Recently, more and more plug-in hybrid and electric vehicles have been put to practical use and the demand for rechargeable batteries with higher capacity has been increasing. Bulk-type all-solid-state batteries (ASSBs) are expected as candidates for next generation batteries, because their volumetric energy density may be improved by a bi-polar stacked structure. Among solid electrolytes used for ASSBs, sulfide solid electrolyte (SSE) have attracted much attention because of its high ionic conductivity and high deformability [1].

One of the main drawbacks of ASSBs using SSE is high interfacial resistance between cathode material and SSE. Calculations of the stability window of SSE have revealed that SSE is electrochemically oxidized and decomposed above 2.5 V vs. Li/Li⁺ and the interfacial resistance is mainly originated from the decomposition layer including an elemental sulfur [2]. These results suggest that suppression of electrochemical decomposition of SSE will reduce the interfacial resistance and, as a result, improve the electrochemical performance of ASSBs.

In order to suppress the electrochemical decomposition of SSE, a method introducing oxide such as Li₂O into SSE has been proposed [3,4]. Although introducing oxide improves the electrochemical stability of SSE, the ionic conductivity decreases because lithium ions are more deeply bound to oxide ions whose electronegativity are higher than sulfide ions. So, it has been difficult to realize the high electrochemical stability and high ionic conductivity at the same time.

In this study, we focused on the synthesis and electrochemical evaluation of surface oxidized sulfide solid electrolyte (O-SSE). O-SSE has the SSE structure with high ionic conductivity inside the particle and has the oxide layer with high electrochemical stability on its surface, which cathode material directly contacts with.

We synthesized O-SSE by annealing SSE (80Li₂S-20P₂S₅) in O₂ atmosphere. For comparison, we synthesized Li₂O-SSE (56Li₂S-20Li₂O-24P₂S₅) by conventional mechano-chemical process.

In order to verify the oxygen distribution within the particles, Li₂O-SSE and O-SSE were analyzed by XPS depth profiling with C₆₀ cluster ion sputtering. P2p peak of Li₂O-SSE showed ca. 132.1 eV, which matched P-S bond, regardless of sputtering time. On the other hand, as sputtering time proceeded, P2p peak of O-SSE shifted from ca. 133.3eV, which matched P-O bond, to ca. 132.1 eV. It was confirmed that the surface oxide layer including P-O bond was formed on the surface of O-SSE.

The ionic conductivity of Li₂O-SSE and O-SSE showed 1.1×10^-4 Scm^-1 and 2.0×10^-4 Scm^-1, respectively. O-SSE showed higher ionic conductivity than Li₂O-SSE.

Discharge capacities of ASSBs, using LiNiCoAlO₂ (NCA) as a cathode material and Li₂O-SSE or O-SSE as a solid electrolyte, were 122.8 mAhg^-1 and 158.5 mAhg^-1, respectively. Discharge capacity of ASSBs improved significantly by using O-SSE as a solid electrolyte.

Therefore, we demonstrated that formation of surface oxide layer on SSE improved the ASSBs performance by improvement of the oxidation stability without reducing bulk ionic conductivity.

[1] A. Hayashi et al, Solid State Ionics, 175 (2004) 683

[2] G. Ceder et al, Chem. Mater., 28 (2016) 266

[3] J. E. Trevey et al, Solid State Ionics, 214 (2012) 25

[4] T. Ohtomo et al, J. Solid State Electrochem., 17 (2013) 2551