New Strategy for Sodium-Ion Battery Electrolyte Design May Jumpstart Development

elemental map of lanthanum enlarge

The colorful pattern is an elemental map of lanthanum (La), showing the distribution of a self-formed secondary phase (Na3Zr2-xLax(PO4)2) from the initial solid electrolyte. A large area scan was obtained using 35 nm pixels, and the higher resolution image shown in the inset was taken using 10 nm pixels.

Rechargeable lithium-ion batteries are a part of modern-day living. They power our smartphones, laptops, and tablets. But for large-scale applications, such as electric vehicles and power grids, larger, longer lasting, and inexpensive batteries are required.

A promising contender is the solid-state sodium-ion (Na-ion) battery. It has several advantages. For one, sodium metal is abundant and cheap, which helps keep production costs down. Solid-state sodium-ion batteries have the potential for impressive energy densities, storing more charge per unit volume than today’s batteries. They also have a built-in safety feature: Because the electrolyte between the battery’s two electrodes (cathode and anode), through which the sodium-ions shuttle back and forth, is a solid material rather than a liquid, there is far less risk of leakage and combustion. Unfortunately, solid-state electrolytes have displayed lackluster conductivities, among other issues, and this has caused a bottleneck in the development of solid-state Na-ion batteries.

Recently, however, a research team that includes scientists from the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory and the Chinese Academy of Sciences (CAS) discovered a way to greatly enhance the performance of a common sodium-ion-based solid-state electrolyte. Scientists at CAS introduced lanthanum (La) ions into the electrolyte’s primary crystal structure, which caused secondary structures to self-form. The group discovered that these structures significantly increased the electrolyte’s conductivity and yielded a battery that was stable across 10,000 cycles. The investigation was done, in part, at Brookhaven’s National Synchrotron Light Source II (NSLS-II), a DOE Office of Science User Facility.

beamline 3-ID at NSLS-II enlarge

Delivering the capability to image nanostructures and chemical reactions down to nanometer resolution requires a new class of x-ray microscope, and the hard x-ray nanoprobe beamline 3-ID at NSLS-II is designed to deliver exactly these world-leading capabilities for x-ray imaging.

“The high spatial resolution and multi-elemental mapping capability of the hard x-ray nanoprobe beamline 3-ID at NSLS-II enabled us to carry out excellent characterization studies of this new solid electrolyte material for sodium batteries,” said Xiao-Qing Yang, a physicist at the Chemistry Division of Brookhaven Lab and the lead Brookhaven researcher on this study.

The electrolyte, a member of the “Na Super Ionic Conductor (NASICON)”-type electrolytes, is a mashup of individual tiny grains with diameters of just a couple hundred nanometers (nm). Each grain has a crystal structure, formed by sodium, zirconium (Zr), silicon, phosphorus (P), and oxygen (O). When the group decided to add lanthanum ions to the electrolyte structure, they predicted that the lanthanum ions might replace some of the zirconium atoms, nudging open the channels in the crystal a bit more so that sodium-ions could more readily pass through.

However, when they studied the lanthanum-doped electrolyte using x-rays in their home lab, they discovered that the lanthanum ions had formed a secondary crystal structure, Na3La(PO4)2, alongside the NASICON main phase. Additional phases, La2O3 and LaPO4, were also observed.

To study it further, the group used ultrahigh resolution x-ray imaging at NSLS-II’s beamline 3-ID – which can yield details down to 10 nm – as well scanning transmission electron microscopy (STEM) imaging, to investigate the grain-level “microstructure” and chemical structure of the composite electrolyte.

“Even though these phases are extremely small and hard to image, with our x-rays we were able to confirm their existence and also to provide a detailed image of them for a quantitative analysis, which would not have been possible with other tools,” said Hanfei Yan, a NSLS-II scientist at beamline 3-ID. 

Their investigations and analyses revealed that the co-existence of the multiple phases can modify the concentration of sodium-ions in the NASICON main phase. The extra structures also alter the chemical composition at the boundaries between grains. These new structural and chemical features produce a sharp increase in conductivity in the electrolyte as a whole, but notably at the boundaries, where conductivity is typically hindered as ions try to make the jump from one grain to the next.

This research is described in a paper published in the February 22, 2017 edition of Advanced Energy Materials.

'A Self-Forming Composite Electrolyte for Solid-State Sodium Battery with Ultralong Cycle Life', Z. Zhang, Q. Zhang, J. Shi, Y. S. Chu, X. Yu, K. Xu, M. Ge, H. Yan, W. Li, L. Gu, Y.S. Hu, H. Li, X. Q. Yang, L. Chen and X. Huang, Adv. Energy Mater. 2017, 7, 1601196, DOI: 10.1002/aenm.201601196

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