Novel multifunctional electronic devices (internet of things (IoT), wearables) do no longer require a conventional energy storage device, but a power source that can be perfectly integrated with the overall architecture of these devices, and ensure an additional structural function [1]. Within this context, transparency, amongst other functions, presents a particular challenge. Despite the potential advantages of integrating such energy storage with windows, screens, glasses and other optoelectronic devices, only a few studies reported on transparent lithium ion batteries [2,3]. In the present work, we report on the development of transparent all solid inorganic thin film lithium ion batteries (TFBs). Our transparent TFBs are realized considering a geometric engineering of battery materials and using advanced microfabrication techniques.

The transparent TFBs are realized on 8’’ glass substrate and present a design with a grid structured active area consisting of LiCoO₂/LiPON/Si based stack (Figure 1.a, 1.b). All materials are obtained by physical vapor deposition techniques. Specific photolithography and etching processes have been carried out to achieve structures below human eye resolution. transparent TFBs are microfabricated with varied architectures: several geometric designs corresponding to transmittances ranging between 25 and 60%, for each design different thicknesses related to active layers have been considered as well (up to 10µm and 0.1µm, respectively for LiCoO₂ and Si based electrodes).

UV-vis transmittance up to 60% have been measured for the obtained transparent TFBs (Figure 1.c). The 2x2 cm² transparent TFBs delivered discharge capacity as high as 0.5mAh upon galvanostatic cyling (at C/2 rate within 4.2-3V voltage range) for the 60% transmittance design. The capacity variation trend (Figure 1.d) shows a first phase (50 cycle) of a gradual decrease with an average capacity loss of 0.15% per cycle, and thereafter a second phase (50 cycles) with almost stable capacity. Particular attention is given to the effects of architecture parameters (geometric design, electrode thickness and cell balancing) on TFB optical and electrochemical properties.

To the best of our knowledge, this work is the first demonstration of transparent all inorganic thin film lithium batteries. While reported studies are limited to battery structures involving liquid or polymer materials, our transparent TFBs may contribute to improve form factor freedom, extend operating temperature ranges and enhance long term stability, the whole being obtained by a fabrication process viable for industrial high volume production.

References

[1] A. Vlad, N. Singh, C. Galande, P.M. Ajayan, Design Considerations for Unconventional Electrochemical Energy Storage Architectures, Adv. Energy Mater. 5 (2015) 1–53. doi:10.1002/aenm.201402115.

[2] Y. Yang, S. Jeong, L. Hu, H. Wu, S. Woo Lee, Y. Cui, et al., Transparent lithium-ion batteries, Proc. Natl. Acad. Sci. USA 108 (2011) 13013. doi:10.1073/pnas.1102873108.

[3] H. Nagai, H. Hara, M. Enomoto, C. Mochizuki, T. Honda, I. Takano, et al., Synchronous Electrochromism of Lithium Ion Battery With Chemically Fabricated Transparent Thin Films, Funct. Mater. Lett. 06 (2013) 1341001. doi:10.1142/S1793604713410014.