Identification of the Limiting Factors to Improve the Cycling Stability of Sulfide-Based ASSBs
Authors: Conforto Gioele , Bohn Moritz , Hüttinger Leon , Kutsch Tobias , Schuster Robin , Gasteiger Hubert Andreas 
Published: December 2024

All-Solid-State Batteries (ASSBs) have emerged as a highly promising post-lithium-ion battery technology, offering notable advancements in safety and energy density. However, an intrinsic disadvantage arises from the usually higher density of inorganic solid electrolyte materials compared to their liquid counterparts, constituting a drawback that hinders gains in overall energy density. To overcome this limitation, the imperative is to transition towards higher specific capacity materials, thereby unlocking the full potential for enhancing the energy density of ASSBs. On the cathode front, research predominantly centers around layered transition metal oxide materials like NMC, already widely employed in traditional lithium-ion batteries (LIBs). Simultaneously, on the anode side, there is a progressive shift from conventional graphite to silicon, which boasts an almost tenfold higher specific capacity, offering a compelling avenue to elevate the overall performance of ASSBs. Contemplating the potential upscaling of ASSBs, the most mature technology currently revolves around sulfide-based materials. Despite the considerable progress in research, only a limited number of studies have explored the cycling stability of ASSB full-cells at this stage.[1,2,3,4,5] This critical evaluation is essential, as it serves as the foundational evidence for the viability of a potentially commercial application.

In this work, we studied the cycling stability of full-cell ASSBs, composed of an Li6PS5Cl solid electrolyte separator sheet, and NMC622-Li6PS5Cl composite cathode, and a microcrystalline-silicon-rich (>94 wt. %) anode. Emphasizing the foundational role of uniaxial pressure during both assembly and operation, we aimed to elucidate its crucial impact on cell performance. It became evident that, beyond just the magnitude, the in-plane distribution of pressure plays a pivotal role in achieving and sustaining effective contacts across multiple solid-solid interfaces. Significantly elevated pressures, reaching up to 500 MPa, proved necessary for the complete densification of the composite cathode and for establishing optimal contact within the 2D area between the anode and the separator. Furthermore, maintaining a homogeneous operational pressure of at least 20 MPa emerged as a key factor in ensuring a stable cycle-life at a C/3 current rate for over 400 cycles until 20% capacity loss, as exemplary showed in Figure 1. This promising cycle life was observed for cells with an areal capacity of 2.9 mAh/cm², highlighting the critical interplay between pressure and overall cycling performance in full-cell ASSBs.

The second part of our investigation focused on determining the primary factors that contribute to the capacity fading observed in the here-examined ASSB full-cells. We endeavored to categorize these factors into two main groups, related to either increasing kinetic overpotentials or to active material losses. Within the first group, kinetic limitations could originate from either the anode or the cathode side, potentially due to ineffective/unstable solid electrolyte interphases at the anode (SEI) and/or at the cathode (CEI). These interphases are known to conduct ions less effectively than the Li6PS5Cl itself. Additionally, contact resistance arising from void formation and diffusion resistance due to a more tortuous lithium diffusion pathway in the active material, possibly caused by cracking, were identified as potential contributors to kinetic overpotential. In the material losses group, scenarios were explored where the active material could undergo chemical inactivity, such as the NMC crystalline transformation from a layered to a rock-salt structure at high potential. Mechanical inactivity might also occur when a portion of the active material becomes ionically and/or electrically disconnected from the rest of the cell stack. Finally, a critical focus was placed on lithium inventory loss, a phenomenon where lithium undergoes electrochemical consumption in parasitic reactions like the reduction of Li6PS5Cl to LiCl, Li3P, and Li2S during SEI growth.

Upon thorough examination, it became evident that a homogeneously applied compressive force is able to render the kinetic overpotential negligible during cycling by maintaining the contact between the several components of the cells. Therefore, the lithium inventory loss emerged as the predominant factor contributing to the observed capacity fading in the studied ASSB full-cells.

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Figure 1