Abstract
The next major leap in energy storage is thought to arise from a practical implementation of all solid-state batteries, which remain largely confined to the small scale due to issues in manufacturing and mechanical stability. Lithium batteries are amongst the most sought after, for the high expected energy density and improved safety characteristics, however the challenge of finding a suitable solid-state electrolyte remains. Lithium rich garnets are prime contenders as electrolytes, owing to their high ionic conductivity (>0.1 mS cm-1), wide electrochemical window (0-6 V) and stability with Li metal. However, the high Young's modulus of these materials, poor wetting of Li metal and rapid formation of Li2CO3 passivating layers tends to give a detrimentally large resistance at the solid-solid interface, limiting their application in solid state batteries. Most studies have focused on La based systems, with very little work on other lanthanides. Here we report a study of the Nd based garnet Ga0.2Li6.4Nd3Zr2O12, illustrating substantial differences in the interfacial behaviour. This garnet shows very low interfacial resistance attributed to dopant exsolution which, when combined with moderate heating (175 °C, 1 h) with Li metal, we suggest forms Ga-Li eutectics, which significantly reduces the resistance at the Li/garnet interface to as low as 67 Ω cm2 (much lower than equivalent La based systems). The material also shows intrinsically high density (93%) and good conductivity (≥0.2 mS cm-1) via conventional furnaces in air. It is suggested these garnets are particularly well suited to provide a mixed conductive interface (in combination with other garnets) which could enable future solid-state batteries.
Highlights
Lithium ion batteries (LIBs) are the electrochemical system of choice for many energy storage applications
A range of Nd based Li garnet materials have been synthesised for the first time, with Ga-NLZO in particular having room temperature ionic conductivity (≥2 × 10−4 S cm−1) similar to La based garnets, despite having significantly smaller lattice parameters
We have demonstrated that GaNLZO enables very low area specific resistance (ASR) values in contact with Li metal which remain stable during cell cycling, a feature which is intrinsic to the garnet material itself rather than requiring additional coatings
Summary
Lithium ion batteries (LIBs) are the electrochemical system of choice for many energy storage applications. To further increase energy density the move to lithium metal negative electrodes, or anodes, is required. This is a particular challenge for cells containing liquid based electrolytes due to hazardous dendritic lithium growth and numerous safety implications.[1,2,3] Liquid electrolytes are toxic, flammable, and unable to be employed with high V cathode materials due to their relatively narrow electrochemical window.[4,5,6,7,8,9]. Solid state batteries (SSB) enable safer, higher energy density and (theoretically, assuming a perfectly dense structure) are able to block Li dendrite growth, a practical solid state electrolyte (SSE) remains an intense research challenge.[14,15,16] Most SSEs, based on oxide and sulphide chemistries, often have either a poor electrochemical window or low ionic conductivity, while requiring time consuming, energy demanding and/or complex synthesis routes.[17,18,19,20,21,22,23,24,25] many SSEs suffer from large interfacial resistance due to poor wetting at the solid–solid interface, limiting the practical use of many SSBs.[25,26,27,28]
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