Micron-sized Li4Ti5O12 was prepared in a single-step solid-state reaction involving TiO2 and Li2CO3, and its electrochemical behavior was evaluated in Li and Li-ion cells containing a polyacrylonitrile (PAN)-based solid polymer electrolyte. The usefulness of Li4Ti5O12 was demonstrated for three distinctive applications : (i) cathode of a 1.5 V rechargeable Li battery, (ii) auxiliary electrode for investigating the electrochemistry of Li insertion cathode materials, and (iii) anode of a Li-ion cell in conjunction with a high voltage cathode, e.g., cubic spinel LiMn2O4. The micron-sized Li4Ti5O12 exhibit ed a capacity of 160 mAh/g at C/20-C/30 rates which is about 7% better than the capacity exhibited by this material prepared according to a previously published procedure. More importantly, the micron-sized oxide showed significantly better high rate capability, yielding 25-50% larger capacity at the 3C to 8C rates. Li/solid polymer electrolyte//Li4Ti5O12 cells underwent extended, full-depth, charge/discharge cycling at greater than or equal to 1C rates with virtually no capacity fade. The auxiliary electrode concept was demonstrated in Li(4+x)Ti5O12 (x similar to 1.2)//solid polymer electrolyte//LiMn2O4 cells. At a 1C discharge rate, more than 150 cycles were demonstrated in these cells with a capacity fade rate of about 0.1% per cycle and an end utilization of similar to 90 mAh/g for spinel LiMn2O4. Balanced Li4Ti5O12//solid polymer electrolyte//LiMn2O4 cells of slightly cathode-limited configuration showed full-depth extended cycling capability at a utilization of similar to 90 mAh/g for LiMn2O4 at 1C rate and a capacity fade rate of about 0.08% per cycle. The capacity fade in the LiMn2O4-containing cells appears to come from this cathode. When fully packaged, specific energy of the Li//PAN electrolyte//Li4Ti5O12 cell would be about 57 Wh/kg and that of the Li4Ti5O12,//PAN electrolyte//LiMn2O4 cell is about 60 Wh/kg.