Nanoscale Sn-Based Protective Layers with Calendaring Process Enhanced the Longevity of Aqueous Zn-Ion Batteries

Aqueous Zn-ion batteries (AZIBs) are drawing interest for their potential to address two challenges: the ever-increasing need for safe batteries and the demand for affordable costs. Even though AZIBs make it possible to utilize Zn metal as an anode for high volumetric capacity (5854 mAh cm^-3), they still endure low Zn utilization. Above all, one of the effective ways to increase low Zn utilization is by employing a Zn powder electrode as the anode. Despite being less researched, the Zn powder electrode offers more significant potential for controllable utilization than adopting Zn foil as an anode in the battery system. In spite of the high utilization, the large surface area of powder electrodes produces massive side reactions like hydrogen evolution, corrosion, and dendrite growth. Therefore, it is crucial to solve these side reactions.<br/>In this study, we employed two strategies to address these concerns: implementing the calendaring process to improve the uniformity and compaction of the electrode and applying the atomic layer deposition (ALD) process to coat the nanoscale protective layer and inhibit undesired side reactions. Uniformity and compactness were reinforced through the calendaring process, directly impacting Zn utilization in the bare Zn powder electrode through depth of discharge (DoD). Meanwhile, hindering side reaction from Zn powder electrode-electrolyte, we selected SnO₂ as an artificial layer to restrain the side reaction. In order to form a uniform and thin protective film on the powder surface, the ALD process was employed. Transmission electron microscopy (TEM) analysis was conducted to investigate the formation of a thin protective film at the nanometer scale, exhibiting uniformity across the surface. Combining with two tactics, we endeavored to resolve the decrease in polarization from contact loss and the reduction of side reactions. Following the application of the SnO₂ coating to the Zn powder electrode samples, symmetrical cell testing in accordance with the calendaring process was conducted, resulting in overpotentials as low as 3 mV after 150 hours with a discharge depth of 40% and the occurrence of the bare Zn powder electrode short-circuit at the same time. Furthermore, a full cell test was carried out using zinc vanadium oxide (ZVO) as the cathode, showing that an uncoated Zn powder electrode exhibited a capacity of 40.93 mAh g^-1, corresponding to 29.14% after 500 cycles, while a Zn powder electrode coated with SnO₂ had a capacity of 138.4 mAh g^-1, equivalent to 54.46%. To analyze the governing side effects like hydrogen evolution reaction, we also adopted differential electrochemical mass spectrometry (DEMS). According to the data, the powder electrode with the SnO₂ coating layer was verified to produce hydrogen gas at a rate that was less than half of the bare Zn powder electrode. Our research findings indicate that coating SnO₂ on Zn powder and combining it with the calendaring process may be an effective method for reducing side reactions, enhancing uniformity, and compacting within the battery system.