Letter pubs.acs.org/JPCL
Cite This: J. Phys. Chem. Lett. 2018, 9, 4976−4980
Lithium-Ion-Transfer Kinetics of Single LiFePO4 Particles Wei Xu,† Yige Zhou,§ and Xiaobo Ji*,† †
State Key Laboratory of Powder Metallurgy, College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China § College of Chemistry and Chemical Engineering, Hunan University, Changsha 410083, China
J. Phys. Chem. Lett. Downloaded from pubs.acs.org by BOSTON COLG on 08/20/18. For personal use only.
S Supporting Information *
ABSTRACT: The Li-ion desertion process of a single LiFePO4 particle with a diameter of 400 nm in aqueous media at high potential is investigated by stochastic collision of the particle at a microelectrode. No extra additive, such as polymeric binder or conductive carbon, is involved in this stochastic measurement. The ion diffusion inside the particle is proved to be the ratedetermining step that limits the discharge rate of batteries using LiFePO4 in an aqueous environment. This result offers guidance for exploring the most effective enhancement of Li-ion batteries. Moreover, this general method can be applied to study the electrochemical behavior of other electrode materials as an alternative and complementary route.
A
of LiMn2O4 in an aqueous environment.19 They also investigated the potassium (de)insertion process of Prussian blue in both single and multiparticle scales. The opposite behavior was found in the two scales, and the result hence highlighted the necessity for the direct study of active materials.20 Furthermore, the single nanoparticle collision method was united with surface plasmon resonance microscopy to reveal the structure−activity relationship of a single LiCoO2 particle.21 Some other experimental methods have been developed to directly investigate single particles of active materials for ion batteries rather than the aggregation of them. In 2016, Hu et al. fabricated an ultrathin single-particle (SP) electrode of LiFePO4 through 3D printing, and the intrinsic electrochemical properties of LiFePO4 nanocrystals were investigated by analyzing the cyclic voltammetry peaks of the SP electrode. It was found that the interfacial rate constant in aqueous electrolyte was much higher than that of organic electrolyte (by one order) while internal Li-ion diffusion coefficients of LiFePO4 were nearly the same in both electrolytes.22 Moreover, in 2018, Tsai et al. used a microscale tungsten probe, attached to a polycrystalline cathode particle-like LiNi1/3Mn1/3Co1/3O2 and LiNi0.8Co0.15Al0.05O2, as the working electrode to explore the electrochemical kinetics of the single particle for its charge−discharge process, which showed that for commercially relevant particle sizes, the interfacial transport is rate-limiting at low state-of-charge; interestingly, the interfacial and bulk transport will dominate at higher state-
variety of advanced analytical methods have been applied to study the performance of nanoparticles from different perspectives, such as microimaging, spectrum analysis, and electroanalysis.1−4 For most of these methods, signals were mostly obtained from ensembles of nanoparticles rather than a single particle, making difficult the extraction of the nature of the particle from the as-studied materials.5−8 In 1956, Micka studied suspended particles of HgS, PbS, CuS, and Ag2O in water by using a mercury electrode and promoted the development of electrochemical analytical methods at the single-particle scale.9 If the concentration of the particle in solution was ultralow and the surface area of the microelectrode was small enough, the stochastic collision of particles, along with the changes of current or open-circuit potential, could be clearly distinguished, and a wealth of effective information would be extracted from these electrochemical signals.10−12 At the very beginning, this method, also called “nano-impact”, was applied only for electrochemical sensing to confirm the existence of metal particles or biological particles in ultralow concentration.13,14 Since then, some new theories were enriched to analyze the signals by prominent researchers, such as Allen J. Bard, Richard Compton, and Richard Crooks, for the determination of the size distribution, concentration, degree of aggregation, and nanoparticle identity of the particles.15−18 With the development of theory, this nanoimpact method was further extended to study the inherent kinetic properties of electrode materials in batteries, which can avoid the serious influences of the nature, amount, and distribution of the additives and the structure of the electrodes. In 2017, Zampardi et al. explored the essential electrochemical behavior of single LiMn2O4 particles utilizing this nano-impact method, demonstrating that the kinetics of the interfacial ion transfer defined a theoretical upper limit for the discharge rates © XXXX American Chemical Society
Received: July 27, 2018 Accepted: August 16, 2018 Published: August 16, 2018 4976
DOI: 10.1021/acs.jpclett.8b02315 J. Phys. Chem. Lett. 2018, 9, 4976−4980
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The Journal of Physical Chemistry Letters
the value of the current is independent of time, but it fluctuates exponentially followed by the overpotential depicted in Figure 1a. In step 3, the current can be expressed as a function of the surface area (Ap = 4πr02) of a particle
of-charge, simultaneously.23 However, compared with the nano-impact technique, these methods often involved tedious preparation of experimental material and intricate instrumentation, thus limiting their widespread application. As LiFePO4 is of particular interest for usage in large-scale energy storage batteries, because of its special structural and chemical stability that lead to safe and long cycle life batteries,24−26 it is extremely essential to investigate the immanent properties of single LiFePO4 nanoparticles for the exploration of the most effective method of optimizing their performance. In this study, the nano-impact method is chosen to study the Li+ (de)insertion kinetics of single LiFePO4 nanoparticles for its convenient and swift acquisition of electrochemical signals. In a typical nano-impact experience, the nanoparticles are dispersed in the electrolyte, and then three electrodes are immersed into the electrolyte followed by a suitable potential which is applied on the working electrode. By virtue of Brownian motion, the particles will be stochastically collided with the interface of the working electrode, resulting in the transitory Faraday currents which appear as an impulse on the chronoamperometry curve. The frequency, duration, and shape of the spikes and the magnitude of the current can provide the most direct information on the electrochemical reactions at a single-particle scale. Four fundamental steps are considered to occur in a complete (dis)charging process:24,27 (1) transfer of electrons
idiff = 4πr0 2Fk
(2)
where r0 is the radius of the particle and k is a constant coefficient. As the value of current is determined only by the surface area of the particle, the impulse of current tends to be rectangular and maintains a nearly fixed value independent of overpotential and time, as shown in Figure 1b. In step 4, the current can be expressed as a function of time of duration (t) ∞
2
idiff = 8πc0Dq0r0 ∑ e−Dλn t n=1
(3)
where c0 is the initial number concentration of ions inside the particle, D the diffusion coefficient of the considered species, and q0 the charge transferred to the electrode per ion released. The analytical result of the current response is shown in Figure 1c, with all parameters set to unity. The shape of the current curve presents a spike feature with a sharp rise followed by a slow decay. At first, to find the suitable potential parameter needed in nano-impact experiment, the redox potential of LiFePO4 was
Figure 1. Depiction of current as a function of time in three steps. Panels a, b, and c are related to steps 1, 3, and 4, respectively.
between particle and the electrode; (2) diffusion of Li ion in the electrolyte; (3) transfer of Li ion through the interface between electrolyte and particle; and (4) internal diffusion of Li ion in the particle. Ascertaining the rate-determining step will point out the most effective way to optimize the rate performance of the active materials. Different theoretical models are employed to analyze the current feature of each step and help us find the rate-determining step of the whole process. Step 2 is excluded first because the ion diffusion rate in aqueous solution is known to be significantly faster than that of its performance in the particle, as shown by ref 22. According to the theoretical models, the current has different expressions in the other three steps (see Theory in the Supporting Information). In step 1, the current can be expressed as a function of the overpotential (η = E − E0) ÄÅ ÑÉÑ ÅÅ (1 − α)nF Ñ (E − E0)ÑÑÑÑ i = nFAek 0C R expÅÅÅ ÅÅÇ ÑÑÖ RT (1) where n is the number of electrons transferred, F the Faraday constant, Ae the surface area of the electrode, k0 the standard heterogeneous rate constant, CR the concentration of the reductor, α the transfer coefficient (α = 0.5 in this case), R the gas constant, T the temperature, E0 the standard potential of the redox reaction, and E the applied potential. It is clear that
Figure 2. Cyclic voltammograms of the LiFePO4 paste electrode at a scan rate of 0.1 mV s−1 in a solution containing 10 mM LiClO4 (a) and 1 M LiClO4 (b). Potentials are referred to a saturated calomel reference electrode (SCE).
Figure 3. Chronoamperometry at E = 1.6 V vs SCE in 10 mM LiClO4 electrolyte with 50 pM LiFePO4 (blue) and without any particle (red) in it.
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in vacuum oven at 80 °C. The electrolyte in the experiment was in aqueous media for its much higher ionic conductivity than that of the conventional organic electrolyte.23,28,29 Under the standard support (1 M LiClO4), one reversible redox wave was observed at about +0.4 V vs SCE, as shown in Figure 2b, which is in good agreement with the literature.30 The peak is confirmed as the oxidation of the ferrum centers from 2+ oxidation state to 3+, and the subsequent desertion of the Li+ ions is observed from the olivine structure (eq 4). LiFePO4 − x e− → x Li+ + Li(1 − x)FePO4 Figure 4. (a) Mean charges obtained from the LiFePO4 particle impacts at different potentials. (b) Mean residence time of the LiFePO4 particle impacts at different potentials. All of the data are obtained in the solution contain 10 mM LiClO4.
(4)
As LiFePO4 particles were easily agglomerated in solutions of high ionic strength, a low concentration of LiClO4 electrolyte was chosen in the collision experiment. Figure 2a depicts the voltammetric response of the modified electrode in the presence of 10 mM LiClO4. The oxidative wave was not shifted in the low support electrolyte when compared with that of the standard support electrolyte, while the area of the reduction peak was reduced significantly and shifted to more negative part because of the relatively low Li+ concentration in the electrolyte. Then the electrochemical responses of individual particles suspended in solution were investigated by the nano-impact
investigated in a three-electrode system using a glassy carbon electrode (r = 3 mm) as the working electrode (WE), a platinum wire electrode as the counter electrode, and a standard calomel electrode (SCE) as the reference electrode. LiFePO4 was mixed with carbon additives and polymer binder in the weight ratio of 70:15:15. Then the mixture was dripped onto the surface of the glassy carbon electrode and then dried
Figure 5. Current feature as a function of time at five different potentials (vs SCE): +1.2, +1.3, +1.4, +1.5, and +1.6 V. The solution contained 50 pM LiFePO4 and 10 mM LiClO4. 4978
DOI: 10.1021/acs.jpclett.8b02315 J. Phys. Chem. Lett. 2018, 9, 4976−4980
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The Journal of Physical Chemistry Letters method. A carbon-fiber microdisc electrode (d = 7 μm) was employed as the working electrode, which was inserted into the electrolyte with the surface of the WE facing downward. In this way, it was ensured that the particle collision onto the surface of the WE was forced by the Brownian motion. In addition, agglomeration that was mainly forced by gravity would not collide with the WE. The counter electrode and reference electrode were platinum wire and SCE, respectively. The concentration of the suspension was 50 pM, and a suitably oxidizing potential was positive enough upon the electrode to drive the occurrence of the electrochemical reaction of LiFePO4. During the chronoamperometry, the small spikelike current peaks above the baseline were recorded with particles present in the solution. Note that no spike was found without particles in the solution, as shown in Figure 3, showing that these spikes were ascribed to the stochastic impact behavior of LiFePO4 particles colliding at the electrode surface, followed by the subsequent oxidative Li+ desertion. We next investigated the duration and the charge passed during the reaction. Figure 4 presents these data as a function of the applied electrode potential. No oxidative feature was observed during a scan at or below +1.1 V (vs SCE). An overpotential of approximately 0.7 V was required to drive the reaction at the individual-particle scale. At least in part, this overpotential likely is the result of a contact resistance between particle and electrode.31 As the potential was increased, the charge of the oxidative features seemed to be nearly consistent. Only about 0.2 pC of charge was passed during a collision event, which represented about 0.27% of the whole charge contained by one particle in the average dimension (d = 400 nm) [Figure S1 of the Supporting Information]. This proportionality value was relatively low compared with the LiMn2O4 particle, which liberated about 10% of the entire charge at a time.19 This might be the result of the ultralow ion diffusion rate in the LiFePO4 particle.32,33 The duration of the single collision event was counted, and an average duration was found to be 18 ± 6 ms, which was significantly greater than the time interval (3 ms) used for recording of the signal, which proved that the spikes represent the oxidation process of a single particle at the electrode rather than a noise signal. Concrete details of some typical examples at five different potentials are shown in Figure 5. It was found that the shape of the spike matched well with the feature discussed above, with a sharp rise followed by a slow delay, while no square-like impulses were observed in all the signals. In addition, at five different potentials, the shapes of the spikes were consistent and the magnitude of current was not sensitive to the potential fluctuation. The current was changed only over time, suggesting that the rate of Li ion diffusion inside the particle limited the Li ion desertion process, in other words, the rate performance of LiFePO4 in aqueous environment. This result came as no surprise because the olivine-type structure of LiFePO4 crystal, resulting in only a one-dimensional path for Li ion transfer, extremely limited the ion diffusion rate inside the particle. By comparison, the square-like current impulse was obtained for the LiMn2O4 crystal in the nano-impact experiment.19 It is not surprising to see this much faster internal ion diffusion because its spinel structure would allow the fast-pass of the ion in multiple directions. In conclusion, the stochastic collision of a single particle upon a microelectrode was applied to investigate the kinetics of the Li+ desertion from single LiFePO4 particles in an aqueous environment. This method, based on simple equip-
ment and operation, can provide abundant information on intrinsic electrochemical properties of active materials swiftly without the influence or presence of binder and conductive carbon additive. Furthermore, this work has demonstrated that at high overpotentials the intrinsic rate-determining step of the Li+ (de)insertion from LiFePO4 is the internal diffusion of ions in the particle.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.8b02315. Experimental Section; size distribution; and details regarding process of current expression in three steps according to different theory models (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Xiaobo Ji: 0000-0002-5405-7913 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (51622406, 21673298, and 21473258), Innovation Mover Program of Central South University (2017CX004 and 2018CX005), and Hunan Provincial Science and Technology Plan (2017TP1001 and 2016TP1009)
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