Revealing and Overcoming Unfavorable Electrochemical Behaviors of Thick LiNbO3-Coated NCM523 for All-Solid-State Lithium Batteries

All-solid-state batteries (ASSBs) are very promising for next-generation energy storage technologies owing to several key advantages including higher power density, better thermal and electrochemical stability, and improved safety for electric vehicles. In this work, bulk-type ASSB cells were prepared with 0–25 nm thick LiNbO _3 coatings, and their electrochemical behaviors at different upper cutoff voltages (upper cutoff potential ≥ 4.40 V) were systematically compared. A thicker coating caused three unfavorable electrochemical behaviors in the first three cycles: (1) a higher overpotential, (2) sluggish discharge kinetics, and (3) capacity fading. The measured electronic conductivity decreased drastically with increasing coating thickness, suggesting that this may have caused behaviors (1–3). To overcome this, a carbon additive was used to improve electronic transport in the composite cathode and successfully suppressed the aforementioned behaviors. Our findings indicate that the combination of a thick LiNbO _3 coating on NCM523 and carbon additive can achieve synergistic effects to improve both the electrochemical properties and durability of ASSB cells.