Avoiding Pitfalls in Rechargeable Aluminum Batteries Research
Avoiding Pitfalls in Rechargeable Aluminum Batteries Research
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DOI：
10.1021/acsenergylett.9b01285
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发表时间：
2019-08
影响因子：
22
通讯作者：
Jiayan Shi;Jian Zhang;Juchen Guo
中科院分区：
文献类型：
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作者：
Jiayan Shi;Jian Zhang;Juchen Guo
近年来，由于对锂电池以外的电池技术的追求，可充电铝（Al）电池已成为一个有吸引力的话题。作为电池阳极，铝具有优异的理论比容量（2980毫安时g−1）和体积容量（8040毫安时cm−3），尽管高压铝电池由于其相对较高的阳极电位而难以实现。典型的可充电铝电池由铝金属阳极、深共晶溶剂电解质和能与电解质中含铝物质发生可逆电化学反应的阴极组成。铝金属可能是唯一可行的阳极选择。因此，阳极的电化学反应是可逆的铝沉积-剥离。迄今为止，唯一能在室温下方便地使Al沉积剥离的电解质是由卤化铝（氯化铝AlCl3或溴化铝AlBr3）和相应的含有机阳离子（如咪唑、吡啶和铵）的卤化铝组成的深共晶溶剂。这种类型的电解质的发现归功于电沉积界对铝的电化学镀的追求。众所周知，只有AlCl3（或AlBr3）与有机卤化物的摩尔比大于1的刘易斯酸性电解质才能可逆地沉积和剥离铝。负责铝沉积的活性物质是刘易斯酸性氯铝酸盐阴离子Al2Cl7−。电解质中存在的另一个主要的氯铝酸盐阴离子是AlCl4−，它在Al沉积中不活跃。另一方面，已知它容易发生电化学氧化以产生氯（Cl2）。AlCl4 -也被报道能够嵌入石墨碳层中，作为可充电铝电池的正极材料引起了人们的极大关注。虽然已经有一些研究报道了新的铝电解质系统，但由于深共晶电解质的成熟度（或缺乏替代品），目前的铝电池研究主要集中在正极材料上。在配套资料中比较了几种具有代表性的可充电铝电池正极材料的性能。深共晶氯铝酸盐电解质在阴极材料研究中起着至关重要的作用，但了解其使用所引起的干扰甚至误导结果也非常重要。氯铝酸盐电解质的问题源于其固有特性：低阳极稳定性，即在充电时产生氯或其他活性副产物，以及电化学增强的腐蚀性。调查人员必须仔细设计和执行实验，并严格解释数据，以获得真实的结果。在接下来的章节中，我们将讨论在可充电铝电池的研究中可能被忽视的一些陷阱，如图1所示。电流收集器的选择。选择合适的阴极集流器至关重要，因为所研究的大多数金属在深共晶氯铝酸盐电解质中会被（电化学）腐蚀，并且腐蚀电流可能被误认为是电池反应产生的电流，正如Reed和Menke在不锈钢的案例中所证明的那样。为了证明合适的电流集电极的重要性，在AlCl3和1-乙基-3甲基咪唑氯（[EMIm]Cl）摩尔比为1.3:1的代表性深共晶电解质中，分析了一些导电衬底的电化学性能。将超纯无水AlCl3与[EMIm]Cl混合制备电解质，并在充满氩气的手套箱中高温真空干燥24 h。用彻底抛光的铝箔作为阳极，用一块浸泡过电解液的干棉絮作为分离器，阴极是待测的裸集流器。为了消除潜在的干扰，使用了定制的swagelak型电池，其主体为聚醚醚酮（PEEK），电极为玻璃碳（GC）棒。我们没有选择聚四氟乙烯（PTFE）作为胞体材料，因为我们之前发现，氟化聚合物在深共晶氯铝酸盐电解质中可能不具有电化学惰性。整个世伟洛克电池不包含与电解液接触的金属部件。在辅助资料中可以找到世伟洛克型细胞的图纸和数字图像以及实验细节。图2显示了集流器的恒流还原和氧化，分别表示为放电和充电，与电池研究一致。给出了相应的循环伏安法（CV）分析
Batteries Research Rechargeable aluminum (Al) batteries have become an attractive topic in recent years driven by the quest for battery technologies beyond lithium. Al has excellent theoretical specific capacity (2980 mAh g−1) and volumetric capacity (8040 mAh cm−3) as a battery anode, although highvoltage Al batteries can be difficult to achieve due to its relatively high anodic potential. A typical rechargeable Al battery is composed of an Al metal anode, a deep eutectic solvent electrolyte, and a cathode capable of reversible electrochemical reaction with the Al-containing species in the electrolyte. Al metal is likely the only viable choice as the anode. Therefore, the electrochemical reaction at the anode is reversible Al deposition−stripping. To date, the only (without ambiguity) electrolytes that can facilely enable Al deposition− stripping at room temperature are deep eutectic solvents composed of aluminum halides (aluminum chloride AlCl3 or aluminum bromide AlBr3) and the corresponding halides with organic cations such as imidazolium, pyridinium, and ammonium. The discovery of this type of electrolyte is attributed to the electrodeposition community in the pursuit of the electrochemical plating of Al. It is well-known that only a Lewis acidic electrolyte, in which the molar ratio between AlCl3 (or AlBr3) and organic halide is higher than 1, can reversibly deposit and strip Al. The active species responsible for Al deposition is the Lewis acidic chloroaluminate anion Al2Cl7 −. The other major chloroaluminate anion existing in the electrolyte is AlCl4 −, which is not active in Al deposition. On the other hand, it is known to be prone to electrochemical oxidation to evolve chlorine (Cl2). 3 AlCl4 − also is reported to be capable of intercalating into the layers of graphitic carbons, which have attracted significant attention as the cathode material in rechargeable Al batteries. Although there have been a few studies reporting new Al electrolyte systems, due to the readiness of (or a lack of alternatives to) the deep eutectic electrolytes, current Al battery investigations are mainly focused on cathode materials. The performance of some representative cathode materials for rechargeable Al batteries is compared in the Supporting Information. The deep eutectic chloroaluminate electrolytes play an essential role in cathode material investigations, but it is also extremely important to understand the interference and even misleading results caused by their use. The problems of the chloroaluminate electrolytes originate from their intrinsic properties: low anodic stability, i.e., generation of chlorine or other active side products during charge, and electrochemically enhanced corrosivity. Investigators must carefully design and execute the experiments as well as rigorously interpret the data to obtain the true results. In the following sections, we discuss some pitfalls that may be overlooked in the research on rechargeable Al batteries, as illustrated in Figure 1. Selection of Current Collectors. Selecting suitable cathode current collectors is of fundamental importance because most of the studied metals can be (electrochemically) corroded in deep eutectic chloroaluminate electrolytes, and the corrosive current can be mistaken as the current from battery reactions, as demonstrated by Reed and Menke in the case of stainless steel. To demonstrate the importance of a suitable current collector, the electrochemical properties of a number of conductive substrates were analyzed in a representative deep eutectic electrolyte composed of AlCl3 and 1-ethyl-3methylimidazolium chloride ([EMIm]Cl) with a molar ratio of 1.3:1. The electrolyte was prepared by mixing ultrapure anhydrous AlCl3 with [EMIm]Cl, which was first dried at elevated temperature under vacuum for 24 h in an argon-filled glovebox. A thoroughly polished Al foil was used as the anode, a piece of dried cotton wool soaked with the electrolyte was used as the separator, and the cathode was the bare current collector being tested. To eliminate potential interference, customized Swagelok-type cells with polyether ether ketone (PEEK) body and glassy carbon (GC) rod electrodes were used. Polytetrafluoroethylene (PTFE) was not selected for the cell body due to our previous finding that fluorinated polymers may not be electrochemically inert in the deep eutectic chloroaluminate electrolyte. The entire Swagelok cell contained no metal parts in contact with the electrolyte. A drawing and digital image of the Swagelok-type cell and the experimental details can be found in the Supporting Information. Figure 2 shows the galvanostatic reduction and oxidation of the current collectors, denoted as discharge and charge, respectively, to be consistent with battery study. The corresponding cyclic voltammetry (CV) analysis are shown