Tailoring Electrolyte for Lithium-Ion Batteries Operating at Low Temperature

High performance of low-temperature electrolyte for lithium-ion batteries using mixed additives

Considering the usage of smart phones, electrical vehicles, and power sources for grid storage application, lithium ion battery (LIB) operating under harsh circumstances have become a serious issue due to the sluggish ionic conductivity and increased viscosity of electrolyte at low temperatures. Ethylene carbonate (EC) is a widely used electrolyte component of commercial LIB owing to the suitable electrochemical stability window, high dielectric constant, and especially, its ability to form stable solid electrolyte interphase (SEI) on the graphite anode, which prevents continuous electrolyte decomposition and resulted in excellent cycling stability on graphite anode. However, its high freezing point (36℃), greatly increases electrolyte solidification temperature and hurts ionic conductivity at low temperatures, thereby leading to a narrow operation temperature range for LIB using EC-based electrolytes. Recently, plenty of efforts have been made to extend LIB operating temperature range, using techniques such as self-heating method with extra Ni-foil in specially designed cell configuration1, or electrolyte modifications by introducing low melting point solvents to increase ionic conductivity at low temperatures. Electrolyte modification is the most effective approach due to its low cost, high flexibility, and convenience. It can easily alter electrolyte physical and chemical properties, affect SEI chemical compositions, and result in positive impact on battery performance. Particularly, SEI is the interphase between anode and electrolyte, which allows Li ion transportation and insulates electron transfer, protects anode surface and prevents electrolyte decomposition. Many different research groups have made a lot of progress of electrolyte modification, including adjusting solvent salt combinations, fluorinating of solvents, and developing novel additives.2 Nevertheless, there are still many challenges need to be addressed for stable low temperature operation.Herein, considering the research importance and current progress of low temperature electrolyte, we screened several organic compounds as co-solvents or additives to improve low temperature performance while keeping other properties of current LIB. For example, propylene carbonate (PC) has comparable dielectric constant and electrochemical window as EC, and much lower freezing point (-49℃), making it very promising candidate for low temperature electrolyte.3 Unfortunately, PC cannot be used as single solvent for LIBs because it usually co-intercalates into graphite layer and cause graphite exfoliation. In order to prevent co-intercalation of PC, a stable SEI needs to be formed. We developed several new electrolytes using different salts, solvents and additives, which have higher reduction potential than PC intercalation voltage and enable stable SEI formation, making PC-based low temperature electrolyte application possible. In addition, advanced characterization techniques were used to characterize the interphase properties to reveal underlying mechanism of improved electrochemical performances by modifying electrolyte composition. These results will be presented at meeting.Reference C.-Y. Wang, Nature, 2016, 529, 515.G. Zhu, Journal of Power Sources, 2015, 300, 29-40.Q. Li, Applied Materials & Interfaces, 2017, 9, 42761-42768. AcknowledgementThe work carried out at Brookhaven National Laboratory was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Vehicle Technology Office of the US Department of Energy through the Applied Battery Research for Transportation (ABRT) program.

The lithium-ion battery (LIB) is a key technology for inexpensive, renewable, and safe energy storage system, but there are a few challenges that need to be addressed to make LIB more efficient. One critical challenge is increasing both energy density and fast charging capability. Increasing electrode thickness can increase the energy density of lithium-ion batteries. However, increasing electrode thickness increases transport limitations and the risk of lithium plating, which limit safe fast charging capability. This work analyzes prospective improvements to the conventional lithium-ion cell electrode structure that may facilitate high energy density and fast charging capabilities. A 2D lithium-ion battery model is applied to five different cell designs to understand the impact of thick electrode structure on performance at different charge and discharge rates for a single cell. These five different cell designs consist of one conventional cell and four cells with electrodes modified by varied electrolyte channel arrangements. The simulated electrodes were taken as Li(Ni1/3Mn1/3Co1/3)O2 (NMC) for the cathode and graphite for the anode. The simulated electrolyte was 1 M lithium hexafluorophosphate (LiPF6) in 1:1 ethylene carbonate (EC) and diethylene carbonate (DEC). All five cells were simulated under discharge at C/10, C/2 and 1C followed by charge at 1C, 3C, and 5C with no rest time prior to charge. Adding electrolyte channels on the anode side was found to facilitate lithium ion transport and reduce the risk of lithium plating. Modification of both electrodes did not present any benefit compared to the conventional cell. Analysis of cell behavior based on mass transport Fourier number and a dimensionless mass flux parameter was performed to compare different electrodes at different charge and discharge conditions. These dimensionless parameters provide a means for the general assessment of prospective electrode structure modifications.

Performance Testing of Lithion 8-Cell, 25 AHr Lithium-Ion Batteries for Future Aerospace Applications

Lithium ion batteries containing lithium titanate (LTO), Li4Ti5O12 are promising energy storage systems for their high rate capabilities, safety, and long cycle-life. However, gas generation and associated swelling, which can be observed under high-temperature operation, present a challenge to the large-scale application of lithium ion batteries made from LTO anodes. Research has suggested various causes for gas generation, including residual moisture in the electrodes and electrolyte [1] and catalysis involving the Ti3+/Ti4+ redox couple combined with water electrolysis.[2,3] It is important to understand the source of gas generation so as to avoid it with proper manufacturing conditions, such as tight control over moisture levels. However, at this time the mechanism of gas evolution from LTO anode based lithium ion battery systems is not well established. Here we analyzed LTO cells following high temperature (80ºC) calendar life storage at 100% SOC. We explored the relationship between volume expansion and impedance increase following high temperature storage. The results in Figure 1 indicate that with increasing gas volume expansion, the performance of the cells degrades. The gas species and quantities resulting from the cell volume expansion were collected and analyzed post-mortem using quadrupole mass spectrometry (MS) and combined gas chromatography / mass spectrometry (GC/MS). With this method, we introduced varying levels of water content into the cathode and anode, as well as the electrolyte. To facilitate determination of the origin of gas generation, we utilized deuterated water (D2O) introduced into the electrodes and electrolyte as well as 5% deuterated dimethyl carbonate (DMC) and ethylene carbonate (EC) based electrolytes. Figure 2 illustrates a comparison of gas compositions obtained from cells with 1600ppm D2O added to the electrolyte and 5wt% deuterated DMC in the electrolyte. The preliminary results illustrate that deuterated DMC in the electrolyte shows negligible breakdown to yield D2 based gas when compared to samples that contain heavy water (D2O) added into the electrolyte. This indicates that the primary source of gas generation in LTO based cells originates from residual moisture in the electrodes and electrolyte. This result reinforces the importance of maintaining low-moisture processing conditions for LTO-based lithium ion batteries. (1) Belharouak, I.; Koenig, G. M.; Tan, T.; Yumoto, H.; Ota, N.; Amine, K. Journal of the Electrochemical Society 2012, 159, A1165. (2) He, Y.-B.; Li, B.; Liu, M.; Zhang, C.; Lv, W.; Yang, C.; Li, J.; Du, H.; Zhang, B.; Yang, Q.-H.; Kim, J.-K.; Kang, F. Sci. Rep. 2012, 2. (3) Bernhard, R.; Meini, S.; Gasteiger, H. A. Journal of the Electrochemical Society 2014, 161, A497. Figure 1

Currently, electrolytes used in lithium-ion batteries are flammable and leaky, causing safety problems when used under high temperatures or extreme conditions. Solid electrolytes can fundamentally avoid the occurrence of such issues, at the same time, it can significantly increase the energy density of solid state lithium batteries. Solid electrolytes are classified into inorganic solid electrolytes and solid polymer electrolytes. Although inorganic solid electrolytes have the high mechanical strength and room temperature ionic conductivity, it has poor flexibility and complicated preparation process. In contrast, solid polymer electrolyte has good elasticity and good interface contact, which can overcome many disadvantages of the inorganic solid electrolyte. Among them, polyethylene oxide (PEO) is first proposed as a polymer electrolyte. Due to its excellent mechanical properties, electrochemical stability and thermal stability, it has been a hot topic for polymer solid electrolytes for decades. However, its low room temperature ionic conductivity limits practical application. Given the problems existing in PEO-based polymer electrolytes, several methods for improving ionic conductivity have been proposed. The primary methods include plasticization, the organic-inorganic composite, and polymer blending. Comparing these methods, it can be seen that adding plasticizers to the polymer can effectively increase the conductivity of the electrolyte, but at the same time it will cause a decrease in the mechanical properties, and the safety issues limit its application in lithium-ion batteries. Inorganic fillers can balance the problems of ionic conductivity and mechanical properties, but the construction of a good filler-polymer dispersion system is complicated in preparing such composite electrolytes, design a new type of multi-dimensional the ion transport network also face tremendous challenges. Undoubtedly, in the future research, inorganic fillers will be the dominant means in the modification of composite electrolytes. Compared with the former two modification methods, polymer blending is easy to operate and friendly to industry, but low ionic conductivity at room temperature still exist. To give full play to the advantages of polymer blending, using it as an auxiliary means to improve the performance of composite electrolytes will be the primary development direction of such modification methods. PEO is the most widely studied polymer electrolyte. Even though a large number of modification methods have emerged in its development history, the ionic conductivity at room temperature is still limited to a relatively low level. To meet the needs of the application in lithium-ion batteries, a high operating temperature (50−70℃) is still needed, the biggest problem remains exist. To solve the problems in polymer solid electrolytes, the research directions in the future may mainly focus on: Expanding the modification method of PEO to other polymer electrolyte host materials, broadening the scope of research, and finding the available polymer electrolytes at room temperature. Concerned about the preparation of ultra-thin polymer electrolytes, reducing the ion transmission path to increase the transmission efficiency of lithium ions between positive and negative electrodes. Effectively combines inorganic solid electrolytes and solid polymer electrolytes to give full play to their advantages. In addition, exploring the effect of inorganic filler morphology on the conductivity of the composite electrolyte will also deserve attention. Many one-dimensional, two-dimensional materials can build ion transport networks so that lithium ions can travel rapidly along the path. With the deepening of research, it is expected that PEO-based polymer electrolytes may replace liquid electrolytes and become an essential cornerstone for building the next generation of safe and reliable lithium-ion batteries.

Laminated Lithium Ion Batteries with improved fast charging capability

Subzero temperature promotes stable lithium storage in SnO2

Study of electrochemical performances of lithium titanium oxide–coated LiFePO4/C cathode composite at low and high temperatures

Novel mobile and stationary applications require batteries with increasing energy density / specific energy, fast-charging capability,1 and long life-time – while maintaining a high safety level. However, there is a dearth of knowledge on the interactions between these parameters. Additionally, the data sheets of commercial cells contain usually only a limited set of parameters, such as a wide temperature range and only one maximum charging C-rate. Therefore, it is highly interesting to correlate electrochemical results with material and electrode parameters. In the present study, we investigated nine types of commercial Li-ion cells (including 18650 and 21700 cells) as well as cells built in our labs. The commercial cells were firstly subject to tests showing their suitability for further experiments. Our experiments reveal interesting specific influences and general trends of materials, electrode and cell properties on specific energy, fast-charging capability, cell resistance, and aging effects. Especially, Li deposition which becomes thermodynamically favorable at negative anode potentials vs. Li/Li+ is to be avoided due to its negative impact on safety and aging.2 In order to judge the fast-charging capability we defined three criteria which have to fulfilled: Charged capacity after fast-charging in comparison to slow charging at 0.1C (>80%)Temperature rise on cell surface (<60°C)Occurrence of Li deposition, estimated from voltage relaxation curves.3,4 Since voltage relaxation curves give only hints on Li deposition while averaging over the whole anode of the cell, it is compared with Post-Mortem analysis,5 measurements of the anode potential vs. Li/Li+ in 3-electrode full cells6 for exemplary cells, and with neutron diffraction data.4 The determination of fast-charging capability is applied to nine types of cylindrical cells for different ambient temperatures (0-40°C) and charging C-rates (0.1-3C), resulting in large data sets for further evaluation. Passing/failing of the three criteria as a function of C-rate and ambient temperature is visualized in fast-charging capability maps (Figure 1c,e). Although the tested commercial cells are from different manufacturers and contain different cathode (NMCs, LFP, NCA, NMC/LMO blend) and anode materials (graphite with different amounts of Si <5%) it is possible to find interesting general trends. For example there is an interaction between the fulfillment of the three fast-charging criteria (Figure 1a) corresponding to either high-energy or high-power cells (Figure 1b,d). In contrast, the change from the 18650 to the 21700 format shows only minor effects on specific energy7 and fast-charging capability. However, the cell format has a significant effect on heating behavior, energy content per cell (~50% more energy in 21700 cells with same electrodes), and therefore most likely a positive impact on costs per produced Wh.7 The study reveals specific influences and general trends in Li-ion cells and gives a perspective for necessary material and electrode improvements to fulfill the requirements for future battery generations.

With the stone energy increasingly dried up and the environment polluted severely, developing renewable clean energy is already in extreme urgency. Exploiting new energy storage and transformation systems has progressively become the focal point in the energy research field. Covalent organic frameworks (COFs) have attracted extensive attention as a new kind of crosslinked polymers owing to the high crystallinity, excellent porosity, and favorable stability. The last decade has witnessed the great progress in crystalline COFs for the application in various arenas. The tailor‐made functional skeleton together with well‐defined periodical alignment has endowed COFs with enormous potential in lithium batteries. In this review, we initially illustrated the design principle of COFs for the application in lithium batteries. Furthermore, we made a comprehensive summary of the fast‐developing COFs field in terms of lithium batteries, including lithium ion and lithium sulfur batteries. Finally, we discussed the remaining challenges and perspectives in this area and also proposed several possible future directions of development for lithium batteries. It is expected that this short review would contribute to the development of COFs materials in energy‐related applications.

Since the commercialization of lithium-ion batteries in 1990, the applications of lithium-ion batteries have spread in a speed faster than any other battery systems. Pouch lithium-ion cells are becoming popular since the year of 2000. In the beginning, the Bellcore type design and process were used and troubled by internal short circuit and other problems. Afterwards, regular winding designs were used for most of pouch cells for 3C applications. Latest, the stacking design and process were introduced to the pouch cells for the high power applications, especially, the EV applications.Amita technologies is one of the companies, who introduced the stacking pouch lithium-ion cells into the power applications in the year of 2000. In order to make thinner and safer lithium-ion stacking power cells, quite a few processes are different from the winding processes. Since the new process manufacturing equipments were not available in the year of 2000, Amita not only need to verify the cell design and process also need to design her own manufacturing equipment. From electrode cutting machine, stacking machine, welding machine, electrolyte injection machine, to the conveyor formation system. The development of an automation manufacturing system was completed in 2010. In 2014, the 2nd generation automation system is being designed and will be set up with a higher efficiency and a better consistency in 2015.Because of the development of EV and renewable energy, it is the interest of many countries to set up its own lithium-ion battery industry. Amita has developed an unique manufacturing model (PDCA, post dry-cell assembly) , by utilizing her own patented design and processes, through which dry cells can be easily made and stored and shipped. PDCA can greatly reduce the difficulties of setting up a lithium-ion battery factory. Since all the materials except the electrolyte have been included in the dry cells, not only the dry cells can be easily made into real cells in the customer side, also the dry cells are perfectly safe to ship and store. Plus, the investment of PDCA becomes less than one fifth of the total production line, and has many other advantages. Unique features of PDCA will be illustrated and explained in details during the presentation.

Herein, we present the use of lithium tetrafluoroborate (LiBF4) as an electrolyte salt for wide-temperature electrolytes in lithium-ion batteries. The research focused on the application of blend salts to exhibit their synergistic effect especially in a wide temperature range. In the study, LiCoO2 was employed as the cathode material; LiBF4 and lithium difluoro(oxalate)borate (LiODFB) were added to an electrolyte consisting of ethylene carbonate (EC), propylene carbonate (PC), and ethyl methyl carbonate (EMC). The electrochemical performance of the resulting electrolyte was evaluated through various analytical techniques. Analysis of the electrical conductivity showed the relationship among solution conductivity, the electrolyte composition, and temperature. Cyclic voltammetry (CV), charge-discharge cycling, and AC impedance measurements were used to investigate the capacity and cycling stability of the LiCoO2 cathode in different electrolyte systems and at different temperatures. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were applied to analyze the surface properties of the LiCoO2 cathode after cycling. The results indicated that the addition of a small amount of LiODFB into the LiBF4-based electrolyte system (LiBF4/LiODFB of 8:2) may enhance the electrochemical performance of the LiCoO2 cell over a relatively wide temperature range and improve the cyclability of the LiCoO2 cell at 60 °C.

The development of new materials for secondary batteries is nowadays one of the most active fields in the international scientific panorama [1].The state of the art for liquid electrolytes in a lithium-ion cell is typically a mixture of organic carbonates such as ethylene carbonate (EC) or dimethyl carbonate (DMC). The mixture ratio varies depending upon the desired cell properties. These solvents contain solvated lithium ions. The latter are provided by lithium salts, most commonly lithium hexafluorophosphate (LiPF6)[2]. Despite the high conductivity of these electrolytes (s >10-3 S·cm-1 at RT), important drawbacks are associated with the volatility and flammability of liquid solvents and the chemical instability of the LiPF6 salt. The latter undergoes hydrolysis in the presence of water traces, releasing fluoride anions. [3]Ionic liquids (ILs) are salts with melting temperatures lower than 100°C. When they are liquid at room temperature, ILs are classified as room temperature ILs (RTILs). Some typical properties of ILs are: a) a low volatility, a high thermal stability and a negligible flammability; b) a high ion density and conductivity; c) a wide electrochemical stability window; and d) a simple synthesis, carried out by choosing the cation and anion which best comply with the requirements of the intended application. These features make ILs very suitable as solvents in electrolytes for Li batteries[4].Recently, a class of solid-state single-ion conducting materials based on lithiated fluorinated-TiO2 (LiFT) was proposed, which demonstrated its applicability as a nanofiller to obtain nanocomposite polymer electrolytes [5,6]. LiFT consists of fluorinated anatase nanoparticles that are directly surface-functionalized with Li+ through an innovative one-step reaction with molten metallic lithium[7]. The conductivity of LiFT is due to Li+hopping processes between coordination sites present at the nanoparticle grain boundaries. These latter events take place in a very effective way.The use of LiFT nanoparticles as the source of Li+ ions and ILs as the plasticizing agents allowed us to obtain innovative composite electrolytes for application in lithium batteries with a high thermal stability and conductivity. Indeed, the use of imidazolium ILs based on TFSI- and BF4 - anions allowed us to investigate the different ability of these anions to coordinate and dissociate the Li+cations present on the surface of LiFT nanoparticles. For these reasons it has to be highlighted that no conventional Li salts are used in the electrolytes here proposed.In this report, composite electrolytes based on LiFT nanopowder, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMImTFSI) ILs are proposed. In details, LiFT nanopowder is doped with a known amount of both ILs to obtain two types of electrolytes with formula LiFT/EMImTFSI0.118 and LiFT/EMImBF4 0.200. The resulting electrolytes contain around 25% wt of ILs and show a powder-like consistence.The correlation between structure, thermal properties and conductivity mechanisms of the resulting LiFT/EMImTFSI0.118 and LiFT/EMImBF4 0.200electrolytes is investigated by a variety of techniques: (a) FT-MIR and FIR at different temperatures; (b) Differential Scanning Calorimetry (DSC); (c) Thermogravimetric Analysis (TGA); and (d) Broadband Electrical Spectroscopy (BES).The materials are thermally stable up to 250°C and their conductivities at 30 and 130°C are, respectively, of 1.2x10-2 cm-1 and 4.1x10-2 Scm-1 for LiFT/EMImTFSI0.118 and 1.7x10-3 Scm-1 and 1.7x10-2 Scm-1, for LiFT/EMImBF4 0.200. Finally, the performance of the proposed materials, tested in operating CR2032 coin cells by galvanostatic cycling in a Li4Ti5O12/{LiFT/EMImTFSI0.118}/LiCoO2configuration is shown in Figure 1. Figure 1 Discharge profiles (inset) and capacity vs. cycle number for a Li4Ti5O12/ {LiFT/EMImTFSI0.118}/LiCoO2, CR2032, coin cell (discharge plotted as negative current).

The development of lithium-ion batteries with simplified assembling steps and fast charge capability is crucial for current battery applications. In this study, we propose a simple in-situ strategy for the construction of high-dispersive cobalt oxide (CoO) nanoneedle arrays, which grow vertically on a copper foam substrate. It is demonstrated that this nanoneedle CoO electrodes provide abundant electrochemical surface area. The resulting CoO arrays directly act as binder-free anodes in lithium-ion batteries with the copper foam functioning as the current collector. The highly-dispersed feature of the nanoneedle arrays enhances the effectiveness of active materials, leading to outstanding rate capability and superior long-term cycling stability. These impressive electrochemical properties are attributed to the highly-dispersed self-standing nanoarrays, the advantages of binder-free constituent, and the high exposed surface area of the copper foam substrate compared to copper foil, which enrich active surface area and facilitate charge transfer. The proposed approach to prepare binder-free lithium-ion battery anodes streamlines the electrode fabrication steps and holds significant promise for the future development of the battery industry.