Extended Battery Compatibility Consideration from an Electrolyte Perspective

The performance of electrochemical batteries is intricately tied to the physicochemical environments established by their employed electrolytes. Traditional battery designs utilizing a single electrolyte often impose identical anodic and cathodic redox conditions, limiting the ability to optimize redox environments for both anode and cathode materials. Consequently, advancements in electrolyte technologies are pivotal for addressing these challenges and fostering the development of next-generation high-performance electrochemical batteries. This review categorizes perspectives on electrolyte technology into three key areas: additives engineering, comprehensive component analysis encompassing solvents and solutes, and the effects of concentration. By summarizing significant studies, the efficacy of electrolyte engineering is highlighted, and the review advocates for further exploration of optimized component combinations. This review primarily focuses on liquid electrolyte technologies, briefly touching upon solid-state electrolytes due to the former greater vulnerability to electrode and electrolyte interfacial effects. The ultimate goal is to generate increased awareness within the battery community regarding the holistic improvement of battery components through optimized combinations.

Single [0001]-oriented zinc metal anode enables sustainable zinc batteries

The optimization of crystalline orientation of a Zn metal substrate to expose more Zn(0002) planes has been recognized as an effective strategy in pursuit of highly reversible Zn metal anodes. However, the lattice mismatch between substrate and overgrowth crystals has hampered the epitaxial sustainability of Zn metal. Herein, we discover that the presence of crystal grains deviating from [0001] orientation within a Zn(0002) metal anode leads to the failure of epitaxial mechanism. The electrodeposited [0001]-uniaxial oriented Zn metal anodes with a single (0002) texture fundamentally eliminate the lattice mismatch and achieve ultra-sustainable homoepitaxial growth. Using high-angle angular dark-filed scanning transmission electron microscopy, we elucidate the homoepitaxial growth of the deposited Zn following the "~ABABAB~" arrangement on the Zn(0002) metal from an atomic-level perspective. Such consistently epitaxial behavior of Zn metal retards dendrite formation and enables improved cycling, even in Zn||NH4V4O10 pouch cells, with a high capacity of 220 mAh g-1 for over 450 cycles. The insights gained from this work on the [0001]-oriented Zn metal anode and its persistently homoepitaxial mechanism pave the way for other metal electrodes with high reversibility.

Gradient Quasi-Solid Electrolyte Enables Selective and Fast Ion Transport for Robust Aqueous Zinc-Ion Batteries

The quasi-solid electrolytes (QSEs) attract extensive attention due to their improved ion transport properties and high stability, which is synergistically based on tunable functional groups and confined solvent molecules among the polymetric networks. However, the trade-off effect between the polymer content and ionic conductivity exists in QSEs, limiting their rate performance. In this work, the epitaxial polymerization strategy is used to build the gradient hydrogel networks (GHNs) covalently fixed on zinc anode. Then, it is revealed that the asymmetric distribution of negative charges benefits GHNs with fast and selective ionic transport properties, realizing a higher Zn2+ transference number of 0.65 than that (0.52) for homogeneous hydrogel networks (HHNs) with the same polymer content. Meanwhile, the high-density networks formed at Zn/GHNs interface can efficiently immobilize free water molecules and homogenize the Zn2+ flux, greatly inhibiting the water-involved parasitic reactions and dendrite growth. Thus, the GHNs enable dendrite-free stripping/plating over 1000 h at 8 mA cm-2 and 1 mAh cm-2 in a Zn||Zn symmetric cell, as well as the evidently prolonged cycles in various full cells. This work will shed light on asymmetric engineering of ion transport channels in advanced quasi-solid battery systems to achieve high energy and safety.

Reliable lateral Zn deposition along (002) plane by oxidized PAN separator for zinc-ion batteries

Aqueous zinc ion batteries (AZIBs) are the promising candidate for energy storage where safety and low cost are the major concerns. However, the uneven and random electrodeposition of Zn has become a serious impediment to the deep recharging of AZIBs. Conventional modifications on zinc substrate can promote homogenous zinc deposition initially, but not sustainably. Here, an oxidized polyacrylonitrile (OPAN) membrane with a conjugated planar structure is proposed as a zinc ion battery separator. This separator can continuously regulate the growth of Zn with (002) texture to inhibit dendrites. In addition, the separator has a fast Zn2+ ion transfer, which can spontaneously repel SO42- and relieve side reactions. As a result, the Zn-symmetric batteries show cycle lifetime of more than 1300 hours at 1 mA cm-2 and 1 mA h cm-2, and kept stable for more than 160 hours even at 65% high discharge of depth (DOD). The MnO2//Zn full celled assembled with an OPAN separator had very little decay for 5000 cycles at 2 A g-1. This work provides a new method for realizing the continuous and uniform deposition of Zn metals, which also provides a new route for batteries with metallic anodes.

Anion chemistry in energy storage devices

Anions serve as an essential component of electrolytes, whose effects have long been ignored. However, since the 2010s, we have seen a considerable increase of anion chemistry research in a range of energy storage devices, and it is now understood that anions can be well tuned to effectively improve the electrochemical performance of such devices in many aspects. In this Review, we discuss the roles of anion chemistry across various energy storage devices and clarify the correlations between anion properties and their performance indexes. We highlight the effects of anions on surface and interface chemistry, mass transfer kinetics and solvation sheath structure. Finally, we conclude with a perspective on the challenges and opportunities of anion chemistry for enhancing specific capacity, output voltage, cycling stability and anti-self-discharge ability of energy storage devices.

Reversible Zn Metal Anodes Enabled by Trace Amounts of Underpotential Deposition Initiators

Routine electrolyte additives are not effective enough for uniform zinc (Zn) deposition, because they are hard to proactively guide atomic-level Zn deposition. Here, based on underpotential deposition (UPD), we propose an "escort effect" of electrolyte additives for uniform Zn deposition at the atomic level. With nickel ion (Ni2+ ) additives, we found that metallic Ni deposits preferentially and triggers the UPD of Zn on Ni. This facilitates firm nucleation and uniform growth of Zn while suppressing side reactions. Besides, Ni dissolves back into the electrolyte after Zn stripping with no influence on interfacial charge transfer resistance. Consequently, the optimized cell operates for over 900 h at 1 mA cm-2 (more than 4 times longer than the blank one). Moreover, the universality of "escort effect" is identified by using Cr3+ and Co2+ additives. This work would inspire a wide range of atomic-level principles by controlling interfacial electrochemistry for various metal batteries.

Challenges and Strategies in the Development of Zinc-Ion Batteries

Although promising, the practical use of zinc-ion batteries (ZIBs) remains plagued with uncontrollable dendrite growth, parasitic side reactions, and the high intercalation energy of divalent Zn²⁺ ions. Hence, much work has been conducted to alleviate these issues to maximize the energy density and cyclic life of the cell. In this holistic review, the mechanisms and rationale for the stated challenges shall be summarized, followed by the corresponding strategies employed to mitigate them. Thereafter, a perspective on present research and the outlook of ZIBs would be put forth in hopes to enhance their electrochemical properties in a multipronged approach.

Realizing Textured Zinc Metal Anodes through Regulating Electrodeposition Current for Aqueous Zinc Batteries

Crystallography modulation of zinc (Zn) metal anode is promising to promote Zn reversibility in aqueous electrolytes, but efficiently constructing Zn with specific crystallographic texture remains challenging. Herein, we report a current-controlled electrodeposition strategy to texture the Zn electrodeposits in conventional aqueous electrolytes. Using the electrolytic cell with low-cost Zn(CH₃ COO)₂ electrolyte and Cu substrate as a model system, the texture of as-deposited Zn gradually transforms from (101) to (002) crystal plane as increasing the current density from 20 to 80 mA cm^-2 . Moreover, the high current accelerates the Zn nucleation rate with abundant nuclei, enabling uniform deposition. The (002) texture permits stronger resistance to dendrite growth and interfacial side reactions than the (101) texture. The resultant (002)-textured Zn electrode achieves deep cycling stability and supports the stable operation of full batteries with conventional V/Mn-based oxide cathodes.

Highly Reversible Zinc Metal Anode in a Dilute Aqueous Electrolyte Enabled by a pH Buffer Additive

Aqueous zinc-ion batteries have drawn increasing attention due to the intrinsic safety, cost-effectiveness and high energy density. However, parasitic reactions and non-uniform dendrite growth on the Zn anode side impede their application. Herein, a multifunctional additive, ammonium dihydrogen phosphate (NHP), is introduced to regulate uniform zinc deposition and to suppress side reactions. The results show that the NH₄ ⁺ tends to be preferably absorbed on the Zn surface to form a "shielding effect" and blocks the direct contact of water with Zn. Moreover, NH₄ ⁺ and (H₂ PO₄ )^- jointly maintain pH values of the electrode-electrolyte interface. Consequently, the NHP additive enables highly reversible Zn plating/stripping behaviors in Zn//Zn and Zn//Cu cells. Furthermore, the electrochemical performances of Zn//MnO₂ full cells and Zn//active carbon (AC) capacitors are improved. This work provides an efficient and general strategy for modifying Zn plating/stripping behaviors and suppressing side reactions in mild aqueous electrolyte.

Proton-Reservoir Hydrogel Electrolyte for Long-Term Cycling Zn/PANI Batteries in Wide Temperature Range

Advanced aqueous batteries are promising for next generation flexible devices owing to the high safety, yet still requiring better cycling stability and high capacities in wide temperature range. Herein, a polymeric acid hydrogel electrolyte (PAGE) with 3 M Zn(ClO₄ )₂ was fabricated for high performance Zn/polyaniline (PANI) batteries. With PAGE, even at -35 °C the Zn/Zn symmetrical battery can keep stable for more than 1 500 h under 2 mA cm^-2 , and the Zn/PANI battery can provide ultra-high stable specific capacity of 79.6 mAh g^-1 for more than 70 000 cycles at 15 A g^-1 . This can be mainly ascribed to the -SO₃ ^- H⁺ function group in PAGE. It can generate constant protons and guide the (002) plane formation to accelerate the PANI redox reaction kinetics, increase the specific capacity, and suppress the side reaction and dendrites. This proton-supplying strategy by polymeric acid hydrogel may further propel the development of high performance aqueous batteries.

Chaotropic Polymer Additive with Ion Transport Tunnel Enable Dendrite-Free Zinc Battery

Zn batteries are considered the new-generation candidate for large-scale energy storage systems, taking both safety and environmental problems into account. They are still restricted by unexpected dendrite/byproducts occurring on the Zn anodes. We hereby screen a powerful polymer type additive, hyaluronic acid (HA), to regulate the typical ZnSO₄ electrolyte for obtaining dendrite-free Zn ion batteries. The intrinsically chaotropic property of the HA molecule can efficiently destruct the original hydrogen-bonds from H₂O-H₂O, thus restricting the common parasitic reactions derived from the large amount of active water molecules. Simultaneously, the abundant functional groups along the long chain from HA additives can construct an effective tunnel for transferring Zn²⁺ smoothly, enabling an obviously improved Zn ion transference number of 0.62. Owning to the above intriguing mechanism for regulating the solvation structure of electrolyte systems, the HA additives can greatly increase the cycling life of Zn-Zn symmetric cells to 2200 and 800 h under the conditions of 1 mA cm^-2/1 mAh cm^-2 and 5 mA cm^-2/5 mAh cm^-2, respectively. Modified performance for both Zn-Ti and Zn-MnO₂ can all be realized by this valid additive, elucidating it can be potentially utilized in large-scale Zn based aqueous energy storage devices.

Self-repairing interphase reconstructed in each cycle for highly reversible aqueous zinc batteries

Aqueous zinc (Zn) chemistry features intrinsic safety, but suffers from severe irreversibility, as exemplified by low Coulombic efficiency, sustained water consumption and dendrite growth, which hampers practical applications of rechargeable Zn batteries. Herein, we report a highly reversible aqueous Zn battery in which the graphitic carbon nitride quantum dots additive serves as fast colloid ion carriers and assists the construction of a dynamic & self-repairing protective interphase. This real-time assembled interphase enables an ion-sieving effect and is found actively regenerate in each battery cycle, in effect endowing the system with single Zn²⁺ conduction and constant conformal integrality, executing timely adaption of Zn deposition, thus retaining sustainable long-term protective effect. In consequence, dendrite-free Zn plating/stripping at ~99.6% Coulombic efficiency for 200 cycles, steady charge-discharge for 1200 h, and impressive cyclability (61.2% retention for 500 cycles in a Zn | |MnO₂ full battery, 73.2% retention for 500 cycles in a Zn | |V₂O₅ full battery and 93.5% retention for 3000 cycles in a Zn | |VOPO₄ full battery) are achieved, which defines a general pathway to challenge Lithium in all low-cost, large-scale applications.

Metallic zinc is an ideal anode material for aqueous rechargeable batteries but reversibility is a challenge. Here, the authors realise a dynamic real-time reconstructed interphase on zinc anode formed by graphitic carbon nitride quantum dot as an electrolyte additive to improve the performance of Zn metal anodes.

Crystallographically Textured Electrodes for Rechargeable Batteries: Symmetry, Fabrication, and Characterization

The vast of majority of battery electrode materials of contemporary interest are of a crystalline nature. Crystals are, by definition, anisotropic from an atomic-structure perspective. The inherent structural anisotropy may give rise to favored mesoscale orientations and anisotropic properties whether the material is in a rest state or subjected to an external stimulus. The overall perspective of this review is that intentional manipulation of crystallographic anisotropy of electrochemically active materials constitute an untapped parameter space in energy storage systems and thus provide new opportunities for materials innovations and design. To that end, we contend that crystallographically textured electrodes, as opposed to their textureless poly crystalline or single-crystalline analogs, are promising candidates for next-generation storage of electrical energy in rechargeable batteries relevant to commercial practice. This perspective is underpinned first by the fundamental─to a first approximation─uniaxial, rotation-invariant symmetry of electrochemical cells. On this basis, we show that a crystallographically textured electrode with the preferred orientation aligned out-of-plane toward the counter electrode represents an optimal strategy for utilization of the crystals' anisotropic properties. Detailed analyses of anisotropy of different types lead to a simple, but potentially useful general principle that "P_ec//P_c" textures are optimal for metal anodes, and "P_ec//S_c" textures are optimal for insertion-type electrodes.

Unveiling the Synergistic Effect of Ferroelectric Polarization and Domain Configuration for Reversible Zinc Metal Anodes

The tendency of zinc (Zn) anodes to form uncontrolled Zn electrodeposits and the occurrence of side-reactions at Zn-electrolyte interfaces are a fundamental barrier hampering broad applications of aqueous rechargeable Zn-based batteries. Herein, a ferroelectric domain-mediated strategy is proposed to manipulate the Zn plating behavior and achieve controllable Zn growth orientation by coating Zn foil with a ferroelectric tetragonal KTN (t-KTN) layer. The ferroelectric domain of t-KTN single crystals exhibits periodic distribution of upward and downward polarizations, corresponding to alternating positively and negatively charged surfaces. The charged ferroelectric surfaces can manipulate the transfer kinetics of Zn ions and the concentration distribution of anions via the interplay between ferroelectric dipoles and adsorbed ions. With the synergistic effect of the ferroelectric polarization and domain configurations, the well-aligned interlamellar arrays composed of electrodeposited Zn are formed in the initial deposition process, which enable selective deposition within interlamellar arrays and eliminate the dendrite growth during the following plating process. As a result, the t-KTN layer-modified Zn anode enables reversible Zn plating/stripping with low voltage hysteresis for over 1200 h at 1 mA cm^-2 in symmetric cells, and the assembled full cell exhibits a significantly enhanced cycling stability of over 5500 cycles at 5 A g^-1 .

Textured Electrodes: Manipulating Built-In Crystallographic Heterogeneity of Metal Electrodes via Severe Plastic Deformation

Control of crystallography of metal electrodeposit films has recently emerged as a key to achieving long operating lifetimes in next-generation batteries. It is reported that the large crystallographic heterogeneity, e.g., broad orientational distribution, that appears characteristic of commercial metal foils, results in rough morphology upon plating/stripping. On this basis, an accumulative roll bonding (ARB) methodology-a severe plastic deformation process-is developed. Zn metal is used as a first example to interrogate the concept. It is demonstrated that the ARB process is highly effective in achieving uniform crystallographic control on macroscopic materials. After the ARB process, the Zn grains exhibit a strong (002) texture (i.e., [002]_Zn //ND). The texture transitions from a classical bipolar pattern to a nonclassical unipolar pattern under large nominal strain eliminate the orientational heterogeneity of the foil. The strongly (002)-textured Zn remarkably improves the plating/stripping performance by nearly two orders of magnitude under practical conditions. The performance improvements are readily scaled to achieve pouch-type full batteries that deliver exceptional reversibility. The ARB process can, in principle, be applied to any metal chemistry to achieve similar crystallographic uniformity, provided the appropriate temperature and accumulated strains are employed. This concept is evaluated using commercial Li and Na foils, which, unlike Zn (HCP), are BCC crystals. The simple process for creating strong textures in both hexagonal and cubic metals and illustrating the critical role such built-in crystallography plays underscores opportunities for developing highly reversible thin metal anodes (e.g., hexagonal Zn, Mg, and cubic Li, Na, Ca, Al).

Horizontally arranged zinc platelet electrodeposits modulated by fluorinated covalent organic framework film for high-rate and durable aqueous zinc ion batteries

Rechargeable aqueous zinc-ion batteries (RZIBs) provide a promising complementarity to the existing lithium-ion batteries due to their low cost, non-toxicity and intrinsic safety. However, Zn anodes suffer from zinc dendrite growth and electrolyte corrosion, resulting in poor reversibility. Here, we develop an ultrathin, fluorinated two-dimensional porous covalent organic framework (FCOF) film as a protective layer on the Zn surface. The strong interaction between fluorine (F) in FCOF and Zn reduces the surface energy of the Zn (002) crystal plane, enabling the preferred growth of (002) planes during the electrodeposition process. As a result, Zn deposits show horizontally arranged platelet morphology with (002) orientations preferred. Furthermore, F-containing nanochannels facilitate ion transport and prevent electrolyte penetration for improving corrosion resistance. The FCOF@Zn symmetric cells achieve stability for over 750 h at an ultrahigh current density of 40 mA cm^-2. The high-areal-capacity full cells demonstrate hundreds of cycles under high Zn utilization conditions.

Manipulating Uniform Nucleation to Achieve Dendrite-Free Zn Anodes for Aqueous Zn-Ion Batteries

The essence of Zn dendrite formation is ultimately derived from Zn nucleation and growth during the repeated Zn plating/stripping process. Here, the nucleation process of Zn has been analyzed using ex situ scanning electron microscopy to explore the formation of the initial Zn dendrite, demonstrating that the formation of tiny protrusions (the initial state of Zn dendrites) is caused by the inhomogeneity of Zn nucleation. Based on this, the uniform Zn nucleation is promoted by the Ni₅Zn₂₁ alloy coating (ZnNi) on the surface of Zn foil by electrodeposition, and the mechanism of ZnNi-promoted even nucleation is further analyzed with the assistance of density functional theory (DFT). The DFT results indicate that the ZnNi displays a stronger binding ability to Zn compared to the bare Zn, suggesting that Zn nuclei will preferentially form around ZnNi instead of continuing to grow on the surface of the initial Zn nuclei. Therefore, the designed Zn metal anode (Zn@ZnNi) can be ultra-stable for over 2200 h at a current density of 2 mA cm^-2 in the symmetric cell. Even at a much higher current density of 20 mA cm^-2, the extra-long life of over 2200 cycles (over 530 h) can be achieved. Moreover, the full cell with the Zn@ZnNi anode exhibits extra-long cycling performance for 500 cycles with a capacity of 207.7 mA h g^-1 and 1100 cycles (148.5 mA h g^-1) at a current density of 0.5 and 1 A g^-1, respectively.

Simultaneous Regulation on Solvation Shell and Electrode Interface for Dendrite-Free Zn Ion Batteries Achieved by a Low-Cost Glucose Additive

Dendrite growth and by-products in Zn metal aqueous batteries have impeded their development as promising energy storage devices. We utilize a low-cost additive, glucose, to modulate the typical ZnSO₄ electrolyte system for improving reversible plating/stripping on Zn anode for high-performance Zn ion batteries (ZIBs). Combing experimental characterizations and theoretical calculations, we show that the glucose in ZnSO₄ aqueous environment can simultaneously modulate solvation structure of Zn²⁺ and Zn anode-electrolyte interface. The electrolyte engineering can alternate one H₂ O molecule from the primary Zn²⁺ -6H₂ O solvation shell and restraining side reactions due to the decomposition of active water. Concomitantly, glucose molecules are inclined to absorb on the surface of Zn anode, suppressing the random growth of Zn dendrite. As a proof of concept, a symmetric cell and Zn-MnO₂ full cell with glucose electrolyte achieve boosted stability than that with pure ZnSO₄ electrolyte.