Tailoring Electrodes and Electrolytes in Emerging Aqueous Ammonium-Ion Batteries for Enhanced Performance
Authors: Wang Ying 
Published: July 2022

The lithium ion battery, a most popular battery technology powering much of our digital and mobile lifestyle, has posed limitations for wider future use, mainly because of concerns raised over their cost, safety, and environmental impact. Most of these concerns come from its use of organic electrolytes that are viscous, flammable, and toxic, as well as the high price of lithium due to the limited lithium sources. Hence, tremendous research efforts have been devoted to the study of aqueous electrolytes, attributed to its being safe, convenient, inexpensive, more durable and less prone to thermal runaways. Its higher ionic conductivity combined with simplicity of the chemistry environment may facilitate long cycle life of the battery too. Moreover, non-metallic charge carriers such as ammonium (NH4
+) ions have attracted much attention, due to the following great merits: (i) favorable sustainability and nontoxicity as it could be synthesized from infinite or unlimited sources (nitrogen and hydrogen in air); (ii) a lighter mass of 18 mAh/g for high energy density batteries; (iii) the smallest hydration radius of 3.31 Å (despite its large ionic radius of 1.48 Å) leading to fast ion diffusion in the electrolyte; (iv) the nonmetallic interaction between NH4
+ carriers and host materials (e.g., hydrogen bond) is more flexible than the rigid metal coordination; (v) nondiffusion-controlled topochemistry between nonmetallic charging carriers and electrode framework during insertion/extraction process leading to pseudo-capacitive-dominated behavior and thus ultrafast kinetics. Nevertheless, a challenge associated with the development of high-performance ammonium ion batteries (AIBs) is that the large ionic radius of NH4
+ requires host materials with larger interlayer spacing or open structure for accommodation and thus limits the choice of electrode materials.

In this work, we employ facile solution processing methods to synthesize various nanostructured electrode materials ranging from oxides to polymer and composite to improve the electrochemical performance of AIBs. Examples include ammonium vanadate, vanadium oxide, polyaniline, and composite consisting of vanadium oxide and polyaniline. For example, a facile in-situ intercalation approach is utilized to prepare a composite material with polyaniline (PANI) intercalated in the interlayers of hydrated V2O5, for application as the electrode in high-performance AIBs. It is found that the interlayer spacing of hydrated V2O5 is expanded to 13.99Å, offering wide channels to accommodate NH4
+ ions during intercalation/deintercalation processes. The resulted AIB exhibits a high capacity of 192.5 mAh/gwhen cycled at a specific current of 1 A/g and retains 39 mAh/g at an extremely specific current of 20 A/g, together with stable cycle life. The diffusion kinetics of the NH4
+ ions, influenced by the hydrogen bonds formed between NH4
+ ion and O2- in the host structure, is effectively enhanced by the unique π-conjugated structure of PANI, leading to high capacity, improved rate capability and improved cycle life of AIBs. This PANI-intercalated V2O5 (PVO) electrode material demonstrates stable and ultrafast NH4
+ ion storage, as revealed by X-ray and Raman spectroscopy characterizations. The performance of AIBs can be further maximized by optimizing the composition of PVO electrodes or the ratio between PANI and hydrated vanadium oxide. Additionally, different aqueous liquid-state electrolytes and hydrogel electrolytes with composition tuned are tested in AIBs, for enhanced cycleability and stability of the batteries. As such, this study opens a new horizon by developing various electrode/electrolyte materials to boost the performance of aqueous ammonium-ion batteries that have great potential for next-generation safe and low-cost electrochemical energy storage.