLZTO) reacts with LiPF6-based state-of-the-art LEs. This work aims to identify the source of this instability and propose approaches to ameliorate it. This is achieved by studying and isolating the effects of organic solvents and Li salts present in the LE on LLZTO. The results indicate that the reactions with the typical solvents are not as detrimental as with the corresponding salts. For example, LiPF6 salt reacts with LLZTO to form LiF, LaF3 and ZrF4, which leads to an increased SE/LE interfacial resistance (Rinterface). The Rinterface after reacting with LiPF6 and LiBOB salts is ~120 and 2000 Ohms.cm2, respectively. However, LiTFSI salt is compatible with LLZTO with Rinterface ~55 Ohms.cm2. Furthermore, optimization of the LiTFSI salt concentration (3 M) results in an Rinterface of ~30 Ohms.cm2. We demonstrate preliminary cycling of Li/LLZTO/LE/NCA cells with ~100% utilization and 98.5% capacity retention over 10 cycles. Overall, this study provides a step towards the successful implementation of hybrid electrolytes in Li metal batteries.

The role of the pH value in water-based pastes on the processing and performance of Ni-rich LiNi0.5Mn0.3Co0.2O2 based positive electrodes

The influence of the preset pH value of the binder solution on aqueous processing of LiNi0.5Mn0.3Co0.2O2 positive electrodes is studied. The processed electrodes are investigated by means of electrochemical charge/discharge cycling, potentiostatic electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy. The results show major impact of the pH value of the binder solution on the cycling performance of lithium ion cells. Low binder solution pH values (here 7.6) lead to faster capacity fading within the first 100 cycles. In comparison, higher binder solution pH values (here 12.5) show similar fading compared to the reference electrodes based on poly(vinylidene difluoride) and N-methyl pyrrolidone as solvent. Positive electrode materials undergo a lithium-proton exchange, when in contact with water or other proton sources. Post-mortem analysis revealed that lower pH values of the binder solution potentially lead to more pronounced lithium-proton exchange. The previously inserted protons are extracted from the active material during electrochemical cycling, potentially initiate decomposition of the conductive salt LiPF6 and hence degrade the cell performance.In combination with LiPF6, inter alia HF might be formed, which then reacts with lithium from LiNi0.5Mn0.3Co0.2O2 or other active and inactive components and precipitate as M-F or other resistive interphases on the LiNi0.5Mn0.3Co0.2O2 surface.

Ultrafast ion transport at a cathode–electrolyte interface and its strong dependence on salt solvation

To access the full performance potential of advanced batteries, electrodes and electrolytes must be designed to facilitate ion transport at all applicable length scales. Here, we perform electrodynamic measurements on single electrode particles of ~6 nAh capacity, decouple bulk and interfacial transport from other pathways and show that Li intercalation into LiNi0.33Mn0.33Co0.33O2 (NMC333) is primarily impeded by interfacial kinetics when using a conventional LiPF6 salt. Electrolytes containing LiTFSI salt, with or without LiPF6, exhibit about 100-fold higher exchange current density under otherwise identical conditions. This anion group effect is explained using molecular dynamics simulations to identify preferred solvation structures, density functional theory calculations of their binding energies and Raman spectroscopy confirmation of solvation structure. We show that TFSI− preferentially solvates Li+ compared to PF6−, and yet its preferred solvation structures provide a lower Li+ binding energy, suggesting a lower desolvation energy consistent with ultrafast interfacial kinetics. Understanding Li transport is important in the development of fast-rate batteries. Here the authors uncover an ultrafast charge transfer across a cathode–electrolyte interface with the aid of single-particle measurements and ascribe the solvation of electrolyte salts to be key for the interfacial kinetics.

Role of Surface Acidity in the Surface Stabilization of the High-Voltage Cathode LiNi0.6Mn0.2Co0.2O2.

Metal oxide coatings have been reported to be an effective approach for stabilizing cathode interfaces, but the associated chemistry is unclear. In this work, thin films of TiO2, ZnO, and Cr2O3, which have different surface acidities/basicities, were used to modify the surface chemistry of LiNi0.6Mn0.2Co0.2O2 and study the acidity’s role in the cathode/electrolyte interphase composition and impedance under high-voltage cycling (4.5 V vs Li/Li+). Cathodes with more acidic surfaces provided higher initial specific capacity and capacity retention with cycling. More basic surfaces had higher initial impedance and greater impedance growth with cycling. These differences appeared to depend on the degree of LiPF6 salt decomposition at the interface, which was related to acidity, with more neutral surfaces having a LiF/LixPOyFz ratio close to unity, but basic surfaces had substantially more LiF. This chemistry was more significant than the cathode electrolyte interphase (CEI) thickness as the more acidic surfaces formed a thicker CEI than the basic surface, resulting in better capacity retention. These results suggest that the Brønsted acidity of cathodes directly influences electrolyte degradation, ion transport, and thus, cell lifetime.

Enhancement of Functional Properties of Liquid Electrolytes for Lithium‐Ion Batteries by Addition of Pyrrolidinium‐Based Ionic Liquids with Long Alkyl‐Chains

AbstractThree ionic liquid belonging to the N‐alkyl‐N‐methylpyrrolidinium bis(trifluoromethanesulfonyl) imides (Pyr1,nTFSI with n=4,5,8) have been added as co‐solvent to two commonly used electrolytes for Li‐ion cells: (a) 1 M lithium hexafluorophosphate (LiPF6) in a mixture of ethylene carbonate (EC) and linear like dimethyl carbonate (DMC) in 1 : 1 v/v and (b) 1 M lithium bis‐(trifluoromethanesulfonyl)imide (LiTFSI) in EC : DMC 1 : 1 v/v. These electrolyte formulations (classified as P and T series containing LiPF6 or LiTFSI salts, respectively) have been analyzed by comparing ionic conductivities, transport numbers, viscosities, electrochemical stability as well as vibrational properties. In the case of the Pyr1,5TFSI and Pyr1,8TFSI blended formulations, this is the first ever reported detailed study of their functional properties in Li‐ion cells electrolytes. Overall, P‐electrolytes demonstrate enhanced properties compared to the T‐ones. Among the various P electrolytes those containing Pyr1,4TFSI and Pyr1,5TFSI limit the accumulation of irreversible capacity upon cycling with satisfactory performance in lithium cells.

Parametrization with Explicit Water of Solvents Used in Lithium-Ion Batteries: Cyclic Carbonates and Linear Ethers.

Molecular dynamics simulations are performed to study carbonates and ethers that are widely used as electrolytes in energy storage devices. The first type contains in their molecular geometry a hydrocarbon tail of ethylene, propylene, and butylene whereas in the second type, the tail comprises 1,2-dimethoxyethane and 1,2-diethoxyethane. The evaluation of optimized potential for liquid simulations (OPLS), CHARMM, and GROMOS force fields for some of the solvents shows poor agreement with experimental thermodynamic and transport properties leading us to parameterize those solvents using the OPLS parameters as the starting point. A systematic procedure that uses the solubility of the solvents as the target property in simulations with explicit water is applied. The transferability of the parameters of the smallest cyclic or linear molecules was used to simulate systems with longer hydrocarbon chains. The optimized parameter reproduce the experimental solubility of butylene carbonate and 1,2-diethoxyethane in water. The interaction parameters were used to obtain the self-diffusion coefficients of ions of the salt LiPF6 at 1 M concentration in mixtures with ethylene carbonate or propylene carbonate. The simulation results for pure components and mixtures with the new parameters are in excellent agreement with the experimental data.

Lithium-electrolyte solvation and reaction in the electrolyte of a lithium ion battery: A ReaxFF reactive force field study.

In the electrode/electrolyte interface of a typical lithium-ion battery, a solid electrolyte interphase layer is formed as a result of electrolyte decomposition during the initial charge/discharge cycles. Electron leakage from the anode to the electrolyte reduces the Li+-ion and makes it more reactive, resulting in decomposition of the organic electrolyte. To study the Li-electrolyte solvation, solvent exchange, and subsequent solvent decomposition reactions at the anode/electrolyte interface, we have extended the existing ReaxFF reactive force field parameter sets to organic electrolyte species, such as ethylene carbonate, ethyl methyl carbonate, vinylene carbonate, and LiPF6 salt. Density Functional Theory (DFT) data describing Li-associated initiation reactions for the organic electrolytes and binding energies of Li-electrolyte solvation structures were generated and added to the existing ReaxFF training data, and subsequently, we trained the ReaxFF parameters with the aim of finding the optimal reproduction of the DFT data. In order to discern the characteristics of the Li neutral and cation, we have introduced a second Li parameter set to describe the Li+-ion. ReaxFF is trained for Li-neutral and Li+-cation to have similar solvation energies, but unlike the neutral Li, Li+ will not induce reactivity in the organic electrolyte. Solvent decomposition reactions are presumed to happen once Li+-ions are reduced to Li-atoms, which can be simulated using a Monte Carlo type atom modification within ReaxFF. This newly developed force field is capable of distinguishing between a Li-atom and a Li+-ion properly. Moreover, it is found that the solvent decomposition reaction barrier is a function of the number of ethylene carbonate molecules solvating the Li-atom.

High Concentration Electrolytes, or Almost Ionic Liquids, for Advanced Batteries

For most of the commercial life of lithium ion batteries, the electrolyte of choice has been LiPF6 salt in mixtures of organic carbonates, such as ethylene carbonate, propylene carbonate, ethyl carbonate, methyl carbonate and ethyl methyl carbonate1 . In the last several years, however, the electrolyte phase for lithium ion batteries has been given increased scrutiny. One productive area of research has been in the general area of high salt concentration in coordinating organic solvents2. In particular, when the mole ratio of electrolyte salt to coordinating solvent becomes less than 1 to 3 or 4, many changes occur in the solution structure. The amount of free (uncoordinated) solvent drops to undetectable levels, as measured, for example by Raman spectroscopy. This means that the "solvent" is completely coordinated to the cation and the solution now resembles an ionic liquid rather than a mixture of coordinated compact ion pairs and free ions in a medium of the excess free solvent molecules like the conventional 1 molar LiPF6 lithium ion battery electrolyte. The transition to the disappearance of free solvent usually occurs at around 3 to 5 molar salt concentration depending on the salt-solvent combination. It has been known for many years that such transitions occur at high concentrations and models have shown that the degree of association of anion and cation into ion pairs may drop many orders of magnitude3 . Thus, we view such solutions as ionic liquids where a quasi-lattice exists of cations surrounded by anions and anions by cations.Several important effects occur with these solutions. Since the solvent molecules are no longer freely available at the negative electrode interface, the composition of the solid electrolyte interphase (SEI) is found to be quite different from that of the conventional electrolyte. The passivation of the aluminum positive electrode, which in the conventional electrolyte depends upon the unique characteristics of LiPF6 is no longer restricted to the use of this salt. It has been found, for example, that lithium bis-(flurosulfonyl) imide salt - Li(FSO2)2N - in various solvents creates a viable SEI on the negative electrode as well as passivates the Al carrier at the positive electrode. It has also been found that coordinating solvents other than organic carbonates may be used and adequate SEI and Al passivation are observed depending on the salt-solvent combination.There are a couple of negative attributes of these nearly ionic liquid electrolytes, however. The solution viscosity is found to increase dramatically with salt concentration after the disappearance of free solvent. The conductivity is also found to decrease substantially after the transition as well. The use of solvents different from the carbonates, such as small molecule nitriles, gives sufficiently lower viscosity and higher conductivity that these solutions may be of use for lithium ion batteries even at high salt concentration.These aspects will be discussed in detail and some promising results for lithium ion batteries will be presented. In addition, the application of these concepts will be considered for lithium and other metal anode batteries.

Full Cell Understanding of Solvents in Battery Systems

Lithium ion batteries are complex systems with multiple components that can be optimized to obtain desired performance capability. These components include electrolyte salts, solvents, and additives, as well as electrode active materials and coatings. Rational optimization of battery components requires an understanding of the fundamental science driving the observed performance. Silatronix® has developed a suite of analytical tools and methods using a combination of full format batteries, lab R&D cells, and model systems to construct a complete understanding of the battery from the cell level performance to the fundamental chemistry. This process includes the recovery and analysis of all parts of a full format pouch cell, including liquid electrolyte, generated gas, and electrode surface layers (Fig. 1).Using this process, Silatronix® has evaluated several battery components, including organosilicon solvents for Li-ion electrolytes, to acquire a comprehensive picture of how these components provide observed performance benefits. For example, OS3®, a novel organosilicon chemistry that merges a silane with a Li+ coordinating nitrile functionality, has provided multiple performance benefits in various advanced Li-ion chemistries. Key experimental results include improved high temperature cycling stability in full format graphite/NMC622 pouch cells with 1%, 2%, or 3% OS3® (Fig. 2a), and using a model system of electrolyte in NMR tubes, we show that enhanced thermal stability of the bulk electrolyte results from HF scavenging and LiPF6 salt stabilization when 2-16% OS3® is present (Fig. 2b). Reduction of gas generation during high temperature testing in pouch cells with a commercial Si-based anode is also shown with 2% OS3® (Fig. 2c), and gas product identification and quantification is determined by GC-TCD for mechanistic analyses. In addition, OS materials change the reactivity of other electrolyte components, particularly at the electrolyte/electrode interfaces that govern critical performance attributes in Li-ion cells such as gas generation and interfacial impedance. These key performance benefits serve to enable several next generation battery chemistries, including a strong synergy with Si-based anodes. Figure 1

Novel Separator Prepared By Electrospinning for Lithium Metal Battery

With the ever-growing demand for high energy density, anode materials with even higher reactivities, such as Li metal, are being researched and drawn extensive attention owing to its high theoretic capacity (3,860 mAh∙g-1) and low electrode potential (−3.04 V versus standard hydrogen electrode). During the charging process of lithium metal batteries (LMBs), lithium will be electrodeposited on the anode current collector as metallic lithium, and be stripped back to the electrolyte in the discharge process. The reaction is purely lithium deposit/stripping, but several side reactions will deteriorate the reversibility, and the major issues include the incomplete dissolution of metallic lithium and formation of solid electrolyte interphase (SEI). After the discharge process, some dead lithium residue will remain on the anode current collector surface. These inactive substances will gradually accumulate after cycling, which will increase the internal resistance and block ions transportation. The dead lithium was composed of metallic Li within the inner layer, and wrapped by insulating SEI on the outer layer. The formation of SEI will consume lithium, and moreover, the metallic lithium embedded by non-conductive SEI can no longer be stripped back to the electrolyte, and both phenomenon will reduce the Coulombic efficiency. An important factor influencing the reversibility of Li is its electrodeposition morphology on the current collector. If the deposited Li patterns are needle-like and the porosity between each grain is large, the amount of metallic lithium embedded in insulating SEI after discharging will be enormous, and therefore, the consumption rate of usable lithium will be very fast. On the other hand, if lithium can be electrodeposited with large and granular grains in a densely-packed formation, the efficiency of lithium dissolution will be enhanced. The overpotential required for nucleation and growth of lithium during the electrodeposition step will affect the deposition pattern, and evidences from theoretical calculation and experiments have confirmed that lower overpotential can deliver better Li deposition morphology. An important component influencing the internal resistance is the separator. Normally, separators soaked with electrolyte will be placed in the middle of cathode and anode, and act as the agent to transport ions during the charge and discharge process. Most microporous membrane separators are made of polyethylene (PE), polypropylene (PP), or their combinations such as PE/PP and PP/PE/PP. For LMBs, if the electrolyte was conventional LiPF6 salt dissolved in the carbonate-based solvent, the Li deposition pattern with commercially available separator was mostly dendritic. To solve this issue, we proposed a novel type of separator synthesized by electrospinning strategy. The polyamic acid (PAA) precursor solution was used to fabricate polyimide (PI) separator. We compared the in-house synthesized separator with the commercial PP/PE/PP separator, and the results showed that both the volume of electrolyte uptake and ionic conductivity were improved. Also, when using electrospun PI as the separator, the overpotential required for Li nucleation in LMB was clearly lower than commercial one, and PI could ameliorate the lithium deposition morphology, leading to better Li reversibility. In this research, scanning electron microscopy (SEM) was employed to analyze the Li deposition morphology and dead Li after cycling. The battery performance with both types of the separator was evaluated by coin cell configuration, and the cycling ability of LMB was successfully promoted with the PI separator.