Stable form of LiMnO.sub.2 as cathode in lithium cell

Abstract

The invention disclosed relates to a new method of forming spinel-related .lambda.-Li.sub.2-x Mn.sub.2 O.sub.4, wherein 0.ltoreq.x.ltoreq.2, solely by electrochemical means with air-stable orthorhombic LiMnO.sub.2 as the starting material. This spinel-related material is hydroscopic, metastable and is typically made by chemical means, followed by electrochemical conversion of spinel-type LiMn.sub.2 O.sub.4. Also disclosed are new secondary lithium ion electrochemical cells employing as initial active cathode material a compound of formula LiMnO.sub.2, having a specific orthorhombic crystal structure.

Description

BACKGROUND OF THE INVENTION

This invention relates to a novel method of forming spinel-related .lambda.-Li.sub.2-x Mn.sub.2 O.sub.4, wherein 0.ltoreq..times..ltoreq.2, in a secondary electrochemical cell, and to the use of a compound of formula LiMnO.sub.2 which has a specific orthorhombic crystal structure, as initial active cathode material in such cells.

The impetus for this invention was the recent, great increase in demand for batteries having high energy density and low weight, such as had already been achieved with metallic lithium systems, but which are also rechargeable. Lithium electrodes do not meet this requirement over extended periods of time because even in an aprotic organic solvent the cycling stability is severely limited, and problems with cell safety arise.

An electrochemical cell in which both the anode and the cathode are lithium intercalation compounds is termed a lithium ion cell. The intercalation compound serves as a host structure for lithium ions which are either stored or released depending on the polarity of an externally applied potential. During discharge, the electromotive force reverses the forced intercalation or de-intercalation thereby producing current.

In a lithium ion cell, the positive electrode generally serves as the initial reservoir of lithium. The capacity of the cell will be limited by the amount of lithium present in the positive electrode on cell assembly. In most cases, only a proportion of the lithium present, during fabrication of the positive electrode, can be reversibly removed (i.e. cycled).

LiMnO.sub.2 is known to exist in several phases. Two phases, whose crystal structures have been well characterized, are a high temperature orthorhombic phase and a tetragonal phase. Both structures involve cubic closest packing but they differ in the arrangement of the ordering of the lithium and manganese cations. The tetragonal form is normally written as .lambda.-Li.sub.2 Mn.sub.2 O.sub.4. It has an atacamite-type structure which is often referred to as being spinel related. Another type of lithium manganate has a spinel structure of composition LiMn.sub.2 O.sub.4.

DESCRIPTION OF THE PRIOR ART

One approach to improving the reversibility of lithium electrodes involves the use of intercalation compounds.

Both LiMn.sub.2 O.sub.4 and .lambda.-Li.sub.2 Mn.sub.2 O.sub.4 have been used as cathodes in lithium ion cells [U.S. Pat. Nos. 5,135,732 and 5,110,696, and J. M. Tarascon and D. Guyomard, J. Electrochem. Soc., 138 2864 (1991)]. .lambda.-Li.sub.2 Mn.sub.2 O.sub.4 has twice the nominal capacity of LiMn.sub.2 O.sub.4 but it is reported to be hydroscopic and metastable [A. Mosbah, A. Verbaere and M. Tournoux, Mat. Res. Bull., 18 1375 (1983)]. Specifically, spinel-related .lambda.-LiMn.sub.2 O.sub.4 is typically made by first heating lithium carbonate with manganese dioxide, or reacting these two compounds in the presence of a lithium source to form spinel-type LiMn.sub.2 O.sub.4, followed by electrochemical conversion to .lambda.-Li.sub.2 Mn.sub.2 O.sub.4. Neither process is very satisfactory. Moreover, lithium is pyrophoric and causes handling problems.

In the reference [J. M. Tarascon and D. Guyomard, J. Electrochem. Soc., 138, 2864 (1991)] it is reported that .lambda.-Li.sub.2 Mn.sub.2 O.sub.4 slowly decomposes in ambient conditions to LiMn.sub.2 O.sub.4. .lambda.-Li.sub.2 Mn.sub.2 O.sub.4 converts to the high temperature orthorhombic form, LiMnO.sub.2, when heated in an argon atmosphere to 600.degree. C. It is prepared by electrochemically, or chemically, intercalating lithium into the LiMn.sub.2 O.sub.4 spinel structure, as described in the two preceding references.

The structure of the high temperature orthorhombic phase of LiMnO.sub.2 was determined in detail by R. Hoppe, G. Brachtel and M. Jansen, [J. Anorg. Allg. Chem., 417 1 (1975)]. This structure is described by the space group Pmnm and has unit cell dimensions a =4.572 .ANG., b=5.757 .ANG. and c=2.805 .ANG.. To our knowledge, the use of LiMnO.sub.2, having this structure, as the active material in the cathode of a reversible electrochemical cell has never been demonstrated. Recently, T. Ohzuku, A. Ueda and T. Hirai [Chemistry Express, 7 193 (1992)] have demonstrated the use of a low temperature form of orthorhombic LiMnO.sub.2 as the active cathode material in a reversible lithium ion cell. The low temperature form of orthorhombic LiMnO.sub.2 was prepared by heating an equimolar mixture of .gamma.-MnOOH and LiOH.H.sub.2 O to 450.degree. C. For the sake of simplicity, the high temperature form of orthorhombic LiMnO.sub.2 with the structure described by Hoppe, Brachtel and Jansen will henceforth be referred to as orthorhombic LiMnO.sub.2.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a novel method of forming spinel-related .lambda.-Li.sub.2-x Mn.sub.2 O.sub.4, wherein 0.ltoreq..times..ltoreq.2, in the cathode of a secondary lithium ion electrochemical cell.

It is another object of the present invention to provide a secondary electrochemical cell of high energy density, whose charge/discharge mechanism is based upon alternating intercalation and de-intercalation of Li.sup.+ ions in the active materials of the positive and negative electrodes.

It is yet another object to provide good chemical resistance to the electrolyte and high cycling stability.

According to one aspect of the invention, a method of forming spinel-related .lambda.-Li.sub.2-x Mn.sub.2 O.sub.4, wherein 0.ltoreq..times..ltoreq.2, in the cathode of a secondary lithium ion cell is provided, which comprises

(a) providing a secondary lithium ion electrochemical cell, said cell comprising a lithium intercalation anode, a suitable non-aqueous electrolyte including a lithium salt, a cathode containing as initial active material orthorhombic LiMnO.sub.2 characterized by the specific orthorhombic crystal structure described by the space group Pmnm and unit cell dimensions a=4.572 .ANG., b=5.757 .ANG. and c=2.805 .ANG., and a separator between the anode and cathode, and (b) charging the cell to de-intercalate sufficient available lithium ions to convert the LiMnO.sub.2 to spinel-type LiMn.sub.2 O.sub.4, and (c) discharging the cell to intercalate sufficient available lithium ions to convert the spinel-type LiMn.sub.2 O.sub.4 to spinel-related .lambda.-Li.sub.2-x Mn.sub.2 O.sub.4.

Preferably, the cell is charged to greater than 40-50% of the nominal cathode capacity, and discharged to 30-40% of the nominal cathode capacity.

According to another aspect of the invention, the use of a compound of formula LiMnO.sub.2 characterized by the specific orthorhombic crystal structure, described by the space group Pmnm and unit cell dimensions a=4.572 .ANG., b=5.757 .ANG. and c=2.805 .ANG., as initial active cathode material in secondary lithium ion electrochemical cells is also provided.

According to yet another aspect of the invention, a secondary lithium ion electrochemical cell comprising a lithium intercalation anode, a suitable non-aqueous electrolyte including a lithium salt, a cathode containing as initial active material orthorhombic LiMnO.sub.2 characterized by the specific orthorhombic crystal structure described by the space group Pmnm and unit cell dimensions a=4.572 .ANG., b=5.757 .ANG. and c=2.805 .ANG., and a separator between the anode and cathode, is provided.

The orthorhombic LiMnO.sub.2 is converted to spinel-type LiMn.sub.2 O.sub.4 on the first charge, and upon discharge spinel-related .lambda.-Li.sub.2-x Mn.sub.2 O.sub.4 is formed. Accordingly, the cell behaves the same way as a cell built with .lambda.-Li.sub.2-x Mn.sub.2 O.sub.4 as the initial cathode material. However, LiMnO.sub.2 is much simpler to prepare than .lambda.-Li.sub.2-x Mn.sub.2 O.sub.4 which is not air-stable. See example 1 below.

The anode of the present invention serves as the recipient substance for Li.sup.+ ions. The anode can be any intercalation compound which is capable of intercalating lithium and has an electrode potential sufficiently reducing to provide an adequate cell voltage over a range of lithium intercalation. Specific examples include transition metal oxides such as MoO.sub.2 or WO.sub.2 [Auborn and Barberio, J. Electrochem. Soc. 134 638 (1987)], transition metal sulfides (see U.S. Pat. No. 4,983,476) or carbon products obtained by the pyrolysis of organic compounds. (The disclosures of the aforementioned two documents are Incorporated Herein by Reference.) As will be apparent hereinafter, various commercially available carbonaceous materials of predetermined structural characteristics have proven useful.

The non-aqueous electrolyte of the present invention can be liquid, paste-like or solid. Preferably, the electrolyte includes a lithium salt with an organic solvent, and is in liquid form. Electrolyte salts useful for this purpose are LiAsF.sub.6, LiPF.sub.6, LiBF.sub.4, LiClO.sub.4, LiBr, LiAlCl.sub.4, LiCF.sub.3 SO.sub.3, LiC(CF.sub.3 SO.sub.2).sub.3, LiN(CF.sub.3 SO.sub.2).sub.2, or mixtures thereof. As a water-free solvent for these salts, there can be used alone or in mixture with others an organic solvent of the group propylene carbonate, ethylene carbonate, 2-methyl tetrahydrofuran, tetrahydrofuran, dimethoxyethane, diethoxyethane, dimethyl carbonate, diethyl carbonate, methyl acetate, methylformate, .gamma.-butyrolactone, 1,3-dioxolane, sulfolane, acetonitrile, butyronitrile, trimethylphosphate, dimethylformamide and other like organic solvents. The electrolyte solution can also contain additives such as Crown ethers e.g. 12-C-4,15-C-5, and 18-C-6, or immobilizing agents such as polyethylene oxide or inorganic gel-forming compounds such as SiO.sub.2, or Al.sub.2 O.sub.3 such as described in U.S. Pat. No: 5,169,736, the disclosure of which is Incorporated Herein by Reference.

The cathode, having orthorhombic LiMnO.sub.2 as the initial active material, has an electrochemical potential sufficiently positive of the negative electrode to produce a useful overall cell voltage. The greater the potential, the greater the resulting energy density. In a lithium ion cell, the positive electrode generally serves as the initial reservoir of lithium. The capacity of the cell will be limited by the amount of lithium, available for de-intercalation, present in the positive electrode on cell assembly. In most cases, only a proportion of the lithium present, during fabrication of the positive electrode, can be reversibly deintercalated. The capacity of a lithium ion cell having a cathode of orthorhombic LiMnO.sub.2 as initial active material increases after the first cycle in which the original structure is converted to spinel-related .lambda.-Li.sub.2-x Mn.sub.2 O.sub.4.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the trace of the X-ray diffraction pattern of orthorhombic LiMnO.sub.2, prepared as described in example 1. The JCPDF powder X-ray diffraction pattern for orthorhombic LiMnO.sub.2 is included for comparison.

FIG. 2 is the trace of the X-ray diffraction pattern, for the same sample as in FIG. 1, taken after 11 months of storage in ambient conditions.

FIG. 3 shows a plot of a typical charge/discharge cycle for an electrochemical cell with a carbon coke anode and a cathode containing orthorhombic LiMnO.sub.2.

FIGS. 4 and 5 are traces of diffraction patterns of cathodes from the two electrochemical cells described in example 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Orthorhombic LiMnO.sub.2 has been found to be air-stable, and easily fabricated into cathodes for use in secondary lithium ion electrochemical cells of high reversible capacity.

The preferred negative electrode is based on a carbonaceous product. Petroleum coke is particularly preferred. Petroleum cokes are non-graphitic carbons which can intercalate up to approximately one lithium ion per twelve carbon atoms.

Suitable anode carbonaceous materials include:

1) carbonaceous material with a d.sub.002 layer spacing of less than or equal to 3.70 .ANG. with true density of greater than or equal to 1.70 g/cm.sup.3 prepared by carbonization of furan resins, as per U.S. Pat. No: 4,959,281. 2) the above doped with 2-5% phosphorous and oxygenated petroleum or coal pitch carbonized and doped with 2-5% phosphorous with the same d.sub.002 layer spacing and true density as per published European application No: EP 0 418 514. 3) carbons formed by the thermal decomposition of gas phase hydrocarbons or hydrocarbon compounds with d.sub.002 from 3.37 to 3.55 .ANG. as per U.S. Pat. No: 4,863,814. 4) carbon formed from mesophase microspheres with d.sub.002 layer spacing up to 3.45 .ANG. as per U.S. Pat. No: 5,153,082. 5) commercial petroleum coke as per U.S. Pat. No: 4,943,497. 6) isotropic graphite composed of a mixture of graphite and carbonized pitch with a degree of graphitization greater than or equal to 0.4 and heat treated fluid coke and commercial graphite whose first lithium electrochemical intercalation is performed at or above 50.degree. C., as per U.S. Pat. No: 5,028,500. (The disclosure of the aforementioned 6 references are incorporated herein by reference).

Typical electrodes in the present invention are fabricated from 80-94 weight percent of active material, carbon coke or LiMnO.sub.2, 5-15 weight percent of a conductivity enhancer such as Super S carbon black, and 1-5 weight percent of a binder such as ethylene propylene diene monomer (EPDM). Other conductivity enhancers such as Shawinigan Acetylene Black, graphites or other conductive materials may be used. In addition, other binders such as Teflon.RTM., poly(vinylidene difluoride), polyolefins or elastomers may be substituted for EPDM.

EXAMPLE 1

A sample of lithium manganese oxide of formula LiMnO.sub.2 characterized by an orthorhombic crystal structure in accordance with the present invention was made by reacting Li.sub.2 CO.sub.3 and .beta.-MnO.sub.2 in proportions such that the atomic ratio between lithium in Li.sub.2 CO.sub.3 and the manganese in the .beta.-MnO.sub.2 was 1:1.

The Li.sub.2 CO.sub.3 and .beta.-MnO.sub.2 were mixed using a mortar and pestle until the mixture was substantially homogenous. The resulting powder was formed into pellets in a 18 mm die, and then heated, in a flow of argon gas, at 600.degree. C. for two hours followed by two intervals of three days each at 800.degree. C. The sample was ground again between the two firings. The product was examined by X-ray diffraction. FIG. 1 shows the trace of the X-ray diffraction pattern of the product from the second firing and the reported diffraction pattern for orthorhombic LiMnO.sub.2, powder diffraction file card #35-0749, from the JCPDS International Centre for Diffraction Data. FIG. 2 shows the trace of an X-ray diffraction pattern of the same sample after 11 months of storage in ambient conditions. The second diffraction pattern is substantially identical to the first. Unlike .lambda.-Li.sub.2 Mn.sub.2 O.sub.4, the orthorhombic form of LiMnO.sub.2 is air and moisture stable.

EXAMPLE 2

An electrochemical cell was assembled with a cathode containing as the initial active material, orthorhombic LiMnO.sub.2 formed by reacting stoichiometric amounts of Li.sub.2 CO.sub.3 and .beta.-MnO.sub.2 in a manner similar to Example 1, except that reaction temperature was increased to 900.degree. C.

The cathode was prepared by drying a slurry made from orthorhombic LiMnO.sub.2 with two weight percent of ethylene propylene diene monomer (EPDM) and 10 wt. % Super S carbon black in cyclohexane. The carbon anode was made from Asbury petroleum coke, with 2 wt. % of EPDM and 5 wt. % of Super S carbon black. Both the anode and cathode pellets were formed by pressing about 200-500 mg of the mixture of EPDM and carbon black coated material in a 17 mm die for 30 s with 3500 pounds of force applied by a hydraulic press. The diameter of the pellets was chosen to fit a commercial coin cell case of the type #2325. These coin cells have a diameter of 23 mm and a height of 2.5 mm, before crimping the seal. The cell stack consisted of a three layer sandwich of the cathode, a cell separator, and a carbon anode. The separator, a non-woven polypropylene felt, prevents the two electrodes from contacting directly. The separator also provides a reservoir for the electrolyte solution.

The electrolyte used was a 50/50 mixture by volume of propylene carbonate and dimethoxyethane containing 1M (CF.sub.3 SO.sub.2).sub.2 N.sup.- Li.sup.+ from 3M company. The salt was dried under vacuum at 160.degree. C. for 24 hours as recommended by the manufacturer. The propylene carbonate was vacuum distilled and the dimethoxyethane was distilled from lithium/naphthalide under helium. The water content of the electrolyte was under 30 ppm as determined by Karl-Fischer titration. The cell was assembled and crimped in a helium filled glove box to prevent the electrolyte solution from reacting with ambient moisture. The coin cell was charged and discharged on custom built cyclers. The fully automated and fully programmable cyclers operate at a constant current anywhere from 0.1 .mu.A to 100 mA, with the current controlled to 0.1 .mu.A, or to 1 part in 4000 for higher currents. The current and voltage are measured every 30 s. The voltage can be resolved to 10 .mu.V over the range 0 to 12 V.

The anode had a thickness of 0.040" and a weight of 0.301 g, which corresponds to a capacity of 52.3 mAh to a composition of LiC.sub.12. The cathode was 0.030 "thick and weighed 0.312 g which corresponds to a theoretical capacity of 79.4 mAh if all the lithium ions in the cathode can be reversibly cycled. The cell completed 64 cycles at a current of 0.28 mA between voltage limits of 4.0 and 1.95 volts. On the 48.sup.the cycle the cell had a reversible capacity of 36 mAh (shown in FIG. 3). The capacity began to decrease significantly at about the 56.sup.the cycle and had diminished to 20 mAh by the 64.sup.the cycle. The voltage versus capacity plot for orthorhombic LiMnO.sub.2 (FIG. 3) is very similar to that of .lambda.-Li.sub.2 Mn.sub.2 O.sub.4, shown in FIG. 6 of Tarascon and Guyomard, [J. Electrochem. Soc., 138 2864 (1991)].

EXAMPLE 3

Another two cells were assembled, as described above, with cathodes containing orthorhombic LiMnO.sub.2, prepared as described in example 1. The first cell had a 0.045" thick anode weighing 0.354 g made with Conoco XP coke, and a cathode of 0.024" thickness, weighing 0.329 g. The cell completed 49 cycles between 1.8 and 4.0 volts at a current of 0.25 mA. The cell was stopped half way into the 50.sup.the charge. After a period of relaxation, the open circuit voltage of the cell was 3.04 volts. The cell was disassembled and the cathode pellet was examined by powder X-ray diffraction. The trace of the diffraction pattern, shown in FIG. 4, matches the JCPDF card file for LiMn.sub.2 O.sub.4.

The second cell had a 0.056" thick anode weighing 0.428 g and a cathode of 0.014" thickness and 0.190 g weight. A capacity of 33.7 mAh was required to charge the cell to 4.0 volts at 0.28 mA. This corresponds to a 70% depth of charge of the cathode and to a composition of Li.sub.0.3 MnO.sub.2 in the active material. This cell was cycled between 1.2 and 3.8 volts at 0.02 mA. The cell was stopped after completing the fifth charge. After a period of relaxation the open circuit voltage of the cell was 3.5 volts. The cell was opened and the cathode was extracted for analysis by X-ray powder diffraction. The trace of the diffraction pattern is shown in FIG. 5. The peaks correspond to a cubic unit cell with a =8.143(3) .ANG.. The diffraction pattern matches that described for Li.sub.0.6 Mn.sub.2 O.sub.4 [T. Ohzuku, M. Kitagawa and T. Hirai, J. Electrochem. Soc., 137 769 (1990) ].

Claims

1. A secondary lithium ion electrochemical cell, comprising a lithium intercalation anode comprising petroleum coke, a non-aqueous electrolyte comprising IM LiN(CF.sub.3 SO.sub.2).sub.2 in a 50/50 mixture by volume of propylene carbonate and dimethoxyethane, a cathode and a separator between the anode and cathode, wherein the initial cathode material comprises a dried slurry of orthorhombic LiMnO.sub.2 characterized by the specific orthorhombic crystal structure described by the space group Pmnm and the unit cell dimensions a=4.572 .ANG., b=5.757 .ANG.and c=2.805 .ANG., and by the X-ray diffraction pattern shown in FIGS. 1 and 2 as active material, two weight percent of ethylenepropylenediene monomer and ten weight percent of carbon black, in cyclohexane.