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

Abstract

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.