WO2001071833A1

WO2001071833A1 - TRANSITION METAL CHALCOGENIDE ELECTRODE COMPONENTS FOR LOW-VOLTAGE RECHARGEABLE Li AND Li-ION BATTERIES

Info

Publication number: WO2001071833A1
Application number: PCT/US2001/005893
Authority: WO
Inventors: Sylvie Grugeon; Stephane Laruelle; Philippe Poizot; Jean-Marie Tarascon
Original assignee: Telcordia Technologies, Inc.
Priority date: 2000-03-22
Filing date: 2001-02-23
Publication date: 2001-09-27

Abstract

A low-voltage rechargeable lithium battery cell comprises at least one electrode member (13, 17) comprising a fine-particle chalcogenide of a transition metal, typically an oxide of Co, Cu, Fe, Ni, Mn, or Zn. The electrode component, preferably having a particle size in the range of about 10 nanometres to 5 micrometres, participates with lithium from the complementary electrode source in a reversible redox reaction providing stable battery cell capacities more than doubled compared to those of prior rechargeable battery cells.

Description

TRANSITION METAL CHALCOGENIDE ELECTRODE COMPONENTS FOR LOW-VOLTAGE RECHARGEABLE Li AND Li-ION BATTERIES

BACKGROUND OF THE INVENTION

The present invention relates to low operating voltage rechargeable lithium and lithium-ion battery cells, and, particularly, to the use of 3d transition metal chalcogenides , preferably oxides, as active electrode components of reversible redox reactions with lithium ions in such cells.

In recent years, because of their large specific energy, volumetric energy density, and broad design flexibility, Li and Li-ion batteries have become the technology of choice for portable electronics demanded by today's mobile society. To ensure long-lasting success for lithium ion battery technology, in particular with respect to the lower operating voltages of emerging electronics, advances in cell component materials and electrochemical applications are needed. Further concentration of developments solely on high-voltage energy sources will likely lead to reliance upon voltage-converting electronics in order to achieve desired ranges of lower operating voltages. Such expedients would unfortunately result in loss of device capacities and efficiencies only recently gained in lithium ion technology.

Some inroads have been made in refining lithium ion insertion materials, such as chalcogenides and spinel intercalation compounds, for use in battery electrode compositions providing higher energy density at lower operating voltages, but, although promising, such developments exhibit detracting properties often countering anticipated gains. For example, the initial operational success in increasing battery cell energy density through the use of lithium cobalt nitride compound electrode compositions has been detracted by the restrictive manufacturing requirements for isolating such compositions from moisture.

Numerous other investigations have concentrated on enhancing the electrochemical characteristics of carbonaceous negative electrode component materials which, as a class, are widely used in lithiated intercalation cells. Specially- synthesized hard carbon materials have in some reported instances achieved nearly 50% increases in reversible capacity over more commonly-used soft carbons. Additionally, lithiated vanadium oxide-based electrodes have been found to display large electrochemical capacities, but have at the same time exhibited limited ability to maintain such capacity levels.

Similarly, a great deal of promise appeared in the low intercalation voltage and reversible capacity nearly double that of carbons which were exhibited by an amorphous tin composite oxide that, unfortunately, also showed a considerable irreversible capacity loss during initial charge/discharge cycling. Other investigations have, in addition, revealed appealing low-voltage reversible reactivity in negative electrode materials, such as Li-alloying Sn0 and composites, e.g., SnFe₃C and the like, as well as SiO. Such materials have, however, exhibited the same deleterious irreversible initial and short-lived capacities suffered by the other prospects, whether due to Li-alloying agglomeration or the formation of electrochemically inactive species. Thus, there has remained a need for economical materials which can be used in the composition of Li and Li-ion battery cell electrodes that can exhibit and maintain high reversible charging capacity at low operating voltages. The present invention has effectively satisfied that need.

SUMMARY OF THE INVENTION

The present invention has arisen from unique investigations into the role of lithium in the high electrochemical reactivity at low voltage exhibited by such useful electrode components as vanadates and in the large reversible capacities of resulting battery cells. Whereas the Li-ion had previously been considered to represent only an inactive "spectator" in any redox electrochemistry involved in battery cell cycling, these studies of the reactivity of chalcogenides, particularly fine-particle binary metal oxides, M-O, of the general group of 3d transition metals, with respect to lithium surprisingly revealed that such metal oxides as CoO can reversibly react at low voltages in the average range of about 2 V with about 2.5 Li per unit formula, resulting in cells having reversible capacities not only at least as high as about 600 mAh/g, but also having stable capacity retention over at least as many as 50 recharging cycles.

These detailed examinations into the electrochemical cell reactivity with Li of a series of Co-oxides, including CoO, Co₂0₃ , C03O₄, and lithiated LiCo0₂, have revealed similar cycling response and most remarkably confirmed that CoO, for example, unlike previously-observed Sn0₂ , does not alloy with Li, but rechargeably cycles by means of some different mechanism. The intercalation process was also discounted as the operational means for reversible recharge cycling, particularly in view of the rocksalt crystalline structure of CoO which, lacking empty sites, should not accommodate Li insertion and extraction.

It was ultimately apparent from the examinations into the electrochemical activity mechanism of Li with the transition metal oxides, such as CoO, that the reversible cycling entails for the most part a redox reaction, e.g., CoO + Li⁺ + e^" → Li₂0 + Co which reversibility relies for feasibility on the fine particle size of the metal oxide and Li₂0 active electrode components. Unlike Sn, which tends toward the capacity- diminishing formation of Li-alloy, the highly-oxidizing fine particles of the resulting transition metal appear to preferentially reform the metal oxide, e.g., CoO, during the charging cycle.

Rechargeable lithium battery cells embodying the present invention may be prepared in the manner of the current procedures employed in the art and are thus formed of respective layer members of positive and negative electrode composition with an intervening member of an electron- insulative, ion-transmissive separator layer material which normally comprises an electrolyte, such as a solution of Li- salt, providing a medium for mobility of Li⁺ ions. The means of cell fabrication may comprise any of the practices of the art, for example, the mechanical compression of cell members within a sturdy container, as generally noted in U.S. Patent

5,196,279, or the lamination of polymeric matrix electrode and separator members utilizing adhesive interlayer compositions or thermal means, such as described in U.S. Patents 5,296,318; 5,456,000; 5,460,904; 5,540,741; 5,587,253; and 5 , 840 , 087 , the disclosures of which are incorporated herein by reference.

The M-0 compound, comprising any of the 3d transition metals, such as Co, Cu, Zn, Mn, Ti , Fe, Cr, or Ni , preferably Co, Cu, Ni , or Fe, is employed in the usual manner of active electrode materials, and is thus normally dispersed in finely- divided form throughout a matrix of polymer or copolymer, such as poly (vinylidene fluoride) (PVdF) or (vinylidene fluoride hexafluoropropylene) (PVdF:HFP), and the composition is solvent-cast or otherwise formed into an electrode member layer. The electrode member may then either be employed as the positive cell electrode in combination with a negative member comprising metallic lithium, lithium alloy, e.g., LiAl, , or other source of Li⁺ ions, or as the negative electrode in a Li- ion cell with a positive electrode comprising a lithiated intercalation composition, such as LiCo0₂, LiMn₂0₄, or LiNi0₂.

The electrode members may be used with cell separator members comprising any of the usual microporous polyolefin, glass fiber, or like insulative materials, or those formed of polymer compositions of the type generally described in U.S. Patent 5,296,318. Common electrolyte solutions, such as noted in that patent, may likewise be employed in cells of the present invention.

The M-0 materials utilized in the rechargeable cells of the present invention may be obtained commercially or may be synthesized by known methods, such as by a glycol-based process generally described in U.S. Patent 4,539,041. From either source, the particle size of the oxide materials is preferably between about 10 nanometres and 5 micrometres, a range which appears to have profoundly advantageous effect on the performance of the resulting cell, particularly with respect to the ability to maintain high energy capacity for numerous cycles.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will be described with reference to the accompanying drawing of which:

FIG. 1 depicts schematically in cross section a typical embodiment of a rechargeable Li/MO battery cell of the present invention;

FIG. 2 depicts graphically the voltage/electrode composition profile of Li/MO cells according to FIG. 1 comprising various metal oxides;

FIG. 3 depicts graphically the voltage/electrode composition profile of a Li/CoO cell embodying the present invention;

FIG. 4 depicts graphically the effect of cycling rate on the capacity of a cell of FIG. 3 over several cycles ,-

FIG. 5 depicts graphically the voltage/electrode composition profile of a Li/Cu₂0 cell embodying the present invention; FIG. 6 depicts graphically the effect of Cu₂0 particle size on the capacity stability of a cell of FIG. 5 over several cycles;

FIG. 7 depicts graphically the voltage/electrode composition profile of a Li/NiO cell embodying the present invention;

FIG. 8 depicts the capacity stability of a cell of FIG. 7 over several cycles;

FIG. 9 depicts graphically the voltage/electrode composition profile of a Li/FeO cell embodying the present invention;

FIG. 10 depicts the capacity stability of a cell of FIG. 9 over several cycles;

FIG. 11 depicts graphically the voltage/electrode composition profile of a CoO/LiCo0₂ Li-ion cell embodying the present invention;

FIG. 12 depicts the capacity stability of a cell of FIG. 11 over several cycles;

FIG. 13 depicts graphically the comparative voltage/electrode composition profiles of a Li/CoO cell and a Li/CoS chalcogenide cell embodying the present invention; and FIG. 14 depicts the comparative capacity stability over several cycles of Li/CoO and Li/MO cells comprising various additional metal oxides .

DESCRIPTION OF THE INVENTION

As seen in FIG. 1, a typical rechargeable lithium electrochemical cell embodying the present invention comprises a positive electrode member 13 , a negative electrode member 17, and an interposed separator member 15. In a preferred embodiment, members 13, 17 comprise respective positive and negative electrode composition layers, and separator 15 comprises a membrane, layer, or sheet material which is electron-insulating and ion-transmissive and which is porous or otherwise capable of absorbing and retaining electrolyte, usually in the form of a non-aqueous solution of a lithium salt .

These cell members are in close, ion-conductive contact, either under compressive force within a can or other tightly- packed inflexible container, or, preferably, the members comprise polymeric films or matrixes by means of which the members are laminated together under heat and pressure to yield flexible, unitary cell structures, as described in the incorporated references . Such latter type cells commonly further comprise respective electrically-conductive current collector members 11, 19 associated with the electrode members, preferably by lamination or component adhesion. This preferred laminated cell structure is usually sealed within an impermeable envelope or other flexible containment package (not shown) with sufficient electrolyte solution to ensure saturation of the electrode and separator members and provide adequate lithium ion mobility between electrode members via the separator. Conductor leads (not shown) provide contact, in the usual manner, between collectors 11, 19 and external utilization devices .

In order to simplify fabrication and minimize parameter variables in the following exemplary lithium/metal oxide (Li/MO) electrochemical cells prepared for testing, such cells were assembled in a Swagelok test cell apparatus from 1 cm² disks of Li foil as the negative electrode member 17; Whatman GF/D borosilicate glass fiber sheet saturated with a 1 M LiN(S0₃CF₃)₂ electrolyte solution in 1:1 ethyl carbonate :propylene carbonate as the separator member 15; and an acetone-cast film of 60 parts finely-divided MO, 15 parts super P carbon black (SP, MMM Carbon, Belgium) , and 16 parts dibutyl phthalate (DBP) dispersed in 9 parts (poly) vinylidene-hexafluoropropylene (PVdF-HFP) (Kynar FLEX, Elf Atochem NA) as the basic positive electrode member 13 which comprised about 6 mg of MO.

Prior to assembly in the test cell, film layer 13 was immersed and rinsed in diethyl ether to remove the DBP plasticizer and thereby create in the film a condition conducive to the absorption of electrolyte solution. The Swagelok test cell with compressed cell members was then arranged in circuit with a "Mac-Pile" automatic cycling/data recording system operating at a preselected cycling rate, e.g., C/5 representing a discharge/recharge cycle of 5 h, in the galvanostatic mode to obtain a characteristic signature voltage/electrode composition profile of the test cell. As a means of characterizing the performance of various transition metal oxides in Li electrochemical cells, and to provide further detailed description of the present invention, the following examples were prepared.

Example I

A number of test cells were prepared as outlined above respectively comprising, as the active positive electrode MO component, CoO, Co₃0₄ , and LiCo0₂. The composition profiles of these cells, representing the calculated number of Li ions having reacted with a Co as a function of cell discharge or recharge voltage, over a few C/5 cycles are depicted in FIG. 2. The similarity of operation in these cells is quite apparent.

Example II

A series of Li/CoO test cells was similarly prepared with about 10 nm particle size CoO. The composition signature of one such cell operating in an optimal manner, i.e., with initial deep discharge as detailed below, over about 20 cycles, represented as the spread between a first cycle 32 and a final cycle 34, is depicted in FIG. 3. The exceptional capacity stability of a number of the cells in the series so operated at differing cycle rates may be seen in FIG. 4. It was observed during the testing of these cells that when operated at the common voltage cycle range of between about 3 V and 1 V, i.e., not discharging below the voltage plateau 36 at about 1 V, a cell rapidly loses capacity after about 8-10 cycles . Under separate microscopic and XRD examination it was seen that in this plateau region the positive electrode composition comprised a significant mixture of Co metal and Li₂0 particles with little solid/electrolyte interface (SEI) passivation. Due to this condition, massive particle agglomeration rapidly occurred with resulting precipitous decline in cell capacity. It was surprisingly discovered, however, that further initial cycle discharge beyond the 1 V plateau through region 38 to a substantially complete discharge at about 0.01 V effectively promoted SEI passivation with subsequent limitation on agglomeration and resulting stable capacity, as seen in FIG. 4.

Example III

Samples of Cu₂0 were prepared by the above-noted glycol process in which a solution of 2 g of Cu(N0₃)*3H₂0 in 75 ml ethylene glycol was heated at 2°C per min to 160°C and refluxed for 3 h. The resulting Cu₂0 precipitate of about lμm particle size was separated by centrifuge and washed with acetone before being incorporated into a positive electrode composition in the basic manner. A second sample of Cu₂0 was prepared with a vastly diluted solution of 0.245 g of Cu(N0 )*3H₂0 in 75 ml ethylene glycol resulting in a yield of 0.15 μm particle size which was used to prepare a second series of test cells. The composition signature of a cell comprising the lμm particle size positive electrode Cu₂0 over a 40 cycle spread from 52 to 54 appears in FIG. 5. The effect of the larger particle size in maintaining a stable cell capacity, apparently due to its better resistance to agglomeration, is depicted in FIG. 6. Example IV

A commercial-grade NiO of about 5 μm particle size was used to prepare positive cell members as above. The composition signature of a cell over a 20 cycle spread from 72 to 74 appears in FIG. 7, with the capacity stability of the cell being represented in FIG. 8.

Example V

A commercial-grade FeO of about 4 μm particle size was used to prepare positive cell members as above. The composition signature of a cell over a 20 cycle spread from 92 to 94 appears in FIG. 9, with the capacity stability of the cell being represented in FIG. 10.

Example VI

A Li-ion electrochemical cell was prepared comprising, as the negative electrode member 17 a film disk of the CoO composition employed as the positive electrode in Example II. The film composition of the positive electrode member 13 of this Li-ion cell was prepared in similar manner with 70 parts LiCo0₂, 11 parts SP carbon black, 12 parts DBP and 6.6 parts PVdF-HFP copolymer. The thickness of the respective electrode member films was controlled to maintain a LiCo0₂:CoO weight ratio of about 6.3 in order to ensure an average cell voltage of about 2.2 V within the range of 4 V to 0.9 V and thus prevent overcharging and eliminate the risk of Li metal plating at the negative electrode which might otherwise occur upon an uncontrolled discharge below 0.01 V. The voltage/composition signature of the cell over a 10 cycle spread from 112 to 114 after the initial charging to 4 V appears in FIG. 11, with the capacity stability of the cell being represented in FIG. 12.

Example VII

A Li/MO-type cell was prepared in the basic manner with the exception that the chalcogenide, CoS, was substituted for the MO. The comparative initial voltage/composition signatures in FIG. 13 of an earlier Li/CoO cell and that of the Li/CoS cell indicate the similarities in the redox activity within these cells.

Example VIII

Electrochemical cells were prepared with further 3d transition metal oxides in the form of Li/MnO and Li/ZnO cells. The capacity stabilities of these unoptimized cells as compared with that of the earlier Li/CoO cell, as shown in FIG. 14, indicate the lesser efficacy, but distinct potential of these highly economical oxide materials .

It is anticipated that other embodiments and variations of the present invention will become readily apparent to the skilled artisan in the light of the foregoing description and examples, and such embodiments and variations are intended to likewise be included within the scope of the invention as set out in the appended claims.

Claims

What is claimed is: 1. A low-voltage rechargeable Li battery cell comprising a positive electrode member, a negative electrode member, and a separator member interposed therebetween c h a r a c t e r i z e d i n t h a t one of said electrode members comprises a transition metal chalcogenide.

2. A battery cell according to claim 1 c h a r a c t e r i z e d i n t h a t said chalcogenide is selected from the group consisting of oxides and sulfides.

3. A battery cell according to claim 2 wherein said chalcogenide is selected from the group consisting of oxides and sulfides of Co, Cu, Fe, Ni , Mn, and Zn.

4. A battery cell according to claim 3 wherein said chalcogenide is selected from the group consisting of oxides of Co, Cu, Fe, and Ni .

5. A battery cell according to claim 1 c h a r a c t e r i z e d i n t h a t said positive electrode member comprises an oxide of a transition metal selected from the group consisting of Co, Cu, Fe, Ni, Mn, and Zn.

6. A battery cell according to claim 5 wherein said negative electrode comprises metallic lithium or a lithium alloy.

7. A battery cell according to claim 1 c h a r a c t e r i z e d i n t h a t said negative electrode member comprises an oxide of a transition metal selected from the group consisting of Co, Cu, Fe, Ni, Mn, and Zn.

8. A battery cell according to claim 7 wherein said positive electrode member comprises a lithiated intercalation compound selected from the group consisting of LiCo0₂, LiMn0₄, and LiNi0₂.

9. A method of making a low-voltage rechargeable Li battery cell which comprises assembling a positive electrode member, a negative electrode member, and a separator member interposed therebetween in close physical contact with a solution of electrolyte salt disposed in at least said separator member c h a r a c t e r i z e d i n t h a t at least one of said electrode members is prepared as a layer of a composition comprising a particulate transition metal chalcogenide.

10. A method according to claim 9 wherein a) said positive electrode member is prepared as a layer of a composition comprising a particulate transition metal o ide ; and b) said cell is initially discharged to a level below about 0.5 volts.

11. A low-voltage rechargeable Li redox battery cell comprising a positive electrode member, a negative electrode member, and a separator member interposed therebetween in close physical contact with a solution of electrolyte salt disposed in at least said separator member c h a r a c t e r i z e d i n t h a t at least one of said electrode members comprises a particulate chalcogenide selected from the group consisting of oxides of transition metals.

12. A battery cell according to claim 11 c h a r a c t e r i z e d i n t h a t said positive electrode comprises a particulate chalcogenide selected from the group consisting of oxides of Co, Cu, Fe, Ni , Mn, and Zn .

13. A battery cell according to claim 12 wherein said negative electrode comprises metallic lithium or a lithium alloy.

14. A battery cell according to claim 11 c h a r a c t e r i z e d i n t h a t said negative electrode member a particulate chalcogenide selected from the group consisting of oxides of Co, Cu, Fe, i , Mn, and Zn .

15. A battery cell according to claim 14 wherein said positive electrode member comprises a lithiated intercalation compound selected from the group consisting of LiCo0₂, LiMn₂0 , and LiNi0₂.

16. A battery cell according to claim 11 c h a r a c t e r i z e d i n t h a t a) each of said electrode members comprises a polymeric matrix; and b) said electrode members are laminated to said separator member to form a unitary cell structure.

17. A battery cell according to claim 16 wherein of said separator member comprises a polymeric matrix.

18. A battery cell according to claim 11 c h a r a c t e r i z e d i n t h a t said particulate chalcogenide has a particle size of less than about 5 micrometres .

1 . A battery cell according to claim 18 wherein said particulate chalcogenide is selected from the group consisting essentially of CoO, Cu₂0, FeO, and NiO having a particle size between about 10 nanometres and 5 micrometres.