Electrochemical performance of lithium/sulfur batteries with protected Li anodes

doi:10.1016/S0378-7753(03)00300-8

Journal of Power Sources

Volumes 119–121, 1 June 2003, Pages 964-972

https://doi.org/10.1016/S0378-7753(03)00300-8 Get rights and content

Abstract

The protection layer was introduced to the surface of the Li anode to enhance the charge/discharge performance of lithium/sulfur batteries. The protection layer was formed by a cross-linking reaction of the curable monomer in the presence of liquid electrolyte and a photoinitiator. When the Li anode is coated with the protection layer, the unit cells with a liquid electrolyte showed an enhanced charge/discharge performance as compared to cells based on a polymer electrolyte, resulting in an average discharge capacity of 270 mAh/g-cathode during 100 cycles. All the charge/discharge tests were performed at room temperatures.

Introduction

Lithium ion batteries and lithium ion polymer batteries have been under intense research and development over the past 10 years due to their advantages such as high energy density, high operating voltage, low self-discharge rate, and no memory effect. However, there is still a need to enhance the capacity of the cathode in order to meet the performance needs of the new energy devices requiring high capacity. In fact, there is a limitation to the extent of lithium intercalation into transition metal oxides, which stimulates the research on lithium/sulfur rechargeable batteries.

Lithium/sulfur rechargeable batteries, which use sulfur as a cathode and Li as an anode, are very attractive for rechargeable lithium batteries due to their high theoretical specific capacity of 1675 mAh/g-active material, high theoretical energy density of 2600 Wh/kg, and low cost. The operating voltage of the lithium/sulfur battery, 2.1 V, is suitable for low-voltage electronic devices. In spite of these advantages of lithium/sulfur batteries, there are still many problems to be overcome for practical applications. Since sulfur is an insulating material, the electronically conducting phase and the lithium ion-conducting phase in the sulfur cathode must be uniformly distributed. For lithium/sulfur batteries, lithium metal is used as the anode, i.e. as the lithium source to provide a high energy density. However, lithium is so reactive that it usually results in poor charge/discharge cycling efficiencies due to severe growth of the SEI layer [1], [2], [3], [4], [5], [6]. For this reason, modification of the surface of the Li anode has been previously studied. For instance, Ogumi and co-workers [7], [8] generated the protection layer on the Li anode by plasma polymerization, and Osaka et al. [9], [10] induced the formation of a Li₂CO₃ layer on the surface of the Li anode by exposing the electrode to carbon dioxide. PolyPlus Co. [11] presented research on glass electrolytes sputtered on the Li anode and applied them to lithium/sulfur batteries.

In this work, the protection layer on the Li anode was newly prepared by a UV cured polymerization method. The protected Li anode was introduced to the lithium/sulfur battery to enhance the charge/discharge performance by reducing the growth of the SEI layer and suppressing the reaction between the Li and soluble polysulfides. The effect of the type of electrolyte on the charge/discharge performance was also investigated.

Section snippets

Preparation of lithium and protected lithium anodes

The Li anode was prepared by laminating Li foil on a Cu current collector in a glove box. A curable, mixed solution consisting of the monomer (poly(ethylene glycol) dimethacrylate), liquid electrolyte (150–300 wt.% of the matrix polymer), and photoinitiator (methyl benzoylformate) was used to make the protection layer on the Li anode by the UV curing method. The thickness of the protection layer was about 10 μm.

Preparation of cathode

Sulfur (99.98%, Aldrich) and Super-P (MMM carbon) as a conducting agent were dried at

Ionic conductivity

The ionic conductivities of the plasticized polymer electrolytes based on P(VdF-co-HFP) as a function of salt concentration are listed in Table 1. The EO:Li ratio refers to the moles of the ethylene oxide repeating unit per mole of lithium ion in the liquid electrolyte. The maximum ionic conductivity at room temperature, 6.5×10⁻⁴ S/cm, was achieved when the EO:Li ratio is 16:1. So, the plasticized polymer electrolyte with an EO:Li ratio of 16:1 was used to assemble the unit cells.

Charge/discharge characteristics of unit cells

Fig. 1a shows

Conclusions

A protected Li anode was introduced to lithium/sulfur batteries to enhance the cycle performance. The unit cells based on the protected Li anode and liquid electrolyte showed a very stable discharge capacity up to 100 cycles, which seemed to be due to the formation of a stable SEI layer. The average discharge capacity of the unit cell was 270 mAh/g-cathode. The protection layer on the Li anode could also suppress the overcharge during the charge process.

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Electrochemical performance of lithium/sulfur batteries with protected Li anodes

Abstract

Introduction

Section snippets

Preparation of lithium and protected lithium anodes

Preparation of cathode

Ionic conductivity

Charge/discharge characteristics of unit cells

Conclusions

J. Power Sources

J. Power Sources

J. Power Sources

J. Power Sources

Electrochim. Acta