Benchmarking Anode Concepts: The Future of Electrically

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Benchmarking Anode Concepts: The Future of Electrically Rechargeable Zinc−Air Batteries Daniel Stock,†,‡ Saustin Dongmo,†,‡ Jürgen Janek,†,‡ and Daniel Schröder*,†,‡ †

Institute of Physical Chemistry, Justus Liebig University Giessen, Heinrich-Buff-Ring 17, D-35392 Giessen, Germany Center for Materials Research (LaMa), Justus Liebig University Giessen, Heinrich-Buff-Ring 16, D-35392 Giessen, Germany

‡

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S Supporting Information *

ABSTRACT: A variety of batteries employing an alkaline zinc anode have been investigated and partially commercialized over the last decades. Of these, electrically rechargeable zinc−air batteries have been considered, since the mid 20th century, as a sustainable alternative for future green energy storage. Despite significant research efforts, it has so far not been possible to commercialize this battery on a large scale because of insufficient performance. The herein presented overview is not yet another review; on the basis of a total of 70 articles published during the last 20 years in lab-scale research, we assess the state-of-the-art performance of alkaline zinc anodes for application in zinc−air batteries. We define descriptors for the underlying analysis focusing on the practical relevance and reveal that the expectations for this battery type are unfortunately too high. Most importantly, the ultimate long-lasting alkaline zinc anode has yet to be identified; this is a challenging, but appealing task for interdisciplinary research. Wh L−1cell)12 or several times more than Zn/NiOOH batteries (135 Wh kg−1cell and 300 Wh L−1cell).13 Electrically rechargeable ZABs are, however, not yet realized in practice despite 50 years of intensive research because of two major degradation phenomena: (i) the fast degradation of the cathode materials such as catalyst, binder, or carbon additive and (ii) the morphological changes of the Zn anode during repeated cycling, leading to an irreversible deformation of the entire anode. Despite the absence of a commercial breakthrough, electrically rechargeable ZABs continue to be listed as a promising candidate in the battery market, owing to their advantages in terms of environmental friendliness, abundance of the constituent elements, and safe operation.14,15 Especially Zn is highlighted as a high-capacity, sustainable, cost-efficient, and recyclable material.5,16,17 Recently, Parker and co-workers pointed to a general mistranslation in the projection of Zn-based battery performance tested on lab-scale research in a half-cell configuration to future practical performance.18 Therein, the authors introduced guidelines on how to evaluate new electrode formulations and concepts in such a way that the results establish a realistic basis for future research. Interestingly, the same gap between lab-scale research and practical applications exists for other next-generation battery technologies as well, such as lithium−oxygen (Li−O2)

T

he concept of the zinc−air battery (ZAB) was developed in the late 19th century.1 The ZAB was patented for the first time in 1878, appearing as a primary battery.2 Since the early 1960s, there have been multiple attempts to develop electrically rechargeable ZABs as promising energy storage devices for electric vehicles and portable applications.1,3−5 In particular, further development of the cathode with the use of novel catalysts made it possible to improve its stability and cycling performance.1,6 Over the last decades, various primary and secondary alkaline batteries using a Zn anode have been investigated and partially commercialized, such as Zn/NiOOH, Zn/MnO2, Zn/Ag2O, and Zn/O2 batteries.3,7−10 The detailed historical milestones during the development of Zn-based batteries can be found in a review article published by Harting and co-workers in 2012.3 Of those Zn-based batteries, ZABs are half-open systems in the sense that one electrode, the cathode, utilizes oxygen (O2) as the active material (AM) at the cathode side from the surrounding gaseous atmosphere.5 Given that only Zn is stored as the AM in the battery, electrically rechargeable ZABs promise high gravimetric and volumetric energy densities. According to the latest reported estimates of what can be practically achieved, ZABs may achieve gravimetric and volumetric energy densities of up to 500 Wh kg−1cell and 1400 Wh L−1cell (based on the practical attainable values of commercially available primary ZABs in coin cell format),6,8,11 which is by good approximation twice as much as today’s commercialized Li-ion batteries (350 Wh kg−1cell and 810 © 2019 American Chemical Society

Received: March 7, 2019 Accepted: May 3, 2019 Published: May 3, 2019 1287

DOI: 10.1021/acsenergylett.9b00510 ACS Energy Lett. 2019, 4, 1287−1300

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Cite This: ACS Energy Lett. 2019, 4, 1287−1300

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or lithium−sulfur (Li−S) systems.19 It is apparent that the demand for comparable assessment of reported battery performance is becoming more important in the whole field of battery research.20 From this point of view, the question arises whether ZABs and alkaline Zn anodes can meet all present-day expectations. What is the state-of-the-art performance of today’s ZABs, and in which direction should future research be directed? In this Review, we analyze the performance of alkaline Zn anodes for application in electrically rechargeable ZABs based on seven descriptors that we chose to project practical benchmark criteria. In selecting suitable criteria, the recently proposed guidelines by Parker et al. were taken into account.18 For this purpose, 70 studies reported in the last 20 years were evaluated. We reveal that the majority of analyzed cycling data typically yields high values for a maximum of only two of the seven selected criteria. Only a small number of studies provide the data necessary to assess the performance metrics that are represented by the seven descriptors. Comparing the mean

The aforementioned descriptors were selected based on the following criteria: For practical operation, the cycled capacity of the anode should be related to the total mass of the anode, which also contains additives and the current collector in contact with the AM. Unfortunately, because of a lack of information, it was not possible to include the mass of the current collectors used. Hence, all defined descriptors are generally chosen to refer to the anode and its mass, excluding the other components of the cell. Thus, on the basis of the anode and the electrolyte mass, the performance of reported batteries, e.g., the gravimetric energy density wgrav, can be estimated and classifieddespite the missing data. Because ZABs are generally considered as high-energy yet low-power batteries, the energy density as the main contribution to some descriptors and not the power density was chosen. The descriptors are subdivided into three categories representing the properties of the reported cell, utilization of the AM, and retention of the respective processes. The descriptors that describe the cell properties include mAM, the mass ratio of the AM such as Zn or ZnO, and manode, the anode mixture including additives such as the binder or conductive agents. It is important to note that both values change from the charged to the discharged state and that anodes reported in the literature are partially prepared in the charged and discharged state. In the following, the mass always refers to the anode’s initial state before electrochemical treatment. In addition, the ratio of QAM, the capacity of the AM, and VE, the volume of electrolyte that is added to the cell, is evaluated. This ratio is of particular importance to judge whether a reported cell is suitable for practical application with high wgrav. Recently, Parker et al. pointed out the importance of VE and that the performance of the Zn anode strongly depends on VE and the test cell geometry.18 The influence of VE on the cycling performance is also known from other battery types, such as the Li−S battery system, where a high VE can mask the decomposition of the electrolyte at the anode during cycling.21,22 In recent years, however, Li−S research has begun to prioritize the electrolyte content as a key parameter, thereby enabling significant improvements within this battery system.19 ZABs with an excess of electrolyte also mask the problem of ongoing side reactions, such as electrolyte decomposition with gas evolution at both electrodes, and possess a low energy density.7,23,24 Furthermore, it does not seem useful to simply minimize VE and QAM in the same step because the same problems persist in a smaller overall volume. Thus, only the ratio between the two is a useful descriptor to assess the practical relevance of a reported cell geometry. The descriptors in the category of retention are NC, the achieved number of cycles, and Φ̅ Q, the averaged Coulombic efficiency, which describe the reversibility of the electrochemical processes. The last category of descriptors contains X̅ AM, the averaged utilization of the AM, and q̅dis, the averaged discharge capacity per mass of anode mixture. Of these two descriptors, the latter is more relevant in practical terms, because the mass of inactive additives is taken into account.18 To combine retention and utilization into one descriptor, NC· q̅dis was defined, which is q̅dis multiplied by NC to evaluate the total capacity that can be gained during the cycle life of an anode. Thus, high NC·q̅dis values can be obtained from a cell showing 10 000 cycles at a X̅ AM value of 1%, but also with a cell showing 100 cycles at 100% X̅ AM. Regarding the energy efficiency, values less than 60% are usually achieved for

Only a small number of studies provide the data necessary to assess the performance metrics. performance of Zn anode concepts applied in electrically rechargeable ZABs with those tested in various alkaline Zn batteries, indicates that promising and already reported Zn anode concepts have mostly not yet been tested in electrically rechargeable ZABs. Instead, simple Zn foils are excessively used as the anode. Thus, future ZAB research should be more directed toward incorporating the best performing Zn anodes to enable long-term stable and efficient battery cycling. Finally, our study can serve as a guide on how to report the relevant data for adequately assessing ZAB performance. Performance Metrics. To evaluate the state-of-the-art performance of alkaline Zn anodes, seven performance descriptors were defined (Table 1). These descriptors are selected on the basis of practical relevance in view of an optimized alkaline full cell battery as well as the availability of information in published articles in the area of research (no patents included). Each of the seven descriptors is selected to address a specific requirement for an alkaline Zn anode (see the Supporting Information). Table 1. Relevant Descriptors for the Evaluation of the State-of-the-Art Performance of Zn Anodes for Application in Alkaline Batteries, e.g., Electrically Rechargeable Zinc− Air Batteriesa category

abbreviation

definition

cell properties

mAM/manode

mass ratio of active material (AM) and anode mixture (AM and additives) ratio of capacity of AM (QAM) and volume of electrolyte (VE) number of cycles averaged Coulombic efficiency averaged utilization of AM averaged discharge capacity per mass of anode mixture product of averaged discharge capacity per mass of anode mixture and number of cycles

QAM/VE retention utilization

combined

NC Φ̅ Q X̅ AM q̅dis NC·q̅dis

a

The descriptors are subdivided into four categories. 1288


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Figure 1. Graphic illustration of the number of publications from which the information was accessible to evaluate the respective descriptor for the performance of the Zn anode. The analysis is grouped into (a) different concepts of Zn anodes applied in alkaline batteries (38 studies) and (b) electrically rechargeable Zn−air batteries using a Zn anode and bifunctional catalysts (32 studies).

of the necessary information for determining the aforementioned descriptors into account, termed publications including all information. In the first group only 9 out of 38 and in the second group 4 out of 32 published studies include all information desired. These studies are highlighted in gray in Tables 2 and 3. On the other hand, the state-of-the-art performance is estimated by taking into account all publications from which it was possible to determine a respective descriptor, termed all publications. The distribution of all descriptors and their corresponding standard deviations are illustrated as boxplots for the performance of Zn anodes applied in an alkaline battery in Figure 2a and in electrically rechargeable ZAB in Figure 2b (see Tables S1 and S2 for a detailed list of the calculated descriptor values). Boxplots are used to show the distribution of the descriptors and to show the difference between the median value (gray line in the boxplot box) and the arithmetic mean (rhombus symbol in purple). A short description of the method used to obtain boxplots and the calculation of the parameters is given in the Supporting Information of this work (Figure S1). The state-of-the-art performance of Zn anode concepts tested in alkaline batteries is depicted in Figure 2a, considering solely publications that include all information. The state-ofthe-art Zn anode can be cycled between 20 and 150 times (first and third boxplot quartile) with an X̅ AM of 26% and Φ̅ Q of 75% (median values). This corresponds to a median q̅dis value of 130 mAh g−1anode, which is only around 16% of the theoretical specific capacity of bulk Zn (819 mAh g−1).5 By contrast, the state-of-the-art Zn anode, considering all publications, shows an increased NC range with first and third boxplot quartile between 1 and 550. The X̅ AM also increases by 16 percentage points to 42% with a Φ̅ Q of 77% (median values). To give a rough estimate of wgrav for both state-of-the-art Zn anodes in a possible application in a ZAB, we multiplied the q̅dis values by Eth, which yields 217 Wh kg−1anode and 322 Wh kg−1anode. The state-of-the-art performance of Zn anodes applied exclusively in electrically rechargeable ZABs is shown in Figure 2b. Taking into account only those studies that include all information, the state-of-the-art Zn anode in a ZAB can be cycled between 10 and 55 cycles (first and third boxplot quartile) with a median value of 21% X̅ AM and Φ̅ Q of around 100%. This results in a median w̅ grav of 203 Wh kg−1anode. Considering all publications, the median value of X̅ AM decreases by approximately 16% and NC increases up to 250 cycles for

electrically rechargeable ZABs because of the sluggish kinetics of the O2 electrochemistry at the gas diffusion electrode (GDE). Because the energy efficiency depends on the cathode rather than on the anode, it was not included as a descriptor in the metrics. At this point it should be mentioned that the selected descriptors are not to be applied exclusively to ZABs and alkaline Zn anodes. They can be transferred more generally to a number of other battery systems with liquid electrolytes. This applies in particular to further metal−air battery systems with aqueous electrolytes, e.g., magnesium−air, iron−air, or aluminum−air batteries. They all basically share the same working principles and performance-limiting phenomena but differ only in the choice of the anode material.6 Thus, the transfer of our selected performance metrics would allow for a direct comparison between the state-of-the-art performances of different types of alkaline metal anodes, which would be beneficial for the community. As shown in Figures 1a,b, our literature survey is subdivided into two groups, which refer to the proportion of reported information that is required to determine the descriptors. In the first group, various concepts for electrically rechargeable Zn anodes are evaluated based on the introduced descriptors. This group refers exclusively to studies that presented novel concepts and compositions of Zn anodes for possible application in alkaline batteries, including also electrically rechargeable ZABs. The second group focuses exclusively on research studies on ZABs employing Zn anodes. The aim of this second group is to exclusively evaluate the state-of-the-art performance of already reported ZABs. In this group, q̅dis and NC·q̅dis are multiplied by the theoretical cell voltage of an alkaline ZAB of Eth = 1.667 V (at standard conditions) to estimate the state-of-the-art gravimetric energy density w̅ grav and NC·w̅ grav. Instead of the actual cell voltage during charge or discharge, Eth was used,8 as some studies did not report values for the cell voltage during cycling. Our estimation reflects that in the ideal case of zero overpotential at the cathode, with negligible overpotential at the Zn anode, the herein calculated values for w̅ grav are maximum values that can be expected but presumably will never be reached in a ZAB.1,25,26 State-of-the-Art Performance of Alkaline Zn Anodes. In order to evaluate the state-of-the-art performance for both groups, on the one hand we take only those publications that provide all 1289


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Table 2. Basic Cell Properties of Published Articles Focusing on Different Concepts of Alkaline Zn Anodesa

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Table 2. continued

a Studies highlighted in gray contain sufficient data to estimate all of the descriptors in Table 1. Based on the reported cell geometry, the studies are divided into different cell types: Cells with a large volume of added electrolyte−compared to relatively small electrode areas are referred to as “beaker cells”. By contrast, “stack-type” cells are composed of vertically aligned planar plates (mostly quadratic) with an electrolyte compartment between. “Coin-type” cells use a cylindrical geometry and a steel container with a relatively small volume of electrolyte. *manode does not contain the mass of the current collector because of missing information; **electrolyte is saturated with ZnO; NPs is the abbreviation used for nanoparticles.

anodes reported in the last 20 years were considered. The anodes differ in their basic underlying concepts and can be divided into foil-, paste-, slurry-, and structure-types. The last anode type comprises advanced concepts with novel anode architecture, for example, the incorporation of a freestanding host structure.29 To put these rather complex Zn anode architectures into practice, various physical, chemical, and electrochemical preparation methods are employed to combine the host structure with different forms of the AM.29−34 Moreover, each anode has a different composition, and different electrolyte compositions are used. Additionally, they differ in the choice of AM (e.g., layered double hydroxides (LDH) beside established materials like Zn and ZnO),1,35,36 additive content (e.g., special polymers as a binder in the slurry),37,38 or coatings on the surface of the AM (e.g., metal oxides or carbon-based materials).36,39,40 In most cases, the performance of the Zn anodes is tested against more stable cathodes than a GDE, for example, the NiOOH/Ni(OH)2 cathode. However, the electrochemical processes on the anode side are identical to those in the ZAB utilizing an alkaline electrolyte.8,41,42

the third boxplot quartile value. Furthermore, the median value of w̅ grav changes from 203 Wh kg−1anode to only 111 Wh kg−1anode. In summary, the state-of-the-art performance of Zn anodes applied in electrically rechargeable ZABs reported is so far limited to a median value of 111 Wh kg−1anode and a maximum reported value of 369 Wh kg−1anode (Table 3) taking into account only the mass of the anode mixture.27 In order to assess wgrav of the entire cell, the mass of the current collector, the electrolyte, the separator, the GDE, and the cell housing have to be included. By now, the practically realized wgrav, which is exclusively related to the anode mixture and which would further decrease by considering additional battery parts, is well below the postulated estimate of what can be practically achieved for the entire cell of 500 Wh kg−1cell reported recently.11 Upon comparison of the state-of-the-art performance of Zn anodes applied in electrically rechargeable ZABs (Figure 2b) with that of Zn anode concepts applied in alkaline batteries (Figure 2a), it was found that the latter one shows 37% higher X̅ AM but 23% lower Φ̅ Q. The latter difference originates from the efficiency of the cathodes used, because NiOOH, for example, shows a lower Φ̅ Q compared to GDEs because of side reactions during charge.23,28 Interestingly, both compared groups show a median value of ∼100 achievable cycles. To identify possible reasons for this difference in performance, the evaluated descriptors have to be interconnected to each other. Thus, in the next sections, both groups will be analyzed in detail in order to discover trends and specific, overarching characteristics in reported Zn anodes applied in alkaline batteries and in ZABs. Zn Anode Concepts Tested in Alkaline Batteries. In this group, 36 publications on various concepts and compositions of Zn

To evaluate the state-of-the-art performance of Zn anodes, we define seven descriptors that focus on the practical relevance for battery applications and allow a useful and comprehensive assessment of reported cells. The cell properties, including the composition of both electrodes and the electrolyte, are given in Table 2. All other 1291


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Table 3. Basic Cell Properties of Published Articles Focusing on Electrically Rechargeable ZABsa

a Studies highlighted in gray contain all data to estimate all of the descriptors in Table 1. *Electrolyte is saturated with ZnO; NPs is the abbreviation used for nanoparticles.

without stating mAM). However, because of the performance of the respective cathode used for battery testing (GDE, NiOOH, and MnO2) there is a large deviation for j reported of ±11 mA cm−2. Approximately 50% of all publications use Zn-containing salts as an additive in the electrolyte to lower the solubility of the discharge product and to mitigate changes to the anode morphology.43,44 The concentration of the Zn-containing salt

data used to determine the descriptors can be found in Table S1. The average ratio between mAM and manode is 85% and was reported in 58% of all publications. This means that 15% of the anode mixture belongs on average to inactive additives, which corresponds to a specific capacity of 696 mAh g−1anode (559 mAh g−1anode) for an anode with Zn (ZnO) as the AM. In addition, the average current density j was determined to be 13 mA cm−2 (excluding all studies normalizing the current to mAM 1292


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Figure 2. Boxplots illustrating the state-of-the-art performance of (a) reported concepts of alkaline Zn anodes and (b) reported electrically rechargeable Zn−air batteries based on the distribution of the descriptors. The median values are given as black numbers and represent the state-of-the-art performance reported. Data that are calculated by means of publications that include all necessary information to evaluate all of the defined descriptors are shown on the left-hand side. Data that are based on publications from which it was possible to determine a respective descriptor are shown on the right-hand side. To highlight the distribution of the data, all calculated data are shown by dot symbols in red and blue, respectively. Rhombus symbols in purple indicate the averaged values (arithmetic mean); they are given in Tables S3 and S4.

slurry-based Zn anode was cycled only with a low utilization X̅ AM of 14%, but in return for 1000 cycles with high Φ̅ Q of 77%. In doing so, the authors used a slurry-approach from a patent registered in 1995.46 The slurry mixture contained ZnO as AM, Ca(OH)2 as trapping additive to lower the solubility of the discharge product, and Bi2O3 as a conductive additive to achieve lower electronic resistance in the network of AM particles. In comparison, we found the highest NC·q̅dis value of all evaluated Zn anode concepts from the report by Yan and co-workers with 6210 Ah kg−1anode.29 The authors used a Cu foam as electronically conductive host structure and electrodeposited Zn as AM on it. However, they performed all tests in a beaker cell with an excess of electrolyte. In this cell geometry, the electrolyte contained more AM, as dissolved Zn-containing salt, than the anode itself, which in turn could yield an increased q̅dis (see Table S1, QE/QAM of 25.5). This example

used depends on the concentration of the KOH solution and is on average 33 mg mL−1. However, only 47% of the publications focusing on the Zn-containing salts as additives report on mAM and only 21% on VE (for comparison, 63% of all publications state mAM and 45% VE). This means that often the ratio between QAM and the additional capacity of AM that is added by the Zn-containing salt in the electrolyte QE (in the discharged state) cannot be identified (see Figure 1). Because of the lack of information, it cannot be excluded that in those cases QAM is directly impacted by the amount of the electrolyte. The absence of sufficient data prevents any further conclusions to be drawn regarding the optimal ratio between QAM and QE. The highest Σq̅ dis value of publications including all information is evaluated from a report by Turney and coworkers (see Table 2, no. 21) with 58 Ah kg−1anode.45 Their 1293


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Figure 3. Key descriptors (QAM/VE, Φ̅ Q, X̅ AM, and q̅dis) for the performance of Zn-based cells from published articles focusing on (a) different concepts of Zn anodes in alkaline batteries and (b) Zn anodes applied exclusively in Zn−air batteries and the associated cycle number NC that can be achieved. Values marked in red are based on an electrolyte volume VE that was roughly estimated from the cell geometry reported.

the same time, q̅dis and QAM/VE could be evaluated much more frequently for publications showing fewer than 250 cycles. This means that 56% of all publications showing more than 250 cycles do not report mAM (compared to only 35% for fewer than 250 cycles) and 78% do not report VE (compared to only 48% for fewer than 250 cycles). In summary, publications with a high NC report a higher utilization and retention of the AM but much more frequently do not state mAM and VE. This interrelation can be explained by assuming that cell geometries with high VE were used. With that, the performance of the anode can be strongly affected by VE and can differ from the performance tested in a conventional cell with limited VE, as recently pointed out by Parker and co-workers.18 To highlight the strong influence of VE, we estimated the least required amount of electrolyte based on a simple calculation: the amount of hydroxide ions provided in the electrolyte should match the amount of utilized AM (see eqs S7−S9 in the Supporting Information). The deviation between our estimate and the reported VE shows that 54% of the publications reporting QAM and VE use on average 70% more VE compared to the least required amount (compare Figures

illustrates why it is important to consider all of the herein presented descriptors as it cannot be excluded that the QE/ QAM ratio influences the initial capacity of the reported Zn anode concept. Reporting all descriptors allows linking them and evaluating the performance on the basis of practically relevant metrics. In order to show the correlation between the descriptors, Figure 3a shows four descriptors (QAM/VE, Φ̅ Q, X̅ AM, and q̅dis) as a function of NC. Because NC was the descriptor reported in nearly all publications, we selected it as the joint connector. In order to not underestimate the influence of the different cathodes used, the plotted data is subdivided into three subgroups of cathodes: NiOOH, GDE, and others (e.g., MnO2, Ag2O). While at least 82% of the studies report on Φ̅ Q and X̅ AM, only 55% and 37% of the studies give detailed information needed to estimate q̅dis and QAM/VE, respectively. It can be seen that on average the two more frequently reported descriptors differ significantly for publications larger or smaller than 250 cycles (gray area). Interestingly, X̅ AM and Φ̅ Q are on average higher for publications showing more than 250 cycles (60% and 76% compared to 37% and 72%, respectively). At 1294


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Mainar and co-workers with an accumulated energy density of 2300 Wh kg−1anode within 10 cycles at X̅ AM of 27%, which corresponds on average to 230 Wh kg−1anode per cycle.92 They presented Nafion-coated Zn particles as the AM to prevent the dissolution of the AM during discharge. Their slurry-based anode possesses a ratio between mAM and manode of 70%. In addition, they applied an advanced electrolyte composition composed of 4 M KOH including 2 M KF, 2 M K2CO3, and saturated with ZnO to ensure that Zn is deposited as a uniform layer during charge.105 Combining both strategies provides evidence for the strong interaction between anode and electrolyte, which is already identified to be important for the overall cell performance of the ZAB.24 Concerning all publications on ZABs, Müller et al. show the highest value of NC·w̅ grav with 100 641 Wh kg−1anode (443 cycles at X̅ AM of 24.8%), which corresponds on average to w̅ grav of 227 Wh kg−1anode per cycle.71 This publication from 1998 is the oldest of all considered studies. Müller and co-workers used a special cell configuration with a 1.4 mm thick slurry-based Zn anode sandwiched between two GDEs so that the two largest faces of the cylindrical anode could be utilized simultaneously. Evaluation and Outlook for Future Research on Alkaline Zn Anode Concepts. Our analysis is divided into publications on various alkaline Zn anode concepts tested against different cathodes and alkaline Zn anodes applied in ZABs. This allows for the general assessment of reported alkaline Zn anode performance and, in particular, the performance of Zn anodes applied in ZABs. Comparing the state-of-the-art performance of Zn anodes applied in electrically rechargeable ZABs with that of Zn anode concepts tested in various alkaline batteries (Figure 2), we conclude that the latter shows 37% higher utilization X̅ AM, but both show a median cycle number NC of ∼100. Regarding the specific capacity, Zn anodes applied in ZABs were reported with a median value of 67 mAh g−1anode, which corresponds to only 8% of the theoretical value of a bulk Zn anode. However, considering solely the alkaline Zn anodes tested in various alkaline batteries reveals that much higher values of around 167 mAh g−1anode can be achieved, which is around 20% of the theoretical value. This means that promising and already reported Zn anode concepts do not yet show sufficient performance for application in ZABs. In particular, novel alkaline Zn anode concepts are tested only against stable cathodes, but not in a ZAB. On the other hand, ZAB research is mostly limited to simple types of Zn anodes, such as Zn foil anodes, which severely limit the possible cell performance. In fact, we feel that ZAB research focused too much on cathode concepts in recent years. Assuming that the anode mixture contributes about 25% to the total mass of the ZAB,1 we can take the highest reported wgrav from all studies by Lee et al. and can estimate 100 Wh kg−1cell over 10 cycles.27 With regard to higher values of NC, we estimate 57 Wh kg−1cell for the ZAB reported by Müller et al. over 443 cycles.71 Considering our detailed evaluation, in comparison with the already commercialized Li-ion batteries (350 Wh kg−1cell), the currently reported expectations and targets for electrically rechargeable ZABs are by far too optimistic. However, testing the performance of novel Zn anode concepts in a ZAB, instead of only in Zn/NiOOH, Zn/ MnO2, or Zn/Ag2O batteries, and avoiding the use of foil-type anodes in the field of catalyst research may accelerate future ZAB research.

S2 and S3). Thus, we conclude that only two times the least required amount of electrolyte is sufficient to adequately operate an alkaline Zn anode. This means that there is no need to use beaker cells with a large excess of electrolyte to test alkaline Zn anodes in an electrochemical half-cell or in a battery. Zinc Anodes Applied in Electrically Rechargeable Zn−Air Batteries. In this group, 32 publications on electrically rechargeable ZABs published in the last 20 years were considered. Only those batteries that utilize a GDE with a bifunctional catalyst were taken into account, because they promise the use of sustainable raw materials.4,71 The properties of the different batteries, including the composition of both electrodes and the electrolyte, are provided in Table 3. In addition, flexible cells were included, which mostly use gelbased electrolytes with a large excess of electrolyte and consist of flexible electrode and electrolyte sheets stacked on top of each other.77 As above, all of the considered batteries are evaluated on the basis of the descriptors in Table 1 (see Table S2 for the calculated values of the descriptors). On average, all types of cells were operated at 13 ± 8 mA cm−2. Particularly striking is that 65% of all reported ZABs use a flat Zn foil as anode. This applies usually to studies from the field of catalyst research that evaluate the performance of novel catalyst materials at the cathode by means of a performance test in a ZAB. Additionally, all publications using Zn foils also use electrolytes composed of 6 M KOH solution with 0.1−0.2 M of a Zn-containing additive (in comparison, 56% of all publications use this electrolyte composition). Unfortunately, far too many of these publications do not contain information on mAM and VE. This means that we were able to evaluate mAM and VE in only 19% and 29% of the publications that made use of a Zn foil (in comparison to 40% for all publications in each case). Although catalyst research does not generally focus on the optimization and performance of ZABs, it is important to state those values in order to determine the actual performance of novel catalyst materials and to make a comparison to already reported catalysts and their use in ZABs possible. Figure 3b correlates all descriptors as a function of NC. In addition, the data are subdivided into three types of anodes: (i) Zn foil and other approaches starting with either (ii) Zn or (iii) ZnO. Overall, only five out of 32 publications allow for an estimate of QAM/VE. None of these five publications, with reasonable values for VE, showed more than 100 cycles, and X̅ AM varies between 100 cycles at 0.01% and 10 cycles at 62.3%. In total, only five publications state X̅ AM and report more than 100 cycles. This trend also applies to Zn foil anodes, which show more than 100 cycles only if X̅ AM is below 0.5%. The underlying reason for this trend in the data could be surface passivation originating from the low surface area of the anode.5,104 Batteries that do not use a Zn foil as the anode showed significantly better overall performance. Especially slurry-based anodes with ZnO as the AM showed good performance but could be cycled with at best 50% of X̅ AM, which implies that the AM is not used to an optimal extent. It seems that anodes with NC > 100 and reasonable X̅ AM are difficult to realize in practice, which could be due to degradation phenomena at the interface between the electrolyte and the anode, such as electrode shape change and irreversible dissolution of the AM during discharge.10,43 The highest NC·w̅ grav value of publications including all information can be evaluated from the report by 1295


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essential for practical applications. We urge the community to report all data within a publication that are needed to use the descriptors defined herein. This can help to streamline research efforts and to enhance the overall performance of electrically rechargeable ZABs in a way similar to that for Li−S battery technologies. In this Review, we analyze the performance of various concepts of alkaline Zn anodes for application in electrically rechargeable ZABs reported in studies published over the last 20 years. To evaluate the state-of-the-art performance of Zn anodes, we define seven descriptors. We suggest that these descriptors focus on the practical relevance for battery applications and allow a useful and comprehensive assessment of reported cells. In addition, they offer the possibility of technological transfer to other battery systems. Our selected descriptors reveal that Zn anodes applied in electrically rechargeable ZABs show on average inferior performance compared to those reported and tested against different alkaline cathodes. Thus, the ZAB research is not focused enough on the Zn anode, and promising concepts of Zn anodes are not yet applied in ZABs; instead, the simple foiltype Zn anode is used excessively. We recommend researchers focus more on advanced anode structures and consider the volume of electrolyte that is added to the cell. Recently published concepts aim for higher utilization of the AM but are on average limited to less than 100 cycles or do not report the volume of electrolyte added.

The aim of future research on the lab scale should incorporate anode concepts in a practically relevant cell geometry demonstrating cycling beyond 100 cycles, combined with higher utilization of the valuable AM Zn of about X̅ AM > 30%. In this context, it might be particularly helpful to define a ZAB standard cell with fixed parameters as a benchmark in order to be able to test them in a comparable way on the lab scale, in a fashion similar to Bonnick et al. introducing a standard cell for the Zn/NiOOH electrode combination.106 We encourage the ZAB community to consider a performance test in a coin-type battery cell with a standardized preparation protocol for new concepts. The coin-type battery cell is characterized by its simple preparation procedure, practical relevance, and control of electrolyte volume and enables the use of high-throughput screening;72,106 in addition, it is relevant not only for batteries with aqueous electrolytes but also for lithium-based batteries.107 Furthermore, the already reported alkaline Zn anode concepts can actually achieve partially high descriptor values with practical relevance (Figure 3a). However, most publications show favorable values only for a maximum of one or two descriptors in single fields, with the remaining descriptors being significantly lower. Recently published Zn anode concepts that exhibit higher X̅ AM are mostly limited to less than 100 cycles or do not report VE. In general, the currently reported Zn anode concepts can be categorized into three groups that are based on13,17,37,41,73 (i) incorporating additives into the AM, such as alloying, or into the anode mixture; (ii) coating the AM or encapsulating the entire surface of the anode; and (iii) modifying the structure of the AM, such as changing the morphology on the nanoscale, or of the anode itself, such as building new 3D architectures on the microscale.41 For example, changing the morphology of the AM, such as in hyper-dendritic Zn particles,56 or preparing Zn monoliths with a three-dimensional pore system,30 aims at enhancing the utilization and the retention of the AM. Coatings primarily serve to protect the AM in the discharged state from dissolving into the electrolyte.6,41 A large number of coating materials, e.g., metal oxides, polymers, or carbon, have already been tested to prevent the phenomena of electrode shape change and enhance the utilization and retention of the AM.41,108 Importantly, a chemically and electrochemically stable protective layer would allow cycle numbers above 100 at high X̅ AM. Further, the electrolyte composition and the choice of separator should be adapted to the working principles of the applied Zn anode so that all parts of the cell are closely interrelated. Therefore, future research should focus on balancing the presented descriptors to enhance the cycling performance of alkaline Zn anodes by combining promising concepts. Finally, one last important point is to be mentioned: only a small number of published cycling data sets (i.e., the electrochemical data) is reported complete. Evaluating only a few descriptors can quickly lead to inaccurate judgment of the anode and thus the battery performance by under- or overestimating just a few descriptors. If, for example, a battery can be cycled over many cycles with high utilization but exhibits a low ΦQ and large excess of liquid electrolyte, a large part of the side reactions and electrolyte decomposition can overshadow the overall performance. Strictly speaking, this would be more of an electrolysis cell than a ZAB. In particular, VE as well as manode are often not provided. However, both are

We recommend researchers focus more on advanced anode structures and consider the volume of electrolyte that is added to the cell. All of this could allow for a comprehensive assessment and improve the overall performance of alkaline Zn anodes for application in electrically rechargeable ZABs. Cooperation between many different disciplines of battery research can enable the full potential of this high energy density battery technology.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.9b00510. Formulas to calculate the descriptors; values for the descriptors representing the state-of-the-art performance of reported Zn anode concepts applied in alkaline batteries; values for the descriptors representing the state-of-the-art performance of reported zinc−air batteries; description and explanation of a boxplot; average values for the descriptors representing the state-of-theart performance of reported Zn anode concepts in alkaline batteries and reported Zn anodes in zinc−air batteries; volume of electrolyte for reported concepts of Zn anodes; volume of electrolyte for reported zinc−air batteries (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 1296


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ORCID

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Jürgen Janek: 0000-0002-9221-4756 Daniel Schröder: 0000-0002-2198-0218 Notes

The authors declare no competing financial interest. Biographies Daniel Stock is currently a Ph.D. candidate at the Institute of Physical Chemistry under the supervision of Prof. Jürgen Janek and Dr.-Ing. Daniel Schröder. His research interests include electrically rechargeable zinc−oxygen batteries and the development of new concepts for battery anodes to increase energy efficiency and cycling stability. Saustin Dongmo received his Ph.D. in chemistry from the University of Oldenburg in Germany in 2017. Then he joined the research group of Prof. Jürgen Janek as a postdoctoral researcher at the Justus-Liebig University Giessen working on secondary zinc−air batteries with the junior group leader Dr.-Ing. Daniel Schröder. His research interests include metal anodes for next-generation batteries. Jü rgen Janek is professor of physical chemistry at Justus-Liebig University Giessen (Germany) and scientific director of the BELLA, a joint lab of BASF SE and KIT in Karlsruhe (Germany). His research spans from fundamental studies on electrode kinetics, interface phenomena, to plasma electrochemistry. Current key interests include solid-state batteries and postlithium cell reactions. Daniel Schröder received his Ph.D. at TU Braunschweig in 2015 working on zinc−oxygen batteries. Afterwards, he joined Justus-Liebig University Giessen as junior group leader. In 2017 he joined Kyoto University for a research stay. His research focuses on understanding redox-flow as well as metal−oxygen batteries with operando and model-based methods. Web page: www.uni-giessen.de/schroeder.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support by the BMBF (Federal Ministry of Education and Research) within the project “Zisabi” (03XP0086) and by the DFG via the GRK (Research Training Group) 2204 “Substitute Materials for sustainable Energy Technologies”. We thank Felix Walther, Dr. Sean Culver, and Dr. Bjoern Luerßen for fruitful discussions.

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ACS Energy Letters

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