Lithium metal oxide electrodes for lithium batteries

This application is a continuation-in-part of application Ser. No. 10/825,931 filed Apr. 15, 2004, now U.S. Pat. No. 7,314,682, which application claims priority based on application Ser. No. 60/465,034 filed Apr. 24, 2003.

The United States Government has rights in this invention pursuant to Contract No. W-31-109-ENG-38 between the U.S. Department of Energy and The University of Chicago representing Argonne National Laboratory.

This invention relates to improved lithium-metal-oxide positive electrodes for lithium cells and batteries, preferably rechargeable lithium-ion cells and batteries. These batteries are used to power a wide range of applications such as portable telecommunications equipment, computers, medical devices, electric vehicles and hybrid-electric vehicles. More specifically, the invention relates to lithium-metal-oxide electrodes with layered-type structures that are chemically preconditioned prior to cell assembly to improve the capacity, cycling efficiency and stability of lithium cells and batteries when charged to high potentials.

State-of-the-art lithium-ion cells have a lithiated carbon negative electrode, or anode, (Li x C 6 ) and a lithium-cobalt-oxide positive electrode, or cathode, Li 1−x CoO 2 . During charge and discharge of the cells, lithium ions are transported between the two host structures of the anode and cathode with the simultaneous oxidation or reduction of the host electrodes, respectively. When graphite is used as the anode, the voltage of the cell is approximately 4 V. The LiCoO 2 cathode, which has a layered structure, is expensive and becomes unstable at low lithium content, i.e., when cells reach an overcharged state at x≧0.5. Alternative, less expensive electrode materials that are isostructural with LiCoO 2 , such as LiNi 0.8 Co 0.2 O 2 , LiNi 0.5 Mn 0.5 O 2 and LiMn 0.33 Ni 0.33 Co 0.33 O 2 are being developed with the hope of replacing at least part of the cobalt component of the electrode. However, all these layered structures, when extensively delithiated become unstable, because of the high oxygen activity at the surface of the particles. Therefore, the delithiated electrode particles tend to react with the organic solvents of the electrolyte or lose oxygen. Such reactions at the surface of layered lithium metal oxide electrodes are detrimental to the performance of the lithium cells and batteries, and methods are required to combat these reactions to ensure that maximum capacity and cycle life can be obtained from the cells.

Considerable efforts have already been made in the past to overcome the stability and solubility problems associated with layered LiCoO 2 and LiNiO 2 electrodes. For example, considerable success has been achieved in the past by stabilizing these electrodes by pre-treating the electrode powders with oxide additives such as Al 2 O 3 or ZrO 2 obtained from metal alkoxide precursors such as solutions containing aluminum ethylhexanoate diisopropoxide (Al(OOC 8 H 15 )(OC 3 H 7 ) 2 or zirconium ethylhexanoisopropoxide (Zr[(OOC 8 H 15 ) 2 (OCH 3 H 7 ) 2 ]) as described, for example, by J. Cho et al in Chemistry of Materials, Volume 12, page 3788 (2000) and J. Cho et al in Electrochemical and Solid State Letters, Volume 4 No. 10, page A159 (2001), respectively, or a zirconium oxide, polymeric precursor or zirconium oxynitrate (ZrO(NO 3 ) 2 .xH 2 O) as described by Z. Chen et al in Electrochemical and Solid State Letters, Volume 5, No. 10, page A213 (2002), prior to the fabrication of the final electrode thereby making the surface of the LiCoO 2 particles more resistant to electrolyte attack, cobalt dissolution or oxygen loss effects.

The loss of oxygen from lithium metal oxide electrodes, such as layered LiCoO 2 and LiNi 1−y Co y O 2 electrodes can contribute to exothermic reactions with the electrolyte and with the lithiated carbon negative electrode, and subsequently to thermal runaway if the temperature of the cell reaches a critical value. Although some success has been achieved in the past to improve the performance of lithium-ion cells by coating electrode particles, the coatings can themselves impede lithium diffusion in and out of the layered electrode structure during electrochemical discharge and charge. Further improvements in the composition of layered lithium-metal oxide electrodes, particularly at the surface of the electrodes, and in methods to manufacture them are still required to improve the overall performance and safety of lithium cells.

This invention relates to improved lithium-metal-oxide positive electrodes for lithium cells and batteries, preferably rechargeable lithium-ion cells and batteries. More specifically, it relates to layered lithium-metal-oxide electrodes, represented by the general formula xLi 2 M′O 3 .(1−x)LiMO 2 (0≦x<1) in which M′ is one or more non-lithium metal ions with an average tetravalent oxidation state and in which M is two or more non-lithium metal ions with an average trivalent oxidation state that are chemically preconditioned prior to cell assembly either by reduction or by acid treatment, or a combination thereof, to improve the capacity, cycling efficiency and cycling stability of lithium cells and batteries when charged to high potentials. The invention extends to methods for synthesizing the preconditioned lithium-metal-oxide electrodes.

The invention consists of certain novel features and a combination of parts hereinafter fully described, illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.

FIG. 1 illustrates the powder X-ray diffraction patterns of a) an untreated LiMn 0.33 Ni 0.33 Co 0.33 O 2 electrode; b) a LiMn 0.33 Ni 0.33 Co 0.33 O 2 electrode treated with NH 3 at 250° C.; and c) a LiMn 0.33 Ni 0.33 Co 0.33 O 2 electrode treated with NH 3 at 350° C.;

FIG. 2 illustrates the electrochemical voltage profiles, at the 15 th cycle, of lithium cells, operated at room temperature (RT) between 4.6 and 2.0 V, with a) an untreated LiMn 0.33 Ni 0.33 Co 0.33 O 2 electrode; and b) a NH 3 -treated LiMn 0.33 Ni 0.33 Co 0.33 O 2 electrode;

FIG. 3 illustrates the electrochemical voltage profiles, at the 15 th cycle, of lithium cells operated at 50° C. between 4.6 and 2.0 V, with a) an untreated LiMn 0.33 Ni 0.33 Co 0.33 O 2 electrode; and b) a NH 3 -treated LiMn 0.33 Ni 0.33 Co 0.33 O 2 electrode;

FIG. 4 illustrates plots of electrode capacity vs. cycle number of lithium cells, operated at room temperature (RT) between 4.45 and 2.5 V, with a) an untreated LiMn 0.33 Ni 0.33 Co 0.33 O 2 electrode; and b) a NH 3 -treated LiMn 0.33 Ni 0.33 Co 0.33 O 2 electrode;

FIG. 5 illustrates plots of electrode capacity vs. cycle number of lithium cells, operated at room temperature (RT) between 4.6 and 2.0 V, with a) an untreated LiMn 0.33 Ni 0.33 Co 0.33 O 2 electrode; and b) a NH 3 -treated LiMn 0.33 Ni 0.33 Co 0.33 O 2 electrode;

FIG. 6 illustrates plots of electrode capacity vs. cycle number of lithium cells, operated at 50° C. between 4.6 and 2.0 V, with a) an untreated LiMn 0.33 Ni 0.33 Co 0.33 O 2 electrode; and b) a NH 3 -treated LiMn 0.33 Ni 0.33 Co 0.33 O 2 electrode;

FIG. 7 a illustrates cyclic voltammograms between 4.6 and 2.0 V vs. metallic lithium of an untreated LiMn 0.33 Ni 0.33 Co 0.33 O 2 electrode;

FIG. 7 b illustrates cyclic voltammograms between 4.6 and 2.0 V vs. metallic lithium of a LiMn 0.33 Ni 0.33 Co 0.33 O 2 electrode treated with NH 3 at 250° C.;

FIG. 7 c illustrates cyclic voltammograms between 4.6 and 2.0 V vs. metallic lithium of a LiMn 0.33 Ni 0.33 Co 0.33 O 2 electrode treated with NH 3 at 350° C.,

FIG. 8 illustrates the powder X-ray diffraction pattern of a) a 0.3Li 2 MnO 3 .0.7LiMn 0.5 Ni 0.5 O 2 composite electrode; b) a 0.3Li 2 MnO 3 .0.7LiMn 0.5 Ni 0.5 O 2 composite electrode treated with NH 3 at 200° C.; c) a 0.3Li 2 MnO 3 .0.7LiMn 0.5 Ni 0.5 O 2 composite electrode treated first with H 2 O at room temperature and then with NH 3 at 200° C.; and d) a 0.3Li 2 MnO 3 .0.7LiMn 0.5 Ni 0.5 O 2 composite electrode treated first with HNO 3 at room temperature and then with NH 3 at 200° C.;

FIG. 9 a illustrates the first charge and discharge voltage profiles of a lithium cell, operated at room temperature, with a 0.3Li 2 MnO 3 .0.7LiMn 0.5 Ni 0.5 O 2 composite electrode;

FIG. 9 b illustrates the first charge and discharge voltage profiles of a lithium cell, operated at room temperature, with a 0.3Li 2 MnO 3 .0.7LiMn 0.5 Ni 0.5 O 2 composite electrode treated with NH 3 ;

FIG. 9 c illustrates the first charge and discharge voltage profiles of a lithium cell, operated at room temperature, with a 0.3Li 2 MnO 3 .0.7LiMn 0.5 Ni 0.5 O 2 composite electrode treated with H 2 O and NH 3 ;

FIG. 9 d illustrates the first charge and discharge voltage profiles of a lithium cell, operated at room temperature, with a 0.3Li 2 MnO 3 .0.7LiMn 0.5 Ni 0.05 O 2 composite electrode treated with HNO 3 and NH 3 ;

FIG. 10 a illustrates a plot of electrode capacity vs. cycle number of a lithium cell, operated at room temperature between 4.6 and 2.0 V, with an untreated 0.3Li 2 MnO 3 .0.7LiMn 0.5 Ni 0.5 O 2 composite electrode;

FIG. 10 b illustrates a plot of electrode capacity vs. cycle number of a lithium cell, operated at room temperature between 4.6 and 2.0 V, with a 0.3Li 2 MnO 3 .0.7LiMn 0.5 Ni 0.05 O 2 composite electrode treated with NH 3 ;

FIG. 10 c illustrates a plot of electrode capacity vs. cycle number of a lithium cell, operated at room temperature between 4.6 and 2.0 V, with a 0.3Li 2 MnO 3 .0.7LiMn 0.5 Ni 0.5 O 2 composite electrode treated with H 2 O and NH 3 ;

FIG. 10 d illustrates a plot of electrode capacity vs. cycle number of a lithium cell, operated at room temperature between 4.6 and 2.0 V, with a 0.3Li 2 MnO 3 .0.7LiMn 0.5 Ni 0.05 O 2 composite electrode treated with HNO 3 and NH 3 ;

FIG. 11 illustrates a schematic representation of an electrochemical cell; and

FIG. 12 illustrates a schematic representation of a battery consisting of a plurality of cells connected electrically in series and in parallel.

FIG. 13 illustrates the powder X-ray diffraction patterns of:

a) an untreated 0.1 Li 2 MnO 3 .0.9LiCo 0.372 Ni 0.372 Mn 0.256 O 2 electrode (Sample H);

b) a 0.1Li 2 MnO 3 .0.9LiCo 0.372 Ni 0.372 Mn 0.256 O 2 electrode treated with an acidic 0.014M NH 4 F (aqueous) solution and dried at 300° C. in air (Sample I);

c) a 0.1Li 2 MnO 3 .0.9LiCo 0.372 Ni 0.372 Mn 0.256 O 2 electrode treated with an acidic 0.014M NH4F (aqueous) solution and dried at 600° C. in air (Sample J);

d) a 0.1 Li 2 MnO 3 .0.9LiCo 0.372 Ni 0.372 Mn 0.256 O 2 electrode treated with a 0.016M NH 4 F solution in methanol and dried at 600° C. in air (Sample K).

FIG. 14 illustrates the first charge and discharge voltage profiles of lithium cells, operated at room temperature, with a) an untreated 1Li 2 MnO 3 .0.9LiCo 0.372 Ni 0.372 Mn 0.256 O 2 electrode (Sample H); b) a 0.1 Li 2 MnO 3 .0.9LiCo 0.372 Ni 0.372 Mn 0.256 O 2 electrode treated with an acidic NH 4 F solution (aqueous) and dried at 300° C. (Sample I); c) a 0.1Li 2 MnO 3 .0.9LiCo 0.372 Ni 0.372 Mn 0.256 O 2 electrode treated with an acidic NH 4 F solution (aqueous) and dried at 600° C. (Sample J); and d) a 0.1 Li 2 MnO 3 .0.9LiCo 0.372 Ni 0.372 Mn 0.256 O 2 electrode treated with a 0.016M NH 4 F solution in methanol and dried at 600° C. in air (Sample K).

FIG. 15 illustrates plots of electrode capacity vs. cycle number of a lithium cell, operated at room temperature between 4.6 and 3.0 V with an untreated 1Li 2 MnO 3 .0.9LiCo 0.372 Ni 0.372 Mn 0.256 O 2 electrode (O), (Sample H); a 0.1Li 2 MnO 3 .0.9LiCo 0.0372 Ni 0.372 Mn 0.256 O 2 electrode treated with an acidic NH 4 F solution (aqueous) and dried at 300° C. (□), (Sample I); a 0.1Li 2 MnO 3 .0.9LiCo 0.372 Ni 0.372 Mn 0.256 O 2 electrode treated with an acidic NH 4 F solution (aqueous) and dried at 600° C., (●) (Sample J); and a 0.1Li 2 MnO 3 .0.9LiCo 0.372 Ni 0.372 Mn 0.256 O 2 electrode treated with a 0.016M NH 4 F solution (aquesous) and dried at 600° C., (●) (sample J); and a 0.1Li 2 MnO 3 .0.9LiCo 0.372 Ni 0.372 Mn 0.256 O 2 electrode treated with a 0.016M Nh 4 F solution in methanol and dried at 600° C. in air, (▪) (Sample K).

It is now well known that state-of-the-art LiMO 2 electrodes with layered structures such as LiCoO 2 , LiNi 0.08 Co 0.2 O 2 , LiAl 0.05 Ni 0.80 Co 0.15 O 2 , LiNi 0.5 Mn 0.5 O 2 , LiMn 0.33 Ni 0.33 Co 0.33 O 2 , LiNi 0.80 Co 0.15 Al 0.05 O 2 and LiCo 0.90 Ti 0.05 Mg 0.05 O 2 electrodes, or variations thereof, for example, those that contain lithium within the transition metal (M) layers are unstable in a lithium cell environment when a large amount of lithium is extracted from their structures, typically when x exceeds 0.5 in Li 1−x MO 2 electrodes, or at a potential higher than approximately 4.2 V vs. metallic lithium. The high reactivity of the delithiated electrode structure has been attributed to the highly oxidizing power of the tetravalent metal ions Ni 4+ , Co 4+ and Mn 4+ , that decreases in the order Ni 4+ >Co 4+ >Mn 4+ . Highly delithiated Li 1−x MO 2 electrodes can therefore react spontaneously with the organic-based electrolyte solvents such as ethylene carbonate, diethyl carbonate or dimethyl carbonate or, in extreme cases, the Li 1−x MO 2 electrode can release oxygen into the cell compartment. The oxidation of electrolyte solvents or the release of oxygen from the delithiated electrodes, which can be represented generically by the formula Li 1−x MO 2−δ (0<δ<1), must of necessity lower the oxidation state of the transition metal ions in the Li 1−x MO 2−δ product, at least at the surface of the electrode particles. Such a reaction takes place predominantly during the initial charge of a C 6 /LiMO 2 lithium-ion cell, when lithium is electrochemically extracted from the LiMO 2 positive electrode and inserted into the negative C 6 (graphite) electrode. These reactions decrease the theoretical capacity of the LiMO 2 electrode; therefore, these electrodes tend to show an enhanced capacity loss on the initial charge/discharge cycle of the lithium-ion cell.

For layered LiMO 2 electrodes that contain Ni, such as LiNiO 2 , LiNi 0.08 Co 0.2 O 2 and LiAl 0.05 Ni 0.80 Co 0.15 O 2 , it has been observed that the electrochemically induced capacity loss can be attributed not only to oxygen loss or reaction with the electrolyte, but also to the migration of the nickel ions into the lithium layer of the delithiated Li 1−x MO 2−δ electrode structure. When nickel diffusion occurs concomitantly with oxygen loss, then the surface of a Li 1−x MO 2−δ electrode can have characteristic features, for example, of a Li 1−x MO 2−δ rocksalt-like structure. For example, lithium extraction and oxygen loss from a Li 1−x NiO 2−δ electrode leaves a surface product that closely resembles a Li z Ni 1−z O phase. Although a Li z Ni 1−z O structure lowers the lithium-ion conductivity at the electrode surface, which can result in an increase in cell impedance, the reduced surface layer serves to protect the Li 1−x MO 2 structure within the bulk of the particles during the extended cycling of the lithium-ion cell. The applicants have found that it is advantageous to pre-reduce chemically the surface of LiMO 2 electrode particles, particularly those containing a slight excess of lithium within the M layers, with a convenient reducing agent to yield a LiMO 2−δ electrode product prior to cell assembly so that the extent of oxygen loss from the electrode and the concomitant loss of capacity could be minimized during the electrochemical charge and discharge of the cell. In this respect, the use of reducing environments to synthesize electrode products has already been disclosed in the literature for different types of electrodes as discussed by M. M. Thackeray et al in U.S. Pat. No. 5,240,794; in these instances the reduced metal ions exist in the bulk of the electrode structures as well as at the surface of the particles.

Layered lithium-metal-oxide structures, Li 2 M′O 3 , in which the M′ ions are tetravalent, are also known to exist. These structures can be reformulated in standard layered notation as Li[M′ 0.67 Li 0.33 ]O 2 in which layers of lithium ions alternate with layers containing both M′- and lithium ions in a 2:1 ratio. One well known example is Li 2 MnO 3 (Li[Mn 0.67 Li 0.33 ]O 2 ) which is electrochemically inactive with respect to lithium insertion and extraction. However, Li 2 MnO 3 can be electrochemically activated by acid treatment, during which some H + —Li + ion exchange occurs and some Li 2 O may be removed from the structure.

This invention relates, in general, to layered lithium-metal-oxide electrodes, represented by the general formula xLi 2 M′O 3 .(1−x)LiMO 2 (0≦x<1) in which M′ is one or more non-lithium metal ions with an average tetravalent oxidation state and in which M is one or more non-lithium metal ions with an average trivalent oxidation state that are chemically preconditioned prior to cell assembly either by reduction or by acid treatment, or a combination thereof, to improve the capacity, cycling efficiency and cycling stability of lithium cells and batteries when charged to high potentials. The invention extends to methods for synthesizing the preconditioned lithium-metal-oxide electrodes. When x=0, the formula xLi 2 M′O 3 .(1−x)LiMO 2 reduces simply to LiMO 2 .

Therefore, in a first embodiment, this invention relates to layered electrodes represented by the two-component formula xLi 2 M′O 3 .(1−x)LiMO 2 in which 0≦x<1, that can be rewritten alternatively as Li (2+2x)/(2+x) M′ 2x/(2+x) M (2−2x)/(2+x) O 2 also for 0≦x<1, in which M is a non-lithium metal ion with an average trivalent oxidation state selected from one or more of the first row transition metals or lighter metal elements in the periodic table, and in which M′ is one or more ions with an average tetravalent oxidation state selected from the first and second row transition metal elements and Sn, the electrodes being chemically preconditioned with a reducing agent to yield Li (2+2x)/(2+x) M′ 2x/(2+x) M (2−2x)/(2+x) O 2−δ compounds in which δ is less than 0.2, preferably less than 0.1. The metal ions, M, are selected preferably from Co, Ni, Mn or Ti ions, optionally in the presence of one or more other cations such as Mg and Al. The M′ cations are selected from elements that form a Li 2 M′O 3 rocksalt-type structure preferably from Ti, Mn, Zr, Ru and Sn and most preferably from Mn and/or Ti that form a layered rocksalt-type structure which is structurally compatible with the LiMO 2 component. The xLi 2 M′O 3 .(1−x)LiMO 2 electrodes have been described previously as having composite electrode structures by J-S. Kim et al in Electrochemistry Communications, volume 4, page 205 (2002). When x=0, the preconditioned electrodes are represented simply by the formula LiMO 2−δ.

In a second embodiment of the invention, the xLi 2 M′O 3 .(1−x)LiMO 2 electrodes can be preconditioned by acid treatment optionally followed by the reduction step. When preconditioning occurs by acid treatment, ion-exchange occurs between the H + and Li + ions. In this instance, the preconditioned xLi 2 M′O 3 .(1−x)LiMO 2 electrodes can be represented prior to reduction by the general formula xLi 2−y H y O.xM′O 2 .(1−x)Li 1−z H z MO 2 in which 0<x<1, 0<y<1, 0<z<1, and in which the lithium ions are partially ion-exchanged by hydrogen ions. In addition, some Li 2 O may be removed from the structure during this process. These preconditioned electrodes can be heated, typically to temperatures above 100° C., to remove at least part of the H 2 O component from the structure.

Of particular significance to this invention are preconditioned xLi 2 M′O 3 .(1−x)LiMO 2 electrodes containing a Li 2 MnO 3 component (i.e., M′=Mn). It has already been reported by M. H. Rossouw et al in the Journal of Solid State Chemistry, volume 104, page 464 (1993) that lithium and oxygen can be removed from Li 2 MnO 3 by acid treatment to create a composite xLi 2 MnO 3 .(1−x)MnO 2 structure without destroying the layered arrangement of the Mn ions; on relithiation, either chemically with LiI or electrochemically in an electrochemical cell, lithium can be inserted into the MnO 2 component of the composite structure to yield xLi 2 MnO 3 .(1−x)LiMnO 2 . These composite electrode structures may contain protons (H + ions) as a result of the H + —Li + ion exchange reactions that occur, for example, during the preconditioning acid-treatment step or when immersed in the non-aqueous electrochemical cells that contain acidic electrolytes such as those containing LiPF 6 salts. For simplicity, these protons are not always included in the description of the preconditioned electrode structures of this invention. The composition of xLi 2 MnO 3 .(1−x)LiMO 2 electrodes can be modified by selection of one or more appropriate metal (M) cations, preferably from one or more of Co, Ni, Mn or Ti, ions, optionally in the presence of one or more other metal cations, selected preferably from the first row of transition metal elements, or from lighter metal elements in the periodic table, such as Mg and Al. Examples of such two-component systems are xLi 2 MnO 3 .(1−x)LiCoO 2 as reported by K. Numata et al in Solid State Ionics, volume 118, page 117 (1999), and xLi 2 MnO 3 .(1−x)LiCrO 2 as reported by B. Ammundsen et al in the Journal of the Electrochemical Society, volume 149, page A431 (2002). Composite electrodes that are showing particular promise typically contain two or more electrochemically-active M cations, for example, Li[Ni x Li (1/3−2x/3) Mn (2/3−x/3) ]O 2 (0<x<½) as reported by Z. Lu et al in Chemistry of Materials, volume 15, page 3214 (2003), which can be reformulated in composite notation as (1−2x)Li 2 MnO 3 .(3x)LiMn 0.5 Ni 0.5 O 2 for the same range of x, and electrodes in which M=Co, Mn and Ni, represented generically as xLi 2 MnO 3 .(1−x)LiCo 1−a Ni b Mn c O 2 in which a=b+c.

A major advantage of using two-component xLi 2 M′O 3 .(1−x)LiMO 2 composite (positive) electrodes, as defined herein, is that the Li 2 M′O 3 component, particularly when present as Li 2 MnO 3 , can be used effectively to supply an excess of lithium to the positive electrode to offset irreversible capacity loss effects associated with the negative electrode of lithium-ion cells, such as lithiated graphite, Li x C 6 (0<x<1). For example, it has been established that when xLi 2 MnO 3 .(1−x)LiNi 0.5 Mn 0.5 O 2 electrodes are charged in lithium cells, the electrochemical reaction occurs first by lithium extraction from the LiNi 0.5 Mn 0.5 O 2 component between 3.0 and 4.2 V, and thereafter by a concomitant extraction of lithium from the Li 2 MnO 3 component. It is believed that the latter process (typically at potentials between 4.2 and 4.6 V) is also accompanied by the chemical loss of oxygen from the surface of the delithiated xLi 2 MnO 3 .(1−x)LiNi 0.5 Mn 0.5 O 2 electrode particles, either as oxygen gas or by reaction with the organic solvents of the electrolyte, for example, organic carbonates such as ethylene carbonate, dimethyl carbonate, diethyl carbonate and the like. In such situations, the result is not only a chemical reduction of the electrode surface by loss of oxygen, but also the loss of some Li 2 O from the Li 2 MnO 3 (Li 2 O.MnO 2 ) component that leaves behind an electrochemically active MnO 2 species, thereby increasing the operating capacity of the electrode. Despite the fact that xLi 2 M′O 3 .(1−x)LiMO 2 composite electrodes exhibit an irreversible capacity loss during the initial cycle, these electrodes provide significantly higher electrode capacities on subsequent cycling than conventional layered LiCoO 2 or LiNiO 2 electrodes and LiMn 2 O 4 spinel electrodes. It would be advantageous to precondition layered LiMO 2 or composite xLi 2 M′O 3 .(1−x)LiMO 2 electrodes chemically prior to assembling them in cells by removing oxygen and/or Li 2 O from the surface of the electrodes to minimize irreversible capacity loss effects and to increase the operating capacity of the electrodes, particularly during the early cycling of cells.

According to a third embodiment, the preconditioned electrodes of the invention can be synthesized by subjecting the parent LiMO 2 or composite xLi 2 M′O 3 .(1−x)LiMO 2 electrodes, for example, either in powder or laminate form, to a suitable reducing agent such as ammonia gas, dilute hydrogen gas, for example, 2-6 percent by volume of hydrogen in argon gas, carbon monoxide gas, or carbon at moderately high temperatures, such as 150-600° C., to reduce the surface of the LiMO 2 or composite xLi 2 M′O 3 .(1−x)LiMO 2 electrodes. Gaseous reducing agents, such as ammonia, are preferred to solid reducing agents such as finely-divided carbon powder because it is easier to control the extent to which the surface of the LiMO 2 electrode is reduced. Alternatively, the LiMO 2 or composite xLi 2 M′O 3 .(1−x)LiMO 2 electrodes can be reduced by heating the electrodes under nitrogen gas or air at elevated temperature, typically at 600° C. or higher, for example, between 900° C. and 1000° C. simultaneously to remove both oxygen and Li 2 O from the surface of the electrode structure.

In a further embodiment of the invention, the LiMO 2 or xLi 2 M′O 3 .(1−x)LiMO 2 composite electrodes of the invention can alternatively be subjected, prior to the reduction process described above, to a preconditioning step in a proton-containing medium, for example, de-ionized water or an acidic solution of an inorganic acid or an organic acid with a pH<7.0 such as phosphoric acid, sulfuric acid, nitric acid, acetic acid, hydrochloric acid or the like, for example, hydrofluoric acid, to exchange some of the Li+ ions within the electrode structure with H + ions, and thereafter to a heating step, preferably for less than 24 hours below 500° C., more preferably below 400° C.

In yet a further embodiment, the LiMO 2 or xLi 2 M′O 3 .(1−x)LiMO 2 composite electrodes may be preconditioned by subjecting the electrodes only to the proton-containing medium, i.e., without a reduction step. For example, it is already known that acid treatment of Li 2 MnO 3 yields a proton-exchanged Li 2−y H y MnO 3 compound that can be reformulated, in component notation, as (1−y/2)Li 2 O.y/2H 2 O.MnO 2 . In this respect, it stands to reason that if the pH is less than 7, proton exchange will occur in the LiMO 2 or xLi 2 M′O 3 .(1−x)LiMO 2 composite electrodes of the invention, irrespective of the choice of the proton-containing solution of an inorganic acid or organic acid, such as phosphoric acid, sulfuric acid, nitric acid, acetic acid, hydrochloric acid and hydrofluoric acid. It is believed that such an ion-exchange process followed by a heat-treatment step may be used effectively to precondition the electrode because it should be easier to remove (or partially remove), by heating, a H 2 O component from the structure rather than a Li 2 O component, in order to increase the MnO 2 component in the initial electrode and therefore its inherent electrochemical capacity. In a final embodiment, this invention extends to include electrochemical lithium cells and batteries that employ the preconditioned LiMO 2 or composite xLi 2 M′O 3 .(1−x)MO 2 electrodes as fully described herein.

The following examples describe the principles of the invention and possible methods of synthesizing the pre-reduced electrodes of this invention as contemplated by the inventors, but they are not to be construed as limiting examples.