LiMn 2-x Cu x O 4 Spineis - 5 V Cathode Materials
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4
was determined from X-ray
diffraction and XANES data to be Li 1.01Mn 1 .67 Cu 0 .32 0 4 suggesting, to a best approximation, that the impurity in the sample was a lithium-copper-oxide phase. Introduction Materials that reversibly intercalate lithium form the cornerstones of the emerging lithium-ion battery industry. The spinel LiMn 2 0 4 is an inexpensive, environmentally benign intercalation cathode that is the subject of intense development (1), although it is not without faults. The achievable electrode capacity (120 mAh/g) is 15-30% lower than that which can be obtained from Li(Co,Ni)0 2 cathodes. Moreover, an unmodified LiMn 2 0 4 electrode exhibits an unacceptably high capacity fade. Several researchers have stabilized the LiMn 20 4 electrode structure to lithium insertion/extraction reactions at -4 V by substituting a small fraction (-2.5%) of the manganese ions with other metal cations (2-4). Recently, we reported a preliminary account of the preparation and electrochemical behavior of a copper substituted spinel, LiMni. 5Cu 0 .504 (5). In this paper, we present electrochemical, structural and spectroscopic data obtained from an examination of various compounds in the LiMn 2 .xCUO 4 system (0 < x < 0.5) which provide a much greater understanding of the behavior of these materials than initially reported (5). Structural properties and variations in the cation charge distribution are used to explain the electrochemical behavior of LiMn 25. Cu5 O 4 electrodes. X-ray absorption studies have been used to determine the oxidation 315 Mat. Res. Soc. Symp. Proc. Vol. 496 0 1998 Materials Research Society
states of the manganese and copper ions. The structure of the spinel component, as determined by a Rietveld refinement of the powder X-ray diffraction pattern, is presented. Experimental LiMn 2 .xCuxO 4 (0< x < 0.5) cathode materials were prepared by conventional solid state and sol-gel methods. In the solid state syntheses, LiOH.H 20 was intimately mixed with the required amounts of CuO and MnO 2 for a given stoichiometry, and then heated for 18 hours in air at 750' C. The product was free-flowing and did not require milling. Nearly phase-pure LiMni. 5Cu 0 .504 was prepared by a sol-gel process by dissolving stoichiometric amounts of CH 3COOLi, Cu(OOCCH 3)2"H2 0, and Mn(OOCCH 3)2 in deionized water, and adding a 4 -times molar amount of NH 4OH. The mixture was stirred with gentle heating for 2 hours, then concentrated to dryness on a rotary evaporator. Cyclic voltammograms were obtained with an EG&G/PAR potentiostat, model 263A; they were recorded at a slow sweep rate of 15 ptV/s. Cycling data were collected on either a Maccor series 4000 or Starbuck multi-channel cyclers. Cathode materials were studied using a lithium foil anode, separated with Whatman BS-65 glass microfibers in a 1 cm2 parallel-plate configuration. Cathode films were prepared from a slurry of LiCuxMn 2.O04 with 10% PVDF and 10% acetylene black (w/w) dissolved in N-methyl-2-pyrrolidinone. The mixture was doctorbladed onto aluminum foil, dried
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