The Road Ahead for Aqueous Lithium‐ion Batteries

Abstract

Aqueous batteries offer enhanced safety due to their nonflammable water-based electrolytes and are more environmentally friendly compared to traditional lithium-ion batteries using organic electrolytes [1]. As compared in Table 1 [2-5], aqueous lithium-ion batteries (ALIBs) offer a balanced performance profile among aqueous battery systems. With an energy density of approximately 100–150 Wh kg−1 and a power density around 500–1000 W kg−1, they surpass aqueous lead–acid batteries, which have lower energy (30–50 Wh kg−1) and power densities (200–300 W kg−1). ALIBs also exhibit higher volumetric energy density (∼250–300 Wh L−1) compared to other aqueous systems. Their self-discharge rate is relatively low at about 1%–2% per month, and they operate at an average voltage of ∼3.0 V, which is higher than that of aqueous sodium-ion (∼1.0–1.5 V), potassium-ion (∼1.0–1.5 V), and zinc-ion batteries (∼1.2–1.8 V). However, achieving competitive energy density and long cycling life comparable to nonaqueous lithium-ion batteries is challenging for ALIBs, which struggle to match the high energy densities of around 200–260 Wh kg−1 and excellent cycle life exceeding 1000 cycles due to material and electrolyte limitations. Unlike organic systems, ALIBs cannot use conventional lithium metal or graphite anodes due to the reactivity and hydrogen evolution reaction (HER) of water. Forming a robust and long-lasting solid-electrolyte interphase (SEI) in aqueous media is inherently challenging but essential for stabilizing the electrode interface. Advanced electrolyte formulations such as water-in-salt or hybrid solvents may compromise cost-effectiveness, limiting scalability. ALIBs also face challenges in current collector corrosion and material compatibility. Traditional current collectors such as aluminum and copper are prone to corrosion in aqueous environments, leading to capacity and power fading. Additionally, the strong reactivity of water limits the selection of suitable anode and cathode materials, as many high-capacity electrodes are prone to water-induced degradation, restricting the performance and longevity of ALIBs. Therefore, overcoming these unique technical barriers—including interface stabilization, material compatibility, and cost-performance trade-offs—is critical to enabling practical deployment of ALIBs. Layered lithium cobalt oxide (LiCoO2), commonly used in organic electrolytes, exhibits a capacity of approximately 120 mAh g−1 in aqueous environments, which is relatively low [6]. Spinel-structured cathodes such as lithium manganese oxide (LiMn2O4, LMO) demonstrate better performance in aqueous electrolytes. For instance, around 29.5 mAh g−1 was reported at 0.5C (approximately 74 mA g−1) in a 2 M Li2SO4 aqueous electrolyte. In more advanced systems, such as ether-in-water hybrid electrolytes, LMO can deliver up to 155.1 mAh g−1 at 3C (444 mA g−1) with excellent cycling stability, highlighting its potential under optimized conditions [7, 8]. Olivine-type lithium iron phosphate (LiFePO4) is considered promising due to its abundance and safety; however, its application in aqueous electrolytes is limited by surface instability [9-11]. These challenges arise because cathode materials effective in organic electrolytes may not perform well in aqueous systems, necessitating the development of suitable cathode materials with redox potentials within the narrow electrolysis window of water to prevent continuous water-splitting reactions. Additionally, fast deintercalation/intercalation is desirable when hydrated ions are introduced into the cathode without the electrolyte desolvation process. Recent efforts have introduced new cathode types, such as Prussian blue and organic compounds, to address these issues. For instance, coating LiCoO2 with a Nafion film substituted with lithium has been conducted to improve structural stability. Moreover, coating olivine LiFePO4 with carbon or carbon nanotubes through a sol-gel method has been explored to enhance electrical conductivity [9-11]. Despite these advancements, further improvements are necessary to optimize the performance of the cathode. The selection of anode materials for ALIBs is limited by the reduction potential of water, which typically occurs before most anode redox reactions [7], as illustrated in Figure 1A. Most anode materials, such as active carbon, Li4Ti5O12 (LTO), etc., have been widely adopted in organic electrolyte-based battery systems due to their excellent electrochemical properties. However, the compatibility of these anodes becomes a prominent challenge when this material system migrates to an aqueous electrolyte environment [2]. The decomposition reaction of water molecules in the conventional operating voltage range of these anodes leads to reduced stability of the battery system. To address this issue, the key to research has turned to the exploration of novel alternative anode materials, such as layered vanadium oxides (e.g., VO2, LiV3O8) that can operate stably at lower operating voltages due to their relatively low redox potentials and do not induce HER. However, their high solubility in aqueous media leads to capacity fading and poor long-term stability. Adjusting electrolytes for these kinds of materials can be very challenging. Therefore, the search for new anode materials with high compatibility with aqueous solutions, such as TiS2 [12], should prioritize materials that exhibit stability in aqueous electrolytes rather than solely considering their redox potential. The development of such electrode materials is important for improving the overall performance and stability of ALIBs. Recent efforts have also focused on tuning the crystallinity, morphology, and surface chemistry of electrode materials to improve compatibility with aqueous environments. Strategies such as nanoscale engineering, composite design, and prelithiation treatments are being actively explored to mitigate structural degradation and enhance reversibility during cycling. (A) Comparison of electrochemical windows with different electrolyte formulations and the working potentials of various cathode and anode active materials; (B) strategies to accommodate the HER and OER of aqueous electrolytes for ALIBs to improve cycling stability. Electrolyte engineering is key to improving ALIBs’ performance by widening the electrochemical stability window (ESW) and enabling SEIs. Although water splitting limits the ESW to 1.23 V, innovations such as concentrated electrolytes, hybrid solvents, and eutectic systems extend this window by modifying Li+ solvation and interfacial chemistry. Techniques such as hydrophobic interfacial layers, in situ SEI (solid electrolyte interphase)/CEI (cathode electrolyte interphase) formation, and surface functionalization further suppress parasitic reactions. Together, these approaches support high-voltage cathodes and low-potential anodes, bringing aqueous batteries closer to the energy density of organic systems with enhanced safety and sustainability. As illustrated in Figure 1A, a range of electrolyte engineering strategies, including the use of high concentrations, hybrid solvents, eutectic solvents, and ionic liquid-water-in-salt (WIS) hybrid solvents, have been demonstrated to markedly expand the ESW of aqueous-based electrolytes [8, 13]. These strategies have the capacity to markedly extend the anodic limit to 5.0 V and diminish the cathodic limit, presenting substantial prospects for the resilience of ALIBs and offering a diverse array of active material options. In hybrid organic–aqueous electrolytes, the choice of lithium salt is crucial for altering the solvation structure, stability, and compatibility. In addition to using LiTFSI and LiFSI, other salts such as LiBr and LiCl facilitate innovative redox chemistry by involving halogens and graphite intercalation, providing high capacities and cost advantages [2]. Nevertheless, challenges such as dissolution, corrosion, and low voltages need to be overcome for practical implementation. Careful salt selection, along with tailored solvents and additives, is essential for cost-effective and high-performance aqueous lithium-ion batteries. In addition to expanding the ESW of the electrolyte, another crucial aspect of electrolyte engineering is the development of stable SEIs [8, 14]. These SEIs are essential for suppressing parasitic reactions such as HER (hydrogen evolution reaction) and OER (oxygen evolution reaction), as well as for enhancing electrode stability. By regulating the presence of electrolyte components such as salt, additives, or solvent at the electrode surface during initial charge cycles, effective SEIs can be formed. A good SEI typically consists of a mixed inorganic–organic layer that is ionically conductive, primarily for Li+ ions, while being electronically insulating. This selective conduction is achieved through uniform ion transport channels, such as LiF-rich regions, which allow Li+ ions to move between defects or through nanochannels, thereby reducing local current hotspots and ensuring a homogeneous Li+ flux. Furthermore, the SEI establishes a physical barrier at the electrode surface, reducing direct contact between the electrolyte and transition metal sites (e.g., Mn in LMO cathodes), which helps limit solvent-induced hydrolysis and subsequent metal dissolution. For instance, introducing tetraethylene glycol dimethyl ether (TEGDME) [15] into a concentrated aqueous electrolyte has been shown to broaden the ESW to 4.2 V (0.6–4.8 V vs. Li/Li+), resulting in a highly stable capacity of 155.1 mAh g−1 at 3C in an LMO|LTO battery. The CEI on LMO comprises compounds such as CH3OCH2CH2OLi, HOCH2CH2OLi, LiOCH2CH2OLi, and RCH2OLi, whereas the SEI on LTO is primarily composed of LiF [8]. These interphases effectively protect the electrodes, enhancing cycling stability and overall battery performance. Additionally, electrolyte additives such as hydrophobic ions can accumulate on electrode surfaces, pushing away water molecules and aiding in the formation of a stable SEI. Although these additives are still being optimized, they show significant potential to further enhance the stability and overall performance of aqueous lithium-ion batteries. In addition to SEI/CEI formation via electrolyte design, surface modification of electrodes using protective coatings (e.g., polymeric layers, LiF-rich layers, or self-assembled monolayers) has proven effective in reducing metal dissolution and improving cycling life. These advances highlight the synergistic role of interface and electrolyte co-design in pushing the practical limits of ALIBs. Overall, matching the redox potentials of electrodes within the ESW, ensuring stability, and preventing side reactions such as HER and OER are key challenges. Future research should focus on designing electrolytes and electrodes to reduce gas evolution and material dissolution. Future development of ALIBs should prioritize key areas (as shown in Figure 1B) to enhance competitiveness with other battery technologies. First, innovation in electrode materials is crucial—identifying stable materials in aqueous environments with high capacity can improve performance. Electrode engineering is crucial for designing cathode or anode materials. It involves finding an anode material with a higher redox potential than the electrolyte's HER potential. Second, electrolyte research remains essential. Advances in interface engineering and ESW-expanding electrolytes have shown significant promise in unlocking higher voltage windows, which is key to narrowing the gap with organic systems. For instance, hybrid electrolytes that combine aqueous solutions with ionic liquids show promise. Third, optimizing interfaces is key—designing stable solid-electrolyte interphases can protect electrodes and enhance cycle life. Lastly, scalability and cost reduction are crucial considerations in ALIBs development; utilizing abundant material resources, such as salts and solvents, can make ALIBs technology more economically viable. Addressing these areas can position ALIBs as a competitive and sustainable energy storage solution. Ze-Ru Wang: resources, writing – original draft. Yi-Lin Zhang: investigation, resources, writing – original draft. Rui-Zhi Zhang: conceptualization, writing – review and editing. Wang-Wu Li: writing – review and editing. Jun Song: writing – review and editing, project administration, writing – original draft. Sai-Lin Liu: project administration, resources, supervision, writing – original draft, writing – review and editing, conceptualization, investigation, funding acquisition. This research was supported by the Australian Research Council (IE240100186, IH20010003). The authors declare no conflicts of interest. The data that support the findings of this study are available upon request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.

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