The Maximum Energy Density of Lithium Ion Capacitors

Abstract

Recently, a new type of supercapacitor–lithium (Li)-ion capacitor (LIC) has attracted much attention, because LIC has not only the advantages of traditional supercapacitor, such as high specific power (e.g. >10 kW/kg) and extremely long cycle life (e.g. >250,000 cycles), but also much higher specific energy (e.g. 10-25 Wh/kg) than that of traditional supercapacitors. In additional, LIC also has high cell voltage, low leakage current, etc.LIC has an asymmetrical structure and is composed of a Li intercalation-type anode electrode (Faradic) and an electric double-layer capacitor-type cathode electrode (non-Faradic), operating in a Li-ion containing electrolyte. The cathode material is activated carbon, and anode material includes graphite, hard carbon, soft carbon, or Li titanate. Compared with ordinary electrochemical energy storage cells, LIC has two following unique characteristics: 1) the anode electrode must be pre-lithiated using a highly concentrated Li source such as Li metal since both electrodes without pre-doped Li-ion. The pre-lithiation can also compensate the active Li loss at anode in the first several cycles and 2) the capacity of anode is much larger than the capacity of cathode (e.g. 7-10× for using hard carbon and 10-15× for using graphite as anode). It is due to the distinct energy storage mechanisms of cathode (non-Faradic) and anode (Faradic), the high-rate performance and cycling durability of anode are not comparable to cathode, resulting in low power density and poor cycling lifespan. Therefore, a general method to improve the power density and cycling lifespan is to construct an ‘unbalanced system’, i.e. the capacity of anode is designed to be higher than that of cathode, which enables the anode to be working at low areal current density and experiencing a shallow charge and discharge to match the rate and cycle life of activated carbon cathode.In this talk, we will present a mathematical formula as Eq. (1) for the theoretical specific energy of LICs for the first time. The theoretical specific energy is only based on “active” materials including cathode, anode, extra Li source, and the required electrolyte during the charge. Other “non-active” materials such as separator, current collectors, binders, tabs, and case can be counted into a value of package efficiency (e.g. 50%). In the specific energy equation, the denominator is the sum of four terms, in which, the 1st term has been normalized by the specific capacitance of cathode, the 2nd term represents capacity ratio between anode and cathode, the 3rd term represents the pre-lithiation, and the 4th term represents the electrolyte consumption when the LIC is at fully charge state. In order to understand the impact of some important parameters to the specific energy of LICs, we vary just one parameter at once and fix all other parameters to default values. They are: cp=100 F/g is the specific capacitance of cathode, ca=372 mAh/g is the specific capacity of anode, cLi=3,860 mAh/g is the specific capacity of extra Li source, Vmax=3.8 V is the maximum cell voltage, Vmin=2.2 is the minimum cell voltage, Vc-max=Vmax+0.2 V vs. Li/Li+ is the maximum potential of cathode, Vc-OCP=3.0 V vs Li/Li+ is the open-circuit potential of cathode, co=1 M/L is the Li salt concentration in electrolyte, g=10 is the capacity ratio between anode and cathode, b=100% is the degree of lithiation for anode, and F=96,485 C/M is the Faraday constant. Fig. 1 shows the specific energy as functions of six different parameters. It can be seen that (a) when the capacity ratio decreased from a default value of 10 to 1, the specific energy can be increased by 54%; (b) when the specific capacity of extra Li source is much greater than that of anode (e.g. cLi>1,000 mAh/g), the specific energy is close to a constant value; (c) when the specific capacitance of cathode is doubled from 100 to 200 F/g, the specific energy increases only 17.5% because the masses of anode and electrolyte are doubled too; (d) when the specific capacity of anode is doubled from 372 to 744 mAh/g, the specific energy will increases 21.7%, which is limited by low capacity of cathode; (e) the specific energy increase rate is less than voltage increase rate (or ∆ε/ε<2∆V/V) because masses of anode, electrolyte, and extra Li source increase proportional to the maximum voltage as shown in Eq. (1); and (f) the specific energy doesn’t increase with decreasing the minimum cell voltage from a default value of 2.2 V due to the mass of anode increases to match the increase of the cathode capacity. Figure 1