The present invention relates to electrochemical storage devices containing a nonaqueous electrolyte with high ionic conductivity and nonvolatile characteristics. More particularly, the present invention relates to liquid type polymeric electrolytes that are superior in ionic conductivity, thermal stability and safety to most of those currently being used in lithium-ion cells today.
The demand for lithium secondary batteries to meet high power and high-energy system applications has resulted in substantial research and development activities to improve their safety, as well as performance. As the world becomes increasingly dependent on portable electronic devices, and looks toward increased use of electrochemical storage devices for vehicles, power distribution load leveling and the like, it is increasingly important that the safety of such devices be paramount, especially as these devices are being used in such environments as airliners and space vehicles. The effort to date has included research in flame-retardants, solid polymer electrolytes and new electrolyte concepts with improved thermostability. Thus, the development of highly conductive electrolytes, free of any problems associated with volatile and combustible solvents, is of paramount importance. Electrolytes based on polymeric structures have basically better heat and chemical resistance than conventional organic carbonate-based electrolytes and can thus reduce many chemical side reactions occurring in lithium secondary batteries. Although, polymeric based electrolytes have many advantages over carbonate solvent based electrolytes, their application in lithium secondary batteries has been limited due to their low ionic conductivity, usually below 10−5 S/cm at room temperature.
To solve this problem, new electrolyte concepts are needed. The new electrolytes should be nonvolatile materials that have excellent electrochemical properties, such as high ionic conductivity of over 10−4 S/cm at room temperature and wide electrochemical stability windows of over 4.5 V (based on lithium metal).
Accordingly, the present inventors have developed a new type of ionically conductive electrolyte based on various polymeric structures, especially having a poly(siloxane-g-ethylene oxide) composition which overcomes the above mentioned problems of volatility, flammability and chemical reactivity inside of the lithium battery. The proposed liquid type poly(siloxane-g-ethylene oxide) materials also have an excellent electrochemical stability window and favorable room temperature ionic conductivity.
Carbonate solvents, such as ethylene carbonate and ethyl-methyl carbonate used for conventional lithium battery electrolytes can easily bum from low temperature sources of ignition and generate flame, carbon dioxide and H2O during thermal degradation. This is a critical problem in high capacity battery applications like lithium batteries for electric vehicles (EV) and satellites. Polymeric materials, however, usually have a somewhat different combustion mechanism than the carbonates. The initial stage of a fire occurs when a heat source decomposes the polymeric materials to flammable volatile products. Thus, for continuous burning to occur, (a) the application of heat must be sufficient to decompose the polymer, (b) the temperature must be sufficient to ignite the products of decomposition, and (c) the amount of heat transferred from the flame back to the polymer must be sufficient to maintain the cycle. In general, polymeric materials are more thermally stable than low molecular weight chemicals like the organic carbonates because they are not volatile and are vaporized at much higher temperatures.
The present inventors have carefully considered the combustion mechanisms of polymeric materials and concluded that to stop the propagation of the burning cycle one needed to develop new polymer materials that are thermally more stable and capable of dissolving the lithium salts to prepare electrolytes for electrochemical devices such as lithium batteries and/or capacitors. The present inventors have developed new structural siloxane polymers with one or more poly(ethylene oxide) side chains. Siloxanes are very thermally stable and are decomposed by heat with difficulty. Only a few flammable by-products are formed during the thermal decomposition of such polymers because their main chain is a Si—O linkage. Thus, its presence in the proposed polymers will delay the initiation of the combustion cycle.
Due to the merits of siloxane-poly(ethylene oxide) graft copolymers, substantial research has been done. See, for example, U.S. Pat. No. 5,112,512 to Nakamura and U.S. Pat. No. 6,124,062 to Horie et al. also describing siloxane-poly(ethylene oxide) graft copolymers (as a polymeric electrolyte material. The '512 patent discloses a crosslinked polymer electrolyte based on the graft copolymers, but its ionic conductivity is too low for room temperature applications. The '062 patent discloses direct use of siloxane-poly(ethylene oxide) grafted copolymers as a liquid for a lithium battery electrolyte with ionic conductivity of around 10 −4 S/cm at ca. 25° C. (See General Formula I). The conductivity of the material disclosed in the '062 patent is disadvantageously low. In addition, the cost of such material is relatively high. The present inventors have discovered a much improved material with higher conductivity and lower cost.
General formula (I) (as disclosed in the '062 patent):
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wherein R is alkyl group.
The present inventors developed an improved liquid state siloxane polymer with one or more poly(ethylene oxide) side chains to ensure thermal stability and electrochemical properties. Poly(ethylene oxides) (PEO) in poly(siloxane-g-ethylene oxide) materials of this invention (see general formula II) are directly bonded to a Si atom. Poly(siloxane-g-ethylene oxide) materials of the present invention are easily synthesized through a simple dehydrocoupling reaction with simple metal carbonate based catalysts and the cost for synthesis of poly(siloxane-g-ethylene oxide) of this invention is much lower than the cost of synthesis of general formula (I) with a propylene spacer between siloxane and PEO. The present inventors also control the viscosity of the materials to get high ionic conductivities of around 10−3 S/cm at room temperature.
where R and R″ are alkyl groups and R′ is hydrogen or alkyl group.
As compared to the material disclosed in the '062 patent, ionic conductivity is improved by changing the chemical structure of siloxane, that is, directly grafting ethylene oxide onto Si atom without any alkyl carbon spacer between them. Changing the structure in this way increases hydrophilicity and solubility, leading to higher conductivity.
A primary objective of the present invention is to provide a safe liquid electrolyte for use in lithium ion type cells and allied devices such as super capacitors and hybrid devices. Specifically, a primary objective is to reduce or eliminate problems associated with volatility, flammability and chemical reactivity of liquid electrolytes.
A further objective of the invention is to provide a lithium-based secondary battery, capacitor or hybrid device with enhanced safety and excellent performance characteristics by use of the present polymeric electrolyte.
Accordingly, the present invention relates to lithium-based secondary batteries with a nonaqueous and nonvolatile polymeric electrolyte with high room temperature ionic conductivity with enhanced safety.
The invention also relates to nonaqueous and nonvolatile polymeric electrolytes processing controlled viscosity for the high-energy lithium secondary battery applications, especially for electric vehicles and satellite applications.
To fulfill the above objectives, the nonaqueous and nonvolatile polymeric electrolyte solutions in the present invention is prepared by using the compositions which comprise poly(siloxane-g-ethylene oxide) in a liquid state with proper viscosity and a dissolved lithium salt.
a and 2b are traces of cyclic voltammetry measurements to determine electrochemical stability of samples made according to the present invention.
a is a data trace from accelerating rate calorimetry (ARC) tests of the electrolyte of the present invention.
b is a comparative data trace from ARC tests of liquid electrolyte LiPF6 in a mixture of ethylene carbonate and ethyl methyl carbonate.
The electrolyte solution of the present invention comprises siloxane polymers with one or more poly(ethylene oxide) as a side chain (see general formula II above). The inorganic siloxane main body ensures flame-retardancy and chemical stability, and poly(ethylene oxide) side chains give ionic conducting phases. A poly(ethylene oxide) group is directly grafted to silicon atoms in the siloxane polymer. The poly(siloxane-g-ethylene oxides) of the present invention are easily synthesized through simple dehydrogenation reactions to avoid problems that come from the use of metal catalysts such as platinum.
The poly(siloxane-g-ethylene oxide) easily dissolves lithium salt and has enough flexibility to transport lithium ions. Through the viscosity control of the poly(siloxane-g-ethylene oxide), the polymeric electrolyte solution provides excellent ionic conductivity of around 10−3 S/cm at room temperature.
The salt to be used in the present invention is not particularly limited, as long as it serves as an electrolyte for a lithium secondary battery, capacitor, or other electrochemical device. Examples of specific lithium salts include, but are not limited to, LiClO4, LiBF4, LiAsF6, LiPF6, LiCF3SO3, Li(CF3SO2)2N, LiC(CF3SO2)3, LiN(SO2C2F5)2, lithium alkyl fluorophosphates, and mixtures thereof. Other salts may be used, particularly alkali metal salts or quaternary ammonium salts having an anion selected from the group consisting of: ClO4−, BF4−, AsF6−, PF6−, CF3SO3−, (CF3SO2)2N−, (CF3SO2)3C−, (C2F5SO2)2N−,PF3(C2F5)3−, PF3(CF3)3−, and B(C2O4)2−.
Further examples (without the intent to limit the scope of the invention) found to be beneficial are lithium bis(chelato)borates having from five to seven membered rings as follows:
The oxygen versus Li ratio (doping level) is preferably 5:1 to 50:1. If the doping level is greater than 50:1, the ionic conductivity of the resulting polymeric electrolyte is undesirably decreased because of the number of ions present in the solid polymer electrolyte. If the doping level is smaller than 5:1, the lithium salt is not sufficiently dissociated in the resulting polymeric electrolyte and the association of lithium ion reduces the ionic conductivity. Preferably, the average molecular weight of the poly(siloxane-g-ethylene oxide) should not exceed 20,000 g/mol. The dynamic viscosity of the poly(siloxane-g-ethylene oxide) is best in the range of 200 cPs, should preferably not exceed 1,000 cPs to 2,000 cPs, and should never exceed 10,000 cPs.
The present invention covers both polymeric electrolytes as well as the lithium secondary batteries that use them in the same way that the carbonate based electrolytes are applied in the state-of-the-art Li-ion batteries today. A lithium secondary battery with the polymeric electrolyte solution of this invention as a conducting medium can be fabricated by injecting the polymeric electrolyte solution into a spiral wound cell or prismatic type cell. It can be also coated onto the surface of electrodes and assembled with a porous separator to fabricate single or multi-stacked cells that are packaged with plastic pouches. Similarly, the electrolyte of the present invention may be used in ultra capacitors (also known as super capacitors) and hybrid devices incorporating aspects of batteries and capacitors.
In situ tests: Type 2032 button cells were assembled for the evaluation of cell performance with a MAG-10 anode, and LiNi0.8Co0.15Al0.05O2 as a cathode. Celgard® 2500 (microporous polypropylene membrane, available from CELGARD Inc.) was used as a separator. These cells contained MAG-10 graphite, with 8 wt % PVDF binder/poly(siloxane-g-3 ethylene oxide), doping level of 32:1 with LiTFSI, electrolyte/LiNi0.8Co0.15Al0.05O2, 8 wt % PVDF, 4 wt % SFG-6 graphite, and 4 wt % carbon black. The effective cell area was 1.6 cm2. Charge and discharge rate were C/10. There was no degradation peak caused by the metal oxide up to 4.1V and the specific charge capacity was over 140 mAh/g.
Poly(siloxane-g-3 ethylene oxide) electrolyte shows an excellent ionic conductivity of 8.6×10−4 S/cm at 37° C.
a and 2b show the electrochemical stability of poly(siloxane-g-3 ethylene oxide) and poly(siloxane-g-7 ethylene oxide) (see general formula IV) electrolytes, respectively. Poly(siloxane-g-ethylene oxides) and Li(CF3SO2)2N were mixed with a doping level of 32:1. The electrochemical stability window of the polymer electrolytes were determined by cyclic voltammetry with type 2030 button cells assembled by sandwiching a glass filter containing the polymer electrolyte between a stainless steel disc as a working electrode and a lithium metal disc as a combined counter and reference electrode. These polymeric electrolytes were found to show excellent electrochemical stability windows over 0 to 4.5V(vs. Li) and only a minimal decomposition peak around 4.5V during the first anodic sweep. In the case of poly(siloxane-g-7 ethylene oxide), which is more stable, the stability window reaches ca. 5.5V.
a) shows thermal stability of poly(siloxane-g-3 ethylene oxide) electrolyte at a doping level of 32:1 with LiPF6, using accelerating rate calorimetry (“ARC”). A heat-wait-search sequence was applied. Sample weight for the test was 500 mg. Each sample was introduced in a 2¼″×¼″ diameter stainless steel bomb for the ARC test.
ARC is more sensitive test method than differential scanning calorimetry (“DSC”). In a heat-wait-search sequence, ARC will search an exothermic reaction for 30 minutes at every 5° C. change from the starting temperature. In DSC, a 5 or 10° C./min heating rate is usually used and the sample quantity is only about 2 to 5 μL. In the case of some chemicals, the exothermic peak by DSC comes later than ARC due to its much faster scan rate than ARC.
Although solid LiPF6 itself decomposes to PF5 and LiF at about 250° C., as measured by the DSC method, (see, N. Katayama et al., J. Power Sources, 2001, in press at time of application) and PF5 reacts with solvents for further thermal decomposition, poly(siloxane-g-3 ethylene oxide) electrolyte has an exothermic peak above 300° C. This means that poly(siloxane-g-3 ethylene oxide) has excellent chemical stability even at an elevated temperature.
b shows comparative data from an ARC test of a widely used liquid electrolyte, 1.2M LiPF6 in ethylene carbonate (EC):ethyl methyl carbonate (EMC) (3:7 weight ratio). This sample was tested in the same way as in
UL intends this standard to serve as a preliminary indication of the acceptability of materials for use as part of electronic goods, lithium batteries, etc. with respect to flammability. The modified UL-94HB test uses a ¼″×5.5″ silanized glass wool wick specimen soaked with the electrolyte samples held at one end in a horizontal position with marks at 1.00″ and 4.94″ from the free end. The specimen contains 5 g of electrolyte. A flame is applied to the free end for 30 seconds or until the flame front reaches the 1″ mark. If combustion continues the duration is timed between the 1.00″ mark and the 4.94″ mark. If combustion stops before the 4.94″ mark, the time of combustion and the damaged length between the two marks are recorded. A set of five specimens is tested. If two specimens from the set of five fail to comply, then a second set of five are tested. All five of this second set must comply. Propagation rate of flame is given by the following equation:
Propagation Rate=Distance (cm)/Elapsed time (min)
Poly(siloxane-g-3 ethylene oxide) electrolyte shows outstanding flame resistance. Its propagation rate is significantly lower than conventional liquid electrolyte for lithium ion batteries and low molecular weight polyethylene oxide electrolyte. This is because the inorganic siloxane backbone naturally acts as a combustion inhibitor.
It should be apparent that the present invention solves the long-felt need to create safe, high energy electrochemical storage devices having liquid electrolytes. Such devices promise to have high energy rate capabilities as well as high energy density. Batteries containing the present electrolyte, especially large batteries such as those used in electric vehicles, would be inherently safer than those with more volatile, flammable and unstable electrolytes.
Having described the present invention, it should be apparent to the reader that many variations of the present invention are possible without departure from the scope of the present invention. The specific implementations disclosed above are by way of example and for the purposes of enabling persons skilled in the art to implement the invention only. Accordingly, the invention is not to be limited except by the appended claims and legal equivalents.
This application is a Continuation-in-Part of application Ser. No. 10/104,352 filed Mar. 22, 2002 now abandoned titled “Solid Polymer Electrolyte and Method of Preparation” to Oh et al., the disclosure of which is incorporated herein by reference.
This invention was made with United States Government support under NIST ATP Award No. 70NANB043022 awarded by the National Institute of Standards and Technology (NIST). The United States Government has certain rights in this invention pursuant to NIST ATP Award No. 70NANB043022 and pursuant to Contract No. W-31-109-ENG-38 between the United States Government and the University of Chicago representing Argonne National Laboratory, and NIST 144 LM01, Subcontract No. AGT DTD Sep. 9, 2002.
Number | Name | Date | Kind |
---|---|---|---|
3172899 | Bailey | Mar 1965 | A |
3530159 | Guinet et al. | Sep 1970 | A |
3734876 | Chu | May 1973 | A |
4259467 | Keogh et al. | Mar 1981 | A |
4830939 | Lee et al. | May 1989 | A |
4849856 | Funari et al. | Jul 1989 | A |
4908283 | Takahashi et al. | Mar 1990 | A |
5037712 | Shackle et al. | Aug 1991 | A |
5112512 | Nakamura | May 1992 | A |
5272021 | Asai et al. | Dec 1993 | A |
5300375 | Chaloner-Gill | Apr 1994 | A |
5362493 | Skotheim et al. | Nov 1994 | A |
5419984 | Chaloner-Gill et al. | May 1995 | A |
5475127 | Klein et al. | Dec 1995 | A |
5538812 | Lee et al. | Jul 1996 | A |
5593787 | Dauth et al. | Jan 1997 | A |
5609974 | Sun | Mar 1997 | A |
5633098 | Narang et al. | May 1997 | A |
5690702 | Skotheim et al. | Nov 1997 | A |
5700300 | Jensen et al. | Dec 1997 | A |
5731104 | Ventura et al. | Mar 1998 | A |
5753389 | Gan et al. | May 1998 | A |
5772934 | MacFadden | Jun 1998 | A |
5882812 | Visco et al. | Mar 1999 | A |
5885733 | Ohsawa et al. | Mar 1999 | A |
5919587 | Mukherjee et al. | Jul 1999 | A |
5961672 | Skotheim et al. | Oct 1999 | A |
6013393 | Taniuchi et al. | Jan 2000 | A |
6015638 | Ventura et al. | Jan 2000 | A |
6124062 | Horie et al. | Sep 2000 | A |
6168885 | Narang et al. | Jan 2001 | B1 |
6181545 | Amatucci et al. | Jan 2001 | B1 |
6245465 | Angell et al. | Jun 2001 | B1 |
6248481 | Visco et al. | Jun 2001 | B1 |
6252762 | Amatucci | Jun 2001 | B1 |
6268088 | Oh et al. | Jul 2001 | B1 |
6337383 | West et al. | Jan 2002 | B1 |
6447952 | Speigel et al. | Sep 2002 | B1 |
6482912 | Boudjouk et al. | Nov 2002 | B2 |
6495287 | Kolb et al. | Dec 2002 | B1 |
6573009 | Noda et al. | Jun 2003 | B1 |
6610109 | Noh | Aug 2003 | B2 |
6653015 | Yoshida et al. | Nov 2003 | B2 |
6872493 | Yamada et al. | Mar 2005 | B2 |
6887619 | West et al. | May 2005 | B2 |
20020028388 | Lee | Mar 2002 | A1 |
20020051911 | Okada | May 2002 | A1 |
20020192554 | Woo et al. | Dec 2002 | A1 |
20030036003 | Shchori et al. | Feb 2003 | A1 |
20030099884 | Chiang et al. | May 2003 | A1 |
20030104282 | Xing et al. | Jun 2003 | A1 |
20030180624 | Oh et al. | Sep 2003 | A1 |
20030198869 | West et al. | Oct 2003 | A1 |
20030198870 | Wariishi et al. | Oct 2003 | A1 |
20040096746 | Wietelmann et al. | May 2004 | A1 |
20040197665 | Amine et al. | Oct 2004 | A1 |
20040197668 | Jung et al. | Oct 2004 | A1 |
20040214090 | West et al. | Oct 2004 | A1 |
Number | Date | Country |
---|---|---|
0 450 981 | Oct 1991 | EP |
0 525 728 | Feb 1993 | EP |
0 581 296 | Feb 1994 | EP |
0 581 298 | Feb 1994 | EP |
0 922 049 | Feb 1998 | EP |
0 932 215 | Jan 1999 | EP |
0 798 511 | Aug 1999 | EP |
1 024 502 | Aug 2000 | EP |
0 932 215 | May 2001 | EP |
1 202 374 | May 2002 | EP |
57-034661 | Feb 1982 | JP |
57-034662 | Feb 1982 | JP |
57-080670 | May 1982 | JP |
57-111957 | Jul 1982 | JP |
57-176669 | Oct 1982 | JP |
59-224072 | Dec 1984 | JP |
60-195877 | Oct 1985 | JP |
60-216461 | Oct 1985 | JP |
61-288374 | Dec 1986 | JP |
62-208169 | Sep 1987 | JP |
63-010466 | Jan 1988 | JP |
63-310569 | Dec 1988 | JP |
02-080462 | Mar 1990 | JP |
02-262274 | Oct 1990 | JP |
02-291603 | Dec 1990 | JP |
03-139566 | Jun 1991 | JP |
60-052893 | Jul 1992 | JP |
05-036441 | Feb 1993 | JP |
05-290816 | Nov 1993 | JP |
07-320782 | Dec 1995 | JP |
08-078053 | Mar 1996 | JP |
09-309544 | Nov 1997 | JP |
11-214032 | Jan 1998 | JP |
11--302384 | Apr 1998 | JP |
10-172615 | Jun 1998 | JP |
11-185804 | Jul 1999 | JP |
11-238523 | Aug 1999 | JP |
11-302383 | Nov 1999 | JP |
11-306856 | Nov 1999 | JP |
11-306857 | Nov 1999 | JP |
2000-058123 | Feb 2000 | JP |
2000-154254 | Jun 2000 | JP |
2000-222939 | Aug 2000 | JP |
2000-277152 | Oct 2000 | JP |
2001-068116 | Mar 2001 | JP |
2001-110455 | Apr 2001 | JP |
2001-185165 | Jul 2001 | JP |
2001-283907 | Oct 2001 | JP |
2001-283913 | Oct 2001 | JP |
2002-053936 | Feb 2002 | JP |
2002-151150 | May 2002 | JP |
2002 155142 | May 2002 | JP |
2002-298913 | Oct 2002 | JP |
2002-343440 | Nov 2002 | JP |
2003-002974 | Jan 2003 | JP |
WO 9621853 | Jul 1996 | WO |
WO 9807729 | Feb 1998 | WO |
WO 0000495 | Jan 2000 | WO |
WO 0008654 | Feb 2000 | WO |
WO 0025323 | May 2000 | WO |
WO 0025323 | May 2000 | WO |
WO 0173884 | Oct 2001 | WO |
WO 0196446 | Dec 2001 | WO |
WO 0199209 | Dec 2001 | WO |
WO 03038970 | Oct 2003 | WO |
WO 03083971 | Oct 2003 | WO |
WO 03083972 | Oct 2003 | WO |
WO 03083973 | Oct 2003 | WO |
WO 03083974 | Oct 2003 | WO |
WO 03090299 | Oct 2003 | WO |
Number | Date | Country | |
---|---|---|---|
20030180625 A1 | Sep 2003 | US |
Number | Date | Country | |
---|---|---|---|
Parent | 10104352 | Mar 2002 | US |
Child | 10167940 | US |