ENABLING SAFE AND HIGH-ENERGY DENSITY ALL SOLID-STATE SULFUR BATTERIES

Alzahrani, Atif

ENABLING SAFE AND HIGH-ENERGY DENSITY ALL SOLID-STATE SULFUR BATTERIES

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Author:: Alzahrani, Atif
Graduate Program:: Materials Science and Engineering
Degree:: Doctor of Philosophy
Document Type:: Dissertation
Date of Defense:: March 06, 2020
Committee Members:: Donghai Wang, Dissertation Advisor/Co-Advisor
Donghai Wang, Committee Chair/Co-Chair
Chao-Yang Wang, Committee Member
Adrianus C Van Duin, Committee Member
Long-Qing Chen, Outside Member
John C Mauro, Program Head/Chair
Keywords:: Li-ion battery
Li-S battery
Solid-state electrolyte
Solid-state battery
Abstract:: All solid-state sulfur battery (ASB) offers a high-energy density alternative to commercial alkali-ion batteries that are based on oxide cathodes, thanks to the high theoretical specific capacity of sulfur (1672 mAh g-1). Additionally, ASB remedies the long-standing parasitic polysulfide shuttling effect that occurs during the operation of conventional organic-based alkali-sulfur battery. Yet, to enable ASB’s production and potentially its application, a handful of challenges associated with ASB need to be addressed, including: homogenization of sulfur, carbon and solid-state electrolyte components in the cathode layer, high production cost of cathode composite layer and spontaneous release of hazardous H2S when sulfide-based solid-state electrolyte is exposed to a moist environment. Firstly, the electrochemical performance of all solid-state lithium-sulfur battery (ASLB) is reliant on the solid-solid interactions between sulfur, carbon and solid-electrolyte components in the cathode layer. Therefore, the mismanagement of these interactions raises the internal cell impedance, causing high cell polarization and ultimately low electrochemical performance. Thus, it is essential to properly homogenize sulfur, carbon and solid-state electrolyte components in order to design high performance ASLB. Herein, we demonstrate a novel highly loaded sulfur in carbon composite (71.4 - 90.9%), utilizing sulfur vapor deposition (SVD) approach, departing from conventional sulfur-carbon composite fabrication methods (i.e sulfur liquid deposition (SLD) and sulfur solid deposition (SSD)). Interestingly, the ASLB prepared by SVD approach achieves a higher discharge specific sulfur capacity of 1792.6 mAh g-1, compared to 1619.2 and 1329.3 mAh g-1 for ASLBs prepared by SLD and SSD methods, respectively, at 0.1C and 60 oC. This excellent performance of ASLB prepared by SVD method is ascribed to the deeply confined, homogenously distributed and smaller size of sulfur particles in carbon composite. Secondly, the major contributor to the high production cost of ASLB is the cathode composite layer. Specifically, the two factors for the elevated cost of the cathode composite layer in ASLB are the absence for a scalable production technique and the requirement of pre-synthesized solid-state electrolyte for the cathode composite layer. In this part, we demonstrate a li-free phosphorus polysulfide-carbon (P2Sx/C) cathode composite layer using a scalable two-stage heating process, as opposed to conventional inefficient ball milling approach. Promisingly, the ASLB prepared by the two-stage heating process shows an improved electrochemical performance compared to ASLB prepared by conventional ball milling approach, and this is ascribed to the formation of homogenous P2Sx/C phase compared to inhomogeneous S/P2S5/C phases. Lastly, most sulfide-based solid-state electrolytes (SSEs) hydrolyze when exposed to a moist environment, triggering a rapid release of hazardous hydrogen sulfide (H2S). Thus, the management of H2S release is vital for further development of SSE. Here, we develop safe and highly conductive 75 Na2S – (25-x) P2S5 – 2/3x P3N5 (2 ≤ x ≤ 8) solid-state electrolytes that significantly suppress the H2S evolution from 400 ppm in the pristine 75 Na2S – 25 P2S5 SSE to lower than 10 ppm for 75 Na2S – (25-x) P2S5 – 2/3x P3N5 (2 ≤ x ≤ 8) SEEs under high relative humidity (RH = 80%). The underlying mechanism behind H2S sequestration is attributed to the formation of an air-stable protective layer composed mainly of Na2S.5H2O that impedes the reaction between moisture and the SSE.

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