Electrochemical Evolution of an Iron Sulfide and Sulfur Based Cathode for All-Solid-State Li-Ion Batteries

Many advanced battery technologies are vying to be the successor of today’s conventional Li-ion batteries. A strong argument can be made that all-solid-state lithium batteries (ASSLB) hold a competitive edge in this technological race because they are comparatively safe, have excellent shelf life, perform stably at high temperatures, and enable the reversibility of high capacity conversion battery materials like FeS2. 1 However, for good performance ASSLB electrodes must often contain a large weight percentage of carbon black (CB) or solid state electrolyte (SSE) for fast transport of charge carriers. Because poor active material (AM) mass loading reduces the gravimetric and volumetric energy densities of ASSLBs, this issue warrants further attention. 2 The success of the ASSLB architecture will depend, in part, on the development of all-solid-state composite electrodes with high energy density. This paper examines the complex evolving electrochemistry of an iron sulfide and sulfur based all-solid-state electrode with the Li2S-P2S5 glass electrolyte. This paper provides indirect evidence for the in-situ electrochemical formation of a FeS2 phase as well as direct evidence for the in-situ activation of the glass electrolyte for higher overall electrode energy density. Utilizinghighcapacityconversionmaterialslikesulfurisapromising approach to increasing the energy density of ASSLBs. Unfortunately, the insulating nature of S and Li2S requires a large weight percentage of SSE for ionic conduction and a large weight percentage of electronically conducting carbon additive. In three studies utilizing So r Li2S, the composite electrodes were comprised of 25% carbon black and 50% solid state electrolyte by weight. 3‐5 Among these three studies the highest electrode specific energy achieved was approximately 600 Wh kg −1 if lithium was considered as the negative electrode. 4 A fourth study improves upon this number by utilizing a polyacrylonitrile-sulfur (PAN-S) active material composite to demonstrateanall-solid-stateelectrodespecificenergyof700Whkg −1 . 6 The volumetricenergydensityofthesecompositeelectrodesisdiminished by the large weight percentage of carbon black or conjugated polymer needed to provide good electronic contact with sulfur species. For transportation applications, more volumetrically dense electrodes are preferred simply because battery stowage space is limited in vehicles. A proposed alternative focuses on replacing carbon black by mechanically milling S or Li2S with Cu. 7‐9 In composite electrodes with mechanochemically prepared S-Cu composites, Cu provides dense electronic conduction pathways and participates electrochemically to form CuS at full charge. For all of these studies, 3‐9 the best performing electrode compositions are composed of no more than 25% S or Li2S by weight. Sulfur’s high theoretical specific capacity of 1672 mAh g −1 offsets poor active material mass loading so that the overall electrode energy densities are still among the highest reported for ASSLBs. It may be possible to electrochemically activate some part of the glass-ceramic electrolyte in order to increase the overall energy density of a com