Article pubs.acs.org/cm
Impact of Particle Size on the Non-Equilibrium Phase Transition of Lithium-Inserted Anatase TiO2 Kun Shen,† Hao Chen,‡ Frits Klaver,† Fokko M. Mulder,† and Marnix Wagemaker*,† †
Faculty of Applied Sciences, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands
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ABSTRACT: The phase transformation behavior in Li-ion battery electrodes is critical for the electrode kinetics and cycle life. Here we reveal unexpected particle size-dependent phase transformation behavior in anatase TiO2 by in situ X-ray diffraction. The equilibrium voltage measured by the galvanostatic intermittent titration technique decreases progressively with a decrease in particle size, which can attributed to the difference in the surface energy of the pristine and lithiated phases. On the basis of the evolution of the domain size and phase fraction of the two phases, we conclude that the first-order phase transition proceeds by continuous nucleation upon lithium insertion. For all particle sizes, the phase boundary is found to migrate under nonequilibrium conditions even under very slow (dis)charge conditions, as reflected by a distinct deviation from the Li solubility limit during the phase transformation. Remarkably, the degree of nonequilibrium increases with a decrease in particle size, which is rationalized by the difference in the observed phase transition behavior between small and large particles. The absence of phase coexistence in smaller particles in combination with the sluggish ionic transport rationalizes the better electrochemical performance of the nanostructured anatase TiO2 compared to that of the microsized material. These results suggest a very low nucleation barrier for the formation and movement of the phase boundary in combination with sluggish ionic migration. Therefore, strategies for improving the rate performance of nanostructured anatase TiO2 should concentrate on improving the interstitial diffusion, for instance by appropriate doping.
A
natase TiO2 is a promising negative electrode for Li-ion batteries because of its ability to store a large amount of Li ions reversibly,1−4 its excellent gravimetric and volumetric storage capacities,5,6 its chemical safety, and its environmentally friendly properties. In the past several decades, the electrochemistry of bulk anatase TiO2 has been widely studied, reporting disadvantages, such as poor ionic and electronic conductivity, and low levels of capacity retention with a high rate of (dis)charge.7,8 Recently, nanostructured materials were found to be very promising in mitigating the disadvantages in bulk materials and provide a variety of favorable properties for electrochemical energy storage phenomena7−9 that are also explored in nanosized anatase TiO2.10−13 One of the most striking features is the large impact of nanosizing on the thermodynamics and kinetics of Li-ion insertion reactions in anatase TiO2. Nanosizing anatase TiO2 results in a curved open cell voltage profile with a much shorter plateau region in comparison with that of micrometer-sized materials.11,14,15 The miscibility gap shifts and shrinks with a decrease in particle size,16 which has been suggested to be a result of interfacial strain, the diffuse interface,17,18 and surface energy effects.14,15 In addition, it was found that the phase transformation behavior in nanosized anatase TiO2 is size-dependent, and its mechanism is significantly different from that of bulk anatase TiO2. During the first-order phase transition, the phases appear not to coexist in nanosized particles, whereas they coexist in >40 nm particles.16 © 2014 American Chemical Society
The aim of this work is to gain insight into the particle sizedependent phase transition mechanism under realistic in situ (dis)charge conditions. Quasi-equilibrium electrochemical measurements using the galvanostatic intermittent titration technique (GITT) were used to study the equilibrium voltage, revealing a systematic dependence on particle size. Detailed structural changes observed by in situ X-ray diffraction indicate a nonequilibrium condition that explains the difference in the observed phase transition behavior between small and large particle sizes as a kinetic rather than a thermodynamic effect.
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METHODS
Materials. Anatase TiO2 crystalline particles with three different dimensions (15, 41, and 130 nm) were obtained from Aldrich. The particle sizes were confirmed by analyzing the particle size broadening of the X-ray diffraction reflections. Sample Preparation and GITT Measurements. Electrodes for the GITT measurements were prepared by mixing the anatase TiO2 (80 wt %) with conducting carbon black additive (ENSAQO, 10 wt %) and a polyvinylidene fluoride (PVDF) binder (10 wt %) in N-methylpyrrolidone (NMP). When the well-mixed slurry had reached the appropriate viscosity, it was cast on a carbon-coated aluminum foil current collector using the “doctor blade” method. The casted Received: November 11, 2013 Revised: January 25, 2014 Published: January 28, 2014 1608
dx.doi.org/10.1021/cm4037346 | Chem. Mater. 2014, 26, 1608−1615
Chemistry of Materials
Article
electrodes were dried in a vacuum furnace at approximately 100 °C for several days. A Swagelok-type cell was used for GITT measurements. The electrodes were mounted under argon (
