Lithium Maganese Oxide Compound and Method of Preparation

The present invention relates to a lithium manganese oxide compound and its production by contacting .beta.-MnO.sub.2 or .lambda.-MnO.sub.2 with lithium dissolved in a solvent in which lithium generates solvated electrons or in which a catalyst is present that is capable of accepting an electron from lithium and delivering it to the .beta.-MnO.sub.2 or .lambda.-MnO.sub.2.

The present invention relates to lithiated manganese oxides, to methods of making such materials and to the use of such materials in the manufacture of the cathodes of electrochemical cells incorporating the materials as cathode-active materials.

More particularly it relates to a process for the manufacture of Li.sub.2 Mn.sub.2 O.sub.4 and the use of Li.sub.2 Mn.sub.2 O.sub.4 in electrical storage batteries. Still more particularly, it relates to a process for the manufacture of Li.sub.2 Mn.sub.2 O.sub.4 by the reaction of .beta.-MnO.sub.2 or .lambda.-MnO.sub.2 with lithium and to using Li.sub.2 Mn.sub.2 O.sub.4 in the manufacturing of the cathode component of rechargeable lithium-ion electrical storage batteries.

Conventionally used nonaqueous electrolyte cells are primary cells which can be used only once. With recent widespread use of video cameras and small-sized audio instruments, there has been an increased need for secondary cells which can be used conveniently and economically for long, repeated use.

Lithium cells useful as electrical storage batteries incorporate a metallic lithium anode and a cathode including an active material which can take up lithium ions. An electrolyte incorporating lithium ions is disposed in contact with the anode and the cathode. During discharge of the cells, lithium ions leave the anode, enter the electrolyte and are taken up in the active material of the cathode, resulting in release of electrical energy. Provided that the reaction between the lithium ions and the cathode active material is reversible, the process can be reversed by applying electrical energy to the cell. If such a reversible cathode-active material is provided in a cell having the appropriate physical configuration and an appropriate electrolyte, the cell can be recharged and reused. Rechargeable cells are commonly referred to in the battery art as secondary cells. It has long been known that useful cells can be made with a lithium metal anode and a cathode-active material which is a sulfide or oxide of a transition metal, i.e., a metal capable of assuming plural different valence states. Dampier, "The Cathodic Behavior of CuS, MoO.sub.3, and MnO.sub.2 in Lithium Cells," J. Electrochem. Soc., Vol. 121, No. 5, pp. 656-660 (1974) teaches that a cell incorporating a lithium anode and manganese dioxide cathode-active material can be used as an electrical power source. The same reference further teaches that a lithium and manganese dioxide cell can serve as a secondary battery.

There has been considerable effort in the battery field directed towards development of cathode materials based on lithium manganese oxides. Both lithium and manganese dioxide are relatively inexpensive, readily obtainable materials, offering the promise of a useful, potent battery at low cost. Nonaqueous electrolyte primary cells using lithium as a negative electrode-active material and nonaqueous solvent such as an organic solvent as an electrolyte have advantages in that self-discharge is low, nominal potential is high and storability is excellent. Typical examples of such nonaqueous electrolyte cells include lithium manganese dioxide primary cells which are widely used as current sources for clocks and memory backup of electronic instruments because of the long-term reliability.

Secondary lithium batteries using an intercalation compound as cathode and free lithium metal as anode have been studied intensively due to their potential technological significance. Unfortunately, these studies have revealed that inherent dangers associated with the use of free lithium preclude the commercial viability of such batteries. Upon repeated cycling, dendritic growth of lithium occurs at the lithium electrode. Growth of lithium dendrites can lead eventually to an internal short-circuit in the cell with a subsequent hazardous uncontrolled release of the cell's stored energy.

One approach to improving the reversibility of lithium-based anodes involves the use of lithium intercalation compounds. 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 thereby producing current.

Batteries using this approach, in which an intercalation compound is used as the anode instead of free lithium metal, are known in the art as "lithium-ion" or "rocking-chair" batteries. Utilization of Li.sub.2 Mn.sub.2 O.sub.4 in lithium-ion secondary batteries is described in detail in the recent review paper, "The Li.sub.1+x Mn.sub.2 O.sub.4 /C Rocking-chair System," J. M. Tarasoon and D. Guyomard, Electrochimica Acta, Vol. 38, No. 9, pp. 1221-1231 (1993).

In this approach, a nonaqueous secondary cell is provided with (a) a negative electrode consisting essentially of a carbonaceous material as a carrier for a negative electrode-active material, said carrier being capable of being doped and dedoped with lithium and (b) a positive electrode comprising lithium manganese complex oxide as an essential positive electrode-active material. This cell has a high expected applicability because dendrite precipitation of lithium does not occur on the surface of the negative electrode, the pulverization of lithium is inhibited, the discharge characteristics are good and the energy density is high.

The output voltage of this lithium-ion battery is defined by the difference in chemical potential of the two insertion compounds. Accordingly, the cathode and anode must comprise intercalation compounds that can intercalate lithium at high and low voltages, respectively.

The viability of this concept has been demonstrated and future commercialization of such cells in D, AA or coin-type batteries has been indicated. These cells include a LiMn.sub.2 O.sub.4, a LiCoO.sub.2, or a LiNiO.sub.2 cathode, an electrolyte and a carbon anode. These lithium-ion batteries are described as being superior to nickel-cadmium cells and do not require a stringent environment for fabrication since the lithium based cathode employed is stable in an ambient atmosphere, and the anode is not free lithium metal, but an intercalation compound used in its discharged state (without intercalated lithium) that is stable in ambient atmosphere when the cells are assembled.

However, a nonaqueous electrolyte secondary cell such as described above has disadvantages in that the cell capacity has proven to decrease because some of the lithium doped into the carbonaceous material used as a negative electrode active material cannot be dedoped upon discharge. In practice, either carbon or graphite irreversibly consumes a portion of the lithium during the first charge-discharge cycle. As a result the capacity of the electrochemical cell is decreased in proportion to the lithium that is irreversibly intercalated into the carbon during the first charge.

This disadvantage can be eliminated by using Li.sub.2 Mn.sub.2 O.sub.4 as all or part of the cathode. Upon the first charge of the cell so manufactured, the Li.sub.2 Mn.sub.2 O.sub.4 is converted to .lambda.-Mn.sub.2 O.sub.4. When the cell is operated over the appropriate range of electrical potential, subsequent discharge cycles of the cell convert .lambda.-Mn.sub.2 O.sub.4 to LiMn.sub.2 O.sub.4, and charge cycles convert LiMn.sub.2 O.sub.4 to .lambda.-Mn.sub.2 O.sub.4. Because excess lithium is available to satisfy the irreversible consumption by carbon or graphite, cells manufactured using Li.sub.2 Mn.sub.2 O.sub.4 have greater electrical capacity.

The capacity of a lithium ion cell is also limited by the quantity of lithium which can be reversibly removed (i.e, cycled) from the cathode. In the cathode materials of the prior art, only about one half mole of lithium per transition metal can be removed reversibly. Thus, they have limited specific capacity, generally no more than about 140 mAh/g.

In principle, one mole of lithium per mole of manganese can be removed reversibly from Li.sub.2 Mn.sub.2 O.sub.4. In practice, however, cells that cycle between Li.sub.2 Mn.sub.2 O.sub.4 and LiMn.sub.2 O.sub.4 suffer more rapid loss of electrical capacity than cells that cycle between LiMn.sub.2 O.sub.4 and .lambda.-Mn.sub.2 O.sub.4. Moreover, cells that cycle between LiMn.sub.2 O.sub.4 and .lambda.-Mn.sub.2 O.sub.4 deliver most of their electrical energy between about 4 volts and about 3 volts, whereas, cells that cycle between Li.sub.2 Mn.sub.2 O.sub.4 and LiMn.sub.2 O.sub.4 deliver most of their electrical energy between about 3 volts and about 2 volts.

Thus, a combination of factors gives a lithium-ion cell that cycles lithium between a carbon or graphite matrix as the anode and LiMn.sub.2 O.sub.4 as the fully discharged cathode many particularly attractive features. Such cells can be assembled conveniently in an over-discharged state using carbon or graphite for the anode and Li.sub.2 Mn.sub.2 O.sub.4 for the cathode. Because the second lithium ion cannot be used effectively for repeated cycling, its consumption to satisfy the irreversible lithium intercalation of the carbonaceous anode material does not entail any additional loss of electrical capacity.

The compounds LiMn.sub.2 O.sub.4 and Li.sub.2 Mn.sub.2 O.sub.4 that are useful in this application are known in the art. Depending upon methods of preparation, their stoichiometries can differ slightly from the ideal. They are precisely identified however by their x-ray powder diffraction patterns. The materials herein referred to as LiMn.sub.2 O.sub.4 and Li.sub.2 Mn.sub.2 O.sub.4 have the diffraction spectra given on cards 35-781 and 38-299, respectively, of the Powder Diffraction File published by the International Centre for Diffraction Data, Newtown Square Corporate Campus, 12 Campus Boulevard, Downtown Square, Pa., 19073-3273, U.S.A.

LiMn.sub.2 O.sub.4 can be prepared from a wide range of lithium sources and a wide range of manganese sources under a wide range of conditions. U.S. Pat. No. 5,135,732 discloses a method for the low temperature preparation of LiMn.sub.2 O.sub.4. LiMn.sub.2 O.sub.4 is one of the raw materials of the present invention.

In contrast, Li.sub.2 Mn.sub.2 O.sub.4 is more difficult to prepare and in fact, known methods for the preparation of Li.sub.2 Mn.sub.2 O.sub.4 are excessively costly. These methods include the electrochemical intercalation of lithium into LiMn.sub.2 O.sub.4 (W. Li W. R. McKinnon, and J. R. Dahn, J. Electrochem. Soc., Vol. 141, No. 9, pp. 2310-2316), the reaction of LiMn.sub.2 O.sub.4 with lithium iodide U.S. Pat. No. 5,266,299), and the reaction of LiMn.sub.2 O.sub.4 with butyl lithium (M. M. Thackeray, W. I. F. David, P. G. Bruce, J. B. Goodenough, Mat. Res. Bull., Vol. 18, pp. 461-472 (1983)).

U.S. Pat. No. 5,196,279 teaches the synthesis of Li.sub.1+x Mn.sub.2 O.sub.4 from LiI and either LiMn.sub.2 O.sub.4 or .lambda.-MnO.sub.2. The reaction is effected by heating mixtures of the solid reactants to 150.degree. C. in sealed ampoules. Li.sub.1+x Mn.sub.2 O.sub.4 is a mixture of Li.sub.2 Mn.sub.2 O.sub.4 and LiMn.sub.2 O.sub.4.

U.S. Pat. No. 5,240,794 discloses a variety of lithium and lithium-ion batteries. These include a range of lithium manganese oxide compositions, including the composition Li.sub.1+x Mn.sub.2 O.sub.4. The patent discloses preparative methods for this composition generally involving mixing precursor lithium compounds and manganese compounds. The mixtures are then heated at elevated temperatures (typically 300.degree. C.) in a reducing atmosphere (typically hydrogen gas) for several hours (typically 24 hours).

It is an object of the present invention to provide a nonaqueous electrolyte secondary cell having an increased cell capacity which comprises a negative electrode consisting essentially of a carrier for a negative electrode active material and a positive electrode comprising a lithium manganese oxide as an essential positive electrode active material.

In accordance with the present invention, the above object can be accomplished by Li.sub.2 Mn.sub.2 O.sub.4 prepared by contacting .beta.-MnO.sub.2 or .lambda.-MnO.sub.2 with lithium suspended or dissolved in a solvent in which lithium generates solvated electrons or in which a catalyst is present that is capable of accepting an electron from lithium and delivering it to the .beta.-MnO.sub.2 or .lambda.-MnO.sub.2 reactant.

As noted above, this invention is directed to a method of manufacturing Li.sub.2 Mn.sub.2 O.sub.4. Specifically such method is accomplished by providing .beta.-MnO.sub.2 or .lambda.-MnO.sub.2, a source of lithium, dissolving lithium from the lithium source in a liquid medium in which lithium generates solvated electrons or the reduced form of an electron-transfer catalyst and contacting the .beta.-MnO.sub.2 or .lambda.-MnO.sub.2 with the lithium-containing liquid medium.

The source of lithium can be any source which makes elemental lithium available for reaction.

In accordance with the present invention lithium is dissolved by a liquid medium solvent in which lithium generates solvated electrons or the reduced form of an electron-transfer catalyst and contacting the .beta.-MnO.sub.2 or .lambda.-MnO.sub.2 with the dissolved lithium. Advantageously the solvent is selected from the group consisting of ammonia, organic amines, ethers, pyridine, substituted pyridines, mixtures of ammonia and amines, and mixtures of ammonia and ethers. Preferably the solvent is ammonia, organic amines, or pyridines. When the solvent is ammonia the contacting step is advantageously carded out at a temperature of from about minus 30.degree. C. to about minus 50.degree. C. Preferably the temperature during the contacting step is maintained at from about minus 33.degree. C. to about minus 45.degree. C. When ammonia is the solvent it is preferred that it be in liquid form.

If the solvent used in the method of this invention is an organic amine, it is advantageously selected from the group consisting of methylamines, ethylamines, propylamines, and butylamines. Advantageously, the method of this invention is carried out wherein the organic amine is a liquid. Preferably the contacting step of the present method is carried out at a temperature of from about minus 25.degree. C. to about 100.degree. C. Preferably the contacting step is carded out from a temperature of from about 20.degree. C. to about 90.degree. C.

If the solvent used in the method of this invention is a pyridine or a substituted pyridine, the contacting step is advantageously carried out at a temperature from about minus 5.degree. C. to about 190.degree. C. Preferably when using pyridine or a substituted pyridine as the solvent the contacting step is carried out at a temperature of from about 30.degree. C. to about 165.degree. C.

Advantageously in the present invention, the solvent is a liquid medium in which an electron-transfer catalyst is dissolved and the liquid medium preferably is a liquid at the reaction temperature.

As discussed above, the use for which the Li.sub.2 Mn.sub.2 O.sub.4 prepared by the method of this invention is uniquely applicable is as a cathode for use in a secondary lithium ion electrochemical cell. Such a cell may be of known design having a lithium intercalation anode, a suitable nonaqueous electrolyte, a cathode of material made by the method of this invention, and a separator between the anode and the cathode. The anode may be of known materials such as transition metal oxides, transition metal sulfides and carbonaceous materials. The nonaqueous electrolyte can be in the form of a liquid, a gel, or a solid matrix that contains mobile lithium ions.

The process of the present invention can optionally be practiced by providing an electron-transfer catalyst to the suspension of .beta.-MnO.sub.2 or .lambda.-MnO.sub.2 before or after the addition of lithium. Advantageously, the catalyst is selected from the group consisting of sulfur, sulfides, reducible sulfur compounds, iodine, iodides, reducible iodine compounds, pyridine, pyridine derivatives, and benzophenone.

Analysis of the products of this invention relies on X-ray diffraction. X-ray diffraction characterizes crystalline phases. It provides very reliable identifications of the compounds present in a sample, but it may not be a good quantitative-analysis method; poorly crystalline compounds or compounds which are present in small mounts (i.e., a few percent) may not be detected. Thus, undetected impurities, including LiMn.sub.2 O.sub.4, could be present at low levels in the Li.sub.2 Mn.sub.2 O.sub.4 so produced. However, it is also true that the chemical characteristics of Li.sub.2 Mn.sub.2 O.sub.4 and the conditions of the X-ray diffraction measurement interact in a way which exaggerates the estimated amount of starting material left in the Li.sub.2 Mn.sub.2 O.sub.4 product. If a Li.sub.2 Mn.sub.2 O.sub.4 sample is exposed to laboratory air for several minutes in the course of making a diffraction measurement, Li.sub.2 Mn.sub.2 O.sub.4 reacts with oxygen and water from the air to produce LiMn.sub.2 O.sub.4 and LiOH as set forth in the following equation:

Successive X-ray spectra taken on the same sample show that the amount of LiMn.sub.2 O.sub.4 present in the sample increases as the duration of its exposure to the atmosphere increases. Because the reaction requires that atoms diffuse between the surface of an individual particle and its interior, the reaction probably produces a growing shell of LiMn.sub.2 O.sub.4 that surrounds a shrinking core of Li.sub.2 Mn.sub.2 O.sub.4. Because the length of the diffusion path increases as the extent of reaction increases, the reaction is fast for pure Li.sub.2 Mn.sub.2 O.sub.4 and much slower for partially converted Li.sub.2 Mn.sub.2 O.sub.4 whose surface is already covered with LiMn.sub.2 O.sub.4.

This Example demonstrates that Li.sub.2 Mn.sub.2 O.sub.4 is produced by the reaction of elemental lithium with .beta.-MnO.sub.2 in refluxing pyridine.

A 500 ml three-neck round-bottom flask was mounted on a ring stand in a hood. It was fitted with an overhead stirrer, a thermometer, a reflux condenser, and provision for purging with argon. The assembled apparatus was vented to the atmosphere through a mineral-oil filled check valve. .beta.-MnO.sub.2 (50.00 g, 575 millimoles) was charged to the flask and covered with 200 ml pyridine. The .beta.-MnO.sub.2 was taken from a bottle labeled: "I.C. No. 6 Beta Manganese Dioxide (A reagent grade manganese dioxide purchased from Fischer Scientific co. and adopted as an international common sample on Sep. 1, 1976.)"

In a dry box, 4.00 g (576 Millimoles) of lithium foil was cut into 1 cm.sup.2 pieces and charged to an Erlenmeyer flask, which was then tightly stoppered. The lithium pieces were transferred quickly in air from the Edenmeyer flask to the round-bottom flask. Stirring was begun, and the temperature increased quickly to the boiling point. By the time the temperature reached 100.degree. C. the solution had become deep purple. Once the boiling point was reached, the heat evolved by the reaction was sufficient to continue refluxing the pyridine for several minutes. Thereafter, a heating mantle was applied, and the suspension was refluxed for an additional 3 hours. At that time, the suspended solids were brown, and the purple color was no longer evident.

The suspended solids were recovered by filtration in an argon atmosphere and washed on the frit with two 50-ml portions of THF. The THF filtrates were yellow-brown. The product was superficially dried on the frit and transferred to a Schlenk tube. Solvent was removed by evacuation in an oil-pump vacuum with intermittent heating using a hot-air gun. The product was analyzed by X-ray diffraction. The spectrum obtained had wide peaks and poor signal-to-noise characteristics. Peaks attributable to Li.sub.2 Mn.sub.2 O.sub.4 were present; no other phases were evident. (Found: 7.5% Li, 46.3% MnO.sub.2, 57.2% Mn, 5.2% C, 0.66% H, 1.0% N. Theoretical for Li.sub.2 Mn.sub.2 O.sub.4 : 7.4% Li, 46.3% MnO.sub.2, 58.5% Mn. Calculated from the Li and Mn analyses, x in Li.sub.1+x Mn.sub.2 O.sub.4 is 1.07; from the MnO.sub.2 and Mn analyses, x is 0.95.