Atomic Layer Deposition of Thin Films for 3D Lithium-Ion Microbatteries

This work demonstrates the synthesis and characterization of several material systems that meet the required functional properties for direct integration in 3D lithium-ion microbatteries—conformal thin film solid-state electrolytes with adequate ionic conductivity and cathodes exhibiting high volumetric capacity, excellent rate capability, and cycle life. The atomic layer deposition (ALD) process optimization allowed for the control of the structural, chemical, and electrochemical properties of Li1+xMn2-xO4, LixAlySizO, as well as their interfacial properties that ultimately dictate electrochemical performance. The synthesis of Li1+xMn2-xO4 combined a plasma enhanced ALD process for MnO2 and a thermal process for ALD LiOH. The PEALD MnO2 showed self-limiting growth, stable composition (within 5% relative composition), and well-controlled growth rate over a temperature range of 205-265�C. As synthesized and amorphous ALD Li1+xMn2-xO4 films were crystallized into the electrochemically active spinel phase. Tuning of the ALD cycle ratio led to controlled Li content in Li1+xMn2-xO4 (x = 0-0.33), exhibiting electrochemical activity in both the 3.0V and 4.0V region depending on the stoichiometry. The Li1+xMn2-xO4 thin films exhibited great rate capability and capacity retention—maintaining 66% of the areal capacity upon increasing the rate by a factor of a 100 as well as 97% capacity retention over 100 cycles at 35.9 �A cm-2 (~5C). The measured volumetric capacity was 52 �Ah cm-2 �m-1 at ~C/2 and 45 �Ah cm-2 �m-1 after 100 cycles at ~5C, offering the potential for superior areal energy densities. Amorphous LixAlySizO thin films were synthesized and integrated on SiGe alloying anodes. Using in-situ transmission electron microscopy, the electrolyte layer was remained intact during the lithiation induced volume expansion of the nanowire for two different electrolyte thicknesses (8 and 20 nm). A novel hybrid solid-state electrolyte consisting of sequential deposition of ALD LixAlySizO and iCVD PV4D4 was synthesized to improve upon the mechanical properties of LixAlySizO for direct integration with high capacity anodes. Integration on Co3O4 thin films resulted in increased capacity-retention with continuous cycling with a discharge capacity 8.4% higher as compared to the uncoated Co3O4 anode after 100 cycles at ~2C.The stoichiometry of ALD LixAlySizO was controlled to allow for crystallization in the β-eucryptite phase for improved ionic conductivities. The crystallized LiAlSiO4 exhibited a well-defined epitaxial relationship of β-LiAlSiO4 (12 ̅10) || Si (100) and β-LiAlSiO4 (101 ̅0) || Si(001). The ionic conductivity of the crystallized thin film was around two orders higher than the amorphous as-deposited films (10-7 vs. 10-9 S/cm) at room temperature. Due to the unique 1-D channel along the c-axis of β-LiAlSiO4, the epitaxial thin film has the potential to facilitate ionic transport if oriented with the c-axis normal to the electrode surface making it a promising electrolyte material for 3D lithium-ion Microbatteries, where a properly oriented film offers the opportunity to achieve higher ionic conductivities