A large amount of impedance data was acquired at each set time point, allowing a systematic and complete analysis of the interface. The temporal profile of potential (d) and impedances (e) at points of interest (color circles in c) of the chronopotentiometric profile are measured simultaneously.
In situ impedance measurements
Fourier Transform Electrochemical Impedance Spectroscopy (FTEIS)
We developed a technique that allows impedance measurements to be made in a much shorter period than the FRA method.11,12 The technique takes advantage of the fact that a potential step is an integrated form of what is called the Dirac delta function, i which is obtained by summing all ac waves with the same amplitude and phase (Figure 1-3). Thus, the application of a potential step followed by taking the first derivative of the received current signal and the subsequent conversion of the time domain derivative signal to the frequency domain by the Fourier transform leads to the calculation of the resistance data in all frequency regions.
Galvanostatic Electrochemical Impedance Spectroscopy (GS-EIS)
Real-time impedance measurements and its applications
Second, the lithium-ion rechargeable batteries were investigated by GS-EIS because charge transfer resistances are definitely and deeply related to kinetics of lithiation or delithiation of active materials. Based on the information on charge transfer resistance profiles along galvanostatic charging processes, a charging strategy with several different C rates is programmed.
Conclusions
Introduction
We have recently developed a technique to obtain a series of impedance data during potential sweep by combining Fourier transform electrochemical impedance spectroscopy (FTEIS) with staircase cyclic voltammetry experiment,42,43 where a real white noise is used as an excitation signal. 44 One can obtain a series of full-frame EIS spectra as a function of the scanned potential while a staircase cyclic voltammogram (SCV) is obtained; The SCV is the same as the cyclic voltammogram (CV) under certain experimental conditions.45,46 Thus, the SCV-FTEIS method has been used for dynamic and transient electrochemical impedance measurements on a variety of electrochemical systems47-57 when performed correctly. A large amount of impedance data recorded during the anodic sweep of the potential made it possible to obtain a number of electrical and electrochemical parameters of zinc oxidation.
Experimental
Electrochemical cell preparation
Cyclic voltammetry & FTEIS
The sampling rate used for both the scaled potential and the resultant current was 50 kHz. The data thus obtained from both the voltage step and the resulting chronoamperometric current were segmented for each possible step.
Results and Discussion
Peaks appear at potentials more positive than their thermodynamic values due to overpotential. Peak A2 is hardly seen in both 0.010 and 0.10 M KOH as the formation of Zn(OH)3- is relatively insignificant due to the depletion of OH- in the reaction zone.61. Finally, significantly large cathodic currents are observed before zinc oxidation starts to occur at both 0.010 and 0.10 M KOH, while a rather small current is observed at 1.0 M KOH, due to the hydrogen evolution reaction according to
We see that Rp for hydrogen release increases steadily as the potential is scanned in the positive direction due to the reduction of the overvoltage for water reduction, until it reaches a maximum of 434.6 at -1.47 V. At this point, the value of Cd is at its maximum due to the pseudo- of capacitance originating from the faradaic reaction. Warburg receivers represent how the mass transports of water molecules and hydroxide ions contribute to hydrogen evolution and zinc oxidation fluxes during potential sweeping in this case.
Conclusions
Introduction
Typical impedance spectra have been obtained at a biased potential after the electrochemical systems of interest have been fully stabilized at the potential.9, 11, 12 With the PS-EIS (potentiostatic EIS) it is difficult to define the state of charge (SOC) at the potential at which impedances are obtained because a wide range of SOC exists at a fixed potential, shown as potential plateaus over time in typical chronopotentiometric potential profiles. Moreover, as another weakness of PS-EIS for investigating the lithiation process of graphite anodes of LIBs, PS-EIS cannot do so. The GS-EIS offers advantages, overcomes the weakness of PS-EIS and provides effective kinetic parameters to provide a useful picture of the galvanostatic lithiation of graphite.
Non-stationary impedance analysis (equivalent to DC-EIS) based on impedance spectra as a function of time was proposed, which was used for the analysis of lead/acid batteries during a galvanostatic charge.13, 14 Also electrochemistry of electrodes for lithium batteries and LIBs were investigated in situ using the DC-EIS in terms of deposition/dissolution of lithium metal anode15 and formation of the SEI layer on graphite anode.10. Kinetic parameters were extracted from coin half-cells based on graphite along lithiation processes in a real-time manner or in situ by the GS-EIS. Based on the information, we proposed a charging strategy called the C-rate switching method (CRS) to store more amount of energy within a set period of time.
Experimental 1. Cell preparation
Impedance measurement by GS-EIS
Sine current waves were applied to constant current and the resultant potential waves were recorded periodically: every 10 minutes for 0.05C and 0.1C; and 5 min for 0.5C and 1C.
C-rate switch charging (CRS charging)
Results and Discussion
The high-frequency semicircle became negligible compared to its low-frequency counterpart when the GIC reached stage 1. a). Then, RCT began to increase from the formation of stage 2L to stage 2 (γ) (III), followed by a sudden increase in RCT observed during the transition from stage 2 to stage 1 (δ) (IV) or after the graphite interspace is completely filled with lithium ions. Independently of the C levels, the potentials at which RCT is minimized corresponded to the GIC II transition from level 4 to level 2L.
In other words, the electrochemical factors affecting lithiation kinetics are enhanced up to stage 2L, while stages 2 to 1 provide electrochemical environments unfavorable to lithiation processes. Based on the lesson from our GS-EIS studies that the resistance associated with charge transfer decreases up to the 2L level and then increases suddenly and exponentially, the question can be asked: what if the charging current is programmed to reduce the effects of RCT by lithiation. The use of high currents on stage 2 to stage 1 should be avoided due to their high resistance.
Conclusions
Introduction
However, the electrode-electrolyte interfaces in LIBs change dynamically during practically used galvanostatic lithiation and delithiation.14-16 For this reason, the electrochemical parameters measuring polarization obtained by PS-EIS cannot describe the time-variant situations where the states of charge ( SOC) ) cannot be defined only by corresponding potentials and the kinetic parameters depend on lithiation or delithiation rates. The galvanostatic EIS (GS-EIS) successfully revealed the rate-dependent kinetics of lithium ion intercalation in graphite.17,18 In this work, we investigated the lithiation kinetics between nano-dimensional and micro-dimensional silicon particles in real situations of battery cell compare charging through our GS-EIS. The apparent kinetic parameters were highly dependent not only on SOC at a fixed rate but also on C rates.
Based on the kinetic information from the operando impedance spectra, a fast charging (lithiation) strategy was designed in such a way that higher C rates are preferred, as far as the lithiasis kinetics can afford this rate.
Experimental Methods
Cell preparation
Lithium foil was the counter electrode and 1.3 M LiPF6 in 3:7 (v/v) ethylene carbonate (EC) :diethyl carbonate (DEC) with 10% fluoroethylene carbonate (FEC) was the electrolyte.
Impedance measurement by GS-IES
Lithiation by C-rate switching (CRS)
Results and Discussion
This does not mean that the total amount of the SEI layer formation on nSi is less than that on µSi. Interestingly, two local maxima of the SEI profiles in Figure 4-3a were observed at the plateau potentials of lithiation (as indicated in blue in Figure 4-3a and b) where amorphous and crystalline phases are formed respectively (equations 7 and 8) . It should be emphasized that the operando measurements of the kinetic parameters by GS-EIS enabled the CRS design.
The insertion of lithium ions into the silicon structure was schematically depicted in two different situations of 0.5C lithiation versus CRS lithiation (Figure 4-4c). With DR and Rp values as functions of lithiation rate and C rates, possible CRS lithiation profiles can be simulated. When the cut-off voltage is reached and the lithiation rate changes at the same time, the voltage suddenly increased due to the rearrangement of the diffusion coefficient ratio (DO/DR) of lithium ions.
Conclusions
CR values (mol cm-3) were calculated from the molar volume of Si and the amount of lithiated Li+.
Electrochemical and impedance investigation of the effect of lithium malonate on the performance of natural graphite electrodes in lithium-ion batteries. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Operando methodology: combination of in situ spectroscopy and simultaneous activity measurements under catalytic reaction conditions.
Conclusions
The variation of the double-layer capacitance (Cd) demonstrates the thickening of the solid electrolyte interface layer (SEI), while the charge transfer resistance (RP) values reveal the mechanism of lithiation in the silicon structure during galvanostatic charging processes. The C-rate switch (CRS) strategy is proposed for fast charging based on kinetic parameters of C-rate dependence. Indeed, these experimental results and findings suggest that the in situ EIS is suitable not only to help understand a battery control system, but also to help find a research direction.
Future Perspective
단 한 번이라도 그동안 고생했다고 환하게 웃으시며 악수를 해주시던 교수님을 보고 싶습니다. UNIST에서의 지난 5년 6개월은 제 삶에 많은 변화를 가져왔습니다. 박사 학위 외에도 가정을 꾸렸습니다.
항상 저를 지지해 준 아내 최현정 씨에게 감사 인사를 전하고 싶습니다. 윤교는 나처럼 되고 싶지만 졸업을 할 수 없다. 영대는 윤교처럼 되고 싶지만 그렇게 하면 졸업도 못 한다.
