It has been well understood that LIB or SIB cells are not ideal stable systems. Because the Fermi level of Li/Na is above the lowest unoccupied molecular orbital of electrolytes, the decomposition of electrolytes cannot be fully avoided (42). The small amount of impurities in electrolytes also contributes some side reactions. When the mass loading of the active electrode material is high enough (1.7 mg in the coin cell), the current from side reactions can be ignored. However, the thin MoS2 layer on one 500 μm by 90 μm cantilever is only around 1 ng; thus, its sodiation/desodiation current will be buried in the currents from side reactions. This is also a common problem for all nanobatteries used for in situ TEM and AFM studies. In this work, ~0.1 mg of additional ball-milled MoS2 powder (the same slurry as used in coin cell test) was added to the base of the cantilever array to increase the signal/noise ratio of the electrochemical current. As shown in Fig. 2A, the MoS2 powder on the array base and the thin film on cantilever were electrically connected. Both of them contribute current upon the intercalation/extraction of Na, but only the MoS2 thin film on the cantilever beams leads to the bending of cantilevers. A small piece of Na metal connected to potentiostat via another platinum (Pt) wire was placed ~2 mm away from the microcantilever array and worked as the reference/counter electrode. Both electrodes were sealed in a homemade electrochemical cell (Fig. 2B) filled with electrolytes. The cell had a glass window at the top, through which a focused laser beam shone on the tip of the microcantilever, and a position-sensitive detector (PSD) was used to monitor the position shifting of the reflected laser beam. The bending of the cantilever was carefully calibrated with a laser Doppler vibrometer. In our system, 1 mV of the signal on PSD corresponds to a bending of 12.3 nm on the cantilever tip, and a surface stress difference of 73.8 mNm−1 according to the Stoney equation. A constant current of 10 μA was used in all the charge/discharge process, which corresponds to ~0.1 A g−1 according to the mass of the MoS2.

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