4G光元：伯克利实验室领导的研究揭示了先前认为相容的电池组中令人惊讶的化学反应。以镁而不是锂为基础的可充电电池有可能通过将更多的能量填充到更小的电池中来扩展电动汽车的范围。但是不可预见的化学障碍减缓了科学的发展。

固体接触液体的地方——带相反电荷的电池电极与周围称为电解质的化学混合物相互作用的地方——是已知的问题点。

现在，在能源储存能量的联合研究中心美国部的一个研究小组，由劳伦斯伯克利国家实验室的科学家（伯克利实验室），发现了一个奇怪的化学反应涉及即使在电池充电情况下镁可以降低电池性能。

这些发现可能对其他电池材料有关，并能引导下一代电池的办法，避免对这些新发现的缺陷设计。

研究小组通过X射线实验、理论建模和超级计算机模拟，充分了解了电极表面几十纳米的液态电解质的化学破坏，从而降低了电池性能。他们的发现发表在了《材料化学》杂志的网络版上。

他们的电池测试功能金属镁作为负电极（阳极）与一个由液体电解质接触（一种溶剂称为）和溶解盐、Mg（TFSI）2。

而他们使用的材料的组合被认为在电池的静息状态兼容，无反应，在伯克利实验室的先进光源（ALS）实验中，X射线源被称为同步，发现这是没有的情况下，新的方向，引导了这项研究。

“人们以为电池的充电过程中发生的这些材料的问题，而是实验表明，已经有一些活动，”David Prendergast说，是纳米材料设备的理论在分子铸造作为该研究的一位领导人。

“在那一点上，它变得非常有趣，”他说。在这些条件下，什么物质可能会导致这些反应稳定？“

这些分子模型显示了电池化学的初始状态，导致镁电池（mg）阳极的不稳定。

分子铸造研究人员对电极和电解质相遇的细节进行了详细的模拟，称为界面，表明在理想条件下也不会发生自发化学反应。然而，这些模拟并没有解释所有的化学细节。

“我们的调查之前，”Ethan Crumlin说，与加斯特ALS科学家协调X射线实验和共同领导了这项研究，“有怀疑这些材料和可能的连接性能导致电池性能差，但他们没有在一个工作组确认。”

市面流行的锂离子电池，其中许多便携式电子设备（如手机，笔记本电脑，和电动工具）和 运输发展电动汽车、飞机的锂离子–锂原子脱落的一个电子成为–来回两电池电极之间的电荷。这些电极材料在原子尺度上是多孔的，当电池充电或放电时，它们交替地装载或排空锂离子。

在这种类型的电池中，负极通常由碳组成，其储存锂离子的能力比其他材料要有限。

因此，通过使用另一种材料来增加储存锂的密度，将有助于制造更轻、更小、更强大的电池。例如，在电极中使用金属锂，可以在同一空间容纳更多的锂离子，尽管它是一种高度敏感性物质，在暴露在空气中时会燃烧，需要进一步研究如何最好地包装和保护它以长期稳定。

镁金属比锂金属具有更高的能量密度，这意味着如果使用镁而不是锂，则可以在同样大小的电池中储存更多的能量。

镁比锂更稳定。当它与空气中的湿气和氧气反应时，它的表面形成了一个自我保护的“氧化层”。但在电池中，这种氧化层被认为会降低效率和缩短电池寿命，所以研究人员正在寻找避免其形成的方法。

更详细地探讨这一层的形成，团队在ALS最近开发了一种独特的X射线技术，称为apxps（常压X射线光电子能谱）。这项新技术对固体和液体界面发生的化学反应非常敏感，这使得它成为探索电池表面化学反应的理想工具，它能满足液体电解质的要求。

模拟结果表明，由于自由浮动氢氧根离子的存在，在液体溶剂中的键减弱，其中含有一个与氢原子结合的氧原子。在这个例子中，原子被颜色编码：氢（白色），氧（红色），碳（浅蓝色），镁（绿色），氮（深蓝），硫（黄色），氟（棕色）。这个过程会降低电池性能。

甚至在电流进入测试电池之前，X射线结果显示电解液的化学分解的迹象，特别是在镁电极的界面上。研究结果迫使研究人员重新思考这些材料的分子尺度图以及它们如何相互作用。

他们确定的是，在镁表面形成的自稳定薄氧化层存在缺陷和杂质，从而产生不必要的反应。

“这不是金属本身，或其氧化物，这是一个问题，”普伦德加斯特说。 “事实上，你可以在氧化表面存在缺陷。这些小的差异成为反应的场所。它以这种方式喂养自己。“

提出氧化镁表面可能的缺陷的另外一轮模拟表明，阳极的氧化表面层中的缺陷可以暴露镁离子，然后镁离子充当电解质分子的陷阱。

如果自由浮动的氢氧根离子 – 含有与氢原子结合的单个氧原子的分子可以形成为微量的水与镁金属反应 – 满足这些表面结合的分子，它们将反应。

这会浪费电解液，随着时间的推移使电池干燥。而这些反应的产物会污染阳极的表面，损害了电池的功能。

在团队的实验和理论成员之间来回进行了几次迭代，以开发与X射线测量一致的模型。实验室的国家能源研究科学计算中心的努力得到数百万小时的计算能力的支持。

研究人员指出，在同一个实验室获得X光技术，纳米技术专长和计算资源的重要性。

结果也可能与其他类型的电池材料相关，包括基于锂或铝金属的原型。普伦德加斯特说：“这可能是定义电解液稳定性的更为普遍的现象。”

Crumlin补充说：“我们已经开始运行新的模拟，可以向我们展示如何修改电解液以减少这些反应的不稳定性。”同样，他也表示，可以调整镁的表面以减少或消除一些不想要的化学反应性。

“而不是允许它创建自己的界面，您可以自己构建它来控制和稳定界面化学。”他补充说。 “现在它导致不可控制的事件。”

伯克利实验室的先进光源，分子铸造和国家能源研究科学计算中心是美国能源部科学用户设施办公室，面向来自全国和世界各地的访问研究人员。

伯克利实验室和新西兰桑迪亚国家实验室能源储存研究联合研究中心的研究人员与马里兰大学的科学家和中国上海微系统与信息技术研究所合作组建了该研究小组。该研究得到美国能源部基础能源科学办公室的支持。

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劳伦斯伯克利国家实验室通过推进可持续能源、保护人类健康、创造新材料、揭示宇宙的起源和命运，来应对世界上最紧迫的科学挑战。伯克利实验室于1931成立，其科学专长已获13项诺贝尔奖。加利福尼亚大学为美国能源部的科学办公室管理伯克利实验室。更多信息，访问www.lbl.gov。

美国能源部的科学办公室是美国物理科学基础研究的最大的支持者，正在努力解决我们这个时代最紧迫的一些挑战。有关更多信息，请访问science.energy.gov。

NEWS CENTER

Scientists Solve a Magnesium Mystery in Rechargeable Battery Performance

Berkeley Lab-led study reveals surprising chemical reactivity in battery components previously considered compatible

News Release Glenn Roberts Jr. (510) 486-5582 • OCTOBER 19, 2017

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Rechargeable batteries based on magnesium, rather than lithium, have the potential to extend electric vehicle range by packing more energy into smaller batteries. But unforeseen chemical roadblocks have slowed scientific progress.

And the places where solid meets liquid – where the oppositely charged battery electrodes interact with the surrounding chemical mixture known as the electrolyte – are the known problem spots.

Now, a research team at the U.S. Department of Energy’s Joint Center for Energy Storage Research, led by scientists at Lawrence Berkeley National Laboratory (Berkeley Lab), has discovered a surprising set of chemical reactions involving magnesium that degrade battery performance even before the battery can be charged up.

The findings could be relevant to other battery materials, and could steer the design of next-generation batteries toward workarounds that avoid these newly identified pitfalls.

The team used X-ray experiments, theoretical modeling, and supercomputer simulations to develop a full understanding of the chemical breakdown of a liquid electrolyte occurring within tens of nanometers of an electrode surface that degrades battery performance. Their findings are published online in the journal Chemistry of Materials.

The battery they were testing featured magnesium metal as its negative electrode (the anode) in contact with an electrolyte composed of a liquid (a type of solvent known as diglyme) and a dissolved salt, Mg(TFSI)2.

While the combination of materials they used were believed to be compatible and nonreactive in the battery’s resting state, experiments at Berkeley Lab’s Advanced Light Source (ALS), an X-ray source called a synchrotron, uncovered that this is not the case and led the study in new directions.

“People had thought the problems with these materials occurred during the battery’s charging, but instead the experiments indicated that there was already some activity,” said David Prendergast, who directs the Theory of Nanostructured Materials Facility at the Molecular Foundry and served as one of the study’s leaders.

“At that point it got very interesting,” he said. “What could possibly cause these reactions between substances that are supposed to be stable under these conditions?”

These molecular models show the initial state of battery chemistry that leads to instability in a test cell featuring a magnesium (Mg) anode. (Credit: Berkeley Lab)

Molecular Foundry researchers developed detailed simulations of the point where the electrode and electrolyte meet, known as the interface, indicating that no spontaneous chemical reactions should occur under ideal conditions, either. The simulations, though, did not account for all of the chemical details.

“Prior to our investigations,” said Ethan Crumlin, an ALS scientist who coordinated the X-ray experiments and co-led the study with Prendergast, “there were suspicions about the behavior of these materials and possible connections to poor battery performance, but they hadn’t been confirmed in a working battery.”

Commercially popular lithium-ion batteries, which power many portable electronic devices (such as mobile phones, laptops, and power tools) and a growing fleet of electric vehicles, shuttle lithium ions – lithium atoms that become charged by shedding an electron – back and forth between the two battery electrodes. These electrode materials are porous at the atomic scale and are alternatively loaded up or emptied of lithium ions as the battery is charged or discharged.

In this type of battery, the negative electrode is typically composed of carbon, which has a more limited capacity for storing these lithium ions than other materials would.

So increasing the density of stored lithium by using another material would make for lighter, smaller, more powerful batteries. Using lithium metal in the electrode, for example, can pack in more lithium ions in the same space, though it is a highly reactive substance that burns when exposed to air, and requires further research on how to best package and protect it for long-term stability.

Magnesium metal has a higher energy density than lithium metal, meaning you can potentially store more energy in a battery of the same size if you use magnesium rather than lithium.

Magnesium is also more stable than lithium. Its surface forms a self-protecting “oxidized” layer as it reacts with moisture and oxygen in the air. But within a battery, this oxidized layer is believed to reduce efficiency and shorten battery life, so researchers are looking for ways to avoid its formation.

To explore the formation of this layer in more detail, the team employed a unique X-ray technique developed recently at the ALS, called APXPS (ambient pressure X-ray photoelectron spectroscopy). This new technique is sensitive to the chemistry occurring at the interface of a solid and liquid, which makes it an ideal tool to explore battery chemistry at the surface of the electrode, where it meets the liquid electrolyte.

Simulations show the weakening of a bond in a liquid solvent due to the presence of free-floating hydroxide ions, which contain a single oxygen atom bound to a hydrogen atom. In this illustration, atoms are color-coded: hydrogen (white), oxygen (red), carbon (light blue), magnesium (green), nitrogen (dark blue), sulfur (yellow), fluorine (brown). This process degrades battery performance. (Credit: Berkeley Lab)

Even before a current was fed into the test battery, the X-ray results showed signs of chemical decomposition of the electrolyte, specifically at the interface of the magnesium electrode. The findings forced researchers to rethink their molecular-scale picture of these materials and how they interact.

What they determined is that the self-stabilizing, thin oxide surface layer that forms on the magnesium has defects and impurities that drive unwanted reactions.

“It’s not the metal itself, or its oxides, that are a problem,” Prendergast said. “It’s the fact you can have imperfections in the oxidized surface. These little disparities become sites for reactions. It feeds itself in this way.”

A further round of simulations, which proposed possible defects in the oxidized magnesium surface, showed that defects in the oxidized surface layer of the anode can expose magnesium ions that then act as traps for the electrolyte’s molecules.

If free-floating hydroxide ions – molecules containing a single oxygen atom bound to a hydrogen atom that can be formed as trace amounts of water react with the magnesium metal – meet these surface-bound molecules, they will react.

This wastes electrolyte, drying out the battery over time. And the products of these reactions foul the anode’s surface, impairing the battery’s function.

It took several iterations back and forth, between the experimental and theoretical members of the team, to develop a model consistent with the X-ray measurements. The efforts were supported by millions of hours’ worth of computing power at the Lab’s National Energy Research Scientific Computing Center.

Researchers noted the importance of having access to X-ray techniques, nanoscale expertise, and computing resources at the same Lab.

The results could be relevant to other types of battery materials, too, including prototypes based on lithium or aluminum metal. Prendergast said, “This could be a more general phenomenon defining electrolyte stability.”

Crumlin added, “We’ve already started running new simulations that could show us how to modify the electrolyte to reduce the instability of these reactions.” Likewise, he said, it may be possible to tailor the surface of the magnesium to reduce or eliminate some of the unwanted chemical reactivity.

“Rather than allowing it to create its own interface, you could construct it yourself to control and stabilize the interface chemistry,” he added. “Right now it leads to uncontrollable events.”

Berkeley Lab’s Advanced Light Source, Molecular Foundry, and National Energy Research Scientific Computing Center are DOE Office of Science User Facilities that are open to visiting researchers from around the nation and world.

Researchers from the Joint Center for Energy Storage Research at Berkeley Lab and Sandia National Laboratories in New Mexico comprised the team, together with scientists from the University of Maryland, and from the Shanghai Institute of Microsystem and Information Technology in China. The research was supported by the U.S. Department of Energy’s Office of Basic Energy Sciences.

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Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel Prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more, visit www.lbl.gov.

DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.