Elsevier

Journal of Power Sources

Volume 138, Issues 1–2, 15 November 2004, Pages 360-363
Journal of Power Sources

Short communication
Fabrication and characterization of a LiCoO2 battery–supercapacitor combination for a high-pulse power system

https://doi.org/10.1016/j.jpowsour.2004.06.050Get rights and content

Abstract

The performance of portable electronic equipment can often be improved by including an electrochemical capacitor alongside the battery. The capacitor extends battery life by reducing its peak output power. A CR2032 coin-type battery cell was used for the LiCoO2 cathode and Li anode. A mixture of 1 M LiPF6-ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 vol, Merk) was used as the electrolyte.

The electrochemical characteristics of a supercapacitor are able to provide a much higher pulse current capability than the battery system. By combining a supercapacitor and a battery, the pulse performance of a battery can be significantly improved according to various pulse times.

Introduction

Recently, lithium-ion batteries have been applied in a wide range of uses including portable computers and mobile telephones just to name a few. Phone batteries must be able to supply repetitive high-current pulses, perhaps up to a 10 C discharge rate. This can severely stress many of the battery systems, particularly during operation at low temperatures. While batteries store a lot of energy and release the stored charge stably, supercapacitors deliver a high current, but only for a very short time. That is, although supercapacitors release their peak current when the devices begin operation, batteries give off a stable current while in use. The performance of portable electronic equipment can often be improved by including a supercapacitor alongside the battery. The capacitor extends a battery's life by reducing its peak output power. A supercapacitor can provide high power density as well as sufficient energy density. A Ragone plot is shown in Fig. 1. Clearly, the supercapacitor offers the best combination of power and energy density. Furthermore, by combining the best features of the battery and the supercapacitor, a superior system may result.

At a high discharge rate, such as the pulse current, the heat generated inside a battery is significant. This not only reduces the efficiency of the electrical energy generation, but also causes deleterious effects (i.e. shortened cycle life and increased internal resistance). With an electrochemical capacitor, it would be expected that only a minimal heat is generated from I2R. Clearly, electrochemical capacitors should possess a high efficiency even at a rapid rate of discharge. Therefore, for pulse power applications, the electrochemical capacitor should help the battery to deliver higher power in a more efficient manner [1].

In this experiment, we used a LiCoO2 battery–supercapacitor combination. LiCoO2, heat-treated above 700 °C (HT-LiCoO2), has a high cycle life compared to other cathode materials, such as LiNiO2 and LiMn2O4. The lattice is formed by oxygen atoms in ABC stacking with alternating layers of Li and Co ions in octahedral interstitial sites between the oxygen planes [2], [3], [4], [5]. The combination of salt encapsulation with an earlier proposed powder engineering technique [6], [7] ensures fine particle size control of LiCoO2 powders obtained using wet chemical methods [8].

Section snippets

Details of experiment

An aqueous solution of Li and Co acetates (Li/Co = 1.1) was frozen by spraying on liquid nitrogen followed by freeze–drying for two days at P = 5 × 10−2 mbar (Alpha 2-4, Christ). A part of the freeze–dried product was mixed with K2SO4 (1:10) and subjected to planetary milling (Pulverisette-5, Fritsch) in ZrO2 media at 600 rpm for 24 h (ball to powder mass ratio 10:1). Thermal decomposition of a precursor and of a precursor mixture with K2SO4 was performed in air first at 400 °C for 10 h, then at 800 

Results and discussion

SEM analysis of the LiCoO2 powders, obtained by matrix isolation of precursor particles by K2SO4, confirmed the efficiency of this method in terms of grain coarsening prevention. Commercial Seimi Co. powder (powder A) formed micron-sized crystallites (Fig. 2 A), while K2SO4-processed powder (powder B) was characterized by a grain size distribution in a range of 30–70 nm (Fig. 2B).

XRD analysis of micron-sized (powder A) and nanocrystalline powders (powder B) revealed the formation of single phase

Conclusion

The electrochemical characteristics of the supercapacitor can provide a much higher pulse current capability than a battery system. By combining a supercapacitor with a battery, the pulse performance can be significantly improved for various pulse times. The better pulse performance of nanocrystalline powder is due to the smaller particle size. Shorter diffusion distances promote faster and uniform Li intercalation into LiCoO2 crystallites during the discharge process compared to that of

Acknowledgements

We would like to express our appreciation for R&D support provided by the Korean government's National Research Laboratory Program (development of monolithic high power hybrid battery).

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