3.1 To beat LIBs and LABs

As highlighted in Table 1, ZIBs demonstrate several advantages over both LIBs and LABs in terms of grid scale energy storage. The demand for cost-efficiency and enhanced safety in large-scale grid energy storage is driving the development of ZIBs, which hold promise of reducing costs to under $50 per kWh, putting them in direct competition with LIBs. Additionally, ZIBs exhibit a substantially higher energy density (eg. achieving 150 Wh kg⁻¹ in Zn-MnO₂ battery) than the commercial LABs (50 Wh kg⁻¹), positioning ZIBs as a competitive choice for grid-scale applications.

3.2 Technique gaps towards the goal

Previous efforts have focused on tackling issues associated with anode dendrites in ZIBs, however, many approaches use coin-cell configurations, flooded electrolytes dosage, and thick zinc foils. Coin cells are widely recognized in fundamental tests and assessing the electrochemical behaviours of materials, including voltage profiles, specific capacities, and electrochemical stability during initial exploration stages. However, their substantial diversity in experimental conditions poses a considerable challenge when attempting to compare and benchmark materials and concepts. Materials and concepts developed on the laboratory scale are typically studied under a range of experimental conditions that often differ significantly from the requirements of a large battery. Alternatively, pouch cells allow for scalability and accommodates diverse electrode loadings and geometries, enhancing represent ativeness for practical applications. Figure 1c and 1d illustrate the impact of cell configuration differences on energy densities, which can be significantly influenced across factors such as electrolyte dosage, cathode loading mass, and N/P ratios, and Zn thickness (areal capacity).¹⁹ Coin cells have limitations in low loading mass (<3 mg cm⁻²), flooded electrolyte (>80 μL mAh⁻¹), and high N/P ratios (>30 : 1), which may not accurately represent practical electrode conditions.²⁰ As a result, achieving a practical energy density of over 50 Wh kg⁻¹ in a coin cell is challenging due to a significant portion of inactive materials, particularly the weight of the packaging (>99 %). In contrast, it is possible to regulate specific parameters in Zn-MnO₂ pouch cells to reach the upper limit of energy density of 150 Wh kg⁻¹, taking into account the total weight of the cell. To meet the desired criteria, it is crucial to control the package weight within 10 % of the total cell weight. Additionally, a high loading mass of cathode active material, such as 16 mg cm⁻² for MnO₂ cathode is required, along with an extreme N/P ratio of 2 : 1 under a lean electrolyte condition of 10 μL mAh⁻¹.²¹ Notably, despite the incorporation of those specific design parameters, there is a lack of comprehensive quantitative analysis regarding how these parameters collectively influence cell-level energy density.²²

4.1 Formulating of practical energy density

4.2 Practical high active materials weight ratio R_weight

For illustrative purposes and to simplify calculations, we use commonly applied parameters found in real pouch cells.⁶ Specifically, the package weight ratio, denoted as $$ , is set to be 10 wt %, referring to the accepted weight ratio in lithium-ion pouch cells. The thickness of the Ti foil is specified as 10 μm, with an areal density of 4.5 mg cm⁻² (equivalent to 4.5 g cm⁻³). The areal density of the separator is taken from lithium-ion separator (1 mg cm⁻²). The anode is a highly conductive Zn metal foil. The density ($$ ) of the electrolyte is established as 1.138 g mL⁻¹, determined from an aqueous 1 M ZnSO₄. The parameter “$$ ” is set at 0.5 based on a one-electron reaction in the MnO₂ cathode. The $$ is calculated using a 1 : 1 weight ratio of 1 M ZnSO₄ electrolyte to MnO₂ cathode material. According to these parameters, we illustrate the correlation derived from the equation (3). As demonstrated in Figure 2a, we explore the relationship between $$ and $$ concerning $$ while keeping the remaining key variable, $$ , fixed at an extreme low value of 1 μl mg⁻¹, as refereed from LIBs. It is evidenced that modifying two key parameters, $$ and $$ , has a significant impact on $$ . A notable enhancement in $$ is observed by decreasing $$ from 10 : 1 to 1 : 1, resulting in an improvement from 10 % to 21.9 % under a maximum ml of 16 mg cm⁻². In contrast, under a constant low $$ of 1 : 1, increasing the loading mass ($$ ) from 2 to 16 mg cm⁻² leads to an increment in $$ from 14.6 % to 21.9 %.

The correlation between electrolyte amount $$ and $$ is shown in Figure 2b. While maintaining a low $$ of 1 : 1, conversely, decreasing $$ yields a more effective enhancement of $$ . While keeping $$ constant at 0.1 μL mg⁻¹, increasing m_l from 2 to 16 mg cm⁻² almost double the $$ , increasing from 17.1 % to 28.4 %. These demonstrations suggest that decreasing $$ or $$ can enhance $$ compared to increasing mass loading $$ . However, it is worth noting that achieving a lean electrolyte condition below 1 μl mg⁻¹ (equals to 0.31 uL mAh⁻¹ based on the aqueous 1M ZnSO₄ electrolyte and MnO₂ cathode) is currently impractical with the existing battery technique. As a result, it remains challenging to attain an $$ beyond 21.9 % for the MnO₂||Zn full battery in real-world conditions.

Equation (2) and Figure 2c demonstrate that increasing both the specific capacity of cathode ($$ ) and real voltage output ($$ ) improves energy utilization ($$ ). For example, increasing $$ from 50 to 308 mAh g⁻¹ (assuming $$ =1.2 V) dramatically enhances the $$ from 16 % to 100 %. Therefore, it is necessary to establish battery testing conditions that approaches the full theoretical capacity of the cathode to reach high $$ . Based on the above discussion by controlling N/P and E/P ratios within a low range, it is evident that the 1e⁻ redox in MnO₂ to form Zn_0.5MnO₂ has reached the limit of 21.9 %, hindering its $$ . To overcome this limitation, increasing $$ to accommodate more Zn²⁺ can enhance the $$ to 40 %, as displayed in Figure 2d.

4.3 Prediction of practical energy density $$

The relationship between the final battery energy density, $$ and $$ is shown in Figure 3a, as depicted by equation (1). Achieving a high $$ of 100 % enables the $$ to approach 150 Wh kg⁻¹ when $$ exceeds 40 %. However, according to calculations in Figure 2a and 2b, our current reports on energy density without controlling battery package weight typically fall below 70 Wh kg⁻¹, with a low $$ less than 43.8 %. As a result, based on these findings, the MnO₂ faces challenges in reaching 150 Wh kg⁻¹. Under optimal conditions with a maximum $$ of 43.8 % (based on a high loading mass of 16 mg cm⁻², low $$ =1 : 1 and $$ =1 μL mg⁻¹) and $$ at 100 %, the maximum $$ can only reach 161.9 Wh kg⁻¹. To address the challenge of low energy density, it is imperative to explore alternative cathode candidates.

Figure 3b compares the theoretical energy density of several representative cathode materials and the maximum $$ . The latter considers only the weight of active cathode and anode materials, along with the electrolyte dosage, using low $$ of 1 : 1 and $$ of 1 μL mg⁻¹. V₂O₅, I₂ (based on 4e⁻ redox) and sulfur show great potential to exhibit high energy density, benefiting from their relatively high theoretical energy density. To identify the existing gap in achieving a high practical energy density, we analyze additional battery components, including package, current collector, and separator. We calculate the minimum $$ required to meet a high cell-level energy density of 150 Wh kg⁻¹, as displayed in Figure 3c. Figure 3c also presents practical $$ values across a broad cathode loading mass range (from 2 mg cm⁻² to 16 mg cm⁻²) under $$ 1 : 1 and $$ 1 μL mg⁻¹ conditions. The analysis indicates that minimum $$ required for MnO₂, V₂O₅, I₂, sulfur is possible, as these values are fall within or below the practical $$ range. It is noted that I₂ (4e⁻ redox) and sulfur hold great advantages in requiring low $$ due to their high theoretical capacity.

Although reaching high $$ is possible in ideal battery manufacturing, achieving the high theoretical capacity is difficult due to irreversible capacity loss in practical testing conditions. Factors such as temperature variations, material impurities, and structural flaws impede energy storage efficiency of the material. In this case, the minimum energy density required for the cathode ($$ ) in research is calculated based on optimal battery parameters (Figure 3d). To reach a high $$ , such as 150 Wh kg⁻¹, all listed cathode materials must deliver an energy density ($$ ) of over 300 Wh kg⁻¹, except for sulfur, which requires a lower threshold of 246.3 Wh kg⁻¹. Prussian blue (PB), polyaniline (PANI) and I₂ (2e⁻ redox) all fail short of meeting this goal for the cathode materials due to their low theoretical capacity. MnO₂ and V₂O₅ are expected to achieve 342.6 and 344.7 Wh kg⁻¹, respectively. These formulas can help establish meaningful benchmarks for laboratory research and guide the development of cathode materials in the future. With these validated equations, recommended battery parameters and promising cathode materials can be proposed for designing high-energy-density ZIBs batteries.

4.4 Coulombic efficiency goals towards long calendar life

As discussed, ZIBs show promising energy densities that can compete with LABs when coupled with high-capacity cathodes. To maintain 80–90% capacity retention after 1,000 cycles, the battery needs to have a high CE of over 99.98%, ideally exceeding 99.99%. Achieving this can be challenging due to capacity loss from both the cathode and anode. To accurately assess Zn reversibility, CE measurement should be conducted using a non-excess Zn working electrode (such as Cu or Ti foils) with Zn foil serving as a reservoir.²³ Importantly, in Zn||Zn cells, using capacity/time control results in a constant CE of 100 %, which lacks practical significance. Asymmetric cells (e.g. Zn||Cu) offer a more direct approach to assess and compare Zn metal CE.

However, achieving these CE objectives for both Zn anode and cathode material in practical batteries is challenging, requiring consideration of irreversible capacity losses on both electrodes and electrolyte. Typically, the evaluation of cathode CE takes priority over the full battery, and the anode-free batteries using a fixed capacity of the anode (Figure 4c), emerges as an ideal approach for assessing the true CE of active cathode materials.

5.1 Cathode materials development

As formulated in Equation (1), (2) and (3), the progression of ZIBs towards a high energy density currently faces dilemmas centered around identifying cathode materials that satisfy several key criteria: 1) High discharge capacity; 2) Elevated working voltage; 3) Structural and crystallographic traits conducive to accommodating Zn²⁺ ions and enhancing typically slow diffusion kinetics; 4) Robust crystal structures capable of withstanding extensive cycling; 5) Economic feasibility and environmental consciousness. Unfortunately, existing materials do not fully align with these specifications, raising concerns about achieving the goals in energy density and cycling life. Cathode materials based on Mn and V chemistry play a key role in the development history of ZIBs and can meet the minimum $$ to achieve 150 Wh kg⁻¹ owing to their high theoretical energy densities (Figure 3c). Although many promising cathode materials, such as MnO₂, V₂O₅, and I₂ have been extensively studied, their performance with zinc anodes is not always satisfactory due to dissolution and side reactions in aqueous electrolytes. In addition, improving electrode preparation techniques to increase the loading mass of active material or reduce side reactions with aqueous electrolytes can help enhance the energy density.^2b

Specifically, significant dissolution of Mn²⁺ was observed in Zn||MnO₂ batteries due to the Jahn–Teller effect in MnO₂, adversely impacting reversibility and cycling performance. Various solutions have been proposed, including pre-adding Mn²⁺ salts, controlling pH values, engineering solvation structure of electrolytes, and developing an artificial solid electrolyte interphase (SEI) layer on electrode surface.^{2c, 25} Nevertheless, these approaches pose additional costs for electrolytes and Mn-based cathodes, raising concerns about the cost of practical applications. Vanadium based cathodes also face structural degradation and active materials dissolution.²⁶ Progress has been made in enhancing stability at high currents exceeding 1 A g⁻¹ via materials structure design. This includes various modification strategies, such as using electrolyte modification (including additives, high concentration systems, hydrogels, etc.²⁷), introducing guest species between cathode material layers for a “pillar effect”,²⁸ including oxygen vacancies (eg. NH₄V₄O₁₀), or modifying natrium super ionic conductor type cathode (eg. Na₃V₂(PO₄)₃).²⁹ While NH₄V₄O₁₀ and Na₃V₂(PO₄)₃) offer great advantages in terms of high voltage operation and ionic conductivity, their widespread deployment faces challenges in scalability and cost-effective production. Simultaneously meeting all requirements is difficult. Practical approaches should include electrolyte design and exploration of new cathode materials and storage mechanisms.³⁰

Besides Zn²⁺ intercalation or conversion, higher capacity can be achieved by utilizing H⁺ storage, or H⁺/Zn²⁺ co-storage.³¹ However, challenges remain in utilizing proton storage due to their influence on irreversible cathode dissolution under small current rates.^2b Therefore, feasible host materials, such as imine compounds³¹ and S-MoO₂³² that can carry out a simultaneous H⁺ and Zn²⁺ insertion/extraction process with enhanced performance should be considered and developed. In addition to proton storage, the optimal choices for achieving high-capacity cathode materials are narrowed down to sulfur and I₂ (with 4e^- redox). However, unlike the intercalation/deintercalation of zinc ions in layered cathode materials, these conversion cathodes present a unique challenge. The solid-to-liquid transition in sulfur and I₂ cathodes results in the shuttling effect of polysulfides and polyiodides, adding complexity to their performance enhancement. Recent reports have partially mitigated the shuttling effect in some cases by anchoring polysulfides or polyiodides through electrolyte design. However, numerous challenges persist for practical applications. For instance, the cycle stability of Zn−S batteries currently poses a bottleneck, and increasing the loading of I₂ remains a challenge.

From the cathode material perspective, the path to increasing capacities seems to lean towards leveraging multi-electron redox activity and integrating additional redox ion pairs. As a result, their achievable $$ appear exceedingly limited compared to the sulfur and I₂. We believe that for materials with limited theoretical energy densities or those that are already approaching theoretical values, it is important to prioritize understanding their energy storage mechanisms and enhancing cycling stability.

5.2 Zn anode reversibility measurement

Measuring Zn reversibility significantly benefits from the use of asymmetric cells. However, zinc CE measurements are influenced by various variables, resulting in varied values reported in the literature, even for identical cell configurations. It is important to note that crucial factors, including current density, electrolyte volume, and Zn thickness, exert a significant influence on CE values. Given side reactions on metallic Zn side in aqueous electrolytes, we recommend initiating testing at lower current densities, such as 0.5 or 1 mA cm⁻², with a capacity of 1 mAh cm⁻². Increasing the applied current density can lead to higher overpotentials, particularly in unmodified bare cells. As a reuslt, careful consideration of Zn electrode parameters, including accurate thickness, surface polishing, and potential trade-offs, is essential. The use of excessively thick Zn foil and a flooded electrolyte is not recommended.³³ In order to get an optimized performance, we suggest using a 50-μm-thick Zn foil with a 50 μL electrolyte, which adequately infiltrates the entire cells. Assuming an electrolyte density is 1.0 g cm⁻³ and an electrode area is 1.27 cm² (diameter is 12.7 mm). As an example, a Zn (50 μm)//V₂O₅ (4 mAh cm⁻²) cell can be assembled to validate new ideas.

In addition to using the asymmetric cell, we suggest an alternative approach to practically assess Zn reversibility, known as the “reservoir” method³⁴ in LIBs, to practically assess the average CE, as illustrated in Figure 4c. Specifically, Zn reservoir (Q_T) is plating on the electrode, while the battery experiences continuous cycles of stripping/plating at a consistent capacity (Q_C) below the initial Zn reservoir (Q_C<Q_T). As the cycling proceeds, side reactions progressively consume the Zn reservoir. After cycling, the Zn reservoir becomes entirely depleted, evident by the abrupt surge in the stripping potential.

5.3 Realistic evaluation process

To ensure their reliability, efficiency, and ability to address renewable energy intermittency in a rapidly evolving energy landscape, it is urgently demanded for building a scientific evaluating process for ZIBs. This evaluation should include several key aspects, including energy density, calendar life, and practical assessment, each playing a crucial role in determining the suitability of batteries for grid-scale applications, as displayed in Figure 5.

The practical energy density of a battery is directly influenced by the physical size and weight of the battery package. The detailed distribution of cell weight can impact the overall energy density. Optimizing weight distribution is essential to maximize energy storage while maintaining structural integrity, a process that can be standardized with a calculation tool (Equation (3)). Apart from reporting a single energy density at a specific cycle number, capacity retention provides direct information into the capacity decay rate of the battery over cycling. Ensuing high-capacity retention of over 80% after thousands of charge–discharge cycles and achieving a low self-discharge rate are imperative for grid-scale applications. However, minimizing self-discharges in aqueous electrolytes poses a great challenge, and future advancements depend heavily on an in-depth study of underlying mechanisms.

For grid-scale storage, evaluating the calendar life of batteries is crucial, but it can be time-consuming. As a result, using a reasonable diagnosis formula (Equation (5) and (6)) to predict cycling life based on the average CE under specific current densities can expedite the experimental process. Effective management of the state of charge (SOC) and the depth of discharge (DOD) within reasonable values during charging/discharging is essential for achieving long calendar life. Avoiding prolonged and high SOC helps prevent degradation and increases the overall lifespan of the battery. Additionally, grid-scale storage systems must accomodate the intermittent nature of renewable resources, typically lasting 4–6 hours. Therefore, adopting small current densities of C/4, C/6, or even C/24 is necessary. However, current research is using much higher rates over 5C³⁵ to avoid side reactions of active cathode materials with electrolytes. Realistically, achieving over 3,000 cycles at C/24, more than 14,000 cycles at C/6, and in excess of 20,000 cycles at C/4 is essential for a potential industrial battery system, which is still challenging forr current ZIBs chemistry.

Practical assessment considers various factors influencing real performance of batteries, including temperature adaptability. Evaluating capacity decay under extreme temperatures is important, as it significantly impacts the practical output ($$ and $$ ) in real environments. As a result, calculating $$ under different temperatures, alongside reporting specific capacity, provides a clear understanding of performance under extreme conditions compared to room temperature. Pouch cells, offering flexibility and scalability, are a preferred choice for grid-scale applications. Parameters, including package weight and electrolyte/cathode/anode ratios, can be optimized using the discussed equations. Assessing battery performance in full pouch configurations, representative of actual installations, ensures results align with practical implementation. In addition to performances, future designs should also minimize the risk of thermal runaway, fires, or explosions, ensuring system and environmental safety with the use of low-risk battery components. Furthermore, cost-effectiveness is critical for grid-scale storage, impacting the feasibility and economics of large-scale deployments. Evaluating the primary production cost of all battery materials, especially electrolytes that occupy most weight of the battery, is essential for overall project viability. For sustainability, assessing the feasibility of battery recycling is crucial, promoting a circular economy by designing materials with recycling in mind to reduce environmental impact.

5.4 Summary

This Minireview emphasizes the significance of considering two factors, denoted as $$ and $$ , beyond normal cathode specific capacity, to bridge the energy density gap between battery research and industrial demand. It highlights the correction of cycling life with N/P ratios to expedite the research-industry transition. Specifically, we offer a comprehensive set of formulas that consider various influencing factors impacting energy density and the weight of active materials. These formulas serve as valuable scientific tools for precise calculations. By quantifying the impact of each factor on energy density, especially considering factors like N/P ratio, E/P ratio, electron transfer number, and energy utilizations, this study provides critical insights into how to enhance energy density via cathode optimization. In addition to calculating practical energy densities, we will also emphasize diagnostics to expedite the calendar life of Zn anodes using the CE formulas under different N/P ratios, assuming 100% CE in the cathode. Furthermore, more attention should be given to the degradation of cathodes caused by various dissolution mechanisms under aqueous solutions to make a more accurate prediction.

The study provides essential guidelines for academic research, particularly in the construction and evaluation of full ZIBs. It emphasizes the improvement of techniques, with a specific focus on controlling electrolyte weight and optimizing electrode configurations. This Perspective opens avenues for future exploration in potential cathode materials and highlights existing limitations in traditional cathodes achieving the high practical energy density. It proposes exploring high energy density alternatives, such as I₂ and sulfur, paving the way for advancements in energy storage technology. Through a holistic approach, this formula-intensive study provides us clear pathways toward the valuable research endeavour aimed at boosting practical integration of ZIBs into grid-scale energy storage.