High Capacity All-Solid-State Batteries Based on Metal Sulfides Conversion Electrodes

Abstract

The steady miniaturization of electronic devices and the emerging need for self-powered miniaturized systems (stand-alone sensors, medical implants, MEMS,…) boost the development of miniaturized energy storage solutions such as all-solid-state thin film batteries (i.e. microbatteries). Up to now, most studies on cathodes focused on intercalation compounds (LiCoO2, LiMn2O4, V2O5,…), despite their limited capacity (650–1150 mAh·cm-3) [1,2]. Nevertheless, other materials reacting with lithium according to a conversion reaction described by the general formula MaXb + (b·n) Li↔aM + bLinX (M = metal; X = O, N, F, S, P, H) are promising [3]. Among them, pyrite (FeS2 or Fe2+S2 2-) which may insert up to 4 Li+ per atom of iron enabling one of the highest theoretical capacity among transition metal sulfides is an ideal candidate: FeS2 + 4Li+ + 4e- → Fe + 2Li2S leading to theoretical capacities of 894 mAh.g-1 and 435 µAh.cm-2.µm-1. Moreover, this reaction that occurs at around 1.5 V vs Li+/Li meets the requirement for lower supply voltages in various microelectronics systems. Nevertheless, the use of pyrite in commercial systems is still confined to lithium primary cells and thermal batteries. Indeed, similarly to lithium–sulfur cells, the main phenomenon hindering the cycle life is the formation of polysulfide species which are soluble in the liquid electrolyte [4], whereas the practical delivered capacity is limited by the size of the pyrite particles. Here, striking results obtained in electrochemical cells combining stacked thin film geometry and the use of a solid state electrolyte will be presented. Lithium microbatteries which comprise a FeS2 positive electrode and a vitreous electrolyte (LiPON) deposited by Radio-Frequency magnetron sputtering, and a lithium negative electrode, allowed demonstrating the efficiency of such a cell design. Physico-chemical characterizations carried out on the deposited pristine materials (EPMA, SEM, Raman and Mössbauer spectroscopies, GIXRD) confirmed the pyrite structure. Besides, ex situ transmission microscopy (HRTEM) analyses at different states of charge couple with EELS, revealed the lithiated pyrite compounds obtained during first lithium insertion/de-insertion. Full Li/LiPON/FeS2 microbatteries delivered excellent capacities of 300 µAh.cm-2.µm-1for more than 800 cycles. Thorough electrochemical characterization suggested that the reversibility of the conversion reaction was affected by a continuous cycling in the low voltage region, indicating a progressive evolution of the phase distribution inside the electrode material [5]. Some recent results on other metal sulfides conversion materials will be also displayed.