Lithium ionic conduction in composites of Li(BH4)0.75I0.25 and amorphous 0.75Li2S·0.25P2S5 for battery applications
Graphical abstract
Introduction
The emerging applications for Li-ion batteries are very demanding regarding high energy density, reasonable lifespan, stability under extreme conditions such as wide temperature range, mechanical stress and fast solicitations for high power devices [[1], [2], [3], [4]]. Solid electrolytes (SEs) can be beneficial in all these aspects compared to current liquid, gel or polymer-based electrolytes, for improved safety and performance of Li-ion batteries [[5], [6], [7], [8], [9]].
The glass system Li2S-P2S5 has been extensively studied as electrolyte for application in all-solid-state Li-ion batteries [[10], [11], [12], [13], [14]]. However, these electrolytes have been reported to create unstable interface with Li [11,15]. The addition of lithium halides to Li2S-P2S5 has been shown to increase the ionic conductivities and improve the contacts at the electrode/electrolyte interface [10,16]. Recently, the LiBH4-Li2S-P2S5 system has attracted attention owing to its adjustable ionic conductivity as function of composition for solid state battery electrolytes [17,18].
The crystal structure and ionic properties of LiBH4 has been the subject of many reports [[19], [20], [21], [22], [23], [24]]. It undergoes a first-order polymorphous transition around 113 °C, from orthorhombic (LT, Pnma, 10−8 S cm-1 at 30 °C) to hexagonal (HT, P63mc, 10−3 S cm-1 at 120 °C) involving a reorientation of the tetrahedral [BH4−] anions. High mobility of the complex anions [BH4−] in the HT-phase, along with short Li-Li distances in a compact structure with transport channels gave fast Li-ionic conduction [[24], [25], [26]]. Thus, LiBH4 is a good Li-ion conductor between the phase transition temperature and the melting point at 280 °C, i.e. an operating domain less than ΔT = 167 °C [22,27]. Below the transition temperature, the HT-phase reverts back to the poorly conducting LT-LiBH4. However, the Li-ion conducting hexagonal phase can be stabilized by partly substituting [BH4−] with halides, e.g. Li(BH4)0.75I0.25, thus suppressing the phase transition and preserving high ionic conductivity down to room temperature (RT), with a value close to 10−4 S cm−1 [[28], [29], [30], [31]]. This phase has been reported to form a stable electrode/electrolyte interface in a lithium battery-cell [26,32].
The present work explores the effect of mixing crystalline hexagonal Li-ion conducting Li(BH4)0.75I0.25 (LI) with amorphous 0.75Li2S·0.25P2S5 (LPS) for solid-state electrolyte applications. The prepared electrolytes were investigated with respect to their structural and ionic properties and application for all-solid-state batteries. The experimental analytical investigations are supported by DFT calculations to gain better understanding of the nature of the interaction between LI and LPS.
Section snippets
Materials synthesis and characterization
LiBH4 (95%), LiI (99.9%), Li2S (99.98%) and P2S5 (99%) were purchased from Sigma-Aldrich and stored in an Ar-filled glove box (<1 ppm O2, H2O). The halide-stabilized hexagonal phase Li(BH4)0.75I0.25 was synthesized according to the procedures described elsewhere [33]. The amorphous 0.75Li2S·0.25P2S5 was prepared by ball-milling for 20 h using a Fritsch Pulverisette 6 (P6) planetary ball-mill with stainless steel vials and balls (ball-to-powder ratio 40:1, 300 or 370 rpm) [15,28,34]. The final
Computational details
Total energies were calculated by the projected-augmented plane-wave (PAW) implementation of the Vienna ab initio simulation package (VASP) [38,39]. These calculations were made with the Perdew, Burke, and Ernzerhof (PBE) exchange correlation functional [40]. Ground-state geometries were determined by minimizing stresses and Hellman-Feynman forces using the conjugate-gradient algorithm with force convergence less than 10−3 eV Å−1. Brillouin zone integration was performed with a Gaussian
Results and discussion
Fig. 1a presents lab-PXD patterns of 0.75Li2S·0.25P2S5 samples before and after mechanical milling. The hand mixed sample shows Bragg peaks from Li2S and P2S5 without any noticeable peaks from any reaction products. After ball-milling, an amorphous phase with no Bragg peaks is formed in agreement with previous studies [15]. Ionic conductivity measurements were performed on 2 different LPS batches prepared at different ball-milling conditions, i.e. rotation speed 300 and 370 rpm. The high energy
Conclusion
A novel strategy was followed to prepare a solid-sate electrolyte based on intimate mixing of crystalline Li(BH4)0.75I0.25 (LI) with amorphous 0.75Li2S·0.25P2S5 (LPS). The temperature dependence of the ionic conductivities were measured and optimized by varying the LI/LPS wt. ratio. The highest RT ionic conductivity (∼10−3 S cm−1) is found for the system with the approximate nominal composition Li(BH4)0.75I0.25·(Li2S)0.75·(P2S5)0.25. It has the lowest activation energy of the investigated
Acknowledgements
This work is financially supported by Research Council of Norway under the program EnergiX, Project no. 244054, LiMBAT - “Metal hydrides for Li-ion battery”. PV acknowledges the Research Council of Norway for providing the computer time (under the project number NN2875k) at the Norwegian supercomputer. AE thanks Dr M. Heere for assistance with synchrotron data collection. We acknowledge the skillful assistance from the staff of SNBL at ESRF, Grenoble, France.
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