**4. Mechanism**

Solid-state thin-film batteries have solid components for the electrodes (cathode and anode) and the electrolyte. They are made by stacking a thin-film electrolyte on the cathode and anode in a vacuum state as shown in **Figure 1**. The principal operation of thin-film batteries works in the same way rechargeable batteries work. The lithium ions migrate from the cathode to the anode and generate electrical energy when the battery is charged and stored in a current collector. During discharge, the lithium ions move from the anode back to the cathode, the electrons move from the cathode to the anode and a current between the cathode and the anode is created as a result of the potential difference between the electrodes [6].

### **5. Fundamentals of thin-film batteries**

Electrodes (both positive and negative) and electrolytes make up the bulk of a current battery's physical components. From an electrical engineering perspective, their properties can be deduced from the first principles. A chemical reaction is what gives a battery its ability to provide electrical energy (the battery converts chemical energy to electrical energy) [67]. Electrodes are required to conduct electrons for electrical energy to be realized in the external circuit. On the other hand, the electrolyte need not. Because if it did, the battery's internals would be accessible to the electrons, rather than just the external circuit, leading to self-discharge and, ultimately, no usable energy at the battery terminals [68]. There is no such thing as an "exceptional" all-solid-state thin-film battery (ASSTFB) [69]. A thin film version of the same three components is used in an ASSTFB, as shown in **Figure 1**.

According to A.G. Buyers [70], the potential energy measured between electrodes for any electrochemical cell, as depicted in **Figure 3**, is the difference in the standard free energy as defined in Eq. (1).

$$
\Delta G = -nFE \tag{1}
$$

where ΔG = change in free energy, *n* = number of electrochemical equivalents transferred in the cell reaction, E = measured voltage (EMF) and *F* = Faraday constant.

For a material to conduct electricity, the charge must be transferred from one location to another. This charge transfer can take the form of electrons, holes (the lack of an electron), ions or ion vacancies [71]. Interstitial hopping mechanisms, such as the Frenkel defect reaction [67], allow metal ions such as Li + to migrate as defined in Eq. (2).

$$\mathbf{M}\_m^\mathbf{x} + \mathbf{V}\_i^\mathbf{x} \to \mathbf{M}\_i^\bullet + \mathbf{V}\_m^\prime \tag{2}$$

Metal ions like Li continue to occupy the same site (denoted by *M<sup>x</sup> <sup>m</sup>*), while vacated sites (denoted by *V<sup>x</sup> <sup>i</sup>* ), newly occupied sites (denoted by *M*• *<sup>i</sup>*) and vacated sites (denoted by *V*<sup>0</sup> *<sup>m</sup>*) are created. Neutral, positive and negative charges are represented by the superscripts x, • and <sup>0</sup> . It follows that the electrolytes need to have enough charge carriers for proper ion movement, including that of lithium ions. As a result, according to the definition in (3), ionic conductivity, *σ*, is proportional to the density of the charge carrier, c.

$$
\sigma = q \propto \mu \propto c \tag{3}
$$

where q = charge of the ions and *μ* = mobility of ions according to the Nernst-Einstein equation, which is defined in Eq. (4).

$$
\mu = \frac{D \propto q}{kT} \tag{4}
$$

From (4) and according to constable [71], it is determined that diffusivities, *D*, fluctuate with temperature as a thermally activated Boltzmann process defined in Eq. (5)

$$D = D\_o e^{-E\_d/kT} \tag{5}$$

where k, T and *Ed* represent the Boltzmann constant, absolute temperature, and diffusivity activation energy, respectively. The interstices should have enough space for the Li + ions to hop around. Metal ion conductivity, in this case, Li+, *σ*, can be calculated using Fick's second law and the Nernst-Einstein equation.

$$
\sigma T = \sigma\_o e^{-E\_d/kT} \tag{6}
$$

*σ<sup>o</sup>* is the pre-exponential constant.

Charge transfer occurs at the interface between the electrodes (cathode and anode) and the electrolyte simultaneously with ion migration through the electrolyte. Therefore, the electrode needs to be highly conductive not only in terms of ions but also electrons. Unfortunately, the electronic conductivity of bare electrode materials is quite low. To provide reasonably good electronic conductivity, some conductive materials such as carbon and carbon derivatives should be incorporated with the electrode materials, and also electrode particles sometimes are coated with carbon layers.

## **6. Advantages over other battery types**

Thin-film lithium-ion batteries offer improved performance due to their higher average output voltage, lighter weights, higher energy density, long cycling life (1200 cycles without degradation) and ability to operate in a wider temperature range (between �20 and 60°C) when compared with the standard lithium-ion batteries [72, 73].

Lithium-ion transfer cells stand out as the most promising systems for meeting the need for high specific energy and high power at a low manufacturing cost [74].

Each electrode in a thin-film lithium-ion battery can accept lithium ions in either direction, creating a Li-ion transfer cell. The components of a battery, including the anode, solid electrolyte, cathode and current leads, must be fabricated into multi-layered thin films using the appropriate technologies to build a thin-film battery [75, 76].

The electrolyte in a thin-film-based system is often a solid electrolyte that can take on the form of a battery. This differs from traditional lithium-ion batteries, which typically use a liquid electrolyte [77]. If the liquid electrolyte is not suitable for use with the separator, its use can be complicated. As a general rule, liquid electrolytes necessitate a larger battery, which is not ideal when trying to get a high energy density in the final product.

Polymer electrolytes, which are commonly used in thin-film flexible Li-ion batteries, can serve multiple functions, including those of electrolyte, separator and binder. Since the problem of electrolyte leakage is thus avoided, flexible systems can be built [78].

Finally, unlike traditional liquid lithium-ion batteries, solid systems can be packed together densely to maximize energy density. Thin-film batteries production have the advantage of high energy densities [79].
