Variation in structure and Li+-ion migration in argyrodite-type Li6PS5X (X = Cl, Br, I) solid electrolytes

Abstract

All-solid-state rechargeable lithium-ion batteries (AS-LIBs) are attractive power sources for electrochemical applications due to their potentiality in improving safety and stability over conventional batteries with liquid electrolytes. Finding a solid electrolyte with high ionic conductivity and compatibility with other battery components is a key factor in raising the performance of AS-LIBs. In this work, we prepare argyrodite-type Li 6 PS 5 X (X = Cl, Br, I) using mechanical milling followed by annealing. X-ray diffraction characterization reveals the formation and growth of crystalline Li 6 PS 5 X in all cases. Ionic conductivity of the order of 7×10 −4 S cm −1 in Li 6 PS 5 Cl and Li 6 PS 5 Br renders these phases suitable for AS-LIBs. Joint structure refinements using high-resolution neutron and laboratory X-ray diffraction provide insight into the influence of disorder on the fast ionic conductivity. Besides the disorder in the lithium distribution, it is the disorder in the S 2− /Cl − or S 2− /Br − distribution that we find to promote ion mobility, whereas the large I − cannot be exchanged for S 2− and the resulting more ordered Li 6 PS 5 I exhibits only a moderate conductivity. Li + ion migration pathways in the crystalline compounds are modelled using the bond valence approach to interpret the differences between argyrodites containing different halide ions.

Facile Synthesis toward the Optimal Structure-Conductivity Characteristics of the Argyrodite Li6PS5Cl Solid-State Electrolyte

“…The lattice parameters of Li 6 PS 5 Cl in the samples annealed at 400–600 °C have similar values as shown in Figure 3b, comparable to previously reported values. 12 Increasing the annealing temperature to 550 °C leads to a clear decrease in the background of the XRD patterns (seen in Figure 1), indicating that the amorphous fraction decreases and the crystalline fraction increases. In addition, the linewidth of the reflections is the smallest at 550 °C.…”

“…ND is in particular important for determining the Li-ion positions and the Cl/S distribution over the 4a and 4c sites which has been shown to be a key parameter for the ionic conductivity. 12,18,21 …”

“…8−10 An important family of the sulfide-based solid electrolytes is the Li-argyrodite Li 6 PS 5 X (X = Cl, Br, and I) providing Li-ion conductivities of up to 10 –3 S cm –1 at room temperature. 11,12 The high conductivity and the low-cost of the starting materials make the Li-argyrodite a viable candidate for application in all-solid-state batteries.…”

Boosting Solid‐State Diffusivity and Conductivity in Lithium Superionic Argyrodites by Halide Substitution

“…Li-argyrodites,L i 6 PS 5 X( X= Cl, Br), fall into this latter family and are easily synthesized using inexpensive precursors.T heir cubic crystal structures F " 43m ÀÁ are comprised of PS 4 3À tetrahedra, with isolated S 2À and X À ions disordered over the 4 a and 4 c Wyckoff sites in the lattice,a nd Li + ion sites that form cagelike Frank-Kasper polyhedra around the anions. [7] While room-temperature Li-ion conductivities initially reported for the Cl (1.9 mS cm À1 ) [8] and Br phases (0.7 mS cm À1 ), [9] were within practical ranges,r ecent values are even higher and comparable to those of liquid electrolytes.T otal ionic conductivities are heavily influenced by the synthesis method, grain boundary contributions,and methods used for conductivity measurements,i ncluding sintering cold-pressed pellets. [10] Solution-based Li 6 PS 5 Xs ynthesis routes typically give lower ion conductivities (10 À5 -10 À4 mS cm À1 ) [11][12][13] owing to phase impurities although recent studies using this method report high values for Li 6 PS 5 Cl and mixed anion Li 6 PS 5 (Cl,Br) argyrodites of 2.4 and 3.9 mS cm À1 ,r espectively; [14] and 3.1 mS cm À1 for Li 6 PS 5 Br.…”

Ionic Conductivity of the NASICON‐Related Thiophosphate Na1+xTi2−xGax(PS4)3

“…Many material classes are currently being investigated,s uch as oxides and phosphates, that is, the garnets [3,4] and NASICON (Na SuperIonic CONductor) materials, [5] as well as thiophosphates.I np articular,t he thiophosphates,f or example, Li 10 GeP 2 S 12 ,L i 6 PS 5 Xa nd Na 3 PS 4 ,a mong others, have shown to be promising materials due to the soft mechanical naturea nd the intrinsically highi onic conductivity. [6,7,[16][17][18][8][9][10][11][12][13][14][15] Typical approaches to attain even higher ionic conductivities within the respective classes involve either the use of aliovalent substitution to increase the number of mobile chargeso rt he introduction of softer,more polarizable anions and aconcurrent widening of the diffusion pathways. [19][20][21] Indeed,t he softer anion lattice with more polarizable anions has been corroborated to lower the activationb arrier, [22][23][24][25] thereby explaining the high ionic conductivity in many of the Li + and Na + conductingt hiophosphates.H owever,i th as re-cently been shown that as ofter lattice not only lowers the migration barrier,but also affects the entropy of migration, which can also lead to an overall lower ionic conductivity.…”