| By using the new solid electrolyte to replace the liquid electrolyte of conventional Lithium Ionic Batteries (LIB), all-solid-state Lithium Batteries could efficiently resolve the issues of traditional LIB such as leakage, explosion, and huge encapsulation etc., which would be used as the next generation energy storage batteries. Superionic conductor Li10GeP2S12 (LGPS) of the highest conductivity of 12 mS/cm at room temperature reported by now is one of the most popular solid electrolyte materials. It was found that the similar family of compounds Li10SnP2S12 (LSnPS) and Li11Si2PS12 with a high conductivity of several mS/cm exhibits the same tetragonal-LGPS crystal structure. The lithium ionic dynamics is deeply bonded with the structure of materials. While the lithium ionic conductive mechanism of LGPS is controversial for a long time. The microscopically detailed lithium ionic diffusion pathways and the temperature dependent Li+ dynamics are needed to be verified experimentally. Clearly and specifically understanding the relationship between the Li+migration mechanism and crystal structure of LGPS family superionic conductors is of profound significance for the designation and synthesis of the new and high-performance alternative solid electrolytes.Superionic conductor Li10GeP2S12 exhibits the highest room-temperature ionic conductivity among the reported solid-state Li ionic electrolytes. The feature of the Li10GeP2S12 structure is its one-dimensional Li ionic channels in which vacancy concentration is high. Besides, there may also exist other ionic diffusion pathway which connects two adjacent one-dimensional channels to form the three-dimensional network. Two distinct Li ion diffusion processes in tetragonal structure materials Li10GeP2S12, along ultrafast 1D tunnel (Ea= 0.16 eV) and in plane perpendicular to 1D tunnel (Ea=0.26 eV), have been separately revealed by both 7Li and 31P multiple solid-state NMR methods. Furthermore, using two magnetically inequivalent 31P resonances as local probe of Li ion motion, we demonstrate that the interstitial Li(4) sites are involved in the in-plane Li ion migration. The ultra fast ID tunnel together with fast in-plane paths for Li ion migration thus ensures extraordinarily high bulk Li+ ion conductivity of Li10GeP2S12.Sn (Tin) is of the same family with Ge (Germanium) on periodic table of the elements. Compared to Ge, Sn has a higher atomic number and bigger atomic radius, as well as economic and environmentally friendly advantages. By substituting the Ge element of LGPS with Sn, we could effectively adjust the PS4 tetrahedron frame and the Lithium diffusion channel size of tetragonal-LGPS structure. Compared to LGPS, the XRD results of Li10SnP2S12 (LSnPS) shows a same space group, however, with some minor modification in crystalline structure. The ionic conductivity of LSnPS is only 1/3 of LGPS at room temperature. By multiple 7Li,31P NMR methods, I have investigated the dependence of ionic conductivity and dynamics of Li ion on the crystal structure, and then interpreted the microscopic Li+diffusion mechanism of LSnPS. It is found that (1) there also are two distinct Li ion diffusion processes in LSnPS materials, i.e. along ultrafast 1D tunnel (Ea=0.18 eV) and in plane perpendicular to 1D tunnel (Ea= 0.13 eV), where Li(4) sites are active in the in-plane Li ion migration; (2) The active energies of low-and high-temperature regimes of LSnPS are slightly higher and remarkably lower than corresponding ones of LGPS; however (3) oveiall in-plane Li migration rate is only about 1/10 of that in LGPS, which makes LSnPS a more anisotropic Li ionic conductor than LGPS at high temperature.Li10GeP2S12 is regarded as a metastable phase occurring in xLi3PS4:yLi4GeS4 system with x= 2, y= 1. Adjusting the ratio of x to y could change the Li ionic concentration and modify the crystalline structure of LGPS. Besides, as tetragonal-LGPS itself is a metastable phase, there is always an orthogonal-LGPS impure phase showing up during the synthesis processes. And this impure phase also appears in the Li10SnP2S12 and Li11Si2PS12, and remarkably reduces the total ionic conductivity of the received materials. The as prepared Li10SiP2S12, which should be of higher conductivity than LGPS by computing prediction, yields an awful Li conductive character because of the nearly 50% high impurity. Finding a way to control the content of impurity phase is of significant importance.Using Li10GeP2S12 as a model compound, we prepared a variety of family outcome samples of xLi3PS4:yLi4GeS4 system (x:y ratio from 2:1 to 1:4), named as Li4x+3yGexPyS4(x+y). By XRD and multiple solid-state NMR methods,the phase-stability and conductivity of Li4x+3yGexPyS4(x+y) have been investigated. It is interestingly found that the superionic conductors (of a "x:y" ratio from 1.4:1 to 1:1.3) exhibit both considerably high conductivity and tetragonal-LGPS structure with little impure phase. And the Li ionic conductive mechanism of a relatively pure Li7.8Ge1.2PS8.8 has been studied as well. Three distinct Li ion diffusion processes in g, along ultrafast 1D tunnel (Ea=0.19 eV) at low temperature and in plane perpendicular to 1D tunnel (Ea=0.28 eV) at high temperature, as well as a third Li ion migration process in the super-high temperature regime, have been separately revealed by multiple solid-state 31P NMR methods. However, we could not do further study on the third process due to the limitation of the working temperature of the probe. |
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