Patent Application
20080305401
The present invention relates to lithium-ion electrolytes, with application to lithium-ion batteries.
Future NASA missions to explore Mars, the Moon, and the outer planets, are expected to require rechargeable batteries that can operate over a wide temperature range (−60° C. to +60° C.) to satisfy the requirements of various applications, including landers, rovers, and penetrators. Some future applications typically may require high specific energy batteries that can operate at very low temperatures, while still providing adequate performance and stability at higher temperatures. In addition, many of these applications envisioned by the Exploration Systems Research and Technology (ESRT) program of NASA will require improved safety, due to their use by humans. The performance of currently known state-of-art lithium-ion systems is severely limited at temperatures below −40° C. and above +30° C. The limited performance at very low temperatures is believed to be due to poor electrolyte conductivity, poor lithium intercalation kinetics over the electrode surface layers, and poor ionic diffusion in the electrode bulk.
In the description that follows, the scope of the term “some embodiments” is not to be so limited as to mean more than one embodiment, but rather, the scope may include one embodiment, more than one embodiment, or perhaps all embodiments.
For some embodiments, anode 102 comprises MCMB (Mesocarbon Microbeads) carbon, and cathode 104 comprises LiNi0.8CO0.2O2 (lithium nickel cobalt oxide). Other embodiments may use other kinds of anodes or cathodes. For example, other embodiments may use cathodes comprising lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), lithium nickel cobalt aluminum oxide (LiNiCoAlO2), lithium iron phosphate (LiPO4), and lithium nickel cobalt manganese oxide (LiNiCoMnO2), and LiFePO4 to name a few examples. As another example, the stoichiometries for nickel, cobalt, and manganese may vary, so that for some embodiments a cathode may comprise LiMnxO4 and LiNiyCozMnuO2, where x, y, z, and u denote variable stoichiometries.
The lithium-ion electrolyte solution comprises a lithium salt, such as lithium hexafluoro phosphate (LiPF6), dissolved into a co-solvent to provide a lithium-ion electrolytic solution (electrolyte). Other embodiments may use other types of lithium salt, such as lithium tetrafluoroborate (LiBF4), lithium bis(oxalate) borate (LiBOB, or LiB(C2O4)2), lithium hexafluoroarensate (LiAsF6), lithium perclorate (LiClO4), lithium trifluoromethanesulfonate (LiCF3SO3), and lithium bistrifluoromethanesulfonate sulfonyl imide (LiN(SO2CF3)2), to name a few. Also, some embodiments may utilize a mixture of such lithium salts.
During discharge, the reaction CLix→C+Li++xe−, where e− denotes an electron, takes place at anode 102, and the reaction xLi++xe−+Li1-xNi0.8Cu0.2O2LiNi0.8CO0.2O2 takes place at cathode 104 (for an embodiment with a cathode comprising LiNi0.8Cu0.2O2). During charge, these reactions are reversed.
A number of ternary co-solvent embodiments were investigated comprising ethylene carbonate (EC), ethyl methyl carbonate (EMC), and a fluoroester (also referred to as fluorinated ester) solvent selected from the group consisting of 2,2,2-trifluoroethyl butyrate (C6H9F3O2 or CH3CH2CH2CO2CH2CF3), Chemical Abstract Services registry number CAS # 371-27-7, abbreviated in these letters patent as TFEB; ethyl trifluoroacetate (C4H5F3O2, or CF3CO2CH2CH3), CAS #383-63-1, abbreviated in these letters patent as ETFA; 2,2,2-trifluoroethyl acetate (C4H5F3O2, or CH3CO2CH2CF3), CAS # 406-95-1, abbreviated in these letters patent as TFEA; and methyl pentafluoropropionate (C4H3F5O2, or CF3CF2CO2CH3), CAS# 378-75-6, abbreviated in these letters patent as MPFP. In particular, these co-solvents were blended in carbonate-based electrolytes to give the following electrolyte solutions:
1.0 M LiPF6 in EC+EMC+TFEB (20:60:20 v/v %);
1.0 M LiPF6 in EC+EMC+TFEB (20:40:40 v/v %);
1.0 M LiPF6 in EC+EMC+TFEB (20:20:60 v/v %);
1.0 M LiPF6 in EC+EMC+ETFA (20:60:20 v/v %);
1.0 M LiPF6 in EC+EMC+TFEA (20:60:20 v/v %);
1.0 M LiPF6 in EC+EMC+TFEA (20:40:40 v/v %); and
1.0 M LiPF6 in EC+EMC+MPFP (20:60:20 v/v %).
A number of experimental glass wound (cylindrical) lithium-ion cells, with MCMB carbon anodes and LiNi0.8CO0.2O2 cathodes, with Li metal reference electrodes, were fabricated to study the above listed electrolytes. Results of those experiments are provided in Table 1. The last two entries in Table 1 are electrolytes comprising 1.0 M LiPF6 in EC+EMC (20:80 v/v %); and 1.0 M LiPF6 in EC+DEC (diethyl carbonate)+DMC (dimethyl carbonate) (1:1:1 v/v %). These two electrolytes serve as baselines for comparison.
As shown in Table 1, all cells displayed good reversibility at room temperature and minimal reactivity during the formation cycling, with the exception of the cell containing the MPFP-based electrolyte. The high coulombic efficiency and comparable irreversible capacity losses are indirectly related to the overall stability of the solutions and the electrode filming characteristics. Note that the fluoroester-based solutions do not display deleterious effects that may limit the reversible capacity or may hinder the kinetics of the lithium intercalation and de-intercalation processes. In some cases, some cells displayed much higher efficiencies and lower cumulative irreversible capacity losses, such as the cells containing TFEB in low proportion, compared with the comparable baseline solution.
Of the electrolytes listed above, the embodiment formulations comprising 1.0 M LiPF6 in EC+EMC+TFEB (20:60:20 v/v %) and 1.0 M LiPF6 in EC+EMC+TFEA (20:60:20 v/v %) yielded the greatest improvements in terms of performance, resulting in improved low temperature performance and greater high temperature resilience. As shown in
The cells were evaluated at the lower temperature of −40° C., as shown in
As seen from
In addition to evaluating the low temperature performance of the cells, the high temperature resilience of the cells was tested by subjecting them to increasingly higher temperature storage periods. Each storage period consisted of storing the cells in a full state of charge for 10 days in an open circuit condition. Although it would be expected that the performance degradation is less pronounced if the cells are stored in a partial state of charge, a fully charged condition was selected for the test to accelerate any deleterious electrochemical electrolyte reactions. As illustrated in Tables 3A, 3B, and 3C, the cell containing the 1.0 M LiPF6 in EC+EMC+TFEB (20:60:20 v/v %) electrolyte sustained the least amount of performance degradation as a result of the high temperature storage testing. For example, after being subjected to 10 days at 55° C., the TFEB-containing cells retained about 75% of the reversible capacity exhibited prior to the storage period, whereas the binary baseline formulation only delivered about 59% of their initial capacity.
As described so far, a number embodiments comprising fluoroester-based co-solvents in Li-ion electrolytes, including trifluoroethyl butyrate (TFEB), ethyl trifluoroacetate (ETFA), trifluoroethyl acetate (TFEA), and methyl pentafluoropropionate (MPFP), were found to provide good performance over a wide range of temperatures. It is believed that these embodiments have good safety characteristics, due to their low flammability associated with their halogenated nature. Of the solvents described so far, trifluoroethyl butyrate (TFEB) was demonstrated to yield the best performance, with improved low temperature capability and high temperature resilience. In addition to the above-described embodiments, other embodiments utilizing TFEB as a co-solvent have been found by developing multi-component electrolytes which also contain non-halogenated esters, film forming additives, thermal stabilizing additives, and flame retardant additives.
Several factors may influence the low temperature performance of lithium-ion cells, including: lithium ion mobility in the electrolyte solution (electrolyte conductivity); cell design; electrode thickness; separator porosity; and separator wetting properties. Of these parameters, the electrolyte properties are presumably the most dominant, in that sufficient conductivity is a necessary condition for good performance at low temperatures. In designing electrolytes with high conductivity at low temperatures, the solvents should possess a combination of several critical properties, such as: high dielectric constant; low viscosity; and adequate coordination behavior; as well as appropriate liquid ranges and salt solubilities in the medium.
Some embodiments comprise a quaternary co-solvent that includes TFEB as well as other non-fluorinated ester co-solvents, such as methyl butyrate (MB), ethyl propionate (EP), and propyl butyrate (PB). These non-fluorinated esters may improve the low temperature performance, due to their low viscosity and melting points. Furthermore, some embodiments make use of vinylene carbonate (VC) in conjunction with fluorinated esters, with the intent of producing robust solid electrolyte interface (SEI) layers on the electrodes. VC has previously been reported to be an effective additive in improving the high temperature cycle life and storage characteristics, and is believed to sacrificially polymerize on the electrode surfaces, producing protective films preventing further electrolyte reaction at the interface.
In addition, some embodiments make use of dimethyl acetamide (DMAc) as an electrolyte additive in conjunction with fluorinated ester co-solvents, which has been demonstrated to be a thermal stabilizing agent, acting to reversibly bind with PF5, preventing LiPF6 and carbonate solvents from decomposition at high temperature. It is envisioned that the use of VC, DMAc, or both, may improve the life characteristics and high temperature resilience of electrolyte formulations that also contain fluorinated ester co-solvents, which may impart improved operating temperature range and safety.
Embodiments may also include other fluorinated ester co-solvents in addition to the ones previously investigated, such as 2,2,2-trifluoroethyl propionate (C5H7F3 or CH3CH2CO2CH2CF3), referred to as TFEP. These new electrolyte formulations were compared with the previously described solutions, namely 1.0 M LiPF6 in EC+EMC+TFEB (20:60:20 v/v %) and 1.0 M LiPF6 in EC+EMC (20:80 v/v %), which were treated as baseline solutions. Accordingly, the following electrolytic solutions were prepared and evaluated:
1.0 M LiPF6 in EC+EMC+TFEB+MB (20:40:20:20 v/v %);
1.0 M LiPF6 in EC+EMC+TFEB+EP (20:40:20:20 v/v %);
1.0 M LiPF6 in EC+EMC+TFEB+PB (20:40:20:20 v/v %);
1.0 M LiPF6 in EC+EMC+TFEB (20:60:20 v/v %)+1.5% VC;
1.0 M LiPF6 in EC+EMC+TFEB (20:60:20 v/v %)+1.5% DMAc;
1.0 M LiPF6 in EC+EMC+TFEB (20:75:5 v/v %);
1.0 M LiPF6 in EC+EMC+TFEP (20:60:20 v/v %); and
1.0 M LiPF6 in EC+EMC+TFEP (20:40:40 v/v %).
These were compared with 1.0 M LiPF6 in EC+EMC+TFEB (20:60:20 v/v %) and 1.0 M LiPF6 in EC+EMC (20:80 v/v %), which served as baselines.
In addition to the embodiments described above, some embodiments comprising TFEB may also comprise flame retardant additives, such as triphenyl phosphate, triethyl phosphate, tributyl phosphate, and bis-(2,2,2-trifluoroethyl)methyl phosphonate, to reduce the flammability of the electrolytes.
Lithium-ion batteries with MCMB carbon anodes and LiNi0.8CO0.2O2 cathodes were fabricated with the electrolytes listed above, and measurements related to the cells are provided in Table 4. As shown in Table 4, all cells with the formulations listed above displayed good reversibility at room temperature, and relatively minimal reactivity during the formation cycling. The high coulombic efficiency and comparable irreversible capacity losses are believed to be indirectly related to the overall stability of the solutions and the electrode filming characteristics.
Note that the fluoroester-based solutions do not display deleterious effects that limit the reversible capacity or hinder the kinetics of the lithium intercalation and de-intercalation processes. From the formation data, it may be observed that: the trifluoroethyl butyrate solutions appear to produce more desirable surface films compared with trifluoroethyl propionate, as suggested by the lower cumulative irreversible capacity losses and higher efficiency values; with regard to the quaternary formulations comprising non-fluorinated ester co-solvents, propyl butyrate displayed the most desirable filming attributes, suggesting enhanced stability compared to methyl butyrate and ethyl propionate; vinylene carbonate appears to be an effective film forming agent, leading to stable cells with low irreversible capacity loss and high efficiency; and DMAc appears to have some reactivity during the formation cycling.
When the cells described were evaluated at −30° C., as shown in
A number of test cells with lithium-ion electrolytes comprising fluorinated ester co-solvents, including trifluoroethyl butyrate (TFEB) and trifluoroethyl propionate (TFEP), were tested, with their performance described in this specification. These electrolytes were found to deliver good performance over a wide temperature range in experimental lithium-ion cells. The general approach taken in the development of these electrolyte formulations was to optimize the type and composition of the co-solvents in ternary and quaternary solutions, focusing upon adequate stability (e.g., EC content for anode passivation, and EMC content for lowering the viscosity and widening the temperature range, while still providing good stability), enhancing the inherent safety characteristics (incorporation of fluorinated esters), and widening the temperature range of operation (the use of both fluorinated and non-fluorinated esters). Furthermore, the use of electrolyte additives, such as VC (SEI promoter) and DMAc (thermal stabilizing additive), are envisioned to provide enhanced high temperature life characteristics. For some embodiments, the concentrations of VC and DMAc may be relatively low, such as for example in the range of 0.10% and 5.0% by weight. It is anticipated that embodiment electrolyte solutions may offer improved performance when incorporated into hermitically sealed, large capacity, prototype cells, especially if all electrolyte components are highly pure.
Various modifications may be made to the described embodiments without departing from the scope of the invention as claimed below. For example, other non-fluorinated ester co-solvents that may be used are: methyl propionate, methyl acetate, ethyl acetate, propyl acetate, ethyl butyrate, butyl butyrate, and propyl propionate. Furthermore, it should be appreciated that experimental results were described for embodiments with specific concentrations of the co-solvents, but embodiments are not limited to these specific concentrations. For example, for some embodiments the EC content may vary from 10% to 50%, the EMC content may vary from 10% to 80%, the fluorinated ester co-solvent content may vary from 5% to 80%, and the non-fluorinated ester co-solvent content may vary from 10% to 80%. Furthermore, for some embodiments the lithium salt concentration (e.g., LiPF6) may vary from 0.50 M to 1.50 M.
These concentration ranges are merely given to provide examples, and some embodiments may have different ranges of concentrations. For example, the lithium salt concentration may be between 0.80 M and 1.20 M, where very low temperature performance benefits from the lower end of this range, and where more moderate temperature and high power applications may benefit from the higher end of this range. As another example, the concentration of the non-fluorinated esters may be in the range of 10% to 80%.
Throughout the description of the embodiments, various mathematical relationships are used to describe relationships among one or more quantities. These relationships are in practice not satisfied exactly, and should therefore be interpreted as “designed for” relationships. One of ordinary skill in the art may design various working embodiments to satisfy various mathematical relationships, but these relationships can only be met within the tolerances of the technology available to the practitioner.
Accordingly, in the following claims, it is to be understood that claimed mathematical relationships can in practice only be met within the tolerances or precision of the technology available to the practitioner, and that the scope of the claimed subject matter includes those embodiments that substantially satisfy the mathematical relationships so claimed.
This application claims the benefit of U.S. Provisional Application No. 60/897,316, filed 25 Jan. 2007.
The invention claimed herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 USC 202) in which the Contractor has elected to retain title.
| Number | Date | Country | |
|---|---|---|---|
| 60897316 | Jan 2007 | US |
