Chemical structure analysis of sulfide-based solid electrolytes by ³¹P solid-state NMR

³¹P solid-state NMR spectra of the four samples are shown in the figure on the right. Large differences in line width in the spectra are seen between crystalline samples and glass samples. Since a crystal has uniform bond length and angle, the line width is narrow for exhibiting a linear Lorentz-type spectral shape. But for a glassy state, the bond lengths and angles are distributed, resulting in a thicker line width and a Gaussian-type spectral shape.
The spectrum of 70LPS-g can be separated into three types of Gaussian components, and it is found that the three units are constituted from the chemical shift range; PS₄^3-, P₂S₇^4-, and P₂S₆^4-. On the other hand, in the spectrum of 70LPS-gcg, which was crystallized once and vitrified again, crystal components of Li₇P₃S₁₁, which have a narrow line width, slightly appear, and the area ratio of the three units is found to change. Thus, solid-state NMR elucidates the difference of in chemical structure even in the same glassy state.
Li₇P₃S₁₁ crystal consists of two units: PS₄^3- unit and P₂S₇^4- unit. Since the number of P atoms are two coming from PS₄^3- unit, and four coming from P₂S₇^4- unit, the area ratio of ³¹P signal is 1:2. With this as constraint conditions, when peak deconvolution was performed for 70LPS-gc, it was found to have glass components coming from PS₄^3-, P₂S₇^4- units with a thicker line width and a Gaussian-type spectral shape, other than Li₇P₃S₁₁ crystal.
On the other hand, 70LPS-gcgc clearly has a larger PS₄^3- component and exhibits a Lorentzian spectral shape with a narrow line width. This result confirms that not only the Li₇P₃S₁₁ crystal, but also the PS₄^3- single-unit crystal is formed. Despite the low degree of crystallinity, the ionic conductivity of 70LPS-gcgc is 1.7 times higher than that of 70LPS-gc, suggesting that the newly-emerged PS₄^3- unit forms a new ion conduction path.