THIN FILMS FOR ADVANCED BATTERIES
   
   This article also appears in the Oak Ridge National Laboratory
   Review (Vol. 25, No. 2), a quarterly research and development
   magazine. If you'd like more information about the research
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   Batteries are a familiar part of everyday life. Many of us depend
   on heavy lead-acid batteries to start our cars; a variety of dry
   cells to operate cameras, toys, and portable sound systems; small
   lithium-iodine batteries to power cardiac pacemakers; and tiny
   lithium-magnesium oxide coin cells to run watches.     
   
   Packing more power into batteries while reducing their size and
   weight has become a goal for researchers because of growing
   interest in smaller, lighter batteries for a variety of purposes.
   Tiny batteries smaller than a button are needed to provide
   electrical energy for computer memory chips, medical implant
   devices, and radiofrequency transmitters in car key sets to prevent
   car theft. Medium-size batteries are useful for consumer
   electronics, such as laptop computers and cellular phones.
   Large-scale, lightweight batteries are needed to power electric
   cars.     
   
   A major issue in the development of new batteries is the materials
   used. Many household batteries contain lead, cadmium, and
   mercury--all toxic materials. New Jersey now has a law that
   requires manufacturers to eliminate mercury from batteries by the
   end of 1995 and mandates that collection programs be set up for
   certain batteries containing mercury, lead, or cadmium.     
   
   The focus of research is to create batteries that contain fewer
   toxic materials, are rechargeable, and are able to pack in more
   energy per unit weight and volume. Batteries using lithium have
   yielded promising results.     
   
   The size and weight of a battery are generally thought to be
   related to the amount of energy it stores. Hence, the larger a
   battery, the more electrical energy it can supply. Electrical
   energy is expressed in watt-hours--the product of the current,
   voltage, and discharge time when the battery is in use. However,
   more important figures of merit for a battery are the energy
   density, which is watt-hours divided by battery volume in liters,
   and specific energy, which is watt-hours divided by battery weight
   in kilograms. Although a lead-acid battery in a car can produce a
   large amount of energy, its energy density and specific energy are
   low. On the other hand, lithium-manganese oxide coin cells in
   watches cannot supply the same amount of energy, but their energy
   density and specific energy are much higher.     
   
   At ORNL a group led by John Bates of the Solid State Division has
   developed a thin-film lithium microbattery for computer memory
   chips that is much thinner than plastic wrap. "The purpose of these
   batteries is to hold the memory until the power comes back," says
   Bates. If such a battery could be scaled up to propel electric
   vehicles, he calculates it would be an improvement over the
   lead-acid battery by a factor of 8 in energy density and a factor
   of 10 in specific energy.      
   
   The main goal of the ORNL work, however, is to find ways to develop
   microbatteries for microelectronics. Integrated circuits on silicon
   chips have made possible the reduction of computer size. As a
   result, modern appliances and cars are now controlled by
   microprocessors. However, partly because of differences in the
   levels of research activity and money, reduction in the sizes of
   batteries has not kept pace with the reduction in the sizes of
   electronic devices.     
   
   For example, consider nonvolatile computer memory chips. They have
   nonrechargeable batteries as backup so that the information stored
   as electronic charges is not lost in case of a power failure.
   However, each lithium battery is many times larger than the chip
   using it. ORNL's development of thin-film lithium batteries offers
   the means to scale down the sizes of batteries to more closely
   match the sizes of microelectronic components. The ORNL work may
   result in the first practical rechargeable microbattery.
   
   
   BATTERY ON A CHIP    
   
   Solid-state processes for depositing thin films on a substrate are
   being used at ORNL and elsewhere to increase the amount of energy
   that can be stored in a battery per unit weight and volume. ORNL's
   Ceramic Thin Films Group in the Solid State Division has made a
   number of thin-film lithium microbatteries that have remarkably
   high specific energies and energy densities. The cells have open
   circuit voltages at full charge of 3.7 to 3.8 volts, the highest
   achieved to date in a thin-film battery. This success has attracted
   the attention of the Eveready Battery Company, which has also been
   developing thin-film batteries. In March 1992, ORNL and Eveready
   signed a cooperative research and development agreement (CRADA) to
   facilitate the commercialization of thin-film batteries.    
   
   According to Bates, the current goal of the ORNL group, which
   includes Nancy Dudney, Greg Gruzalski, and Chris Luck, is to make
   thin-film batteries that can be deposited directly either on the
   reverse side of a computer memory chip or onto the chip's
   protective ceramic package. "The idea," he says, "is to incorporate
   the battery into the integrated circuits of computer memory chips
   during their manufacture. This approach would eliminate the
   manufacturing step of soldering large batteries onto circuit
   boards."     
   
   Bates believes that these solid-state thin-film batteries will also
   offer these advantages over competitive technologies: 
   
        - They are rechargeable.
   
        - They have high power and energy densities.
   
        - They can be fabricated to virtually any size onto a variety
          of substrate materials, such as semiconductors, ceramics,
          and plastics.
   
        - They can be fabricated using standard deposition techniques
          and mild deposition conditions.
   
        - They can operate over a wide temperature range, even at
          temperatures near the melting point of lithium; ORNL cells
          have been operated between -15C and 150C. 
   
        - They contain no liquid components and produce no gases.  
   
   "Our thin-film battery," Bates says, "can be deposited onto any
   substrate that can withstand a temperature of 50C, including a
   variety of polymers. In addition, the shape of the battery can
   conform to the shape of the support. For example, it could be
   fabricated on a cylinder. Our battery can be made as small as any
   multilayer device or as large as the deposition equipment allows.
   For example, a thin-film battery could be deposited onto an entire
   sheet of window glass."    
   
   Bates and his colleagues have been searching for the right
   combination of materials to make reliable backup power sources for
   computer memory chips based on complementary metal oxide
   semiconductor (CMOS) technology. Currently, standby power for
   computer memory chips is supplied by nonrechargeable lithium-
   manganese oxide or other lithium batteries, which are soldered to
   circuit boards or incorporated into a self-contained package. These
   batteries are larger than the chips for which they provide backup
   energy. Someday they may be replaced by smaller thin-film lithium
   batteries based on the ones developed at ORNL using a newly
   discovered material for the electrolyte.    
   
   The ORNL group is also adapting this microbattery technology for
   use in miniature radiofrequency transmitters expected to be
   commercially available someday. The group hopes to contribute to
   the development of scaled-up batteries for consumer electronics and
   electric cars (see sidebar below).
   
   
   WHY THIN-FILM LITHIUM CELLS?    
   
   Why did Bates and his colleagues choose to investigate thin-film
   lithium cells? First, they were attracted to lithium's advantages.
   Because lithium has a small atomic mass and the highest
   electrochemical potential for a metal, it makes a good reactant for
   a battery that must have high cell voltage and high specific power.
   They chose thin films for batteries because they make an effective
   cell which can be manufactured by the same processes used by the
   electronics industry. Battery cell components can be prepared as
   thin (~1m) sheets built up as layers. The area and thickness of
   the sheets determine battery capacity.     
   
   Deposition of thin films increases the contact area of the cell
   components, resulting in a high fraction of reactants. Thin films
   result in higher current densities and cell efficiencies because
   the transport of ions is easier and faster through thin-film layers
   than in bulk materials.     
   
   The major challenge to the development of the lithium cells was to
   find an electrolyte that satisfactorily conducted ions and was
   stable in contact with lithium. In the fall of 1991, groups at
   Eveready and the University of Montpellier in France reported the
   development of solid-state rechargeable thin-film cells with
   lithium anodes.  
   
   The French group's cells had low current densities and short
   lifetimes, but the Eveready cells had excellent current densities
   and were charged and discharged hundreds of times. The Eveready
   cells use a titanium sulfide cathode and an oxysulfide-based
   electrolyte. In both cases, however, the cells required an extra
   layer of lithium iodide to protect the electrolyte from attack by
   lithium. At ORNL Bates' group has developed an even better cell
   using an oxynitride-based electrolyte, which is stable with a
   lithium anode. 
   
   
   BUILDING A BETTER BATTERY    
   
   The ORNL research program began in November 1986 at a Materials
   Research Society meeting when Jim Roberto, director of the Solid
   State Division, heard a talk on microbatteries by Minko Balkanski
   of the Universiti Pierre et Marie Curie in Paris, France. There
   Roberto discussed with Bates the possibility of ORNL conducting
   research in support of microbattery development. In April 1987,
   Bates and Dudney submitted a seed money proposal entitled
   "Micropower Sources" and obtained internal funding from ORNL. By
   November 1987, they had their first vacuum chamber for film
   deposition. In June 1988, Bates and Dudney submitted a proposal on
   "Microionics: Materials and Devices" and received support for
   fiscal year 1989 from the Director's R&D Fund at ORNL. In November
   1988 Bates' group fabricated the first vanadium oxide cell ever
   made at ORNL and possibly anywhere else. It marked the beginning of
   a series of successes for the group in microbattery development.  
    
   
   "Our work started from scratch," Bates says. "We had no deposition
   equipment and no experience in thin films. We began our electrolyte
   studies using a lithium phosphosilicate system."    
   
   The electrodes of the ORNL thin-film battery are lithium (Li) and
   noncrystalline vanadium oxide (V2O5). Vanadium oxide was selected
   as the cathode because it is an intercalation compound that permits
   a lithium ion to move into and out of a framework without causing
   more than a small expansion or contraction of the structure.
   Lithium ions move into the V2O5 structure during discharge of the
   cell and are forced out of the structure during recharge. The
   amorphous material is preferred over the crystalline form because
   three times more lithium ions can be inserted into the amorphous
   cathode, thus making a battery that has a higher capacity.     
   
   In a thin-film lithium cell (see the schematic diagram of its
   operation on p. 53), lithium ions leave the anode, diffuse through
   the electrolyte film, and reach the V2O5 cathode. At the same time,
   electrons travel through the external circuit to power a device and
   then to the cathode to "combine" with the Li+ ions so that the
   compound retains a net neutral charge. The cell reaction can be
   represented asxLi + V2O5 = Lix V2O5.    
   
   In their effort to develop an improved electrolyte, the ORNL
   scientists took into account the fact that many inorganic compounds
   are better ionic conductors in the amorphous state than in the
   crystalline form. For example, the conductivity of amorphous
   lithium phosphate having the composition 0.6Li2O:0.4P2O5 is 109
   times as high as that of crystalline lithium orthophosphate
   (Li3PO4).     
   
   To avoid the need for a layer of lithium iodide to protect the
   electrolyte from being attacked by lithium, Bates consulted the
   literature for some clues. "I learned that adding nitrogen to
   sodium metaphosphate glasses improves their durability in contact
   with air and water vapor. So we decided to sputter the lithium
   orthophosphate in nitrogen rather than the standard gas mixture of
   argon and oxygen. The resulting film was an oxynitride that
   contained about 3 at. % nitrogen. It seems that nature much prefers
   oxygen to nitrogen in compounds such as these, but the presence of
   this small amount of nitrogen greatly improves the performance of
   the electrolyte."     
   
   Currently, the electrolyte used exclusively in ORNL's thin-film
   lithium batteries is lithium phosphorus oxynitride (LiPON), the
   first film of which was grown at ORNL in February 1991. The LiPON
   electrolyte, which is deposited over the cathode, outperforms
   competitive electrolytes, such as the oxysulfide electrolytes
   employed by Eveready.     
   
   To analyze the composition of the electrolyte, the group has relied
   on resonance ion backscattering performed by Ray Zuhr of the Solid
   State Division, electron spectroscopy for chemical analysis and
   Auger electron spectroscopy carried out by Ashok Choudhury of the
   Metals and Ceramics Division, and proton-induced gamma emission
   analysis conducted by their collaborator, Dave Robertson, of the
   University of Kentucky at Lexington.    
   
   "Tests show that the nitrogen-containing electrolyte has 30 times
   the lithium ion conductivity of a film of pure amorphous lithium
   ortho-phosphate," Bates says. "This is important because every
   battery has an internal resistance to ion transport. Our goal is to
   overcome this resistance by increasing the ion conductivity of the
   electrolyte. The result will be a better battery."
   
   
   HOW FILM IS DEPOSITED    
   
   The thin-film lithium cell is fabricated at ORNL by depositing
   successive layers of the cathode, electrolyte, and anode using
   direct-current and radiofrequency magnetron sputtering and thermal
   evaporation. The battery is only 6 microns thick, or one-third the
   thickness of plastic wrap (a micron is a millionth of a meter), and
   cells have been deposited on alumina or glass substrates.    
   
   Magnetron sputtering is done in a vacuum chamber at near room
   temperature in pure argon, oxygen, and nitrogen gases. A thin
   2.5-cm-diam disk of a target material is loaded into a commercial
   magnetron sputter source. The targets are vanadium metal for the
   cathode and Li3PO4 for the electrolyte. A high voltage is applied
   to the target material. At low gas pressures, the voltage generates
   a plasma discharge. The positively charged ions in the plasma are
   accelerated to the target material because of its negative charge,
   and upon bombardment, some of the target atoms are ejected, or
   sputtered.     
   
   The sputtered atoms then condense on the battery substrate
   positioned about 5 cm from the sputter target. Permanent magnets
   positioned beneath the target enhance the sputtering efficiency by
   confining the electrons in the plasma close to the target surface,
   thereby increasing the ionization of the atoms in the sputtering
   gas.      
   
   "Currently, the deposition rates are quite low because pushing for
   a more rapid rate of film growth would sacrifice the film's
   uniformity and quality," Bates says. "But this production problem
   should be considerably reduced when larger sputter sources are
   employed."    
   
   The final cell layer, the lithium anode, is deposited by thermal
   evaporation under vacuum.
   
   
   CRADA WITH EVEREADY    
   
   The ORNL group has apparently solved the problem of lithium attack
   on the thin-film electrolyte. However, a remaining problem that
   must be addressed is protecting the lithium from corrosion in air.
   Currently, thin-film batteries must be stored in a protective argon
   atmosphere. "We must find a way to seal up the battery and make it
   self-contained," says Bates.    
   
   The researchers are now working on a project to package thin-film
   batteries under a CRADA signed in March 1992 between Energy Systems
   and the Eveready Battery Company. Eveready had approached ORNL
   about a cooperative research effort because the company had not
   found a packaging solution and because the smaller-scale deposition
   equipment at ORNL allows researchers to try out ideas rapidly. The
   deposition system at Eveready is much larger because it is designed
   for small production runs, not research.      
   
   "We will work with Eveready on determining which material could
   best seal up the battery without altering the properties of our
   films," Bates says. "We are testing the results of depositing a
   variety of materials onto the lithium as a sealant film."    
   
   Bates predicts the group will develop a self-contained thin-film
   battery of more than 3.5 volts during 1992. Then he hopes the group
   may become involved in research on scaling up thin-film batteries
   for use as lightweight, energy-efficient sources of power for
   electric cars (see sidebar "From Chips to Cars" below).
   
   
   FUTURE OF THIN-FILM BATTERY RESEARCH    
   
   The use of LiPON in thin-film batteries will make possible higher
   voltage cells based on lithium cobalt oxide (LiCoO2) or lithium
   manganese oxide (Li2Mn2O4). Raising the voltage of these cells is
   important because it increases their energy density.     
   
   Another goal of the ORNL research is to enhance the current density
   and pulse capability of the thin-film cells. One problem in the
   ORNL cell is that the current appears to be limited by lithium ion
   transport in the vanadium oxide cathode.     
   
   The discovery of lithium phosphorus oxynitride and its excellent
   properties as an electrolyte has opened up a new area of research
   for thin-film materials, which will be explored in a Basic Energy
   Sciences program of the Department of Energy. In addition, the new
   field of microionics could become a major target of research at
   ORNL.    
   
   "We hope to continue to focus our basic research on improving
   battery technology," Bates says. "Our work in microbatteries and
   our work with other groups on microelectronics could be extended to
   development of, for example, miniature radiofrequency transmitters
   and remote microsensors for environmental or biomedical
   applications."     
   
   In conclusion, the program started by Bates, Dudney, and others
   could expand to scale up microbatteries being developed for
   computer chips into macrobatteries that could be useful in many
   ways, including powering electric vehicles. Use of thin films for
   batteries to power everything from chips to cars may someday be
   seen as a classic example of getting more from less.


   SIDEBARS

      Battery Basics

   A battery is one of two kinds of electrochemical devices that
   convert the energy released in a chemical reaction directly into
   electrical energy.     
   
   In a battery, the reactants are stored close together within the
   battery itself, whereas in a fuel cell the reactants are stored
   externally. This conversion of chemical energy to electrical energy
   is potentially 100% efficient, whereas the conversion of chemical
   energy to mechanical energy via a thermal conversion (e.g.,
   internal combustion of gasoline in cars) always results in heat
   transfer losses limiting the intrinsic efficiency.     
   
   The first electric battery may have been made in ancient Egypt, but
   historians often credit Italian physicist Alessandro Volta, who in
   1800 assembled a series of silver and zinc disks that sandwiched
   cardboard disks soaked in saltwater. The disks served as
   electrodes, and the saltwater was the electrolyte.    
   
   Familiar batteries of today are the flashlight (or dry cell)
   battery, which uses manganese oxide and zinc for the electrodes and
   a paste as an electrolyte, and the lead-acid car battery, which
   uses lead and lead oxide for the electrodes and sulfuric acid for
   the electrolyte. The lead-acid battery, which was invented in 1860,
   is unsurpassed for vehicle uses because it can deliver a large
   current.     
   
   The electrodes are the positively charged pole (cathode) and the
   negatively charged pole (anode) of a storage battery. The
   electrolyte is a chemical compound that separates the electrodes
   and conducts ions released during discharge. The electrolyte forces
   the electrons to flow in the device's external circuits.     
   
   Chemical energy is converted into electrical energy by an oxidation
   reaction in which electrons are released to an external circuit
   through the anode. Simultaneously, a reduction reaction removes
   electrons from the external circuit through the cathode. The
   electrolyte functions to "control" the rate of reaction between the
   anode and cathode by forcing electrons to move through external
   circuits, producing energy. However, the electrodes must not touch
   each other to avoid internal short circuits. (See the examples of
   battery reactions shown in the schematic.)     
   
   Some cells operate as two independent half cells in which reactions
   occur by ion exchange with the cell electrolyte. For others, a net
   cell reaction results in the formation of a new compound such as
   lithium iodide or sodium sulfide. Such a reaction requires the
   transport of an ion through the electrolyte from one electrode to
   the other.    
   
   Among the important characteristics of battery cells are
   
        - Cell voltage--ideal or open circuit voltage, which is higher
          than the actual cell voltage when the current flows through
          the cell (cell efficiency is determined by the ratio of
          actual voltage to the ideal voltage) 
   
        - Power output--the magnitude of the current that can be
          delivered at a given voltage 
   
        - Power density--power output per weight or volume of the
          battery system, which includes the cell materials,
          reactants, and necessary packaging 
   
        - Capacity--the amount of chemical reactants that can be
          stored and effectively used in the battery. Batteries rarely
          can be used until fully discharged, so that one of its
          reactants is totally consumed
   
        - Shelf life--the amount of time a battery can retain its  
          original properties when not in use
   
        - Rechargeability--the ability of the battery to be restored
          to a useful condition.    
   
   Batteries are designed differently for various applications. "For
   computer memory backup power," Bates says, "the battery should have
   the appropriate cell voltage, a long shelf life, rechargeability,
   and the ability to be integrated with the memory chips during
   electronic fabrication. For vehicle applications, the battery
   should offer high energy density, large current output, full
   rechargeability, rapid recharge, safety, and a reasonable
   cost."


      From Chips to Cars

   In the fall of 1991, the U.S. government gave a jump start to the 
   nation's battery research by combining its resources with those of 
   the Big Three automakers in Detroit.    
   
   Because of renewed concerns about air pollution and future oil
   shortages, there has been a resurgence of interest in electric
   cars. But to be acceptable to the public, such vehicles will need
   batteries that are low in cost and able to keep the vehicle
   operating for many miles before a recharge is needed. In addition,
   the time to recharge the battery should be short.    
   
   DOE has established an Advanced Battery Consortium in which
   national laboratories, universities, and other companies will
   collaborate on battery research with Ford, Chrysler, and General
   Motors. The latter company is already developing electric vehicles
   that would use lead-acid batteries. One reason for this move by DOE
   is to make the United States more competitive in battery research
   and electric vehicle development; nations taking the lead in these
   areas are Great Britain, Germany, Japan, and Canada.    
   
   ORNL is expected to be named a lead laboratory for DOE and the
   consortium in at least one area of electric vehicle development:
   lightweight body materials. ORNL may also play a role in the
   development of electric vehicle battery materials.    
   
   The potential competitors for electric vehicle propulsion include
   lead-acid (Pb/acid) batteries, sodium-sulfur (Na/S) batteries, and
   lithium-iron sulfide (Li/FeS) batteries. The lithium-iron sulfide
   battery has higher specific energy and energy density than the
   other two. However, according to Bates, a scaled-up thin-film
   lithium battery would be an improvement over the lithium-iron
   sulfide battery by a factor of about 4 for specific energy and by
   a factor of about 5 for energy density (see table).      
   
   "Improved power density is achieved by using a lithium battery,"
   Bates says. "By making a thin-film battery, we hope to increase the
   maximum discharge current and decrease the time required to
   recharge the battery."
   
   Carolyn Krause
   
   
   (keywords: thin films, batteries, microbatteries)
   

   ------------------------------------------------------------------------
     
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   Date Posted:  2/7/94  (ktb)