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1、ESSCAP2008 Lithium Ion capacitorcharacterization and modelingAbstractTo develop electrical busses for applications with a fast recharge system in stations, PVI has been testing some electrical energy components like supercapacitors and batteries with high power density. Neverthless, supercapacitors
2、limit average autonomy between two recharge points, due to their poor energy density. On the other side cycle life of batteries are very dependent on current. To surmount these problems, PVI in collaboration with the FCLAB laboratory and the AMPERE laboratory, are studying Lithium-ion capacitor (LIC
3、) for applications with fast recharge.We take to assess how the storage system meets busses power and energy requirements in heavy electric vehicles. We note that the advantage of LIC technology compared to conventional supercapacitor lies in the fact that the energy density and the nominal voltage
4、are higher. In this study, the Li-ion capacitor is characterized and modelled. The characterization and modelling methods are the same of supercapacitor with double layer activated carbon technology. The LIC efficiency will be discussed.I. INTRODUCTIONLithium Ion capacitor is a new storage device wh
5、ich combines high power density and high energy density compared to conventional supercapacitor of the market. It has four time higher energy density than conventional supercapacitor. The structure of the LIC is composed by two electrodes. The positive one is formed by activated carbon as in double
6、layer capacitor. The negative electrode uses lithium ion doped carbon. This new electrode technology boosts the capacity of the negative electrode and increases the electrical potential difference.The electrolyte is based on the Li Ion.Figure 1 shows the elementary structure of EDLC and Li-ion capac
7、itor structure. It can be seen that the negative LIC electrode is formed by Li doped Carbone. The equivalent capacitance is formed by the positive electrodecapacitance Cdl in series with the negative one Cli. The equivalent capacitor can be expressed as following:（1）where Cli Cdl Ceq Cdl （2）Fig. 1:
8、Elementary structure of EDLC and Li-ion capacitor (JM Energy 1)The Li Ion capacitor studied in this paper ( figure 2) is fabricated by JM Energy. Their parameters are: nominal capacitance: 2000F; volume 124ml; weight: 208g, maximum operation voltage 3.8V and minimum voltage of 2.1V.Fig. 2: Lithium I
9、on capacitorII. LI ION CAPACITOR CHARACTERIZATIONA. DC characterizationLi-ion capacitor is charged and discharged under constant current constraints for several current values. Figure 3 represents the Li-ion capacitor voltage evolution as a function of time. The current of charge and discharge is fi
10、xed at 100A at ambient temperature; the device voltage varies between its nominal voltage 3.8V and 2.2V. Charge discharge result shows that voltage curves can be fitted with linear curve.The DC ESR and C measuring methods are based on the Li-ion capacitor discharge at constant current. At the first
11、approximation, Li-ion capacitor can be characterized by an equivalent series resistance and by an equivalent capacitance. The leakage current and the Li-ion capacitor inductive behavior can be neglected. The method used to determine ESR and C is presented in figure 4 and 5.Fig. 3: Li-ion capacitor v
12、oltage evolution during charge and discharge, the current is regulated at 100AThe equivalent capacitance value is determined from the Li-ion capacitor discharge and charge at constant current between Vmax = 3.8V and Vmin = 2.2V (figure 4). The equivalent capacitance value is calculated using the fol
13、lowing expression:(3)where I is the current of discharge, t = t1 and V=V2 - V1. Where V1 = Vmin + 40%*(Vmax - Vmin) and V2 = Vmin + 80%*(Vmax - Vmin).Fig. 4: Li-ion capacitor discharge at constant currentIn the equation (3), I is the current, t can be the duration of charge or discharge and V is the
14、 Li-ion capacitor voltage variation. Using this expression we have determined the equivalent apacitance of charge and discharge and for 50A, 100A, 150A and 200A. Table I gives the Li Ion capacitor of charge (Ceqch) and discharge (Ceqdis).TABLE I. EQUIVALENT CAPACITANCE OF CHARGE AND DISCHARGE FOR SE
15、VERAL CURRENT VALUESCurrent(A)Ceach(F)Ceqdis(F)Ceqdis/Ceqch502118210099%1002083200496%1502024195596%These results shows that Li-ion capacitor coulombic efficiency (Ceqdis/Ceqch) is very high (96%) compared with battery.For DC ESR measurement is based on the discharge at constant current (cf. dischar
16、ge voltage figure 5). The time between the end of charge and the start discharge is fixed at 30 mn this duration can be reduced because of the very low Li-ion capacitor self discharge compared with double layer capacitor. The ESR is calculated by using the following expression:(4)U3 is the drop voltage obtained from the intersection of the
