Degradation and (de)lithiation processes in the high capacity battery material LiFeBO ₃

Lithium iron borate (LiFeBO ₃) 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 LiFeBO ₃ 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 LiFeBO ₃ and partial oxidation of Fe(ii), resulting in the creation of a stable lithium-deficient phase with a similar crystal structure to LiFeBO ₃. Considerable LiFeBO ₃ degradation occurs during electrode fabrication, which greatly reduces the accessible capacity of LiFeBO ₃ under all but the most stringently controlled conditions for electrode fabrication. Comparative studies on micron-sized LiFeBO ₃ and nanoscale LiFeBO ₃-carbon composite showed a very limited penetration depth (∼30 nm) of the degradation phase front into the LiFeBO ₃ core under near-ambient conditions. Two-phase reaction regions during delithiation and lithiation of LiFeBO ₃ 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 LiFeBO ₃ was ultimately achieved by introducing carbon to minimize the crystallite size and strictly limiting air and moisture exposure to inhibit degradation.