8 - Hybrid energy storage devices: Li-ion and Na-ion capacitors

Abstract

To accelerate any electric vehicle or electric motor a high power with high energy density-based energy storage system is required. Secondary batteries (Li-ion) (energy density of 130–250Whkg−1 and power density of <1200Wkg−1) and electrochemical capacitors (energy density: <15Whkg−1 and power density: >20,000Wkg−1) are incapable to fulfill the requirement of high energy density and high-power density in a single system. For low power consumption devices (laptops, smartphones, tablets, power backup systems, etc.) the secondary batteries are suitable and acceptable but not for high power consumption applications (e-vehicles, bikes, power tools, etc.). To overcome this, researchers look forward to making a new device by which both high energy density and power density can be achieved. A hybrid energy storage system (HESS) is the coupling of two or more energy storage technologies in a single device. In HESS a battery type of electrode is used in which the redox process is followed. On the other side capacitor type of electrode material is used in which a double layer is formed during the process. HESS is the alternative and trade between supercapacitors and batteries. HESS devices show sufficient energy and power densities, self-discharge rate, efficiency, lifetime, etc. Lithium-ion, sodium-ion, potassium-ion, etc., based batteries, capacitors, and fuel cells are frequently used as HESS. An arrangement of transition-metal and carbon-based materials are a well-known combination of hybrid systems. These systems offer possibilities to develop highly-efficient electrodes. Lithium-ion-based hybrid batteries are already commercialized for the e-vehicles by the Nissan motor corporation, Tesla Model S and X, BMW iX3, etc. In this chapter, the Na-ion and Li-ion-based hybrid energy storage devices will be discussed. The used electrode materials for hybrid energy storage systems and some basic understanding of electrochemical energy storage devices will be discussed.