To obtain high energy and power densities of planar MSCs, a large number of nanostructured materials including activated carbon, [10,11] carbon nanotube, [12][13][14] graphene, [15][16][17] metal oxide, [18][19][20] conductive polymer, [21,22] and their composites [23,24] have been used to construct MSCs. Those with superior volumetric capacitance and power performance are desired.In recent years, electrode engineering approaches to improve the performance of MSCs have also been reported. [25][26][27][28][29][30] A narrow interspace between interdigital electrodes reduces the distance of ions diffusion and the equivalent series resistance of electrodes, leading to improved capacitance and power density of MSCs. [31] The interspace width is dependent on the preparation technologies. Specifically, for current screen printing [26] and inkjet printing, [32,33] the interspace width is over 40 µm. Photolithography technology has achieved an interspace width of 15 µm. [3,34,35] High-precision 3D printing [36,37] and electro-deposition [38,39] have attained an interspace width of 120 and 5 µm, respectively. Laser-based technology having advantages of flexible machining and precise patterning without additional assistance of templates or masks, [40] can accomplish the one-step fabrication of MSCs with interdigital electrodes. [41][42][43] The pioneering work was from Gogotsi group, which reported the preparation of MXene MSCs The capacitance of microsupercapacitors (MSCs) can double if both sides of substrates are used to construct MSCs. Nevertheless, achieving electric connections of MSCs through substrates is a challenge due to the difficulty in precisely positioning each MSC couple that has two of the same MSCs units on two sides. In this work, taking advantage of the synchronous etching on both sides of transparent polyethylene terephthalate substrates by femtosecond laser pulses, a double-sided configuration is attained with high precision in the alignment of back-to-back MSC couples and versatile doubleside MSCs are realized via arbitrary on-and through-substrate connections of MXene MSC units. The MXene double-side MSC fabricated by the series connection of 12 spiral pattern MXene MSC units with interdigital electrodes of 10 μm width interspace can output a large working voltage of 7.2 V. Additionally, femtosecond laser etching brings the transformation of MXene into titania near-etched edges with a lateral distance less than 1 µm. Such a small laser-affected area has little influence on the capacitive performance, which is one of advantages for femtosecond laser over conventional lasers. This research is valuable for one-step manufacturing of highly integrated MSCs in the field of miniaturized energy storage systems.
