Dongli Zeng

Structure of aluminum fluoride coated Li[Li1/9Ni1/3Mn5/9]O2 cathodes for secondary lithium-ion batteries

The structural properties of layered Li[Li1/9Ni1/3Mn5/9]O2 positive electrodes nominally coated with aluminum fluoride are studied. Coatings were prepared by using aqueous solutions with various concentrations of aluminum and fluorine and are compared with samples treated under similar conditions but with aqueous HCl solutions. Samples were investigated following heat treatment at 120 °C and 400 °C with powder X-ray diffraction, transmission electron microscopy including energy dispersive X-ray spectroscopy (TEM/EDS), elemental analysis via inductively coupled plasma-optical emission spectroscopy (ICP-EA), and both 6Li and 27Al magic angle spinning NMR spectroscopy. The TEM/EDS and 27Al NMR data provide support for an aluminum-rich amorphous coating that, following drying at 120 °C, comprises six coordinated, partially hydrated aluminum environments. Heat treatment at 400 °C results in a phase that resembles partially fluorinated γ- or γ′-Al2O3, at least locally. An Al : F ratio of 2 : 1 is obtained in stark contrast to the ratio used in the original solution (1 : 3). No AlF3 is detected by PXRD and instead some evidence for a protonated phase (formed by ion exchanging protons for lithium) is detected along with Li[Li1/9Ni1/3Mn5/9]O2 after drying. This phase disappears on heating to 400 °C, suggesting some reorganization of bulk Li[Li1/9Ni1/3Mn5/9]O2 and possibly some incorporation of Al into the structure. This is in agreement with the 6Li NMR spectra, which indicate that the local environments that are found in the Ni-free end member of the series Li[Li(1/3−2x/3)NixMn(2/3−x/3)]O2 (i.e. Li2MnO3) are enhanced on sintering.

Degradation and (de)lithiation processes in the high capacity battery material LiFeBO3

Lithium iron borate (LiFeBO3) is a particularly desirable cathode material for lithium-ion batteries due to its high theoretical capacity (220 mA h g−1) and its favorable chemical constituents, which are abundant, inexpensive and non-toxic. However, its electrochemical performance appears to be severely hindered by the degradation that results from air or moisture exposure. The degradation of LiFeBO3 was studied through a wide array of ex situ and in situ techniques (X-ray diffraction, nuclear magnetic resonance, X-ray absorption spectroscopy, electron microscopy and spectroscopy) to better understand the possible degradation process and to develop methods for preventing degradation. It is demonstrated that degradation involves both Li loss from the framework of LiFeBO3 and partial oxidation of Fe(II), resulting in the creation of a stable lithium-deficient phase with a similar crystal structure to LiFeBO3. Considerable LiFeBO3 degradation occurs during electrode fabrication, which greatly reduces the accessible capacity of LiFeBO3 under all but the most stringently controlled conditions for electrode fabrication. Comparative studies on micron-sized LiFeBO3 and nanoscale LiFeBO3–carbon composite showed a very limited penetration depth (∼30 nm) of the degradation phase front into the LiFeBO3 core under near-ambient conditions. Two-phase reaction regions during delithiation and lithiation of LiFeBO3 were unambiguously identified through the galvanostatic intermittent titration technique (GITT), although it is still an open question as to whether the two-phase reaction persists across the whole range of possible Li contents. In addition to the main intercalation process with a thermodynamic potential of 2.8 V, there appears to be a second reversible electrochemical process with a potential of 1.8 V. The best electrochemical performance of LiFeBO3 was ultimately achieved by introducing carbon to minimize the crystallite size and strictly limiting air and moisture exposure to inhibit degradation.