Designing Cathodes and Cathode Active Materials for Solid‐State Batteries

Abstract

CAM) in its lithiated form, that is, as present in a discharged cell. In its delithiated form, when the cell is charged, it is the only cell component that is contributing to storing energy (in conjunction with a hypothetical in situ lithiumplated anode formed during charging), thus making it the material required to be present in large quantity to achieve a high-performing cell. All other components, which may be required for large scale processing, only decrease the specific energy of the cell and are, therefore, engineered to minimize their content without affecting the function of the cell. This is evident in the research efforts made to increase the CAM content in the cathode layer, decrease the separator thickness as much as possible, and the pursuit to plate lithium metal in situ (in "anode-free" cells, which are more correctly described as "zero excess lithium metal" cells) without the use of an anode active material. [4] Thus, the CAM type and content in the cell ultimately determine the maximum specific energy that the system can provide.Moreover, the CAM contributes a significant proportion to the overall cell costs, [5] hence the necessity of steady tailoring toward reduced costs and higher energy density. So far, CAM development has mainly targeted performance optimization with LEs in LIBs. For instance, cathode electrolyte interface (CEI) formation, [6] Solid-state batteries (SSBs) currently attract great attention as a potentially safe electrochemical high-energy storage concept. However, several issues still prevent SSBs from outperforming today's lithium-ion batteries based on liquid electrolytes. One major challenge is related to the design of cathode active materials (CAMs) that are compatible with the superionic solid electrolytes (SEs) of interest. This perspective, gives a brief overview of the required properties and possible challenges for inorganic CAMs employed in SSBs, and describes stateof-the art solutions. In particular, the issue of tailoring CAMs is structured into challenges arising on the cathode-, particle-, and interface-level, related to microstructural, (chemo-)mechanical, and (electro-)chemical interplay of CAMs with SEs, and finally guidelines for future CAM development for SSBs are proposed.

Phase Transformation and Microstructural Evolution of CuS Electrodes in Solid‐State Batteries Probed by In Situ 3D X‐Ray Tomography

“…The observed formation of cracks and voids during cycling of SSBs is believed to be a major cause for poor electrode utilization and capacity loss. This can be mitigated to some extent by optimizing the pellet fabrication pressure and the stack pressure (pressure during cycling), [4][5][6]17] but studies on how the stack pressure impacts the inner microstructure are still scarce, especially under in situ conditions. [4,8] The results discussed so far were obtained at a stack pressure of 26 MPa.…”

“…cell degradation. [2,16] Relevant aspects of designing the structure of cathodes for SSBs have recently been discussed by Minnmann et al [17] In search of alternative CAMs, copper sulfide has been recently studied in SSBs. Copper sulfides are naturally occurring minerals and show specific properties that make them appealing for SSBs.…”

Mapping and Modeling Physicochemical Fields in Solid-State Batteries

“…29−31 Because electrode cracks can occur regardless of the type of electrolyte used (liquid or solid), 31 we intend not to extensively discuss this topic in the Perspective, but rather direct the readers to a large body of existing works and reviews. 32 However, we do note that electrode cracks in solid-state batteries, different from that of conventional batteries, can produce "dead" cathode particles which are isolated from electron and/or ion transfer pathways. For quasisolid and all-solid-state batteries, in addition to the electrodes, SSEs can also crack, which leads to further electrode− electrolyte interface reactions and/or loss of ion-transport pathways.…”

“…Besides the commonly observed irreversible reactions of the electrodes and between the electrode and electrolyte, material cracks are also a major cause of capacity decay. The electrode materials can crack during cycling processes because of repeated volume expansion and contraction. , An electrode crack exposes fresh surfaces that can react with the liquid electrolyte, which decreases the Coulombic efficiency. − Because electrode cracks can occur regardless of the type of electrolyte used (liquid or solid), we intend not to extensively discuss this topic in the Perspective, but rather direct the readers to a large body of existing works and reviews . However, we do note that electrode cracks in solid-state batteries, different from that of conventional batteries, can produce “dead” cathode particles which are isolated from electron and/or ion transfer pathways.…”

Alleviating Anisotropic Volume Variation at Comparable Li Utilization during Cycling of Ni-Rich, Co-Free Layered Oxide Cathode Materials

“…Thus, Co-free NMX compositions may instead be more practical in lower cost applications where the cathode material does not limit the power density of the system. NMX materials could also hold promise for use in conjunction with solid electrolytes (SEs) because of the lower anisotropic volume changes, which may be beneficial to the chemomechanics of the system . The use of novel halide SEs, which are stable over a wide electrochemical potential window, could enable NMX-based systems with higher energy density and stable cycling by enabling operation at high voltage to achieve high Li utilization while avoiding chemical decomposition of the electrolyte, while the stable structural evolution helps to maintain contact between the CAM and SE.…”