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Summer 2016

Packing in the Lithium

New class of longer-lasting battery electrodes proven possible

Kimberly Lundberg

Researchers calculated the possible lithium-ion pathways through the new cathode material. The calculations show that the open nature of the framework allows lithium ions to travel unimpeded through the cathode. This is an imperative feature for battery function. The top panel shows the lithium pathways, and the bottom panel shows the location of these pathways in the material. Copyright 2016, American Chemical Society

Researchers across America are searching for the next big breakthrough towards safer, longer-lasting, and more durable batteries. Next-generation batteries will drive our electric cars farther and run our personal electronics longer. Research scientists dedicate their expertise to improving one part of a battery, so when all is said and done, every aspect of a battery will be optimized. This idealized battery has yet to be conceived.

In today’s popular lithium-ion battery, the amount of lithium the battery contains directly affects how long it can operate on a single charge (known as its capacity), such that more lithium equals more texts or highway miles. In a lithium-ion battery, a cathode works as the positive electrode and stores lithium ions within its structure. Metal atoms within the cathode framework interact with lithium ions and control how many ions the material can store. Limited cathode capacity remains a major hurdle in improving batteries. Therefore, researchers at the NorthEast Center for Chemical Energy Storage (NECCES), an Energy Frontier Research Center, are dedicated to improving batteries by developing cathodes that accommodate more lithium ions than previous technologies.

New battery cathodes have been developed by researchers at NECCES. The cathodes store two lithium ions per metal center instead of the usual one. If two lithium ions interact with a metal atom instead of just one, the cathode should have twice as much capacity. The math is easy enough but designing such a cathode is a difficult endeavor.

NECCES researchers first designed a layered molybdenum-containing material, δ-(MoO2)2P2O7, which includes two important features:

  • Two lithium ions are incorporated for every molybdenum metal center so the battery can power electronics longer; and
  • The phosphate component (P2O7) helps stabilize the structure so the battery has a longer lifetime (think: how many times a battery can be charged before losing performance).

Researchers used X-ray diffraction to observe structural changes in the material as lithium ions were inserted and removed. When researchers inserted the second set of lithium ions, they noticed the orderliness of the material’s structure began to deteriorate, like a building suddenly no longer having well-defined floors or walls. Structural degradation led to lower-than-predicted capacities for the material. Consequentially, the cathode displayed a long lifetime only when the second lithium ion wasn’t inserted. Researchers learned that molybdenum-based materials can work as cathodes but also recognized a better candidate was needed.

In 2016, NECCES researchers developed a new molybdenum- and phosphate-containing cathode, Li3Mo4P5O24, which features an open 3D framework and contains four types of molybdenum metal centers. Each type of molybdenum should interact with two lithium ions. They chose the 3D framework to allow unimpeded movement of lithium ions in and out of the cathode, which is mandatory for the battery to work smoothly. As hoped, and unlike the previous material, this cathode didn’t structurally degrade as the second lithium ion was inserted.

To explore how lithium ions traverse the framework, the NECCES scientists calculated which pathways were sizable enough to accommodate lithium ion movement, just as a semi-truck driver only uses routes with underpasses taller than his trailer. As you can see in the image, there are many allowed pathways (marked in yellow) by which lithium ions can travel through the material. This complex and mostly continuous network allows lithium ions to access the molybdenum atoms and percolate through the material unencumbered.

To prove each type of molybdenum can interact with lithium ions, NECCES researchers turned to an experimental method called cyclic voltammetry (CV). CV counts how many electrons are transferred at certain energies. Lithium-ion batteries work by transferring one electron for every lithium ion that interacts with the cathode. Therefore, researchers can indirectly count lithium ions by counting electrons. A large number of electrons measured at a certain energy is a telltale sign that lithium ions are moving into the cathode and finding a molybdenum partner. This produces a peak in an otherwise flat CV plot. Researchers observed seven distinct peak pairs in the CV. This indicates that each type of molybdenum can interact with and store at least one lithium ion. The seven peaks reveal that three out of four molybdenums can store two lithium ions.

With this study, the NECCES scientists have developed a new and promising cathode material for lithium-ion batteries. The cathode features an open framework that facilitates up to two molybdenum-lithium ion interactions per metal center. Although the measured capacity of this material is slightly lower than the theoretical capacity, NECCES scientists are optimistic that it can be increased by improving their synthesis methods.

Acknowledgments

Wen et al. 2016. This work was supported as part of the NorthEast Center for Chemical Energy Storage, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. Also acknowledged is the National Science Foundation Chemistry Research Instrumentation and Facilities for synchrotron X-ray diffraction support. Use of the Advanced Photon Source at Argonne National Lab was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.

Wen et al. 2013. This research is supported as part of the NorthEast Center for Chemical Energy Storage, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. Use of the National Synchrotron Light Source at Brookhaven National Lab is supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.

More Information

Wen B, J Liu, NA Chernova, X Wang, Y Janssen, F Omenya, PG Khalifah, and MS Whittingham. 2016. “Li3Mo4P5O24: A Two-Electron Cathode for Lithium-Ion Batteries with Three-Dimensional Diffusion Pathways.” Chemistry of Materials 28:2229-2235. DOI: 10.1021/acs.chemmater.6b00177

Wen B, NA Chernova, R Zhang, Q Wang, F Omenya, J Fang, and MS Whittingham. 2013. “Layered Molybdenum (Oxy)Pyrophosphate as Cathode for Lithium-Ion Batteries.” Chemistry of Materials 25:3513-3521. DOI: 10.1021/cm401946h

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Energy Density Comes from Storing More Lithium

New material packs in twice the lithium, offers insights into creating higher-capacity batteries

Scientists calculated possible routes a lithium ion could take through a new electrode material with an open framework. Left: lithium paths; right: the location of these paths in the material. Image credit: Cortland Johnson, Pacific Northwest National Lab (modified from American Chemical Society, copyright 2016)

Everything from driving to grandma’s house to calling your kids would be easier if batteries held more energy and lasted longer between charges. Creating such lithium-ion batteries means improving the cathode, the positively charged electrode. Scientists devised a new, promising material. It is predicted to hold a charge longer than materials used in today’s lithium-ion batteries. Its composition lets it store more lithium, which equates to higher capacity. The team is now working to boost the performance of this molybdenum-and-phosphate-based material. One day, this material could change our expectations of a battery’s capacity. The research was done at the NorthEast Center for Chemical Energy Storage (NECCES), an Energy Frontier Research Center led by Binghamton University.

More Information

Wen B, J Liu, NA Chernova, X Wang, Y Janssen, F Omenya, PG Khalifah, and MS Whittingham. 2016. “Li3Mo4P5O24: A Two-Electron Cathode for Lithium-Ion Batteries with Three-Dimensional Diffusion Pathways.” Chemistry of Materials 28:2229-2235. DOI: 10.1021/acs.chemmater.6b00177

Wen B, NA Chernova, R Zhang, Q Wang, F Omenya, J Fang, and MS Whittingham. 2013. “Layered Molybdenum (Oxy)Pyrophosphate as Cathode for Lithium-Ion Batteries.” Chemistry of Materials 25:3513-3521. DOI: 10.1021/cm401946h

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.