Electrochemical Characterization of Li4Ti5O12 By Single Particle Measurements Using a Particle - Current Collector Integrated Microelectrode

A single particle measurement is known as a characterization method of electrode material [1]. In a conventional single particle measurement, electrical contact between a microelectrode and an electrode particle is made while observing with an optical microscope.

Although the conventional single particle measurement can be electrochemically performed under an optical microscope by contacting the single active material particle with a Pt microfilament, the applicability of this technique is limited with respect to the size and shape of particles. Recently, we overcame the problem by using “a particle - current collector integrated microelectrode” , in which the single active material particle was directly-bonded on the tip of the microelectrode [2]. In this study, new measurement systems were constructed using the integrated microelectrode embedded in a three-electrode flange cell and a constant temperature bath in order to evaluate the electrochemical properties of Li₄Ti₅O₁₂ (LTO) single particle, which includes the temperature dependency of the electrode kinetics.

Particle - current collector integrated microelectrodes were prepared as follows using a focused ion beam process unit (FIB): After attaching a tungsten probe coated with fluorinated resin to a manipulator in the FIB, the tip of the probe was cut off to expose the inner conductive probe. An ionic beam of platinum was positioned and deposited on contact between the top of the cut probe and a single LTO particle. The particle - current collector integrated microelectrode as the working electrode was assembled in a three-electrode flange cell with Li metals as the counter and reference electrodes. The electrolyte employed was 1mol/L Li(TFSI)/EC+PC(1:1 v/v%). Electrochemical measurements of the LTO single particle were carried out at 10 ^oC -40 ^oC.

Cyclic voltammograms of LTO single particle exhibited the well-defined reversible redox peaks at around 1.55V vs. Li/Li⁺ when the scan was initiated from reduction direction. The obtained phenomenon, higher anodic peak current than cathodic peak current, is indicative of faster reaction during Li⁺ extraction (i.e. discharge) than Li⁺ insertion (i.e. charge). At 40 ^oC, single LTO particle was found to deliver its 95% capacity upon discharge at 1000C. On the contrary, the capacity retention upon charge at 1000C retained only 37%. These tendencies in charge/discharge were also confirmed at all temperatures tested.

In this presentation, the peculiar reaction behavior mentioned above will be discussed in terms of reaction kinetics of LTO with lithium, through electrochemical impedance spectroscopies at various temperatures.

References

[1] I. Uchida et al., J. Power Sources, 68, 139 (1997) .

[2] Y. Sakurai et al., 65th Annual ISE Meeting, Mon S05, Lausanne, Switzerland (2014).