Mechanochemical Synthesis of Lithium Sulfide for Li-S Batteries
Authors: Prosini Pier Paolo , Tarquini Gabriele , Cento Cinzia , Di Carli Mariasole 
Published: February 2020

Lithium–sulfur (Li–S) batteries are receiving intense interest because their promise for low-cost and high-energy electrochemical storage [1]. Sulfur has a high theoretical specific capacity (1675 mAh g−1) and energy density (2600 Wh kg−1). In addition, it is abundant, inexpensive and nontoxic. Despite these advantages Li-S batteries have not been commercially produced. Major drawbacks are the insulating nature of orthorhombic sulfur and its discharge product (Li2S), the dissolution of lithium polysulfide formed during the electrochemical process in organic liquid electrolyte, and the large volumetric expansion of sulfur during lithiation. These factors result in a low utilization of active material and poor cycle life [2]. Furthermore this kind of battery must use lithium metal as the anode. Fully-lithiated lithium sulfide (Li2S) is more desirable than sulfur as a high capacity cathode material because it allows the use of a variety of lithium-free anode materials. Recently, a number of studies on Li2S with promising results have emerged. Some authors ball-milled commercial Li2S to get different particle sizes, and the resulting particles are mostly micrometer-sized [3-4]. Furthermore, high-energy ball milling was also used for the preparation of compounds with improved chemical properties. Iio et al. [5] have synthesized lithium ion conducting materials in the system Li3N-SiS2 by mechanical milling at room temperature using Li3N as a lithium source. In the present study we synthesized Li2S by high energy ball milling using elemental sulfur and Li3N. To identify crystalline phases of samples, X-ray diffraction measurements (Rigaku Miniflex diffractometer) were performed for powdered samples after mechanical milling for a given period of time. In the figure below is showed the X-ray diffraction patterns of the sample obtained after 1 h of ball milling. It is possible to observe that the sulfur was completely converted into Li2S. The Scherrer's formula was used to evaluate the crystal size of the crystallite: d = k*λ/B*cosθ where d is the size of the crystallites, k is a constant that depends on the shape of the crystallites, λ is the wavelength of the radiation used, B is the peak width at half height and θ is the angle of diffraction of the peak. The value of the crystallite size calculated with the Sherrer’s formula was about 17-18 nm. The so obtained material was used to produce an electrode and the electrochemical properties were evaluated in a two electrodes lithium cell. LiClO4 dissolved in a mixture of ethylene glycol dimethyl ether and  diethylene glycol dimethyl ether 50:50 was used as the electrolyte. We found that lithium can be electrochemically extracted and reversible reinserted into the material.  The capacity was limited to about 180 mAh g-1 with a good capacity retention with cyclation . Further study will be addressed to increase the specific capacity of the material and to couple the electrode with suitable anode materials to obtain a full lithium-ion cell.

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