# A Review on Nanocomposite Materials for
## 2 Basics of rechargeable Batteries  
A rechargeable battery or a secondary battery is a type of battery that can be discharged and charged (i.e., cycled) multiple times over its lifetime as opposed to a primary battery, which can only be discharged one time only before being discarded. The smallest packaged unit of a battery is the cell. Most applications require higher voltages (i.e., PHEV/EV battery packs are in the range of 100-400 V, SLI batteries are \(\sim\)12 V) than a single cell (i.e., \(\sim\)3-4 V for Li-ion) can provide; therefore, multiple cells are assembled into modules (i.e., in a typical EV battery module there are 4 cells in a 2 series and 2 parallel formation) in parallel or in series configuration depending on the application. Battery pack is the final product formed by connecting multiple modules together. A battery pack is custom designed for a specific application and includes battery management system, sensors for voltage, current, and temperature measurements, cooling system in addition to battery modules. A typical Li-ion rechargeable battery cell is composed of a cathode (positive electrode), an anode (negative electrode), a separator filled with an electrolyte and the current collectors for both anode and cathode. The electrodes (i.e., cathode and anode) in Li-ion batteries are composite materials, which include active materials (i.e., typically, metal oxides for cathode and graphite for anode) for storing Li ions, conductive carbon additive to improve the electronic conductivity and a binder to keep the active materials together and attached to the current collector. The cell components that are not involved in Li storage are called the inactive materials (i.e., current collector, separator, electrolyte, binder and carbon additive). The inactive components do not directly contribute to the cell capacity so it is critical to minimize their content; however, the inactive components play a remarkable role in the operation and safety of a battery. The cross-sectional view of a cylindrical Li-ion battery is given in Figure 4. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Metrics** & **Status in 2012** & **Target for 2022** \\ \hline Battery cost (\$/kWh) & 500 & 125 \\ Pack specific energy (Wh/kg) & 80–100 & 250 \\ Pack energy density (Wh/L) & 200 & 400 \\ Pack specific power (W/kg) & 500 & 2000 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The U.S. DOE “EV Everywhere” energy storage targets for battery systems [8]. The energy and power characteristics of an electrochemical cell can be determined by utilizing thermodynamic and kinetics relations. The net useful energy of an electrochemical cell can be calculated by the following thermodynamic equation [20]:  
\[\Delta G=-nFE \tag{1}\]  
where \(\Delta G\) is the Gibbs free energy, \(n\) is the number of electrons transferred per mole of reactants, \(F\) is the Faraday constant, and \(E\) is the cell potential. The Equation (1) implies that, the higher the cell potential (i.e., \(E\)) the higher the capacity of an electrochemical cell assuming number of electrons transferred per mole of reactants are the same (i.e., this is the case for different Li-ion chemistries such as LiCoO\({}_{2}\) and LiFePO\({}_{4}\)). To maximize the useful energy of an electrochemical cell, the difference between the cathode and anode potentials (i.e., open circuit voltage, \(E_{\text{OCV}}\)) should be as large as possible, and this is the main reason behind the research efforts on developing high voltage batteries [21]. Although the thermodynamic relations are useful in understanding the limits of an electrochemical cell, the practical batteries do not operate under equilibrium conditions hence deviate from thermodynamic equilibrium condition when current is withdrawn due to kinetic limitations (i.e., polarization effects, \(\eta\)).