The next generation of energy storage devices relies on a broad and adaptable range of volumetric and gravimetric energies to compete with the challenges in stationary, mobile and portable electronics electricity supply. Here, all solid state batteries based on Li-garnet structures are interesting model systems as these allow for complete solid state microbattery prototypes that can be transferred to thin film structures for powering of portable electronics on chip in the future. Reducing electrolyte thickness and using microfabrication tools allows us to fabricate advanced and optimized microbattery designs able to overcome the most important issues found on microbattery operation, namely their mechanical stability, volume expansion and strain adaptation during lithiation/delithiation processes. In addition, solid state electrolytes open the possibility to integrate new high capacity electrode materials with conventional low stability in standard liquid or polymer based Li-batteries.

Promising power densities of >10 mWcm^-1 have been already reported on first prototypes of 3D-printed microbatteries ^[1]; however, thin film fabrication, and especially on fast conducting Li-based Garnets, remains a challenge. From a materials perspective, cubic Garnet Li₇La₃Zr₂O₁₂-structures has been reported on bulk pellets to be one of the most promising candidates for solid state battery electrolytes, mainly due to its high Li⁺ conduction and stability. Although originally stable in its tetragonal phase at room temperature, one can stabilize its more conductive cubic phase by adding different dopants in either the A (Li) site or C (Zr) site. Li⁺ conductivities of up to 1 mScm^-1 has been reported on cubic-stabilized Garnet Li₇La₃Zr₂O₁₂, positioning the material as one of the best solid state Li conductors ever published ^[2-4]. Nevertheless, its transferability to thin film structures still remains challenging. First studies on the deposition of thin Li₇La₃Zr₂O₁₂ films by different techniques reflect the challenge on the cubic garnet phase stability and high conduction, and the stability range, Li content and resulting Li conduction still remains unclear.  

Here, we present a detailed stability/conductivity mapping study of Al- and Ta-doped Li₇La₃Zr₂O₁₂ thin films grown by pulsed laser deposition. Their crystallization and ionic transport characteristics are discussed as a function of deposition conditions towards thin film phase stabilization and Li⁺ mobility maximization. Special attention is placed on the Li loss compensation both during target processing ^[5] and thin film growth ^[6] vs. phase stability. We observe an optimum range of co-existing cubic Li₇La₃Zr₂O₁₂ and the Li-deficient La₂Zr₂O₇ phase in the near order Raman spectroscopy, which is directly connected to the Li-content and deposition film conditions selected. We carefully discuss the direct implication of this on the Li-conductivity.

Finally, novel types of thin film microbatteries tunable and adaptable to electro-chemo-mechanical variations ^[7] during operation will be discussed. Here, we report processing of novel “Tortilla wrapped Li⁺ microbattery” structures with engineered strain fields to control the electro-chemo-mechanic variations and compensate mechanical stresses during operation, which allow to avoid cracking upon operation on silicon substrates.

References

^[1] Sun, K., et al., Advanced Materials 25(33): 4539-4543.

^[2] Knauth, Solid State Ionics 180(14–16), 911.

^[3] Thangadurai et al., Chemical Society Reviews 43(13), 4714.

^[4] Afyon, Rupp et al., J. Mater. Chem. A, 3, 18636.

^[5] Buschmann, Janek et al., Phys. Chem. Chem. Phys. 13, 19378.

^[6] Park et al., Thin Solid Films 576, 55.

^[7] Shi, Rupp et al., Nat. Mater. 14, 721, 2015.