Lithium ion batteries are widely used for portable electronics, particularly laptops and mobile phones, due to their superior energy and power densities. As modern microelectronic devices and microelectronic mechanical systems (MEMS) advance further, the bottleneck to further miniaturization is often energy storage.¹ While solid state thin film batteries offer remarkable electrochemical performance and long term stability, they are limited by areal capacity (mAh/cm²).² Solid state three dimensional lithium ion micro-batteries (3D micro-batteries, for short) are an attempt to realize the exceptional electrochemical performance of thin film batteries by fabricating solid state lithium ion cells on high specific surface area. The challenge to realizing these battery designs is the preparation of conformal, ultrathin solid-state electrolytes that physically separate the two electrodes while allowing rapid ion transport. Using initiated chemical vapor deposition, 1µm-thick crosslinked polymer thin films composed of poly (methacrylic acid-co-ethylene glycol diacrylate) were synthesized. These films were transformed to lithium-bearing polyelectrolytes through H⁺/Li⁺ ion exchange in a 1M LiOCH₃ solution in methanol. Fourier transform infrared spectroscopy (FTIR) characterization was used to determine the chemical composition of the copolymer film and confirm successful ion exchange through analysis of the vibrational stretch of the carbonyl (C=O) bonds in the acidic and salt forms of the polymer electrolyte. The effect of crosslinking densities in these films (the mole fraction of ethylene glycol diacrylate) on electrochemical properties was studied. The optimal ionic conductivity of the Li⁺-exchanged polymer was determined to be 3.75x10^-7 S/cm at 60°C with an activation energy of 0.8785eV. The electrochemical stability of this electrolyte is still under investigation through cycling in thin film cells where vapor-deposited lithium metal as the anode and Au as the working electrode. Current efforts to further enhance ionic conductivity and to integrate these materials into full solid state lithium metal batteries will also be presented.

1. Letiche, M.; Eustache, E.; Freixas, J.; Demortiere, A.; De Andrade, V.; Morgenroth, L.; Tilmant, P.; Vaurette, F.; Troadec, D.; Roussel, P.; Brousse, T.; Lethien, C., Atomic Layer Deposition of Functional Layers for on Chip 3D Li-Ion All Solid State Microbattery. Adv. Energy Mater. 2017, 7 (2), 12.

2. Ferrari, S.; Loveridge, M.; Beattie, S. D.; Jahn, M.; Dashwood, R. J.; Bhagat, R., Latest advances in the manufacturing of 3D rechargeable lithium microbatteries. Journal of Power Sources 2015, 286, 25-46.