Applied Materials Today
3D-printed architecture of Li-ion batteries and its applications to smart wearable electronic devices
Graphical abstract
Introduction
The rapid downscaling of transistors in the semiconductor industry has led to the reformation of the current hand held smart phones into smart wearable devices (wearables) such as eyeglasses, watches, and fabrics. Epidermal electronics may also become possible in the near future.[1], [2], [3], [4], [5] In the context of the Internet of Things (IoT) technology, these smart wearables can communicate 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 healthcare, 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], [9], [10], [11], [12], [13] Though, LIBs have a higher energy density, higher safety standards, and extended cycle life compared to that of other types of batteries, they pose some restrictions preventing them being integrated into wearable devices. The current wearables require batteries with versatile design architectures and flexibility.[6,7,[14], [15], [16], [17], [18], [19]] The conventional 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 useful 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 regarded as the future of manufacturing due of its high efficiency and low cost.[25], [26], [27], [28], [29] In particular, the extrusion based 3D-printing is used in many applications, including functional hydrogels for wound healing,[30,31] 3D-scaffolds for cell growth,[32] thermoelectric, energy storage and harvesting devices such as nanogenerators and supercapacitors.[33,34] The extrusion based 3D-printing uses digitally designed patterns and rheologically modified inks loaded into a syringe barrel for printing over any substrate. The patterns printed through the printer can be either filaments or drops depending on the ink rheology and design requirements. Batteries for wearable devices can be manufactured using this printing technology and easily integrated into the device structure.
Manufacturing batteries with 3D-printing technology offer several advantages when compared to the conventional slurry coating technique. For instance, an array of electrodes with a specific geometry can be printed in a short time with pinpoint accuracy and uniform coating thickness by reducing the material wastage and the cost of production. Some processing steps in electrode fabrication like electrode shaping and calendering can be eliminated which reduce the production time and a minimal amount of solvent NMP might be used for the ink formulation to achieve the desired viscosity for extrusion based 3D-printing, which decreases the quantity of hazardous chemicals used. Since 3D-printing is a completely automated process, it will contribute to reduce the manpower cost.
Many research groups have been working on 3D-printed batteries in recent years. For instance, Gao et al. have demonstrated the printed several layers of carbon composite electrodes with high mass loading in a compact space achieving high energy density without polymer binders. [34] Sun et al. worked on fabricating an interdigitated Li-ion micro-battery over a glass substrate with LFP and LTO as active materials.[35] A similar work on the interdigitated architecture type of Li-ion battery has been reported by Fu et al. by incorporating graphene oxide sheets to the electrode inks, which acts both as a conductive additive and viscosity modifier.[36] Wang et al. have demonstrated fabricating all fiber based batteries using LFP and LTO active materials using 3D-printing technology and also demonstrated their easy adaptation with commercial textile fabrics. [37] Reyes et al. proposed printed batteries for wearable energy devices able to power LCD displays for few seconds using a fused deposition modelling based 3D-printer by directly blending active materials and conductive additives into the polylactic acid polymer.[38] Blake et al. demonstrated 3D-printed ceramic polymer electrolyte by employing Al2O3 nanoparticles as a reinforcement agent to PVdF polymer matrix; these electrolytes could provide electrochemical performances comparable to that of the commercially available separator even at a high current rate.[39] The term “fully printed batteries” refers to the batteries whose components fabricated entirely by a 3D-printer. Most studies on fully printed batteries concentrate on fabricating electrodes alone rather than the current collector and packaging materials.[35,40] While several studies such as by Chen et al. reported on printing copper through an electrochemical printer,[41] further research is required to investigate its electrical conductivity before incorporating this material into the battery manufacturing process.
Inspired by the previous research, in this study, we adopt 3D-printing technology to fabricate batteries with versatile designs combined with finely tuned battery component inks providing an acceptable electrochemical performance. In this paper, we demonstrate the design and fabrication of the proposed all printed LIBs. We used electrode inks that are compatible with 3D-printing technology and investigated their rheological properties, and their adhesion to the substrate material. Furthermore, the electrochemical performance and the tolerance for the cell deformation of these all printed batteries were investigated. The novelty of this work lies in the following. First, the proposed printed batteries can be directly integrated with other components of wearable devices thereby ensuring a compact well embedded architecture with a reduced size and weight. This modification makes a new path for a next generation of wearable devices. Printed batteries in the shape of glass frames, wrist bands, and hearing aids are demonstrated in this work. A brief summary of entire printing process and design architectures with wearable device applications is shown in Fig. 1. Second, the composition of the inks used for the 3D-printing process is similar to those of slurries used for the conventional battery manufacturing except that their rheology is modified for printing compatibility. In contrast, the existing studies on 3D-printed batteries use either unconventional materials such as coagulants or viscosifiers that are not suitable for large scale applications and may compromise the electrochemical performance of batteries.[35,42] Third, we adopted well established and commercially accepted LiNi0.8Co0.15Al0.05O2 (NCA) and natural graphite as cathode and anode active materials, respectively, owing to their high discharge capacity, low cost, and low toxicity.[43,44]
Section snippets
Preparation of inks
Commercially available NCA (3-8 µm, EcoProBM, Ltd.) and natural graphite powders (15-20 µm, BTR New Energy Material, Ltd.) were used as a cathode and anode active material, respectively. Denka Black (Denka Company, Ltd.) was used as an electrically conductive additive, PVdF powder (Mw 602,000, Solvay Chemicals) was used as a binder, and N-Methyl-2-Pyrrolidone (NMP, 99.5%, Daejung Chemicals and materials Co) was used as a solvent. A highly concentrated cathode ink was prepared by grinding the
Results and discussions
Ink rheology plays a significant role in extrusion based 3D-printing technology. Hence, fine-tuning of inks without any significant loss in the battery performance ought to be carefully investigated. The battery inks contain active materials, conductive additives, binders, and the necessary solvents to homogeneously disperse them. The presence of different components with a wide particle size range in the inks may result in nozzle clogging due to agglomeration. The amount of solvent in the inks
Conclusion
This paper described the fabrication of highly flexible LIBs of various forms for wearable device applications by adopting an advanced 3D printing technology. The rheologically modified inks exhibited a good shear-thinning behaviour, facilitating the extrusion-based 3D printing process with a high homogeneity and post-printing stability. The printed electrodes and separator membrane exhibited a reasonable electrochemical performance. All printed batteries displayed a specific discharge capacity
CRediT authorship contribution statement
Sekar Praveen: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft. P. Santhoshkumar: Formal analysis. Youn Cheol Joe: Conceptualization, Data curation. Chenrayan Senthil: Formal analysis, Writing - review & editing. Chang Woo Lee: Project administration, Funding acquisition, Supervision, Conceptualization, Methodology, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (no. 2017H1D8A2031138).
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