Development of Multilayered Inorganic Protections on Li Electrode to Improve Performance and Safety of Li-S Battery
Authors: Pecquenard Brigitte , Mauvy Fabrice , Le Cras Frédéric , Barchasz Céline , Benayad Anass 
Published: February 2020

Over the past 25 years, intense research efforts were focused on lithium-ion batteries due to their advantages such as high energy density, high operating voltage and low self-discharge rate. However, their gravimetric energy density that reaches 250 Wh.kg-1 today is still insufficient to meet some requirements, in particular for electric cars and electrified aircrafts. Among possible alternatives, Li/S system seems to be very promising. Indeed, elemental sulfur is characterized by a high theoretical specific capacity of about 1675 mAh.g-1 of sulfur material [1]. The discharge potential is around 2.1 V (vs. Li+/Li), and the complete Li/S system should allow to reach a gravimetric energy density close to 500 Wh.kg-1. Moreover, elemental sulfur is readily available and non-toxic, advantages that should allow to manufacture cheap and safe high energy batteries requirements for instance. However, despite actual progress, this promising system still suffers from several limitations: low discharge capacity, poor cycle life, low coulombic efficiency, high self-discharge and the compulsory use of the highly reactive lithium metal negative electrode, which may lead to dendrites formation, short-circuits and explosions [2]. Most of research works have been devoted to the positive electrode and the electrolyte and in particular to sulfur/carbon composites [3-6] or to the optimization of ether-based electrolyte compositions [7-9]. Regarding inorganic or organic protective coatings on the lithium metal electrode, numerous efforts were achieved on protective coating of lithium metal electrode either for Li/ion batteries or more recently for Li/air secondary batteries. The proposed solutions are mainly dealing with surfactants, polymers and inorganic materialsto protect the electrode and/or to better control the SEI formation. Hence, the use of polyether surfactants such as PEGDME significantly suppresses the inactivation of deposited lithium. Among inorganic materials, LiPON is commonly used to protect the battery electrodes such as lithium [10]. This material is widely used as electrolyte in all-solid-state thin film batteries [11]. The deposition of LiPON on lithium electrode leads to the formation of a stable electrolyte interface which effectively improves the reaction between lithium and the liquid electrolyte, as impedance measurements indicate that LiPON layer is more ionically conductive than the film formed by electrolyte decomposition. Quite recently, LiPON was associated with LATP in Li/air batteries [12-13]. The use of this glass-ceramic electrolyte allows fast ion conduction but necessitates the use of a LiPON interlayer to protect LATP from metallic lithium, the ceramic being not stable in contact with lithium. Only few studies are related to negative electrode in Li/S batteries [14]. This study aims at developing inorganic layers to protect for lithium metal negative electrode in order to prevent the dendrites formation as well as to improve the system performances (increase of the discharge capacity, reduction of self-discharge and shuttle mechanism). Several ceramic electrolyte materials have been developed and tested to prevent the formation of dendrites as well as the contact of lithium metal electrode with the dissolved active materials. Thorough physico-chemical characterizations were carried out on pristine ceramic materials as well as on ceramic after immersion in the liquid electrolyte to check the chemical stability of the latters (ICP, DRX, SEM, impedance measurements, XPS). Finally, the effect of the multilayered inorganic electrolyte on the electrochemical behavior of Li/S cells will be presented.

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