At present, the replacement of fossil fuels by alternative CO₂-emission-free energy sources, such as solar and wind, is substantially hindered by the lack of low-cost and large-scale energy storage technologies. Although lithium-ion batteries (LIBs) represent the most mature and widely deployed electrochemical energy storage technology for mobility and portable electronics, the uneven worldwide distribution of known lithium reserves and the high production costs greatly reduce the economic appeal of LIBs for large-scale stationary storage.(1−5) In this regard, inexpensive Al–graphite dual-ion batteries (AGDIBs) have attracted great attention over the past few years.(6−26) With energy densities of 30–70 Wh kg^–1, AGDIBs are suitable for stationary storage.(27−30) The constituents of AGDIBs include highly abundant elements (H, O, N, C, and Al) and are easy to manufacture. While recent research efforts on AGDIBs have been mainly focused on testing various graphite cathodes, further progress of this technology is inherently limited by the low charge storage capacity of the chloroaluminate ionic liquids used as anolytes (often but incorrectly called electrolytes). In this Viewpoint, we discuss the critical interplay between the capacity of the anolyte and the energy density of AGDIBs along with their measurements at different current densities. The basic configuration of an AGDIB contains a graphite cathode, AlCl₃-EMIMCl (1-ethyl-3-methylimidazolium chloride) ionic liquid anolyte, and metallic aluminum current collector, as demonstrated in Figure 1a. AGDIBs operate as an electrochemical energy storage system by employing the reversible intercalation of AlCl₄^– anion species into the positive graphite electrode upon charging (i.e., the oxidation of the graphite network). Concurrently, the aluminum electroplating reaction takes place on the negative side of AGDIBs. The working principle of AGDIBs can be represented by the following cathodic and anodic half-reactions during charging:(I)(II) On the negative electrode On the positive electrode Figure 1. (a) Schematic of the charging process of aluminum GDIBs. (b) Charge storage capacity of the chloroaluminate ionic liquid anolyte (AlCl₃:EMIMCl) versus its acidity (r). The curve is computed from eq 1. The chloroaluminate ionic liquid anolyte used in AGDIBs is defined as a mixture of aluminum chloride and other chlorides countered by a bulky organic cation, such as 1-ethyl-3-methylimidazolium chloride (EMIM) or 1-butyl-3-methylimidazolium chloride (BMIM). As a result of the acid–base reaction between AlCl₃ (Lewis acid) and Cl^– (Lewis base), the solid mixture liquefies at room temperature, forming AlCl₄^– anions that are charge-balanced with asymmetric organic cations, such as EMIM⁺. The chloroaluminate ionic liquid with an excess of AlCl₃ over EMIMCl is composed of both AlCl₄^– and Al₂Cl₇^– ions. Importantly, only Al₂Cl₇^– ions enable the electroplating of aluminum, which, therefore, occurs only in acidic chloroaluminate melts (i.e., an excess of AlCl₃).(31−40) Consequently, the specific charge storage capacity of the acidic melt is a function of the concentration of Al₂Cl₇^– ions in the ionic liquid. Electroplating, and consequently the charging process, stops when no Al₂Cl₇^– ions remain in the ionic liquid, at which point the melt becomes neutral (AlCl₃:EMIMCl = 1). The highest molar ratio (r) of AlCl₃ and EMIMCl that forms an ionic liquid is ca. 2:1. AlCl₃ does not dissolve in the chloroaluminate melts at higher molar ratios. In this context, we note that the mechanism of AGDIBs is substantially different from the operation of “rocking-chair” metal-ion batteries: there is no unidirectional flow of Al³⁺ ions from the cathode to the anode and vice versa. In fact, the Al species are depleted from the chloroaluminate ionic liquid during the charge of AGDIBs and are being consumed by both electrodes (Figure 1a). Notably, the electrochemistry of AGDIBs is not restricted to AlCl₃/EMIMCl ionic liquids. Other possible ionic melts have also been recently tested in AGDIBs, such as AlCl₃/1-methyl-3-propylimidazolium chloride (MPIMCl),(41) AlCl₃/benzyltriethylammonium chloride (TEBACl),(42) AlCl₃/1,2-dimethyl-3-propylimidazolium chloroaluminate (DMPICl),(43) AlCl₃/NaCl,(44) and AlCl₃/LiCl/KCl(45) or AlCl₃/urea/EMIMCl,(46) AlCl₃/urea,(47−49) and AlCl₃/Et₃NHCl.(50,51) Figure 1b illustrates the impact of acidity (r) on the charge storage capacity of the chloroaluminate ionic liquid anolyte. The theoretical capacity of the ionic liquid (C_an) as an anolyte, considering its whole mass/volume, can be calculated as follows:(1)(2)where F = 26.8 × 10³ mAh mol^–1 (the Faraday constant), and (number of electrons used to reduce 1 mol of the Al₂Cl₇^– ions); M_AlCl3 is the molar mass of AlCl₃ in g mol^–1, M_ACl the molar mass of the Cl^– source (for example, EMIMCl) in g mol^–1, r the AlCl₃:ACl molar ratio, and ρ the density of the chloroaluminate-based anolyte in g mL^–1. Gravimetric Volumetric The theoretical gravimetric charge storage capacities of AlCl₃:EMIMCl ionic liquids equal, for instance, to 48 mAh g^–1 and 19 mAh g^–1 for r = 2 and r = 1.3, respectively. These capacities determine and limit the overall energy density of AGDIBs, as previously pointed by us(7,27) and others.(52−59) To the best of our knowledge, it had not been verified whether these capacities are achievable experimentally, i.e. whether Al₂Cl₇^– ions can be fully depleted for Al electroplating at practically relevant experimental conditions. For this, the anolyte-limited cell must be assembled, that is, the cell with the significant excess of graphite cathode. On the contrary, the commonly reported AGDIB tests employ an up to 10-fold excess of anolyte (or even higher). We note that such a 10-fold anolyte access can be recalculated into an impractical overall charge-storage capacity of the cell (<5 mAh g^–1). In the anolyte-limited cell, depletion of Al₂Cl₇^– ions r will eventually cause a drop in the potential at the negative electrode of the battery. It is thus also important to conduct tests in a three-electrode configuration in order to differentiate between changes in the potentials originating at positive and negative electrodes. Following these considerations, we prepared an anolyte-limited full cell, in which an excess of graphite flakes was used in combination with the anolyte, i.e. having roughly an order of magnitude higher cathodic capacity in comparison to that needed to match the theoretical charge storage capacity of the anolyte. AGDIBs were assembled using a three-electrode cell composed of a glassy carbon current collector, graphite flake cathode, Al foil reference electrode, separator impregnated with chloroaluminate ionic liquid anolyte, and Al foil current collector (Figure 2a). The cells were cycled between 2.5 and 0.1 V vs Al³⁺/Al. An example of the electrochemical measurement at a current density of 60 mA g^–1 is shown in Figure 2b using a chloroaluminate ionic liquid with a molar ratio of 2.0. In these measurements, the simultaneous acquisition of the potential profiles of both positive and negative electrodes was recorded, in addition to that of the full cell. Figure 2. (a) Three-electrode cell configuration of the AGDIBs (the location of the reference electrode is shown on the left). (b and c) Galvanostatic charge curves for the chloroaluminate ionic liquid (r = 2) anolyte (E_CE), graphite (E_WE), and full cell (E_Cell) measured versus the Al foil reference electrode in anolyte-limited (b) and graphite-limited (c) cell configurations. The cells were charged at room temperature with a current density of 60 mA g^–1 and an upper voltage limit of 2.5 V vs Al³⁺/Al. In the case of the graphite-limited cell configuration, a 5-fold excess of the anolyte over graphite was used. As follows from Figure 2b, the voltage profile at the negative electrode (E_CE) remains relatively stable during charging for 15 min, with a small overpotential of up to 200 mV. However, upon further charging, this voltage drops sharply when the Al plating process ends. The plating termination is also reflected in the sharp increase in the overall potential of the cell of up to 2.5 V vs Al³⁺/Al. The potential at the positive electrode (E_WE), however, is relatively stable for the full charging cycle. In contrast to the anolyte-limited cell, the voltage profile at the negative electrode (E_CE) for the graphite-limited cell is very stable during the entire charge, with a minimal overpotential of <50 mV (Figure 2c). As indicated above, the graphite-limited cell configuration is used for the assessment of the charge storage capacity of graphite in the majority of research studies on AGDIBs. Using the ionic liquid formulations with r = 1.3–2.0, we performed rate capability measurements of anolyte-limited full cells at different current densities ranging from 5 A g^–1 to 20 mA g^–1. Figure 3a presents the galvanostatic curves of the anolyte with r = 2 during charging (see Figure S1 for details). The electrochemical data for r = 1.3 and 1.8 are shown in the Supporting Information (Figures S2 and S3). Figure 3b summarizes these results in terms of the obtained charge storage capacities at different current densities. The results reveal two major points. First, a more acidic anolyte yields, as expected, a higher anolyte capacity. For instance, the charge storage capacity of the most commonly used anolyte with r = 1.3 is ca. 21 mAh g^–1 at 20 mA g^–1. In contrast, the anolyte with the highest acidity (r = 2.0) possesses a capacity of ca. 46 mAh g^–1. These results point to the fact that the highest energy density of the AGDIBs can be obtained using a 2.0 molar ratio, and therefore, future works on AGDIBs should be focused on the most acidic formulations. Second, the charge storage capacity of the anolyte is highly dependent on the applied current density. For instance, at a high current density of 1 A g^–1, minimal charge storage capacities are obtained (ca. 10–14% from theoretical values). The latter point is reflected in the pronounced deviation of the voltage profiles at the negative electrode at high current densities (Figures 3a and S1–S3). Conversely, minimal polarization is observed at the graphite positive electrode. These results indicate that the frequent statements regarding the high power density of AGDIBs need to be taken with care. Specifically, we note that at high current densities a significant drop in the energy density of AGDIBs is expected and originated from the rate capability limitations of the chloroaluminate ionic liquid anolyte when its acidity (concentration of Al₂Cl₇^–) is drastically reduced. In fact, our observations show that the charge storage capacities of the anolyte significantly deviate from the theoretical value at charge current densities greater than 20 mA g^–1. Figure 3. (a) Galvanostatic Al plating (discharge) curves of the chloroaluminate ionic liquid anolyte with r = 2.0 measured at different current densities in combination with graphite as the working electrode and Al foil as the reference electrode. (b) Charge storage capacities of the chloroaluminate ionic liquid anolyte with r = 1.3, 1.8, and 2.0 measured at different current densities. The green line shows the theoretical charge storage capacity of the anolyte computed from eq 1. In conclusion, we reiterate that commonly reported tests of AGDIBs employ large excess of ionic liquid anolyte (cathode-limited cells) and, despite nominally dealing with full cells, do not provide correct and practically relevant information on achievable energy and power densities, as well as cycling stability, energy and Coulombic efficiencies of these batteries. At best, such tests yield the theoretical capacities and other characteristics of the cathode material only, similar to, for instance, Li-ion half-cell tests of novel cathodes (with thick Li foil as a counter electrode). We further note that ionic liquids used in AGDIBs are not just electrolytes (ion-conductors) but represent an electrochemically active, capacity- and rate-limiting battery component. It is also apparent that future work should focus on finding ways of minimizing the amount of this anolyte toward the capacity-matched quantity, ideally with just ca. 10% of the excess anolyte, without sacrificing the power density and cyclability of the battery. Most likely, a successful solution to this problem will invoke a radically new battery design, vastly different from that employed in commercial Li-ion batteries and nearly all tests of new battery materials. All these advancements will need to come at low capital costs in order to retain the overall cost-competitiveness that is commonly attributed to AGDIBs (based on low costs of electrochemically active constituents). Similar considerations apply also to other nonrocking-chair batteries, such as magnesium–sodium and magnesium–lithium dual-ion batteries. The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsenergylett.9b02832.

• Additional electrochemical data (PDF)

Additional electrochemical data (PDF) Views expressed in this Viewpoint are those of the authors and not necessarily the views of the ACS. The authors declare no competing financial interest. Electronic Supporting Information files are available without a subscription to ACS Web Editions. The American Chemical Society holds a copyright ownership interest in any copyrightable Supporting Information. Files available from the ACS website may be downloaded for personal use only. Users are not otherwise permitted to reproduce, republish, redistribute, or sell any Supporting Information from the ACS website, either in whole or in part, in either machine-readable form or any other form without permission from the American Chemical Society. For permission to reproduce, republish and redistribute this material, requesters must process their own requests via the RightsLink permission system. Information about how to use the RightsLink permission system can be found at http://pubs.acs.org/page/copyright/permissions.html. This research is part of the activities of SCCER HaE, which is financially supported by Innosuisse - Swiss Innovation Agency. This article references 59 other publications.

中文翻译：

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铝-石墨双离子电池的氯铝酸盐离子液体电解质的局限性

目前，由于缺乏低成本和大规模的储能技术，大大阻碍了用替代的无CO ₂排放的能源（例如太阳能和风能）替代化石燃料。尽管锂离子电池（LIB）代表了用于移动性和便携式电子设备的最成熟且部署最广泛的电化学能量存储技术，但已知锂储量在全球范围内分布不均以及高昂的生产成本极大地降低了LIB在大规模固定式设备上的经济吸引力存储。（1-5）在这方面，近几年来廉价的铝石墨双离子电池（AGDIB）引起了极大的关注。（6-26）能量密度为30-70 Wh kg ^–1，AGDIBs适合固定存储。（27-30）AGDIBs的成分包括高度丰富的元素（H，O，N，C和Al），并且易于制造。尽管最近关于AGDIB的研究工作主要集中在测试各种石墨阴极上，但是该技术的进一步发展固有地受限于用作阳极电解液的氯铝酸盐离子液体（通常但错误地称为电解质）的低电荷存储能力。在此观点中，我们讨论了阳极电解液容量与AGDIB能量密度之间的关键相互作用，以及它们在不同电流密度下的测量结果。AGDIB的基本配置包含石墨阴极AlCl ₃-EMIMCl（1-乙基-3-甲基咪唑鎓氯化物）离子液体阳极电解液和金属铝集电器，如图1a所示。AGDIBs作为电化学能量存储系统通过采用的AlCl的可逆嵌入操作₄ ^-在充电阴离子种到正电极石墨（即，石墨网络的氧化）。同时，铝电镀反应发生在AGDIB的负极。AGDIB的工作原理可以由充电过程中的以下阴极和阳极半反应表示：（一世）（二）在负极上在正极上。图1.（a）铝制GDIB的充电过程示意图。（b）氯铝酸盐离子液体阳极电解液（AlCl ₃：EMIMCl ）的电荷存储容量与其酸度（r）的关系。该曲线是从等式1计算得出的。AGDIB中使用的氯铝酸盐离子液体阳极电解液定义为氯化铝和其他被大量有机阳离子（例如1-乙基-3-甲基咪唑鎓氯化物（EMIM）或1-乙基）抵消的氯化物的混合物。丁基-3-甲基咪唑鎓氯化物（BMIM）。由于AlCl ₃（路易斯酸）和Cl ^–（路易斯碱）之间的酸碱反应，固体混合物在室温下液化，形成AlCl ₄ ^–与不对称有机阳离子进行电荷平衡的阴离子，例如EMIM ⁺。用过量的AlCl的氯铝酸盐离子液体₃上方EMIMCl由两者的AlCl的₄ ^-和Al ₂氯₇ ^-离子。重要的是，仅由Al ₂氯₇ ^-离子使铝，其中，因此，只发生在酸性氯熔体电镀（即，过量的AlCl的₃）（31-40）。因此，酸性的特定的电荷存储容量。熔体是Al ₂ Cl ₇浓度的函数^–离子液体中的离子。电镀，并且因此充电过程中，停止时不含Al ₂氯₇ ^-离子保留在离子液体，在该点，熔体变成中性（ALCL ₃：EMIMCl = 1）。形成离子液体的AlCl ₃和EMIMCl的最高摩尔比（r）为。2：1。AlCl ₃在较高的摩尔比下不溶于氯铝酸盐熔体。在这种情况下，我们注意到AGDIB的机理与“摇椅”金属离子电池的操作有很大不同：没有Al ^3+的单向流动离子从阴极到阳极，反之亦然。实际上，在AGDIB充电过程中，铝物质已从氯铝酸盐离子液体中耗竭，并且被两个电极消耗（图1a）。值得注意的是，AGDIB的电化学不限于AlCl ₃ / EMIMCl离子液体。最近在AGDIB中还测试了其他可能的离子熔体，例如AlCl ₃ / 1-甲基-3-丙基咪唑鎓氯化物（MPIMCl），（41）AlCl ₃ /苄基三乙基氯化铵（TEBACl），（42）AlCl ₃ / 1,2 -二甲基-3-丙基咪唑鎓氯铝酸盐（DMPIC1），（43）AlCl ₃ / NaCl，（44）和AlCl ₃ / LiCl / KCl（45）或AlCl ₃ /脲/ EMIMCl，（46）AlCl ₃/ urea，（47-49）和AlCl ₃ / Et ₃ NHCl。（50,51）图1b说明了酸度（r）对氯铝酸盐离子液体阳极电解液的电荷存储容量的影响。考虑到其整体质量/体积，离子液体（C _an）作为阳极电解液的理论容量可以计算如下：（1）（2）其中˚F = 26.8×10 ³毫安摩尔^-1（法拉第常数），以及（用于减少1个摩尔的Al的电子数量₂氯₇ ^-离子）; 中号_{的AlCl 3}是的AlCl的摩尔质量₃以g摩尔^-1，中号_AC1的所述氯的摩尔质量^-以克摩尔来源（例如，EMIMCl）^-1，- [R的的AlCl ₃：访问列表的摩尔比，和ρ的氯铝酸盐基阳极电解液的密度，单位为g mL ^–1。重量体积理论上，对于r = 2和r = 1.3，AlCl ₃：EMIMCl离子液体的理论重量电荷存储容量分别等于48 mAh g ^–1和19 mAh g ^–1。这些容量决定并限制了AGDIB的整体能量密度，正如我们先前指出的（7，27）和其他方法一样。（52-59）据我们所知，尚未验证这些容量是否可以通过实验实现，即Al ₂ Cl ₇是否^–可以在实际相关的实验条件下将铝中的离子完全消耗掉。为此，必须组装阳极电解液受限的电池，即具有明显过量的石墨阴极的电池。相反，通常报道的AGDIB测试使用多达10倍过量的阳极电解液（甚至更高）。我们注意到，可以将这种10倍的阳极电解液访问量重新计算为该电池不切实际的总体电荷存储容量（<5 mAh g ^–1）。在阳极电解液限制细胞，将Al的耗尽₂氯₇ ^-离子ř最终会导致电池负极的电位下降。因此，以三电极配置进行测试也很重要，以便区分源自正极和负极的电势变化。基于这些考虑，我们制备了一个阳极电解液受限的全电池，其中与阳极电解液结合使用了过量的石墨薄片，即与匹配理论电荷存储容量所需的容量相比，其阴极容量大约高出一个数量级。阳极电解液。AGDIB使用三电极电池组装而成，该电池由玻璃状碳集电器，石墨鳞片阴极，铝箔参比电极，浸渍有氯铝酸盐离子液体阳极电解液的隔膜和铝箔集电器组成（图2a）。³⁺ /铝 图2b中显示了使用摩尔比为2.0的氯铝酸盐离子液体在60 mA g ^–1的电流密度下进行电化学测量的示例。在这些测量中，除了完整电池的电位记录外，还记录了正极和负极电位分布的同时获取。图2.（a）AGDIB的三电极电池配置（参比电极的位置显示在左侧）。（b和c）氯铝酸盐离子液体（r = 2）阳极电解液（E _CE），石墨（E _WE）和全电池（E _Cell）的恒静电荷曲线）是在阳极液受限（b）和石墨受限（c）电池配置中相对于铝箔参比电极测量的。电池在室温下充电，电流密度为60 mA g ^–1，相对于Al ³⁺ / Al的电压上限为2.5V 。在石墨受限的电池构型的情况下，使用的阳极电解液是石墨的5倍过量。从图2b中可以看出，在充电15分钟的过程中，负电极（E _CE）上的电压分布保持相对稳定，且小过电位高达200 mV。然而，在进一步充电时，当镀铝过程结束时，该电压急剧下降。电镀终止也反映在电池的总电势相对于Al高达2.5 V的急剧增加上³⁺ /铝 但是，在整个充电周期中，正极的电势（E _WE）相对稳定。与阳极限制型电池相比，石墨限制型电池在负极（E _CE）上的电压分布在整个充电过程中非常稳定，最小过电势<50 mV（图2c）。如上所述，在有关AGDIB的大多数研究中，石墨限制的电池配置用于评估石墨的电荷存储容量。使用r = 1.3–2.0的离子液体制剂，我们在5 A g ^–1到20 mA g的不同电流密度下进行了阳极电解液限制的全电池的速率能力测量^–1。图3a给出了充电期间r = 2时阳极电解液的恒电流曲线（有关详细信息，请参见图S1）。r = 1.3和1.8的电化学数据显示在支持信息中（图S2和S3）。图3b根据在不同电流密度下获得的电荷存储容量总结了这些结果。结果揭示了两个要点。首先，如预期的那样，酸性更高的阳极电解液产量更高。例如，r = 1.3的最常用阳极电解液的电荷存储容量约为ca。21 mA g ^–1在20 mA g ^–1时。相反，具有最高酸度（r = 2.0）的阳极电解液的容量约为ca。46毫安克^–1。这些结果表明，使用2.0摩尔比可以获得AGDIB的最高能量密度，因此，未来在AGDIB上的研究应集中在最酸性的配方上。第二，阳极电解液的电荷存储容量高度取决于所施加的电流密度。例如，在1 A g ^–1的高电流密度下，获得的电荷存储容量最小（理论值的10-14％）。后一点反映在高电流密度下负极上电压曲线的明显偏差（图3a和S1-S3）。相反，在石墨正电极处观察到最小的极化。这些结果表明，关于AGDIB的高功率密度的频繁陈述需要格外小心。具体来说，我们注意到，在高电流密度下，预计AGDIB的能量密度会显着下降，这是由于氯铝酸盐离子液体阳极电解液的酸度（Al ₂ Cl _7的浓度为^–）大大减少了。实际上，我们的观察表明，在大于20 mA g ^–1的充电电流密度下，阳极电解液的电荷存储容量会明显偏离理论值。图3.（a）在以石墨为工作电极和以铝箔为参比电极的情况下，在不同电流密度下测得的r = 2.0的氯铝酸盐离子液体阳极电解液的恒电流镀铝（放电）曲线。（b）具有r的氯铝酸盐离子液体阳极电解液的电荷存储容量在不同的电流密度下测得的= 1.3、1.8和2.0。绿线显示了从等式1计算得出的阳极电解液的理论电荷存储容量。总之，我们重申，通常报道的AGDIB的测试使用了大量的离子液体阳极电解液（阴极限制的电池），尽管名义上涉及满电池，不要提供有关这些电池可达到的能量和功率密度以及循环稳定性，能量和库仑效率的正确且实用的信息。这种测试充其量只能产生阴极材料的理论容量和其他特性，例如类似于新型阴极的锂离子半电池测试（以厚的Li箔作为对电极）。我们进一步注意到，AGDIB中使用的离子液体不仅是电解质（离子导体），还代表了电化学活性，容量和速率限制的电池组件。同样显而易见的是，未来的工作应集中在寻找将这种阳极电解液的量减少到与容量匹配的量最小的方法，理想情况下大约为。10％的过量阳极电解液，而不会牺牲电池的功率密度和可循环性。成功解决此问题的方法很可能会带来全新的电池设计，这与商用锂离子电池和几乎所有新电池材料测试中所采用的设计大不相同。为了保持总体成本竞争力（通常归因于AGDIB）（基于电化学活性成分的低成本），所有这些进步都需要以较低的资本成本实现。类似的考虑因素也适用于其他非摇椅电池，例如镁钠和镁锂双离子电池。可从https://pubs.acs.org/doi/10.1021/acsenergylett.9b02832免费获得支持信息。

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