Electronic Conductivity in the Li_(4/3)Ti_(5/3)O_4Li_(7/3)Ti_(5/3)O_4 System and Variation with State-of-Charge as a Li Battery Anode

摘要

Lithium titanate spinel (Li_(4/3)Ti_(5/3)O_4,LTO) has been studied extensively in recent years as a Li ion battery anode mate-rial. Its high theoretical capacity of 175 mAh g~(-1), "zero-strain" characteristics, and a flat 1.55 V potential (vs. Li~o) due to the two-phase coexistence of Li_(4/3)Ti_(5/3)O_4 and its lithiated counterpart Li_(7/3)Ti_(5/3)O_4 allow for a moderately high cell-level energy density and long cycle life when paired with high voltage (＞3.5 V vs. Li~0) cathodes such as those based on Mn, Ni, and Co oxides. A recent scanning transmission electron microscopy study clarifies the Li positions in the two phases, the structure of the topotactic interface, and the charge distributions in each phase. Extremely high charge/discharge rates have been reported for nanocrystalline LTO. However, the insulating nature of LTO has been an area of concern, with reported electronic conductivities ranging from from 10~(-8) to ＜10~(-13) S cm~(-1). Correspondingly, there have been numerous studies on doping or coating with conductors to improve electronic conductivity. At the same time, there are indications that the lithiated phase itself may have high electronic conductivity, including electrochemical tests of sintered or powder-based electrodes that are free of conductive additive, and DFT-GGA computations from which it was concluded that there is a 2 eV bandgap for Li_(4/3)Ti_(5/3)O_4 and metallic behavior for Li_(7/3)Ti_(5/3)O_4 There are numerous measurements of electronic conductivity in as-prepared (delithiated) LTO, but surprisingly no data for the lithiated phase except for an approximate value "in the range of 10~(-2) S cm~(-1)" mentioned by Scharner et al. Table 1 summarizes the available electronic conductivity data. Since the ionic conductivity appears to lie in between the electronic conductivities of the endmem-bers, (e.g., 2.5 × 10~(-5) S cm~(-1)), a transition from electronic-to ionic-limited chemical diffusion might be expected during use. The state-of-charge at which this occurs will depend on the electronic conductivity of the endmembers as well as their physical distribution. In this Communication, we report the first direct measurement of the electronic conductivity of the fully lithiated phase Li_(7/3)Ti_(5/3)O_4 as well as its temperature dependence. In addition, the state-of-charge dependence of electronic conductivity across the two-phase coexistence field is reported.