The thin-film lithium-ion battery is a form of solid-state battery. Its development is motivated by the prospect of combining the advantages of solid-state batteries with the advantages of thin-film manufacturing processes. Thin-film construction could lead to improvements in specific energy, energy density, and power density on top of the gains from using a solid electrolyte. It allows for flexible cells only a few microns thick. It may also reduce manufacturing costs from scalable roll-to-roll processing and even allow for the use of cheap materials.
Background Lithium-ion batteries store chemical energy in reactive chemicals at the anodes and cathodes of a cell. Typically, anodes and cathodes exchange lithium (Li+) ions through a fluid electrolyte that passes through a porous separator which prevents direct contact between the anode and cathode. Such contact would lead to an internal short circuit and a potentially hazardous uncontrolled reaction. Electric current is usually carried by conductive collectors at the anodes and cathodes to and from the negative and positive terminals of the cell (respectively). In a thin-film lithium battery the electrolyte is solid and the other components are deposited in layers on a substrate. In some designs, the solid electrolyte also serves as a separator.
Components of thin film battery
Cathode materials Cathode materials in thin-film lithium-ion batteries are the same as 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 (Voc), and energy densities (DE).
Deposition methods for cathode materials There are various methods being used to deposit thin-film cathode materials onto the current collector.
Pulsed laser deposition In pulsed laser deposition, materials are fabricated by controlling 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 In chemical vapor deposition, volatile precursor materials are 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 glassy 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. LiBON, lithium boron oxynitride, is another amorphous glassy material used as a solid electrolyte material in thin-film flexible batteries. 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 not block the transport of lithium ions while preventing the physical contact of the anode and cathode materials, e.g. short-circuiting. In a liquid cell, this separator would be a porous glass or polymer mesh that allows ion transport via the liquid electrolyte through the pores, but keeps the electrodes from contacting and shorting. However, in a thin-film battery the electrolyte is a solid, which conveniently satisfies both the ion transportation and the physical separation requirements without the need for a dedicated separator.
Current collector Current collectors in thin-film batteries must be flexible, have high surface area, and be 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.
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