High lithium ion conducting Li2S–GeS2–P2S5 glass–ceramic solid electrolyte with sulfur additive for all solid-state lithium secondary batteries
Graphical abstract
Research highlights
► High conductivity over 1 × 10−3 S cm−1 for solid electrolyte using SSBM method. ► Solid electrolyte stability against LiCoO2 improved with elemental sulfur addition. ► Performance of all-solid-state batteries improved with double layer electrolyte.
Introduction
Lithium secondary batteries prevail as one of the most widely accepted power sources for portable electronics and mobile devices because of their superior performance over other rechargeable batteries [1]. While they have great potential for high energy density and long cycle life, as well as being lightweight, most of these batteries uses liquid electrolytes that are flammable and hazardous [2], [3]. Replacing the liquid electrolytes with solid state electrolytes (SSEs) would effectively eliminate the safety concern associated with the liquid electrolyte [4], [5]. However, low ionic conductivity and interfacial instability stand in the way of the commercialization of all-solid-state rechargeable lithium-ion batteries [6], [7].
Ball-milling has recently emerged as a more enticing method for SSE development over the melt quenching method [8]. The ball-milling process is relatively low cost and produces ultra-fine powders good for achieving high interfacial contact area between active materials and SSE powders [9]. Ball-milled SSE powders are often heat treated to achieve a crystalline structure capable of even higher conductivities than those reached by amorphous powder [10], [11], [12]. Furthermore, it has been shown that addition of a network former such as GeSe2 to the Li2S–P2S5 system results in even higher conductivity [9].
We have prepared sulfide-based lithium ion conducting SSE by the single-step ball-milling (SSBM) procedure [13], which combines ball-milling and heat treatment (HT) into one step. The Li2S–GeS2–P2S5 glass–ceramic electrolytes produced by SSBM exhibit high conductivities over 1 × 10−3 S cm−1 at ambient temperature. The all-solid-state cells with these SSE were confirmed to work as lithium secondary batteries. We also report on the inclusion of elemental sulfur into the Li2S–GeS2–P2S5 system which showed improved cycling stability and first cycle coulombic efficiency of Li/LiCoO2 all-solid-state cells. While it is well known that sulfur is highly insulating and its incorporation into electrolytes typically shows decreased ionic conductivity, it has a highly polarizable character that has potential to improve stabilization of electrolyte in contact with the electrode. Elemental sulfur was chosen for enhancing the cycling stability of the electrolyte for its highly polarizable character, as high polarizability of anions is well known to aid the formation of strong covalent bonds between the anions of the framework, effectively orienting charge density away from the interstitial ions and improving ion conduction [14].
Section snippets
Experimental
SSEs were prepared by SSBM of Li2S–GeS2–P2S5 for 20 h [9], [13]. Reagent-grade powders of Li2S (Aldrich, 99.999%), P2S5 (Aldrich, 99%), and GeS2 (City Chemical, 99.99%) were used as starting materials. Appropriate concentrations of materials were combined into a zirconia vial (Spex) at a net weight of 1 g with two zirconia balls (1 × 12 mm, 1 × 15 mm in diameter) for grinding. High energy ball milling (Spex8000) took place for 20 continuous hours in an Ar-filled dry box. Heat treatment of SSBM SSE
Results and discussion
A schematic diagram for ternary component Li2S–GeS2–P2S5 is shown in Fig. 2. As x in Li4−xGe1−xPxS4 increases, GeS2 decreases and P2S5 increases with relatively small changes of Li2S, and finally x = 1.00 corresponds with 75Li2S–25P2S5 (mol%).
Fig. 3(A) shows the recorded conductivities of the Li4−xGe1−xPxS5 (Li2S–GeS2–P2S5) electrolyte in the range of 0.70 < x < 1.00. We show that the SSBM procedure is superior to conventional ball-milling by approximately a factor of two for achieving extremely high
Conclusion
Glass–ceramic Li2S–GeS2–P2S5 (Li4−xGe1−xPxS4) electrolytes with various compositions from x = 0.70 to x = 1.00 were prepared by a simple SSBM process. The Li2S–GeS2–P2S5 SE showed high conductivities of maximum 1.2 × 10−3 S cm−1 for x = 0.95 in Li4−xGe1−xPxS4. Structural analysis showed that inclusion of GeS2 leads to the enlarged lattice structure followed by occurrence of amorphous structure. HT of the SSBM material with 1 wt.% elemental sulfur resulted in increased capacity (129 mA h g−1 at the second
Acknowledgments
This work has been supported by DARPA/DSO. Dr. Yoon Seok Jung acknowledges the Korea Research Foundation Grant funded by the Korean Government [KRF-2008-357-D00066].
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