Rechargeable hybrid battery/supercapacitor system

A rechargeable hybrid battery/supercapacitor electrical storage system capable of providing high energy and high power densities comprises an intercalation electrode (17) and a capacitor electrode (13) combined with a separator (15) and electrically-conductive current collector elements (11, 19) to form a unitary cell structure (10). An electrolyte solution of a dissociable salt absorbed into the porous structure of the separator (15) provides complementary ion species which respectively reversibly intercalate into the one electrode (17) and capacitively adsorb at the surface of the other electrode (13) upon the application of charging current. The high density stored electrical energy may be recovered at high power over extended periods upon demand of a utilizing device and may be rapidly restored to stable capacity through numerous charging cycles.

BACKGROUND OF THE INVENTION
 The present invention relates to electrical energy storage systems which
 may be recharged over numerous cycles to provide reliable power sources
 for a wide range of electrical utilization devices. The invention is
 directed in particular to a rechargeable storage system which is capable
 of exhibiting both high energy density normally associated with batteries,
 and high power density and long operative life typical of supercapacitors.
 In the present invention, such a system comprises a multi-layer energy
 storage device structure which incorporates respective positive and
 negative electrode elements comprising pseudocapacitor or double-layer
 supercapacitor materials and rechargeable intercalation battery materials
 in a unitary, flexible structure which may be sized and shaped as desired
 to be most compatible with utilization devices while providing
 advantageously high energy and power densities.
 Modern applications requiring mobile electrical energy sources, ranging
 from personal telecommunications devices to electric vehicles, are
 proliferating at an exponential rate. The demands of these applications
 range widely, for example, in voltage or power level, but all are
 preferably served by light-weight storage devices which can rapidly
 provide consistently high energy density over long time spans and can be
 quickly recharged to operational energy levels. To date, these extensive
 mobile energy needs are being met, in a fashion, by one or the other of
 the two available types of storage devices, viz., rechargeable batteries,
 such as lithium-ion intercalation systems, or supercapacitors of either
 faradic pseudocapacitive or non-faradic double-layer reaction type.
 The choice between these battery or supercapacitor systems is normally
 dictated by the more pressing of the application's demand for high energy
 density, available from batteries, or for the rapid delivery of high
 power, provided by supercapacitors. Attempts to meet requirements for both
 high energy and high power densities in a single application have led in
 some instances to the utilization of both device types arranged together
 in such a manner that the battery is available to recharge the
 supercapacitor between periods of high power demand. The disadvantage of
 such a practice in the excessive weight factor alone is clearly apparent.
 Additional limitations on this expedient are reflected in the time
 requirement for battery charging, as well as in the multiplicity of cells
 and in battery life cycle which may often be shortened by the physical
 rigors of the intercalation battery charging operation.
 The system of the present invention represents a remarkable advancement in
 means for meeting the requirements of mobile electrical energy utilization
 in that it combines the desirable characteristics of both the battery and
 the supercapacitor in a single integrated device of light weight and
 extended energy capacity. Comprising opposing electrodes of, for example,
 an activated carbon supercapacitor element and an intercalatable battery
 composition, particularly a transition metal oxide spinel material having
 a structure which exhibits rapid ion diffusion and little physical
 distortion from intercalation, the system is able to exhibit both the high
 energy storage capability of batteries and the high speed power delivery
 and exceptional cycle life of supercapacitors. An additional advantage of
 this unique combination of faradic battery intercalation and capacitive
 surface charging is the realization of intercalation systems which would
 not otherwise be available due to the sparsity of receptive
 counter-electrode materials able, for instance, to accommodate cations of
 considerable size, e.g., alkaline earth cations.
 The hybrid systems of the present invention can utilize most of the
 respective compositions of previous rechargeable intercalation batteries
 and supercapacitor devices. Such earlier devices are typically
 represented, e.g., in U.S. Pat. Nos. 5,418,091 and 5,115,378. As in these
 earlier systems, intercalating electrodes may comprise metallic sulfides,
 oxides, phosphates, and fluorides, open-structured carbonaceous graphites,
 hard carbons, and cokes, and alloying elements, such as aluminum, tin, and
 silicon. Similarly, surface-active capacitor materials, typically high
 surface area closed-structure activated carbon powders, foams, fibers, and
 fabrics may be used in the counter-electrodes. The additional active
 electrolyte element of the hybrid systems may likewise be formulated of
 prior available materials, with particular utility being enjoyed in the
 non-aqueous solutions of intercalatable alkali and alkaline earth cations,
 usually incorporated in significantly fluid form in fibrous or polymer
 matrix containment materials, thus maintaining an environment conducive to
 mobility of both species of electrolyte ions. The laminated polymeric
 layer format typified by the secondary batteries described in U.S. Pat.
 No. 5,460,904 and related publications serves well for the structures of
 the present invention.
 SUMMARY OF THE INVENTION
 A hybrid battery/supercapacitor structure of the present invention
 comprises, in essence, negative and positive electrode members with an
 interposed insulative ion-transmissive separator member containing a fluid
 electrolyte. These functional members are preferably in the form of
 individual layers or membranes laminated together to form a flexible,
 unitary structure. The negative "battery" electrode member layer comprises
 a composition of an intercalatable material, preferably a spinel compound
 dispersed in a polymeric matrix of, for example, a copolymer of
 poly(vinylidene fluoride-co-hexafluoropropylene). To provide low
 resistance electrical current conduction between electrodes, the battery
 layer may be thermally laminated to a conductive current collector
 element, such as a reticulated metal foil. The positive "supercapacitor"
 counter-electrode member layer is similarly fabricated of an activated
 carbon composition in a matrix of the copolymer along with a current
 collector foil.
 Interposed between the electrode members is the separator member which may
 comprise any of the previously employed high-porosity, microporous, or
 absorptive polymer film layers or membranes within which is dispersed a
 solution of electrolyte salt comprising an intercalatable cation, e.g., 1
 M solution of LiPF.sub.6 in a mixture of 2 parts ethylene carbonate and 1
 part dimethyl carbonate. Such an electrolyte ensures essential ionic
 conductivity and mobility within the system structure. In the present
 invention this mobility serves the notable purpose of enabling the rapid
 flow of both ion species of the electrolyte salt to and from the
 respective electrodes during charge and discharge of the device. The high
 degree of fluidity enables a relatively unrestricted migration of the
 larger, previously inactive and unutilized anion species to adsorption at
 the positive electrode where they participate in the capacitive charging
 at that system member.
 Thus the usual cation migration to effect intercalation within the negative
 electrode during a charging cycle, which normally serves as the sole mode
 of energy storage in prior battery structures, is augmented by anion
 migration from the electrolyte to the positive electrode surface to effect
 a capacitive charging, e.g., of the non-faradic double-layer type. This
 combined effect of faradic intercalation battery charging and non-faradic
 capacitor charging rapidly builds a high energy density which may be
 recovered in an equally rapid manner to yield high power density upon
 application demand. By judicial choice of electrode materials, that is,
 those respective intercalation and capacitor electrode member compounds
 presenting desired electrical charging potential differences, varied
 voltage levels may be achieved in the hybrid storage device.

DESCRIPTION OF THE INVENTION
 A laminated hybrid battery/supercapacitor structure 10 typical of the
 present invention is generally depicted (not to scale) in FIG. 1 and
 includes a positive electrode member comprising a current collector foil
 11, preferably in the form of an open mesh aluminum grid with an extending
 terminal tab 12, which is laminated under heat and pressure to electrode
 element 13 comprising an activated carbon layer, such as a carbon fiber
 fabric or a composition of powdered carbon in a polymeric binder matrix.
 A negative electrode member comprises a current collector foil 19,
 preferably in the form of a open mesh copper grid having a terminal tab
 16, similarly laminated to an intercalation electrode element 17
 comprising, for example, a polymeric matrix in which is dispersed a
 finely-divided, e.g., in the sub-micrometer range, intercalation compound
 such as a preferred spinel, Li.sub.4 Ti.sub.5 O.sub.12. The structure of
 this preferred compound advantageously presents intercalation sites of
 sufficient dimension that a system cation, e.g., Li.sup.+, may be rapidly
 accommodated and diffused within the crystal structure without introducing
 expansion stresses which could lead to loss of energy storage capacity and
 useful life after extended charge/discharge cycling. While the noted
 spinel is remarkable in this respect, numerous other intercalation
 materials, such as those mentioned in the noted publications and hereafter
 in this specification, are entirely satisfactory for use in active
 compositions for the negative electrode of the present system.
 A separator member comprising a membrane 15 of, for example, an ultra-high
 molecular weight micro-fibrillar polyolefin, a hyperporous copolymeric
 membrane, or other type of inert electron-insulating, ion-transmissive
 medium capable of absorbing electrolyte solution is interposed between
 electrode elements 13, 17 of the composite electrode members. The
 separator member of the system is preferably at least partially of
 thermoplastic or thermoadhesive composition in order to facilitate
 lamination by the application of heat and pressure to soften the surfaces
 of the separator membrane and effect its firm bonding to the system
 electrodes. In a testing mode, as noted below, the hybrid device may
 include a reference electrode 14.
 Upon completion of the laminated cell structure, electrolyte solution of
 the type earlier described may be applied for a time sufficient to allow
 its absorption into the porous structure of separator 15 in order to
 provide the essential ion mobility within the system. Preferred
 electrolytes comprise non-aqueous solutions of dissociable salts providing
 intercalatable cation species, such as alkali, e.g., Li+, alkaline earth,
 e.g., Mg.sup.++, lanthanide, Al.sup.+++, or Zn.sup.++ moieties. These
 electrolytes likewise provide for operation of the system such
 complementary anion species as PF.sub.6.sup.-, BF.sub.4.sup.-, or
 ClO.sub.4.sup.-.
 A representative embodiment of the present invention may be more
 particularly fabricated and employed as shown in the following examples.
 EXAMPLE 1
 A separator membrane 15 is prepared in the manner which has served
 successfully in the fabrication of rechargeable Li-ion batteries, such as
 described in the earlier-noted patent specifications. In particular, the
 membrane is cast from a composition comprising a solution of 6 g of 88:12
 poly(vinylidene fluoride-co-hexafluoropropylene) (VdF:HFP) copolymer of
 about 380.times.10.sup.3 MW (available commercially from Atochem North
 America as Kynar FLEX 2801) and 10 g of a compatible organic plasticizer,
 dibutyl phthalate (DBP), in about 40 g of acetone. An additional 4 g of
 powdered fumed silica is dispersed into the solution in a mechanical
 blender, and the composition is cast and dried to a flexible membrane of
 about 0.075 mm thickness. The composition may comprise alternative
 plasticizers, such as dimethyl phthalate, diethyl phthalate, or tris
 butoxyethyl phosphate, and other inorganic filler adjuncts, such as fumed
 alumina or silanized fumed silica, may be used to enhance the physical
 strength of the separator membrane and, in some compositions, to increase
 the subsequent level of electrolyte solution absorption.
 EXAMPLE 2
 A positive electrode coating composition is prepared by suspending 10 g of
 a high surface area (1500 m.sup.2 /g) activated carbon powder in a
 solution of 20 g of 88:12 VdF:HFP copolymer (Atochem Kynar FLEX 2801) and
 30 g of plasticizer (DBP) in about 160 g of acetone. The mixture is
 stirred in a mechanical blender for about 10 minutes to ensure homogeneity
 and is then cast and allowed to dry in air at room temperature for about 1
 hour. The resulting tough, flexible supercapacitor electrode membrane 13
 is readily cut to desired test cell size of about 50 mm.times.80 mm. An
 aluminum open mesh grid about 50 .mu.m thick, e.g., a MicroGrid precision
 expanded foil marketed by Delker Corporation, is cut to form a current
 collector element 11 (FIG. 1) of about 50 mm.times.100 mm and placed in
 face contact with membrane 13 so as to provide an extending collector
 terminal tab 12 of about 20 mm. This assemblage is passed between rollers
 heated to about 125.degree. C. A at a pressure of about 45 N per linear cm
 of roller contact where the polymeric electrode composition of membrane 13
 is softened sufficiently to penetrate the open mesh of the grid collector
 11 and establish a bond which firmly embeds the collector to form a
 unitary positive electrode member.
 EXAMPLE 3
 A negative intercalation electrode membrane is similarly prepared from a
 casting composition comprising a suspension of 10.5 g of pulverized
 Li.sub.4 Ti.sub.5 O.sub.12 and 1.2 g of Super-P conductive carbon powder
 in a solution of 2.8 g of the VdF:HFP copolymer of Example 1 and 4.3 g of
 DBP in about 20 g of acetone. A 50 mm.times.80 mm electrode membrane 17 is
 likewise laminated to a current collector 19 of Microgrid expanded copper
 foil having an extending terminal tab 16. In preparing this electrode
 member and the counter-electrode of Example 2 care is taken to provide a
 reasonable balance in the respective amounts of active capacitor and
 intercalation materials comprising the final electrodes. Such a balance is
 based upon the predetermined energy storage capacity of the respective
 electrodes and is effected primarily by adjusting the cast thickness of
 the membranes. Thus, in these examples where the intercalation electrode
 provides the higher specific capacity, viz., about 150 mAh/g as compared
 to the 30 mAh/g of the capacitive charging electrode, that negative
 electrode membrane may be cast at a thickness providing the spinel at
 about 20% of the mass of the positive electrode activated carbon compound.
 EXAMPLE 4
 To complete the fabrication of a unitary hybrid battery/supercapacitor cell
 device embodying the present invention, the respective positive and
 negative electrodes members prepared in Examples 2 and 3 are arranged with
 an interposed separator membrane of Example 1, and the assemblage is
 laminated in the previous manner using a heated roller apparatus, such as
 a commercial card laminator, at a temperature of about 135.degree. C. In
 order to avoid short-circuiting in the device, terminal tabs 12, 16 of the
 collector elements are formed from laterally spaced portions of grids 11,
 19.
 The final operation in the fabrication process entails activation of the
 hybrid cell device by addition of electrolyte solution in order to achieve
 ionic conductivity and to provide a sufficient reservoir of ion species to
 maintain the charge/discharge cycle activity. In this respect it should be
 noted that the present system utilizes not only cation species, e.g.,
 Li.sup.+, as an active charge transfer medium during reversible
 intercalation at the negative electrode in the manner of prior secondary
 batteries, but also relies upon anion species, e.g., PF.sub.6.sup.-, which
 effect charge storage in double-layer supercapacitor reactivity at the
 positive electrode. Thus, whereas one or the other of the intercalating
 electrodes of prior battery systems could represent a source of cations,
 the electrolyte serves as the primary source of both ion species in the
 present hybrid system. It is therefore important to provide sufficient
 electrolyte to support full and repeated charging over extended cycling. A
 useful measure of such electrolyte is an excess of 2 to 5 times
 stoichiometric amounts.
 Sufficient such activation of cells having structures comprising separator
 members of preformed hyperporous films, microfibrillar membranes, or
 fibrous mats, such as 0.5 mm Whatman borosilicate fiber filter sheet, may
 be readily achieved by simple saturation of the separator with electrolyte
 solution. With cell structures of the present exemplary type comprising
 electrode and separator members of plasticized polymer composition, the
 addition of electrolyte is preferably accomplished through application of
 an electrolyte solution after extraction of the plasticizer from the
 copolymer composition of the structure by immersing the laminated cell
 structure 10 of FIG. 1 in a solvent for the plasticizer which has
 significantly little affect on the copolymer matrix material. For the
 described VdF:HFP copolymers, such a solvent may be diethyl- or dimethyl
 ether, methanol, hexane, or the like. The microporous structure formed
 throughout separator membrane 15 by moderately slow evaporation of the
 extraction solvent provides the laminated cell 10 with an abundance of
 sites for retention of any of the non-aqueous electrolyte solutions
 commonly employed in prior secondary polymeric batteries and
 supercapacitors.
 Notably, there may be employed in the electrolyte solution such organic
 solvents as propylene carbonate, diethoxyethane, diethyl carbonate,
 dimethoxyethane, sulfolane, and dipropyl carbonate and mixtures thereof.
 Also, in the formulation of the activating electrolyte solutions, useful
 lithium salts include LiClO.sub.4, LiN (CF.sub.3 SO.sub.2).sub.2
 LiBF.sub.4, LiCF.sub.3 SO.sub.3, and LiSbF.sub.6 which may be employed in
 solution concentrations of between about 0.5 and 2 M. Of particular
 utility are the exceptional ethylene carbonate/dimethyl carbonate
 compositions of LiPF.sub.6 and mixtures with LiBF.sub.4 described in U.S.
 Pat. No. 5,192,629.
 EXAMPLE 5
 In preparation for using such an electrolyte, laminated polymeric cell
 structure 10 is immersed in a body of diethyl ether where, under mild
 agitation for about 10 minutes, the DBP component of the electrode and
 separator membrane compositions is extracted. After removal from the
 extracting bath and air-drying at room temperature, the cell is activated
 under a moisture-free atmosphere by immersion in a 1 M electrolyte
 solution of LiPF.sub.6 in a 2:1 mixture of ethylene carbonate
 (EC):dimethyl carbonate (DMC) for about 10 minutes during which it imbibes
 the electrolyte solution into the microporous laminate structure to
 substantially replace the original DBP plasticizer. Following a mild
 wiping with absorbent material to remove surface electrolyte, the
 activated battery/supercapacitor cell 10 is hermetically sealed, but for
 the extending terminal tabs 12, 16, within a polyolefin envelope (not
 shown) to maintain a moisture-free environment.
 EXAMPLE 6
 A sample of the foregoing hybrid battery/supercapacitor cell device is
 modified during fabrication for testing purposes by insertion of a silver
 wire electrode 14 into separator membrane 15 in order to provide a common
 reference datum against which to measure the voltage characteristics
 during charge/discharge cycling of the device. For comparison purposes,
 respective prior art Li/C Li-ion intercalation battery and C/C
 double-layer supercapacitor cells are similarly modified in preparation
 for charge/discharge cycle testing. With a comparative voltage of
 approximately -0.05 V vs. Standard Hydrogen Electrode (SHE), the Ag pseudo
 electrode provides a practical near-zero datum against which to plot cell
 operation as progressive voltage changes at the respective electrodes
 during charge and discharge cycle activity.
 In this manner, for example, the voltage levels of deintercalation and
 intercalation of Li.sup.+ ions at the respective positive and negative
 Li.sub.x Mn.sub.2 O.sub.4 and graphite electrodes during charge and
 discharge of a prior battery cell incorporating laminated PVdF:HFP
 membranes and LiPF.sub.6 :EC:DMC electrolyte may be traced as shown in
 FIG. 2 where the voltage levels 24, 26 at those electrodes reach about
 +1.0 V and -3.01 V to yield a full charge battery voltage of about 4.01 V.
 Conversely, as seen in FIG. 2, during discharge of the battery cell with
 deintercalation of Li.sup.+ ions from the negative graphite electrode to
 intercalation at the positive spinel electrode, the respective electrode
 voltages move back toward the base datum.
 EXAMPLE 7
 In similar manner, the voltage change characteristics during
 charge/discharge cycling are plotted for a prior art laminated polymer
 matrix double-layer supercapacitor comprising activated carbon electrode
 membranes. The structure is substantially similar to FIG. 1, utilizing
 PVdF:HFP copolymer and the electrolyte of Example 6. As shown in FIG. 3,
 the symmetrical, regular change of respective charging electrode voltages
 34, 36 from the base datum 22 to +1.25 V and -1.25 V during each cycle is
 typical of capacitive charging and reflects the similar composition of
 those electrodes.
 EXAMPLE 8
 The hybrid battery/supercapacitor embodiment test cell of Example 6 is
 cycled in the previous manner and the electrode voltage characteristics
 are plotted against the Ag pseudo standard base datum 42, as shown in FIG.
 4. As was discovered from such an evaluation, it may be seen that the
 respective positive supercapacitor composition electrode and negative
 battery intercalation composition electrode retain their distinctive
 charge/discharge voltage characteristics 44, 46 between base datum and
 full charge levels of +1.25 V and -1.5 V. These representative charge
 cycling profiles confirm the functioning of the hybrid cell as a means
 wherein, upon charging, the Li.sup.+ ions of the electrolyte intercalate
 into the Li.sub.4 Ti.sub.5 O.sub.12 spinel of the negative electrode to
 provide the high energy density charge 46 of a battery while the
 PF.sub.6.sup.- electrolyte ions adsorb at the positive activated carbon
 electrode to yield the high power density supercapacitor charge 44. As
 shown in FIG. 5, the combined functions of the electrodes which provide
 individual charging voltage accumulations along traces 54, 56 yield a
 cumulative linear charge voltage range 58 providing high energy and power
 density between about 1.5 V and 2.75 V with respect to datum 52.
 The extraordinarily stable capacity of the hybrid battery/supercapacitor
 after extended operational cycling at a constant 1.12 mA may be seen at
 trace 62 of that property in FIG. 6. This advantageous characteristic
 reflects the generally high stability of supercapacitor devices and, in
 the present embodiment, is enhanced by the remarkable property of the
 Li.sub.4 Ti.sub.5 O.sub.12 spinel of the negative electrode which enables
 intercalation without imparting degrading physical expansion stresses to
 the spinel structure.
 Other intercalation materials which may be employed with varying degrees of
 success in the hybrid cells of the present invention include sulfides,
 such as TiS.sub.2, FeS.sub.2, and ZrS.sub.2 ; oxides, such as MnO.sub.2,
 LiMn.sub.2 O.sub.4, MoO.sub.3, WO.sub.3, TiO.sub.2, Co.sub.3 O.sub.4,
 Fe.sub.2 O.sub.3, and Cr.sub.3 O.sub.8 ; phosphates, such as LiFePO.sub.4
 and LiMnPO.sub.4 ; fluorides, such as FeF.sub.2 and FeF.sub.3 ;
 carbonaceous materials, such as graphite, coke, and hard carbon; and
 alloying metals and compounds, such as Al, Sn, SnO.sub.2, and Si. In order
 to obtain significant operating voltage ranges with the present hybrid
 system, it is preferred to select intercalation compounds and materials
 having intercalating voltage ranges which extend significantly below the
 capacitive charging ranges of selected supercapacitor electrode materials,
 e.g., activated carbon. The intercalating voltage ranges of a number of
 useful negative electrode substances as compared to the SHE datum are
 shown in FIG. 7. It has also been found satisfactory to employ the simpler
 expedient of mechanical Swagelock test block cells in the evaluation of
 electrode and electrolyte materials and system operation. Test data are
 seen to be comparable to those obtained with the more fully developed
 laminated cells described in the foregoing examples.
 It is expected that other variants in the structure and fabrication of the
 hybrid battery/supercapacitor systems disclosed in the foregoing
 description will occur to the skilled artisan through the exercise of
 ordinary aptitude, and such variants are nonetheless intended to be
 included within the scope of the present invention as set out in the
 appended claims.