Chemical and Structural Changes of Li2s-P2S5 Solid Electrolyte during Heat Treatment
Authors: Aoki Yasuhito , Ogawa Kengo , Nakagawa Takeshi , Hasegawa Yuichi , Sakiyama Yoko , Kojima Toshikatu , Tabuchi Mitsuharu 
Published: February 2020

1. Introduction
Solid electrolyte is the key material for the all solid state lithium ion battery, which possesses higher safety than current lithium ion battery with organic electrolyte solvent. In addition to the safety advantage, solid electrolyte can operate under wider range of temperature, and has higher chemical stability against high voltage cathode material. So far, many researches on oxide and sulfide solid state electrolyte are reported, and particularly, sulfide electrolyte attracts many interests because of its higher lithium ion conductivity (1-3). Among various solid state electrolytes, Li2S-P2S5 based sulfide electrolyte is one of the most promising materials, and it is known that ion conductivity of sulfide electrolyte depends on its chemical component ratio and heat treatment temperature (4, 5). In order to optimize the component ratio and heat treatment condition, the detail analysis during the heat treatment is quite important. In this study, chemical changes and crystallization behaviors of Li2S / P2S5= 70 : 30 system are investigated.
2. Experimental
The sulfide electrolytes were synthesized from Li2S and P2S5 by mechanical milling using a planetary ball mill apparatus. The mixture of reagent grade crystal of Li2S and P2S5 with the ratio 70 : 30 was put into the zirconia pot and mixed about 30 hours under N2 atmosphere. The obtained glass sample was heated under Ar atmosphere from room temperature to 270 °C for 1 hour to obtain glass ceramics sample. For the in situ measurement during heat treatment, the glass sample was put into glass capillary in the Ar atmosphere glove box and sealed under the same atmosphere. The structural changes of the glass sample during heat treatment were investigated by in situX-ray diffraction (crystallization behavior) and Raman spectroscopy (chemical structural changes). The generated gases during heat treatment can be detected by TPD-MS (temperature programmed desorption MS).
3. Results and discussions
Figure 1 shows Raman spectra of glass and glass ceramics sample. The observed Raman bands are assigned to PS stretching of PS4
3- and P2S7
4- structure. From the comparison of Raman spectra, spectral sharpening of each Raman bands of glass ceramics sample is observed and the change is derived from the crystallization of the glass sample. To investigate what happened during the crystallization process, the gas generated during the heat treatment were analyzed (Figure 2). H2S is detected from around 100 °C to 250 °C and S compounds are also detected from 200 °C, and the generation maximum is around 350 to 400 °C. In the in situ Raman spectra (Figure 3), the Raman band of polysulfide can be detected around 500 cm-1 from 200 °C, and the temperature of polysulfide generation is coincide with the detection temperature of S compounds by TPD-MS. The origin of sulfur generation around 350 to 400 °C and the relationship with crystallization behavior will be discussed in the presentation considering the data of in situXRD diffraction patterns.
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Figure 1