1. Field of the Invention
Embodiments of the invention relate to electrolytes for use with lithium-sulfur batteries.
2. Description of the Related Art
Many consider rechargeable lithium-ion batteries to be favorable energy storage devices for both existing and future upcoming hybrid electric-vehicles (HEV) and full electric vehicles (EV). Unfortunately, many lithium-ion batteries are unsatisfactory for one or more of a number of reasons. For example, they may lack a desired high capacity, or they may lack a long cycle lifetime. In many cases these drawbacks are the result of use of a cathode that is inadequate for the task of high capacity, long cycle duty. Common cathode materials include cobalt oxide, manganese oxide, mixed oxides with nickel, iron phosphate, and vanadium oxide.
After decades of intensive development, lithium ion (Li-ion) batteries are still incapable of meeting the energy density requirements of emerging applications such as electric vehicles. The exploration of new electrochemistry and new materials is thus necessary for the creation of high-energy battery systems. The rechargeable lithium-sulfur (Li—S) battery is a promising candidate because sulfur has a high theoretical specific capacity of 1675 mAh/g and a high specific energy of 2600 Wh/kg.
Many solutions have been proposed for increasing the conductivity of the sulfur. Typically these solutions involve incorporating the sulfur into cathodes in conjunction with carbon or a conducting polymer. Unfortunately, neither the carbon nor the conducting polymer, taken alone, is able to ameliorate the polysulfide shuttle effect.
Extensive attempts have been devoted to improving the electrochemical performance of sulfur electrodes. These include attempts at electrolyte modification, use of additives, and anode protection. Recently, considerable attention has focused on immobilizing the polysulfides within the cathode by addition of metal oxides, such as Mg0.6Ni0.4O, V2O5, SiO2, and Al2O3. Performance of the sulfur cathodes obtained in these attempts was largely suboptimal because the approaches relied on simple, inhomogeneous mixtures of metal oxides and sulfur.
The Li—S system operates by conversion of sulfur through a multistep redox reaction, forming different lithium sulfide products (LisSx, 1≦x≦8). Ether-based electrolytes are normally used in Li—S batteries because of their ability to dissolve insulating polysulfides and thus improve their reaction kinetics. However, this dissolution can also lead to loss of active material from the cathode, causing capacity fading, and to a shuttle phenomenon that leads to poor coulombic efficiency. The formation of insoluble, insulating Li2S on the surface of both the cathode and the lithium anode also contributes to poor sulfur utilization and capacity fading because of its poor reversibility.
Considerable effort has been devoted to engineering carbon/sulfur (C/S) composites that are capable of trapping soluble polysulfides by physical or chemical adsorption or of enabling the reversible reaction of Li2S at the positive electrode. Electrolyte additives, e.g. LiNO3 and P2S5, were reported to passivate lithium metal and suppress the redox shuttle of polysulfides, resulting in unproved coulombic efficiency. P2S5 was also reported to promote the dissolution and reversible reaction of Li2S. Nevertheless, none of these approaches are sufficient to fully address the dissolution of polysulfides and the accumulation of Li2S.
Since the dissolution of polysulfides is inevitable, Li—S liquid batteries that directly use dissolved polysulfides as a catholyte, as reported decades ago, have been re-considered recently; their capacity and cyclability are still not satisfactory. Alternately, increasing sulfur loading in the cathode might be expected to increase the cell capacity and mitigate the effect of losing active mass to dissolution; however, even lower sulfur utilization and faster capacity fading have usually been reported, possibly due to the poorer conductivity and formation of more insoluble products in the cathode.
Embodiments presented herein provide a new approach for a high-performance lithium-sulfur battery by combining conventional C/S cathode with liquid electrolyte containing dissolved electrochemically active material. The dissolved electrochemically active material includes sulfur in the form of at least one of a soluble lithium polysulfide and/or an organodisulfide compound or compounds having the formula RSSR′, where R and R′ are the same or different, and where they may be C1-C6 alkyl, straight or branched (for example, dimethyl disulfide (DMDS), diethyl disulfide (DEDS), dipropyl disulfide (DPDS), and isopropyl disulfide (IPDS)). Through use of a sufficient concentration of an active sulfur species and amount of electrolyte, the unfavorable formation of insoluble Li2S is avoided and the capacity, cyclability, and rate capability of the cell are drastically improved.
Embodiments of the invention provide a liquid electrolyte for a lithium-sulfur battery. Electrolytes of the invention include a protecting additive; a lithium salt in addition to the protective additive; at least one electrolyte solvent; and a dissolved electrochemically active material comprising sulfur. The protective additive may be, for example, LiNO3, P2S5, or fluorinated ether. Use of LiNO3 is reported, for example, in U.S. Pat. No. 7,553,590, which is incorporated by reference herein. Use of P2S5 as a protectingadditive is reported, for example, in Lin, Z., et al., “Phosphorous Pentasulfide as a Novell Additive for High-Performance Lithium-Sulfur Batteries,” Adv. Func. Mat. 2013: 23(8), 1064-69, and fluorinated ether is reported in “Improved Performance of Lithium-Sulfur Batter with Fluorinated Electrolyte,” Electrochem. Comm., December 2013: 37, 96-99, both of which are incorporated by reference herein. The dissolved electrochemically active material is in the form of at least one of a soluble lithium polysulfide (Li2Sx) with between about 1 to 10 molar sulfur atoms (i.e. Li2S, Li2S2, Li2S3, Li2S4, Li2S5, Li2S6, Li2S7, Li2S8, Li2S9, Li2S10, or mixtures of those) and/or about one or more organodisulfide compounds (for example, dimethyl disulfide (DMDS), diethyl disulfide (DEDS), dipropyl disulfide (DPDS), and isopropyl disulfide (IPDS)). These ingredients are discussed in more detail below. Various embodiments may comprise, consist of, or consist essentially of these components.
A. Protecting Additive
Embodiments of the invention typically include a protecting additive. This additive tends to increase cycling stability and coulombic efficiency in the electrolyte. In typical embodiments the protecting additive is present in the electrolyte in a concentration between 0.1 and 1M, 0.1 and 0.5 M, and 0.5 and 1 M. Ideally the protective additive will be at least 99.999% pure prior to addition to the electrolyte, though a certain level of purity is not required unless otherwise stated in the claims. The protective additive may be, for example, LiNO3, P2S5, or fluorinated ether.
B. Lithium Salt
Embodiments of the invention will include at least one lithium salt in addition to the LiNO3. Suitable lithium salts include, for example, but are not limited to, LiPF6, LiBF4, LiAsF6, LiClO4, LiCF3SO3, and LiN(CF3SO2)2 (also referred to as “LiTFSI”). These lithium salts dissolve in the electrolyte and help form a charge transfer medium.
These lithium salts are typically included in the electrolyte, either alone or in combination, in concentrations between 0.1-2 M. Other embodiments include one or more lithium salts in concentrations between 0.1-1 M, 0.1-0.5 M, 0.5-2 M, 1-2 M, or 1.5-2 M.
C. Solvent
Electrolytes of the invention further include one or more nonaqueous solvents. These solvents may be ethers (both cyclic and/or acyclic), sulfones (for example, ethyl methyl sulfone), or combinations of those. Suitable solvents include, for example, but are not limited to dioxolane, dimethoxyethane, and combinations of those. When in combination the solvents may be included in a ratio of 1:1, 1:2, 2:1, or other amounts. Preferably the solvent will include less than 20 ppm water.
D. Dissolved Electrochemically Active Material Including Sulfur
Embodiments of the invention include at least one dissolved electrochemically active sulfur material that is dissolved in the electrolyte. The material may be, for example, a soluble lithium polysulfide (Li2Sx). The lithium polysulfide may have between about 1 to 10 molar sulfur atoms (i.e. Li2S, Li2S2, Li2S3, Li2S4, Li2S5, Li2S6, Li2S7, Li2S8, Li2S9, Li2S10, or mixtures of those).
In other embodiments the material is one or more organodisulfide compound. Preferably the organodisulfide is a liquid at room temperature. For example, suitable compounds include dimethyl disulfide (DMDS) and diethyl disulfide (DEDS). Suitable organodisulfides have the formula RSSR′, where R and R′ may be the same or different, and where they may be C1-C6 alkyl, straight or branched. By “C1-C6 alkyl” it is meant a hydrocarbon including one to six carbons, saturated or unsaturated, along with a sufficient number of hydrogen sufficient to render the moiety neutral when attached to a sulfur.
Although not wishing to be bound by theory, the applicant believes that in the alkyl disulfide-based electrolyte, sulfur reacts chemically with RSSR′ to form alkyl polysulfide RSxR′, where x is between 3-17. (mainly RS3R) intermediates, which then could receive 2 e− and be reversibly reduced to alkyl thiolate (RS−) and disulfide anion (RSS−) during discharge process without the unfavorable formation of lithium polysulfide and insoluble Li2S2/Li2S.
The dissolved electrochemically active sulfur material is typically present in an amount in the form of at least one of a soluble lithium polysulfide (Li2Sx) with between about 0.1 to 4 mole/L of sulfur atoms. For RSSR′, with a weight percentage of from 1.0% to 50%, the concentration is between about 0.2 to 1M.
E. Preparation of Electrolyte
Electrolytes may be prepared by dissolving the protecting additive and additional lithium salt(s) in a solvent. Lithium sulfide and stoichiometic amount of sulfur and/or organodisulfides are then added until the desired concentration of each is reached. The solution is stirred, then allowed to sit until the reaction that creates the electrode has run to completion or near-completion. The time and temperature used for the mixture depend on the concentration of polysulfide. If the concentration is low, it can dissolve and react completely in several minutes. If the concentration is high, one may need to heat to about 75° C. and stir for several hours in a glovebox. Mixture of organodisulfides is not accomplished due to their imiscibility in the electrolyte.
In the examples given below, a reference electrolyte referred to as a “non-sulfur containing” electrolyte including 0.1 M LiTFSI+0.2 M LiNO3 in DOL/DME (1:1, v:v) is prepared by dissolving required amounts. The required amount will depend on the amount of electrolyte that is being prepared; for example, preparing 1 L of electrolyte would use 0.1M LiTFSI and 0.2M LiNO3 in DOL/DME (1:1, v:v). To form polysulfide-containing electrolytes of the embodiments of the invention, stoichiometric amounts of elemental sulfur and Li2S are added to form polysulfide-containing electrolytes of different sulfur concentration ([S]) and different average polysulfide chain length. When adding stoichiometric amounts of elemental sulfur Li2S, one can calculate the xin Li2Sx, which is the average chain length. In the electrolyte there are typically a mixture of different Li2Sx.
The solution is stirred for 6 hours at 75° C. followed by 48 hours at room temperature to complete the reaction and dissolution and form dark red polysulfide-containing electrolytes of moderate viscosity. Less stirring and lower heat may also suffice. Organodisulfides for the examples discussed herein were purchased from Sigma and directly added as additives in the reference electrolyte to form organodisulfide-containing electrolytes.
Embodiments of the invention are better understood by characterization of their abilities and comparison with various electrolytes known in the prior art. To that end a number of examples are presented below.
A. Preparation of Sulfur Cathodes
Embodiments of the invention do not require sulfur cathodes prepared by a particular method. However, for illustrative purposes only, and to explain how cathodes are prepared for the following examples, the following method is offered. Sulfur cathodes are prepared by ball milling 50 wt % elemental sulfur, 40 wt % Super P carbon black, and 10 wt % PVDF binder in NMP solution at 300 rpm for 3 hours to make a slurry, followed by spreading the slurry on aluminium foil using a common doctor-blade coating method. After drying at 55° C. under vacuum overnight, the electrodes are cut into circular pieces of 1.13 cm2 (12 mm diameter) with sulfur loading of about 0.6 mg cm-2 and incorporated into CR-2016 coin-type cells with a precisely controlled amount of either reference or sulfur species-containing electrolyte. All the electrolyte preparation and cell assembly steps are performed in an Ar-filled glove box with O2 and H2O less than 1 ppm. Of course, those of skill in the art will recognize that additional methods may be used.
B. Experimental Conditions
The coin-type cells can be galvanostatically cycled on battery testing systems (Neware BTS-5V1 mA or Arbin BT-2000) under room temperature. In one instance, the cutoff potentials for charge and discharge were set at 2.6 V and 1.6 V vs. Li+/Li, respectively, and cyclic voltammetry (CV) scanning was carried out on a CHI660 system using coin-type cells and with a scanning rate of 0.1 mV s−1.
C. Representative Battery Cell
D. Electrolyte Preparation
In one embodiment, polysulfide-containing electrolytes with the desired sulfur concentration ([S]) and average polysulfide chain length are prepared by chemically reacting stoichiometric amounts of sulfur and Li2S in a polysulfide-free electrolyte of 0.1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)+0.2 M LiNO3 in dioxolane (DOL)/dimethoxyethane (DME) (1:1, v:v). One or more nonaqueous solvents may be selected from the group consisting of acyclic ethers, cyclic ethers, and sulfones can also be used as electrolyte solvents.
Electrolytes of embodiments of the invention may also be made, for example, by in-situ reaction of stoichimetric amounts of sulfur and lithium metal in the polysulfide-free electrolyte of the preceding paragraph. Cathodes containing 50 wt % sulfur are prepared by ball milling, and no novel porous carbon/sulfur composites are used. The resulting cathodes have an average loading of 0.6 mg S/cm2 with an area of 1.13 cm2.
The cyclic voltammetry (CV) curves in 10 μL of polysulfide-free and polysulfide-containing (Li2S9, [S]=2 M) electrolyte at 0.1 mV s−1 scanning rate are depicted in
As shown in
The concentration of sulfur species and the amount of electrolyte show significant effects on the performance of Li—S batteries, independent of polysulfide chain length.
When the amount of electrolyte is doubled, the mass of S added to each cell is also doubled, but the initial discharge capacity hardly changes. Capacity drops fairly quickly in the first 10 cycles and stabilizes at 760 mAh g−1 (based on S in the cathode only) and 500 mAh g−1 (based on total S in both cathode and electrolyte) when [5] is 1 M, and at and 1250 mAh g−1 (based on S in the cathode only) and 650 mAh g−1 (based on total S in both cathode and electrolyte) when [5] is 2 M.
All cells tested in polysulfide-containing electrolyte stabilized within about 10 cycles with capacities below 836 mAh g−1 based on total S from cathode and electrolyte (50% utilization of S with capacity of 1672 mAh/g), meaning there is less than 1e− per S transferred during cycling. The ultimate product during discharge is primarily slightly soluble Li2S2 and some higher-order polysulfide such as Li2S4. Conductive surfaces in the positive electrode may be passivated by significant precipitation of Li2S2during discharge, induced by the polysulfides added to the electrolyte, leading to huge polarization and causing the cell to reach the cut-off voltage before much Li2S2 can be further reduced to insoluble Li2S. This is confirmed by the sharp drop of the discharge curves at the end of discharge in
Although not wishing to be bound by theory, the applicant suggests that avoiding the irreversible formation of Li2S, the cell can be reversibly cycled between elemental sulfur and Li2S2 through multiple soluble polysulfides. These reactions are dominated by the interfacial charge transfer and are highly reversible and kinetically fast. The depth of discharge (DOD) of cells with polysulfide-free electrolyte was controlled to avoid formation of Li2S. Cyclability was improved when an appropriate capacity cut-off 600 mAh g−1 was selected, but cell capacity decreased as shown in
The rate performance of Li—S batteries using the polysulfide-containing electrolyte was tested. When the rate is increased to 5 C (8.4 A g−1), the sample still delivered a capacity of more than 600 mAh g−1 (based on S in the cathode only) and 310 mAh g−1 (based on total S in both cathode and electrolyte) with a coulombic efficiency of close to 100%, and relatively low polarization with a second voltage plateau at ˜1.8 V, indicating remarkable high-rate capability as shown in
In this case the electrolyte shows only one cathodic peak at around 2.0 V in CV curves (
To show the different discharge-charge mechanism of sulfur cathode in polysulfide-free and alkyl disulfide-based electrolyte, we took photos at different discharge capacities for the initial discharge in both electrolytes, thereby monitoring this process. The results are shown in
Those skilled in the art will understand that the various embodiments presented herein may be varied by those of skill in the art who have the advantage of reviewing this disclosure. Those variations are included within the spirit and the scope of the various embodiments of the invention.
This application claims priority to U.S. Provisional Patent Application No. 61/737,606, filed on Dec. 14, 2012. That application is incorporated by reference herein.
Portions of this invention were made using funds from the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-EE0005475. The United States government may have certain rights in this invention.
Number | Date | Country | |
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61737606 | Dec 2012 | US |