|Two Studies Reveal Details of Lithium-Battery Function|
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.
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.
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.
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.
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