The recent trend in zinc (Zn) anode aqueous batteries has been to explore layered structures like manganese dioxides and vanadium oxides as Zn‐ion intercalation hosts. These structures, although novel, face limitations like their layered counterparts in lithium (Li)‐ion batteries, where the capacity is limited to the host's intercalation capacity. In this paper, a new strategy is proposed in enabling new generation of energy dense aqueous‐based batteries, where the conversion reactions of rock salt/spinel manganese oxides and carbon nanotube‐nested nanosized Zn electrodes are exploited to extract significantly higher capacity compared to intercalation systems. Accessing the conversion reactions allows to achieve high capacities of 750 mAh g−1 (≈30 mAh cm−2) from manganese oxide (MnO) and 810 mAh g−1 (≈30 mAh cm−2) from nanoscale Zn anodes, respectively. The high areal capacities help to attain unprecedented energy densities of 210 Wh per L‐cell and 320 Wh per kg‐total (398 Wh per kg‐active) from aqueous MnO|CNT‐Zn batteries, which allows an assessment of its viable use in a small‐scale automobile.

Despite this widespread interest, commercialization of devices based on ZnO has been limited due to difficulty controlling its electronic properties in part due to an incomplete understanding or control of the effects of doping and native defects. [7] A better understanding of the origins of these properties must be developed to promote the commercialization of ZnO-based devices.These considerations are particularly important for ZnO in rechargeable zinc metal alkaline batteries, abbreviated "Zn alkaline batteries", which are attractive for electric grid integration of renewable energy sources because of the low cost, inherent safety, environmental friendliness, and high energy density of Zn. However, rechargeable Zn alkaline batteries have not yet been widely commercialized due to poor cycle life of zinc anodes at high utilization. Failure analyses show ZnO passivation of Zn metal to be a leading failure mechanism. [1][2][3]11] The exact passivation mechanism of Zn metal by ZnO has stood uncontrolled and unexplained for decades since the first reports of so-called "type I" and "type II" ZnO. [12][13][14][15][16] Recent studies have attempted empirical solutions to the passivation problem by use of Zn sponge architecture, [17] water-in-salt electrolyte, [18] or conductive carbon frameworks, [19] but these improvements often require significant extra costs and increased battery volume, negating key benefits of Zn battery chemistry. The properties, behavior, and effects of the different types of ZnO formed in alkaline batteries are clearly complex, and they must be better understood to improve Zn battery performance.ZnO used in Zn batteries is historically generated via the indirect (French) process in which Zn metal is vaporized and exposed to oxygen at high temperatures (>1000 °C) to form wurtzite ZnO. This "classical ZnO" is well-known to have stoichiometric composition, no electrochromic activity, no electrochemical intercalation capacity, high transparency to infrared and red wavelengths, [20] and resistivity of ≈10 2 Ω-cm for singlecrystals [21] or >10 8 Ω-cm for powders. [22] Recent publications [9,23] show new behavior from ZnO nanocrystals (1 to 10 nm) in organic-solvent dispersions, Zinc oxide is of great interest for advanced energy devices because of its low cost, wide direct bandgap, non-toxicity, and facile electrochemistry. In zinc alkaline batteries, ZnO plays a critical role in electrode passivation, a process that hinders commercialization and remains poorly understood. Here, novel observations of an electroactive type of ZnO formed in Zn-metal alkaline electrodes are disclosed. The electrical conductivity of battery-formed ZnO is measured and found to vary by factors of up to 10 4 , which provides a firstprinciples-based understanding of Zn passivation in industrial alkaline batteries. Simultaneous with this conductivity change, protons are inserted into the crystal structure and electrons are inserted into the conduction band in quantities up to ≈10 20 cm −3 and ≈1 mAh g ZnO −1. Electron inserti...

Zinc (Zn)–manganese dioxide (MnO2) rechargeable batteries have attracted research interest because of high specific theoretical capacity as well as being environmentally friendly, intrinsically safe and low-cost. Liquid electrolytes, such as potassium hydroxide, are historically used in these batteries; however, many failure mechanisms of the Zn–MnO2 battery chemistry result from the use of liquid electrolytes, including the formation of electrochemically inert phases such as hetaerolite (ZnMn2O4) and the promotion of shape change of the Zn electrode. This manuscript reports on the fundamental and commercial results of gel electrolytes for use in rechargeable Zn–MnO2 batteries as an alternative to liquid electrolytes. The manuscript also reports on novel properties of the gelled electrolyte such as limiting the overdischarge of Zn anodes, which is a problem in liquid electrolyte, and finally its use in solar microgrid applications, which is a first in academic literature. Potentiostatic and galvanostatic tests with the optimized gel electrolyte showed higher capacity retention compared to the tests with the liquid electrolyte, suggesting that gel electrolyte helps reduce Mn3+ dissolution and zincate ion migration from the Zn anode, improving reversibility. Cycling tests for commercially sized prismatic cells showed the gel electrolyte had exceptional cycle life, showing 100% capacity retention for >700 cycles at 9.5 Ah and for >300 cycles at 19 Ah, while the 19 Ah prismatic cell with a liquid electrolyte showed discharge capacity degradation at 100th cycle. We also performed overdischarge protection tests, in which a commercialized prismatic cell with the gel electrolyte was discharged to 0 V and achieved stable discharge capacities, while the liquid electrolyte cell showed discharge capacity fade in the first few cycles. Finally, the gel electrolyte batteries were tested under IEC solar off-grid protocol. It was noted that the gelled Zn–MnO2 batteries outperformed the Pb–acid batteries. Additionally, a designed system nameplated at 2 kWh with a 12 V system with 72 prismatic cells was tested with the same protocol, and it has entered its third year of cycling. This suggests that Zn–MnO2 rechargeable batteries with the gel electrolyte will be an ideal candidate for solar microgrid systems and grid storage in general.