Cathodes for rechargeable lithium-ion batteries

Abstract

The present invention includes an apparatus and method of making and using a composition that includes the replacement of electrochemically inactive additives with a conductive and electrochemically active polymer that is attached so as to make an electrical contract to the redox couples of the electrochemically active oxide particles into/from which Lithium is reversibly inserted/extracted in a battery discharge/charge cycle.

Claims

1. A cathode consisting essentially of: one or more carbon coated olivine CLiM(XO.sub.4), wherein M is selected from Mn, Co, Ni or mixtures thereof and X is selected from Si, P, As, S or mixtures thereof and wherein the lithium is inserted/extracted reversibly into/from the one or more carbon coated olivine CLiM(XO.sub.4); and a cathode filler material consisting essentially of an electronically conductive polymer of repeating units of substituted or unsubstituted monomers of aniline, thiophene, pyrrole, phenyl mercaptan, furan, [(ferrocenyl)amidopropyl] pyrrole or mixtures thereof, wherein the cathode filler material is attached to the one or more carbon coated olivine CLiM(XO.sub.4), and wherein the electronically conductive polymer is a binder material.

2. The cathode of claim 1, wherein X is Si, As or S.

3. The cathode of claim 1, wherein X is P to form the carbon coated olivine CLiM(PO.sub.4).

4. The cathode of claim 1, wherein the lithium content is variable.

5. The cathode of claim 1, wherein the electronically conductive polymer comprises conjugated regions.

6. The cathode of claim 5, wherein the electronically conductive polymer comprises repeating units of substituted or unsubstituted monomers of thiophene, phenyl mercaptan, furan, pyrrole or mixtures thereof.

7. The cathode of claim 5, wherein the electronically conductive polymer comprises monomers which are ring-substituted with one or more straight or branched alkyl, alkoxy, or alkyloxylalkyl groups, which contain from 1 to 30 carbon atoms.

8. A rechargeable electrochemical cell comprising an anode, and the cathode of claim 1, in communication with an electrolyte.

9. The cathode of claim 1, wherein M is Mn and X is P, Si, As or S.

10. The cathode of claim 1, wherein M is Co and X is P, Si, As or S.

11. The cathode of claim 1, wherein M is Ni and X is P, Si, As or S.

12. The cathode of claim 1, wherein M is Mn and X is As or S.

13. The cathode of claim 1, wherein M is Co and X is As or S.

14. The cathode of claim 1, wherein M is Ni and X is As or S.

15. The cathode of claim 1, wherein M is Mn and X is P or Si.

16. The cathode of claim 1, wherein M is Co and X is P or Si.

17. The cathode of claim 1, wherein M is Ni and X is P or Si.

18. A cathode comprising: a cathode filler material consisting essentially of an electronically conductive polymer obtained by polymerization of organic monomers of [(ferrocenyl)amidopropyl]pyrrole and pyrrole, wherein the electronically conductive polymer is a binder material, electrically attached to one or more carbon coated olivine CLiM(XO.sub.4), wherein M is selected from Fe, Mn, Co, Ni or mixtures thereof, and X is selected from Si, P, As, S or mixtures thereof and wherein the lithium is inserted/extracted reversibly into/from the one or more carbon coated olivine CLiM(XO.sub.4).

19. A cathode comprising: an electronically conductive polymer cathode filler material consisting essentially of pyrrole and [(ferrocenyl) amidopropyl] pyrrole in a ratio of about 50:50, and carbon coated olivine CLiM(XO.sub.4) particles attached to the electronically conductive polymer cathode filler material, wherein M is selected from Fe, Mn, Co, Ni or mixtures thereof and X is selected from Si, P, As, S or mixtures thereof and wherein the lithium is inserted/extracted reversibly into/from the carbon coated olivine CLiM(XO.sub.4).

20. The cathode of claim 19, wherein the carbon coated olivine CLiM(XO.sub.4) particles eliminate the need for a polymer binder.

21. The cathode of claim 19, wherein the carbon coated olivine CLiM(XO.sub.4) particles and the electronically conductive polymer have a similar voltage range.

22. The cathode of claim 19, wherein the electronically conductive polymer conducts electrons and stores lithium-ions to provide additional capacity.

23. A cathode consisting essentially of: one or more carbon coated olivine CLiM(XO.sub.4), wherein M is selected from Fe, Mn, Co, Ni or mixtures thereof and X is selected from Si, As, S or mixture thereof and wherein the lithium is inserted/extracted reversibly into/from the one or more carbon coated olivine CLiM(XO.sub.4); and a cathode filler material consisting essentially of an electronically conductive polymer of repeating units of substituted or unsubstituted monomers of aniline, thiophene, pyrrole, phenyl mercaptan, furan, [(ferrocenyl)amidopropyl] pyrrole or mixtures thereof, attached to the one or more carbon coated olivine CLiM(XO.sub.4), wherein the electronically conductive polymer is a binder material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures and in which:

(2) FIG. 1a is a an illustration the structure of pyrrole and of a ferrocenyl complex tethered to a pyrrole by an amidopropyl chain and FIG. 1b illustrates a polypyrrole polymer having pyrrole and [(ferrocenyl)amidopropyl]pyrrole units;

(3) FIG. 2 is a graph of the voltage verses state of charge in a charge/discharge cycle for a cell with a Lithium anode and a cathode of polypyrrole (PPy) polymer having ferrocene tethered to half of the pyrrole units;

(4) FIG. 3a depicts an apparatus for fabrication of a PPy/oxide composite anode by electrodeposition, while FIG. 3b is a graph of several cycles shown in a typical voltammogram;

(5) FIG. 4 is a SEM image of the CLiFePO.sub.4/PPy composite cathode of the present invention;

(6) FIG. 5 is a graph that compares the charge/discharge voltage verses the state of charge curves with a Lithium anode at C/5 rate for CLiFePO.sub.4 with PPy against those of CLiFePO.sub.4 with PTFE and CLiFePO.sub.4 with C and PTFE in the weight ratio of about 75:20:5;

(7) FIG. 6 is a graph that compares the voltage verses state of charge curves at different discharge rates after charging at C/5 rate with a Lithium anode for a cell having a CLiFePO.sub.4, PPy composite cathode with those for a cell having a 70:20:5 weight ratio CLiFePO.sub.4:C:PTFE composite cathode; and

(8) FIG. 7 is a graph that compares the capacities and reversible capacity loss between 2.5 and 4.1V of a CLiFePO.sub.4, PPy composite cathode with those of a 70:20:5 weight ratio CLiFePO.sub.4:C:PTFE composite cathode, each with a Lithium anode, at different C-rates of discharge after a charge at C/5 rate.

DETAILED DESCRIPTION OF THE INVENTION

(9) While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The terminology used and specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and do not delimit the scope of the invention.

(10) To enhance capacity and rate capability the present invention replaces the electrochemically inactive additives (e.g., C and PTFE) of conventional cathode by active polymer. The use of the integrated cathode material advocated by the present invention, while advantageous from an operating perspective, does add some complexity to the electrode manufacturing process. However, since the present method of manufacturing composite cathodes is itself complex, requiring the selective combination of transition-metal oxide particles, a soft conductor like carbon black, and a binding agent like PTFE, the marginal increase in complexity required to manufacture the integrated cathode material of this invention is an insignificant tradeoff for the performance benefits gained when the integrated cathode material is used.

(11) Present-day secondary (rechargeable) lithium batteries use a solid reductant as the anode and a solid oxidant as the cathode. It is important that the chemical reactions at the anode and cathode of a lithium secondary battery be reversible. On discharge, the metallic anode supplies Li.sup.+ ions to the Li.sup.+-ion electrolyte and electrons to the external circuit. The cathode is a host compound into/from which the working Li.sup.+ ion of the electrolyte can be inserted/extracted reversibly as a guest species over a large solid-solubility range (Goodenough 1994) (see generally U.S. Pat. No. 5,910,382 to Goodenough, et al. incorporated herein by reference). When the Li.sup.+ ions are inserted as a guest species is into the cathode, they are charge-compensated by electrons from the external circuit. On charge, the removal of electrons from the cathode by an external field releases Li.sup.+ ions back to the electrolyte to restore the parent host structure. The resultant addition of electrons to the anode by the external field attracts charge-compensating Li.sup.+ ions back into the anode to restore it to its original composition.

(12) The present invention provides new materials for use as cathodes in lithium secondary (rechargeable) batteries. It will be understood that the anode for use with the cathode material of the invention may be any lithium anode material, such as a host for lithium or elemental lithium itself. Preferably, the anode material will be a host for lithium insertion/extraction. Where both the anode and cathode are hosts for the reversible insertion or removal of the working ion into/from the electrolyte, the electrochemical cell is commonly called a rocking-chair cell. An implicit additional requirement of a secondary battery is maintenance not only of the electrode/electrolyte interfaces, but also of electrical contact between host particles and between these particles and the current collector throughout repeated discharge/recharge cycles.

(13) The redox couples of interest for a cathode are associated with antibonding states of d-orbital parentage at transition-metal cations M or 4f-orbital parentage at rare-earth cations Ln in an oxide. The stronger is the cation-anion covalent mixing, the higher is the energy of a given redox couple. Modulation of the strength of the cation-anion covalence at a given M or Ln cation by nearest-neighbor cations that compete for the same anion valence electrons is known as the inductive effect. Changes of structure alter primarily the Madelung energy as is illustrated by raising of the redox energy within a spinel [M.sub.2]O.sub.4 framework by about 1 eV on transfer of Li.sup.+ ions from tetrahedral to octahedral interstitial sites. Changing the counter cation, but not the structure, alters primarily the inductive effect, as is illustrated by a lowering of the Fe.sup.3+/Fe.sup.2+ redox energy by 0.6 eV on changing (MoO.sub.4).sup.2 or (WO.sub.4).sup.2 to (SO.sub.4).sup.2 polyanions in isostructural Fe.sub.2(XO.sub.4).sub.3 compounds. Raising the energy of a given redox couple in a cathode lowers the voltage obtained from cells utilizing a common anode. Conversely, lowering the redox energy of a cathode raises the cell voltage with respect to a common anode.

(14) The invention provides new cathode materials containing oxide polyanions, including the oxide polyanion (PO.sub.4).sup.3 as at least one constituent, for use in secondary (rechargeable) batteries. For example, the cathode materials of the presented invention may have the general formula LiM(PO.sub.4) with the ordered olivine structure or the more open rhombohedral NASICON framework structure of general formula M.sub.2(XO.sub.4).sub.3. The cathode materials of the present invention have the general formula LiM(PO.sub.4) for the ordered olivine structure or for the rhombohedral NASICON framework structure, A.sub.xM.sub.2(PO.sub.4).sub.y(XO.sub.4).sub.3y, where 0<y, M is a transition-metal atom, A is Li or Na and X is Si, As or S and acts as a counter cation.

(15) Insertion of lithium into carbon-coated CFePO.sub.4 was reversible over the several cycles studied. The Li.sub.xFePO.sub.4 material of the present invention represents a cathode of good capacity and contains inexpensive, environmentally benign elements. With small oxide particles attached to the conductive PPy polymer, high rates of charge and discharge are realized over the range of ambient temperatures that a rechargeable Lithium battery is expected to encounter. The output voltage remains flat with little decrease as the discharge rate is increased; the capacity loss at higher rates of discharge is reversible, i.e., is recovered on cycling at lower rates.

(16) Generally, in one aspect, the invention provides an ordered olivine compound having the general formula LiMPO.sub.4, where M is at least one first row transition-metal cation. The alkali ion Li.sup.+ may be inserted/extracted reversibly from/to the electrolyte of the battery into/from the interstitial space of the host MPO.sub.4 framework of the ordered-olivine structure as the transition-metal M cation (or combination of cations) is reduced/oxidized by charge-compensating electrons supplied/removed by the external circuit of the battery in a discharge/charge cycle. In particular, M may be Mn, Fe, Co, Ti, Ni or a combination thereof. Examples of combinations of the transition-metals for use as M include, but are not limited to, Fe.sub.1x Mn.sub.x, where 0<x<1.

(17) Additional formulas for the ordered olivine electrode compounds include, but are not limited to, LiFePO.sub.4, LiMnPO.sub.4, LiCoPO.sub.4, LiNiPO.sub.4, and mixed transition-metal compounds such as LiFe.sub.1x Mn.sub.x PO.sub.4, where 0<x<1. However, it will be understood by one of skill in the art that other compounds having the general formula LiMPO.sub.4 and an ordered olivine structure are included within the scope of the invention.

(18) Redox energies of the host M cations can be varied by a suitable choice of the XO.sub.4 polyanion, where X is taken from Si, P, As, or S and the structure may contain a combination of such polyanions. Tuning of the redox energies allows optimization of the battery voltage with respect to the electrolyte used in the battery. Replacement of the oxide ion O.sup.2 of conventional cathode materials by a polyanion (XO.sub.4).sup.m to take advantage of (1) the larger size of the polyanion, which can enlarge the free volume of the host interstitial space available to the alkali ions, and (2) the covalent XO bonding, which stabilizes the redox energies of the M cations with M-OX bonding so as to create acceptable open-circuit voltages with environmentally benign Fe.sup.3+/Fe.sup.2+ and/or V.sup.4+/V.sup.3+ redox couples.

(19) One example of polymeric material that may be used in the present invention includes pyrrole (C.sub.4H.sub.4NH), which contains both unsaturated carbon bonds and nitrogen. The nitrogen is part of a five-member aromatic ring. In addition derivatives of Pyrrole may be used, e.g., [(ferrocenyl)amidopropyl]pyrrole. Generally, polymerized pyrrole is a good organic conductor and polypyrrole (PPy), is stable under normal operating conditions of temperature, load, voltage and current. One embodiment of the present invention includes a 50/50 ration of [(ferrocenyl)amidopropyl]pyrrole to pyrrole.

(20) Any conductive polymer suitable for forming a suspension in aqueous media may be used. Examples of suitable conductive polymers include polypyrrole, polyaniline and polythiophene or derivatives thereof. Other examples of such suitable polymers include, but are not limited to, polypyrrole, polyaniline, polythiophene, [(ferrocenyl)amidopropyl]pyrrole, polyfuran and derivatives thereof. Such polymers are prepared from the monomers pyrrole, aniline, thiophene, furan, thiophene, [(ferrocenyl) amidopropyl]pyrrole and furan: or from a copolymer including a structural polymer selected from polystyrene, polyacrylate, polyurethane and derivatives thereof. Additional components that may be added to the conductive polymer colloidal compositions include oxidants, additional conductive particles, preservatives and stabilizers. Oxidants are employed to initiate polymerization between monomers in the colloidal compositions. Any suitable oxidant used to initiate polymerization between monomers may be employed.

(21) Additional examples of polymers include polymers containing conjugated regions, or composed entirely, of repeating units which are substituted or unsubstituted aniline, thiophene, pyrrole, and/or phenyl mercaptan (C.sub.6 H.sub.5 SH), polyaniline, polythiophene, polypyrrole, poly(p-phenylene sulfide), and copolymers of these polymers. Polymers useful in the present invention also include polymers of any of the corresponding monomers which are ring-substituted with one or more straight or branched alkyl, alkoxy, or alkoxyalkyl groups, which can contain from 1 up to about 30 carbon atoms, particularly where such substituents are cross-linkable with each other as described in more detail hereinbelow. It will also be recognized that polymers incorporated within the compositions of matter of the present invention may also be copolymers of any one or more of such monomers with other comonomers having ethylenic unsaturation, including but not limited to ethylene, propylene, vinyl chloride, styrene, vinyl alcohol, vinyl acetate and combinations thereof.

(22) Carbon coating of the oxide particle allows an attachment of the conductive polymer to the oxide particle to give electrical conduction between the polymer and the redox couple of the oxide. One method of making the Ppy/CLiFePO.sub.4, where CLiFePO.sub.4 designates carbon-coated LiFePO.sub.4 particles, electrode is through electrodeposition. For example, a three-electrode cell can be used both for electrodeposition and subsequent cyclic voltammetry experiments. Normally, one or more polymers are deposited potentiostatically onto a micro-electrode (e.g., platinum electrode) in the form of thick films, e.g., 15-20 m in depth. A platinum flag can act as the counter electrode. A silver flag or a commercial Ag/AgCl electrode can served as a reference electrode. Persons of ordinary skill in the art will recognize that the electrode may be fabricated using other methods.

(23) Additionally, the incorporation of an oxide into a polypyrrole matrix allows elimination of the inactive carbon-black and TEFLON binder from a conventional cathode, which increases the specific capacity and rate capability while lowering the overpotentials at high discharge rates. PPy/ferrocene films of the present invention had specific capacities up to 65 mAh/g with a short plateau near 3.5 V vs Li.sup.+/Li.sup.0 due to the Fe(III)/Fe(II) redox couple of the ferrocene group.

(24) The present invention includes a rechargeable electrochemical cell having an anode and a cathode in communication with an electrolyte, wherein the cathode includes a conductive polymer attached to one or more transition-metal oxides and wherein Lithium is reversibly inserted/extracted into/from the one or more transition-metal oxides.

(25) The one or more transition-metal oxides may be LiMPO.sub.4. Other compounds may include LiFe(PO.sub.4), Li.sub.3Fe.sub.2(PO.sub.4).sub.3, LiNi.sub.0.5+Mn.sub.0.5+O.sub.2, LiNi.sub.2/3Mn.sub.1/3O.sub.2, Li.sub.3n Fe.sub.2n Ti.sub.n (PO.sub.4).sub.3. LiFePO.sub.4; LiNi.sub.0.5+Mn.sub.0.5+O.sub.2 (01/6); and nanowires of these oxides or combinations thereof.

(26) In embodiments having the composition LiMPO.sub.4 the M is a cation of a metal selected from the group consisting of Fe, Mn, Co, Ti, Ni or mixtures thereof. The conductive polymer may include any conductive polymer and may contain monomers of the same or different compositions. For example, the conductive polymer of the present invention may be [(ferrocenyl)amidopropyl]pyrrole, pyrrole or a combination thereof. In one embodiment the ratio of [(ferrocenyl)amidopropyl]pyrrole to pyrrole is 50/50; however other ratios may be used. The conductive polymer may also include polypyrrole, polyaniline, polythiophene, polyfuran or mixtures thereof.

(27) Another example of the present invention includes a cathode having a conductive polymer in contact with a compound having a formula LiMPO.sub.4, wherein Lithium is reversibly inserted/extracted into/from the oxide. Another cathode of the present invention includes a conductive polymer and one or more transition-metal oxides, wherein Lithium is reversibly inserted/extracted into/from the one or more transition-metal oxides.

(28) The present invention also provides a battery having an anode and a cathode in communication with an electrolyte, wherein the cathode includes a conductive polymer and one or more transition-metal oxides, wherein Lithium is reversibly inserted/extracted into/from the one or more transition-metal oxides.

(29) The battery of the present invention may have one or more transition-metal oxides in the form of LiMPO.sub.4. Other compounds may include a spinel like Li[Li.sub.yMn.sub.2yxNi.sub.xO.sub.4F.sub., Li.sub.3Fe.sub.2(PO.sub.4).sub.3, or LiNi.sub.0.5+Mn.sub.0.5O.sub.2, (01/6); in the form of particles or nanowires. In embodiments having the composition LiMPO.sub.4 the M is a cation of a metal selected from the group consisting of Fe, Mn, Co, Ni or mixtures thereof. The conductive polymer used in the battery of the present invention may include any conductive polymer and may contain monomers of the same or different compositions. For example, the conductive polymer of the present invention may be [(ferrocenyl)amidopropyl]pyrrole, pyrrole or a combination thereof. In one embodiment, the ratio of [(ferrocenyl)amidopropyl]pyrrole to pyrrole is 50/50; however other ratios may be used.

(30) In addition the present invention includes a method of storing rechargeably electricity including supplying electricity to a rechargeable electrochemical cell having an anode and a cathode in communication with an electrolyte. The cathode includes a conductive polymer and one or more transition-metal oxides and wherein Lithium is reversibly inserted/extracted into/from one or more transition-metal oxides. In some embodiments, the conductive polymer includes [(ferrocenyl)amidopropyl]pyrrole, pyrrole or a combination thereof.

(31) FIG. 1 is an illustration of the monomers and PPy chains of the conductive polymer of the present invention. FIG. 1a illustrates the structure of [(ferrocenyl)amidopropyl]pyrrole and pyrrole, while FIG. 1b illustrates the structure of the PPy chains with tethered ferrocene molecules. FIG. 2 is a graph of the voltage verses the state of charge for electrodes of the present invention. FIG. 3 illustrates an apparatus for electrode fabrication by electrodeposition of the present invention and the corresponding cyclic voltammogram of the electrodeposition process.

(32) FIG. 4 is a SEM image of composition the CLiFePO.sub.4/PPy of the present invention. FIG. 5 is a graph that compares the charge/discharge voltage verses state of charge curve taken at 5/C rate of a cell having a CLiFePO.sub.4, PPy composite cathode of this invention with that of a conventional CLiFePO.sub.4:C:PTFE composite cathode. FIG. 6 is a graph that compares the voltage verses the state of charge curves taken on discharge at different rates after charging at C/5 rate of a cell having a CLiFePO.sub.4, PPy composite cathode of this invention with those of a cell having a conventional CLiFePO.sub.4:C:PTFE composite cathode. FIG. 7 is a graph that compares the capacities and reversible capacity loss between 2.5 and 4.1V of a CLiFePO.sub.4, PPy composite cathode with those of a 70:20:5 weight ratio CLiFePO.sub.4:C:PTFE composite cathode, each with a Lithium anode, at different C-rates of discharge after a charge at C/5 rate.

(33) It will be understood that particular embodiments described herein are shown by way of illustration and not as limitations of the invention. The principal features of this invention can be employed in various embodiments without departing from the scope of the invention. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, numerous equivalents to the specific procedures described herein. Such equivalents are considered to be within the scope of this invention and are covered by the claims.

(34) All of the compositions and/or methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the compositions and methods of this invention have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations can be applied to the compositions and/or methods and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit and scope of the invention. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope and concept of the invention as defined by the appended claims.