Cycle Degradation Analysis by High Precision Coulometry for Sulfide-Based All-Solid-State Battery Cathode under Various Potentials

Abstract

All-solid-state batteries (ASSBs) using sulfide-based solid electrolytes (SEs) are promising energy storage devices beyond the present liquid-type lithium-ion batteries (LIBs) using organic solvents, which are expected to realize the adaptation of new systems such as higher-voltage cathodes. However, in recent years, undesirable side reactions are being reported in the nanometer-order region at the interface of cathode active materials/SEs. Therefore, we evaluated the cycle durability of the all-solidstate cathode half-cells using an argyrodite-structured sulfide-based solid electrolyte at various potentials at 60 ℃ and then measured the coulombic behavior by high precision coulometry.Furthermore, the interfaces of the cathode active material/SE were observed using secondary electron microscopy (SEM), transmission electron spectroscopy (TEM), and electron diffraction (ED). In conclusion, a strong correlation was found between the coulombic behavior and material decomposition at the interface.

Degradation Analysis by X-ray Absorption Spectroscopy for LiNbO3 Coating of Sulfide-Based All-Solid-State Battery Cathode

“…The sample preparation, the ASSB cell fabrication, and the durability test have been explained in detail in a previous paper ,,, and will only be briefly described here.…”

“…Floating test sequence at constant potential (CP) was conducted at potentials of 4.25−4.55 V vs Li/Li + for 120 h at 60 °C. During the cycle test, physical cell degradation, such as cracking of NCM523 particles and the reduction of the contact with the SE, due to the expansion and contraction of the cathode active material, cannot be avoided at higher potential, 23 whereas the number of charge/discharge cycles in floating tests can be reduced to a minimum to focus on chemical degradation. Before the floating test, the halfcells were charged to 4.25, 4.35, 4.45, and 4.55 V vs Li/Li + at a constant current (CC) of 200 μA cm −2 (defined as C-rate of 0.1).…”

“…We have also conducted several studies on ASSB using sulfide-based SE, especially focusing on cathode of argyrodite-structured Li 6 PS 5 Cl/LiNbO 3 -coated LiNi 0.5 Co 0.2 Mn 0.3 O 2 (NCM523) system, with investigation of the range of operation temperature and cathode potential. ,,, In durability tests, the discharge capacity retention rate was >99% after continuous charging (floating) at 4.25 V vs Li/Li + and 60 °C for about 1 month, which showed extremely high durability of ASSB cathode . Although the retention rate had an excellent value of ∼80% even after floating in a harsh condition at higher potential of 4.55 V vs Li/Li + and 60 °C for about 1 month, the oxidization decomposition of sulfide SE into phosphates (PO x ) and sulfates (SO x ) components was clearly observed by using time-of-flight secondary-ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS). , To confirm the origin of the oxidization, NCM523 in obviously degraded cell at 4.55 V vs Li/Li + was analyzed by X-ray diffraction (XRD) and transmission electron microscope/electron energy loss spectroscopy (TEM/EELS).…”

Microscopic Degradation Mechanism of Argyrodite-Type Sulfide at the Solid Electrolyte–Cathode Interface

“…5,6 In cycling testing, physical degradation such as cracking of cathode active material due to expansion and contraction is added to lead more complex degradation mode. 17 Therefore, in this study, we adopted the floating test to reduce to a minimum cycle number. Before the floating test, the half cells were charged to 4.25 and 4.55 V vs Li/Li + at a constant current (CC) of 200 μA cm −2 (defined as a Crate of 0.1) and discharged to 3.0 V vs Li/Li + at a CC of 0.1 C-rate.…”

“…However, ASSBs employing sulfide-based SEs have some problems: vulnerability of the sulfides, − expansion–contraction influence of active materials, − low reduction durability for lithium metal anodes, and oxidative decomposition side reactions at the interface of the SE/cathode active material. ,,,− Among these, we focused on the cathode side as it often determines ASSB performance in terms of cell resistance and capacity. The chemical/physical/mechanical issues of ASSB cathodes employing inorganic SEs are simplified by Banerjee et al We investigated the degradation mechanism of the sulfide-based argyrodite-type SE Li 6 PS 5 Cl/LiNbO 3 -coated LiNi 0.5 Co 0.2 Mn 0.3 O 2 (NCM523) employing various techniques, such as electrochemical impedance spectroscopy, time-of-flight secondary-ion mass spectrometry (ToF-SIMS), , electron energy loss spectroscopy, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), ,, electron diffraction (ED), and X-ray absorption spectroscopy (XAS) . The results indicated that almost no degradation was observed on the NCM523 cathode active material side and that degradation near the interfacial region of the SE/LiNbO 3 coating was more dominant.…”