Thin Film Micro-Batteries by Nancy J
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Thin Film Micro-Batteries by Nancy J. Dudney tags, and smarter cards. In principle Most of the thin films used in current Building a Battery by with a large deposition system, a thin commercial variations of this thin Vapor Deposition film battery might cover a square meter, film battery are deposited in vacuum but in practice, most development is chambers by RF and DC magnetron hin film batteries are built layer targeting individual cells with active sputtering and by thermal evaporation by layer by vapor deposition. areas less than 25 cm2. For very small onto unheated substrates. In addition, The resulting battery is formed battery areas, <1 mm2, microfabrication many publications report exploring a T 2 of parallel plates, much as an ordinary processes have been developed. variety of other physical and chemical battery construction, just much thinner. Typically the assembled batteries have vapor deposition processes, such as pulsed The figure (Fig. 1) shows an example of a capacities from 0.1 to 5 mAh. laser deposition, electron cyclotron thin film battery layout where films are The operation of a thin film battery resonance sputtering, and aerosol spray deposited symmetrically onto both sides is depicted in the schematic diagram coating, for one or more components of of a supporting substrate. The full stack (Fig. 2). Very simply, when the battery is the battery. The active materials used of films is only 10 to 15 µm thick, but allowed to discharge, a Li+ ion migrates for the thin film cathodes and anodes including the support at least doubles from the anode to the cathode film by are familiar intercalation compounds, the overall battery thickness. When diffusing through the solid electrolyte. but the microstructures and often the the support is thin, the entire battery When the anode and cathode reactions cycling properties of the thin films may can be flexible. At least six companies are reversible, as for an intercalation be quite distinct from those of battery have commercialized or are very close compound or alloy, the battery can electrodes formed from powders. The to commercializing such all-solid-state be recharged by reversing the current. thin film cathodes are dense and thin film batteries and market research1 The difference in the electrochemical homogeneous with no added phases predicts a growing market and a variety potential of the lithium determines the such as binders or electrolytes. When of applications including sensors, RFID cell voltage. deposited at ambient temperatures, the films of cathodes, such as LiCoO2, V2O5, LiMn2O4, LiFePO4 are amorphous or nanocrystalline. But even in this form, they often act as excellent cathodes with large specific capacities and good stability for hundreds to thousands of cycles.3 Annealing the cathode films at temperatures of 300° to 800°C may be used to induce crystallization and grain growth of the desired intercalation compound. Crystallizing the cathode film generally improves the Li chemical diffusivity in the electrode material, and hence the power delivered by the 10-15µm battery, by 1-2 orders of magnitude. The microstructure is also tailored by the deposition and heat treatment. Figure 3 shows a fracture edge of an annealed LiCoO2 cathode film on an alumina FIG. 1. Schematic cross section of a thin film battery fabricated by vapor substrate. The columnar microstructure, deposition onto both sides of a substrate support. which is typical of a vapor deposited film, sinters at high temperatures leaving small fissures between the dense columns. Such crystalline films also may have a preferred crystallographic orientation. For LiCoO2 films the crystallographic texture differs for films deposited by sputtering4 versus pulse laser ablation processes.5 To improve the manufacturability of the thin film batteries, it would be beneficial to eliminate or minimize the temperature or duration of the annealing step. Several efforts have lead to low temperature fabrication of thin film batteries on polyimide substrates, but the battery capacity and rate are lower than those treated at high temperatures.6,7 For the battery anode, many designs use a vapor-deposited metallic lithium film as both the anode and current collector. When used with a FIG. 2. Schematic illustration of a thin film battery. The arrows indicate the mechanically and chemically stable discharge reaction where a Li ion diffuses from the lithium metal anode to solid electrolyte, the lithium anodes fill a vacancy in an intercalation compound that serves as the cathode. The give excellent charge–discharge compensating electron is conducted through the device. 44 The Electrochemical Society Interface • Fall 2008 1 µm-thick films are adequate for a pinhole-free barrier over most thin film electrodes. Further, the electronic resistivity of Lipon is very high, greater than 1014 Ωcm. The ternary composition diagram (Fig 4) shows the composition range giving the highest lithium ion conductivity. The lithium content is slightly below that for the orthophosphate structure and spectroscopic studies indicate that much of the nitrogen substitutes for oxygen and forms links between two or three phosphate groups.14 The diagram also shows that the Lipon composition is well outside of the normal glass forming region. So far, synthesis of this material is limited to vapor deposition with an energetic FIG. 3. Schematic cross section of a thin film battery fabricated by vapor source of nitrogen. When deposited by deposition onto both sides of a substrate support. magnetron sputtering, film deposition rates can exceed 10 nm/m with good cycling at high rates, and long battery microstructure or boundaries. Al- results. Higher Lipon deposition rates life with no lithium dendrites. The chief though the nitrogen partial pressure are reported for alternative processes drawback for metallic lithium anodes over-whelms that of the oxygen in the such as plasma enhanced chemical is the restricted maximum operating plasma, only a small amount of N re- vapor deposition16 and nitrogen ion temperature to <180°C, which avoids places the oxygen in the composition, beam assisted deposition.17 melting of the lithium. Alternatives that but this nitrogen has a profound A variety of other inorganic and allow higher fabrication and operating effect on the ion conductivity and polymer electrolytes are being evaluated, temperatures include a Li-ion battery electrochemical stability. With a but none have been as widely tested anode or a “Li-free” construction. For N/O ratio as small as 0.1, the ionic as the Lipon electrolyte in thin film lithium-free batteries, the metallic conductivity is 1 to 2 µS/cm, which is rechargeable batteries. Inorganic glasses lithium anode is omitted during the about 40-fold higher than glassy films include binary and ternary mixes of fabrication, but is subsequently plated of N-free Li3PO4. More importantly, lithium borate, phosphate, silicate, and at the anode current collector when the the ionic transport number of Lipon is vanadate, but most compositions do not battery is charged for the first time.8 one, the electrochemical window is en- appear to meet conductivity or stability Lithium-ion anodes that have been hanced to 5.5V versus Li/Li+, and Lipon standards. Glass-ceramics with higher proposed include single-phase and is stable at both elevated temperatures ionic conductivities18 might be attractive composite films based on reversible and in contact with metallic lithium.15 as a self supporting electrolyte if reactions of lithium with Sn, Si, Ge, The lithium ion conductivity, although fabricated as <100 µm sheets. Promising and C. For the pure metals, only very 100X less than for many liquid polymer electrolytes are based on block thin films, ~50 nm, can tolerate the electrolytes, is sufficient because just copolymer compositions.19 extreme volume changes associated with insertion and extraction of large amounts of lithium.9,10 Good cycling has been reported for various oxide, nitride and oxynitrides of Si, Sn, and Zn3,11 which likely form composite films upon cycling. Intercalation 12 compounds, such as Li4Ti5O12, might also be used for thin film lithium-ion batteries. Lipon, a Glassy Electrolyte The thin film solid electrolyte invented at Oak Ridge National Laboratory in the early 1990s is the most widely used solid electrolyte for thin film batteries. The key insight by J. B. Bates was that addition of nitrogen to the glass structure might enhance the chemical and thermal stability of a lithium glass, as it does for sodium phosphate and sodium silicate glasses. The lithium phosphorus oxynitride electrolyte, which is now known as Lipon, is deposited by RF magnetron FIG. 4. Expanded region the LiO0.5-PO2.5-PN1.67 composition diagram (mole percentages) showing results of thin electrolyte films obtained by magnetron sputtering from a ceramic target of sputtering. The orange shaded areas indicates the compositions giving increasing Li3PO4 using a nitrogen process gas ionic conductivities exceeding 0.5, 1.0, and 2.0 µS/cm. Dashed lines indicate 13,14 to form the plasma. The films are constant ratios of (N+O)/P. The blue shading indicates the approximate amorphous and free of any columnar compositions for glasses formed from a melt. The Electrochemical Society Interface • Fall 2008 45 Dudney (continued from previous page) Cycle Life, Self Discharge, and Temperature Performance Thin film batteries with several different cathode and anode materials have been cycled to hundreds and many thousands of deep cycles with little loss of capacity. This is attributed to: the stability of the Lipon electrolyte film, the ability of the thin film materials to accommodate the volume changes associated with the charge-discharge reactions, and the uniformity of the current and charge distribution in the thin film structure. With cycling, the batteries gradually become more resistive with aging rates that depend FIG. 5. Ragone plot for thin film lithium batteries comparing the energy and on the particular electrode materials, power delivered by constant current discharge.