Zn-based ABs (ZnABs), the first electrochemical battery, dated back to the voltaic pile invented by A. Volta in the late 19th century. Since then, because of the very good electrochemical reversibility of zinc, dozens of Zn-based batteries were developed. Nowadays, one-third of the world battery market is composed of Zn-based batteries, which highlights its importance as a power source for a wide range of applications. On the basis of the cathode and electrolyte, ZnABs can be divided into alkaline zinc batteries (AZABs; such as Zn-Ni, alkaline Zn-MnO2, Zn-Ag, and Zn-air), near-neutral Zn-ion batteries (NZIBs; such as Zn-MnO2 and Zn-V2O5 ZIB and electrolytic Zn-Mn battery), and zinc-based redox flow batteries (such as Zn-Br, Zn-V, Zn-Ce, and Zn-I). In recent years, some breakthroughs have been achieved in aqueous Zn-based batteries (see Fig. 4A). The specific capacity versus operation voltage data for different types of batteries are displayed in Fig. 4B, based on the mass of cathode.
Alkaline Zn-based ABs. Alkaline Zn-based batteries, including Zn-Ni/Co, Zn-MnO2, Zn-Ag2O, and Zn-air batteries, which depend on the reversible redox reaction of Zn/ZnO with a redox potential of −1.35 V versus SHE, represent an old and mature battery technology, but recently, they are gaining a lot of attention. This is mainly caused by weak stability arising from the unavoidable formation of Zn dendrite, shape change, corrosion, and passivation. To solve the above issues, various strategies (23), including alloying with other metals (e.g., Bi, Sn, and In) to suppress corrosion, hybridization or surface modification with additives [such as BaO, Bi2O3, and In(OH)3 and Ca(OH)2] to suppress H2 evolution, geometry and structure design (such as Zn fibers, rods, bars, and sheets with different thickness and lengths) to mitigate shape change and zinc dendrite formation, and electrolyte additives (such as KF, K2HPO4, K2CO3, polyethylene glycol, and saturated ZnO) to reduce Zn dissolution and inhibit Zn dendrite were used. In 2017, Rolison and co-workers (52) investigated 3D Zn sponges as the anode materials at high DoD (DoDZn). As shown in Fig. 4C, thanks to the monolithic, porous, and nonperiodic architecture of Zn sponges, such 3D Zn anode achieves high utilization of 91% DoDZn and is dendrite free with repeated 50,000 cycles at <1% DoDZn. Inspired by this case, various long-life Zn anodes, based on 3D skeleton construction strategy (such as Ni nanowire, carbon cloth, Cu foam, and graphene foam) have been developed. In addition, quasi–solid-state design not only endows the batteries with high flexibility but also stabilizes the Zn anode via suppressing the corrosion and dissolution of Zn anode.
For cathode materials, MnO2 as a nontoxic, low-cost, earth-abundant, and high-capacity (617 mAh g−1) material is a promising candidate for AZABs. In alkaline Zn-Mn batteries, by-products of spinal-phase Mn3O4 (formed by Mn2+ and mother MnO2) and ZnMn2O4 [formed by MnOOH and Zn(OH)42−] accumulate after repeated cycles at deep DoD, leading to the capacity fade and eventual battery failure. In general, doping with Bi, Cu, Ni, Co, etc., elements or integrating the corresponding oxides with the cathode and using LiOH electrolyte is an effective method to enhance the capacity and rechargeability of MnO2 (23). In 2017, Banerjee and co-workers (74) realized the two-electron utilization (617 mAh g−1) of MnO2 with 6000 cycles of life span using a Cu-intercalated Bi-birnessite cathode. As shown in Fig. 4D, the key to rechargeability relies on the redox potentials of Cu to reversibly intercalate into the Bi-birnessite–layered structure during the dissolution and precipitation process for stabilizing and enhancing charge transfer characteristics. In witnessing deep utilization with high stability, such electrochemical tuning strategies unprecedentedly solved the main challenges of using MnO2 cathode in alkaline batteries.
Ni/Co-based cathode materials, because of their excellent electrochemical reversibility and acceptable theoretical capacity, have been widely used for Zn-Ni battery since the late 19th century. Among various ABs, Zn-Ni/Co batteries are particularly advantageous due to their unique merits of higher operation voltage (around 1.7 to 1.8 V; see Fig. 4B), impressive theoretical energy density (~372 Wh kg−1), high-power ability, and low cost. However, because of the poor life of zinc anode, after over 100 years, the Zn-Ni batteries were commercialized by PowerGenix (now named ZincFive Inc.) until 2003. In general, the commercial Zn-Ni battery prepared from β-phase Ni(OH)2 cathode delivers an energy density of 70 to 100 Wh kg−1, a peak power density of 2000 W kg−1, and a life span of around 500 cycles. The overall electrochemical performance is far from satisfactory for the ever-increasing demand for power storage. Aside from poor stability in zinc anode, the researchers tend to attribute such poor electrochemical performance to the reluctant specific capacity, irreversibility, and poor electroactivity in Ni/Co-based cathode. Fortunately, some achievements in designing nano-architecture Ni/Co-based cathode materials have raised hopes in recent years. In 2014, Dai and co-workers (75) inaugurated an ultrafast high-capacity Zn-Ni battery based on ultrathin NiAlCo layered double hydroxide/carbon nanotube (LDH/CNT) nanoplate cathode, in which Al and Co co-doping stabilized α-Ni(OH)2. Attributing to the high capacity (354 mAh g−1), high rate (278 mAh g−1 at 66.7 A g−1), and good stability (94% of capacity retention after 2000 cycles) of the NiAlCo LDH/CNT cathode, the assembled Zn-Ni battery delivered an energy density of 274 Wh kg−1 and a power density of 16.6 kW kg−1, together with good cycling stability (85% capacity retention after 500 cycles). Inspired by this work, so far, various Ni-based and Co-based materials, such as NiAlCo-LDH/CNT (75), Ni3S2 (76), Co3O4 (77), and NiCo2O4 (78), have been extensively explored for Zn-Ni batteries.
Besides the compositional optimization, rational nano-architecture design (e.g., nanoparticles, nanowires, nanorods, and nanosheets) can provide unique merits in mechanical and electrical properties, such as higher surface area and shorter pathways for transport of ions and electrons, and conquer the intrinsic challenges of bulk materials, such as poor electrical conductivity and large volume expansion. In addition, surface modification, such as surface coating of PANI and surface doping of phosphate ions, can further enhance the electrical conductivity of electrodes (76, 78). However, it should be noted that the developed advanced self-standing cathodes are still far from practical application, although they have achieved remarkable gravimetric capacity, high rate, and long life. Their areal capacity is generally lower than 1.0 mAh cm−2, which is much lower than the industrial-level areal capacity of ∼35 mAh cm−2 (79). Therefore, further developments of Ni-based and Co-based materials for Ni-Zn or Co-Zn batteries, which simultaneously have high gravimetric capacity, high rate capability, and long life with high mass loading, remain difficult to achieve.
Neutral ZIBs. NZIBs, which use neutral or weak acidic Zn2+-containing aqueous media as the electrolyte, are attracting increasing global attention in recent years because of their potential for large-scale electrical energy storage. As early as 1986, a rechargeable Zn-MnO2 battery using MnO2 cathode and Zn anode in 2 M ZnSO4 electrolyte was first investigated by Yamamoto et al. (80), but the reaction mechanism was unclear. Until 2012, Kang and co-workers (81) found the reversible intercalation of Zn2+ into α-MnO2 and proposed the concept of NZIBs combining with a zinc anode and a mild ZnSO4 or Zn(NO3)2 aqueous electrolyte. Intensive efforts have been devoted to NZIBs since then with the purpose of revealing the reaction mechanism and developing advanced electrode materials. Unlike AZABs, the charge storage in the anode depends on the reversible plating/stripping of Zn/Zn2+ with a redox potential of −0.763 V versus SHE. Although the severe corrosion and dissolution of Zn are eliminated, the biggest challenges are in suppressing the formation of zinc dendrite. Up to now, various efforts, including surface modification, structural optimization (37), and electrolyte optimization (47), have been explored to eliminate the formation of zinc dendrite. For instance, a high-rate flexible quasi–solid-state ZIB constructed from graphene foam supported Zn array anode, and a gel electrolyte can deliver long-term durability of 2000 cycles with 89% of the initial capacity (37). Moreover, it was recently pointed out by Archer and co-workers (82) that graphene, with a low lattice mismatch for Zn, is effective in driving the deposition of Zn with a locked crystallographic orientation, which prompts exceptional reversibility of the Zn anode.
Another issue that has hindered the application of NZIBs is the lack of robust cathode host materials for fast and reversible Zn2+ storage, due to the high charge density and high hydrated ionic radius of Zn2+. So far, although various cathode materials, such as manganese oxides, V-based, PBAs, and organic materials have been proposed (14), the development of cathode materials for ZIBs is still in its infancy stage. This is mainly ascribed to the following four aspects: (i) The reaction mechanism still remains controversial, (ii) fast capacity decay, (iii) unsatisfactory specific capacity, and (iv) poor rate performance. V-based compounds, especially vanadium oxides, are attractive host materials for Zn2+ storage. Because of their inherent features of the multiple valence states of vanadium and the large open-framework structure, V-based materials have merits of high capacity (even up to 400 mAh g−1), fast dynamics, and low cost. As for the reaction mechanism, it is generally considered as the insertion/extraction of Zn2+ in the host materials during the corresponding discharge/charge process. Recently, with the observation of zinc hydroxide sulfate [Zn4(SO4)(OH)6·nH2O, ZHS] in Zn-V systems, H+ is also regarded as the charge carrier to participate in the electrochemical reaction (83). On the basis of the simultaneous H+ and Zn2+ insertion/extraction process, the Zn/NaV3O8·1.5H2O battery proposed by Chen and co-workers (51) delivers a superior reversible capacity (380 mAh g−1) and a high durability (82% of capacity retention after 1000 cycles). Apart from the study of mechanism, more work is needed on advanced materials for improving the specific capacity, rate performance, and cycling life of V-based cathode. Up to now, some optimization strategies, including morphological and structural control (such as designing various nanoarchitectures; preinsertion of Li, Na, K, Zn, Ca, etc., metal ions; and adjustment of structural water), integrating with conductive additives, designing binder-free electrode, and optimizing electrolytes, have been attempted (83). Dozens of V-based compounds, such as V2O5·nH2O (84), Zn0.25V2O5·nH2O (85), and Zn2(OH)VO4 (37), have been developed. In general, the V-based oxides can present an ultrahigh discharge capacity of over 400 mAh g−1, while their operating voltage is relatively poorer than that of Mn-based materials (Fig. 4B). For example, the Zn/Zn0.3V2O5·1.5H2O battery fabricated by Wang et al. (86) delivers an average discharge voltage of 0.8 V, and a high specific capacity of 426 mAh g−1 at 0.2 A g−1, together with an unprecedented cycling stability (maintains 214 mAh g−1 after 20,000 cycles at 10 A g−1).
PBAs, similarly to LiAB, NaAB, and KAB systems, can also be used as cathode materials for NZIBs. In 2015, a PBA-based NZIB built on ZnHCF was first proposed by Liu and co-workers (87), with a relatively high operation voltage of ~1.7 V, a discharge capacity ~65.4 mAh g−1, and an energy density of 100 Wh kg−1. Since then, various other PBA-based NZIBs, such as CuHCF-Zn (56 mAh g−1, 1.73 V), FeHCF-Zn (120 mAh g−1, 1.1 V), NiHCF-Zn (56 mAh g−1, 1.2 V), and MnHCF-Zn (137 mAh g−1, 1.7 V), have been developed (14). However, note that because of the low capacity, the energy density of PBA-based NZIBs is still not competitive. Besides the inorganic materials mentioned above, some organic ones, such as PANI (191 mAh g−1, 1.0 V) (88) and calixquinone (335 mAh g−1, 1.0 V) (89), have been developed. Up to now, the development of organic cathode materials for NZIB is still in its preliminary stage. With the abundant choices of functional group and molecular weight, there remains an immense potential to optimize the electrochemical performance of organic electrodes.
Manganese oxides, with the merits of abundant crystallographic polymorphs (α, β, γ, δ, λ, ε, and todorokite types), high theoretical capacity (308 mAh g−1), low cost, and being earth abundant, have been regarded as promising cathode candidates for NZIBs. In general, with the applicable exploration of various MnO2 polymorphs for NZIBs, there are mainly four-stream concepts as shown in Fig. 4E on the energy storage mechanism: (i) Zn2+ insertion/extraction, (ii) H+ insertion/extraction accompanied with the deposition of ZHS, (iii) coinsertion/extraction of both H+ and Zn2+ in different charge/discharge steps, and (iv) electrolysis/electrodeposition of MnO2/Mn2+, which have been systematically summarized in reports (14, 41).
Although the reaction mechanism remains under dispute, the first three mechanisms assume that the severe dissolution of Mn2+ in the discharge process is responsible for the fast capacity decay. Up to now, some effective strategies, including preaddition of Mn salt in electrolyte (50), surface coating [such as N-doped carbon (90) and PEDOT (91)], and incorporation of closely bonded ions [such as K0.8Mn8O16 (92)], have been used to suppress the dissolution of Mn2+ and enhance the cycling stability of NZIBs. Specially, the cycling stability of the Zn-MnO2 battery could be greatly enhanced by preaddition of Mn2+, achieving a 10,000-cycle life span without obvious capacity decay (93). It should be noted that such excellent cycling stability is attributed not only to the stabilized MnO2 by suppressing the dissolution of Mn2+ but also from the extra capacity provided by the redeposited MnO2 in the charge process (94). In addition, large volumetric change and structural collapse caused by repeated insertion of hydrated Zn2+ ions also result in a rapid capacity fading. Thereby, various efforts, such as morphological control of porous structure (90), coupling with graphene and CNTs (95), and structure stabilization by cationic doping and PANI intercalation (96), have been explored. For example, PANI-intercalated MnO2 nanolayer can deliver a stable discharge capacity of around 125 mAh g−1 over 5000 cycles (96).
Although great progress has been achieved as can be seen from Fig. 4A, the current Zn-based alkaline batteries and neutral or weak acidic Zn2+ batteries have shown limited output voltages (<1.8 V) and discharge capacity below 450 mAh g−1. In our latest research, we found a latent high-voltage MnO2 electrolysis process in a conventional ZIB and proposed a previously unknown electrolytic Zn-Mn system (see Fig. 4F), via enabled proton and electron dynamics (41). The four-step MnO2 electrolysis process was first analyzed by density functional theory calculations. This Zn-Mn electrolytic system presents an output voltage as high as 1.95 V, an imposing gravimetric capacity of about 570 mAh g−1, and density of ~409 Wh kg−1 based on both anode and cathode active materials. A prototype redox flow-battery stack was also built in our Zn-Mn electrolytic battery. In summary, the output voltage (~2 V), energy efficiency (88%), and cost of the electrolyte [3 to 5 US$ (kW h)−1] outperform other redox pairs integrated AB systems (Fig. 4G), such as Zn-Fe, Zn-Br2, Zn-Ce, and all vanadium flow batteries (41). It is expected that with further judicial development, such as the use of a more selective electrolyte, Zn efficiency improvement, and efficient flow-stack battery design, this Zn-Mn electrolytic flow battery design will be applicable for practical energy storage and, particularly, for large-scale grid energy storage.