Abstract
Several “Beyond Li-Ion Battery” concepts such as all solid-state batteries and hybrid liquid/solid systems envision the use of a solid electrolyte to protect Li-metal anodes. These configurations are very attractive due to the possibility of exceptionally high energy densities and high (dis)charge rates, but they are far from being realized practically due to a number of issues including high interfacial resistance and difficulties associated with fabrication. One of the most promising solid electrolyte systems for these applications is Al or Ga stabilized Li7La3Zr2O12 (LLZO) based on high ionic conductivities and apparent stability against reduction by Li metal. Nevertheless, the fabrication of dense LLZO membranes with high ionic conductivity and low interfacial resistances remains challenging; it definitely requires a better understanding of the structural and electrochemical properties. In this study, the phase transition from garnet (Ia3̅d, No. 230) to “non-garnet” (I4̅3d, No. 220) space group as a function of composition and the different sintering behavior of Ga and Al stabilized LLZO are identified as important factors in determining the electrochemical properties. The phase transition was located at an Al:Ga substitution ratio of 0.05:0.15 and is accompanied by a significant lowering of the activation energy for Li-ion transport to 0.26 eV. The phase transition combined with microstructural changes concomitant with an increase of the Ga/Al ratio continuously improves the Li-ion conductivity from 2.6 × 10–4 S cm–1 to 1.2 × 10–3 S cm–1, which is close to the calculated maximum for garnet-type materials. The increase in Ga content is also associated with better densification and smaller grains and is accompanied by a change in the area specific resistance (ASR) from 78 to 24 Ω cm2, the lowest reported value for LLZO so far. These results illustrate that understanding the structure–properties relationships in this class of materials allows practical obstacles to its utilization to be readily overcome.
Highlights
In some “Beyond Li-Ion Battery” concepts, Li metal is used as the anode, e.g., in Li/air, Li/sulfur, and some redox flow batteries.[1]
No composition other than LLZO was observed, which is in agreement with PXRD and neutron powder diffraction (NPD) data
For Ga3+ content > 0.10 pfu a change in space group symmetry to I4̅3d is observed. For the latter SG there is strong evidence that Ga and Al are enriched onto the tetrahedral 12a site; this is observed in both single crystal X-ray diffraction and data from combined refinement (SC-XRD and NPD) and supported by density functional theory (DFT) calculations
Summary
In some “Beyond Li-Ion Battery” concepts, Li metal is used as the anode, e.g., in Li/air, Li/sulfur, and some redox flow batteries.[1]. A single Ga3+ or Al3+ ion was placed onto the 24d site of the Ia3̅d crystal structure with parameters taken from SC-XRD measurements, and an enumeration algorithm was used to generate structures with one Li placed into each of the distinct remaining sites (i.e., 24d, and 96h).[24] Total energy calculations were performed in the Perdew−Burke−Ernzerhof (PBE) generalized-gradient approximation (GGA), implemented in the Vienna Ab initio Simulation Package (VASP).[25,26] The projector augmented-wave (PAW) method is used for representation of core states.[27] An energy cutoff of 520 eV and a kpoint density of at least 1000/(number of atoms in the unit cell) was used for all computations, with a background charge added to compensate for the lack of Li. During the relaxation the structures with the Li in an octahedral site always relaxed to the nearest tetrahedral site in good agreement with previous calculations.[28] The total energy difference between structures with Li in the tetrahedral site closest to and farthest from the supervalent cation was calculated (see below)
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