Recently, the demand for rechargeable batteries with higher capacity has been increasing. Bulk-type all-solid-state batteries (ASSBs) are expected as candidates for one of the next generation batteries. 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, which restricts high rate performance [2].

We previously demonstrated that formation of surface oxide layer on sulfide solid electrolyte (O-SSE) reduced the interfacial resistance and improved the ASSBs performance due to improvement of the oxidation stability of the SSE [3]. For further enhancement of rate capability, we herewith better controlled the formation of surface oxide layer to reduce the interfacial resistance while suppressing the bulk conductivity reduction.

It is known that in the system Li₂S-P₂S₅, the composition of Li₂S and P₂S₅ affects not only ionic conductivity but also the interfacial resistance [4]. So, first, we optimized Li₂S-P₂S₅ ratio of O-SSE. SSE with three different compositions, x Li₂S – (100 – x) P₂S₅ (x = 70, 75, 80), were synthesized, and then the surface oxide layer was formed by annealing in O₂ atmosphere (to be referred as O-7030, O-7525, O-8020, respectively). Among batteries using these electrolytes in cathode layer, the battery using O-7525 showed the lowest interfacial resistance with LiNiCoAlO₂ cathode.

In the next step, the new synthesis process of O-SSE was developed to maximize exchange yield of the near-surface sulfur with oxygen and to minimize the thickness of oxide layer so that the abrupt oxide-sulfide junction was to be formed. In the previously reported process, SSE powder was sealed in a vessel in an Ar-filled glove box and O₂ gas was introduced into the vessel during annealing. It took several hours to react the entire powder with O₂ gas, and after annealing, ionic conductivity decreased due presumably to oxygen diffusion deep into SSE particles during long annealing time.

For improving reactivity, the vessel was sealed under vacuum before O₂ gas introduction, so that O₂ gas uniformly spread into the powder. Thanks to this new process, significantly shorter anneal time, only within 10 min, was needed to have the same level of total oxygen content per SSE particle. This prevented the oxygen atoms from diffusing into inside the SSE particles and resulted in more abrupt sulfide core / oxide shell structure with thin oxide layer, and the ionic conductivity of O-SSE improved from 3.7×10^-5 S cm^-1 to 1.8×10^-4 S cm^-1.

These composition optimizations and the new synthesis process of O-SSE improved the battery discharge capacity from 40.7 mAh g^-1 to 86.3 mAh g^-1 at a high current density of 2.0 mA cm^-2. This improvement can be attributed to the simultaneous improvement of both the interface stability against cathode materials and the ionic conductivity of O-SSE.

In conclusion, we demonstrated that the composition and synthesis process optimization of O-SSE improved the ASSBs performance, especially discharge characteristic at a high current density.

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

[2] N. Ohta et al, Electrochem. Commun., 9 (2007) 1486.

[3] I. Sasaki et al, 233rd ECS Meeting (2018).

[4] T. Ohtomo et al, J. Power Sources, 233 (2013) 231.