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energies comparable to that of cLiCoO2 at low current dis- charge, but so far, have not matched the very high power per- formance achieved with the LiCoO2 cathodes. An example of the discharge curves for a LiMn O –Li battery is given in 24 Fig. 4. Unfortunately the LiMn2O4 batteries have not proven to be as reproducible as the LiCoO2 batteries in terms of the cell resistivity and cycle stability. This is believed to be largely due to difficulties in optimizing and controlling the composition of the sputter deposited cathodes. Research is continuing to address these problems. The third kind of battery included in Fig. 3 has an unan- nealed, nano-crystalline cathode film, nLixMn2−yO4. The performance of these batteries has been reported in an ear- lier work [11]. The compositions of the as-deposited film are significantly Mn deficient, leading to a lithium substituted spinel structure when crystallized. Although the 4.5–2.5 V capacity of this cathode is comparable to the well-crystallized cathode, the Li+ diffusivity is greatly reduced in the disor- dered material, thereby limiting the power that can be real- ized with this thin-film cathode. The bright side, however, is that this cathode can be used to fabricate thin-film batteries on low temperature substrates or temperature sensitive de- vices. Although the nLixMn2−yO4 batteries cannot deliver a high power discharge, they have proven to be robust with N.J. Dudney / Materials Science and Engineering B 116 (2005) 245–249 247 Fig. 4. Constant current discharge curves for a battery with a 2 m-thick LiMn2O4 cathode. The cathode was crystallized at 550◦C. The currents ranged from 10 A to 2 mA/cm2 . This corresponds to a 0.2 mA/cm2, or 0.6 C, continuous dis- charge rate. When discharged at higher rates, the available en- ergy decreases due to the polarization of the cathode and the internal resistance of the electrolyte and its interfaces. In re- cent studies, LiCoO2 batteries have been cycled to 4.4 V with good results [8,9]. This high-voltage stability is attributed to the ability of submicron grains to accommodate the vol- ume changes associated with further Li extraction. Cycling to 4.4 V gives a 26% increase in the specific energy over that for a 4.2 V limit shown in Fig. 3. In addition, a study [10] of batteries with cLiCoO2 cathodes of 5 nm to 4 m thick indicates that the energy densities are ultimately limited by the rapidly decreasing Li ion diffusivity as the lithium con- tent approaches compositions of Li>0.96CoO2 at cell voltages <3.8 V versus Li. Batteries with a thin crystalline LiMn2O4 spinel cathode, cLiMn O in Figs. 2 and 3, cycled between 4.5 and 3.0 V give 24 Fig. 5. Charge of a Li–LiCoO2 thin-film battery to 4.2 V. The absolute and relative charge capacities are indicated by the solid and dashed curves, re- spectively. long cycle lives, even when cycled at temperatures up to 100◦C. For many applications, achieving a high charge rate for the battery is as important as the discharge power. Results in Fig. 5 show rapid charge of a Li–LiCoO2 thin-film bat- tery with a thick cathode. The maximum current was limited to either 1 or 10 mA/cm2 and the battery potential was not allowed to exceed 4.2 V. The results show that a full charge was approached in just 20 min. To date, no deleterious effects have been attributed to such high maximum charge currents. Several different experiments have been used to quantify the self-discharge of our thin-film batteries. The open cir- cuit cell voltage has been monitored over prolonged periods for Li batteries with LiCoO2 and LiMn2O4 cathodes ranging from 500 A ̊ thick to more useful cathodes of 1–2 m thick- ness. These batteries were stored at various states of charge under an inert atmosphere near 25 ◦C. Comparison of the bat- tery potentials with low current discharge curves suggests a self-discharge rate of 1–3 Ah/cm2/year. For a battery with a 2 m-thick cathode, this corresponds to <2% self-discharge per year. If the self-discharge is attributed to the electronic conduction through the Lipon electrolyte film, the resistiv- ity of Lipon must be >1014 cm. A more rigorous test is shown in Fig. 6. Here the fully charged batteries were stored at room temperature under open circuit conditions for dif- ferent periods of time before being suddenly discharged in a high current pulse (∼4 C-rate). The decrease in capacity reflects not only the self-discharge, but also any deleterious aging effects leading to an increase in cell resistance or a Fig. 6. Capacities recorded for three Li–LiCoO2 thin-film batteries when rapidly discharged after prolonged storage in the fully charged state.PDF Image | Solid-state thin-film rechargeable batteries
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