Structural and Dynamical Origin of Ionic Conductivity As Studied By Solid-State NMR

Abstract

Ionic conductors are promising electrolyte of solid-state batteries. Their major advantages are high ionic conductivity, electrochemical stability and inflammability. Various works have been done to explain the origin of high ionic conductivity. For example, the high ionic conductivity in the order of 10-3 Scm-1 at room temperature of Li7P3S11 metastable crystal, which is synthesized by annealing (Li2S)70(P2S5)30 glass at 513 K [1, 2], has attracted much interest. By analyzing its synchrotron X-ray powder diffraction pattern, Yamane et al. showed that Li ions are situated in the conduction pathways formed by PS4 tetrahedra and P2S7 ditetrahedra units [3]. The conduction pathway has been attributed to as the structural origin of high conductivity. We thus applied 6/7Li and 31P solid-state NMR to examine structural/dynamical origins of high conductivity of glass and glass-ceramics Li2S-P2S5 samples. (Li2S) x (P2S5)100 – x (mol %; x = 70 and 75) glasses were prepared by mechanical milling. Li2S and P2S5 crystalline powders were used as the starting materials. The glass-ceramics sample was prepared by heating the glass at 290 °C for 2 h. All the processes were performed in a dry Ar atmosphere. The NMR measurements were made using a JEOL ECA600 NMR spectrometer at 14 T. The MAS NMR spectra were observed by using a single pulse with the Hahn-echo. The spin-lattice relaxation times (T 1) were measured under MAS by using the conventional saturation-recovery method. To examine the origin of the linewidth of signals of 31P NMR, we applied the two-dimensional (2D) Radio Frequency Driven Recoupling (2D-RFDR). It was performed with an experimentally optimized p pulse length of 2.2 ms using XY16 phase cycling. While 6/7Li and 31P spin-lattice relaxation times exhibit that 31P relaxation of the glass sample is governed by 7Li motion, those of the glass-ceramics sample indicate 31P motion is appreciable at temperatures higher than ca. 310 K. The results showed that dynamics of the PS4 tetrahedra and P2S7 ditetrahedra units in (Li2S)70(P2S5)30 glass-ceramic is not appreciable at temperatures below ca. 310 K, where the ionic conductivity is low. At higher temperatures, however, significant motion especially for the P2S7 ditetrahedra unit is apparent in both of 31P-T 1 and 31P MAS lineshapes. High-resolution 31P MAS NMR spectra of the glass-ceramics sample bear the 31P signals of PS4, P2S7, and P2S6. In addition to these signals reported previously, we found two signals, which have not yet been assigned. Interestingly, these two signals are merged into the P2S7 signal at higher temperatures. Further, we applied 31P-31P dipolar correlation experiment to examine the 31P linewidth, which is reduced by motion at higher temperatures. It was shown that the linewidth of the P2S7 unit is attributable to distribution of local structures of and around the P2S7 ditetrahedra unit. With these, we concluded that the significant motional fluctuation of the P2S7ditetrahedra unit at above 310 K allows facile diffusive motion of lithium ions, leading to the high ionic conductivity [4]. As has been described, NMR is a powerful tool to correlate the local environment of ions and ion transportation (= ionic conductivity) and can be applied to other systems including F- (or other ion) conductive systems. 1) F. Mizuno et al., Adv. Mater. 17, 918-921 (2005). 2) F. Mizuno et al., Electrochem. Solid-State Lett. 8, A603-A606 (2005). 3) H. Yamane et al., Solid State Ionics 178, 1163–1167 (2007). 4) M. Murakami et al., J. Phys. Chem. C 119, 24248-2425 (2015). Acknowledgment: This work was financially supported by the RISING project of the New Energy and Industrial Technology Development Organization (NEDO).