ELECTROLYTES IN SUPPORT OF 5V LI ION CHEMISTRY

[0001] 1. Field of the Invention

[0002] The present invention relates to electrolytes that have very wide electrochemical stability window, and can therefore support Li ion chemistries that occurs near or above 5.0 V. More particularly, this invention relates to the compounds that can be incorporated into electrolytes as electrolyte co-solvents, electrolyte additives, or electrolyte solutes, the result of such incorporation being that the electrolytes can support the reversible Li ion intercalation/de-intercalation chemistry at potentials above 4.5 V. Still more particularly, this invention relates to the compounds that can be incorporated into the electrolyte as electrolyte co-solvents, electrolyte additives, or electrolyte solutes, which, upon the initial charging of cathode, decompose sacrificially to form a passivation film. This passivation film prevents sustaining decomposition of electrolyte components but does not hinder the reversible Li ion intercalation/de-intercalation chemistry at potentials above 4.5 V.

[0003] The invention of such an electrolyte will enable the use of high voltage cathode materials, affording new rechargeable battery chemistries with higher energy density as well as delivering energy of higher quality in the form of direct electricity current at higher voltages, which are unavailable otherwise from the state-of-the-art electrolytes. The state-of-the-art electrolytes, comprising mainly organic carbonate esters, decompose at potentials below 4.5 V on those high voltage cathode surfaces and cause sustaining capacity fading accompanied with increasing cell impedances.

[0004] The said high voltage cathodes include, but are not limited to, transition metal-oxides with spinel lattice structures or metal phosphates with olivine lattice structures, or metal fluorides with conversion reaction natures.

[0005] More particularly, the novel compounds of the present invention goes beyond the battery application and could benefit any electrochemical devices that pursue higher operating potentials. The presence of the novel compounds in the electrolyte can stabilize the highly oxidizing surface of the positive electrode and hence enable new chemistry that is otherwise impossible with the current state-of-the-art electrolyte technology. Such electrochemical devices include, but are not limited to, rechargeable batteries, double layer capacitors, pseudo-capacitors, electrolytic cells, fuel cells, et cetera.

[0006] Still more particularly, the batteries or the electrochemical devices comprise, but are not limited to, (1) an anode such as lithium or other alkaline metals, alloys of lithium or other alkaline metals, intercalation hosts such as layered structured materials of graphitic, carbonaceous, oxides or other chemical natures, non-intercalating hosts of high surface area or high pseudo-capacitance, et ceteras; (2) a cathode such as an intercalation host based on metal oxides, phosphates, fluorides or other chemical natures, or non-intercalating hosts of high surface area or high pseudo-capacitance, et ceteras; and (3) an electrolyte of the present invention. These electrolytes comprise (a) one or more electrolyte solutes with various cations and anions, (b) a solvent or a mixture of solvents based on organic carbonates or other compounds, and (c) one or more additives. Any of (a), (b) and (c) could be from the claimed structures of the present invention.

[0007] 2. Description of the Prior Arts

[0008] Li ion chemistry is established upon reversible intercalation/de-intercalation of Li ion into/from host compounds. The voltage of such an electrochemical device is determined by the chemical natures of anode and cathode, where Li ion is accommodated or released at low potentials in the former, and at high potentials in the latter. Apparently, the reversibility of the cell chemistry and the resultant energy density are limited by the stability of the electrolyte to withstand the reductive and oxidative potentials of these electrodes. In today's market, a majority of Li ion batteries use organic carbonate as electrolyte solvents, which decompose oxidatively above 4.5 V vs. Li, and set an upper limit to the candidate cathode chemistry. Despite the fact that 5 V Li ion chemistry has already been made available from such cathodes like olivine structured LiCoPO₄ (˜5.1 V) and spinel structured LiNi_0.5Mn_1.5O₄ (˜4.7 V), their advantages such as high energy density and quality cannot be realized due to the lack of an electrolyte that is able to withstand high voltage operation.

[0009] Early attempts have been made by one of the inventors to identify an electrolyte system that can resist oxidation beyond 5.0 V, and unsymmetrical sulfones were shown to be such a system on spinel LiMn₂O₄ surface (K. Xu, et al, J. Electrochem. Soc., 1998, Vol. 145, L70; J. Electrochem. Soc., 2002, Vol. 149, A920). However, intrinsic shortcomings of sulfone as a major electrolyte component, including its failure to form a protective layer on graphitic anode, slow Li ion kinetics, and poor electrode active material utilization caused by high viscosity, prevented wide application.

[0010] Additional improvements were also made on mitigating the oxidizing nature on the cathode surfaces through surface coating approaches, and various metal oxides or phosphates were shown to be effective in elongating the service life of the carbonate-based electrolytes (J. Liu, et al, Chem. Mater, 2009, Vol. 21, 1695). But these coating approaches have their own intrinsic shortcomings as well. They not only add additional cost to the manufacturing of the cathode materials, but also induce further interphasial resistance to the Li ion migration at electrolyte/cathode junction. Moreover, overall coverage of cathode particle surface with those inert coatings will inevitably decrease the energy density of the device.

[0011] It is therefore of significant interest to the battery industry to find a technology that can effectively enable the 5.0 V class cathode to be applied in Li ion batteries, without the aforementioned shortcomings.

[0012] To be more specific, it is therefore of significant interest to the battery industry to find a technology that can effectively enable the 5.0 V class cathode to be applied in Li ion batteries, while there is no major negative impact on the original electrolyte and cathode materials. Such negative impact have been exhibited in the prior arts, and include but are not limited to, the failure of electrolyte to form desired interphasial chemistry on graphitic anode, the slowed Li ion kinetics and difficult electrode wetting due to high electrolyte viscosity, the increased electrolyte/cathode interphasial impedance, additional processing cost of material manufacturing, and sacrificed cathode energy density, et cetera.

[0013] It is therefore still of significant interest to the battery industry to identify such electrolytes that can stably support reversible Li ion chemistry, without those shortcomings exhibited by the prior arts.

[0014] It is therefore still of significant interest to the battery industry to identify such compounds that, once incorporated as an electrolyte component, can assist in forming a protective layer on the surface of the 5.0 V class cathodes.

[0015] It is therefore still of significant interest to the battery industry to identify such compounds that could serve the aforementioned purposes either as electrolyte solvent, co-solvent, solute, or both molecular and ionic additives.

[0016] This invention will provide such a technology of the electrolytes with all those desired merits.

[0017] Therefore, it is highly desirable to develop electrochemical cells that can reversibly store and release electricity at voltages above 4.5 V.

[0018] More specifically, it is highly desirable to develop electrochemical cells that can reversibly store and release electricity at voltages in the neighborhood of or above 5.0 V.

[0019] Still more specifically, it is highly desirable to develop the aforementioned electrochemical cells, which include, but are not limited to, rechargeable batteries that are based on Li ion chemistry, or electrochemical double-layer capacitors that comprise high surface area electrodes.

[0020] Still more specifically, it is highly desirable to develop the aforementioned electrochemical cells based on Li ion chemistry, which comprise of 5.0 V class cathode materials such as, but are not limited to, spinel metal oxide LiNi_0.5Mn_1.5O₄ or olivine phosphate LiCoPO₄, and materials of other chemical natures.

[0021] Still more specifically, it is highly desirable to develop the aforementioned electrochemical cells based on electrochemical double layer capacitance, which comprise high surface area materials as electrodes, such as, but are not limited to, activated carbon, aligned or random carbon nanotubes, various aerogels and materials of other chemical natures.

[0022] Further more specifically, it is highly desirable to formulate novel electrolyte compositions that would enable the aforementioned electrochemical cells.

[0023] Further more specifically, it is highly desirable to identify and develop novel compounds that, once incorporated into electrolytes either as electrolyte solvent, co-solvent, solute or molecular and ionic additives, would assist in stabilizing the electrolyte against oxidative decompositions, without negatively impacting the properties and performances of the electrochemical cells as in the prior arts.

[0024] It is therefore the primary objective of the present invention to identify and develop such novel compounds.

[0025] It is another objective of the present invention to develop the electrolyte compositions utilizing the said novel compounds either as solvent, co-solvent, solute, or molecular and ionic additives. Electrolytes so formulated will have an extra wide electrochemical stability window, and are capable of supporting electrochemical processes occurring at high potentials without degrading.

[0026] It is still another objective of the present invention to assemble electrochemical cells utilizing the said electrolyte solutions. The said electrochemical cells include, but are not limited to, rechargeable batteries or electrochemical double-layer capacitors that have been described above. The cells thus developed should deliver superior performances as compared with the state-of-the-art technologies in terms of the energy density and energy quality.

[0027] These and additional objectives of the invention are accomplished by adopting one or more novel compounds either as solvent, co-solvent, solute, or molecular and ionic additives in the non-aqueous electrolytes.

[0028] More particularly, these and additional objectives of the invention are accomplished by adopting one or more novel compounds in the non-aqueous electrolytes, which are soluble in the non-aqueous, organic electrolyte solvents to certain concentrations.

[0029] Still more particularly, these novel compounds, upon dissolution in the non-aqueous electrolytes, will form desirable interphasial chemistry on cathode surfaces.

[0030] Still more particularly, these novel compounds, upon dissolution in the non-aqueous electrolytes, will either form desirable interphasial chemistry on anode surfaces, or will not negatively impact the other electrolyte components to form desirable interphasial chemistry on anode surfaces.

[0031] With the electrolyte solutions comprising these novel compounds either as solvent, co-solvent, solute, or molecular and ionic additives in the non-aqueous electrolytes, all the said objectives can be achieved.

[0032] Before describing the present invention in detail, it is necessary to define the terminologies used in this invention so that it helps to understand the spirit of this invention. It is to be understood that the definition herein is for the purpose of describing particular embodiments only, and is not intended to be limiting.

[0033] In the present disclosure, the said “organic” refers to a structure that contains hydrocarbon moieties.

[0034] In the present disclosure, the said “inorganic” refers to a structure that contains no hydrocarbon moieties.

[0035] In the present disclosure, the said “halogen” refers to fluorine, chlorine, bromine and iodine.

[0036] In the present disclosure, the said “alkyl” refers to a hydrocarbon structure, with or without unsaturations, or their perhalogenated or partially halogenated derivatives.

[0037] The said “solvent” refers to molecular components of the electrolyte.

[0038] The said “solute” or “salt” refers to ionic components of the electrolyte, which will dissociate into cationic and anionic species upon dissolution in the solvents or mixture of co-solvents.

[0039] The said “co-solvents” refers to molecular components of the electrolyte whose concentrations are at least 10% by weight.

[0040] Furthermore, the said “additives” are the molecular components of the electrolyte whose concentrations are lower than 10% by weight.

[0041] The said “molecular” refers to compounds that cannot be dissociated into any ionic species in non-aqueous electrolyte solvents.

[0042] The said “ionic” refers to compounds that can be dissociated into a cation species that bears positive charge and an anion species that bears equal but negative charge in non-aqueous electrolyte solvents.

[0048] Table 1 summarizes the skeleton structures of the electrolyte additives of this invention.

[0049] Table 2 shows selected examples of these additives that were used to form electrolytes.

[0050] Table 3 showed selected electrolyte solution formulations by using the base electrolyte and the novel compounds disclosed in the present invention, as either electrolyte solvent, co-solvent, solute, or additives.

[0051] As a primary aspect of the invention, the novel compounds of the present invention are constructed on the basis of the molecular or ionic compounds whose skeleton structures were shown in structures 1 through 8 in Table 1, where: R¹, R², R³, R⁴, R⁵ and R⁶ designate substituent, which can be identical or different from each other. These are hydrogen, hydroxyl, or halogen. The hydroxyl is hydroxide salts with metal ions of various valences. Examples of which include, but are not limited to, Li⁺, Na⁺, ½Mg²⁺, ¼Al³⁺, et cetera. These are normal or branched alkyls with carbon number from 1 through 30, with or without unsaturation. These are halogenated normal or branched alkyls with carbon number from 1 through 30, with or without unsaturation which can be partially halogenated or perhalogenated, normal or branched alkyls with carbon number from 1 through 30, with or without unsaturation; which can be partially halogenated or perhalogenated normal or branched alkyls with carbon number from 1 through 30, where the halogen substituents can be identical or different selected from F, Cl, Br or I, or mixture of all halogens.

TABLE 1
Structure of Novel Compounds in the Present Invention

[0052] Examples of R¹, R², R³, R⁴, R⁵ and R⁶ include, but are not limited to, trifluoromethyl, trichloromethyl, 1,1,1-trifluoroethyl, perfluoroethyl, perfluoro-iso-propyl, 1,1,1,3,3,3,-hexafluoropropyl, perfluoro-tert-butyl, perfluorododecayl, et cetera. As a way to illustrate, Table 2 listed, selected compound that were included in the compound families as described in Table 1.

TABLE 2
Example of Novel Compounds covered in Table 1

[0053] Preferentially, but not intended to be limiting, the novel compounds can be dissolved in typical non-aqueous electrolyte solvent or mixture of solvents.

[0054] Preferentially but not intended to be limiting, the novel compounds can serve in the electrolyte either as major solvents, or co-solvents at contents above 10% by weight, or as salts at concentrations as high as 3.0 m, or as additives at concentrations below 10% by weight.

[0055] The above-mentioned typical non-aqueous electrolyte solvents comprise, but are not limited to, organic carbonate esters such as ethylene carbonate (EC), propylene carbonate (PC), dimethylcarbonate (DMC), ethylmethylcarbonate (EMC), diethylcarbonate (DEC), 1-(trifluoromethyl)ethylene carbonate (CF₃-EC), et cetera; or organic acid esters such as alkyl carboxylates, lactones, et cetera; and inorganic acid esters such as alkyl sulfonates, alkyl sulfurates, alkyl phosphonates, alkyl nitrates, and et cetera; or dialkyl ethers that are either symmetrical or unsymmetrical, or alkyl nitriles.

[0056] The above-mentioned typical non-aqueous electrolytes also comprise electrolyte solutes that are based on a cation and an anion. The cation selections include but are not limited to, alkali metal salts such as lithium (Li), sodium (Na), potassium (K), et cetera, or alkali earth metal salts such as beryllium (Be), magnesium (Mg), calcium Ca), et cetera, or tetraalkylammonium or phosphonium (R₄N, R₄P); whereas the anion selections include but are not limited to (PF₆), hexafluoroarsenate (AsF₆), tetrafluoroborate (BF₄), perfluoroalkylfluorophosphate (PF_XR_F(6-X)), perfluoroalkylfluoroborate (BF_XR_F(4-X)), bis(trifluoromethanesulfonyl)imide ((CF₃SO₂)₂N), bis(perfluoroethanesulfonyl)imide ((CF₃CF₂SO₂)₂N), bis(oxalato)borate ((C₂O₄)₂B), (difluorooxalato)borate (C₂O₄FB). The salts are selected by combining these cation and anions.

[0057] The above-mentioned typical electrolyte solutes are selected from the novel compounds of the present invention.

[0058] More preferentially but not intended to be limiting, the novel compounds of this invention comprise at least one fluorine in the structure.

[0059] With the purpose of illustrating only and no intention to be limiting, novel compounds of this invention can be selected from the following list: tris(1,1,1,3,3,3-hexafluoro-iso-propyl)phosphate (compound II in Table 2), tris(perfluoroethyl)phosphate, tris(perfluoro-iso-propyl)phosphate (compound 12 in Table 2), bis(1,1,1-trifluoroethyl)fluorophosphate (compound 10 in Table 2), tris(1,1,1-trifluoroethyl)phosphite (compound 9 in Table 2); hexakis(1,1,1-trifluoroethoxy)phosphazene (compound 14 in Table 2), and tris(1,1,1-trifluoroethoxy)trifluorophosphazene (compound 15 in Table 2), et cetera.

[0060] In yet further aspects of the invention, electrochemical devices that are filled with the novel electrolyte solution formulated in this invention are fabricated. These devices include, but are not limited to, (1) lithium batteries with lithium metal cells as anode, and various transition metal oxides, phosphates and fluorides as cathode; (2) Li ion batteries with carbonaceous such as graphitic, carbon nanotube, graphene as anode, or non-carbonaceous such as titania or other Li⁺ intercalating hosts as anode, and various transition metal oxides, phosphates and fluorides as cathode; (3) electrochemical double-layer capacitors with both carbonaceous and non-carbonaceous electrodes of high surface area or high pseudo-capacitance; and (4) dual intercalation cells in which both cation and anion intercalate simultaneously into lattices of anode and cathode materials of either carbonaceous or non-carbonaceous natures, respectively.

[0061] The above cells are assembled according to the procedures that can be readily performed by one with ordinary skill in the art. These electrochemical devices containing the novel electrolyte solutions as disclosed in the present invention can enable high voltage rechargeable chemistries that would be otherwise impossible with the state-of-the-art electrolyte technologies.

[0062] Having described the invention, the following examples are given to illustrate specific applications of the invention including the best mode now known to perform the invention. They are intended to provide those of ordinary skills in the art with a complete disclosure and description of how to make and use the novel solvents and additives of this invention. These specific examples are not intended to limit the scope of the invention described in this application.

[0063] To a flask containing 500 mL of diethyl ether, 175 g of 1,1,1,3,3,3-hexafluoroisopropanol is added and stirred until a complete solution is made. To the stirring solution of diethyl ether and 1,1,1,3,3,3-hexafluoropropanol, 8.28 g of solid lithium hydride is added through a solid-addition funnel and allowed to react at room temperature. After 1 hour, the reaction mixture is chilled to the range of 0-5° C. by immersion in a water/ice bath. Once chilled, 53.21 g of phosphorus oxychloride is carefully added. The reaction is considered complete once no more insoluble lithium chloride is formed during reflux of the reaction mixture. The final product, tris(1,1,1,3,3,3-hexafluoroisopropyl)phosphate, is recovered by distillation after filtering off the precipitation.

[0064] The synthesis of precursor tris(iso-propyl)phosphate was conducted in a similar manner as described in Example 1. The intermediate phosphate was then subjected to either elemental fluorination or electrochemical fluorination to achieve the perfluorinated product. The final product, tris(perfluoro-iso-propyl)phosphate, is recovered by after purification.

[0065] The synthesis of 1,1,1-trifluoroethoxide lithium was similar to the procedure as described in Example 1. 53.21 g of phosphorus oxychloride is then carefully added. The reaction is considered complete after refluxing. The final product, tris(1,1,1-trifluoroethyl)phosphate, is recovered by distillation after filtering off the precipitation.

[0066] To a flask containing 500 mL of diethyl ether, 69.1 g of 1,1,1-trifluoroethanol is added and stirred until a complete solution is made. Then 5.48 g lithium hydride was gradually added through a solid addition funnel. After 1 hour, the reaction mixture is chilled to the range of 0-5° C. by immersion in a water/ice bath. Once chilled, 40 g of phosphonitrillic chloride trimer was carefully added with vehement stirring. The purification process was similar to what described in Example 1. After repeated distillation, the final product is a colorless liquid with boiling point of 100° C. at 0.1 ton.

[0067] This example summarizes a general procedure for the preparation of novel electrolyte solutions comprising the solvents, solutes and additives of this invention, whose structures have been listed in Table 1. Both the concentration of the lithium salts, the co-solvent ratios, and the relative ratios between the additives to solvents can be varied according to needs.

[0068] The salts selected include, but are not limited to, LiPF₆, LiAsF₆, LiBF₄, LiP(C_nF_2n+1)_xF_6-x (0≦n≦10, 0≦x≦6), LiB(C_nF_2n+1)_xF_4-x (0≦n≦10, 0≦x≦4), LiIm, LiBeti, LiBOB, and LiBF₂C₂O₄, triethylmethylammonium (Et₃MeNPF₆), any one or more of the novel compounds whose structures were listed in Table 1, and mixtures thereof.

[0069] The solvents selected include, but are not limited to, EC, PC, DMC, DEC, EMC, FEC, CF₃-EC, any one or more of the novel compounds whose structures were listed in Table 1, and mixtures thereof.

[0070] The additives selected include any one or more of the novel compounds whose structures were listed in Table 1 or Table 2, and mixtures thereof.

[0071] The resultant electrolyte solution should contain at least one of those novel compounds that are disclosed in the present invention.

[0072] In one instance, 1000 g base electrolyte solution of 1.0 m LiPF₆/EC/EMC (30:70) was made in glovebox by mixing 300 g EC and 700 g EMC followed by adding 151.9 g LiPF₆. The aliquots of the base electrolyte solution was then taken to be mixed with various amount of Tris(1,1,1,3,3,3-hexafluoro-iso-propyl)phosphate as synthesized in Example 1. The concentration of Tris(1,1,1,3,3,3-hexafluoroisopropyl)phosphate ranges from 0.1 ppm up to 5%.

[0073] In a similar instance, 1000 g base electrolyte solution of 1.0 m LiPF₆/FEC/EC/EMC (15:15:70) was made in glovebox by mixing 150 g FEC, 150 g EC and 700 g EMC followed by adding 151.9 g LiPF₆, and aliquots of the base electrolyte solution was then taken to be mixed with various amount of Tris(1,1,1,3,3,3-hexafluoroisopropyl)phosphate as synthesized in Example 1. The concentration of Tris(1,1,1,3,3,3-hexafluoroisopropyl)phosphate ranges from 0.1 ppm up to 5%.

[0074] In another similar instance, 1000 g base electrolyte solution of 1.0 m LiPF₆/Tris(1,1,1,3,3,3-hexafluoroisopropyl)phosphate/EC/EMC (15:15:70) was made in glovebox by mixing 150 g Tris(1,1,1,3,3,3-hexafluoroisopropyl)phosphate as synthesized in Example 1, 150 g EC and 700 g EMC followed by adding 151.9 g LiPF₆.

[0075] In other similar instances, the novel electrolyte solutions with other novel compounds at varying concentrations were also made with tris(perfluoro-iso-propyl)phosphate (compound 12 in Table 2), or hexakis(1,1,1-trifluoroethoxy)phosphazene (compound 14 in Table 2), or Tris(1,1,1-trifluoroethyl)phosphate, et ceteras.

[0076] With purpose of illustrating only and no intention to be limiting, Table 3 listed some typical electrolyte solutions prepared and tested. It should be noted that the compositions disclosed in Table 3 may or may not be the optimum compositions for the electrochemical devices in which they are intended to be used, and they are not intended to limit the scope of the present invention.

TABLE 3
Novel Electrolyte Solutions with Ionic Additives
Salt		Additive
Concentration		Concentration
(m)	Solvent Ratio (by Weight)	(by Weight)
LiPF6 (1.0)	EC/FEC/EMC (15:15:70)	1% Compound 9
LiPF6 (1.0)	EC/Tris(1,1,1-	1% Compound 11
	trifluoroethyl)phosphate/EMC
	(20:10:70)
LiPF6 (1.0)	EC/EMC (30:70)	1% Compound 14
LiPF6 (1.0)	EC/EMC (30:70)	1% Compound 9
LiBF4 (1.0)	EC/EMC (30:70)	1% Compound 11
LiBOB (1.0)	EC/γBL/EMC/MB	1% Compound 9
	(15:15:30:30)
Et3MeNPF6 (2.0)	EC/EMC (30:70)	1% Compound 16
LiPF6 (1.0)	EC/(1,1,1,3,3,3-	None
	hexafluoroisopropyl)methyl
	carbonate/EMC (20:10:70)

[0077] This example summarizes the general procedure of the assembly of electrochemical cell. These electrochemical cells include Li ion cell, double layer capacitor, or dual intercalation cell. Typically, a piece of Celgard polypropylene separator was sandwiched between an anode and a cathode. The cell was then activated by soaking the separator with the electrolyte solutions as prepared in Example 5, and sealed with appropriate means. All above procedures were conducted under dry atmospheres in either glovebox or dryroom.

[0078] With the invention having been described in general and in details and the reference to specific embodiments thereof, it will be apparent to one ordinarily skilled in the art that various changes, alterations, and modifications can be made without departing from the spirit and scope of the invention and its equivalents as defined by the appended claims.