Lithium metal oxide electrodes for lithium batteries

[0001] 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.

[0002] 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.

[0003] 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.

[0004] State-of-the-art lithium-ion cells have a lithiated carbon negative electrode, or anode, (Li_xC₆) and a lithium-cobalt-oxide positive electrode, or cathode, Li_1−xCoO₂. 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 LiCoO₂ 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 LiCoO₂, such as LiNi_0.8Co_0.2O₂, LiNi_0.5Mn_0.5O₂ and LiMn_0.33Ni_0.33Co_0.33O₂ 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.

[0005] Considerable efforts have already been made in the past to overcome the stability and solubility problems associated with layered LiCoO₂ and LiNiO₂ 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 Al₂O₃ or ZrO₂ obtained from metal alkoxide precursors such as solutions containing aluminum ethylhexanoate diisopropoxide (Al(OOC₈H₁₅)(OC₃H₇)₂ or zirconium ethylhexanoisopropoxide (Zr[(OOC₈H₁₅)₂(OCH₃H₇)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(NO₃)₂.xH₂O) 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 LiCoO₂ particles more resistant to electrolyte attack, cobalt dissolution or oxygen loss effects.

[0006] The loss of oxygen from lithium metal oxide electrodes, such as layered LiCoO₂ and LiNi_1−yCo_yO₂ 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.

[0007] 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 xLi₂M′O₃.(1−x)LiMO₂(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.

[0036] It is now well known that state-of-the-art LiMO₂ electrodes with layered structures such as LiCoO₂, LiNi_0.08Co_0.2O₂, LiAl_0.05Ni_0.80Co_0.15O₂, LiNi_0.5Mn_0.5O₂, LiMn_0.33Ni_0.33Co_0.33O₂, LiNi_0.80Co_0.15Al_0.05O₂ and LiCo_0.90Ti_0.05Mg_0.05O₂ 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−xMO₂ 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 Ni⁴⁺, Co⁴⁺ and Mn⁴⁺, that decreases in the order Ni⁴⁺>Co⁴⁺>Mn⁴⁺. Highly delithiated Li_1−xMO₂ 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−xMO₂ 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−xMO_2−δ (0<δ<1), must of necessity lower the oxidation state of the transition metal ions in the Li_1−xMO_2−δ product, at least at the surface of the electrode particles. Such a reaction takes place predominantly during the initial charge of a C₆/LiMO₂ lithium-ion cell, when lithium is electrochemically extracted from the LiMO₂ positive electrode and inserted into the negative C₆ (graphite) electrode. These reactions decrease the theoretical capacity of the LiMO₂ electrode; therefore, these electrodes tend to show an enhanced capacity loss on the initial charge/discharge cycle of the lithium-ion cell.

[0037] For layered LiMO₂ electrodes that contain Ni, such as LiNiO₂, LiNi_0.08Co_0.2O₂ and LiAl_0.05Ni_0.80Co_0.15O₂, 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−xMO_2−δ electrode structure. When nickel diffusion occurs concomitantly with oxygen loss, then the surface of a Li_1−xMO_2−δ electrode can have characteristic features, for example, of a Li_1−xMO_2−δ rocksalt-like structure. For example, lithium extraction and oxygen loss from a Li_1−xNiO_2−δ electrode leaves a surface product that closely resembles a Li_zNi_1−zO phase. Although a Li_zNi_1−zO 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−xMO₂ 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 LiMO₂ 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.

[0038] Layered lithium-metal-oxide structures, Li₂M′O₃, in which the M′ ions are tetravalent, are also known to exist. These structures can be reformulated in standard layered notation as Li[M′_0.67Li_0.33]O₂ 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 Li₂MnO₃ (Li[Mn_0.67Li_0.33]O₂) which is electrochemically inactive with respect to lithium insertion and extraction. However, Li₂MnO₃ can be electrochemically activated by acid treatment, during which some H⁺—Li⁺ ion exchange occurs and some Li₂O may be removed from the structure.

[0039] This invention relates, in general, to layered lithium-metal-oxide electrodes, represented by the general formula xLi₂M′O₃.(1−x)LiMO₂ (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 xLi₂M′O₃.(1−x)LiMO₂ reduces simply to LiMO₂.

[0040] Therefore, in a first embodiment, this invention relates to layered electrodes represented by the two-component formula xLi₂M′O₃.(1−x)LiMO₂ 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)}O₂ 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 Li₂M′O₃ 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 LiMO₂ component. The xLi₂M′O₃.(1−x)LiMO₂ 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.

[0041] In a second embodiment of the invention, the xLi₂M′O₃.(1−x)LiMO₂ 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 xLi₂M′O₃.(1−x)LiMO₂ electrodes can be represented prior to reduction by the general formula xLi_2−yH_yO.xM′O₂.(1−x)Li_1−zH_zMO₂ 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 Li₂O 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 H₂O component from the structure.

[0042] Of particular significance to this invention are preconditioned xLi₂M′O₃.(1−x)LiMO₂ electrodes containing a Li₂MnO₃ 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 Li₂MnO₃ by acid treatment to create a composite xLi₂MnO₃.(1−x)MnO₂ 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 MnO₂ component of the composite structure to yield xLi₂MnO₃.(1−x)LiMnO₂. 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 LiPF₆ salts. For simplicity, these protons are not always included in the description of the preconditioned electrode structures of this invention. The composition of xLi₂MnO₃.(1−x)LiMO₂ 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 xLi₂MnO₃.(1−x)LiCoO₂ as reported by K. Numata et al in Solid State Ionics, volume 118, page 117 (1999), and xLi₂MnO₃.(1−x)LiCrO₂ 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_xLi_(1/3−2x/3)Mn_(2/3−x/3)]O₂ (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)Li₂MnO₃.(3x)LiMn_0.5Ni_0.5O₂ for the same range of x, and electrodes in which M=Co, Mn and Ni, represented generically as xLi₂MnO₃.(1−x)LiCo_1−aNi_bMn_cO₂ in which a=b+c.

[0043] A major advantage of using two-component xLi₂M′O₃.(1−x)LiMO₂ composite (positive) electrodes, as defined herein, is that the Li₂M′O₃ component, particularly when present as Li₂MnO₃, 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_xC₆ (0<x<1). For example, it has been established that when xLi₂MnO₃.(1−x)LiNi_0.5Mn_0.5O₂ electrodes are charged in lithium cells, the electrochemical reaction occurs first by lithium extraction from the LiNi_0.5Mn_0.5O₂ component between 3.0 and 4.2 V, and thereafter by a concomitant extraction of lithium from the Li₂MnO₃ 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 xLi₂MnO₃.(1−x)LiNi_0.5Mn_0.5O₂ 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 Li₂O from the Li₂MnO₃ (Li₂O.MnO₂) component that leaves behind an electrochemically active MnO₂ species, thereby increasing the operating capacity of the electrode. Despite the fact that xLi₂M′O₃.(1−x)LiMO₂ composite electrodes exhibit an irreversible capacity loss during the initial cycle, these electrodes provide significantly higher electrode capacities on subsequent cycling than conventional layered LiCoO₂ or LiNiO₂ electrodes and LiMn₂O₄ spinel electrodes. It would be advantageous to precondition layered LiMO₂ or composite xLi₂M′O₃.(1−x)LiMO₂ electrodes chemically prior to assembling them in cells by removing oxygen and/or Li₂O 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.

[0044] According to a third embodiment, the preconditioned electrodes of the invention can be synthesized by subjecting the parent LiMO₂ or composite xLi₂M′O₃.(1−x)LiMO₂ 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 LiMO₂ or composite xLi₂M′O₃.(1−x)LiMO₂ 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 LiMO₂ electrode is reduced. Alternatively, the LiMO₂ or composite xLi₂M′O₃.(1−x)LiMO₂ 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 Li₂O from the surface of the electrode structure.

[0045] In a further embodiment of the invention, the LiMO₂ or xLi₂M′O₃.(1−x)LiMO₂ 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.

[0046] In yet a further embodiment, the LiMO₂ or xLi₂M′O₃.(1−x)LiMO₂ 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 Li₂MnO₃ yields a proton-exchanged Li_2−yH_yMnO₃ compound that can be reformulated, in component notation, as (1−y/2)Li₂O.y/2H₂O.MnO₂. In this respect, it stands to reason that if the pH is less than 7, proton exchange will occur in the LiMO₂ or xLi₂M′O₃.(1−x)LiMO₂ 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 H₂O component from the structure rather than a Li₂O component, in order to increase the MnO₂ 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 LiMO₂ or composite xLi₂M′O₃.(1−x)MO₂ electrodes as fully described herein.

[0047] 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.

[0048] LiMn_0.33Ni_0.33Co_0.33O₂ powder was synthesized from LiOH.H₂O and Mn_0.33Ni_0.33Co_0.33(OH)_x(x˜2) precursors. LiOH.H₂O was used as supplied by Aldrich (98% pure). The Mn_0.33Ni_0.33Co_0.33(OH)_x precursor was prepared by coprecipitation of the required stiochiometric amounts of metal nitrates M(NO₃)₂.xH₂O (M=Mn, Ni, and Co). After intimate mixing and grinding, the powdered mixture was pressed into a pellet and placed in a muffle furnace. A two-step heating procedure was used to fire the pellet according to the following procedure. First, a low temperature calcination step was performed at 480° C. for 5 hr. The pellet was then ground again and recompacted into pellet form. Second, a high temperature sintering step was performed at 900° C. for 3 hr followed by rapid quenching of the pellet between two copper plates. The pellet was ground a final time. Samples of the resulting LiMn_0.33Ni_0.33Co_0.33O₂ powder were heated at either 250 or 350° C. for approximately 20 hr in a tubular furnace under flowing NH₃ gas. The X-ray diffraction patterns of the parent LiMn_0.33Ni_0.33Co_0.33O₂ compound and the NH₃-treated LiMn_0.33Ni_0.33Co_0.33O₂ samples are shown in FIG. 1(a-c), respectively; there were no significant differences in the X-ray patterns of the untreated- and NH₃-treated LiMn_0.33Ni_0.33Co_0.33O₂ sample, indicating that there were no substantial changes to the bulk of the electrode structure.

[0049] Electrochemical evaluation of LiMn_0.33Ni_0.33Co_0.33O₂ and preconditioned LiMn_0.33Ni_0.33Co_0.33O₂ positive electrodes was carried out as follows. The electrodes for the lithium cell were fabricated from an intimate mixture of 84 wt % of LiMn_0.33Ni_0.33Co_0.33O₂ electrode powder, 8 wt % polyvinylidene difluoride (PVDF) polymer binder (Kynar, Elf-Atochem), 4 wt % acetylene black (Cabot), and 4 wt % graphite (SFG-6, Timcal) slurried in 1-methyl-2-pyrrolidinone (NMP) (Aldrich, 99+%). An electrode laminate was cast from the slurry onto an Al current collector foil using a doctor-blade. The laminate was subsequently dried, first at 75° C. for 10 h, and thereafter under vacuum at 70° C. for 12 h. The electrolyte was 1 M LiPF₆ in ethylene carbonate (EC):diethyl carbonate (DEC) (1:1 mixture). The electrodes were evaluated at both room temperature and 50° C. in coin-type cells (size CR2032, Hohsen) with a lithium foil counter electrode (FMC Corporation, Lithium Division) and a polypropylene separator (Celgard 2400). Cells were assembled inside a He glovebox (<5 ppm, H₂O and O₂) and cycled on a Maccor Series 2000 tester under galvanostatic mode with a constant current density of either 0.3 mA/cm² or 0.1 mA/cm².

[0050] The electrochemical voltage profiles, at the 15^th cycle, of lithium cells, operated at room temperature between 4.6 and 2.0 V, with an untreated LiMn_0.33Ni_0.33Co_0.33O₂ electrode and a NH₃-treated (250° C.) LiMn_0.33Ni_0.33Co_0.33O₂ electrode are shown in FIG. 2 (a and b), respectively. It is clearly apparent that the capacity delivered by the electrochemical cell containing the NH₃-treated LiMn_0.33Ni_0.33Co_0.33O₂ electrode is significantly superior to that of the cell with the untreated LiMn_0.33Ni_0.33Co_0.33O₂ electrode in accordance with the principles of the invention.

[0051] The electrochemical voltage profiles, at the 15^th cycle, of lithium cells, operated at 50° C. between 4.6 and 2.0 V, with an untreated LiMn_0.33Ni_0.33Co_0.33O₂ electrode and a NH₃-treated LiMn_0.33Ni_0.33Co_0.33O₂ electrode are shown in FIG. 3 (a and b), respectively. It is clearly apparent that the capacity delivered by the electrochemical cell containing the NH₃-treated (250° C.) LiMn_0.33Ni_0.33Co_0.33O₂ electrode is significantly superior to that of the cell with the untreated LiMn_0.33Ni_0.33Co_0.33O₂ electrode in accordance with the principles of the invention. FIG. 4 (a and b) represents plots of electrode capacity vs. cycle number of lithium cells operated at room temperature between 4.6 and 2.0 V, with an untreated LiMn_0.33Ni_0.33CO_0.33O₂ electrode and a NH₃-treated LiMn_0.33Ni_0.33Co_0.33O₂ electrode, respectively, whereas FIG. 5 (a and b) represents plots of electrode capacity vs. cycle number of lithium cells operated at room temperature between 4.45 and 2.5 V, with an untreated LiMn_0.33Ni_0.33Co_0.33O₂ electrode and a NH₃-treated LiMn_0.33Ni_0.33Co_0.33O₂ electrode, respectively. FIG. 6 (a and b) represents plots of electrode capacity vs. cycle number of lithium cells operated at 50° C., between 4.6 and 2.0 V, with an untreated LiMn_0.33Ni_0.33Co_0.33O₂ electrode and a NH₃-treated LiMn_0.33Ni_0.33Co_0.33O₂ electrode, respectively. These data clearly illustrate the superior capacity delivered consistently by the NH₃-treated LiMn_0.33Ni_0.33Co_0.33O₂ electrodes compared to the untreated LiMn_0.33Ni_0.33Co_0.33O₂ electrodes in accordance with the principles of this invention. Of particular note is that the coulombic efficiency of the cells containing the NH₃-treated LiMn_0.33Ni_0.33Co_0.33O₂ electrode is notably superior to that of the cell containing the untreated LiMn_0.33Ni_0.33Co_0.33O₂ electrode (FIGS. 4 and 5). The NH₃ preconditioning step also increases the first-cycle reversible capacity of the LiMn_0.33Ni_0.33Co_0.33O₂ electrodes from 83% (unconditioned electrode, Sample A) to 88% (preconditioned electrodes, Samples B and C) as shown in Table 1.

[0052]

TABLE 1
Charge/discharge capacities and reversible capacity (%)
of the initial cycle of lithium cells with unconditioned-
and preconditioned LiMn0.33Ni0.33Co0.33O2 electrodes.
	Initial Charge	Initial Discharge	Reversible
Electrode	Capacity (mAh/g)	Capacity (mAh/g)	Capacity (%)
Sample A	229	191	83
Sample B	228	200	88
Sample C	229	201	88

[0053] FIGS. 7a, 7b and 7c (samples A-C, respectively) represent plots of cyclic voltammograms between 4.6 and 2.0 V vs. metallic lithium of an untreated LiMn_0.33Ni_0.33Co_0.33O₂ electrode, a LiMn_0.33Ni_0.33Co_0.33O₂ electrode treated with NH₃ at 250° C., and a LiMn_0.33Ni_0.33Co_0.33O₂ electrode treated with NH₃ at 350° C., respectively. It is significant that the second oxidative sweep of the untreated LiMn_0.33Ni_0.33Co_0.33O₂ electrode between 3.5 and 4.25 V (FIG. 7a) occurs at a lower potential than the initial sweep, indicating that the surface of the electrode had been reduced electrochemically during the first cycle. By contrast, the difference in potential between the first and second oxidative sweep of an LiMn_0.33Ni_0.33Co_0.33O₂ electrode treated with NH₃ at 250° C. (FIG. 7b) and at 350° C. (FIG. 7c) is less than it is for the untreated LiMn_0.33Ni_0.33Co_0.33O₂ electrode (FIG. 7a), consistent with the principles of the invention that NH₃ treatment reduces the electrode surface to provide enhanced electrode stability as demonstrated by the improved coulombic efficiency of the NH₃-treated electrodes (FIGS. 4 and 5). These data illustrate that preconditioning the electrode produces improved cycling characteristics. Although it is difficult to determine the precise degree to which the parent electrode is reduced by the chemical preconditioning process, particularly at the surface, excessive reduction will damage the electrode structure and degrade the electrochemical properties of the electrode. It is therefore, believed that optimum electrochemical performance will only be achieved for mild levels of reduction, i.e., for δ less than 0.2, preferably less than 0.1 in the Li_(2+2x)/(2+x)M′_2x/(2+x)M_{(2−2x)/(2+x)}O_2−δ electrodes of this invention.

[0054] An electrode material with the composite formula 0.3Li₂MnO₃.0.7LiMn_0.5Ni_0.5O₂ was prepared directly from Ni_1−xMn_x(OH)₂ and LiOH.H₂O precursors using the required Li:Mn:Ni ratio. The Ni_1−xMn_x(OH)₂ precursor was prepared by precipitation from a basic LiOH solution of Ni(NO₃)₂ and Mn(NO₃)₂ (pH˜11). The reagents were intimately mixed in an acetone slurry, dried in an oven overnight, and subsequently fired at 480° C. for 12 h and then at 900° C. for 5 h in air. Thereafter, the 0.3Li₂MnO₃.0.7LiMn_0.5Ni_0.5O₂ product was rapidly quenched (also in air). The X-ray diffraction pattern of the resulting powder is shown in FIG. 8a (Sample D). The electrode powder was preconditioned according to the following procedures prior to cell assembly: 1) subjecting the powder to flowing NH₃ gas in a tubular furnace at 200° C. for approximately 20 hrs, the X-ray diffraction pattern of this product being shown in FIG. 8b (Sample E); 2) washing the powder in de-ionized water (p˜6.5) before subjecting the powder to flowing NH₃ gas in a tubular furnace at 200° C. for approximately 20 hrs, the X-ray diffraction pattern of this product being shown in FIG. 8c (Sample F); 3) treating the powder with 0.1 M HNO₃ (pH˜1.8) before subjecting the powder to flowing NH₃ gas in a tubular furnace at 200° C. for approximately 20 hrs, the X-ray diffraction pattern of this product being shown in FIG. 8d (Sample G). There were no significant differences in the X-ray patterns of Samples D, E, F and G, indicating that there were no significant changes to the bulk structure of the individual compounds. Refinement of the lattice parameters of Samples D, E and G using rhombohedral symmetry (hexagonal setting), which is typical of layered-type compounds such as LiCoO₂ (Table 2), showed that there was an insignificant change of the lattice parameters after NH₃-treatment (Sample D). However, the slight expansion of the c-axis in Sample G is consistent with the removal of some lithium from the composite structure as a result of acid-treatment.

[0055]

TABLE 2
Lattice parameters of Samples D, E, and G.
	Electrode	a (Å)	c (Å)
	Sample D (untreated)	2.8655(3)	14.254(3)
	Sample E (NH3-treated)	2.8666(3)	14.259(3)
	Sample G (HNO3/NH3-treated)	2.8711(3)	14.271(4)

[0056] The construction of lithium cells containing 0.3Li₂MnO₃.0.7LiMn_0.5Ni_0.5O₂ and preconditioned 0.3Li₂MnO₃.0.7LiMn_0.5Ni_0.5O₂ positive electrodes was carried out by the same procedure as described in Example 2. Cells were cycled under galvanostatic mode between 4.6 and 2.0 V at constant current density (0.1 mA/cm²).

[0057] The voltage profiles of the first charge/discharge cycle of lithium cells with:

[0058] an untreated 0.3Li₂MnO₃.0.7LiMn_0.5Ni_0.5O₂ electrode (Sample D);

[0059] an NH₃-treated 0.3Li₂MnO₃.0.7LiMn_0.5Ni_0.5O₂ electrode (Sample E);

[0060] an H₂O/NH₃-treated 0.3Li₂MnO₃.0.7LiMn_0.5Ni_0.5O₂ electrode (Sample F); and

[0061] an HNO₃/NH₃-treated 0.3Li₂MnO₃.0.7LiMn_0.5Ni_0.05O₂ electrode (Sample G)

[0062] are shown in FIG. 9(a-d), respectively.

[0063] The capacities obtained during the first charge and discharge and the cycling efficiency of the initial cycle of these cells are provided in Table 2 and illustrated graphically in FIG. 10(a-d), respectively.

[0064]

TABLE 3
Charge/discharge capacities and reversible capacity (%) of the
initial cycle of lithium cells with 0.3Li2MnO3•0.7LiMn0.5Ni0.5O2
and preconditioned 0.3Li2MnO3•0.7LiMn0.5Ni0.5O2 electrodes.
	Initial Charge	Initial Discharge	Reversible
Electrode	Capacity (mAh/g)	Capacity (mAh/g)	Capacity (%)
Sample D	259	203	78
Sample E	254	199	78
Sample F	248	204	82
Sample G	205	195	95

[0065] The data in Table 3 demonstrate that an improvement in the irreversible capacity loss during the initial cycle can be obtained by preconditioning xLi₂M′O₃.(1−x)LiMO₂ composite electrodes, particularly those containing a Li₂MnO₃ component by subjecting the electrodes to an acidic medium, e.g., deionized water (pH˜6.5) or an HNO₃ solution (pH˜1.8) and that the improvement is significantly more pronounced when the electrodes are preconditioned with acid at low pH (Sample G, 95% efficiency). The excellent 95% capacity retention of electrode sample G when charged to the high potential of 4.6 V vs. Li⁰ is indicative of a stabilized electrode with a significantly reduced surface reactivity compared with electrode samples D, E and F.

[0066] An electrode material with the formula 0.1 Li₂MnO₃.0.9LiCo_0.372Ni_0.372Mn_0.256O₂ was prepared as follows. The (Mn_0.330Ni_0.335Co_0.335)(OH)_x(x˜2) precursor was prepared by coprecipitation in a similar manner to the procedure described in Example 1. Li₂CO₃ was intimately with the (Mn_0.330Ni_0.335Co_0.335)(OH)_x(x˜2) precursor in a ratio of ratio Li₂CO₃:(Mn_0.330Ni_0.335Co_0.335)(OH)_x=0.55:1 (or Li:(Mn+Ni+Co)=1.1:1). The powder mixture was calcined at 700° C. for 16 hours in air and then at 950° C. for 12 hours in air to make 0.1Li₂MnO₃.0.9LiCo_0.372Ni_0.372Mn_0.256O₂ (Sample H). A 0.1Li₂MnO₃.0.9LiCo_0.372Ni_0.372Mn_0.256O₂ sample was treated with an acidic 0.014M NH₄F (aqueous) solution, the pH of which was 5.54, and thereafter dried at either at 300° C. or 600° C. in air for 6 hours (Samples I and J, respectively). A 0.1Li₂MnO₃.0.9LiCu_0.372Ni_0.372Mn_0.256O₂ sample was also treated with a 0.016M NH₄F solution in laboratory grade methanol containing trace amounts of water (typically up to 0.1%), the pH of which was 6.24, and thereafter dried at 600° C. in air for 6 hours (Sample K). The X-ray diffraction patterns of Samples H, I, J and K are shown in FIG. 13(a-d). There were no significant differences in the X-ray patterns of Samples H, I, J and K, indicating that no significant changes had occurred to the bulk structure of the individual compounds during the preconditioning reactions.

[0067] The construction of lithium cells containing 0.1Li₂MnO₃.0.9LiCo_0.372Ni_0.372Mn_0.256O₂ electrodes of Example 5 was carried out in a similar manner to the procedure described in Example 2. Cells were cycled galvanostatically between 4.6 and 3.0 V at constant current density (first two cycles at 0.1 mA/cm² and following cycles at 0.5 mA/cm²).

[0068] The voltage profiles of the first charge/discharge cycle of lithium cells with an untreated 0.1Li₂MnO₃.0.9LiCo_0.372Ni_0.372Mn_0.256O₂ electrode; a 0.1 Li₂MnO₃.0.9LiCo_0.372Ni_0.372Mn_0.256O₂ electrode treated with an acidic NH₄F solution (aqueous) and dried at 300° C.; a 0.1Li₂MnO₃.0.9LiCo_0.372Ni_0.372Mn_0.256O₂ electrode treated with an acidic NH₄F solution (aqueous) and subsequently dried at 600° C.; and a 0.1 Li₂MnO₃.0.9LiCo_0.372Ni_0.372Mn_0.256O₂ electrode treated with an acidic NH₄F solution (methanol+trace H₂O) and subsequently dried at 600° C. are shown in FIG. 14(a-d), respectively. The capacities obtained during the first charge and discharge and the cycling efficiency of the initial cycle of these cells are provided in Table 4; the cycling stability of the untreated electrode and preconditioned electrodes is shown graphically in the capacity vs. cycle number plots (30 cycles) in FIG. 15.

[0069]

TABLE 4
Charge/discharge capacities and reversible capacity
(%) of the initial cycle of lithium cells with
0.1Li2MnO3•0.9LiCo0.372Ni0.372Mn0.256O2
and preconditioned
0.1Li2MnO3•0.9LiCo0.372Ni0.372Mn0.256O2 electrodes.
	Initial Charge	Initial Discharge	Reversible
Electrode	Capacity (mAh/g)	Capacity (mAh/g)	Capacity (%)
Sample H	230	184	80.1
Sample I	230	194	84.4
Sample J	233	197	84.5
Sample K	228	189	83.3

[0070] The electrochemical data of Example 6 are fully consistent with the principles of this invention. The data demonstrate that when xLi₂M′O₃.(1−x)LiMO₂ (0≦x<1) electrodes, as represented by 0.1 Li₂MnO₃.0.9LiCo_0.372Ni_0.372Mn_0.256O₂, are chemically preconditioned prior to cell assembly in a proton-containing, acidic solution with a pH<7.0, the capacity, cycling efficiency and cycling stability of lithium cells, when charged to high potentials, are improved. For example, the improved coulombic efficiencies of the initial charge/discharge cycle of cells with preconditioned electrodes Samples I, J, and K (83.3-84.5%) when charged to the high potential of 4.6 V vs. Li⁰ are indicative of stabilized electrodes with reduced surface reactivity compared to that of the untreated control sample H (80.1%). Furthermore, the capacity vs. cycling plots in FIG. 15 provide unequivocal evidence that higher electrochemical capacities can be obtained from acid-treated electrodes Samples I, J and K on cycling and, moreover, that improved cycling stability can be achieved by using acidic media in which the salt of a relatively strong acid and a relatively weak base, such as NH₄F, is dissolved in a non-aqueous solvent such as an alcohol, for example, methanol, as in Sample K. In this respect, it should be noted that alcohols are readily miscible with water, and therefore are likely to contain trace amounts of H₂O, even when in high purity form. Because of the miscibility of water with non-aqueous solvents such as methanol, ethanol, or the like, it therefore stands to reason and is obvious to those skilled in the art that the amount of water in such co-solvents can be tailored to control the pH of the acid solution (pH<7) for the purposes of this invention. The invention therefore includes using acidic solutions of salts dissolved in one or more aqueous or non-aqueous solvents, such as ammonium salts NH₄NO₃, NH₄C₁, NH₄F, and NH₄HF₂ (NH₄F.HF) dissolved in methanol, ethanol, or the like, to provide acidic solutions having a pH<7.0, to precondition the electrodes and prior to heating the electrodes above 100° C., but preferably below 600° C., in order to completely dry the electrodes and to stabilize the electrode surface as effectively as possible.

[0071] Although the precise reasons for the improved electrochemical behavior of Samples I, J and K over Sample H are not yet fully understood, the applicants believe that amongst the major reasons for the improved behavior brought about by the preconditioning reactions are: 1) a greater stability of the preconditioned electrode surface, 2) possible H⁺—Li⁺ ion-exchange reactions and 3) a removal of some lithia (Li₂O) from the xLi₂M′O₃ component of the electrode structure, as described hereinbefore. It is also well known that NH₃ or NH₄-bearing species can act as reducing agents, and therefore that reduction of the electrode surface can occur, particularly when the electrodes are dried at elevated temperatures. Moreover, when acids such as HNO₃, HCl and HF are used, then the replacement of divalent surface oxygen ions, O²⁻, by monovalent NO₃⁻, Cl⁻ or F⁻ ions would have the effect of reducing the nearest neighbor transition metal ions (M and M′) of the xLi₂M′O₃.(1−x)LiMO₂ (0≦x<1) electrode structure. In this respect, transition metal oxy-nitrato, oxychloride and oxyfluoride compounds are already known to exist, such as FeONO₃, TiO(NO₃)₂, FeOCl, FeOF, VOCl₂ and VOF, and it is therefore believed that such oxy-nitrato-, oxychloride- and oxyfluoride species may form at least at the electrode surfaces of this invention, thereby imparting greater structural stability to these surfaces and reducing the reactivity of the electrode surfaces towards the electrolyte at high potentials, typically above 4.2 or 4.3 V. The same principles may also apply, in likewise fashion, to other inorganic acids and organic acids, such as phosphoric acid, sulfuric acid and acetic acid.

[0072] The examples and results of this invention demonstrate that improved electrochemical performance of layered lithium metal oxide electrodes as defined herein, can be significantly improved by preconditioning the electrodes either with a reducing agent such as ammonia or by acid treatment or a combination thereof, particularly when the electrodes are subjected to high potentials in excess of 4.2 V during charge. This invention, therefore, also relates to non-aqueous electrochemical lithium cells containing such preconditioned positive electrodes, a typical cell shown schematically in FIG. 11, represented by the numeral 10 having a negative electrode 12 separated from a positive electrode 16 by an electrolyte 14, all contained in an insulating housing 18 with suitable terminals (not shown) being provided in electronic contact with the negative electrode 12 and the positive electrode 16. Binders and other materials normally associated with both the electrolyte and the negative and positive electrodes are well known in the art and are not fully described herein, but are included as is understood by those of ordinary skill in this art. FIG. 12 shows a schematic illustration of one example of a battery in which two strings of electrochemical lithium cells, described above, are arranged in parallel, each string comprising three cells arranged in series. The invention also includes methods of making the preconditioned positive electrodes, cells and batteries including same.

[0073] While there has been disclosed what is considered to be the preferred embodiments of the present invention, it is 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 and that additional improvements in the capacity and stability of the electrodes can be expected to be made in the future by improving and optimizing the processing techniques whereby lithium metal oxide electrodes are chemically preconditioned either by acid treatment or reduction processes, or a combination thereof, prior to the construction of electrochemical lithium cells.