Composite Electrode (LiNi_0.6Mn_0.2Co_0.2O₂) Engineering for Thiophosphate Solid-State Batteries: Morphological Evolution and Electrochemical Properties

Abstract

Solid-state batteries using thiophosphate solid electrolytes are believed to be the next generation solid electrolyte batteries, but they suffer from several issues, including also the engineering of the composite positive electrode. To date, the relationship between the morphology of the composite electrode and its electrochemical performance remains undetermined. By using Focused Ion Beam–Scanning Electron Microscopy (FIB-SEM) tomography, we investigated the engineering of several composite electrodes of polycrystalline NMC 622 (LiNi0.6Mn0.2Co0.2O2) particles and amorphous Li3PS4 solid electrolyte. Based on the morphological analysis of the electronic and ionic networks, it is possible to determine that the electrochemical performance limitation in this cell emerges from the surface contact between the NMC and the LPS as the sintering of the solid electrolyte (SE) is far from optimal. Additionally, similar analyses performed after the first charge show that the fading of the electrochemical is linked to the morphological evolution in the composite positive electrode.

Operando Focused Ion Beam–Scanning Electron Microscope (FIB-SEM) Revealing Microstructural and Morphological Evolution in a Solid-State Battery

“…The pristine surface is presented in Figure S2e . From the primary observation, the interface between the solid electrolyte and the composite electrode is hardly visible, indicating optimal contact during shaping as demonstrated previously. , Image analyses obtained from the cross-section reveal a surface fraction of 39.6% for NMC, 38.6% for LPS, and 21.8% for porosity which is in agreement with our previous investigation . The overall cross-section of the cell stack is presented in Figure S3 .…”

“…As already demonstrated, the polycrystalline particles of NMC622 fracture during the shaping of the cell components. Then, during the first charge (i.e., delithiation), particles that were intact before starting cycling begin to fracture along the multiple grain boundaries caused by the overall volume shrinkage of NMC622 ,, as shown in Figure b, c. This type of fracture has already been observed in Li-rich NMC positive electrode materials by an X-ray ptychography technique, and their extent was found to be about the size of the particles …”

“…Solid-state batteries are believed to be the next breakthrough in electrochemical energy storage systems if their chemo-mechanical degradations are controlled. − Different solid electrolytes are under investigation, but sulfide ones stand out in the category. − However, the main difficulties in solid-state batteries rely on the transport properties, both ionic and electronic, within the composite electrode, which can only be achieved thanks to intimate contact between the electroactive materials and the solid electrolyte. Even if the intimate contact before cycling can be controlled, unfortunately during cycling, the electroactive materials are breathing, due to Li in (de)tercalation processes, causing mechanical fracture and microstructural changes including interfacial contact loss . LPS-family (Li 2 S–P 2 S 5 ) could help to buffer the breathing processes since its elastic limits are favorable and could help maintaining the composite electrolyte cohesion during cycling. , As high energy density is targeted in solid-state batteries, NMC layer oxide materials should be a material of choice; nevertheless, it suffers some volume changes (4.4% at 4.2 V vs Li + /Li , ) during cycling: an increase of the c -axis lattice parameter with a decrease of the a -axis parameter causes mechanical stress on the secondary particles.…”

Operando Investigation on the Role of Densification and Chemo‐Mechanics on Solid‐State Cathodes

“…Composite cathodes with identical active material loadings (areal capacities) but different solid electrolyte particle sizes were processed. The particle size influences the microstructure as previously described [ 10,12,13,18,19 ] but also has implications on densification mechanisms [ 17,20 ] and the mechanical response of the cell during cycling. Combining synchrotron imaging, operando energy dispersive x‐ray diffraction, and modeling we systematically investigate how lithiations and mechanical gradients evolve in composite solid‐state cathodes and impact active material utilization.…”

“…These dynamic loads can induce internal strain at the cathode and drive interfacial contact loss, particle fracture, and increases in the porosity or void region in the composite cathode. [ 17 ] In this paper, we investigate how dynamic mechanical responses influence active material utilization in composite solid‐state cathodes. Model cathodes were constructed to control both the microstructure and mechanics of the cathode.…”