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Thin film lithium ion batteries are similar to lithium-ion batteries, but they are composed of thin materials, some only nanometers or micrometers thick, which allow for the finished battery to be just millimeters thick. They have been developed and advanced primarily within the last decade. These batteries consist of a substrate, electrolyte, current collector, anode, cathode, and a charge separator. There has been much research into the determination of the most effective components for this type of battery. It has been shown recently that even ordinary printer paper can be used as a charge separator and a substrate. . These thin film batteries are an improvement on the common secondary, or rechargeable, lithium ion batteries in many ways. These batteries exhibit the same voltage and current as their bulky counterparts, but their thinner dimensions allow for greater applications such as making thinner electronic devices, like cell phones and laptops and even implantable medical devices and reducing the weight of common devices that are run on battery power because of the batteries’ high energy density. These batteries can be formed into any shape and they can be stacked, to be used in parallel, thus even further reducing the space needed for a battery.

Background
Lithium-ion batteries are a newer battery technology that is preferable due to their ability to be recharged. Also they have high energy density and last longer than many similar battery technologies. In the battery cell lithium ions flow through the electrolyte from the anode to the cathode while the battery is being discharged. Upon recharging the battery the lithium ions move back to the anode. This Li-ion battery design is effective for large devises. However, as a more mobile, technology driven society we rely heavily on portable electronics, which require thin batteries for power. Research into thin film batteries has developed over the recent years to accommodate for this demand.

Cathode materials
Cathode materials in thin film lithium ion batteries are the same of what is seen in classical lithium ion batteries. They are normally metal oxides that are deposited as a film by various methods.

Metal oxide materials are shown below as well as their relative specific capacities (Λ), open circuit voltages (V oc ), and energy densities (D E ).

Deposition methods for cathode materials
There are various methods being used in order to deposit a thin film cathode material onto the current collector.

Pulsed Laser Deposition (PLD)
In Pulsed Laser Deposition materials are fabricated with varying parameters such as laser energy and fluence, substrate temperature, background pressure, and target-substrate distance.

Magnetron Sputtering
In Magnetron Sputtering the substrate is cooled for deposition.

Chemical Vapor Deposition (CVD)
In Chemical Vapor Deposition volatile precursor materials is deposited onto a substrate material.

Sol-Gel Processing
Sol-gel processing allows for homogeneous mixing of precursor materials at the atomic level.

Electrolyte
The greatest difference between classical lithium ion batteries and thin, flexible, lithium ion batteries is in the electrolyte material used. Progress in lithium ion batteries relies as much on improvements in the electrolyte as it does in the electrode materials, as the electrolyte plays a major role in safe battery operation. The concept of thin film lithium ion batteries was increasingly motivated by manufacturing advantages presented by the polymer technology for their use as electrolytes. Lipon, lithium phosphorus oxynitride, is an amorphous polymer material used as an electrolyte material in thin film flexible batteries. Layers of Lipon are deposited over the cathode material at ambient temperatures by RF magnetron sputtering. This forms the solid electrolyte used for ion conduction between anode and cathode. Solid polymer electrolytes offer several advantages in comparison to a classical liquid lithium ion battery. Rather than having separate components of electrolyte, binder, and separator, these solid electrolytes can act as all three. This increases the overall energy density of the assembled battery because the constituents of the entire cell are more tightly packed.

Separator Material
Separator materials in lithium ion batteries must have the ability to transport ions through their porous membranes while maintaining a physical separation between the anode and cathode materials in order to prevent short-circuiting. In a thin film based system, the electrolyte is normally a solid electrolyte, capable of conforming to the shape of the battery. Typically this material is a polymer based material and as mentioned above, this polymer commonly acts as both the separator and electrolyte. Since thin film batteries are made of all solid materials, this affords to use of simpler separator materials in these systems such as Xerox paper rather than in liquid based Li-ion batteries.

Current Collector
Current collectors in thin film batteries must be flexible, have high surface area, and cost-effective. Silver nanowires with improved surface area and loading weight have been shown to work as a current collector in these battery systems, but still are not as cost-effective as desired. Extending graphite technology to lithium ion batteries, solution processed carbon nanotubes (CNT) films are being looked into for use as both the current collector and anode material. CNTs have the ability to intercalate lithium and maintain high operating voltages, all with low mass loading and flexibility.

Advantages and Challenges
Thin film lithium ion batteries offer improved performance by having a higher average output voltage, lighter weights thus higher energy density, and longer cycling life than typical rechargeable batteries. In the thin film lithium ion battery, both electrodes are capable of reversible lithium insertion, thus forming a Li-ion transfer cell. Li-ion transfer cells are the most promising systems for satisfying the demand of high specific energy and high power. In order to construct a thin film battery it is necessary to fabricate all the battery components, as an anode, a solid electrolyte, a cathode and current leads into multi-layered thin films by suitable technologies.

In a thin film based system, the electrolyte is normally a solid electrolyte, capable of conforming to the shape of the battery. This is in contrast to classical lithium ion batteries, which normally have liquid electrolyte material. Liquid electrolytes can be challenging to utilized if they are not compatible with the separator. Also liquid electrolytes in general call for an increase in the overall volume of the battery, which is not ideal for designing a system that has high energy density. Additionally, in a thin film flexible Li-ion battery, the electrolyte, which is normally polymer-based, can act as the electrolyte, separator, and binder material. This provides the ability to have flexible systems since the issue of electrolyte leakage is circumvented. Finally, solid systems can be packed together tightly which affords an increase in energy density when compared to classical lithium ion batteries.

Separator materials in lithium ion batteries must have the ability to transport ions through their porous membranes while maintaining a physical separation between the anode and cathode materials in order to prevent short-circuiting. Furthermore, the separator must be resistant to degradation during the battery’s operation. In a thin film Li-ion battery, the separator must be a thin and flexible solid. Typically today, this material is a polymer-based material. Since thin film batteries are made of all solid materials, allows one to use simpler separator materials in these systems such as Xerox paper rather than in liquid based Li-ion batteries.

Scientific Development
Development of thin solid state batteries allows for roll to roll type production of batteries which would decrease production costs. Solid-state batteries can also afford increased energy density due to decrease in overall device weight. Where as the flexible nature allows for novel battery design and incorporation into electronics. Development is still required in cathode materials which will resist decreased capacity due to cycling.

Applications
The advancements made to the thin film lithium ion battery have allowed for many potential applications. The majority of these applications are aimed at improving the currently available consumer and medical products. Thin film lithium ion batteries can be used to make thinner portable electronics, because the thickness of the battery required to operate the device can be reduced greatly. These batteries have the ability to be an integral part of implantable medical devices, such as defibrillators and neural stimulators, “smart” cards, radio frequency identification, or RFID, tags and wireless sensors. They can also serve as a way to store energy collected from solar cells or other harvesting devices. Each of these applications is possible because of the flexibility in the size and shape of the batteries. The size of these devices doesn’t have to revolve around the size of the space needed for the battery anymore. The thin film batteries can be attached to the inside of the casing or in some other convenient way. The opportunities in which to use this type of batteries are endless.

Solar Cell Storage Devices
The thin film lithium ion battery can serve as a storage device for the energy collected from a solar cell. These batteries can be made to have a low self discharge rate, which means that these batteries can be stored for long periods of time without a major loss of the energy that was used to charge it. These fully charged batteries could then be used to power some or all of the other potential applications listed below.

Smart Cards
Smart cards are basically the same size as a credit card, but they contain a microchip that can be used to access information, give authorization, or process an application. These cards can go through harsh production conditions, with temperatures in the range of 130 to 150°C, in order to complete the high temperature, high pressure lamination processes. These conditions can cause other batteries to fail because of degassing or degradation of organic components within the battery. Thin film lithium ion batteries have been shown to withstand temperatures of -40 to 150°C. This use of thin film lithium ion batteries is hopeful for other extreme temperature applications.

RFID tags
Radio Frequency Identification (RFID) tags can be used in many different applications. These tags can be used in packaging, inventory control, used to verify authenticity and even allow or deny access to something. These ID tags can even have other integrated sensors to allow for the physical environment to be monitored, such as temperature or shock during travel or shipping. Also, the distance required to read the information in the tag depends on the strength of the battery. The farther away you want to be able to read the information, the stronger the output will have to be and thus the greater the power supply to accomplish this output. As these tags get more and more complex, the battery requirements will need to keep up. Thin film lithium ion batteries have shown that they can fit into the designs of the tags because of the flexibility of the battery in size and shape and are sufficiently powerful enough to accomplish the goals of the tag. Low cost production methods, like roll to roll lamination, of these batteries may even allow for this kind of RFID technology to be implemented in disposable applications.

Implantable Medical Devices
Thin films of LiCoO2 have been synthesized in which the strongest x ray reflection is either weak or missing, indicating a high degree of preferred orientation. Thin film solid state batteries with these textured cathode films can deliver practical capacities at high current densities. For example, for one of the cells 70% of the maximum capacity between 4.2 V and 3 V (approximately 0.2 mAh/cm2) was delivered at a current of 2 mA/cm2. When cycled at rates of 0.1 mA/cm2, the capacity loss was 0.001%/cycle or less. The reliability and performance of Li LiCoO2 thin-film batteries make them attractive for application in implantable devices such as neural stimulators, pacemakers, and defibrillators.

Implantable medical devices require batteries that can deliver a steady, reliable power source for as long as possible. These applications call for a battery that has a low self-discharge rate, for when it’s not in use, and a high power rate, for when it needs to be used, especially in the case of an implantable defibrillator. Also, users of the product will want a battery that can go through many cycles, so these devices won’t have to be replaced or serviced often. Thin film lithium ion batteries have the ability to meet these requirements. The advancement from a liquid to a solid electrolyte has allowed these batteries to take almost any shape without the worry of leaking, and it has been shown that certain types of thin film rechargeable lithium batteries can last for around 50,000 cycles. Another advantage to these thin film batteries is that they can be stacked and used in parallel to give a larger voltage equal to the sum of the individual battery voltages. This fact can be used in reducing the “footprint” of the battery, or the size of the space needed for the battery, in the design of a device.

Wireless Sensors
Wireless sensors need to be in use for the duration of their application, whether that may be in package shipping or in the detection of some unwanted compound, or controlling inventory in a warehouse. If the wireless sensor can’t transmit its data due to low or no battery power, the consequences could potentially be severe based on the application. Also, the wireless sensor must be adaptable to each application. Therefore the battery must be able to fit within the designed sensor. This means that the desired battery for these devices must be long-lasting, size specific, low cost, if they are going to be used in disposable technologies, and must meet the requirements of the data collection and transmission processes. Once again, thin film lithium ion batteries have shown the ability to meet all of these requirements.

Thinner Electronics
The reduction of the battery footprint can be the foothold to thinner and lighter electronics based on these thin film flexible lithium ion batteries. Since the batteries have such a high energy density in such a thin film, a thin film battery can replace a thicker, heavier, less energy dense battery in order to accomplish the same task. Today’s society is fast-moving, technology driven and ever interconnecting. These thinner, lighter electronic devices can help shape the future of the way we use and think about technology. With other technological advancements being made, the possibility of even smaller, thinner and lighter electronic devices than those currently found today is not as far away as was once thought. These developments may be the step that leads to some part of the technology seen in futuristic television shows and movies, like Avatar, for example.