New Analytical Tool for Electrochemical Interfaces – in Situ Neutron Reflectometry

Interfacial reactions between intercalation electrodes and electrolyte have attracted much attention to achieve high current drain and long-term stability of lithium batteries, fuel cells, and metal-air batteries.  However, there is no common strategy for the design of materials produced under an electrochemical potential field.  Neutron reflectometry (NR) is a new experimental technique that gives useful information of the concentration gradient of diffusing species from the electrode to the electrolyte on battery operation [1].  Here we have detected a defect-rich phase in the 10–20 nm region from the surface of lithium intercalation electrodes, Li₄Ti₅O₁₂, LiMn_1/3Co_1/3Ni_1/3O₂, and LiFePO₄.  The defective phase formation has been directly related to the electrochemical activity of nanosized electrode materials.

  Epitaxial films of each electrode material were grown on Nb:SrTiO₃(111) substrates using a pulsed laser deposition apparatus [1-2].  Thicknesses of the films were 25 to 45 nm.  NR measurements were performed on SOFIA (BL-16, J-PARC), which is a time-of-flight reflectometer [3].  A deuterated propylene carbonate electrolyte containing 1 M LiPF₆ was used as an electrolyte solution to prevent incoherent scattering of ¹H.  The neutron reflectivity spectra were collected at pristine and at open circuit voltage condition.  The models of interfacial structure were refined by the Parratt32 data analysis program using roughness, thickness, and scattering length density (SLD) of each layer as parameters.

  The SLD in the surface region of Li₄Ti₅O₁₂(111) increased from the pristine to the cell-construction conditions.  As ⁿLi ions have a negative coherent scattering length of -1.9 fm, the increase in the SLD indicated a formation of lithium vacancies in the surface region prior to electrochemical cycling.  The lithium vacancy-type (V_Li') defect formation was confirmed by lattice contraction observed by surface X-ray diffraction [4].  Similar to Li₄Ti₅O₁₂, a lithium vacancy-type defect phase was detected for LiMn_1/3Co_1/3Ni_1/3O₂(1-18).  In contrast, LiFePO₄(100) films showed a decrease in the SLD value at the electrochemical interface formation, indicating a defect phase of interstitial lithium type (Li_i^.).  The type of defects generated may be related to the electrochemical potentials of the electrode and the electrolyte.  The difference in electrochemical potential causes a gradual change in the structure at the interfacial region to equilibrate the potentials of the two materials.  Lithium is the mobile species in these electrodes; therefore, lithium may participate in charge compensation with formation of the defect phase.  The defective phase was related to an extremely high lithium storage capacity of nanosized Li₄Ti₅O₁₂[5] and an isotropic lithium diffusion in nanosized LiFePO₄.  The results suggest how to create a defect phase at the electrochemical interface and the concept of interfacial defect phase control is extended to nanosize material control for next-generation battery materials.

References

1. M. Hirayama, et al., Electrochemistry, 78 (2010) 413.
2. M. Hirayama, et al., Dalton Trans., 40 (2011) 2882.
3. M. Koji et al., J. Phys.: Conf. Ser., 272 (2011) 012017.
4. M. Hirayama, et al., J. Am. Chem. Soc., 132, (2010) 15268; K. Sakamoto, et al., Phys. Chem. Chem. Phys., 12 (2010) 3815; K. Sakamoto, et al., Chem. Mater., 21 (2009) 2632.
5. Wagemaker, et al., J. Am. Chem. Soc.,  131 (2009) 17786.