This invention relates to secondary, rechargeable lithium and lithium-ion batteries and, more particularly, relates to a continuous method for preparing Li.sub.1+x Mn.sub.2-x O.sub.4 intercalation compounds for use as the positive electrode in such batteries where x is from about 0.022 to about 0.20.
Lithium-cobalt oxide is currently used as the positive electrode material in commercial four-volt lithium-ion cells. On the basis of their lower cost, raw material abundance, additional safety, environmental acceptability, and electrochemical performance, Li.sub.1+x Mn.sub.2-x O.sub.4 intercalation compounds have shown exceptional promise as positive electrode materials in such cells. However, for the commercial success of Li.sub.1+x Mn.sub.2-x O.sub.4 as a cathode material a process has not previously been found that will rapidly and economically produce a material with the required electrochemical performance properties. This invention addresses this issue.
LiMn.sub.2 O.sub.4 (Li.sub.1+x Mn.sub.2-x O.sub.4 where x=0) was synthesized as early as 1958 D. G. Wickham and W. J. Croft, J. Phys. Chem. Solids 7 (1958) 351-360!, by intimately mixing Li.sub.2 CO.sub.3 and any manganese oxide, taken in the molar ratio of Li/Mn=0.50, reacting the mixture at 800.degree.-900.degree. C. in air, and repeatedly grinding and reacting the mixture at this temperature until the sample reached constant weight. Acid leaching of LiMn.sub.2 O.sub.4 to produce .lambda.-MnO.sub.2, which possesses the LiMn.sub.2 O.sub.4 crystal framework, and the subsequent usage of .lambda.-MnO.sub.2 as the positive electrode material in a lithium cell were reported by Hunter J. C. Hunter (Union Carbide), U.S Pat. No. 4,246,253, Jan. 20, 1981; J. C. Hunter (Union Carbide), U.S. Pat. No. 4,312,930, Jan. 26, 1982; J. C. Hunter, J. Solid State Chem. 39 (1981) 142-147.!. Hunter electrochemically reduced his .lambda.-MnO.sub.2 to LiMn.sub.2 O.sub.4, which occurred at 4V, but they did not cycle his cell. He also noted that lithium and manganese compounds other than those specified by Wickham and Croft may be used in the synthesis, provided that they decompose to lithium or manganese oxides under the reaction conditions used. Thackeray, et al. M. Thackeray, P. Johnson, L. de Picciotto, P. Bruce and J. Goodenough, Mat. Res. Bull. 19 (1984) 179-187; M. Thackeray, L. de Picciotto, A. de Kock, P. Johnson, V. Nicholas and K. Adendorff, J. Power Sources 21 (1987)1-8! showed that Li intercalation into the LiMn.sub.2 O.sub.4 spinel structure is electrochemically reversible, giving two voltage plateaus at 4.1 V and 3.0 V vs Li, which correspond to the intercalation/de-intercalation of the first and second Li ions, respectively, into .lambda.-MnO.sub.2.
Various investigators studied the synthesis of LiMn.sub.2 O.sub.4 by thermal reaction of a lithium and manganese compound, and found it could be effected over a large temperature range--i.e., 300.degree.-900.degree. C. The ability of the products to intercalate and de-intercalate Li was also investigated. The so-called "low" temperature materials, made at less than about 550.degree. C., are poorly crystalline, have a distorted spinel structure, and cycle at about 3V but not at 4V vs Li W. J. Macklin, R. J. Neat and R. J. Powell, J. Power Sources 34 (1991) 39-49; T. Nagaura, M. Yokokawa and T. Hashimoto (Sony Corp.), U.S. Pat. No. 4,828,834, May 9, 1989; M. M. Thackery and A. de Kock (CSIR), U.S. Pat. No. 4,980,251, Dec. 25, 1990; V. Manev, A. Momchilov, A. Nassalevska and A. Kozawa, J. Power Sources, 43-44 (1993) 551-559!. These are not the materials of focus in this patent application.
The so-called "high" temperature materials, made at about 600.degree.-900.degree. C. in an air atmosphere, are quite crystalline. They show cycling capability at about 4V vs Li, but cycle much worse at 3V vs Li, losing capacity rapidly J. M. Tarascon, E. Wang, J. K. Shokoohi, W. R McKinnon and S. Colson, J. Electrochem. Soc. 138 (1991) 2859-2868!. Even when LiMn.sub.2 O.sub.4 is synthesized at low temperature, as in a sol-gel process, it can be cycled in the 4V regime if it is first fired/annealed at high temperatures--e.g., 600.degree.-800.degree. C. P. Barboux, F. K. Shokoohi and J. M. Tarascon (Bellcore), U.S. Pat. No. 5,135,732, Aug. 4, 1992!. High temperature LiMn.sub.2 O.sub.4 materials will be the focus the remainder of this application.
Investigators have generally found that synthesis of a single-phase product in their (static) muffle furnaces required many hours or even days of reaction time, which they often coupled with regrinding of the heated product and reheating of the reground powder P. Barboux, F. K. Shokoohi and J. M. Tarascon (Bellcore), U.S. Pat. No. 5,135,732, Aug. 4, 1992; W. J. Macklin, R. J. Neat and R. J. Powell, J. Power Sources 34 (1991) 39-49; A. Mosbah, A. Verbaire and M. Tournoux, Mat. Res. Bull. 18 (1983) 1375-1381; T. Ohzuku, M. Kitagawa, and T. Hirai, J. Electrochem. Soc. 137 (1990) 769-775!. Without such laborious synthesis procedures, various byproducts are produced in addition to LiMn.sub.2 O.sub.4 --i.e., Mn.sub.2 O.sub.3, Mn.sub.3 O.sub.4 and Li.sub.2 MnO.sub.3. These substances are undesirable in lithium cells, creating low capacities and high fade rates.
Apart from the production of undesirable byproducts, the synthesis parameters also affect the molecular/crystal structure and physical properties of the LiMn.sub.2 O.sub.4, and these material properties greatly affect the battery capacity and cyclability of the material. Momchilov, Manev and coworkers A Momchilov, V. Manev, and A Nassalevska, J. Power Sources 41 (1993) 305-314! varied the lithium reactant, the MnO.sub.2 reactant, the reaction temperature and reaction time prior to cooling in air. They found it advantageous to make the spinels from lithium salts with the lowest possible melting points and from MnO.sub.2 samples with the greatest surface areas. The advantages were faster reaction times and more porous products, which gave greater capacities and better cyclability (i.e., less capacity fade with cycle number). However, the reaction times were the order of days in any case. These investigators also found V. Manev, A. Momchilov, A. Nassalevska and A. Kozawa, J. Power Sources, 43-44 (1993) 551-559; A. Momchilov, V. Manev, and A. Nassalevska, J. Power Sources 41 (1993) 305-314.! that the optimum reaction temperature was approximately 750.degree. C. At higher temperatures the material lost capacity, presumably due to a decreased surface area and from oxygen loss, which reduced some of the manganese in LiMn.sub.2 O.sub.4. At the lower reaction temperatures, synthesis required even longer times, and evidence of spinel distortion occurred, which apparently caused lower capacities. These investigators also demonstrated advantage in preheating the reaction mix at temperatures just above the melting point of the lithium reactant before reacting at the final temperature.
Tarascon and coworkers J. M. Tarascon, W. R. McKinnon, F. Coowar, T. N. Bowmer, G. Amatucci and D. Guyomard, J. Electrochem. Soc. 141 (1994) 1421-1431; J. M. Tarascon (Bellcore), International Patent Application WO 94/26666; U.S. Pat. No. 5,425,932, Jun. 20, 1995! found that high capacity and long cycle life were best achieved by (1) employing a reactant mixture in which the mole ratio of Li/Mn is greater than 1/2 (i.e., Li/Mn=1.00/2.00 to 1.20/2.00 so that x in Li.sub.1+x Mn.sub.2-x O.sub.4 =0.0 to 0.125), (2) heating the reactants for an extensive period of time (e.g., 72 h) at 800-900.degree. C., (3) cooling the reacted product in an oxygen-containing atmosphere at a very slow rate, i.e., preferably at 2.degree. to 10.degree. C./h, to about 500.degree. C., and, finally, (4) cooling the product more rapidly to ambient temperature by turning off the furnace. The cooling rate from more than 800.degree. C. to 500.degree. C. can be increased to 30.degree. C./h if the atmosphere is enriched in oxygen. These investigators found that the lattice parameter, a.sub.o, of the product was an indicator of the product efficacy in a battery, and that a should be less than about 8.23 .ANG.. By comparison, for LiMn.sub.2 O.sub.4 made with Li/Mn=1.00/2.00 and with air cooling, a=8.247 .ANG..
Manev and coworkers V. Manev, A. Momchilov, A. Nassalevska and A. Sato, J. Power Sources 54 (1995) 323-328! also found that a Li/Mn mole ratio greater than 1.00/2.00 is advantageous to both capacity and cyclability. They chose 1.05/2.00 as the optimum ratio. These investigators also found that as the amount of pre-mix/reactants in the muffle furnace was scaled up from .about.10 g to .about.100 g, the capacity decreased significantly. This they traced to a depletion of air in the furnace and a resultant partial reduction of the product. The problem was alleviated by flowing air through the furnace. When the air flow was too great, the capacity of the product decreased again, so the air flow had to be optimized to be beneficial. Manev and coworkers found the most beneficial cooling rate to be several tens of degrees per minute, which is more than 100 times faster than that of Tarascon and coworkers. After optimizing all conditions, which included the use of lithium nitrate and a very porous chemical manganese dioxide as reactants, Manev and coworkers obtained a product Li.sub.1+x Mn.sub.2-x O.sub.4 (with x=0.033) that gave a very high capacity and low fade rate. The use of lithium nitrate has negative impact on the process since poisonous NO.sub.x fumes are expelled during the synthesis. When Manev developed a successful synthesis process that utilized lithium carbonate rather than lithium nitrate V. Manev, Paper given at 9th IBA Battery Materials Symposium, Cape Town, South Africa, Mar. 20-22, 1995. (Abstract available)!, this new process once again involved a reaction time of several days.
Howard W. F. Howard, Jr., in Proceedings of the 11th Int'l Seminar on Primary and Secondary Battery Technology & Application, Feb. 28-Mar. 3, 1994, Deerfield Beach, Fla., sponsored by S. P. Wolsky & N. Marincic! discussed possible LiMn.sub.2 O.sub.4 production equipment, mainly from a cost viewpoint. Although he developed/presented no data, Howard suggested that a rotary kiln transfers heat faster than a static oven, which serves to shorten reaction times The desirable slow cooling rate coupled with long thermal reaction times is very difficult to accomplish on a large scale, as in pilot-plant or commercial operation. Therefore, it would be highly desirable to shorten the reaction and cooling times while avoiding the unwanted byproducts and preserving the needed Li.sub.1+x Mn.sub.2-x O.sub.4 stoichiometry and structure, the latter being evidenced by a smaller lattice parameter.