Capacity fade analysis of a battery/super capacitor hybrid and a battery under pulse loads ��� full cell studies RAJESWARI CHANDRASEKARAN, GODFREY SIKHA and BRANKO N. POPOV* Department of Chemical Engineering, University of South Carolina, Columbia, SC 29208, USA (*author for correspondence, fax: +803-777-8265, e-mail: popov@engr.sc.edu) Received 31 October 2004 accepted in revised form 20 April 2005 Key words: capacity fade, interfacial impedance, lithium ion battery/supercapacitor hybrid, pulse discharge amplitude, rate capability Abstract A detailed analysis of the capacity fade of a battery/supercapacitor hybrid and a battery alone has been carried out at 55 ��C by discharging them at three different pulse rates. The applied peak current amplitudes were 5C (7 A), 3C (4.2 A), and 1C (1.4 A), respectively. The results indicated that for hybrids the pulse discharge run time was extended for all pulse discharge rates. The ohmic resistances estimated as a function of the pulse discharge rates were smaller for hybrids when compared with those for batteries. The variation of the ohmic resistance under pulse discharge with cycling, irrespective of the pulse discharge rate was smaller for hybrids than that for the batteries. The batteries and hybrids cycled at the lowest pulse discharge rate (high pulse discharge time) have larger capacity fade when compared with the capacity fade of the batteries and the hybrids discharged using higher discharge rates (low pulse discharge time). Impedance, cyclic voltammogram, and the rate capability studies were carried out on batteries cycled alone and on batteries cycled as part of the hybrid. The battery showed larger increase in the interfacial impedance with cycling when compared with the hybrid system. 1. Introduction Hybrid energy storage devices are more efficient than a battery in supplying the total power for use in digital cellular phones, space communications, power distribu- tion systems, uninterrupted power supply, electric and hybrid vehicles, portable computers and military appli- cations [1]. The load in these systems is usually not a constant, but of the pulse type. A lithium ion battery has a high energy density of about 105 J/kg. However, it cannot meet high power demands when discharged at high currents. A lithium ion battery-super capacitor hybrid system is preferred over a lithium ion battery for higher rates of discharge due to the higher power density of an ultra capacitor ( 106 W/kg) compared to that of a lithium ion battery ( 100 W/kg). Also, since the inter- nal resistance of the super capacitor is smaller than that of the battery, the super capacitor shares the major part of the load current during high power demands, thus making the hybrid system more beneficial. Dougal et al. have analyzed the interrelations between battery, ultracapacitor and the load in terms of their power and energy partitions [1]. Performance optimiza- tion of a battery-capacitor hybrid system has been recently studied by Sikha and Popov [2]. Characteriza- tion of a lithium ion battery-super capacitor hybrid using impedance measurements were done by Chu and Braatz [3]. Comparison between a battery-capacitor hybrid and a battery under pulse loads using Ragone plots was done by Holland et al. [4]. Capacity fade analysis of lithium ion batteries at elevated temperatures under constant current discharge has been done by Ramadass et al. [5, 6]. This explains the high rate of capacity loss with cycling at temperatures above room temperature. High temperature operation of lithium ion batteries is ob- served in modern electronics where they are discharged under temperatures up to 60 ��C. In recent literature, experimental and theoretical work has been reported on the performance of lithium ion batteries under constant current discharge at room and elevated temperatures [7���9]. However, a detailed analysis of the capacity fade of the battery coupled with the capacitor under pulse type of loads has not been presented in literature. The objective of this work was to study the capacity fade of battery-super capacitor hybrid systems and single batteries cycled up to 400 cycles. The capacity fade of both systems was investigated by using three different discharge pulse protocols at an elevated tem- perature of 55 ��C. Capacity check, rate capability studies, electrochemical impedance analysis and cyclic voltammetric studies were also carried out for both hybrid and battery systems. Journal of Applied Electrochemistry (2005) 35:1005���1013 �� Springer 2005 DOI 10.1007/s10800-005-6728-8

2. Experimental 2.1. Full-cell studies The hybrid system in our experiments consisted of a single Sony US 18650 lithium ion cell with a rated capacity of 1500 mA h in parallel with a set of Maxwell PC super capacitors with an effective capacitance of 50 F. Two 100 F capacitors were connected in series to obtain a 50 F capacitance with a capacitor configuration index [2] of 0.5. Clubbing the two capacitors in series approximately gives the equilibrium potential of a single lithium ion battery. The hybrids were cycled at an elevated temperature of 55 ��C. The hybrid system was charged to 4.2 V using the conventional constant current ��� constant voltage (CC- CV) protocol. Constant current charging was carried out at 0.7 A or C/2 rate and subsequently the voltage was held constant at 4.2 V until the current dropped to 50 mA. The hybrids were discharged at three different pulse rates with peak current amplitudes of 5C (7 A), 3C (4.2 A), and 1C (1.4 A), respectively. Here C refers to the capacity of the hybrid system and is equal to 1.4 A h for our system. The capacity of the capacitor is almost negligible. The duty ratio of the discharge pulse and the pulse frequency were fixed at 0.1 and 1 Hz, respectively. The cut-off voltage on discharge for the lithium ion battery in this study is 2.5 V. Below this voltage there is no intercalated lithium available at the carbon electrode for discharge. The same set of protocols were applied to the battery alone systems. An Arbin BT-200 battery cycler was employed for cycling, rate capability and cyclic voltammetry studies. The high cycling temperature of 55 ��C was maintained using a Tenny Model T6S environmental chamber. A Solatron SI 1255 HF Frequency Response Analyzer and Potentiostat/Galvanostat Model 273A were used for the electrochemical impedance studies for both the fresh and the cycled Sony 18650 cells. Capacity checks were done for both the fresh and cycled cells which were part of the hybrid by charging them using the CC-CV protocol as mentioned before, followed by a constant current discharge at C/2 rate. All capacity checks were done at room temperature, and for the hybrid systems the battery was separated from the capacitor before the capacity check was done. This was done to make sure that the capacity fade of the battery and the hybrid systems were compared based on the degradation of the battery in both systems. Rate capability and cyclic voltammetry studies were carried out for both the fresh and the cycled lithium ion cells used in the hybrid system. Rate capability analysis was done by charging the lithium ion cells, both fresh and those cycled as part of the hybrids, using a CC-CV protocol and then discharging at constant currents of C/8, C/4, C/2 and C rates. Cyclic voltammetric studies were carried out at both low and high scan rates of 0.05 and 0.2 mV s)1, respectively. Electrochemical Imped- ance Spectroscopy (EIS) was done for both the fully charged and discharged states of the battery. A sinusoi- dal wave perturbation with amplitude of 5 mV was applied over a frequency range of 0.1 Hz���0.1 MHz to obtain the impedance spectra. For comparison, the above outlined experiments were also carried out on a Sony US 18650 lithium ion battery alone system, both fresh and cycled under the same protocol. The ohmic resistance at each pulse was calculated by dividing the voltage drop at that pulse by the amplitude of the pulse discharge current. Since there is not a large variation in the ohmic resistance at each pulse during an entire discharge at a given pulse rate, the average of the ohmic resistance during an entire pulse discharge at each rate is used in our studies. 3. Results and discussion 3.1. Discharge characteristics Table 1 gives the percentage capacity fade at different cycle numbers for both the battery and hybrid cycled at three different pulse discharge rates. As shown in Table 1 at 200 cycles, the battery in the hybrid always had higher capacity fade than the battery alone system irrespective of the discharge rate used. The systems cycled at higher pulse discharge rates showed lesser capacity fade than those cycled at lower pulse discharge rates, irrespective of whether it is a battery or a hybrid system. However with further cycling, the hybrids which were cycled at low pulse discharge rates show better performance when compared to that of the battery alone system. The capacity fade of the 1C battery after 300 cycles is 45.5%, while the highest capacity fade after 400 cycles of the battery in the 1C hybrid is 47.7%. Since the rate of capacity fade of 1C battery is the highest, its capacity fade at 400 cycles will be higher than that of the battery in the 1C hybrid at the same cycle number. The capacity Table 1. Capacity fade (%) of the battery in the hybrid and the battery under pulse discharge Cycle number Capacity fade (%) Battery (pulse discharge rate) Battery in the hybrid (pulse discharge rate) 1C 3C 5C 1C 3C 5C 100 11.232 2.527 1.126 15.696 11.332 10.288 200 27.345 8.304 4.383 27.808 16.275 14.897 400 45.484 (at 300 cycles) 26.506 9.087 47.724 30.153 35.282 1006