Improved performance of Li-S battery with hybrid electrolyte by interface modification

doi:10.1016/j.ssi.2016.11.001

Solid State Ionics

Volume 300, February 2017, Pages 67-72

https://doi.org/10.1016/j.ssi.2016.11.001 Get rights and content

Highlights

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Li-S battery with hybrid electrolytes is of free shuttle effect.
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A carbon coating layer is one-sided introduced onto the solid state electrolyte.
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The modified cell demonstrates a high initial discharge capacity of 1409 mA h g^− 1 and a high reversibility.

Abstract

A carbon coating layer facing sulfur cathode is introduced on one side of the solid state ionic conductor in the ceramic-liquid hybrid electrolyte based Li-S cell. Both the cycling performance and the rate capability are improved as a result of enhanced electron transport for the conversion of the dissolved sulfur-containing active materials as well as the wettability of the ceramic electrolyte towards the liquid electrolyte. The interface modified hybrid Li-S cell exhibits an initial discharge capacity of 1409 mA h g^− 1 and remains at 1000 mA h g^− 1 which is 235 mA h g^− 1 higher than the pristine one, after 50 cycles at 0.2C rate.

Introduction

Lithium metal is one of the most promising candidates as an anode material for next-generation energy storage systems due to its high specific capacity (3860 mA h g^− 1) and the low negative electrochemical potential (− 3.04 V versus the standard hydrogen electrode) [1], [2]. When coupled with a sulfur cathode, a rechargeable lithium-sulfur (Li-S) battery is expected to yield high theoretical specific energy of 2600 W h kg^− 1, based on a redox reaction that reversibly interconverts sulfur (S₈) and Li₂S [3]. In addition, sulfur is low cost, highly natural abundant and environmental friendly. However, the practical realization of Li–S batteries is currently hindered by numerous scientific and technical challenges.

The most critical problem is believed to be associated with the intermediate lithium polysulfides (Li₂S_n, n = 4–8) that are highly soluble in many organic solvents based electrolytes [4], [5], [6]. The dissolved polysulfides, formed at the beginning of discharge, could shuttle between the cathode and anode through the commonly used porous polymer separator. This shuttle process induces irreversible loss of active materials from the cathode, corrosion of Li metal anode, low coulombic efficiency, rapid capacity fading, and high self-discharge rates [7], [8]. Nevertheless, the dissolution of polysulfides is inevitable and essential for effective utilization of the active material in a Li-S cell [9], [10]. In the discharge process, the soluble polysulfides dissolve into the electrolyte solution enabling the subsequent sulfur to be exposed to the conductive agent and the reduction process to progressively move forward. In the charge process, the dissolved polysulfide species can facilitate the electrochemical conversion of the insoluble discharge products Li₂S₂/Li₂S [11], [12]. We have previously reported a novel type of Li-S cell employing a hybrid electrolyte (HE) [13]. The hybrid electrolyte was composed of the liquid electrolyte and a lithium ion conductor that acts as the separator. The NASICON-type structured Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) solid electrolyte, which physically block the dissolution and diffusion of polysulfides could solve the shuttling problem. Meanwhile, the liquid electrolyte takes advantage of the dissolved polysulfides which offer fast charge/discharge rates and favor fast electrochemical kinetics of the cathode.

However, a redistribution of polysulfides on the electrode is unavoidable no matter what kind of treatment is done on the initial sulfur cathodes [14]. The redistribution of sulfur in the cell would cause the capacity fading once the soluble polysulfides detach from the cathode into the liquid electrolyte [15], [16], [17]. Gradually, sulfur loses intimate contact with carbon matrix. Electronically isolated and electrochemically inactive Li₂S does not take part in the electrochemical oxidation during charge state owing to the lack of carbon matrix. The formation of irreversible Li₂S becomes severe as cycling proceeds and the available conducting surface is decreased gradually, leading to the decreasing of discharge capacity. To make full use of the dissolved sulfur-containing active materials, a carbon coating layer facing sulfur cathode is introduced onto one side of the solid electrolyte. As shown in Fig. 1, the carbon-coating layer serves as an upper current collector to facilitate electron transport as well as to improve the wettability of solid electrolyte towards the liquid electrolyte. The migration of the polysulfides is physically blocked by the solid electrolyte and the shuttle effect is prevented. A lithium sulfur battery with enhanced electrochemical performance is demonstrated with an initial discharge capacity of 1409 mA h g^− 1 and a reversible specific capacity of 1000 mA h g^− 1 after 50 cycles at 0.2C rate.

Section snippets

Carbon coated Li_1.5Al_0.5Ge_1.5(PO₄)₃ pellet fabrication

The Li_1.5Al_0.5Ge_1.5(PO₄)₃ ceramic electrolyte is synthesized by the solid state reaction method. The detailed preparation process is previously reported [13], [18]. The obtained ceramic pellet, with the thickness of 0.7 mm and diameter of 17.4 mm, has a conductivity of 1.7 × 10^− 4 S/cm at room temperature. This value is in good agreement with results reported by several previous literatures [19], [20], [21], [22]. The electronically conductive carbon is a mixture of acetylene black (AB) and carbon

Results and discussion

Fig. 2a presents cross-sectional image of the carbon coated LAGP ceramic electrolyte. The carbon coating layer is stacked well on the surface of the LAGP ceramic and is about 5 μm thick on average. The carbon material is composed of a uniformly dispersed mixture of AB and CNT, as shown in the TEM image (Fig. 2b). Fig. 2c shows the top surface morphology of the carbon coating layer which shows a uniform porous structure without visible agglomeration. The digital photo of the pristine ceramic

Conclusion

In summary, this work clearly demonstrates that by modifying the ceramic electrolyte with a carbon coating layer, a highly efficient interface is achieved in the ceramic-liquid hybrid Li-S cell. The carbon coating layer obviously increases the conducting surface between cathode and the ceramic electrolyte where high reutilization of the adsorbed dissolved polysulfide species is realized, leading to significant enhancement in the reversibility and rate capability. The resultant Li-S cell shows

Acknowledgements

We are grateful to the support of the Natural Science Foundation of China (NSFC) No. 51402330, No. 51372262, No. 51472261 and the high resolution earth observation system major special project youth innovation foundation of China No. GFZX04060103. The authors thank Prof. B. V. R. Chowdari (School of Materials science and Engineering, Nanyang Technological University, Singapore) for helpful discussion.

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Improved performance of Li-S battery with hybrid electrolyte by interface modification

Highlights

Abstract

Introduction

Section snippets

Carbon coated Li1.5Al0.5Ge1.5(PO4)3 pellet fabrication

Results and discussion

Conclusion

Acknowledgements

J. Power Sources

J. Power Sources

Solid State Ionics

J. Power Sources

J. Power Sources

J. Power Sources

Nature

Nat. Commun.

Nat. Commun.

Acc. Chem. Res.

Acc. Chem. Res.

Adv. Energy Mater.

J. Electrochem. Soc.

J. Electrochem. Soc.

J. Am. Chem. Soc.

J. Electrochem. Soc.

Phys. Chem. Chem. Phys.

Carbon coated Li_1.5Al_0.5Ge_1.5(PO₄)₃ pellet fabrication