# A Review on Nanocomposite Materials for
## 1 Introduction  
Rechargeable (rechargeable battery and secondary battery terms are used interchangeably throughout this article) batteries are one of the key technologies in today's information-rich and mobile society. For a long time, the most important application for rechargeable batteries has been the starter-ignition-lighting (SLI) batteries in motor vehicles. With the rapid market penetration of the portable electronics such as laptops, smartphones and tablets since 2000s, the rechargeable battery market significantly expanded. More recently, interest in plug-in hybrid electric vehicles (PHEV) and electric vehicles (EV) has been driving the further expansion of the rechargeable battery market, specifically Li-ion batteries. Additionally, large-scale utilization of rechargeable batteries for peak shaving and load leveling applications in the power grid has been envisioned by many, but is still in very early phases [1]. Apart from the aforementioned applications, rechargeable batteries are also critical for e-bikes and power tools [2]. Considering the importance of the rechargeable batteries in a variety of applications, the future of these batteries looks brighter than ever. Rechargeable batteries comprise three quarters of the global battery market, and the most important rechargeable battery chemistries are lithium-ion (Li-ion), lead acid, nickel-metal-hydride (NiMH) and nickel-cadmium (Ni-Cd) as shown in Figure 1[3]. Li-ion and lead acid batteries dominate the market, and Li-ion is the fastest-growing market. The lead acid batteries are mostly utilized in the SLI, stationary (i.e., back-up power) and deep cycle applications, whereas Li-ion batteries are preferred in portable electronics. In recent years, Li-ion chemistry is making inroads to the lead acid market (i.e., SLI batteries), but the lead acid battery market still continues to grow due to its low cost and maturity [4]. The NiMH and Ni-Cd batteries have much smaller market share compared to lead acid and Li-ion batteries, and their share continues to shrink because Li-ion batteries are rapidly replacing these types of batteries. The NiMH batteries are still utilized in some hybrid electric vehicles (HEV) such as the Toyota Prius, while Ni-Cd batteries are employed in power tools due to their high rate capability. The introduction of the Li-ion chemistries with high rate capability such as LiFePO\({}_{4}\) significantly dropped the market share of the Ni-Cd batteries. The popularity of Li-ion chemistry is associated with its high gravimetric and volumetric energy density as well as the significant cost reduction during the last decade. The main characteristics of the most common battery chemistries are summarized in Table 1[5, 6]. Despite the significant growth of the rechargeable battery market since 2000s, further improvements in energy density and cost reduction are required for EV and grid level energy storage applications. The progress in rechargeable battery technologies is rather slow, the energy density of the rechargeable batteries has only increased 3% per year on average during the last 60 years, and the rate of energy density increase has been slightly higher since the introduction of Li-ion batteries in 1991 as seen in Figure 2[7]. However, the current rate of increase (i.e., 5-8%) in the energy density of secondary batteries is not sufficient to meet the targets set by the United States (U.S.) Department of Energy (DOE) [8].