Abstract:
A method of synthesizing a lithium manganese oxide in accordance with the formula Li 1-x  Mn 2  O 4  in which 0≦x&lt;1 and having a spinel-type crystal structure includes forming a mixture in finely divided solid form of at least one lithium salt such as Li 2  CO 3  which decomposes when heated in air to form an oxide of lithium, and at least one manganese salt such as MnCO 3  which decomposes when heated in air to form an oxide of manganese. The mixture is heated in an oxidizing atmosphere to a temperature in the range 200°-600° C. to decompose the salts and to obtain the Li 1-x  Mn 2  O 4  having a spinel-type crystal structure.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     THIS INVENTION relates to a method of synthesizing an essentially single phase lithium manganese oxide in accordance with the formula Li 1-x  Mn 2  O 4  in which O≦x&lt;1 and having a spinel-type crystal structure. In particular, the invention relates to a method of synthesizing such oxide to produce an oxide which is suitable for use as a cathode in an electrochemical cell of the Li/Li y  MnO 2  type, with an anode comprising lithium or a suitable lithium-containing alloy. The invention also relates to the oxide when produced by the method; and to an electrochemical cell comprising said oxide as its cathode. 
     According to the invention, a method of synthesizing a lithium manganese oxide having a spinel-type crystal structure comprises forming a mixture in finely divided solid form of at least one lithium salt is defined herein and at least one manganese salt as defined herein, and heating the mixture in an oxidizing atmosphere to a temperature in the range 200°-600° C. to cause said salts to react with each other and to obtain said lithium manganese oxide having a spinel-type crystal structure by simultaneous decomposition and cubic close packed oxygen lattice construction. 
     Certain forms of the lithium manganese oxide having a spinel-type structure produced by the method can be expressed by the formula Li 1-x  Mn 2  O 4  in which O≦x&lt;1, but it is to be noted that this Li 1-x  Mn 2  O 4  can have a (Mn 2 )O 4   n-   framework structure in which the quantity of Mn cations varies from the stoichiometric value. 
     By a `lithium salt as defined herein` is meant a lithium salt which decomposes when heated in air to form an oxide of lithium an, correspondingly, by a `manganese salt as defined herein` is meant a manganese salt which decomposes when heated in air to form an oxide of manganese. 
     The salt of lithium may be a member of the group consisting of Li 2  CO 3 , LiNO 3  and mixtures thereof, the salt of manganese being a member of the group consisting of Mn(NO 3 ) 2 , MnCO 3  and mixtures thereof. Forming the mixture may be in a stoichiometric ratio so that there is an at least approximate molar ratio of Li:Mn of 1:2, optionally with a slight excess of either salt, i.e. such that the ratio is 1:1.7-1:2.5, preferably 1:1.9-1:2.1. Forming the mixture may be by milling, e.g. in a ball mill containing alumina grinding media or in a mortar and pestle so that the mixture has an average particle size of at most 250 microns. Instead, forming the mixture of the lithium and manganese salts may be by making a slurry in a solvent selected from the group consisting of water, ethanol and mixtures thereof and thereafter drying the mixture until the solvent content is at most 10% by mass, e.g. by drying at 60°-90° C. in a drying oven for 12 hours. 
     The heating of the mixture may be in air to a temperature of 300°-420° C., e.g. 400° C.; the mixture being held at the maximum temperature, preferably with an accuracy of ±10° C., for a period of at least 2 hrs, e.g. 2-5 hours. Heating may typically be at a rate of 60° C./hr; and may be followed by cooling by quenching in air or slow cooling at the natural furnace cooling rate. The heating may be of the mixture in powder form. However, the method conveniently includes the step of, prior to the heating, compacting the mixture, by pressing it at a pressure of 5-10 MPa to form a unitary artifact, so that, after the heating, the lithium manganese oxide of the formula Li 1-x  Mn 2  O 4  is in the form of a self-supporting unitary artifact. 
     Spinel compounds have structures that can be represented by the general formula A(B 2 )X 4  in which X atoms are arranged in a cubic-close -packed fashion to form a ne.g.atively charged anion array comprised of face-sharing and edge-sharing X tetrahedra and octahedra. In the formula A(B 2 )X 4 , the A atoms are tetrahedral-site cations and the B atoms are octahedral-site cations, i.e. the A cations and B cations occupy tetrehedral and octahedral sites, respectively. In the ideal spinel structure, with the origin of the unit cell at the centre (3m), the close-packed anions are located at the 32e positions of the space group Fd3m. Each unit cell contains 64 tetrahedral inerstices situated at three crystallographically non-equivalent positions 8a, 8b and 48f, and 32 octahedral interstices situated at the crystallographically non-equivalent positions 16c and 16d. In an A(B 2 )X 4  spinel the A cations reside in the 8a tetrahedral interstices and the B cations in the 16d octahedral interstices. There are thus 56 empty tetrahedral and  16 empty octahedral sites per cubic unit cell. For the present invention A(B 2 )X 4  is represented by Li(Mn 2 )O 4 . 
     Therefore, the B cations of the (B 2 )X 4   n-  framework structure may be re.g.arded as being located at the 16d octahedral positions and the X anions located at the 32e positions of the spinel structure. The tetrahedra defined by the 8a, 8b and 48f positions and octahedra defined by the 16c positions of the spinel structure, thus, form the interstitial space of the (B 2 )X 4   n-   framework structure for the mobile Li cations, for diffusion therethrough during the electrochemical discharge and charge reactions. 
     Furthermore, the cathodes of the present invention need not necessarily be stoichiometric compounds. For example, cathodes may be synthesized in which defects are created by varying the quantity of Li ions at the A sites to generate a lithium-defici.e.nt spinel Li 1-x  (Mn 2 )O 4  with 0≦x&lt;1; alternatively cathodes may be synthesized in which defects are created by varying the quantity of Mn cations in the framework structure such that additional Li cations may enter the framework. In certain instances, these additional Li cations may partially occupy the 16d octahedral sites normally occupi.e.d by the Mn cations. Under such circumstances, these partially occupi.e.d octahedra may be considered to form part of the interstitital space. Conversely, cathodes may also be synthesized, in which part of the interstitial spaces defined by the 8a, 8b and 48f tetrahedral and 16c octahedral interstices of the spinel structure may be occupi.e.d by Mn cations, thereby rendering these particular sites at least partially inaccessible to the mobile Li cations. It follows that, in compounds of the formula Li 1-x  Mn 2  O 4  of the lithium manganese oxide synthesized by the method of the present invention, the Mn:O atomic ratio need not be precisely 1:2, but will be about 1:2, so that the formula Li 1-x  Mn 2  O 4  is defined as covering also compounds in which the Mn:O ratio is slightly greater than 1:2 and compounds in which said ratio is slightly less than 1:2, the formula Li 1-x  Mn 2  O 4  merely being used for convenience of expression. 
     The Li 1-x  Mn 2  O 4  spinel-type oxide product of the present invention can be described, broadly, as Li y  MnO 2  in which y is not greater than 0.5. When this Li y  MnO 2  is assembled into a cell of the type Li (anode)/electrolyte/Li y  MnO 2  (cathode), charging will involve a reduction of the value of y to a theoretical minimum value of 0 in the fully charged state. While the electrolyte may be a lithium-containing molten salt electrolyte, it is conveni.e.ntly a room-temperature electrolyte such as LiClO 4 , LiAsF 6  or LiBF 4 , dissolved in an organic solvent such as propylene carbonate or dimethoxyethane. It is also in principle possible to discharge such cells further than a partially discharged state in which y in LiyMnO 2  is 0.5, up to a practically useful value for y of 1. Although y values in excess of 1 are possible, the electrochemical reaction when y exceeds 1.0 will be associated with a sharp drop in voltage from an open circuit value of approximately 3 V to below 2 V, which limits usefulness. In practice, the value of y will be controlled between conveni.e.nt values, e.g. a value of y=0.2 in the nominally fully charged state and a value of y=1.0 in the nominally fully discharged state. 
     The invention also extends to lithium manganese oxide in ( accordance with the formula Li 1-x  Mn 2  O 4  whenever synthesised by the method described above, particularly for use as a cathode in an electrochemical cell. 
     The invention also extends to an electrochemical cell having a cathode comprising a lithium manganese oxide of formula Li 1-x  Mn 2  O 4  as described above, an anode which comprises lithium metal or a lithium-containing alloy, and an electrolyte whereby the anode is electrochemically coupled to the anode. 
     The cell may, thus, be of the type described above. 
     The cells of the invention may be primary cells or rechargeable secondary cells. Secondary cells can in principle be loaded with the Li 1-x  Mn 2  O 4 , i.e. Li y  MnO 2  in which y is not more than 0.5, but may be loaded, if more conveni.e.nt, with y at some other value. In practice the Li 1-x  Mn 2  O 4  of the cathode will usually be compacted, as mentioned above, optionally with a suitable binder, and for cathode use a suitable electronically conductive material to act as current collector may be incorporated in the compacted cathode. Thus, polytetrafluoroethylene (PTFE) may be used as a binder and acetylene black (carbon) may be used as current collector. 
     Accordingly, in a particular embodiment of the cell, the lithium manganese oxide of the cathode may be present in the cathode in admixture with a binder and with an electronically conductive current collector in a compacted artifact, the electrolyte being a room-temperature electrolyte comprising a member of the group consisting of LiClO 4 , LiAsF 6  and LiBF 4  dissolved in an organic solvent selected from the group consisting of propylene carbonate and dimethoxyethane. 
     Without being bound by theory, the Applicant believes that an advantage of the present invention, whereby particular utility of the Li 1-x  Mn 2  O 4  as a cathode of a cell of the type in question is obtainable, arises from the relatively poorly developed crystallinity and high surface area thereof as discussed in more detail hereunder. This poorly developed crystallinity is to be contrasted with the well developed crystallinity obtained when a lithium salt such as Li 2  CO 3  is heated in a similar fashion in air with, for example, MnO 2 , Mn 2  O 3  or Mn 3  O 4 , to a temperature in the range of 700° C. to 900° C. The advantage of using e.g. Mn(NO 3 ) 2  or MnCO 3  precursors in the reaction as opposed to manganese oxides, such as those mentioned above, is that the nitrate or carbonate compounds decompose rapidly within a period of 2-5 hours at relatively lower temperatures, particularly in the presence of the lithium salt, to produce a virtually amorphous intermediate, which can be expressed as Li 2  0.4 MnO from which the Li 1-x  Mn 2  O 4  product, and in particular, its cubic close packed oxygen lattice, must be reconstructed. This accounts for the poorly developed crystallinity and strain in the individual particles. When starting from manganese oxide precursors e.g. as mentioned above, it is believed that the retention of much of the oxygen content makes it more difficult to obtain a single-phase Li 1-x  Mn 2  O 4  product at these relatively low temperatures, with the required electrochemical properties. 
     The oxidizing atmosphere under which the heating takes place may be an oxygen-containing atmosphere, conveniently air. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     By way of comparison, reference is made to the accompanying drawings. In the drawings FIGS. 1-10 comprise X-ray diffraction patterns, i.e. plots of intensity against 2-theta for the 2-theta range of 10°-70° for the Li 1-x  Mn 2  O 4  produced in accordance With the invention, and for samples of Li 1-x  Mn 2  O 4  produced in accordance with controls using CuK-alpha radiation. These plots are as follows: 
     FIG. 1, a control, shows LiMn 2  O 4  produced from 4Mn 3  O 4  +3LiCO 3  heated to 800° C. in air; 
     FIG. 2, a control, shows LiMn 2  O 4  produced from 2Mn 2  O 3  +Li 2  CO 3  heated to 800° C. in air; 
     FIG. 3, a control, shows LiMn 2  O 4  produced from 4MnCO 3  +Li 2  CO 3  heated to 800° C. in air; 
     FIG. 4, a control, shows LiMn 2  O 4  produced from 4MnCO 3  +Li 2  CO 3  heated to 800° C. in air; 
     FIG. 5, a control, shows LiMn 2  O 4  produced from 2Mn(NO 3 ) 2 .4H 2  O+LiNO 3 .3H 2  O heated to 800° C. in air; 
     FIG. 6, a control, shows a product of the reaction 4Mn 3  O 4  +3Li 2  CO 3  heated to 400° C. in air for 14 hours; 
     FIG. 7, a control, shows a product of reaction 2Mn 2  O 3  +Li 2  CO 3  heated to 400° C. in air for 14 hours; 
     FIG. 8, a control, shows a product of reaction 4MnCO 3  +Li 2  CO 3  heated to 400° C. in air for 14 hours; 
     FIG. 9, the invention, shows Li 1-x  Mn 2  O 4  produced from 4MnCO 3  +Li 2  CO 3  heated to 400° C. in air for 14 hours; 
     FIG. 10, the invention, shows Li 1-x  Mn 2  O 4  produced from 4Mn(N02) 3 .4H 2  O+LiNO 3 .3H 2  O heated in air to 400° C. for 5 hours; 
     FIG. 11, a control, shows the X-ray diffraction patterns for the 2-theta range 10°-80° of samples taken at various stages of the reaction sequence: Mn 2  O 3  reacted with Li 2  CO 3  at 400° C. for 5 hours, at 550° C. for 26 hours, at 700° C. for 19 hours, at 850° C. for 24 hours and finally at 000° C. 5 hours; 
     FIG. 12, the invention, shows the X-ray diffraction patterns of samples taken at various stages of the reaction sequence: MnCO 3  reacted with Li 2  CO 3  at 400° C. for 5 hours, at 550° C. for 26 hours, at 700° C. for 19 hours, at 850° C. for 24 hours and finally at 1000° C. for 5 hours. 
     Further in the drawings: 
     FIG. 13 shows a schematic representation of an electrochemical cell according to the invention; 
     FIG. 14 shows discharge curves of an electrochemical cell having a cathode according to the invention and a control, being a plot of cell voltage (V) against the amount of lithium in the cathode; and 
     FIG. 15 shows a charge/discharge cycle of a cell according to the invention, being a plot of voltage (V) against time minutes. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With regard to above-mentioned drawings, it should be noted that FIGS. 1-5 display diffraction patterns characteristic of very highly crystalline LiMn 2  O 4  products, when prepared at 800° C. When the temperature is dropped to 400° C. as in FIGS. 6 and 7, sharp peaks characteristic of highly crystalline LiMn 2  O 4  and unreacted manganese oxide starting materials are observed In FIG. 8 at 400° C. peaks characteristic of a two-phase product consisting of Li 1-x  Mn 2  O 4  and Mn 2  O 3  (a decomposition product of the MnCO 3  starting material) are observed. 
     With regard to FIGS. 9 and 10, particularly FIG. 9, in  contrast to FIGS. 6-8, the X-ray diffraction patterns exhibit peaks characteristic of essentially single-phase Li 1-x  Mn 2  O 4  product of low crystallinity (broad peaks), which patterns and peaks are believed to be attributable to strain in the crystals, introduced during formation thereof. 
     FIG. 12 when compared with FIG. 11 clearly indicates the single phase spinel pattern of Li 1-x  Mn 2  O 4  from 400° C. to 1000° C. The broad peaks at 400° C. and 550° C. can be attributed to the strain in the Li 1-x  Mn 2  O 4  particles that is established during the reconstruction of the cubic close packed oxygen sub-array. The X-ray patterns in FIG. 11 corresponding to the preparations at 400° C. and 550° C. contrast strongly with those in FIG. 12, e.g. impurity peaks such as that at 33° 2-theta , (which are deleterious for cell performance) of unreacted Mn 2  O 3  are clearly evident; and the peaks in FIG. 11 are significantly sharper, e.g. at 400° and 550° C. 
     Heating e.g. Li 2  CO 3  or LiNO 3  with MnCO 3  to completion in accordance with the reactions: ##STR1## would provide LiMn 2  O 4  with a poorly developed crystallinity and high surface area. However, in terms of the present invention, the heating and reactions are not to completion so that, instead of LiMn 2  O 4 , the phase Li 1-x  Mn 2  O 4  is obtained. This arises from the fact that the reaction of the starting salts leads to an intermediate, `Li 2  0.4MnO` which is substantially amorphous and has a very poorly developed initial de.g.ree of (20 crystallinity. This in turn leads to the production of Li 1-x  Mn 2  O 4  With a correspondingly poorly developed crystallinity as characterized by the substantially broader peaks in the powder X-ray diffraction pattern obtained therefor. This product has been found to exhibit a high degree of electrochemical activity in primary electrochemical cells of the type described above, and has been found to operate reversibly, which makes it attractive as a cathode material for a secondary cell of the type described above Furthermore, the reaction whereby the Li 1-x  Mn 2  O 4  is synthesized can, within limits, be tailored in accordance with the present invention, e.g. by controlling the reaction temperature and reaction time, to produce materials with an optimized electrochemical activity for their precise intended purpose in electrochemical cells. 
     The invention will now be described with reference to the following illustrative example of a cell proposed in accordance with the invention, and with reference to the accompanying diagrammatic drawing, FIG. 13, which shows a schematic representation of a cell according to the invention. In the drawing, is shown a cell of the type Li (anode)/1 Molar LiClO 4  in propylene carbonate (electrolyte) /Li y  MnO 2  (in which the Li y  MnO 2  is provided by the Li 1-x  Mn 2  O 4  of the present invention) pTFE, acetylene black (cathode). The cell is designated 10 and the anode, electrolyte and cathode are respectively designated 12, 14 and 16 and are contained in an insulating housing 18 with the anode separated from the cathode by the electrolyte and suitable terminals in electronic contact respectively with the anode and the cathode. In the cathode, the Li 1-x  Mn 2  O 4  in powder form is mixed with PTFE and acetylene black. 
     In the cathode, the PTFE is a binder and the acetylene black is a current collector. The Li 1-x  Mn 2  O 4  in powder form is mixed in a mass proportion of 70-80% Li 1-x  Mn 2  O 4  with 30-20% PTFE and acetylene black, with the PTFE and acetylene black in a mass ratio of 1:2, and compacted at 5-10 MPa. 
     Stoichiometric LiMn 2  O 4  has a cubic unit cell with a lattice parameter `a` of 8.24 Angstrom units. Lithium can be extracted from this unit cell without disturbing the Mn 2  O 4  spinel framework either chemically, for example using Molar HCl, or electrochemically. Using chemical methods, it is possible to synthesize the cubic Mn 2  O 4  phase commonly known as lambda MnO 2 , which has a lattice parameter `a` of 8.03 Angstrom units. Cubic lithium-deficient phases of Li 1-x  Mn 2  O 4  with O≦x&lt;1 can be prepared With intermediate `a` values. When preparing Li 1-x  Mn 2  O 4  cathodes at 400° C. according to the invention using lithium salts and manganese salts such that the Li:Mn molar ratio is 1:2, it is often observed that a spinel material is formed which is lithium-deficient. In these cases, it may be desirable to wash off the unreacted lithium salt component, e.g. using water, in case it is undesirable in the cell. 
     The following is a Table of surface areas determined by BET methods using nitrogen gas, in m 2  /g of Li 1-x  Mn 2  O 4  compared with gamma-MnCO 3  samples, of the type conventionally used in primary Li/MnCO 3  electrochemical cells. The improved electrochemical performance obtained from a Li 1-x  Mn 2  O 4  cathode prepared at 400° C. (see Sample 3 in the Table) in accordance with the invention, compared with an LiMn 2  O 4  control cathode prepared at e.g. 850° C. (Sample 6 in the Table) can be attributed to the remarkable increase in surface area of the Li 1-x  Mn 2  O 4  particles Compared with the surface area of LiMn 2  O 4  particles made at 850° C., despite a relatively small difference in the average particle size, i.e. about 3 microns for Sample 3 and about 6 microns for Sample 6. For example, the surface area of a Li 1-x  Mn 2  O 4  cathode prepared at  400° C. from MnCO 3  and Li 2  CO 3  is 78.3 m 2  /g compared to an Li/Mn 2  O 4  cathode made from MnCO 3  and Li 2  CO 3  at 850° C. which has a surface area of only 2m 2  /g. For comparison, a typical gamma-MnCO 3  cathode used in primary Li/MnCO 3  cells, heated to 350° C. has a surface area of 29.6 m 2  /g. 
     
                       TABLE______________________________________Sample No                         m.sup.2 /g______________________________________Control 1   Electrolytic manganese dioxide                             50.1   (gamma-MnO.sub.2)Control 2   Sample No 1 heated to 350° C. for 24 hours                             29.6Invention 3   Li.sub.1-x Mn.sub.2 O.sub.4 made at 400° C.                             78.3   MnCO.sub.3 + Li.sub.2 CO.sub.3Invention 4   Li.sub.1-x MnO.sub.4 made at 550° C. from                             31.9   MnCO.sub.3 + Li.sub.2 CO.sub.3Control 5   Li.sub.1-x Mn.sub.2 O.sub.4 made at 700° C.                             8.1m   MnCO.sub.3 + Li.sub.2 CO.sub.3Control 6   Li.sub.1-x Mn.sub.2 O.sub.4 made at 850° C.                             2.0m   MnCO.sub.3 + Li.sub.2 CO.sub.3Control 7   Li.sub.1-x Mn.sub.2 O.sub.4 made at 1000° C.                             0.7m   MnCO.sub.3 + Li.sub.2 CO.sub.3Control 8   LiMn.sub.2 O.sub.4 made at 700° C. from                             2.2   Mn.sub.2 O.sub.3 + Li.sub.2 CO.sub.3______________________________________ 
    
     Typical X-ray diffraction patterns of Sample No. 3, the invention, and Sample No. 6, a control, are shown in FIGS. 9 and 4, respectively. 
     Representative initial discharge curves of Li y  MnO 2  cells having 
     (1) an Li 1-x  Mn 2  O 4  cathode (i.e. an Li y  MnO 2  cathode in which y is about 0.5) made at 400° C.; and 
     (2) an LiMn 2  O 4  cathode made at 850° C. are shown in FIG. 14. These curves clearly illustrate, in terms 10 of increased capacity, the advantage of cathodes in accordance with the invention. It should be noted that, in the Li of the cathode made at 400° C., 0.5 Li ions are inserted into each MnCO 3  framework unit during discharge whereas, in the Li y  MnO 2  cathode prepared at 850° C., only 0.2 Li ions are inserted (to a cut-off voltage of 2.0 V). The Li y  MnO 2  of the cathode of the present invention has a theoretical fully charged state in which y is 0; and a fully discharged state in which y is 1. In FIG. 14, Plot 1 is according to the invention and Plot 2 is the control. 
     FIG. 15 shows the 4th charge/discharge cycle of a Li/Li 1-x  Mn 2  O 4  cell according to the invention. The cut-off voltages for cycling were 3.6 V (charge) and 2.0 V (discharge). The cell comprised 34.6 mg of Li 1-x  Mn 2  O 4  cathode admixed with 8.6g of PTFE and 17.2 g of acetylene black The discharge current was 300 μA/cm 2  and the charge current was 150 μA/cm 2 . During this cycle the discharge time was 1080 minutes and the charge time 2040 minutes. This corresponds to an insertion of 1.05 Li ions into each Mn 2  O 4  unit during discharge and extraction of 1.00 Li ions during charge, which highlights the excellent reversibility of the cell. 
     An advantage of the invention is that it provides a cell, potentially suitable for primary or rechargeable use, of simple design, low cost and good shelf life, operable over a wide range of values of y in Li y  MnO 2 , which wide range of values and acceptable charging/discharging rates are promoted by the relatively poor crystallinity and high surface area of the Li 1-x  Mn 2  O 4  cathode. 
     The Applicant believes that the exceptional utility promised by the present invention for use as a reversible or secondary cathode in electrochemical cells of the type in question, arises from the reversible behavior of the cathode with regard to insertion and extraction of Li +   ions respectively during charging and discharging. This reversibility is believed to arise from the exceptionally high surface areas associated with the relatively poor crystallinity displayed by the Li 1-x  Mn 2  O 4  of the present invention. This is associated with enhanced internal porosity of particles of the Li 1-x  Mn 2  O 4 , promoting said reversibility. A yet further advantage of the present invention is the advantageously high capacity of the Li 1-x  Mn 2  O 4  to receive Li +   ions inserted therein during charging, leading to the attainment of enhanced capacities.