Investigations on Tunnel-Structure MnO2 for Utilization as a High-Voltage and Long-Life Cathode Material in Aqueous Ammonium-Ion and Hybrid-Ion Batteries

Recently, nonmetal NH4 + ions have attracted extensive attention for use as charge carries in the field of energy storage due to their abundant resources, environmental friendliness, and low cost. However, the development of aqueous ammonium-ion batteries (AAIBs) is constrained by the absence of high-voltage and long-life materials. Herein, different tunnel-structure MnO2 materials (α-, β-, and γ-MnO2) are utilized as cathodes for AAIBs and hybrid-ion batteries and compared, and α-MnO2 is demonstrated to exhibit the most remarkable electrochemical performance. The α-MnO2 cathode material delivers the highest discharge capacity of 219 mAh g-1 at a current density of 0.1 A g-1 and the best cyclability with a capacity retention of 95.4% after 10 000 cycles at 1.0 A g-1. Moreover, aqueous ammonium-ion and hybrid-ion (ammonium/sodium ions) full batteries are successfully constructed using α-MnO2 cathodes. This work provides a novel direction for the development of aqueous energy storage for practical applications.

Mo-Pre-Intercalated MnO2 Cathode with Highly Stable Layered Structure and Expanded Interlayer Spacing for Aqueous Zn-Ion Batteries

Although manganese-based oxides possess high voltage and low cost, the sluggish reaction kinetics and poor structural stability hinder their applications in aqueous rechargeable Zn-ion batteries (ZIBs). Herein, a molybdenum (Mo) pre-intercalation strategy is proposed to solve the above issues of δ-MnO₂. The pre-intercalated Mo dopants, acting as the interlayer pillars, can not only expand the interlayer spacing but also reinforce the layered structure of δ-MnO₂, finally achieving enhanced reaction kinetics and superb cycling stability during carrier (de)intercalation. Moreover, oxygen defects, introduced due to Mo-pre-intercalation, play a critical role in the fast reaction kinetics and capacity improvement of the Mo-pre-intercalated δ-MnO₂ (Mo-MnO₂) cathode. Therefore, the Mo-MnO₂ cathode displays a high energy density of 451 Wh kg^-1 (based on cathode mass), excellent rate capability, and admirable long-term cycling performance with a high capacity of 159 mAhg^-1 at 1.0 A g^-1 after 1000 cycles. In addition, the energy storage mechanism of Zn²⁺/H⁺ stepwise reversible (de)intercalation is also revealed by ex situ experiments. This work provides an insightful guide for boosting the electrochemical performance of Mn-based oxide cathodes for ZIBs.

Inhibition of Zinc Dendrites Realized by a β-P(VDF-TrFE) Nanofiber Layer in Aqueous Zn-Ion Batteries

Uncontrollable Zn dendrite formations and parasitic side reactions on Zn electrodes induce poor cycling stability and safety issues, preventing the large-scale commercialization of Zn-ion batteries. Herein, to achieve uniform Zn deposition and suppress side reactions, an electrospun ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) copolymer, a P(VDF-TrFE) nanofiber layer, is introduced as an artificial solid-electrolyte interface on a Cu substrate acting as a current collector. The aligned molecular structure of β-P(VDF-TrFE) can effectively suppress localized current density on the Cu surface, lead to uniform Zn deposition, and suppress side reactions by preventing direct contact between electrodes and aqueous electrolytes. The half-cell configuration formed by the newly fabricated electrode can achieve an average coulombic efficiency of 99.2% over 300 cycles without short-circuiting at a current density of 1 mA cm^-2 and areal capacity of 1 mAh cm^-2. Stable cycling stability is also maintained for 200 cycles at a current density of 0.5 A g^-1 in a full-cell test using MnO₂ as a cathode.

Optimization of Electrolytes for High-Performance Aqueous Aluminum-Ion Batteries

Aqueous rechargeable batteries based on aluminum chemistry have become the focus of immense research interest owing to their earth abundance, low cost, and the higher theoretical volumetric energy density of this element compared to lithium-ion batteries. Efforts to harness this huge potential have been hindered by the narrow potential window of water and by passivating effects of the high-electrical band-gap aluminum oxide film. Herein, we report a high-performing aqueous aluminum-ion battery (AIB), which is constructed using a Zn-supported Al alloy, an aluminum bis(trifluoromethanesulfonyl)imide (Al[TFSI]₃) electrolyte, and a MnO₂ cathode. The use of Al[TFSI]₃ significantly extends the voltage window of the electrolyte and enables the cell to access Al³⁺/Al electrochemistry, while the use of Zn-Al alloy mitigates the issue of surface passivation. The Zn-Al alloy, which is produced by in situ electrochemical deposition, obtained from Al[TFSI]₃ showed excellent long-term reversibility for Al electrochemistry and displays the highest performance in AIB when compared to the response obtained in Al₂(SO₄)₃ or aluminum trifluoromethanesulfonate electrolyte. AIB cells constructed using the Zn-Al|Al[TFSI]₃|MnO₂ combination achieved a record discharge voltage plateau of 1.75 V and a specific capacity of 450 mAh g^-1 without significant capacity fade after 400 cycles. These findings will promote the development of energy-dense aqueous AIBs.

Novel Insights into Energy Storage Mechanism of Aqueous Rechargeable Zn/MnO2 Batteries with Participation of Mn2

Aqueous rechargeable Zn/MnO₂ zinc-ion batteries (ZIBs) are reviving recently due to their low cost, non-toxicity, and natural abundance. However, their energy storage mechanism remains controversial due to their complicated electrochemical reactions. Meanwhile, to achieve satisfactory cyclic stability and rate performance of the Zn/MnO₂ ZIBs, Mn²⁺ is introduced in the electrolyte (e.g., ZnSO₄ solution), which leads to more complicated reactions inside the ZIBs systems. Herein, based on comprehensive analysis methods including electrochemical analysis and Pourbaix diagram, we provide novel insights into the energy storage mechanism of Zn/MnO₂ batteries in the presence of Mn²⁺. A complex series of electrochemical reactions with the co-participation of Zn²⁺, H⁺, Mn²⁺, SO₄^2-, and OH^- were revealed. During the first discharge process, co-insertion of Zn²⁺ and H⁺ promotes the transformation of MnO₂ into Zn_xMnO₄, MnOOH, and Mn₂O₃, accompanying with increased electrolyte pH and the formation of ZnSO₄·3Zn(OH)₂·5H₂O. During the subsequent charge process, Zn_xMnO₄, MnOOH, and Mn₂O₃ revert to α-MnO₂ with the extraction of Zn²⁺ and H⁺, while ZnSO₄·3Zn(OH)₂·5H₂O reacts with Mn²⁺ to form ZnMn₃O₇·3H₂O. In the following charge/discharge processes, besides aforementioned electrochemical reactions, Zn²⁺ reversibly insert into/extract from α-MnO₂, Zn_xMnO₄, and ZnMn₃O₇·3H₂O hosts; ZnSO₄·3Zn(OH)₂·5H₂O, Zn₂Mn₃O₈, and ZnMn₂O₄ convert mutually with the participation of Mn²⁺. This work is believed to provide theoretical guidance for further research on high-performance ZIBs.