Preparation of fully flexible lithium metal batteries with free-standing β-Na 0.33 V 2 O 5 cathodes and LAGP hybrid solid electrolytes Jong Su Han a , Gil Chan Hwang b , Hakgyoon Yu a , Du-Hyun Lim a , Jung Sang Cho c, **, Matthias Kuenzel d,e, **, Jae-Kwang Kim a, *, Jou-Hyeon Ahn f a Department of Energy Convergence Engineering, Cheongju University, Cheongju, Chungbuk 28503, Republic of Korea b Department of Earth System Sciences, Yonsei University, Seoul 03722, Republic of Korea c Department of Engineering Chemistry, Chungbuk National University, Chungbuk 28644, Republic of Korea d Helmholtz Institute Ulm (HIU), Helmholtzstrasse 11, 89081 Ulm, Germany e Karlsruhe Institute of Technology (KIT), P.O. Box 3640, 76021 Karlsruhe, Germany f Departmentof Chemical Engineering and Research Institute for Green Energy Convergence Technology, Gyeongsang National University, 501 Jinju-daero, Jinju 52828, Republic of Korea A R T I C L E I N F O Article history: Received 23 August 2020 Received in revised form 6 November 2020 Accepted 16 November 2020 Available online 21 November 2020 Keywords: Flexible lithium metal battery NVP Free-standing electrode LAGP Hybrid solid electrolyte A B S T R A C T Safe and flexible batteries are expected to be the enabler for advancing the technology of wearable electronics to an unforeseen level in near future. However, to date the energy density of such devices is rather limited due to the rather large proportion of dead weight and volume to provide good flexibility. To overcome this hurdle, a disruptive change in the battery manufacturing process is needed. Herein, we not only introduce a simple phase inversion method for the preparation of free-standing and flexible β- Na 0.33 V 2 O 5 cathodes without metal current collector, but also demonstrate the possibility to integrate those into fully flexible lithium metal batteries. Additionally, employing a LAGP-based hybrid solid electrolyte enables excellent high temperature stability and thus, enhanced safety characteristics of the device. Such integrated flexible batteries exhibit fast and stable lithium-ion storage capabilities, with a large specific capacity of 228 mA h g 1 at 0.1 C and excellent cycling stability translating into an outstanding specific energy of 407.8 Wh kg 1 on electrode level. © 2020 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. Introduction Portable electronics have a bright future with an ever-rising interest in wearable and smart devices that assist in improving life quality and are useful for healthcare. For advanced applications the flexibility of such devices is crucial. Hence, it is essential to develop as well flexible energy storage media with improved electrochem- ical properties that could power this next generation of portable electronic devices [1–3]. Compared to other energy storage devices, such as hydrogen storage systems and supercapacitors, secondary lithium-ion batteries (LIBs) with good flexibility would be the most viable option to power these devices thanks to their good power density and far superior energy density. However, flexible LIBs have not been commercialized yet and rigid batteries are still employed instead. Although there have been reports on flexible LIBs on lab scale, sustaining the flexibility with at the same time good electrochemical properties remains challenging and often results in reduced capacities in comparison to the rigid ones [4–10]. Especially, the energy density of flexible batteries is rather low due to the inherent limitation of thickness to maintain flexibility. Since cathode materials play a vital role in determining the capacity of LIBs, it is desirable to develop novel efficient cathode materials that could assist in providing both enhanced capacity as well as flexibility to the next generation of flexible LIBs. To achieve this goal, different strategies have been proposed aiming on tailoring the battery electrodes as thin layers, structuring or pattering the current collector and infiltrating the active materials into porous substrates such as fabrics, paper or plastic [11–14]. However, all these methods have in common to reduce the battery’s energy density because the introduction of additional dead weight and non-conductive materials, increasing volume and weight of the cell and might even react with the electrolyte [11]. On the positive electrode side, vanadium pentoxide (V 2 O 5 ) has been employed as active material due to its large abundance, reduced cost and high theoretical capacity *Corresponding author. **Corresponding author at: Helmholtz Institute Ulm (HIU), Helmholtzstrasse 11, 89081 Ulm, Germany. https://doi.org/10.1016/j.jiec.2020.11.011 1226-086X/© 2020 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. Journal of Industrial and Engineering Chemistry 94 (2021) 368–375 Contents lists available at ScienceDirect Journal of Industrial and Engineering Chemistry journal home page : www.elsevier.com/loca te/jiec