Key parameters in design of lithium sulfur batteries

Highlights

• Studied the effect of the carbon to sulfur ratio on the performance of Li–S batteries.

• Investigated the influence of sulfur loading density and electrolyte volume on cell capacity.

• Provided a guideline for the design of high performance Li–S batteries.

Abstract

In this paper, we discuss some key parameters in design of lithium sulfur batteries with high energy density, in particular carbon/sulfur ratio, sulfur loading density, electrolyte volume and electrode deformability, which directly impact the battery performance in terms of capacity, cycleability, and processibilities, etc. We find that a carbon to sulfur ratio of 1:2 (w/w) in the electrode is good for high specific capacity, giving a capacity of 538 mA h g⁻¹ which is 3 times as high as that of LiCoO₂ cathode. The issue of fragileness for electrode at high sulfur loading density is mitigated by replacing carbon black with fine graphite powders. With these optimizations the carbon–sulfur electrode gives an energy density of 5.88 mW h cm⁻² which corresponds to 60% of that of the commercial LiCoO₂ electrode.

Introduction

Lithium-ion (Li-ion) batteries have dominated the power supply market of portable electronic devices since the first launch of commercial lithium cobalt oxide/carbon (LiCoO₂/C) cells by Sony in 1991. Though being widely used the Li-ion batteries are seeing their bottlenecks in energy density limited by the lithium capability of lithium transition metal oxide (Li-M-O, M = Ni, Co and Mn) cathodes which typically deliver a theoretical specific capacity in the range of 120–320 mA h g⁻¹ [1], [2]. To meet the ever-growing demands on rechargeable batteries with higher energy density, non-lithium intercalated transition metal oxides, such as Me_xO_y (Me = V [3], [4], [5] or Cr [6]) and group 16 (VIA) elements (primarily oxygen [7], sulfur[8] and selenium [9]), have recently been exploited as the cathode materials for next-generation lithium batteries. For these non-lithium intercalated cathodes, lithium is used as the counter anode. In this case extra care in safety is required, as the growth of dendrites may cause short-circuit and explosion if the electrode is not properly engineered. Lithium–sulfur (Li–S) system is considered as the safest among batteries using lithium metal as anode, as its discharged products, lithium polysulfides, are soluble in the electrolyte that significantly suppress the formation of lithium dendrites [10]. Li–S battery is also a choice for its high energy density. Theoretically, sulfur can deliver a specific capacity of 1672 mA h per gram sulfur (mA h g⁻¹-sulfur) that is nearly 5 times as high as that of Li-ion batteries [11]. Furthermore, Li–S battery is a cost-effective choice, as sulfur is obtained from the byproducts of oil refining with the annual world production of 69 million tons (Mt) in 2011 [12] which is sufficient for 4 billions of 60 kW automobiles.

So far, major efforts in the Li–S battery research have been the development of sulfur composites with various microstructures for the cathode, and some interesting findings were reported [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. For sulfur cathode, one has to deal with its poor intrinsic electrical conductivity (1 × 10⁻¹⁵ S m⁻¹) by incorporating large portion of carbon black, which lowers the sulfur content and reduces the gravimetric and volumetric energy density at cell level [24]. High sulfur content (up to 70 wt%) can be achieved by incorporating sulfur into highly ordered mesoporous carbon, followed by coating with polyethylene glycol [25], or using a lower load of graphene or carbon nanotubes as substitute of carbon black [26], [27], [28], [29], [30]. More recently, the concept of forming a yolk-shell structure with internal void space to tolerate the volume expansion of sulfur during discharge was proposed by Cui et al. [13], [23], [31] and the composites exhibited distinctly improved cycling performance (>1000 cycles). Though these new cathode materials shed a light on Li–S battery research, the actual progress was far insufficient for practical application. For example, in these studies the sulfur loading density was less than 1 mg cm⁻² [32], [33], [34], [35], [36], [37], [38]. In this case the theoretical overall cathode capacity per unit area is about 60% of that of LiCoO₂ cathode in a commercial Li-ion batteries and the energy density is merely about 35% due to the lower working voltage of Li–S system. To compete with lithium-ion batteries using LiCoO₂ electrode, one would need a minimum sulfur loading density of 3 mg cm⁻² in the cathode. Some high sulfur loading density cases have been reported [39], [40], [41], [42]; however, a high sulfur loading density often associates with mechanical failure of the electrode film, e.g. cracking and/or peeling, leading to poor battery performance. It remains a challenge to develop high quality sulfur electrode for high performance Li–S batteries.

Apart from cathode materials, the selection of electrolyte is crucial for Li–S batteries [43]. The electrolyte is the media that transports lithium ions and dissolves polysulfides (PS) which significantly impacts the overall electrochemical performances of Li–S battery [44]. Electrolyte with low viscosity and high PS solubility generally favors the cell reaction; however, the dissolution of PS imposes negative impact on coulombic efficiency and capacity retention. Such negative impact can be suppressed by the introduction of LiNO₃ additive into the electrolyte [45]. During the discharge process, sulfur is gradually reduced to soluble PS (Li₂S₈, Li₂S₆ and Li₂S₄). Li₂S₄ is further reduced to insoluble sulfides (Li₂S₂ and Li₂S) that precipitate on the carbon surface at the end of discharge. In the reversed charge process, insoluble PS is first oxidized into soluble PS which is further converted into sulfur and eventually precipitates on the carbon surface when charge is completed. The excess amount of PS in the electrolyte may cover the sulfur surface as a nonconductive coating and adversely impact the following discharge performance. Moreover, the amount of electrolyte also affects the cell performance. Larger volume of electrolyte generally leads to a higher initial discharge capacity [46]; however, it reduces the energy density of the cell. Therefore, it is critical to balance the volume of electrolyte and the amount of sulfur for high performance Li–S batteries.

In this work, we prepared a series of carbon/sulfur (C/S) composites with a different weight ratio (3:1, 2:1, 1:1, 1:2 and 1:3) and evaluated their electrochemical performances in Li–S batteries. A high sulfur content and crack-free electrode with a loading density of 10 mg cm⁻² was fabricated, as carbon black was substituted by fine graphite powders. The effect of the volume of electrolyte on the electrochemical performances was also investigated.