Applied Materials Today 20 (2020) 100688 Contents lists available at ScienceDirect Applied Materials Today journal homepage: www.elsevier.com/locate/apmt 3D-printed architecture of Li-ion batteries and its applications to smart wearable electronic devices Sekar Praveen, P. Santhoshkumar, Youn Cheol Joe, Chenrayan Senthil, Chang Woo Lee ∗ Department of Chemical Engineering & Center for the SMART Energy Platform, College of Engineering, Kyung Hee University, 1732 Deogyeong-daero, Giheung, Yongin, Gyeonggi, 17104, South Korea a r t i c l e i n f o Article history: Received 12 February 2020 Revised 5 April 2020 Accepted 30 April 2020 Keywords: 3D-printing Li-Ion battery Rheological properties Shape conformability Wearable electronics a b s t r a c t The evolution of wearable electronics technology, currently used in various smart wearable devices such as watches and eyeglasses based on applications that range from healthcare to fashion, has provided customers an access to data directly from these devices. From the energy consumption point of view, several challenges are yet to be addressed. However, the conventional Li-ion batteries (LIBs) are confined to particular shapes and sizes that limit their incorporation into certain wearable device applications. This study proposes a highly efficient 3D-printing technology for fabricating printed LIBs of any shape suitable for a wide range of wearable devices. In particular, the proposed technology is based on modu- lating inks containing active materials, conductive additives, and binders to obtain a non-Newtonian fluid for achieving a homogeneous flow of the inks through the printer nozzle. The individually printed elec- trodes and separator membranes are assembled and sealed in a plastic sheet with the injection of a small electrolyte for membrane soaking. All-printed LIBs display a specific discharge capacity of 184 mAh g −1 at a current rate of 0.1 C and maintained a consistent electrochemical performance upon bending. This promising technology can be adopted for the fabrication and integration of batteries for future wearable devices. © 2020 Elsevier Ltd. All rights reserved. 1. Introduction The rapid downscaling of transistors in the semiconductor in- dustry has led to the reformation of the current hand held smart phones into smart wearable devices (wearables) such as eye- glasses, watches, and fabrics. Epidermal electronics may also be- come possible in the near future.[1–5] In the context of the In- ternet of Things (IoT) technology, these smart wearables can com- municate with other devices and provide real time data to the wearer. These leverages appease users to widely accept wearable devices into their daily lives. However, several challenges are yet to be addressed before wearables can be recognized as devices of choice.[6,7] One challenge is providing sufficient energy to power wearables, especially in the context of applications such as health- care, where continuous monitoring is required. Over the past three decades, Li-ion batteries (LIBs) have become a reliable choice for powering the majority of electronic devices.[8–13] Though, LIBs have a higher energy density, higher safety standards, and ex- tended cycle life compared to that of other types of batteries, they pose some restrictions preventing them being integrated into ∗ Corresponding author. E-mail address: cwlee@khu.ac.kr (C.W. Lee). wearable devices. The current wearables require batteries with ver- satile design architectures and flexibility.[6,7,14–19] The conven- tional LIBs are restricted to coin, cylindrical, prismatic, and pouch types, which has led to designing smart devices depending on the battery shape and thus limiting their style. Low-flash-point liquid electrolytes used in LIBs pose another challenge as they may compromise the safety of devices worn close to the body, especially near sensitive parts such as eyes and ears. This problem can be overcome by replacing such electrolytes with solid-state or gel polymer electrolytes.[6,16,20,21] Although solid-state electrolytes are considered very safe when it comes to wearables, gel polymer electrolytes are preferred because of their better ionic conductivity. Adopting technologies from other fields may help addressing the cell manufacturing complications in designing batteries for wearable devices. Screen printing is one of such technologies; it uses a stencil mask pattern and rheologically optimized inks to manufacture desired shapes.[22,23] Spray painting is another use- ful technology for making batteries over any curved surface.[24] However, these technologies require predefined masks for every design, formation of aerosols, and irregular coating of inks, which prevents them from making multiple arrays at the same time. 3D- printing technology is an additive manufacturing technology that is https://doi.org/10.1016/j.apmt.2020.100688 2352-9407/© 2020 Elsevier Ltd. All rights reserved.