Solid State Ionics 135 (2000) 33±45 www.elsevier.com/locate/ssi Thin-®lm lithium and lithium-ion batteries J.B. Bates, N.J. Dudney* , B. Neudecker, A. Ueda, C.D. Evans Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6030, USA Abstract Research over the last decade at Oak Ridge National Laboratory has led to the development of solid-state thin-®lm lithium and lithium-ion batteries. The batteries, which are less than 15 mm thick, have important applications in a variety of consumer and medical products, and they are useful research tools in characterizing the properties of lithium intercalation compounds in thin-®lm form. The batteries consist of cathodes that are crystalline or nanocrystalline oxide-based lithium intercalation compounds such as LiCoO224 and LiMn O , and anodes of lithium metal, inorganic compounds such as silicon±tin oxynitrides, Sn34 N and Zn 32 N , or metal ®lms such as Cu in which the anode is formed by lithium plating on the initial charge. The electrolyte is a glassy lithium phosphorus oxynitride (`Lipon'). Cells with crystalline LiCoO2 cathodes can deliver up to 30% of their maximum capacity between 4.2 and 3 V at discharge currents of 10 mA/cm2 , and at more moderate discharge±charge rates, the capacity decreases by negligible amounts over thousands of cycles. Thin ®lms of crystalline lithium manganese oxide with the general composition Li11x Mn22y O4 exhibit on the initial charge signi®cant capacity at 5 V and, depending on the deposition process, at 4.6 V as well, as a consequence of the manganese de®ciency±lithium excess. The 5-V plateau is believed to be due to oxidation Mn of ions to valence states higher than 1 4 accompanied by a rearrangement of the lattice. The gap between the discharge±charge curves of cells with as-deposited nanocrystalline Li11x Mn22y O4 cathodes is due to a true hysteresis as opposed to a kinetically hindered relaxation observed with the highly crystalline ®lms. This behavior was con®rmed by observing classic scanning curves on charge and discharge at intermediate stages of insertion and extraction of Li1 ions. Extended cycling of lithium cells with these cathodes at 25 and 1008C leads to grain growth and evolution of the charge±discharge pro®les toward those characteristic of well crystallized ®lms. 2000 Published by Elsevier Science B.V. Keywords: Solid-state thin-®lm battery; Lithium battery; Lithium-ion battery; LiCoO224 ; LiMn O ; Hysteresis 1. Introduction now moving toward production of these devices for application in consumer and medical electronics. The Research on thin-®lm solid-state lithium batteries long cycle and shelf life of these batteries results begun a decade ago at the Oak Ridge National from the properties of the glassy lithium phosphorus Laboratory [1±4] has resulted in the development of oxynitride (`Lipon') electrolyte [5,6] which is stable practical thin-®lm rechargeable lithium and lithium- in contact with metallic lithium at potentials from 0 ion batteries, and several companies in the US are to nearly 5.5 V and has an acceptable conductance in thin-®lm form. Responding to the need for thin-®lm *Corresponding author. Fax: 1 1-423-574-4143. batteries that can tolerate heating to 250±2608Cso E-mail address: [email protected] (N.J. Dudney). they can be integrated into circuits using the solder 0167-2738/00/$ ± see front matter 2000 Published by Elsevier Science B.V. PII: S0167-2738(00)00327-1 34 J.B. Bates et al. / Solid State Ionics 135 (2000) 33 ±45 re¯ow process, we have synthesized several inor- situ X-ray diffraction measurements or that are to be ganic anode materials [7,8] that result in thin-®lm subjected to extended cycling at high rates are lithium-ion cells which are stable at these tempera- covered with a protective coating consisting of a tures. Recently, we have investigated batteries with multilayer of parylene and titanium. Details about in situ plated lithium anodes [9] which deliver the ®lm deposition and characterization can be found in high energies of lithium cells while, prior to the several references and on our web page [1±5,16]. initial plating, also are not affected by the solder re¯ow conditions. Thin-®lm batteries have been important tools in our studies of thin ®lms of lithium 3. Results and discussion intercalation compounds, allowing us to investigate the electrochemical properties of the pure phases free 3.1. Crystalline LiCoO2: lithium, lithium-ion, and of binders [1±4,10±14] and to perform in situ lithium free cells experiments such as X-ray diffraction [11] and X-ray absorption [15] as a function of the lithium con- Polycrystalline ®lms of LiCoO2 deposited by rf centration in these materials. magnetron sputtering exhibited a strong preferred The purpose of this paper is to summarize the orientation or texturing after annealing at elevated results of recent studies of lithium, lithium-ion, and temperatures [17±19]. For ®lms thicker than about 1 lithium free thin-®lm cells with crystalline LiCoO2 mm, more than 90% of the grains were oriented with cathodes and to brie¯y describe some of the interest- their (101) and (104) planes parallel to the substrate ing properties of nano- and microcrystalline ®lms in and less than 10% with their (003) planes parallel to the lithium manganese oxide system. Published the substrate. For example, the comparison of the results and work in progress on the structure and X-ray diffraction pattern of a 4-mm thick ®lm with electrochemical properties of thin ®lms of layered that of a powdered (random) sample (Fig. 2), shows Li±Ni±Mn oxides [13,14] and amorphous [1,10] and that within the sensitivity of the measurement, none crystalline V25 O will not reviewed. The paper will of the grains had their (003) planes oriented parallel conclude with a brief discussion of applications and to the substrate. As the ®lm thickness decreased the status of manufacturing scaleup. below 1 mm, the percentage of (003)-oriented grains increased until at a thickness of about 0.05 mm, 100% of the grains were (003) oriented [19]. These 2. Experimental procedures extremes in texturing were caused by the tendency to minimize volume strain energy for the thicker ®lms A schematic drawing illustrating the layout of a and the surface energy for the very thin ®lms [18]. thin ®lm battery and a micrograph of a fracture The strain energy arises from the difference in cross-section are shown in Fig. 1. For our laboratory thermal expansion between the LiCoO2 ®lm and the cells, the substrate is typically a 2.54 3 1.27 3 0.1- substrate on heating to the anneal temperature. cm thick polycrystalline alumina plate, but cells have Examples of discharge curves of a Li±LiCoO2 been fabricated on silicon, metal foils, and plastics. battery are shown in Fig. 3. At low currents, the For the latter two substrates, the batteries are quite discharge capacity corresponds to about 0.5 Li per ¯exible. The cathodes, which are deposited by rf CoO2 or 137 mAh/g. The shift to lower potentials magnetron sputtering of the parent compounds, are with increasing current is due to cell resistance. characterized using a variety of physical and chemi- Although lithium ion diffusion should be much faster cal techniques prior to depositing the electrolyte. through cathodes with a high percentage of (101)- After depositing the lithium phosphorus oxynitride and (104)-oriented grains than through cathodes with (`Lipon') electrolyte, the lithium anode is deposited predominately (003)-oriented grains [18,19], it was by thermal evaporation to a thickness corresponding not possible to verify this expectation because the to three to four times overcapacity. The lithium-ion resistance of most cells was independent of cathode cells on the other hand typically are cathode heavy. thickness up to 3 mm [19]. For cells with 1-cm2 Cells that are to be exposed to air in order to make in cathodes deposited below 708C, the average resist- J.B. Bates et al. / Solid State Ionics 135 (2000) 33 ±45 35 Fig. 1. Top: schematic cross-section illustrating the layout of a thin-®lm battery. Bottom: photograph of the fracture cross-section of a thin-®lm battery with a nanocrystalline as-deposited LiMn24 O cathode fabricated on an alumina substrate. The apparent cracks in the Lipon electrolyte ®lm and pullouts from the cathode and substrate were caused by fracturing. 36 J.B. Bates et al. / Solid State Ionics 135 (2000) 33 ±45 ance of about 110 V was due to the electrolyte and electrolyte±cathode interface. These cells could de- liver signi®cant fractions of their maximum capacity and energies at discharge rates of 5 mA/cm2 or higher as illustrated by the graphs in Fig. 4. The most important process variable in achieving low resistance cathode ®lms is the deposition tempera- ture. Deposition at temperatures above about 1008C leads to larger grains and increased void fraction. This reduces the contact area between the electrolyte and cathode as well as the contact area between the grains resulting in higher cell resistances. The secondary performance of thin-®lm lithium batteries is illustrated in the graphs of capacity versus cycle number in Fig. 5. The total capacity loss of the cell with the thinner cathode having a capacity of 65 mAh was less than 2% after more than 4000 Fig. 2. X-ray diffraction patterns of a 4-mm thick LiCoO2 ®lm and a commercial powdered sample. Peaks marked with * are due to the alumina substrate. The absence of the (003) re¯ection and the strong intensity of the (101) re¯ection from the ®lm indicate a high degree of preferred orientation.