Lithium-ion batteries are considered to play an important role in future energy storage, especially for mobile applications such as vehicle propulsion. One approach to overcome both safety and durability problems of state-of-the-art Lithium-ion batteries is the use of solid electrolytes, which must satisfy the criteria of having high Li ion conductivity and a wide electrochemical stability window. In 2011, a new solid electrolyte Li10GeP2S12, a metastable phase occurring in the system xLi4GeS4:yLi3PS4, was reported . Tetragonal LGPS combines the most important prerequisites for a high-performance Li electrolyte: a room-temperature conductivity of 12mScm−1, an activation energy of 0.24eV, and an electrochemical window of up to 4V vs. Li/Li+ . While tetragonal LGPS has only been studied by means of MD and ab initio calculations since its original publication, a fundamental study on the Li ion dynamics occurring in tetragonal LGPS has not been reported yet.
Here we present a fundamental study on the structure and dynamics of LGPS electrolytes [2,3]. Both Li10GeP2S12 and Li7GePS8, a new member of the solid solution of tetragonal LGPS with a Ge:P ratio of 1:1, were prepared and structurally characterized. The Li ion dynamics occurring in the materials was studied by several complementary techniques sensitive to (i) long-range Li diffusion (PFG-NMR), (ii) atomic-scale jumps (NMR relaxometry), and (iii) long-range charge transport (impedance spectroscopy). This combination of techniques allows us to connect diffusion (at the macroscopic scale) to ionic hopping in the LGPS lattice (at the microscopic scale). In the studied temperature range between 110K and 450K, the Li ion dynamics is well described by an Arrhenius law with an activation energy as low as 0.22(1)eV.
Synthesis and Structural characterization
Fig. 1: A: Structural characterization of Li7GePS8 and Li10GeP2S12. (A): Left: unit cell of tetragonal Li10GeP2S12 with thermal ellipsoids (p=0.8) as obtained from single crystal X-ray diffraction. Right: measured and simulated TEM precession electron diffraction (PED) patterns of Li7GePS8 for selected orientations. (B) XRPD patterns with single-phase Rietveld refinement. The asterisks denote reflections of the orthorhombic modification (side phase). (C): 31P MAS NMR spectra of Li7GePS8 and Li10GeP2S12. The asterisk denotes the 31P signal of a minority phase of low crystallinity.[less]
Fig. 1: A: Structural characterization of Li7GePS8 and Li10GeP2S12. (A): Left: unit cell of tetragonal Li10GeP2S12 with thermal ellipsoids (p=0.8) as obtained from single crystal X-ray diffraction. Right: measured and simulated TEM precession electron diffraction (PED) patterns of Li7GePS8 for selected orientations. (B) XRPD patterns with single-phase Rietveld refinement. The asterisks denote reflections of the orthorhombic modification (side phase). (C): 31P MAS NMR spectra of Li7GePS8 and Li10GeP2S12. The asterisk denotes the 31P signal of a minority phase of low crystallinity.
Both Li7GePS8 and Li10GeP2S12 were synthesized from stoichiometric amounts of Li2S, Ge, P, and a slight excess of S. The starting materials were mechanically treated in a high-energy ball mill and then heated in evacuated quartz tubes at temperatures between 420°C and 550°C. Figure 1 summarizes the structural characterization of Li10GeP2S12 and Li7GePS8 in a comparative fashion. The structure of Li10GeP2S12 was determined using single-crystal X-ray diffraction. The obtained structure is shown in Fig. 1(a). The previously reported structural model, which had been obtained from Rietveld refinement of X-ray powder diffraction data, was largely verified (space group, atomic coordinates). However, a fourth Li position was clearly revealed. This Li site, which connects the 1D Li channels along the c-axis (see Fig. 1(a)), has significant impact on the Li ion dynamics since it allows for 3D conduction (see Ref.  for further details).
The measured TEM-PED patterns of Li7GePS8 show good agreement with the structural model for Li7GePS8 obtained by Rietveld refinement, and the Ge:P ratio as determined by TEM-EDX (individual crystallites) and SEM-EDX (powder) is in line with the sum formulae given here. The occupancy of the mixed occupied GeP site (Fig. 1(a)) was independently determined from 31P NMR and Rietveld refinement of the X-ray powder diffraction data and both yielded results which were in line with the expectations according to the formula Li11−x(Ge2−xPx)PS12. Hereby, x=1 corresponds to Li10GeP2S12 and x=0.5 to Li7GePS8 (see Ref.  for further details).
Li ion dynamics
The long-range Li ion dynamics (see Figs. 2(a),(b)) was assessed by two complementary methods: the dc-conductivity (=long-range charge transport) was determined using impedance spectroscopy as well as 7Li PFG NMR, which is sensitive to Li dynamics on the micron scale and gives direct access to Li tracer diffusion coefficients. Both the bulk conductivity (Fig. 2(b)) and the Li tracer diffusion coefficients (Fig. 2(a)) are activated by 0.22(2)eV, suggesting that both phenomena describe the same process, i.e. Li ion dynamics in tetragonal LGPS. For comparison, the Li diffusion coefficients as obtained from MD simulations (Mo et al. Energy & Environmental Science, Chem Mater. 24, 15-17 (2012)) are included in Fig. 2(a). The experimentally assessed 3D diffusion coefficients are in very good agreement with the values obtained in the simulations for diffusion perpendicular to the c-axis, Dab. This is expected since the macroscopic 3D self-diffusion will be limited by the slower process Dab (see Ref.  for further details).
7Li NMR relaxometry is sensitive to site-to-site hopping of Li ions on the Ångström scale and thus probes short-range Li dynamics. Hereby, transversal relaxation (7Li line shape analysis, see Fig. 2(c) and 2(f)) is sensitive to relatively slow hopping (jump rates τ–1 in the order of 104 s−1), while longitudinal relaxation (Fig. 2(e)) is sensitive to fast hopping (jump rates τ–1 in the order of 109s−1). Li jump rates τ–1 determined from both techniques are shown in Fig. 2(d). Li hopping in tetragonal LGPS is characterized by an activation energy of 0.22(1)eV.
The Li ion dynamics in LGPS was characterized by means of long-range-sensitive (dc-conductivity, 7Li PFG NMR) and short-range-sensitive (7Li NMR relaxometry) techniques. The results of these complementary techniques can be compared using the Einstein-Smoluchowski equation Duc=α2/6×τ–1 (jump distance a, jump rate τ–1) and the Nernst-Einstein equation Dσ=kBT/(Nq2)×σdc (Li+ conductivity σdc, number density of Li+ N, charge of Li+ q, Boltzmann’s constant kB). The correlation factor f and the Haven ratio HR connect the quantities Duc and Dσ with the tracer diffusion coefficient Dtr via Dtr=f×Duc=HR×Dσ. f and HR are on the order of unity for simple diffusion mechanisms. Figure 3 shows the Dtr as determined from PFG NMR together with Dσ calculated from the dc conductivity and Duc derived from the jump rates whereby the number density of Li, N, and the jump distance a were taken from the LGPS crystal structure.
Obviously, the results from all techniques are in perfect agreement. The high conductivity observed for LGPS is thus unambiguously traced back to Arrhenius-activated ionic hopping of Li in the LGPS lattice in a wide temperature range and a dynamic range of six orders of magnitude.
Reproduced from Refs.  and  with permission from the Royal Society of Chemistry.