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Two Studies Reveal Details of Lithium-Battery Function Print
Wednesday, 27 February 2013 00:00

Our way of life is deeply intertwined with battery technologies that have enabled a mobile revolution powering cell phones, laptops, medical devices, and cars. As conventional lithium-ion batteries approach their theoretical energy-storage limits, new technologies are emerging to address the long-term energy-storage improvements needed for mobile systems, electric vehicles in particular. Battery performance depends on the dynamics of evolving electronic and chemical states that, despite advances in material synthesis and structural probes, remain elusive and largely unexplored. At Beamlines 8.0.1 and 9.3.2, researchers studied lithium-ion and lithium-air batteries, respectively, using soft x-ray spectroscopy techniques. The detailed information they obtained about the evolution of electronic and chemical states will be indispensable for understanding and optimizing better battery materials.

A Battery of Tests
for Better Batteries

The prosaic battery has often been overlooked as little more than an afterthought in a consumer-driven society quickly fascinated by the latest electronic gadget. But that is now changing as the development of safe, low-cost, high-efficiency, sustainable energy-storage devices is now recognized as one of the key challenges for green-energy applications from plug-in electric vehicles to the electric grid, with projected market expansion of ten times this decade. Electrochemical batteries provide one of the most promising high-efficiency solutions for electrical energy storage. However, at present, the technology level is well below what is required for this new phase of large-scale energy storage.

The formidable challenge in developing high-performance electrochemical systems for energy storage stems from the complexity of the device operations. Our lack of understanding of the battery materials' physical evolution and chemical pathways during the charging and discharging processes hinders speedy optimization and development. At the ALS, numerous international collaborations utilize our unique soft x-ray spectroscopy capabilities to tackle fundamental questions that are directly related to practical performance. Here, we highlight two specific studies: one involving lithium-ion batteries and another on lithium-air batteries.

When charging a lithium-ion battery, lithium ions must be extracted from the cathode material (delithiation) by an external voltage source and inserted (intercalated) into an anode material (lithiation). The same process occurs in reverse when discharging. Cathode materials are key to improved performance, partially because there is not yet a candidate that can maintain high power and stable cycling with a capacity comparable to that of anode materials.

In the search for safe, high-performance cathode materials, perhaps the most striking discovery is lithium iron phosphate (LiFePO4 or simply LFP). LFP is a natural mineral of the olivine family and a surprisingly good cathode material despite some unfavorable properties. Delithiation in LFP is nominally a two-phase process, with a higher energy barrier than a single-phase transformation. It has structural peculiarities that affect its conductivity. It is also believed to have one-dimensional lithium diffusion channels, prone to impurity obstacles. Therefore, LFP's impressive high-rate performance challenges our conventional wisdom on understanding, choosing, and developing cathode materials.

Crystal structure of LFP. Lithium ions (green) diffuse into and out of the olivine framework through a one-dimensional channel.

To better understand LFP, researchers combined x-ray absorption spectroscopy (XAS) at Beamline 8.0.1, theoretical calculations, and material synthesis to tackle the subtle evolution of the key electronic states in LFP cathodes under different lithiation levels in nanoparticles and single crystals. The sensitivity and high resolution of XAS provide spectroscopic "fingerprints" of the lithiation process, with abundant information on phase transformation, valence, spin states, and local structural distortions. Thus, this technique provides systematic and in-depth information on the interplay between lithiation and electronic structure evolution, shedding light on the phase transformation and lithium diffusion mechanism in LFP cathodes.

Reconstructed soft x-ray absorption spectroscopy data can provide a "fingerprint" of candidate materials for lithium-ion battery components. Red and purple spectra correspond to fully lithiated and delithiated states of LFP, respectively. The graph provides abundant information on the two-phase transformation (crossing point at center), the evolution of electronic states, and a quantitative definition of the energy levels of the Fe 3d states, which is important for understanding the puzzling charge-discharge behavior of this cathode material.

By taking oxygen from the air rather than incorporating an internal oxidizer, lithium-air batteries can have up to four times the energy density of conventional lithium-ion batteries by weight. However, they cannot charge/discharge as efficiently or as many times (less than 100 cycles), and there are still many questions surrounding reaction mechanisms during oxygen reduction and evolution. Attempts to unravel these questions are complicated by equipment requiring ultrahigh-vacuum (UHV) conditions that allow only ex situ characterization and by the use of liquid electrolytes that place electrodes in contact with many other chemical species and potentially cause parasitic reactions.

At Beamline 9.3.2, the researchers overcame these challenges with ambient-pressure soft x-ray photoelectron spectroscopy (APXPS) on a solid-state lithium-air cell. The cathode (the electrode of primary interest) was a mixed ionic and electronic conducting material, allowing lithium ions, electrons, and oxygen to converge over the entire electrode surface. With this apparatus, the researchers were able to examine the chemistry of lithum-oxygen reaction products in situ as a function of applied voltage in both UHV and in 380-mTorr oxygen.

Left: The solid-state lithium-oxygen cell consisted of a conductive, platinum-coated alumina substrate, a negative anode made of lithiated lithium titanate (LLTO) encased in a solid electrolyte of lithium phosphorus oxynitride (LiPON), and a positive cathode made of LixV2O5. Right: Graph of cell voltage vs capacity with APXPS O 1s and V 2p spectra at 380-mTorr oxygen at both full charge (high cell voltage) and discharge (low cell voltage).

The study provides the first evidence of reversible lithium peroxide (Li2O2) formation and decomposition on an oxide surface and lays the foundation for the characterization of reaction mechanisms for both conventional lithium-ion and lithium-air batteries using in situ APXPS. Future work will take advantage of this cell design to understand the influence of other gas environments and electrode materials on intercalation and reaction-product formation, providing fundamental insights improving energy-storage technologies.

 


 

Research conducted by: X.S. Liu, Y.D. Chuang, Z. Liu, Z. Hussain, and W.L. Yang (ALS); J. Liu, X.Y. Song, G. Liu, T.J. Richardson, and D. Prendergast (Berkeley Lab); R.M. Qiao (ALS and Shandong University, China); Y. Yu (Max Planck Institute for Solid State Research, Germany); H. Li, L.M. Suo, and Y.S. Hu (Chinese Academy of Sciences); G.J. Shu and F.C. Chou (National Taiwan University); T.C. Weng, D. Nordlund, and D. Sokaras (SLAC National Accelerator Laboratory); Y.J. Wang, H. Lin, B. Barbiellini, and A. Bansil (Northeastern University); S.S. Yan (Shandong University, China); S. Qiao (Fudan University, China); F.M.F. de Groot (Utrecht University, The Netherlands); Y.-C. Lu, E.J. Crumlin, J.R. Harding, E. Mutoro, and Y. Shao-Horn (Massachusetts Institute of Technology); and G.M. Veith, L. Baggetto, and N.J. Dudney, (Oak Ridge National Laboratory).

Research funding: Laboratory Directed Research and Development Program, Berkeley Lab; National Science Foundation; and U.S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy. Operation of the ALS is supported by the DOE Office of Basic Energy Sciences.

Publications about this research: X.S. Liu, J. Liu, R.M. Qiao, Y. Yu, H. Li, L.M. Suo, Y.S. Hu, Y.D. Chuang, G.J. Shu, F.C. Chou, T.C. Weng, D. Nordlund, D. Sokaras, Y.J. Wang, H. Lin, B. Barbiellini, A. Bansil, X.Y. Song, Z. Liu, S.S. Yan, G. Liu, S. Qiao, T.J. Richardson, D. Prendergast, Z. Hussain, F.M.F. de Groot, and W.L. Yang, "Phase transformation and lithiation effect on electronic structure of LixFePO4: An in-depth study by soft x-ray and simulations," J. Am. Chem. Soc. 134, 13708 (2012); and Y.-C. Lu, E.J. Crumlin, G.M. Veith, J.R. Harding, E. Mutoro, L. Baggetto, N.J. Dudney, Z. Liu, and Y. Shao-Horn, "In situ ambient pressure x-ray photoelectron spectroscopy studies of lithium-oxygen redox reactions," Sci. Rep. 2, 715 (2012).

ALS Science Highlight #265

 

ALSNews Vol. 339