Scalable and safer printed Zn//MnO2 planar micro-batteries for smart electronics

The rapidly growing field of miniaturized smart electronics has forced us to search for compatible microscale power sources with reliable electrochemical performance, various form factors, manufacturing scalability, and safety [1–5]. Among the several power sources reported to date, planar microbatteries, which are characterized by geometrical superiority over simplestacked ones, have recently garnered considerable attention due to the simple miniaturization, facile serial/parallel integration, mechanical flexibility, and removal of conventional separator membranes [6]. Lithium-based thin-film micro-batteries have been extensively investigated; however, the complex manufacturing processes and flammable organic electrolyte-induced safety concerns pose a formidable barrier to their practical applications. To address this issue, aqueous-based non-lithium planar micro-batteries are suggested as a promising alternative beyond the aforementioned lithium-based ones. They can be fabricated through various printing techniques including inkjet, screen, gravure, and 3D printing [7]. A recent study published in Natl. Sci. Rev. by Wu. et al. [8] reported a new class of screen-printed, aqueous Zn//MnO2 planar micro-batteries as a breakthrough approach.TheZn//MnO2 planar micro-batteries, which were based on interdigital patterns of Zn ink as an anode and MnO2 ink as a cathode, with high-conducting graphene ink as a metal-free current collector, showed outstanding electrochemical performance,

The rapidly growing field of miniaturized smart electronics has forced us to search for compatible microscale power sources with reliable electrochemical performance, various form factors, manufacturing scalability, and safety [1][2][3][4][5]. Among the several power sources reported to date, planar microbatteries, which are characterized by geometrical superiority over simplestacked ones, have recently garnered considerable attention due to the simple miniaturization, facile serial/parallel integration, mechanical flexibility, and removal of conventional separator membranes [6]. Lithium-based thin-film micro-batteries have been extensively investigated; however, the complex manufacturing processes and flammable organic electrolyte-induced safety con-cerns pose a formidable barrier to their practical applications. To address this issue, aqueous-based non-lithium planar micro-batteries are suggested as a promising alternative beyond the aforementioned lithium-based ones. They can be fabricated through various printing techniques including inkjet, screen, gravure, and 3D printing [7].
A recent study published in Natl. Sci. Rev. by Wu. et al. [8] reported a new class of screen-printed, aqueous Zn//MnO 2 planar micro-batteries as a breakthrough approach. The Zn//MnO 2 planar micro-batteries, which were based on interdigital patterns of Zn ink as an anode and MnO 2 ink as a cathode, with high-conducting graphene ink as a metal-free current collector, showed outstanding electrochemical performance, aesthetic diversity, mechanical flexibility, and modularization.
The Zn//MnO 2 micro-batteries were fabricated by a low-cost and scalable screen-printing technique as illustrated in Fig. 1a. The screen-printing enabled seamless integration of the Zn//MnO 2 micro-batteries with various complexshaped planar geometries, resulting in the fabrication of multiple parallel interdigitated micro-batteries via in-series/ in-parallel connections (Fig. 1b), individual micro-batteries (Fig. 1c), flexible patterns with multiple connections (Fig. 1d), and flexible tandem concentric circular (Fig. 1e) and linear-structured micro-batteries free from conventional metal-based interconnectors (Fig. 1f).

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Natl Sci Rev, 2020, Vol. 7, No. 1 RESEARCH HIGHLIGHTS (Fig. 1g). They delivered a high volumetric capacity of 19.3 mAh/cm 3 and, notably, a volumetric energy density of 17.3 mWh/cm 3 , outperforming those (≤10 mWh/cm 3 ) of conventional lithium thin-film batteries. The Zn// MnO 2 micro-batteries also provided long-term cyclability, high capacity retention of 83.9% after 1300 cycles at a current density of 5 C, which far exceeds those of stacked Zn//MnO 2 batteries reported to date. Furthermore, the Zn//MnO 2 planar micro-batteries exhibited exceptional flexibility without capacity loss under serious deformation and high voltage/high capacity through facile serial and parallel connection of bipolar cells. The serial or parallel Zn//MnO 2 planar micro-batteries were assembled with unit cells one by one, which were packaged by dropping electrolyte onto the project area of interdigital microelectrodes.
The low-cost, environmentally benign Zn//MnO 2 micro-batteries with in-plane geometry presented in this study hold great promise as a highperformance, safe, flexible, and shapeversatile printed microscale power source that can be directly integrated with various miniaturized electronics. This study will be of broad interest to scientists and engineers involved in nanotechnology, chemistry, material science, and energy storage, and contributes to enriching development perspectives and directions of planar microscale power sources for potential use in future microelectronics. Research directions on printable batteries are currently focused on (i) synthesis of highly conducting and stable battery component inks with tunable rheological properties associated with electrochemical performance, (ii) design of battery shapes and configurations with fully printable techniques, (iii) development of industrially scalable printing techniques, and (iv) monolithic/seamless integration of printable batteries with electronic devices [2,9].

Sang-Young Lee
Department of Energy Engineering, Ulsan National Institute of Science and Technology, Korea