Dense coating of Li4Ti5O12 and graphene mixture on the separator to produce long cycle life of lithium-sulfur battery

doi:10.1016/j.nanoen.2016.09.030

Nano Energy

Volume 30, December 2016, Pages 1-8

https://doi.org/10.1016/j.nanoen.2016.09.030 Get rights and content

Highlights

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A dense and hybrid coating is fabricated on separator for long life Li-S Battery.
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The coating contains Li₄Ti₅O₁₂ nanospheres embedded in graphene layers.
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The coating combines physical barrier with chemical adsorption for polysulfides.
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A fast lithium ion transport is achieved by Li₄Ti₅O₁₂ nanospheres.

Abstract

The high solubility of polysulfides in the electrolyte, together with the resulting poor cycling performance, is one of the main obstacles to the industrial production and use of lithium-sulfur (Li-S) batteries. We have developed a novel hybrid and dense separator coating that greatly improves the cycling and rate performance of the battery. The coating is fabricated by mono-dispersed Li₄Ti₅O₁₂ (LTO) nanospheres uniformly embedded in graphene layers. In this hybrid dense coating, the LTO nanospheres have a high chemical affinity for polysulfides and an excellent ionic conductivity to produce highly efficient ionic conductive channels, while the graphene layers play twin roles as a physical barrier for polysulfides and an upper current collector. The unique hybridization guarantees a very dense coating that does not significantly add the volume of the battery and meanwhile achieves an ideal combination of an effective barrier for polysulfide diffusion with a fast ion transport. For a normal coating, a loose and very thick structure is needed to meet these requirements. Cells using a pure sulfur electrode with the dense coating separator show an ultra-high rate performance (709 mA h g⁻¹ at 2 C and 1408 mA h g⁻¹ at 0.1 C) and an excellent cycling performance (697 mA h g⁻¹ after 500 cycles at 1 C with 85.7% capacity retention). The easy achieving of such excellent performance indicates the possibility of producing an industrially practical Li-S battery.

Graphical abstract

Introduction

It is clear that, with the development of electric vehicles (EVs) and other devices needing portable high power energy sources, traditional lithium-ion batteries cannot meet future demands [1], [2], [3], [4]. Because of their high theoretical specific capacity (1675 mA h g⁻¹) and specific energy density (2600 W h kg⁻¹), lithium-sulfur (Li-S) batteries have attracted much attention as the next-generation batteries [1], [5], [6], [7]. In addition, the cheap and nontoxic nature of sulfur is an added attraction. However, Li-S batteries have many obstacles to industrialization, such as the low electrical conductivity and volume expansion (76%) of sulfur [8], high solubility of polysulfides in the electrolyte, the electrically insulating nature of the discharge products Li₂S/Li₂S₂ [9], and the problem of the safety of the lithium metal anode [10], [11]. Among them, the shuttle effect, which refers to soluble polysulfides being reduced at the anode and reoxidized at the cathode, results in severe self-discharge and a fast loss of capacity [12], [13].

In order to mitigate the shuttle effect, multi-dimensional nanostructured or amino-functionalized carbon skeletons [5], [14], [15], [16], [17], [18], [19], [20], [21], [22], polymer coatings [23], [24], [25], [26], [27], [28] and metal oxides [8], [29], [30], [31], [32] have been used to immobilize the polysulfides, but the complex structure design and expensive preparation process have limited their practical use. Manthiram et al. first proposed the concept of an interlayer between separator and cathode to improve the utilization of sulfur and hinder polysulfide diffusion by using a carbon nanotube membrane [33]. Subsequently, carbon nanofibers [34], carbonized leaves [35], a carbonized eggshell membrane [36] and a Ti₃C₂ nanosheet/glass fiber composite [37] were investigated as interlayers. At present, modifying separators with mesoporous carbon [38], graphene oxide [39] or graphene [40] has been widely considered as an alternative structure. However, these carbon layers only work as a physical barrier and the blocking effect for polysulfides is limited. More recently, it has been proposed that a TiO₂/graphene hybrid coating on the electrode, which combines the chemical adsorption of polysulfides by TiO₂ with the physical barrier of graphene, would improve the cyclic stability greatly [41]. Nevertheless, commercial TiO₂ has an ultra-low ionic conductivity and blocks the diffusion of lithium ions. In addition, anatase TiO₂ that could be used as an anode for a lithium-ion battery has high catalytic activity that initiates the decomposition of the electrolyte, resulting in the generation of a large amount of CO₂ [42].

The above studies clearly show that interlayers or separator coating with different components can greatly suppress the shuttle effect of polysulfides. However, these modifying layers with loose structure obviously add weight and volume to the batteries, decreasing their energy density, especially the volumetric energy density. Constructing a dense coating on the separator can greatly reduce the volume of battery and may have a better blocking effect for the diffusion of polysulfides. In a previously reported work, Zhou et al. fabricated a dense graphene coating on separator by vacuum-filtration method, which possesses only physical blockage for diffusion of polysulfides [43], and such a coating would also easily block the diffusion of lithium ions. The key problem to answer is how to achieve a high ionic conductivity in dense coating.

Nano sized Li₄Ti₅O₁₂ (LTO) has a high ionic conductivity, large specific surface area and stable structure [44], [45], and is normally used as an anode for a lithium-ion battery and beneficial for lithium ion diffusion and transport in dense layer. Here, we report the fabrication of a hybrid dense coating of mono-dispersed LTO nanospheres uniformly embedded between graphene (G) layers on the separator that synergistically suppresses the shuttling of the polysulfides and improves the electrochemical performance (unimpeded ion transport) of the battery (Fig. 1). The G layer acts as a physical barrier and LTO plays a key role in the chemical adsorption of polysulfides. Furthermore, high efficiency ion transport channels are achieved in such a dense coating as a result of the presence of the LTO nanospheres (Fig. 1a–c). The mono-dispersed LTO nanospheres could be sufficiently and uniformly distributed in the graphene layer. In addition, the mono-dispersed LTO nanospheres possess a large specific surface for improving efficiency of chemical adsorption and ionic conductivity of the hybrid layer. Due to the construction of high efficiency ion transport channels by mono-dispersed LTO nanospheres in hybrid coating, the dense coating (oriented stack of graphene) layer prepared by vacuum-filtration shows a much better inhibition of the shuttle effect without compromising the power performance of Li-S batteries, than does a layer with a loose coating (unordered stack) prepared by blade-coating (Fig. 1c and d). Cells using a pure sulfur electrode with such a dense coating on the separator show an ultra-high rate performance (709 mA h g⁻¹ at 2 C and 1408 mA h g⁻¹ at 0.1 C) and an excellent cycling performance (697 mA h g⁻¹ after 500 cycles at 1 C with 85.7% capacity retention). The strong chemical adsorption of LTO for polysulfides and the formation of a dense hybrid coating produce excellent performance, which has the possibility of use in next generation Li-S batteries.

Section snippets

Synthesis of LTO nanospheres

LTO nanospheres were prepared by the method previously reported by our group [46]. Briefly, TiN nanopowder was dispersed in deionized water and then hydrogen peroxide and an ammonia solution were added to it and the mixture was stirred for 30 min. Deionized water and ethanol were mixed with this mixture to hydrolyze the peroxo-titanium complex. After which, LiOH·H₂O and polyvinyl pyrrolidone were introduced to form a uniform dispersion, which was subsequently dried at 80 °C for 30 h. The dried

Results and discussion

The morphology of the prepared LTO is shown in Fig. 2a, in which mono-dispersed nanospheres with homogeneous size of around 100 nm can be seen. The XRD pattern (Fig. S1) of LTO is consistent with JCPDS card No. 49-0207, suggesting a spinel structure. After the LTO (10 mg) was added to the Li₂S₆ (60 μL, 0.2 M) solution in DOL/DME (1:1 by volume, 5 mL), it could be seen that the color of the solution changes from bright yellow to nearly transparent after storing for 24 h (Fig. 2b), proving the obvious

Conclusions

Constructing a dense coating on the separator greatly reduces the volume and improves the volumetric energy density of Li-S battery. It may also produce better blocking effect for the diffusion of polysulfides. On the other hand, the diffusion of lithium ions through the dense coating may be hindered. We have developed a dense hybrid coating on the separator, which is composed of mono-dispersed LTO nanospheres uniformly embedded in graphene layers. This coating successfully achieves an ideal

Acknowledgments

The authors appreciate support from the National Key Basic Research Program of China (2014CB932400), the National Natural Science Foundation of China (Nos. 51525204, 51672156, U1401243 and 51302146), NSAF (U1330123), the Youth research funds of Graduate School at Shenzhen, Tsinghua University (QN20150002), and the Shenzhen Basic Research Project (Nos. ZDSYS20140509172959981 and JCYJ20150529164918735).

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¹: These two authors are equal major contributors to this work.

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Dense coating of Li4Ti5O12 and graphene mixture on the separator to produce long cycle life of lithium-sulfur battery

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Synthesis of LTO nanospheres

Results and discussion

Conclusions

Acknowledgments

Energy Storage Mater.

Energy Storage Mater.

Energy Storage Mater.

Nano Energy

J. Power Sources

Nano Energy

Electrochim. Acta

Carbon

Nat. Mater.

Philos. Trans. A Math. Phys. Eng. Sci.

Adv. Energy Mater.

Nat. Mater.

Nat. Commun.

Energy Environ. Sci.

Nat. Nanotechnol.

Adv. Mater.

J. Mater. Chem.

Adv. Energy Mater.

ACS Nano

Adv. Mater.

Small

Nat. Commun.

Adv. Energy Mater.

Energy Environ. Sci.

Nano Lett.

Sci. China Mater.

Nat. Commun.

Dense coating of Li₄Ti₅O₁₂ and graphene mixture on the separator to produce long cycle life of lithium-sulfur battery