A Long-Cycle-Life Self-Doped Polyaniline Cathode for Rechargeable Aqueous Zinc Batteries.

Rechargeable batteries offer promising solutions to the energy shortage issue.[1] In comparison to non-aqueous Li-ion batteries, aqueous rechargeable batteries possess the merits of using low-cost and high-safety water-based electrolytes. Aqueous electrolytes also provide higher ionic conductivities than non-aqueous ones, which promotes better rate performance of the cell.[2] These superiorities make the system suitable for stationary grid-level energy storages. The zinc metal is a convenient anode for aqueous batteries due to its low redox potential (-0.76 V vs. standard hydrogen electrode), high theoretical capacity (5845 mAh cm-3, 819 mAh g-1) and good compatibility with water.[3]-[5] Its abundant earth storage, non-toxicity and stability in air furthermore benefits large-scale productions. Using neutral or slightly acidic solutions to replace the alkaline electrolytes can suppress the dendritic growth of Zn anode.[4] One urgent requirement of zinc battery research is to find a suitable cathode. Benefiting from the long π-electron conjugated systems, conducting polymers hold higher conductivities compared to metal oxides and organic materials.[6] The highly conductive polyaniline (PANI) is a potential cathode, but it tends to deactivate in low acidic electrolytes (i.e. pH > 1) due to the de-protonation of the polymer. Herein, we synthesized a sulfo self-doped PANI electrode by a facile electrochemical copolymerization process. The -SO3 - self-dopant functions as an internal proton reservoir to ensure locally high acidic environment and facilitate the redox process in the weak acidic ZnSO4 electrolyte. In a full zinc cell, the self-doped PANI cathode delivers a high capacity of 180 mAh g-1, excellent rate performance of 70% capacity retention at 50 times current density increase, and long cycle life of over 2000 cycles with coulombic efficiency close to 100%. The detailed charge storage processes were investigated by X-ray photoelectron spectroscopy and X-ray diffraction. They suggest partial proton insertion into self-doped PANI during discharge, which leads to the internal protonation of -N= units and facilitates further reduction. This helps the polymer to maintain high electrochemical activity and stable capacity retention over cycling, in direct contrast to the fast capacity decay with the non-self-doped PANI cathode. Overall, our work utilizes the smart pH adjustment ability of self-dopants to overcome the discrepancy of active conditions between PANI cathode and Zn anode. More importantly, it demonstrates the potential applications of conducting polymers as cathode materials for rechargeable zinc batteries, with the specific energy storage behavior tunable thanks to the rich chemistry of their family.

Rechargeable aqueous zinc batteries are promising energy‐storage systems for grid applications. Highly conductive polyaniline (PANI) is a potential cathode, but it tends to deactivate in electrolytes with low acidity (i.e. pH >1) owing to deprotonation of the polymer. In this study, we synthesized a sulfo‐self‐doped PANI electrode by a facile electrochemical copolymerization process. The −SO3− self‐dopant functions as an internal proton reservoir to ensure a highly acidic local environment and facilitate the redox process in the weakly acidic ZnSO4 electrolyte. In a full zinc cell, the self‐doped PANI cathode provided a high capacity of 180 mAh g−1, excellent rate performance of 70 % capacity retention with a 50‐fold current‐density increase, and a long cycle life of over 2000 cycles with coulombic efficiency close to 100 %. Our study opens a door for the use of conducting polymers as cathode materials for high‐performance rechargeable zinc batteries.

VOPO4 ⋅x H2 O has been proposed as a cathode for rechargeable aqueous zinc batteries. However, it undergoes significant voltage decay in conventional Zn(OTf)2 electrolyte. Investigations show the decomposition of VOPO4 ⋅x H2 O into VOx in the electrolyte and voltage drops after losing the inductive effect from polyanions.PO4 3- was thus added to shift the decomposition equilibrium. A high concentration of cheap, highly soluble ZnCl2 salt in the electrolyte further prevents VOPO4 ⋅x H2 O dissolution. The cathode shows stable capacity and voltage retentions in 13 m ZnCl2 /0.8 m H3 PO4 aqueous electrolyte, in direct contrast to that in Zn(OTf)2 where the decomposition product VOx provides most electrochemical activity over cycling. Sequential H+ and Zn2+ intercalations into the structure are revealed, delivering a high capacity (170 mAh g-1 ). This work shows the potential issue with polyanion cathodes in zinc batteries and proposes an effective solution using fundamental chemical principles.

VOPO4⋅x H2O has been proposed as a cathode for rechargeable aqueous zinc batteries. However, it undergoes significant voltage decay in conventional Zn(OTf)2 electrolyte. Investigations show the decomposition of VOPO4⋅x H2O into VOx in the electrolyte and voltage drops after losing the inductive effect from polyanions.PO43− was thus added to shift the decomposition equilibrium. A high concentration of cheap, highly soluble ZnCl2 salt in the electrolyte further prevents VOPO4⋅x H2O dissolution. The cathode shows stable capacity and voltage retentions in 13 m ZnCl2/0.8 m H3PO4 aqueous electrolyte, in direct contrast to that in Zn(OTf)2 where the decomposition product VOx provides most electrochemical activity over cycling. Sequential H+ and Zn2+ intercalations into the structure are revealed, delivering a high capacity (170 mAh g−1). This work shows the potential issue with polyanion cathodes in zinc batteries and proposes an effective solution using fundamental chemical principles.

Currently, there is considerable interest in developing advanced rechargeable batteries that boast efficient distribution of electricity and economic feasibility for use in large-scale energy storage systems. Rechargeable aqueous zinc batteries are promising alternatives to lithium-ion batteries in terms of rate performance, cost, and safety. In this investigation, we employ Cu3(HHTP)2, a two-dimensional (2D) conductive metal-organic framework (MOF) with large one-dimensional channels, as a zinc battery cathode. Owing to its unique structure, hydrated Zn2+ ions which are inserted directly into the host structure, Cu3(HHTP)2, allow high diffusion rate and low interfacial resistance which enable the Cu3(HHTP)2 cathode to follow the intercalation pseudocapacitance mechanism. Cu3(HHTP)2 exhibits a high reversible capacity of 228 mAh g−1 at 50 mA g−1. At a high current density of 4000 mA g−1 (~18 C), 75.0% of the initial capacity is maintained after 500 cycles. These results provide key insights into high-performance, 2D conductive MOF designs for battery electrodes.

π‐Conjugated polymers provide a viable solution to the solubility and conductivity challenges of small‐molecule cathodes in rechargeable aqueous zinc batteries (RAZBs). However, their low active‐site utilization leads to unsatisfactory practical capacities. Herein, we have constructed a π‐conjugated polymer, poly(1,4,5,8‐tetraazaanthracene‐9,10‐dione‐[e]pyrene) (PTAP), by fusing p‐benzoquinone, pyrazine, and pyrene units. The pyrene units serve as fused‐ring π‐electron reservoirs, accommodating electrons during discharge, and facilitating the reduction of all C═O/N sites in the benzoquinone and pyrazine units. When employed as the cathode in RAZBs, PTAP delivers a considerable capacity of 416.3 mAh g −1 , corresponding to an active‐site utilization exceeding 95%. PTAP's semiconductivity minimizes the requirement for conductive additives, achieving superior specific capacity compared to previously reported organic cathodes based on the total cathode mass. Additionally, its rigid polymeric structure effectively suppress dissolution, enabling excellent cycling and static stability. This work presents a high‐capacity cathode candidate. Importantly, an effective electron reservoir strategy may inspire new designs of high‐performance organic electrode materials.

With the rapid consumption of fossil energy resources and the increasing pollution problems, the efficient use of energy and the development of renewable energy sources is urgently needed, which demand for the large-scale electrical energy storage system with properties of high-security, low-cost, environmentally benign, remarkable capacity and long-life span. In this regard, achieving some breakthroughs in this system will have extremely important strategic impacts on future energy structure adjustments and smart grid construction. Although lithium ion battery has dominated the energy market for decades, the intrinsic drawbacks of high cost and safety issues, to a great extent, impedes its utilization in scale-up energy storage system. And recent years witnessed the significant progress of rechargeable multivalent metal ions batteries, such as Mg2+-ion, Al3+-ion, Ca2+-ion and Zn2+-ion batteries. With the unique features of high safety, low-cost, environmental friendly, abundant resources stockpile, low redox potential (–0.76 V vs. SHE), high capacity (up to 820 mAh g–1 theoretically) and energy density (5851 mAh mL−1) of metallic Zn as well as remarkable ionic conductivity of aqueous electrolyte, aqueous rechargeable zinc battery has attracted plenty of interest in recent years and exhibits great potential to be an important candidate for the next generation of high-safe and large scale energy storage system. Many researchers have paid great attention to the development of high performance aqueous rechargeable zinc battery system, including cathode materials, electrolytes and anode. And some great achievements have been obtained. Henceforth, in this review, we have systematically reviewed the research progress of aqueous rechargeable zinc battery, summarized and discussed the existing problems faced by this system and the related solutions. Based on the author’s best knowledge and understanding of aqueous rechargeable zinc battery, we have looked forward to its future research direction and made some perspectives, which is expected to pave the pathway for the understanding and further development of aqueous rechargeable zinc battery. In our view, the development of advanced solid/gel electrolyte should be attached great significance, which is a promising way to solve a series of problems existing in anode and cathode, such as cathode material dissolution, zinc anode dendrite growth, passivation and corrosion, as well as the by-products existing in the system. Moreover, the research on solid/gel electrolyte also offers some opportunities of constructing wearable and flexible electronic devices, such as smart watch, which is greatly beneficial to put forward to the practical application of aqueous rechargeable zinc battery. In addition, development of high performance cathode materials with remarkable strength of both high-voltage and high-capacity, and in-depth researches on the modification and optimization of zinc anode are the further trends of high-performance aqueous rechargeable zinc battery.

Rechargeable aqueous zinc batteries have gained considerable attention for large‐scale energy storage systems because of their low cost and high safety, but they suffer from limitations in cycling stability and energy density with advanced cathode materials. Here, a high‐performance V5O12·6H2O (VOH) nanobelt cathode uniformly located on a stainless‐steel substrate via a facile electrodeposition technique is reported. We show that the hydrated layered VOH cathode enables highly reversible and ultrafast Zn2+ cation (de)intercalation processes, as confirmed by various electrochemical, X‐ray diffraction, X‐ray photoelectron spectroscopy, and transmission electron microscopy analyses. It is demonstrated that the binder‐free VOH cathode can deliver a discharge capacity of 354.8 mAh g−1 at 0.5 A g−1 with a high initial Coulombic efficiency of 99.5%, a high energy density of 194 Wh kg−1 at 2100 W kg−1, and a long cycle life with a capacity retention of 94% over 1000 cycles. In addition, a flexible quasi‐solid‐state Zn–VOH battery is constructed, achieving a reversible capacity of ≈300 mAh g−1 with a capacity retention of 96% after 50 cycles and displaying excellent electrochemical behaviors under different bending states. This work sheds light on the development of rechargeable aqueous zinc batteries for stationary grid storage applications or flexible energy storage devices.

Reaction mechanisms and optimization strategies of manganese-based materials for aqueous zinc batteries

A porous puckered V2O5 polymorph as new high performance cathode material for aqueous rechargeable zinc batteries

Organic cathode materials (OCMs) have been widely applied in aqueous rechargeable zinc batteries (ARZBs), but there are still many novel structures to be explored for better electrochemical performance and clearer mechanism. Herein, we have studied an organic biological dye, namely thionin (Thn‐CH3COO), as a bipolar‐type OCM for ARZBs based on its electroactive phenothiazine unit. In the optimal electrolyte of 2 M Zn(CF3SO3)2, Thn‐CH3COO exhibited the lowest solubility and thus the highest cycling stability (94 % capacity retention after 100 cycles), with a reversible capacity of 162 mAh g−1 (the theoretical value is 186 mAh g−1) and a discharge plateau at ∼0.8 V vs. Zn2+/Zn. Various characterization and DFT calculations have revealed that both CH3COO− and CF3SO3− anions and H+ cations participated the p‐type and n‐type reactions, respectively. In brief, the novel structure, competitive performance, and exhaustive mechanism investigation will facilitate the further development of ARZBs based on OCMs.

Polyaniline (PANI) is a promising cathode material for aqueous rechargeable zinc batteries (ARZBs), mainly benefitting from its good electrical conductivity. The high conductivity of PANI requires high doping level, yet the introduced nonactive dopants (e.g., SO4 2- ) limit the gravimetric capacity of PANI (usually <180 mAh g-1 ). Herein, an electro-active dopant (decavanadate anion, V10 O28 6- ) is employed to fabricate the PANI cathode (PANI-V10 O28 ) for ARZBs. The doped decavanadate anion with the sub-nanometer structure can fully expose the V-based active sites, exhibiting good electrochemical activity. Due to the steric hindrance effect as well as the strong interaction between decavanadate anions and PANI chains, the active dopants are trapped in the polymer chains, demonstrating good structural and electrochemical stability. PANI-V10 O28 achieves a record-high gravimetric capacity of 355 mAh g-1 at 0.1 A g-1 , which is significantly higher than other reported PANI cathodes. Experimental results suggest that the charge storage mechanism of PANI-V10 O28 includes reversible injection/extraction of Zn(H2 O)2 Cl4 2- ions in PANI, as well as the protonation/deprotonation of V10 O28 6- . This work enriches the doping chemistry of conducting polymer and pushes the development of organic cathodes for ARZBs to a new stage.

Rechargeable aqueous zinc batteries (RAZBs) are promising for large-scale energy storage because of their superiority in addressing cost and safety concerns. However, their practical realization is hampered by issues including dendrite growth, poor reversibility and low coulombic efficiency (CE) of Zn anodes due to parasitic reactions. Here, we report a non-concentrated aqueous electrolyte composed of 2 m zinc trifluoromethanesulfonate (Zn(OTf)2) and the organic dimethyl carbonate (DMC) additive to stabilize the Zn electrochemistry. Unlike the case in conventional aqueous electrolytes featuring typical Zn[H2O]62+ solvation, a solvation sheath of Zn2+ with the co-participation of the DMC solvent and OTf− anion is found in the formulated H2O + DMC electrolyte, which contributes to the formation of a robust ZnF2 and ZnCO3-rich interphase on Zn. The resultant Zn anode exhibits a high average CE of Zn plating/stripping (99.8% at an areal capacity of 2.5 mA h cm−2) and dendrite-free cycling over 1000 cycles. Furthermore, the H2O + DMC electrolytes sustain stable operation of RAZBs pairing Zn anodes with diverse cathode materials such as vanadium pentoxide, manganese dioxide, and zinc hexacyanoferrate. Rational electrolyte design with organic solvent additives would promote building better aqueous batteries.

Rechargeable aqueous zinc batteries (RAZB) have been re-evaluated because of the superiority in addressing safety and cost concerns. Nonetheless, the limited lifespan arising from dendritic electrodeposition of metallic Zn hinders their further development. Herein, a metal-organic framework (MOF) was constructed as front surface layer to maintain a super-saturated electrolyte layer on the Zn anode. Raman spectroscopy indicated that the highly coordinated ion complexes migrating through the MOF channels were different from the solvation structure in bulk electrolyte. Benefiting from the unique super-saturated front surface, symmetric Zn cells survived up to 3000 hours at 0.5 mA cm-2 , near 55-times that of bare Zn anodes. Moreover, aqueous MnO2 -Zn batteries delivered a reversible capacity of 180.3 mAh g-1 and maintained a high capacity retention of 88.9 % after 600 cycles with MnO2 mass loading up to 4.2 mg cm-2 .

The controlled quinone introduction and conformation modification of polyaniline cathode materials for rechargeable aqueous zinc-polymer batteries