High Specific Charge and Long Cycle Life of Simple Li–S Batteries

By demonstrating that carbon–sulphur electrodes based on simple physical mixing of carbon black and elemental sulphur can yield performance values at least as good as those of most electrodes based on structured carbons and advanced preparation methods, we underline the importance of often-neglected experimental parameters that influence the performance of Li–S batteries,

Li–S batteries are being studied since several decades, motivated by predictions of high specific charge (1672 mAh/g) and high specific energy (2,500 Wh/kg) [1]. They are among the most promising systems for the next generation of rechargeable lithium batteries. However, the development of Li–S batteries faces a number of challenging problems on the way to maximum performance. One of them is the insulating nature of sulphur, necessitating the addition of conductive additives to the electrode material. Whereas a wide range of factors contribute to the overall performance of the Li–S system, reported optimization procedures typically focus on the development of novel structured cathodes, in which the conductive environment of sulphur is optimized. The most widely used conductive additive for electrodes is carbon, many different forms of which are being employed for positive electrodes in Li–S batteries [2–5].

In this contribution, we demonstrate the importance of various other parameters that influence the performance of Li–S batteries and conclude that carbon–sulphur electrodes based on simple physical mixing of carbon black and sulphur can be at least as good as most of the electrodes based on the advanced carbons prepared by complicated composite-preparation methods [4]. Specifically, the effects of varying the amount of electrolyte, the salt concentration and the type of electrolyte additive are presented and factors that improve cycling stability are discussed. Some of the parameters, such as sulphur-particle size and the type of conductive additive used in the electrodes, affect the cell's performance less than might have been expected [6]. For a Li–S cell based on cheap commercially available carbon black (Super P) with a sulphur content of 60 % and loading of ~2 mg/cm², a specific charge of ~800 mAh/g after 100 cycles (see Figure a) and b)) and a life time of 500 cycles (see Figure c)) have been reached.

Electrodes were prepared by mixing elemental sulphur, Super P and polyethylene oxide (PEO) binder to the final composition of electrode of 60 % sulphur, 30 % carbon (Super P) and 10 % PEO. After drying electrodes were punched into 13-mm-diameter discs and assembled into the cells with Celgard 2400 as separator, metallic lithium as counter electrode and electrolyte containing 1 M LiTFSI in DME:Diox (2:1) with or without addition of 0.5 M LiNO₃. Electrodes were galvanostatically cycled at C/5 rate (1C is defined as I = 1672 mA/g) between 1.8 and 2.7 V vs. Li+/Li.

References

[1] X. Ji, K.T. Lee, L.F. Nazar, Nat. Mat. 8, 500–506 (2009).

[2] H. Wang, Y. Yang, Y. Liang, J.T. Robinson, Y. Li, A. Jackson, Y. Cui, H. Dai, Nano Lett. 11, 2644–2647 (2011).

[3] Y. Yang, G. Yu, J.J. Cha, H. Wu, M. Vosgueritchian, Y. Yao, Z. Bao, Y. Cui, Nano Lett. 5, 9187–9193 (2011).

[4] Y. Yang, G. Zheng, Y. Cui, Chem. Soc. Rev. 42, 3018–3032 (2013).

[5] S. Evers, L.F. Nazar, Acc. Chem. Res. 46, 1135–1143 (2013).

[6] S. Urbonaite, P. Novák, J. Power Sources 249, 497–502 (2014).

Acknowledgements

BASF SE is acknowledged for financial support within the “BASF Scientific Network on electrochemistry and Batteries”.