Rechargeable batteries, regarded as one of the most efficient energy storage technologies, have experienced tremendous research interest over the past decades.(1,2) The simple configuration with basic components of cathode, anode, electrolyte, separator, and current collectors makes them easy to manufacture and use, where the success of Li-ion batteries (LIBs) is the best example. However, most research work focused on the exploration of new electrode materials and new battery chemistries; the importance of the electrolyte was not recognized until it was found that ethylene carbonate (EC) is able to prevent the exfoliation of the graphite to render improved LIBs.(3) Similarly, ether electrolyte can cointercalate with Na⁺ into graphite to realize the storage of sodium into the graphite anode.(4) Recently, it has been widely accepted that the future development of battery technology will overwhelmingly depend on the further design and discovery of tailored electrolyte systems. In fact, the electrolyte, which is sandwiched between the highly oxidative cathode and highly reductive anode, plays a key role in transferring ionic species while insulating electronic conducting internally to ensure the normal operation of batteries. In addition to solid electrolytes, most electrolytes used today are in the liquid state because of the easy access with electrode materials and the low impedance across the electrode/electrolyte interface.(5) The typical electrolytes are usually formed by dissolving salts in polar solvents, where cations and anions of salts are dissociated by aqueous or nonaqueous solvents via solvation sheaths. To obtain a good electrolyte formulation, comprehensive parameters ranging from physical properties (wide liquidus range, low viscosity, high ionic conductivity, good thermal stability, low cost, environmentally benign, etc.), chemical characteristics (simple synthesis, inert toward inactive or active components, etc.), and electrochemical requirements (wide electrochemical stability window, thin and stable solid electrolyte interphase, etc.) should be taken into account. Thus, electrolytes significantly influence the overall performance of batteries including practical capacity, rate capability, cycling stability, intrinsic safety, and so on. Early efforts in optimization of electrolyte properties mainly lie in adjusting the electrolyte composition, such as varying the combinations of salts and solvents, mixing different types of solvents or salts, adopting diversified functional additives, and so on. However, the regulation of electrolyte concentration did not arouse tremendous interest because of the so-called “1 molarity (M) legacy” of nonaqueous electrolytes where the maximum of ionic conductivities almost always occurs near the salt concentration of 1 M for most systems. Several research works on electrolyte concentrations open a new field of research on electrolytes and pave the way for the further development of battery technologies. The magic of electrolyte concentration was perhaps first noticed by McKinnon and Dahn in 1985 who reported the cointercalation of propylene carbonate (PC) with Li⁺ into the ZrS₂ layered material can be circumvented with a saturated solution of LiAsF₆ in PC, which is not possible in the 1 M electrolyte.(6) The confinement of electrolyte concentration was first broken by Angell et al.(7) in 1993 who mixed lithium salts with small quantities of polymers and found the decreasing trend of ionic conductivity was reversed when increasing the salt concentration beyond a certain threshold. They defined the new ionic conductors as “polymer-in-salt” electrolytes. In 2003 Inaba et al.(8) examined the effects of electrolyte concentration on the interfacial reactions between graphite and PC-based solutions and demonstrated that the poor compatibility between graphite and PC could be improved without the need for a film-forming agent such as EC as long as the concentration of lithium salts is sufficiently high. In 2004 Chen et al.(9) mixed two solids of LiTFSI and acetamide to form a room-temperature molten salt which was later known as the concentrated electrolyte. The tested superior physicochemical properties indicate the potential application in lithium batteries. In 2010 Watanabe and co-workers(10) started to investigate the glyme-based superconcentrated electrolytes, which they regarded as a new family of room-temperature ionic liquids (later named solvated ionic liquids) because of the similar physicochemical properties. In 2013, Hu et al.(11) proposed that if either the weight or volume ratio of salt to solvent exceeds 1.0 then the new class of electrolyte can be denoted as “solvent-in-salt (SIS)” with unusual properties to distinguish it from traditional electrolytes. Meanwhile they demonstrated the benefits of this SIS electrolyte in a Li||S battery, including the low solubility of polysulfides, a high Li⁺ transference number, as well as the suppression of Li dendrite growth. This is the first demonstration that the superhigh concentrated electrolyte has a positive effect on stabilization of Li metal and opens a new way to improve the reversibility of Li anode and suppress the lithium dendrite growth. Afterward, this SIS concept and/or concentrated electrolytes became widely extended to many other battery chemistries, including but not limited to Li–O₂/air batteries, Li/Na/K-ion batteries, multivalent cation batteries, and aqueous batteries. Superior battery functions achieved through the SIS concept are well-summarized in recent review articles.(12−14) It is worth mentioning that in 2015 Xu, Wang, and co-workers(15) chose special solvent water and prepared a “water-in-salt (WIS)” electrolyte, expanding the narrow electrochemical stability window of water from 1.23 to 3.0 V in aqueous LIBs. Yamada et al.(16) soon independently reported a similar system termed “hydrate melt” to further improve the stability of water via employing a second salt at even higher concentrations in aqueous LIBs. Both works triggered interest in further expanding the electrochemical stability window of water.(17,18) Even though the superconcentrated electrolytes could bring tremendous benefits for battery performance, the high cost necessitated by the high salt concentration could be a disadvantage for practical applications. Zhang et al.(19) proposed “localized high concentration electrolyte,” maintaining the solvation structure of a superconcentrated electrolyte in the local environment while keeping the parent electrolyte in a dilute state. Very recently, Hu, Lu, and co-workers(20) found it is not necessary to employ a high-concentration electrolyte in every battery system; they employed “ultralow-concentration electrolyte” for Na-ion batteries to further reduce the cost and expand the operating temperature range. While it is not possible to discuss all the related research work in this Editorial, representative work on the electrolyte concentration is shown in Figure 1a.(21−25) Figure 1. (a) Representative research work on the electrolyte concentration for rechargeable batteries (Source: Web of Science, Clarivate Analytics, accessed 2020-10-16).(6−11,15,19−25) (b) Solvation and interfacial structures of superhigh and ultralow concentrated electrolytes. Besides the above-mentioned initial trials to tailor the electrolyte concentration from superhigh to ultralow, many efforts were also dedicated to unravel the mystery of electrolyte concentration. As part of the study, two particular structures, viz., solvation structure and interfacial structure, are deemed to influence the electrolyte properties as well as electrode/electrolyte interfaces that together can affect the battery performance (Figure 1b).(5,26−28) The solvation structure illustrates the interactions among the cations, anions, and solvent molecules, such as the cation–anion Coulomb interaction (Li⁺–X^–) and the ion–dipole interaction (Li⁺–solvent). The competition between these two interactions directly influences the existence of the solvates, mainly in the form of solvent-separated ion pairs (SSIPs), contact ion pairs (CIPs), and cation–anion aggregates (AGGs), which in turn defines a series of parameters such as viscosity, solubility, reactivity, ionic conductivity, ionic transport mode, and so on. The interfacial structure existing at the inner-Helmholtz layer describes the correlations between the electrolyte components and charged electrode surfaces, and hence, it determines the SEI.(26) At present, most of the research work is also focused on the origin of the interfacial chemistry involving the electrolyte from dilute to high concentration, switching from solvent molecules to anions. However, a claim has been made that an aqueous SEI does not necessarily require an anion to provide the chemistry source.(29) The interphase chemistry still remains an important area worthy of future investigations. The intensified investigations of electrolyte concentration have indeed helped to create new electrolyte systems and provided a deeper understanding of the fundamental science. The investigations will continue to advance our understanding of both electrolyte nature and interfacial electrochemistry. We believe this field will receive increasing attention in the coming decades as we expect unusual properties have yet to be discovered that will further improve the performance of batteries. Views expressed in this editorial are those of the authors and not necessarily the views of the ACS. The authors declare no competing financial interest. The authors thank Mr. Yuqi Li for drawing Figure 1. This article references 29 other publications.

中文翻译：

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电解液浓度之谜：从超高到超低

在过去的几十年中，可再充电电池被认为是最高效的能量存储技术之一，受到了极大的研究兴趣。（1,2）带有正极，负极，电解质，隔膜和集电器的基本组件的简单配置使其易于使用制造和使用方面，锂离子电池（LIB）的成功就是最好的例子。但是，大多数研究工作集中在新电极材料和新电池化学的探索上。直到发现碳酸亚乙酯（EC）能够防止石墨剥落以改善LIB为止，电解质的重要性才被认识。（3）同样，醚电解质可以与Na ⁺共嵌入（4）最近，已经广泛接受的是，电池技术的未来发展将绝大部分取决于定制电解质系统的进一步设计和发现。实际上，夹在高氧化性阴极和高还原性阳极之间的电解质在传递离子物质的同时，在内部对电子导电进行绝缘以确保电池正常工作时，起着关键作用。除固体电解质外，当今使用的大多数电解质还处于液态，这是由于易于与电极材料接触以及跨电极/电解质界面的阻抗低。（5）典型的电解质通常是通过将盐溶解在极性溶剂中形成的，其中盐的阳离子和阴离子通过溶剂化鞘被水性或非水性溶剂解离。为了获得良好的电解质配方，需要综合参数，包括物理性能（宽液相线范围，低粘度，高离子电导率，良好的热稳定性，低成本，对环境无害等），化学特性（简单合成，对惰性或活性呈惰性）组件等）和电化学要求（宽的电化学稳定性窗口，稀薄且稳定的固体电解质界面）等都应考虑在内。因此，电解质显着影响电池的整体性能，包括实际容量，倍率能力，循环稳定性，本质安全性等。优化电解质性能的早期努力主要在于调节电解质组成，例如改变盐和溶剂的组合，混合不同类型的溶剂或盐，采用多样化的功能性添加剂等。然而，由于非水电解质的所谓“ 1摩尔浓度（M）遗留”，对于大多数系统而言，离子电导率的最大值几乎总是在1 M的盐浓度附近发生，因此，电解质浓度的调节并未引起人们的极大兴趣。有关电解质浓度的多项研究工作为电解质的研究开辟了新领域，为电池技术的进一步发展铺平了道路。⁺可以用LiAsF ₆饱和溶液规避进入ZrS ₂层状材料在PC中，这在1 M电解质中是不可能的。（6）1993年，Angell等人（7）首先打破了电解质浓度的限制，他将锂盐与少量聚合物混合，发现离子的下降趋势当盐浓度增加到某个阈值以上时，电导率反转。他们将新的离子导体定义为“盐中聚合物”电解质。Inaba等人（8）在2003年研究了电解质浓度对石墨与PC基溶液之间界面反应的影响，并证明无需使用成膜剂即可改善石墨与PC之间的不良相容性。 EC只要锂盐浓度足够高即可。2004年Chen等。（9）将LiTFSI和乙酰胺的两种固体混合形成室温熔融盐，该熔融盐后来称为浓电解质。经测试的优异物理化学性能表明其在锂电池中的潜在应用。在2010年，Watanabe和同事（10）开始研究基于甘醇二甲醚的超浓缩电解质，由于其相似的理化性质，他们将其视为室温离子液体的新家族（后称溶剂化离子液体）。Hu等人（11）在2013年提出，如果盐与溶剂的重量或体积比超过1.0，则新的电解质类别可以称为“盐中溶剂（SIS）”，具有非同寻常的特性，可以区分它来自传统的电解质。同时，他们展示了这种SIS电解液在Li || S电池中的好处，⁺迁移数，以及抑制锂枝晶生长。这是首次证明超高浓度电解质对锂金属的稳定具有积极作用，并为改善锂阳极的可逆性和抑制锂枝晶的生长开辟了一条新途径。此后，这种SIS概念和/或浓电解质被广泛地扩展到许多其他电池化学中，包括但不限于Li–O ₂/空气电池，锂/钠/钾离子电池，多价阳离子电池和水性电池。通过SIS概念获得的卓越电池功能在最近的评论文章中得到了很好的总结。（12-14）值得一提的是，2015年，Xu，Wang和他的同事（15）选择了特殊的溶剂水，并准备了“盐（WIS）”电解质，将含水的LIB中水的狭窄电化学稳定性窗口从1.23 V扩展到3.0V。Yamada等人（16）很快独立地报道了一个类似的系统，称为“水合物熔体”，它通过在水性LIB中使用更高浓度的第二种盐进一步提高水的稳定性。两项工作都引起了人们对进一步扩大水的电化学稳定性窗口的兴趣。（17,18）尽管超浓缩电解质可以为电池性能带来巨大的好处，高盐浓度所必需的高成本对于实际应用可能是不利的。Zhang et al。（19）提出了“局部高浓度电解质”，在局部环境中保持超浓缩电解质的溶剂化结构，同时保持母体电解质处于稀释状态。最近，Hu，Lu和同事（20）发现没有必要在每个电池系统中使用高浓度电解液。他们为Na离子电池使用了“超低浓度电解液”，以进一步降低成本并扩大工作温度范围。虽然无法在本社论中讨论所有相关研究工作，但有关电解质浓度的代表性工作如图1a。（21-25）图1所示。（a）关于可充电电池的电解质浓度的代表性研究工作（资料来源：Web of Science，Clarivate Analytics，于2020-10-16访问）。（6-11,15,19-25）（b）溶剂化和界面结构超高和超低浓缩电解质。除了上述将电解质浓度从超高调整为超低的初步试验之外，还付出了许多努力来揭示电解质浓度的奥秘。作为研究的一部分，认为两种特殊的结构即溶剂化结构和界面结构会影响电解质的性能以及共同影响电池性能的电极/电解质界面（图1b）。（5,26- 28）溶剂化结构说明了阳离子，阴离子和溶剂分子之间的相互作用，⁺ –X ^–）和离子-偶极相互作用（Li ⁺-溶剂）。这两种相互作用之间的竞争直接影响溶剂化物的存在，主要形式是溶剂分离的离子对（SSIP），接触离子对（CIP）和阳离子-阴离子聚集体（AGG），这反过来又定义了一系列诸如粘度，溶解度，反应性，离子电导率，离子传输模式等参数。亥姆霍兹内层存在的界面结构描述了电解质成分与带电​​电极表面之间的相关性，因此决定了SEI。（26）目前，大多数研究工作都集中在界面的起源上。化学过程涉及电解质从稀到高浓度，从溶剂分子转变成阴离子。然而，有人声称，水性SEI不一定需要阴离子即可提供化学来源。（29）相间化学仍然是一个重要领域，值得未来研究。电解质浓度的深入研究确实有助于创建新的电解质系统，并提供了对基础科学的更深刻理解。这些研究将继续增进我们对电解质性质和界面电化学的理解。我们相信该领域将在未来几十年中得到越来越多的关注，因为我们预计尚未发现的异常特性将进一步改善电池的性能。本社论中表达的观点只是作者的观点，不一定是ACS的观点。作者宣称没有竞争性的经济利益。