Thin film rechargeable Li batteries

SOUD STATE ELSEVIER

Solid State Ionics 69 (1994) 357-368

NmCS

Thin film rechargeable Li batteries Steven D. Jones, James R. Akridge Eveready Battery Company, Westlake, OH 44145, USA

Frough K. Shokoohi Bellcore, Red Bank, NJ 07701, USA

Abstract

Thin film rechargeable lithium microbatteries that have been previously reported in literature are reviewed. A more detailed discussion of the microbatteries recently developed independently at Eveready Battery Company (EBC) and Bellcore is presented. The EBC microbattery consists ofa TiS2 cathode, an oxide-sulfide electrolyte, and a Li anode. It has an OCV near 2.5 V and routinely goes more than 1000 cycles between the limits of 1.8 V and 2.8 V, with greater than 70% cathode utilization, at current densities up to 300 p.A/cm2. Also, it can be cycled at - 10"C at a current density of 100 pA/cm 2, is capable of supplying 2 s pulses of several mA/cm 2, and has excellent long term stability both on shelf and while cycling. The Bellcore microbattery consists of an LiMn204 cathode, a lithium borophosphate (LiBP) or lithium phosphorus oxynitride (LiPON) electrolyte, and a Li anode. It has an OCV near 4.2 V and cycles more than 150 times between 3.5 V and 4.3 V, with nearly 75% cathode efficiency, at current densities up to 70 p.A/cm2.

1. Introduction

Demand for low power, battery operated devices is on the rise, particularly in portable equipment needing longer operation with high reliability. Advances in the microelectronics industry and the miniaturization of electronic devices have reduced the current and power requirements of some of these devices to extremely low levels. This has made possible the use of thin film solid state microbatteries as power sources for these devices. Therefore, it is important to develop long lasting, high energy density, thin film batteries which can be used as an integral part of microelectronic circuits. Various advantages offered by thin film microbatteries for these applications include: (1) they are manufactured by the same techniques as currently used in the microelectronics industry, (2) the extreme thinness of the layers reduces the internal resistance caused by a solid state electrolyte to a manageable level, (3) the sequential vacuum deposition process provides cleaner surfaces and more intimate

contact between layers, and (4) the microbattery can be constructed in almost any 2-dimensional shape. The major difficulty in fabricating a microbattery lay in the ability to prepare layers of electrodes of high intercalation capability and electronic conductivity, as well as thermal, electrochemical, and physical stability, in thickness less than one micrometer, using processes compatible with the microelectronics fabrication techniques. Both electrode and electrolyte materials should not undergo morphology changes during cycling, leading to the formation of chemically unstable, high surface area materials, even at the high rate cycling desirable in some on-chip microbattery applications. The fabrication of such multilayer systems would allow integration of the backup power unit with the electronic circuit board. Lithium batteries are recognized as the most viable rechargeable battery system, since lithium is the lightest and the most electropositive element, producing the highest specific capacity. Several thin film rechargeable battery programs have been launched over the last decade. Earlier work included thin film

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358

S.D. Jones et al. / Solid State lonics 69 (1994) 357-368

AgMo6S s Chevrel phase as the positive electrode [ 1 ].

This material converts to M06S8 during the first few cycles, reversibly intercalating Li between 1.7-2.5 V versus Li thereafter. V2Ox and/or MoO× have also been investigated as the positive electrodes versus Li metal films as the negative electrode. Much work has been devoted to utilization of the layered materials such as TiS2 and its derivative TiO~S~ where x + y > 2, as the positive electrode. The latter system has an effective operating voltage of 1.2-2.6 V against metallic Li. Lithium spinels Li[M2104, where M=V, Ti, and Mn, are attractive electrode materials for Li batteries. LiMn204 is particularly interesting, since it can reversibly intercalate one Li ion per mole, without altering the MnO2 framework. This system has a 4 V operating voltage versus Li metal negative electrode and good electrochemical behavior is expected due to the favorable kinetics for fast Li + diffusion through the three dimensional channels of the Mn204 spinel structure. This chapter reviews in a somewhat chronological order the thin film rechargeable Li microbatteries that have been investigated. The performance and limitations of these various systems are discussed. The anode in the microbatteries being discussed is thin film Li metal deposited by vacuum evaporation in all cases. Finally, data from two new microbattery systems being independently developed at Eveready Battery Company and Bellcore is presented. It will be seen that the state-of-the-art microbatteries currently being developed overcome many of the problems of the earlier microbatteries and demonstrate outstanding secondary performance, long shelf-life, and high current densities.

2. Background Thin film rechargeable Li microbatteries have been investigated for many years. The first totally thin film cell was a Li/TiS2 microbattery developed by Kanehori et al. [2 ]. The TiS2 cathode was deposited by low pressure CVD using TiCI4 and H2S as the source gas. Films of random or ordered 110 crystal orientation could be obtained depending on the deposition parameters. The films were composed of small platelike crystals that randomly intersected each other. The density of the cathode film was about 65% oftheoret-

ical. The Li + diffusion coefficient was determined to be 4 × 10-13 cm2/s in the randomly oriented film and 1.1 × 10 -11 cm2/s for the 110 oriented film. Both these values are significantly smaller than the 10-8 cm2/s determined earlier for a single crystal of TiS2 [ 3 ]. The electrolyte film was sputter deposited from a target of Li3.6Sio.6Po.404 (0.6Li4SiO4-0.4Li3PO4) pressed powder and Li20 pellets using a working gas of 0.6Ar-0.402. The resulting glassy electrolyte had a RT ionic conductivity of 5× 10 - 6 S / c m and an electronic conductivity of 5 × 10-10 S/cm at 92°C. This indicates that the Li ÷ transference number is very close to 1.0. Extrapolating the electronic conductivity of the electrolyte to RT, a significant selfdischarge must be taking place in the microbattery. For example, at 2.0 V the leakage current should be 0.1 IxA/cm 2 through the 4 ~tm thick electrolyte layer. Discharge of the microbatteries using either type of cathode material at 3 ~tA/cm 2 gave about 80% of the theoretical capacity to a 1.5 V cutoff. Higher current densities reduced the cell efficiency to about 55% at 16 ~tA/cm 2. Cells were cycled at 20% DOD and a current density of 16 ~tA/cm 2 for over 2000 cycles. The discharge capacity had decreased about 25% after this time. These cycling results were important because they indicated that dendritic growth of the Li or exfoliation of the layers caused by volume changes of the cathode during intercalation and de-intercalation do not occur with a thin film solid state microbattery. An improvement on the above microbattery was later made by producing the TiS2 films with plasmaenhanced CVD [ 4 ]. The PCVD technique resulted in films consisting of the same small plate-like crystals in the 110 orientation as films produced by the previously used CVD technique but with a more stoichiometric composition and slightly higher density (70% of theoretical). The maximum diffusion coefficient for Li + was found to be 2 × 10 -9 cm2/s. This was still lower than that obtained from a single crystal but this could be explained by a higher defect concentration in the film. The discharge capacity of a microbattery fabricated as above but using PCVD for the TiS2 was about 90% of theoretical at current densities from 16-30 I~A/cm 2. This was a significant improvement over earlier microbatteries using CVD TiS2 that got about 55% of the theoretical capacity at 16 ~tA/cm 2. The increased capacity of the microbatteries using the PCVD TiS2 appears to be caused by

S.D. Jones et al. / Solid State lonics 69 (1994) 357-368

the larger Li + diffusion coefficient in the PCVD films. Later, another microbattery that used the same electrolyte as the above but with an amorphous cathode material was reported by Kirino et al. [ 5 ]. The cathode was either a-WO3 or the mixture 0.63WOa0.37V205. These cathode films were prepared by reactive sputtering in a H2-Ar plasma from targets of either WO3 or a mixture of WO3 and V205. The reducing atmosphere was used because it produced the highest electronic conductivity in the amorphous films. Depending on the H2-Ar mixture, the deposition product is not WOa but WO2 which is a good Li + ion conductor with good electronic conductivity. The diffusion coefficient for Li + in both types of film was about 10 -9 cm2/s. The OCV of the two types of cells appeared to be near 2.2 V, however, the authors state that the OCV of the WOa cell was 1.8 V and the OCV of the mixture decreased from 2.2 to 1.8 V as the amount of WOa was increased. Both types of cells produced about 2.5 mA/cm 2 when short circuited. Cells were cycled between 1.0 and 2.0 V at a current density of 16 l,tA/cm 2. The cell using the a-WO3 cathode discharged to roughly Lio.45WO3 while the cell using the 0.63WO3-0.37V205 cathode discharged to only about 2/3 of this initial capacity. During cycling the 0.63WOa-0.37V205 cell lost about 50% of its initial capacity within the first 25 cycles and 75% within the first 50 cycles. The cell using the a-WOa cathode lost about 55% of its initial capacity within the first 100 cycles but no additional loss was observed out to 400 cycles. A glassy electrolyte made by the evaporation of lithium metaborate, LiBO2 (0.5 Li20-0.5B203 ) was investigated by Balkanski et at. [ 6,7 ] in thin film cells. The composition of the electrolyte was determined to be 0.44Li20-0.56B203 and had a conductivity of about l0 -9 S/cm. The cathode was a polycrystalline T-In2Se3 made by molecular beam deposition. The cell showed an OCV of 1.2 V. The highest current density produced by this cell was less than 0. l ~tA/cm 2. The low current density was blamed on the high resistivity of both the cathode and electrolyte. Balkanski et al. [6,8,9] also investigated an electrolyte with the composition 0.06Li2SO4-0.31Li200.63B203. It was deposited by flash evaporation and had a conductivity of about 10 -s S/cm at RT. The cathode was composed of InSe with poor polycrystallinity that was deposited by flash evaporation onto a

359

substrate either at RT or at 160°C with a post-treatment at 200°C for 10 h. The diffusion coefficient for Li + in these films was about 10 -13 cm2/s. The cell had an OCV of 2.0 V. Cycling curves at current densities less than 0.1 I.tA/cm 2 between the voltage limits of 1.3 to 2.1 V show an early polarization of approximately 100 mV and an average operating voltage of 1.6-1.5 V, while the initial charge curves are only about half the capacity of the discharge. The low ionic diffusivity of the cathode would appear to be the cause of the poor discharge performance of this system. The low charge capacity can be attributed to a percentage of the Li atoms remaining trapped in the disordered lattice of the quasi-amorphous cathode material. Meunier et al. [ 10-12] investigated a similar electrolyte with the compos/tion 0.3 ILi2SO4-0.31Li200.38B203. This electrolyte was formed by sputtering from a pressed powder target prepared from bulk glass of the same composition. The film had a conductivity of 1.6× l0 -7 S/cm. The cathode was sputtered from a TiS2 target and had the composition TiO~Sr The oxygen is supposedly present in the target due to hydrolysis of the TiS2 during construction and installation of the target. The amount of oxygen in the sputtered film depended on the amount of prior sputtering that the target had been subjected to. X-ray analysis showed the films to be amorphous with some small crystallites scattered around in the amorphous matrix. The film had a density of about 70% that of crystalline material. T h e TiO~Sy film is a semiconductor and has an electronic conductivity of about 2.5 × 10- a S/cm at RT compared to 28 S/cm for TiS2. The Li + diffusion coefficient was approximately l0 -15 cm2/s, this is about six orders of magnitude lower than TiS2. Microbatteries were constructed using a cathode with the composition TiOi.5So.7 and had an OCV of 2.6 V. The initial discharge to 1.25 V gave about 80% cathode efficiency. The following charge then recovers about 75% of this initial capacity. The cell could then be cycled between 2.6 and 1.25 V ( x = 0.2-0.8 in LixTiOS) for more than 600 cycles at 1 ~tA/cm 2 with little additional loss in capacity. Microbatteries were tested for more than 3000 h without trouble, indicating the stability of the materials in the microbattery. The cells could be cycled at up to 60 ~tA/cm 2 but with a large loss in capacity. For example at 60 iiA/cm 2 the capacity obtained was only 23% of what was obtained at l ~tA/cm 2. This capac-

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S.D. Jones et al. / Solid State lonics 69 (1994) 357-368

ity loss was attributed to the low diffusion coefficient of the cathode material. Another rechargeable microbattery was investigated by Ohtsuka et al. [ 13,14 ]. The cathode was deposited by sputtering from an MoO3 target. Only a few lines were seen in the X-ray diffraction pattern of the film and the closest fit indicated that the material was M0902~ (MoO2.sg). However, the electronic conductivity of the film was much closer to that of MoO2 than MoO2.sg. It was therefore suspected that the film was composed of mostly MoO2.s9 with a surface coating of MOO2. The electrolyte composition used in the microbatteries was 0.82Li20-0.08V2Os0.10SiO2 (Li6.1oVo.61Sio.39Os.36).This film was made by sputtering in a 1 : 1 At: O2 atmosphere from a target of similar composition to the final electrolyte. The electrolyte film had an RT conductivity of 1.0 × l 0 - 6 S/cm. The X-ray diffraction pattern of the film showed one broad peak that, according to the authors, makes the material not totally amorphous but of very low crystallinity. The film appeared to be microscopically composed of a mixture of the T-phase solid solutions, Li20, V2Os and SiO2. The electronic conductivity of the electrolyte film was estimated to be 2.0 × 10-10 S/cm. The OCV appeared to be about 2.8 V. The cell was cycled between 1.0 and 3.0 V at 20 ~tA/cm 2. After the first cycle the discharge capacity dropped rapidly for a few cycles and then started to recover. From the 10th cycle until the test was terminated at the 247th cycle the discharge capacity remained relatively constant at about 40% of its initial capacity. However, the charge capacity was always about 50% larger than the discharge capacity. This was caused by a relatively large self discharge that was occurring due to the electronic conductivity of the electrolyte. The calculated leakage current density was estimated to be 4 ~tA/cm 2. This would then result in the true discharge current density being 24 ~tA/cm 2 and the true charge current being 16 ~tA/cm 2. Using these current density values the discharge and charge capacities were equal. This electrolyte was later coupled with an MnO2 cathode [ 15 ]. The cathode was fabricated by sputtering with an Ar-O2 mixed gas and consisted mainly of a mixture of T-MnO2 and ~-MnO2. However, the microbattery had an OCV of only 2.5 V instead of the expected 3.6 V. The authors suggested that this low voltage and the cell's discharge performance indi-

cated that the cathode film was composed of MnO2 covered by a layer of electrochemically inactive Mn203. The microbattery was cycled between 1.0 and 3.0 V at a current density of 10 ixA/cm 2 for 10 cycles. By this time, the discharge capacity had faded to 56% of its initial capacity. However, the discharge and charge capacities were close to equivalent, indicating that the self discharge seen above with the same electrolyte was not present. The microbattery was then cycled between 1.5 and 3.5 V at a current density of 10 ~tA/cm 2 for almost 100 cycles. The discharge capacity of the microbattery continued to slowly decrease and the electrolyte again showed the leakage current caused by its relatively high electronic conductivity. According to the authors, it appeared that during the fabrication of the microbattery an insulating film was formed at one of the interfaces and that this film was disrupted at the higher charge voltage limit. Recently a glassy electrolyte was investigated by Bates et al. [ 16 ] in rechargeable microbatteries. The electrolyte was sputter deposited from a Li3PO4 target using N2 as the processing gas. A small amount of nitride is incorporated into the oxide film and this increases the conductivity of the film substantially. An example composition was given as Li2.9PO3.3No.5 with a conductivity of 3.3 X 10 -6 S/cm. The cathode was amorphous V2Os deposited by sputtering a V target in an Ar-O2 mixture. The amorphous films could be obtained only with carefully controlled sputtering conditions, including the age of the V target. It was found that as the target aged, the sputtered film became more crystalline. When crystalline V205 was used as the cathode the current density that the microbattery could be cycled at was greatly reduced. The cell OCV was about 3.75 V. The authors stated that the microbattery could be discharged at up to 100 ~tA/cm 2. The microbattery was tested out to 23 cycles with voltage limits of 2.75 and 3.75 V at 20 gA/cm 2 discharge and 5 ttA/cm 2 charge and out to 8 cycles at the larger voltage limits of 1.5 and 3.6 V at 15 ~tA/ cm 2 discharge and 5 gA/cm 2 charge. The cathode efficiency during testing was not given for this system.

3. The EBC Li/TiS2 microimttery As the above discussion shows, the fabrication of a

S.D. Jones et al. / Solid State lonics 69 (1994) 357-368

thin film rechargeable lithium microbattery has been investigated for many years. However, no one had yet reported the development of a microbattery with properties suitable for commercial use as a long term rechargeable power supply for a microdevice. The first commercially feasible microbattery has been developed at Eveready Battery Company (EBC) in the last few years [ 17-20]. A cross-section of the EBC microbattery is shown in Fig. 1. The EBC microbattery is normally fabricated on a glass substrate but alumina, mylar and paper have also been used. The preferred substrate has a surface roughness of no more than 2 ~tm and is not electronically conductive. However, electronically conductive substrates can be used if a nonconductive coating, such as LiI, is applied to the substrate before fabrication of the microbattery. Flexible substrates such as the mylar and paper mentioned above show that the EBC microbattery has good physical integrity with the ability to be bent with the substrate and still cycle well. This flexibility may allow the EBC microbattery to be used in applications where the substrate is not perfectly rigid, such as on a credit card. The contacts/current collectors of the EBC m i c r o battery are sputtered Cr. Any electronically conductive material can be used as long as it does not react with the dry-box atmosphere where the cell is fabricated or with the other battery components. For example, the use of sputtered Al was discontinued as contact material with the EBC microbattery because of the oxide crystals that formed on its surface. These crystals produced a rough surface for deposition of the subsequent films and increased the IR of the interface.

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361

The cathode is sputter deposited from a TiS2 target. The deposited cathode film appears amorphous to X-ray analysis but SEM shows it to be made up of very small crystallites. The film has good stoichiometry (TiS2.o9) and a density of about 45% of theoretical. The small crystaUite size of these sputtered films compared to the plasma-enhanced CVD films reported by Kanehori et al. (0.3-0.4 I~m versus 10-12 ~tm) results in a much larger active surface area. This is probably one of the reasons that Li + intercalates as easily into this cathode as it does. Also, it is believed that the low density of the cathode material reduces the volume expansion of the cathode during intercalation and enables the EBC Microbattery to be cycled thousands of times at high DOD. The glassy electrolyte that is being used in the EBC microbattery is sputter deposited from a target made from pressed powder with the composition 6LiI4Li3PO4-P2Ss. The exact composition of the electrolyte film is not known. The RT conductivity of the thin film electrolyte is 2 × 10 -5 S/cm. Several other electrolytes were investigated but their use was discontinued for reasons such as high sulfide content, low conductivity, or high internal stress. For example, an electrolyte was sputtered from a target consisting of 5LiI-4Li2S-I.95P2Ss-0.05P2Os. The resulting electrolyte had a conductivity of approximately 10 -5 S/cm and microbatteries were fabricated using it. However, the high sulfide content of the target was very abusive to the sputtering system and this forced a change in the target composition. Another electrolyte was sputtered from a target with the composition 4LiI-LieS-3Li20-B203-P2Ss. This electrolyte had a conductivity of approximately 10 -6 S/cm. Its use was discontinued because its sulfide content was still too high and because its conductivity was nearly an order of magnitude lower than other electrolytes. Yet another electrolyte was sputtered from a target with the composition 4LiI-Li2S2Li3PO4-P2Ss. This electrolyte again had a conductivity of approximately 10-5 S/cm. However, microbatteries could not be constructed using this electrolyte due to high internal stress in the electrolyte that caused it to fracture and thereby produce shorted cells upon deposition of the Li. It was found necessary to use a vacuum evaporated film of LiI between the oxide-sulfide electrolyte and the Li anode. The LiI is used as a protective layer be-

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S.D. Jones et al. / Solid State lonics 69 (1994) 357-368

cause impedance spectroscopy showed that when the lithium was deposited directly onto the electrolyte they reacted with each other and formed a high resistance film, probably Li2S. The LiI is not used as the microbattery electrolyte because of the possibility of forming electronically conductive "color centers" if the stoichiometry gets shifted during deposition of the lithium. Also, the conductivity of the LiI is relatively poor, being only 10 -7 S/em. Because of this, the LiI layer is kept as thin as possible. The total ionic conductivity of the two layers, electrolyte plus LiI, is approximately 10 -6 S/cm. The electronic conductivity of the combination was not measured directly but voltage losses of microbatteries stored at RT indicate it is very low. From this data the volume resistivity is estimated to be 10- ~3 10 '4 ~-cm. The EBC microbattery has a total thickness of about 10 pm and an OCV near 2.5 V. The primary discharge of the microbattery shows essentially 100% cathode utilization to a 1.8 V cutoff at current densities from 10 to 135 pA/cm 2 (Fig. 2). For these microbatteries this calculates to an energy density of 140 Wh/l and power density up to 270 W/1. The energy density can be increased substantially by increasing the cathode thickness. The microbattery also is able to supply 2-second pulse currents as high as 4 mA/ cm 2. The secondary performance of the EBC microbattery is exceptional with the microbattery routinely achieving well over 1000 cycles between the voltage limits of 1.8 and 2.8 V at current densities as high as 300 p A / c m 2 and efficiencies greater than 70%. For example, after 5000 cycles at 100 ~ A / c m 2 an EBC microbattery still shows a cathode utilization of more

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S.D. Jones et al. / Solid State lonics 69 (1994) 357-368

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correspond to a current density of 20 ~tA/cm 2 due to the larger active area involved with this configuration. Microbatteries stored at RT for close to 3 years show only a 5% reduction in their OCV. Also, microbatteries have been continuously cycled for more than 34 months and 11000 cycles with only about a 10% loss in cathode utilization. These results indicate that the microbattery components are stable with one another whether in a static condition or actively being cycled.

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TEMPERATURE (°C) Fig. 5. Cathode efficiencyof the EBC microbatteryat various temperatures. (100 ~tA/cm2). This type of performance will allow the EBC microbattery to be designed to supply days or even months of power at a low drain rate and then be fully recharged in a matter of minutes. It has also been found that the EBC microbattery can be cycled at the relatively high current density of 100 ~ A / c m 2 down to - 1 0 ° C (Fig. 5). At the low temperature, the efficiency of the EBC microbattery is significantly reduced due to the increased cell IR but recovers back to its initial value when the microbattery is brought back to RT. High temperature performance was also investigated up to 90°C. As would be expected for a solid state system, the microbattery performed better as the temperature was increased. Higher temperatures were not explored due to the temporary packaging available for the microbattery at that time, but there was no indication that the microbattery would not function up to the melting point of Li. Five EBC microbatteries have also been tested in both series and parallel configurations. (Five cells were used because that is the amount of microbatteries deposited at one time on a microscope slide. ) The series configuration has an OCV near 12.5 V and was cycled between the voltage limits of 14 V and 7 V for over 5000 cycles at a current density of 100 pA/cm 2. The cumulative cathode efficiency was about 70%. This low value was probably the result of one of the cells being only marginally good. The cells in the parallel configuration have been on test for more than 6 months and have gone more than 500 cycles at close to 100% cathode utilization. The total current through the parallel microbatteries is 100 ~tA which would

4. The Beilcore Li/LiMn204 microbattery Two important parts of making rechargeable Li/ LiMn204 thin film batteries are fabrication of the LiMn2Oa positive electrode and compatibility of the solid electrolyte fdm with the LiMn204 cathode while having sufficient electrochemical stability at the cell operating voltage of 4 V. Thin films of LiMn204, a ternary compound which consists of components having widely different volatilities, must be vacuum deposited in the fight stoichiometry to form the correct spinel phase that is suitable for Li intercalation. The spinel phase must he formed at low enough temperatures where the substrate remains unaffected. For a microbattery on a chip, processing temperatures must not exceed 600°C for Si, and 400°C for III-IV (GaAs, InP, etc. ) substrates. However, LiMn204 spinel is a higher temperature material and is routinely synthesized by solid state reaction at 800°C. Lower temperature treatments have not produced promising materials for Li battery applications in the past. Fabrication of I ttm spinel LiMn204 electrodes for thin film Li battery applications was first reported by Bellcore in 1991. Films were deposited by electron beam evaporation from the spinel source materials followed by a 20 min, 800°C post-deposition anneal in air [21 ]. Although high quality spinel LiMn204 films were prepared, the process temperature had to be lowered for practical applications. Bellcore recently reported fabrication of single phase I ttm thin films of LiMn204 spinel. These films were made at temperatures lower than 400°C, through electron beam evaporation and in-situ anneal in oxygen [ 22 ]. Fig. 6 shows the X-ray diffraction pattern of Bellcore's low temperature LiMn204 spinel films compared to the 800°C bulk spinel used as the depo-

364

S.D. Jones et al. / Solid State lonics 69 (I 994) 357-368

Grain Size (~tm)

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size and the ability to operate at more than ten times higher current density than those annealed at 800°C. These spinel films can be made at temperatures lower than 300oc by annealing the films for longer than two hours. This section mainly describes the cells which contain low temperature 1 Ilm LiMn204 spinel films as their positive electrode. The electrochemical performance of LiMn204 films was first tested against Li in liquid electrolyte. 1 ~tm LiMn204 films intercalate nearly one Li per mole of active cathode material, at the rate of 200 ~tA-cm -2, with only about 40 mV polarization between the charge and discharge half cycles. Typical room temperature cycling behavior of a 1 ~tm thin film o f LiMnzO4 spinel cathode, in the 3.5 V to 4.3 V range, is shown in Fig. 8. The films perform well for over 5.0

(degrees)

Fig. 6. X-ray diffraction patterns of LiMn204 spinel films compared 1o bulk material.

sit•on source. Low temperature heat treatment resuits in 2-6 times smaller grain size than the 800°C films and as much as 20 times smaller than the bulk material [23]. Fig. 7 shows the specific capacity of the low temperature films versus anneal temperatures and grain sizes. The high temperature post-deposition annealed samples show the maximum capacity for films that were annealed at 800°C. The low temperature samples show the highest capacity for films that were annealed for two hours at 400"C. Also, the 4000C anneal produces material with half the grain

i = 10 ~A.cm -2

~

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3,0 2.5

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1

I = 200 laA-cm"2 0.0

0'.2

01.4 x in

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i 0.8

1.0

LixMn204

Fig. 8. Secondary performance of a thin film LiMn204 cathode in EC+DEE+LiC104 between 3.5 V and 4.3 V.

365

S.D. Jones et al. / Solid State lonics 69 (1994) 357-368

180

''"

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Fig. 9. Capacity versus cycle number of a thin film LiMn204 cathode in liquid electrolyte.

Fig. 10. Secondary performance of a thin film LiMn204 cathode in liquid electrolyte between 3 V and 4.8 V.

200 cycles. Fig. 9 shows room temperature capacity versus cycle number at 200 lxA-cm -2 at 25"C as well as 55"C. At the higher temperature, electrolyte oxidation is more severe above 4 V and the reaction kinetics leading to impurity formation are favored. At 25"C the cell capacity fades slightly with cycling, but the cell retains over 83% of its initial capacity after 250 cycles. The 55"C cycling results in slightly faster capacity fade initially but after the first 100 cycles the capacity remains stable, retaining over 70% of its initial value. The better kinetics and the decrease of polarization at higher temperature should result in a larger capacity at 55"C, compared to that at room temperature. However, equal capacity and a more rapid capacity fade is observed at 55"C. This is mainly due to the lower charge cut off voltage used at 55°C and an increased polarization caused by a partial oxidation of the electrolyte in cells containing the EC + DEE + LiC104 electrolyte. The current versus voltage measurements of LiMn204/Li cells show two peaks at 4.0 V and 4.15 V corresponding to the removal of a total of 0.9 Li per mole of LiMn204 cathode. Testing LiMn204 films in Bellcore's liquid electrolyte composition (EC+ DMC(2: 1 ) + IM LiPF6), which is resistant to oxidation at high voltages, allows charging of the cells to 4.8 V with minimum contribution to the current from electrolyte oxidation. Fig. 10 shows the cycling curve for a LiMn204 ( 1 ~tm)/EC + DMC + LiPF6/Li cell, between 3 V and 4.8 V at 200 ~tA-cm-2. Although the initial several tens of cycles mostly show capacity at the average voltage of 4.1 V, continued

cycling results in appearance and growth of a fully reversible peak at 4.5 V. Tarascon and coworkers [ 24 ] recently showed the presence of two fully reversible peaks at 4.5 V and 4.9 V for LiMn204, representing intercalation/de-intercalation of an additional 0.06 Li from the electrode. The high voltage redox peaks (4.5 V and 4.9 V) observed during intercalation/deintercalation of LiMn204 are inherent to the spinel The origin of the 4.5 V peak is attributed to the presence of Mn ions in the tetrahedral sites of the spinel [25 ]. Within the LiMn204 spinel, the degree of cation mixing is limited to less than 10%, thereby limiting the capacity of the 4.5 V and 4.9 V peaks. The presence and amplitude of these peaks, and the associated electrochemical properties of the spinel cathode, depend strongly on the preparation condition of the films. A systematic search for the most suitable glassy electrolyte has lead to the fabrication of I ~tm thin films of lithium borophosphate (LiBP) and lithium phosphorus oxynitride (LiPON) by electron beam evaporation in a low pressure background gas. Electrolyte films of B203: Li2CO3: Li3PO4 show that doping of the glass with as little as 0.03% phosphorus allows incorporation of larger amounts of Li, thereby increasing the ionic conductivity of the glassy electrolyte. Using LiNOa instead of Li2COa as the source of Li + incorporates nitrogen into the glass, leading to higher content and higher mobility of the Li + in the t'rims. A similar effect was observed by Bates and coworkers [ 16 ] when they sputtered lithium phosphate in a nitrogen atmosphere. The oxide-based glassy

goD. Jones et al. / Solid State lonics 69(1994) 357-368

366

lithium electrolyte films are compatible with the spinel LiMn204 cathode, have higher chemical and mechanical stability above 4 V than the sulfide-based glasses, and have an ionic conductivity of 1-5 X 10 - 6 S/cm. This is a workable conductivity range for solid state thin film batteries. Thin film LiMn204/Li batteries need 1 Ixm thin pinhole- and crack-free glassy electrolyte films. This is achieved through electron beam evaporation from LiBP glasses, or in-situ melting and evaporation of Li~PO4 in nitrogen to form LiPON on heated substrates. Fig. I 1 shows the ionic conductivity of 1 Ixm thin Li-borophosphate films as a function of substrate temperature. 1-2 l~m films with good mechanical properties are fabricated at temperatures up to 300°C. At higher temperatures, the Li conductivity decreases due to a partial crystallization and the resulting grain boundary effects. The electrochemical stability of Li glass films are tested through current-voltage measurement of the films against Li anodes, as shown in Fig. 12 for LiPON. Despite the higher stability of these oxide glasses at these voltages, small currents (I~A) corresponding to the oxidation of the electrolyte are measurable even below 4 V. Thin film batteries based on LiMn20,/Glass Elec./ Li are fabricated on quartz substrates, as shown in Fig. 13. The current collectors are 0.1 ~m gold. Fig. 14 shows the discharge capacity with cycling of Bellcore's solid thin film lithium battery at room temperature, between 3.5 V and 4.3 V. Performance of this battery is compared to that of the LiMn20, film ver-

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sus Li in liquid electrolyte. Over 150 cycles have been obtained for the thin film battery at room temperature. Nearly 0.75 Li is reversibly intercalated into the LiMn20, positive electrode at 30 lxA-cm-2. The lower capacity observed for the thin film solid cell is because of the lower charge cut off voltage (4.3 V ) and the large 0.25 V polarization observed at room temperature due to the lower ion transport kinetics across the interfaces. At 55°C lower polarization allows partial recovery of the cell capacity between 3.5 V and 4.3 V. However, solid electrolyte oxidation above 4 V becomes more severe at this temperature, depleting cell cycle life. Thin film batteries have been cycled at current densities of up to 70 IxA-cm-2.

5. Summary

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Fig. 11. Conductivityof amorphous LiBP thin films versus substrat¢temperature.

Thin film microbatteries using many types of electrolytes in combination with a variety of cathodes have been investigated since the early 80's with little success. The microbatteries recently developed by Eveready and Bellcore show sufficient current density, rechargeability, and shelf-life for many possible applications. However, a problem that was never addressed in thin film work was the development of a material to package the microbattery and protect its components (particularly the Li) from reacting with the outside environment. The packaging material must be inert to the components that the microbattery is constructed from, must be roughly the same thickness as the other layers that form the microbat-

S.D. Jones et aL / Solid State lonics 69 (1994) 35 7-368

367

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tery ( 1-3 Ilm), and must block the ingress of gases such as 02, N2 and H 2 0 . This final step is now being worked on by several different groups and when completed should allow the commercialization of thin film rechargeable Li microbatteries. In the near future it will be possible to incorporate a microbattery with many types of microdevices during their manufacture to provide a rechargeable long-term power source for the device.

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T = 25°C

16o E

I = 200 i~A/cm 2

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120

160

Fig. 14. Capacity versus cycle number of the Bellcore microbattery.

368

S.D. Jones et al. / Sofid State lonics 69 (1994) 357-368

References [ 1 ] J.M. Tarascon, T.P. Orlando and M.J. Neal, J. Electrochem. Soc. 135 (1988) 804. [2] K. Kanehori, K. Matsumoto, K. Miyauchi and T. Kudo, Solid State Ionics 9/10 (1983) 1445. [ 3 ] M.S. Whittingham, Prog. Solid State Chem. 12 (1978) 41. [4] K. Kanehori, Y. Ito, F. Kirino, K. Miyauchi and T. Kudo, Solid State Ionics 18/19 (1986) 818. [ 5 ] F. Kirino, Y. Ito, K. Miyauchi and T. Kudo, Nippon Kagaku Kaishi 3 (1986) 445. [6] M. Balkanski, C. Julien and J.Y. Emery, J. Power Sources 26 (1989) 615. [7] J.Y. Emery, in: Microionics, ed. M. Balkanski (Elsevier, Amsterdam, 1991 ) p. 41. [8] C. Julien, I. Samaras, M. Tsakiri, P. Dzwonkowski and M. Balkanski, Mater. Sci. Eng. B 3 (1989) 25. [9]C. Julien and M. Balkanski, in: Microionics, ed. M. Balkanski (Elsevier, Amsterdam, 1991 ) p. 3. [ 10 ] G. Meunier, R. Dormoy and A. Levasseur, Mater. Sci. Eng. B3 (1989) 19. [ 11]G. Meunier and R. Dormoy, in: Microionics, ed. M. Balkanski (Elsevier, Amsterdam, 1991 ) p. 73. [12] G. Meunier, R. Dormoy and A, Levasseur, U.S. Patent 5,202,201, April 13, 1993. [ 13 ] H. Ohtsuka and J. Yamaki, Prog. Batteries Solid Cells 8 (1989) 108.

[ 14] H. Ohtsuka, and J. Yamaki, Solid State lonics 35 (1989) 201. [ 15 ] H. Ohtsuka, S. Okada and J. Yamaki, Solid State Ionics 40/ 41 (1990) 964. [16] J.B. Bates, N.J. Dudney, G.R. Gruzalski, R.A. Zuhr, A. Choudhury, C.F. Luck and J.D. Robertson, J. Power Sources 43/44 (1993) 103. [ 17] S.D. Jones, J.R. Akridge, S.G. Humphrey, C.C. Liu and J. Sarradin, Mater. Res. Soc. Symp. Proc. 210 ( 1991 ) 31. [ 18 ] S.D. Jones and J.R. Akridge, Proc. Int. Symp. Ionic Mixed Conduct. Ceram., Proc.-Electrochem. Soc. 91-12 (1991) 145. [19] S.D. Jones and J.R. Akridge, Solid State Ionics 53-56 (1992) 628. [20 ] S.D. Jones and J.R. Akridge, J. Power Sources 43/44 (1993) 505. [ 21 ] F.IC Shokoohi, J.M. Tarascon and B.J. Wilkens, Appl, Phys. Lett. 59 (1991) 1260. [22] F.K. Shokoohi and J.M. Tarascon U.S. Patent 5,110,696 (1992). [23 ] EK. Shokoohi, J.M. Tarascon, B.J. Wilkens, D. Guyomard and C.C. Chang, J. Electrochem. Soc. 139 (1992) 1845. [24] J.M. Tarascon and D. Guyomard, Electrochim. Acta 38 (1993) 1221. [25] J.M. Tarascon, W.R. McKinnon, F. Coowar, T.N. Bowmer, G. Amatucci and D. Guyomard, J. Electroehem. Soc. 141 (1994), in press.