(Invited) Conformal Deposition of Li-Based Thin Films for 3D Lithium-Ion Microbatteries

Abstract

Lithium-ion batteries have been the premiere power source in the mobile electronics industry due to their superior energy density and cycle life as compared to other battery chemistries. In the Era of the Internet of Things (IoT) there is an increasing need to find a viable power source to support millimeter-size and smaller scale devices in the fields of environmental sensing, biomedical devices, and wearable electronics. Three-dimensional battery architectures mitigate the limitations of traditional thin film batteries through improvement of ion transport properties allowing for higher areal loading of active materials. Atomic layer deposition not only offers the ability to synthesize a wide range of functional materials, but crucially is a non-line-of-sight deposition technique that allows for conformal deposition necessary for fabrication of 3D lithium-ion batteries. In this work, two ALD processes were developed to synthesize conformal thin films applicable to 3D battery fabrication—LiAlSiO4, a solid-state electrolyte, and LiMn2O4, a high-rate thin film cathode.LiAlSiO4 is a solid-electrolyte of interest, in which its quartz-like framework produces unique 1D channels for Li-ion transport along the c-axis, exhibiting ionic conductivities as high as 4.7x10-5 S/cm at room temperature,1, 2 where the ionic conductivity in its thin film form has shown to increase upon decreasing thicknesses, 3 In this work, ALD of LixAlySizO was demonstrated with careful tuning of the stoichiometry to allow for crystallization into the ionically conductive β-Eucryptite LiAlSiO4 phase. The rapid thermal annealed ALD film developed a well-defined epitaxial relationship to the silicon substrate: β-LiAlSiO4 (12 ̅10) || Si (100) and β-LiAlSiO4 (101 ̅0) || Si(001). The extrapolated room temperature ionic conductivity was found to be 1.2 x 10-7 S/cm in the [12 ̅10] direction with an activation energy of 0.63 eV, as compared to amorphous ALD LASO films with ionic conductivities on the order of 10-9 S/cm.Beyond the electrolyte layer, an equally crucial material is a thin film cathode with optimal interfacial properties to ensure homogeneous active material utilization, adequate cycle-life, and high rate performance. To realize this goal, LiMn2O4 was demonstrated utilizing O2 plasma and Tris(2,2,6,6Tetramethyl-3,5-heptanedione) Mn(III), where lithium incorporation was performed through a thermal ALD LiOtBu/H2O process. Thin films with stoichiometries of Li1+xMn2-xO4, where x>0 were observed (as-characterized via XPS) with LiOH ALD sub-cycle percentages lower than 1% of the total ALD supercycle. LiMn2O4 thin films exhibited great rate capability (66% capacity retention after increasing the rate from C/2 to 50C) as well as capacity retention (97% capacity retention after 100 cycles at 5C).Lastly, a proof-of-concept nanobattery consisting of a SiGe nanowire (anode) coated with LixAlySizO (electrolyte) and LiMn2O4 (cathode) was fabricated. Both materials show great promise for direct integration with three-dimensional lithium-ion microbatteries.1. V. Thangadurai and W. Weppner, Ionics 8 (3), 281-292 (2002). 2. P. G. Bruce, Solid State Electrochemistry. (Cambridge University, Press, 1995).3. S.-i. Furusawa, H. Tabuchi and T. Tsurui, Solid State Ionics 178 (15), 1033-1038 (2007).