Elsevier

Energy Storage Materials

Volume 15, November 2018, Pages 209-217
Energy Storage Materials

A Li-ion sulfur full cell with ambient resistant Al-Li alloy anode

https://doi.org/10.1016/j.ensm.2018.04.003Get rights and content

Abstract

Lithium (Li) metal as anode for Li-S batteries has encountered some issues, eg., dendrite formation and ambient instability, both of which imposed safety problems on the operation and manufacturing of Li metal sulfur batteries. Exploring safer Li metal replacement is thus of fundamental and technical importance for enabling Li-metal-free sulfur batteries. Aluminium (Al) is an appealing Li-alloy anode material for the sake of its high capacity, natural abundance, and safety. Pairing Al-Li alloy with sulfur (S) could be a promising strategy to achieve high-energy rechargeable batteries with improved safety. Herein we show the suppressed dendrite growth and the enhanced ambient stability of Al-Li alloy anode. A Li-metal-free Li-ion sulfur battery was assembled with an Al-Li alloy anode, a sulfurized polyacrylonitrile cathode and a carbonate electrolyte. This Li-ion sulfur full cell exhibited good reversibility and stability, with a slow decaying rate at 0.09% per cycle. The specific energy of the full cell based on the total weight of active materials is estimated to be in a range of 589–762 Wh/kg.

Graphical abstract

Herein we report Li-metal-free Li-ion sulfur battery assembled with an Al-Li alloy anode, a sulfurized polyacrylonitrile cathode and a carbonate electrolyte. Pairing Al-Li alloy with sulfur (S) could be a promising strategy to achieve high-energy rechargeable batteries with improved safety due to the improved dendrite suppression and ambient resistance of the anode.

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Introduction

Rechargeable batteries based on sulfur cathodes have attracted considerable attention because of the extraordinary capacity (1675 mAh/g) and natural abundance of sulfur. The sulfur cathode can pair with a range of metal anodes, including lithium (Li) [1], [2], [3], sodium (Na) [4], [5], magnesium (Mg) [6], [7] and aluminium (Al) [8], [9]. Among many, Li-S battery is more advantageous on account of the high theoretical capacity of Li metal (3860 mAh/g), high cell voltage (average 2.15 V), and extraordinary energy density of 2500 Wh/kg. As a result, Li-S batteries have triggered remarkable attention since the report by Nazar et al. [3]. In spite of such merits of Li-S cells, however, a number of great challenges including the dendrite and the ambient instability of Li metal, as well as polysulfides shuttling need to be addressed [10]. The dissolution of polysulfides have been studied extensively [11], [12], while the anode part is still uncertain.

The Li dendrite initiated by the inhomogeneous Li deposition could trigger short circuit of the cell, and cause explosion issues. On the other side, the solid electrolyte interphase (SEI) at lithium surface could break off due to the repetitive Li stripping/plating, resulting in the consumption of electrolyte, as well as the low coulombic efficiency. A range of optimized approaches are underway to tackle with this problem. For instance, constructing protective layers containing poly(vinylidene-co-hexafluoropropylene) (PVDF-HFP) and LiF composites on Li metal was proposed [13], [14]. These kinds of protective layers could effectively suppress the random Li deposition and avoid the consumption of excess electrolyte. Electrolyte additives, like LiF-rich layer [15], were also studied to stabilize the SEI on Li metal and alleviate the uncontrolled Li deposition. Surface coating on the electrode materials could suppress the structure change during charge/discharge, and further stabilize the cell performance [16]. Other approaches like electrolyte modification, interfacial engineering, and solid-state electrolyte incorporation could also be found to improve the safety and performance of Li anode [17]. However, the sensitivity of Li metal to air and moisture is still a severe challenge that has not been fully solved.

Meanwhile, Li-based alloy anode represents possible approaches to minimizing the lithium dendrite problem [18], [19]. Among them, aluminium (Al) has shown a great potential because of its high capacity (2980 mAh/g and 8046 mAh/cm3), abundance in earth's crust (8.32%), safety and low market price (~ $0.5–1.5 USD/lb) [20]. The merit of Al alloy lies in the moderate potential versus Li°/Li+. The measured potential of Al alloy vs Li°/Li+ is around 0.2–0.3 V, and it could disadvantage the lithium dendrite growth that occurs with silicon or graphite anodes (<0.05 V vs Li°/Li+), which is crucial for battery safety. It is worth noting that the electrode expansion/shrinkage upon alloying/de-alloying could potentially be a challenge for the alloy anode [21], [22], [23]. Even though, Al has much smaller volume change (~ 96%), compared with other alloy anodes, such as Si alloy (320%), Sn alloy (260%) and Sb alloy (200%) [24]. Another advantage for Al alloy is the existence of surface Al2O3, which could likely provide a stable oxide cover to isolate the alloy from air or moisture, and slow down the rigorous oxidation. It is expected that Al-Li alloy might be a safe replacement anode for lithium metal in the Li-metal-free sulfur battery in order to simultaneously address both the dendrite and air instability issues of lithium metal.

In this work, we report a rechargeable 1.5 V Li-ion sulfur battery with Al-Li alloy anode and sulfurized polyacrylonitrile (SPAN) cathode, where Li ions as charge carrier to shuttle between both electrodes. SPAN was confirmed to undergo solid-state reaction in the carbonate electrolyte without any shuttle phenomenon, which would happen in ether electrolyte [25]. Al-Li alloy as anode material could operate as alternative anodes to minimizing the lithium dendrite problem, along with the merits of mitigating oxidation of Al-Li surface in air. In the meanwhile, the low potential (0.28 V, vs. Li°/Li+) of the alloying reaction of Al anode with Li ions (Li+) enables a relatively high cell voltage of 1.5 V, in contrast to Al-S cell (<1 V). It is further demonstrated that the cell performance is largely improved by applying pre-lithiation to either the Al anode or the sulfur cathode. The optimised cells have initial cathode capacity of 550 mAh/g at 200 mA/g and endure 200 cycles with a decaying rate of 0.09% per cycle. The specific energy based on the total weight of active materials in the full cell ranges from 589 Wh/kg to 762 Wh/kg, which is a function of the specific electrode compositions.

Section snippets

Preparation of sulfurized polyacrylonitrile (SPAN) particles

SPAN was fabricated by heating a mixture of polyacrylonitrile and sulfur with a mass ratio of 1:3 in a tube furnace under the nitrogen atmosphere at 500 °C.

Preparation of SPAN cathodes

Then as-prepared SPAN material was mixed with binder (Alginate, Sigma Aldrich) and carbon black at a ratio of 70 wt%: 15 wt%: 15 wt%. The mixture was grounded in water to get homogenous slurry. The slurry casted onto the carbon-coated aluminium foil and dried in a vacuum oven under 80 °C for 24 h. The slurry-loaded foil was punched into

Results and discussion

The alloy/de-alloy process of Al-Li half-cell was measured using galvanostatic charge/discharge method. As shown in Fig. 1(a), the alloy reaction between Al and Li was around 0.28 V while the de-alloy process occurred at 0.42 V vs. Li/Li+. The Al-Li alloy could consist of three different structures, including AlLi, Al2Li3 and Al4Li9. The composition of the Al-Li alloy was dependent on the charging time (alloy formation) as displayed by the ex situ XRD in Fig. S1. With the reaction time

Acknowledgements

We acknowledge the support from Faculty of Engineering, The University of New South Wales, and the Australian Research Council (DP160103244). The authors acknowledge the facilities and the scientific and technical assistance from Mark Wainwright Analytical Centre, The University of New South Wales.

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