No abstract

No abstract

Thin-film rechargeable lithium batteries with amorphous and crystafline LiCoO2 cathodes were investigated, The lithium cobalt oxide films were deposited by radio-frequency (RF) magnetron sputtering of an LiCoO2 target in a 3:1 Ar/02 mixture gas. From proton-induced -y-ray emission analysis (PIGE) and Rutherford backscatterung spectrometry (RBS), the average composition of these films was determined to be Li115CoO216 or, within experimental uncertainty, LiCoO2 + 0.08 Li20. The x-ray powder diffraction patterns of films annealed in air at 500 to 7 00°C were consistent with the regular hexagonal structure observed for crystalline LiCoO2. The discharge curves of the cells with amorphous LiCoO2 cathodes showed no obvious structural transition between 4.2 and 2.0 V, while the discharge curves of the cells with polycrystalline cathodes were consistent with a two-phase potential plateau at -3.9 V with a relatively large capacity. Two lower capacity plateaus were observed at ---4.2 and 4.1 V with the 600 and 700°C annealed cathodes; the -dq/dV peaks were broader and weaker for the 600°C annealed cathodes and were not present at all with the 500°C annealed films. The chemical diffusion coefficients of Li in the cathodes obtained from ac impedance measurements at cell potentials of -4 V ranged from ,1012 cm2/s for the as-deposited amorphous cathodes to --10 cm2/s for the films annealed at 700°C. The capacity loss on extended cycling of the thin-film cells varied with the crystallinity and thickness of the cathodes and with temperature. With the highly crystalline, 700°C annealed material, losses on cycling between 4.2 and 3.8 V at 25°C ranged from 0.0001%/cycle (>1O cycles) to 0.002%/cycle for cells with cathodes from 0.05 to 0.5 m thick.

No abstract

The large number of recent papers on LiMn 2 O 4 spinel and related compounds attest to the intense interest in the attractive properties of these intercalation materials for possible use as reversible cathodes (positive electrodes) in lithium and lithium-ion batteries. Materials referred to as "defect spinels" include compositions extending from LiMn 2 O 4 to Li 2 Mn 4 O 9 and Li 4 Mn 5 O 12 . [1][2][3][4][5][6][7][8][9][10][11] The chemistry of these compounds is both complex and sensitive to the particular processing conditions, but recent work helps to clarify the stable phase equilibria. [8][9][10] In our laboratory, crystalline thin-film lithium manganese oxide cathodes have been fabricated by electron-beam evaporation and sputtering followed by a high-temperature anneal. 12-15 These films, when incorporated into a thin-film solid-state lithium cell, have ϳ4 V capacities of 50-120 mAh/g and have been cycled many hundreds of times with little degradation. Others have also prepared crystalline thin films of ϳLiMn 2 O 4 by physical vapor deposition and anneal techniques. [16][17][18][19][20] They report comparable capacities and in one case good cycling, 17 but all of these cells used a liquid electrolyte making them subject to undesired cathode-electrolyte reactions, particularly at high voltages and temperatures.The goal of the work reported here was to deposit LiMn 2 O 4 thinfilm cathodes at modest temperatures <150ЊC in order to permit thinfilm battery construction onto lower-temperature substrate materials, battery stacks, and host devices. Magnetron sputter deposition was chosen for fabrication of the thin-film cathodes as this energetic deposition technique often enhances density, crystallinity, and adhesion of films at low temperatures. It became clear, however, that films deposited at low temperatures were poorly crystalline. 21 Nevertheless, upon optimization of the deposition parameters, we find that even the nanocystalline (n) Li x Mn 2Ϫy O 4 cathodes are useful for low-power, thin-film battery applications, due to their high energy density and exceptional rechargeability.This report describes the cycling properties of this cathode when incorporated into our all-solid-state, thin-film battery. 22 We show cycling results extending to thousands of cycles at 25 and 100ЊC and to cell potentials from 5.3 to 1.5 V. The specific capacity and cell polarization are discussed with respect to the film composition and proposed free energy of mixing diagrams. ExperimentalThe thin-film cathodes were deposited by radio frequency (rf) magnetron sputtering from 2 in. diam LiMn 2 O 4 ceramic disks. Several targets were purchased (Cerac); others were fabricated in-house by cold pressing and sintering (1200ЊC, air). Powders were synthesized at 600ЊC from Li 2 CO 3 and MnO 2 , as well as purchased. All targets were 70-80% of theoretical density, and their X-ray diffraction (XRD) pattern matched the file for LiMn 2 O 4 (no. 35-782). Targets were either 0.125 or 0.25 in. thick, several being bonded to 0.125 in. copper bac...