**Abstract**

Thin-film batteries are solid-state batteries comprising the anode, the cathode, the electrolyte and the separator. They are nano-millimeter-sized batteries made of solid electrodes and solid electrolytes. The need for lightweight, higher energy density and long-lasting batteries has made research in this area inevitable. This battery finds application in consumer electronics, wireless sensors, smart cards medical devices, memory backup power, energy storage for solar cells, etc. This chapter discussed different types of thin-film battery technology, fundamentals and deposition processes. Also discussed in this chapter include the mechanism of thin-film batteries, their operation and the advantages of thin-film batteries over other batteries. The vast applications of thin-film batteries drive research in this area. These applications are discussed extensively.

**Keywords:** batteries, deposition, electrodes, seperator, solid state, substrate, thin films

## **1. Introduction**

Thin-film batteries are nano- to millimeter-sized solid-state batteries comprising the anode, the cathode, the electrolyte and the separator. The anode is the negative electrode that is oxidized after giving up electrons to the external circuit. It is the anode that generates ions that move through the electrolyte. The cathode is the positive electrode that accepts electrons from the external circuit and is reduced in the process. During the charging and discharging process, ions are inserted into and extracted from the cathode. The electrolyte is the medium for charge transfer between the cathode and the anode. Thin-film electrolyte is usually chemically stable, ionically conductive and electrically insulating and is required also to build good contact with the cathode and anode surfaces. The separator prevents physical contact between the anode and the cathode without blocking the transport of ions. Most times in thin-film batteries, the solid electrolyte acts both as an ion transport medium and physically separates the cathode and the anode as shown in **Figure 1**.

**Figure 1.**

*Thin-film batteries with a solid-state cathode, anode, and electrolyte.*

Thin-film batteries are manufactured using physical and chemical deposition techniques [1]. They include magnetron sputtering, pulsed laser deposition, molecular layer deposition [2], atomic layer deposition, vacuum evaporation [3], thermal evaporation, electron beam and sputtering [4]. These techniques follow four pathways called thin-film battery technologies [5].

The mechanism of the thin-film batteries is that ions migrate from the cathode to the anode charging and storing absorbed energy and migrating back to the cathode from the anode during discharge and thereby releasing energy [6].

The recent research in and development of smarter societies have necessitated the integration of smart devices with improved safety, specific energy, power and reduced-size materials [3]. This has given rise to the demand for using thin-film rechargeable batteries for electrical energy storage with good energy and power densities, excellent mechanical strength, good and long cycle life and appreciable temperature tolerance for small portable consumer electronics, especially in cell phones, laptops and notebook computers, smart cards, mobile applications, for electric cars, communication and other electrical equipment [7].

This book chapter reviews the fundamentals of thin-film batteries and the use of these batteries in various applications.
