www.advsustainsys.comion-based chemistries exclusively. Though few reviews document electrode materials and their performances for rechargeable aqueous Zn-and Al-ion batteries, [23,[25][26][27] a comprehensive understanding of ion storage mechanisms remains poorly enunciated. We believe the key to successful implementation of MV-ion batteries requires a simultaneous understanding of various elemental chemistries, in context with each other. Here, we summarize reported electrochemical reactions for Zn-ion and Al-ion electrodes. We compare activation mechanisms for ion transport and the role of water in various crystal lattices, and analyze specific challenges for electrode materials such as limited cation mobility, spontaneous dissolution, poor cycling, O 2 interaction, untoward proton/hydronium coinsertion, ineffective electrode-electrolyte interface formation, and current-collector corrosion. We illustrate how these challenges are dependent on some of the battery design parameters, with an aim to find optimized solutions or alternative strategies to increase the viability of Zn-ion aqueous batteries (ZIABs) and Al-ion aqueous batteries (AIABs) in BESS. Aqueous Rechargeable Electrodes for Zn-Ion StorageIn this review, we discuss rechargeable electrodes which can insert Zn 2+ ions. Since, electrodes for zinc-nickel batteries, [28,29] alkaline Zn-MnO 2 batteries, [30,31] or the hybrid zinc aqueous batteries, [32] store other ions (and not Zn 2+ ), we do not discuss these systems here. Majority of the literature for ZIAB electrodes spans various polymorphs of manganese dioxide, [33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] vanadium compounds, [49][50][51][52][53][54][55][56][57][58][59][60][61] and hexacyanoferrates, [62][63][64][65] and we discuss few Zn 2+ storage mechanisms for these electrodes. Additionally, we also highlight key similarities, missing links in the reaction mechanism, and the role of water, if any. 2.

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Hybrid electrochemical capacitors (HECs) are capable of storing more energy than supercapacitors while providing more power compared to lithium-ion batteries (LIBs). The development of Li-intercalating materials is critical to organic electrolyte based HECs, which generally give larger potential output than aqueous electrolyte based HECs. This article reports on a simple binder-free Nb 2 O 5 @graphene composite that exhibited excellent HEC performance as compared with other Li intercalating electrode materials. The composite exhibited enhanced cyclability with a capacity retention of 91.2% compared to 74.4% of the pure Nb 2 O 5 half-cell when tested at a rate of 2000 mA g À1 (10 C). The composite displayed a lower polarization effect when cycled at increasing scan rates (1-10 mV s À1 ). The enhanced rate capability could be ascribed to the use of a highly conductive graphene support. As a result, the HEC composed of the Nb 2 O 5 @graphene composite and activated carbon (AC) delivered a maximum energy and power density of 29 W h kg À1 and 2.9 kW kg À1 . The performance is better than most reported HECs with other Li-intercalating electrode materials.

Although "water-in-salt" electrolytes have opened a new pathway to expand the electrochemical stability window of aqueous electrolytes, the electrode instability and irreversible proton co-insertion caused by aqueous media still hinder the practical application, even when using exotic fluorinated salts. In this study, an accessible hybrid electrolyte class based on common sodium salts is proposed, and crucially an ethanol-rich media is introduced to achieve highly stable Na-ion electrochemistry. Here, ethanol exerts a strong hydrogenbonding effect on water, simultaneously expanding the electrochemical stability window of the hybridized electrolyte to 2.5 V, restricting degradation activities, reducing transition metal dissolution from the cathode material and improving electrolyte-electrode wettability. The binary ethanol-water solvent enables the impressive cycling of sodium-ion batteries based on perchlorate, chloride, and acetate electrolyte salts. Notably, a Na 0.44 MnO 2 electrode exhibits both high capacity (81 mAh g -1 ) and remarkable long cycle life >1000 cycles at 100 mA g -1 (a capacity decay rate per cycle of 0.024%) in a 1 M sodium acetate system. The Na 0.44 MnO 2 /Zn full cells also show excellent cycling stability and rate capability in a wide temperature range.The gained understanding of the hydrogen-bonding interactions in the hybridized electrolyte can provide new battery chemistry guidelines in designing promising candidates for developing low cost and long lifespan batteries based on other (Li + , K + , Zn 2+ , Mg 2+ , and Al 3+ ) systems.

Batteries exploiting zinc/aluminum electrochemistries in water‐based electrolytes are attractive due to cost, safety and elemental abundance considerations, but attaining reversibility at high energy densities and long‐life operation is challenging. In article https://doi.org/10.1002/adsu.201800111, William Manalastas Jr., Madhavi Srinivasan and co‐workers outline nascent systems, key challenges and proposed solutions for potentially enabling grid‐level implementation of aqueous Zn/Al batteries.

material is highly warranted. Several prospective materials have been proposed as anode insertion hosts to reversibly accommodate Li-ions; for example, anatase TiO 2 (≈1.7 V vs. Li with theoretical capacity of 335 mAh g −1 ) [ 6 ] and monoclinic TiO 2 -B (≈1.55 V vs. Li with theoretical capacity of 335 mAh g −1 ), [ 7,8 ] Li 4 Ti 5 O 12 (≈1.5 V vs. Li with theoretical capacity of 175 mAh g −1 ), [ 9,10 ] LiCrTiO 4 (≈1.5 V vs. Li with theoretical capacity of 157 mAh g −1 ), [ 11,12 ] TiP 2 O 7 (≈2.6 V vs. Li with theoretical capacity of 121 mAh g −1 ), [ 13,14 ] LiTi 2 (PO 4 ) 3 (≈2.6 V vs. Li with theoretical capacity of 138 mAh g −1 ), [ 15,16 ] Li 3 V 2 (PO 4 ) 3 (≈1.7 V vs. Li with theoretical capacity of 132 mAh g −1 ), [ 17 ] Nb 2 O 5 (≈1.7 V vs. Li with theoretical capacity of 403 mAh g −1 ) [ 18 ] and TiNb 2 O 7 (≈1.6 V vs. Li with theoretical capacity of 388 mAh g −1 ) [ 19 ] ( Figure 1 ). The mentioned insertion-host materials showed much higher insertion potential (>1.5 V vs. Li) and very less practical capacity (<300 mAh g −1 ) than graphitic anodes (≈0.1 V vs. Li), which results in a drastic reduction of overall energy density when coupled with a high performance cathode. On the other hand, displacement and alloy-based anodes exhibit a higher capacity than insertion electrodes, but they experience huge irreversible capacity loss in the fi rst cycle, large volume changes, and poor long-term cycleability, which renders them as "show case" anodes. [ 5,20 ] This situation clearly shows the importance of the development of low-voltage insertion-type anodes to realize the construction of high energy density Li-ion power packs to fulfi l the requirements for HEV and EV applications. [ 4 ] Recently, Goodenough and co-workers [ 21 ] reported the possibility of using a garnet framework Li 3 Nd 3 W 2 O 12 as a low-voltage (≈0.3 V vs. Li) insertion anode for LIB applications. Generally, such garnet materials are considered as fast Li-ion conductors with ionic conductivities of >10 −4 S cm −1 in ambient conditions. [ 22 ] The crystal structure of the lithium garnet framework can be described as Li 3 A 3 B 2 O 12 in which Li occupies square anti-prismatic, octahedral and tetrahedral sites in 3:2:3 ratio. [ 23 ] The tetrahedral Li-sites are bridged by empty octahedra sharing opposite faces with two tetrahedral sites; every face of a Lisite is bridged to neighboring Li-sites by the means of octahedral sites to provide a 3D interstitial space. This interstitial space can accommodate 9 moles of Li, but a practical limit of 7 moles of Li has been set. In the present case, the W 6+/4+The synthesis of carbon-coated Li 3

cost and thermal stability of Mn. [4] However, both families of cathodes face significant challenges in maintaining their structural integrity and rate capability upon cycling, with rapid capacity fade, significant voltage hysteresis, and impedance rise often accompanying the gradual transformation in local and/or long-range structure upon Li (de)intercalation. [5][6][7][8] At the material level, side reactions with the electrolyte, TM reduction and dissolution, TM migration and structural transformation as well as irreversible O redox chemistry, have all been suggested to contribute to these issues. [9][10][11][12][13][14][15] To minimize capacity loss in DRX, increasing Mn redox contribution at the expense of O-based redox processes is an effective strategy, as the former chemistry is inherently more reversible than the latter. [15][16][17][18] Fluorine substitution into the O anion sublattice not only stabilizes O redox, as shown by a number of recent studies, [10,[18][19][20][21][22] but also lowers the overall anionic valence and allows for an increased amount of Mn to be incorporated into the material. In contrast to the layered structure, F substitution into the cubic DRX structure can easily be achieved during synthesis. Substitution levels up to 10 at.% and 33 at.% have previously been reported on fluorinated Mn-based DRX samples prepared by solid-state and high-energy ball milling synthesis methods, respectively. [10,18,19,20] Although electrochemical performance improvements have been observed even at low levels of FThe capacity of lithium transition-metal (TM) oxide cathodes is directly linked to the magnitude and accessibility of the redox reservoir associated with TM cations and/or oxygen anions, which traditionally decreases with cycling as a result of chemical, structural, or mechanical fatigue. Here, it is shown that a capacity increase over 125% can be achieved upon cycling of high-energy Mn-and F-rich cation-disordered rocksalt oxyfluoride cathodes. This study reveals that in Li 1.2 Mn 0.7 Nb 0.1 O 1.8 F 0.2 , repeated Li extraction/reinsertion utilizing Mn 3+ /Mn 4+ redox along with some degree of O-redox participation leads to local structural rearrangements and formation of domains with off-stoichiometry spinel-like features. The effective integration of these local "structure-domains" within the cubic disordered rocksalt framework promotes better Li diffusion and improves material utilization, consequently increased capacity upon cycling. This study provides important new insights into materials design strategies to further exploit the rich compositional and structural space of Mn chemistry for developing sustainable, high-energy cathode materials.