# A Review on Nanocomposite Materials for
## 1 Introduction
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Properties**} & \multicolumn{4}{c}{**Rechargeable Battery Chemistry**} \\ \cline{2-5}  & **Li-ion** & **Lead Acid** & **NiMH** & **Ni-Cd** \\ \hline Specific energy density (Wh/kg) & 90–190 & 30–55 & 60–120 & 45–80 \\ Cycle life (80\% of initial capacity) & 500–2000 & 200–300 & 300–500 & 1000 \\ Cell voltage (V) & 3.3–3.8 & 2.0 & 1.2 & 1.2 \\ Self-discharge/month at 25 °C & \textless{5\%} & 5–15\% & 30\% & 20\% \\ Safety & Protection circuit is mandatory & & Thermally stable & \\ Commercial in & 1991 & 1881 & 1990 & 1950 \\ Toxicity & Low & High & Low & High \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of different battery chemistries [5, 6]. Figure 1: Market share of rechargeable battery chemistries in 2009 [3]. There are two ways to improve the energy density of the secondary batteries: (i) by introducing new battery chemistries; and (ii) by employing enhanced battery designs and manufacturing processes (i.e., engineering improvements) to reduce the weight and volume of the inactive components in the existing battery chemistries. A closer look at Figure 2 shows that a quantum leap in energy density of secondary batteries was usually achieved when new battery chemistries were introduced. Therefore, it is critical to develop advanced chemistries and materials to facilitate the higher market penetration of secondary batteries. The U.S. DOE \"EV Everywhere\" initiative was launched in 2012, and the energy storage targets for battery systems identified by this initiative are given in Table 2[8]. Although significant progress has been made in improving the cost and energy density of batteries, it is estimated the targets can only be achieved by utilizing advanced Li-ion chemistries and materials. Among the targets listed in Table 2, the cost target for the Li-ion batteries is critical to make PHEV and EV cost competitive with the gasoline powered cars. As shown in Figure 3, cost of Li-ion battery packs has significantly dropped during the last 10 years, mostly due to efforts by the EV market leaders such as Tesla (i.e., Model S) and Nissan (i.e., Leaf). The most recent cost estimate in 2016 for Li-ion battery packs is \(\sim\)$190/kWh indicating that the U.S. DOE cost target for battery energy systems for 2022 is well within reach [9]. Figure 3: Cost of Li-ion battery packs in EV. Adapted with permission from [10], © Nature Publishing Group, 2015. Figure 2: Increase in the energy density of the secondary batteries since 1900s. Adapted with permission from [7], © Royal Society of Chemistry, 2011. The operation of a secondary battery relies on various interfacial processes at the nanoscale; therefore, manipulating the material properties at the nanoscale is essential for developing advanced materials. The most common strategies to improve the battery materials are particle size reduction, morphology control, composite formation, doping, functionalization, coating, encapsulation and electrolyte modification [11]. The battery electrodes (i.e., anode and cathode) that determine the overall performance of a battery are composite materials. A typical composite battery electrode includes active material, conductive carbon additive and a binder. Each of these components is vital to the operation of a rechargeable battery, and their performances can be improved by utilizing nanocomposites. In this review article, the basics of rechargeable batteries and the key developments in nanocomposite materials are discussed with a focus on Li-ion batteries due to their technological significance. Specifically, the nanocomposite-based developments in cathode, anode, binder and separator materials for Li-ion batteries are discussed in detail. Readers can also benefit from the following review articles based exclusively on cathodes [12; 13], anodes [14; 15], binders [16; 17] and separators [18].