Tutorial - Electrochemical Techniques in CR2032 battery Research

　　Research Background With the popularity of electric vehicles and electronic products, CR2032 battery products have developed rapidly. The huge market potential and remaining development limitations have stimulated researchers' enthusiasm for batteries. However, most of the researchers currently studying batteries are not from electrochemistry majors, but come from fields such as materials and physical chemistry, so they do not necessarily have the system skills. knowledge of electrochemistry. For CR2032 battery research, understanding electrochemical data, standard electrochemical tests and professional electrochemical analysis are all necessary electrochemical knowledge and skills. For this reason, it is necessary to summarize the knowledge of electrochemical tests and electrochemical analysis to provide a basis for the majority of CR2032 battery research Provide theoretical support. Introduction to the results Recently, doctoral candidate Xuming Yang of City University of Hong Kong and Professor Andrey L. Rogach (joint correspondence) published the latest research work of "Electrochemical Techniques in CR2032 battery Research: ATutorial for Nonelectrochemists" on Adv.EnergyMater. This work introduces basic electrochemical concepts, conducts detailed discussion and analysis of testing technologies such as voltage, current, and impedance, and provides a learning outline and guidance tutorial for CR2032 battery researchers. Research highlights (1) Introduced basic electrochemical concepts: voltage, current, capacity and test time; (2) Introduced common electrode material evaluation indicators: energy/power density, Coulombic efficiency and cycle life; (3) In view of the above Concepts and indicators, common electrochemical testing methods are introduced: cyclic voltammetry test, galvanostatic charge and discharge test, AC impedance test, constant voltage intermittent titration method and galvanostatic intermittent titration method. The above methods are classified into potentiometric technology, current technology and impedance technology, and discuss the shortcomings in current research work and literature reports. Graphic introduction 1. Introduction to basic electrochemical concepts Voltage and Faradaic capacity are essential properties of electrode materials. Voltage is determined by the Gibbs free energy change between reactants and products, while Faradaic capacity is determined by the number of transferable electrons. Specifically The values can be derived from equations (1) and (2), where E is the voltage, ΔrG is the Gibbs free energy change of the reaction, QF is the Faraday capacity, M is the relative molecular mass of the electrode, and F is the Faraday constant. The energy capacity (QE) is the result of voltage and Faradaic capacity and can be derived from equation (3). However, the above three equations only indicate the calculation method of theoretical parameters. The actual experimental results usually deviate from the theoretical calculation results because the actual situation is usually affected by polarization, side reactions and incomplete reactions. Side reactions will consume part of the charge, and the resistance Energy is lost in the form of heat. In addition, Coulomb efficiency and energy efficiency are also important concepts in CR2032 battery research.

　　2. Potential technology Cyclic voltammetry (CV) is the most important potential technology in electrochemical testing. The use of cyclic voltammetry in CR2032 battery research usually involves solid-liquid interfaces, ion diffusion, multiple reactions, etc. The typical curve obtained from the cyclic voltammetry test is shown in Figure 1(b). A pair of Gaussian peaks corresponds to an electrode reaction. The ratio of the peak current (ip) and its corresponding voltage difference (ΔEp) can be used to judge the electrochemical reaction. Reversibility. When the CV curve is converted into a voltage vs. capacity curve (as shown in Figure 1c), the charge and discharge plateau corresponds to the voltage corresponding to the peak current in the CV curve. When current is measured using current density (Ag-1) instead of current (A), the absolute value obtained is the specific capacity (mAhg-1). It should be noted in particular that specific capacity can only be used as a measurement when CR2032 battery performance is independent of active material loading. In addition, cyclic voltammetry testing needs to specify a suitable voltage window and scan rate. For this purpose, there are two criteria for specifying the voltage window and scan rate: (1) Within the specified voltage window range, the electrode reaction should occur, and within The current should drop to zero at the end; (2) At the specified scan speed, the integrated capacity should be close to the theoretical capacity of the electrode material (based on this, the CR2032 battery scan rate is generally within the range of 0.1-10mVs-1).

　　Figure 1 (a) Voltage curve of cyclic voltammetry test (b) Current response and voltage curve (c) Voltage and integrated current curve The current response has a double electric layer response (idl), surface response (is) and volume response (iB) ) three types (as shown in Figure 2). The response of the electric double layer is similar to that of a physical capacitor. The size only depends on the capacity and scanning speed. Since there is no chemical reaction, it does not belong to the Faradaic process. In the CR2032 battery system, the electric double layer current is usually very small, and the surface response (is) and volume response (iB) together constitute the Faradaic current. They have different responses to the sweep speed. For details, please refer to the following formula:

　　The difference between surface response and volume response is that charge transport in the solid electrode bulk phase is much slower than transport in the liquid electrolyte. Faradaic processes on surfaces have capacitive properties, often called pseudocapacitance or electrochemical capacitance. The biggest difference between electrochemical capacitors and physical capacitors is: (1) The current response of the pseudocapacitor is related to the voltage, which is determined by the Gibbs free energy change of the surface redox reaction; (2) The chemical reaction process is not completed instantaneously. This means that the pseudocapacitive current response is not perfectly proportional to the sweep speed. The current contribution from redox processes in the active material bulk phase is generated by charge transport and is approximately proportional to the square root of the sweep velocity. For entirely diffusion-controlled electrode reactions, the linear relationship between peak current and square root of sweep rate is often used to calculate the diffusion coefficient. Reaction kinetics is an important characteristic of electrode materials. For this reason, most batteries are designed to have pseudocapacitive characteristics and still maintain large capacity characteristics. Under this concept, nanomaterials are often used as electrode materials to improve rate performance because nanomaterials themselves have the characteristics of large specific surface area, resulting in high pseudocapacitance contribution and fast diffusion-controlled current response. For simple diffusion-controlled electrochemical systems (mainly cathode materials), CV tests at different scan rates can usually obtain the diffusion coefficient. However, when the pseudocapacitive contribution cannot be ignored, its contribution value can be obtained by the following equation:

　　In the above equation, a and b are not constants, but parameters that can change according to the scanning speed. Under small changes in scan speed, a and b can be regarded as constants, and the b value can be derived from the curve of ipvsv1/2. The b value ranges from 0.5 to 1, corresponding to diffusion control and pseudocapacitive response respectively. Therefore, it is not difficult to understand that the b value can reflect whether the pseudocapacitance reaction or diffusion control dominates the CR2032 battery system. However, when the b value is at an intermediate value (such as b=0.8), the proportion of pseudocapacitance and diffusion control cannot be clearly determined. Compare. For this purpose, the specific proportion of pseudocapacitance contribution and diffusion control can be quantitatively calculated through the following equation:

　　However, the disadvantage of this method is that it requires cumbersome and complicated operations, but it can be simplified through matrix operations, such as using MATLAB or other calculation programs to run automatically. Another fatal flaw of this method is that this equation can only be used when increasing the scan speed causes polarization and thus causes a negligible peak shift. To this end, the following equation is proposed to quantitatively distinguish the two parts of the capacity contribution, which can still work even when the polarization cannot be ignored.

　　As shown in Figure 2f, a linear curve between the reciprocal of the square root of the scan speed (v-1/2) and the integrated capacity can be fitted. The obtained intercept represents the pseudocapacitive contribution capacity, which has nothing to do with the scan speed; the slope represents diffusion. The capacity contributed by the control is proportional to the square root of the sweep speed.

　　Figure 2 (a) Three current response modes (b), (c) Cyclic voltammetry test curves at different scan speeds (d) log2(ip) vslog2(v) curve (e) Pseudocapacitance contribution (f) Integrated capacity Linear fitting vs the reciprocal of the square root of the sweep speed 3. Current technology Constant current charge and discharge test is a test method commonly used to evaluate CR2032 battery capacity, reversibility, cycle performance and rate performance. It is tested under constant current (Figure 3a), And it is necessary to specify an electrochemical window that is the same as the CV test, and finally obtain a slope curve with an obvious platform (Figure 3b). In order to more easily see the potential platform, the integrated capacity curve (dQ/dVvsV) can be made. The peak generated corresponds to the platform of the GCD curve and is consistent with the peak of the CV curve. It is worth mentioning that the dQ/dV curve will inevitably produce a lot of burrs during the integration process. In order to avoid this problem, you can smooth the GCD curve in advance (remove white noise) and increase the interval between value points (by getting k>1 (Qn+k-Qn)/(Vn+k-Vn) Reduce error) Constant current charging and discharging usually require normalization of the active material mass or electrode area to obtain the weight specific capacity or area specific capacity. In CR2032 battery testing, a common unit of measurement for capacity is gravimetric capacity (mAhg-1), and gravimetric current density (A/g) or C is often used (1C refers to the current value that fully charges the CR2032 battery in 1 hour) Make current measurements. It should be pointed out that when making a full CR2032 battery to match the positive and negative electrode capacities, due to charge conservation, the area specific capacity (mAhcm-2) should be matched instead of the weight specific capacity (mAhg-1). Similarly, when conducting comparative experiments, what should be controlled is the area specific capacity (mAhcm-2) rather than the load capacity (mgcm-2). That is, C is more suitable as a current measure. The weight specific current density only affects the capacity when the mass Use only when it is negligible. However, in a large amount of current research work, weight-to-capacity is usually used for convenience, which means that a large number of excellent research results can only be obtained when the load is extremely small.

　　Figure 3 (a) Actual test current curve (b) Actual recorded voltage response and capacity relationship curve (c) Constant current charge and discharge test capacity voltage integral curve during the charge and discharge process (d) Cycle performance (e) Rate performance (f) The relationship between power density and corresponding energy density. The galvanostatic intermittent titration method (GITT) can measure the diffusion coefficient in each equilibrium state during the charge and discharge process. It needs special explanation: the discharge time should be short enough so that Eτ and A linear relationship can be established between τ1/2 (Figure 4c). Only in this way can the diffusion coefficient of the discharge process be calculated by the following equation:

　　The diffusion coefficient of the charging process can also be calculated by the following equation:

　　However, Vm refers to the molar volume of the intermediate product, which changes with the concentration of ion embedded electrode material; A is the electrochemical area, not the geometric area of the pole piece, nor the surface area of the active material, because the above two areas are both Connected to binders and conductive additives, charge uptake in electrode materials is often anisotropic, meaning that not all areas are electrochemically active for charges.

　　Figure 4 (a) Galvanostatic intermittent titration curve (b) Voltage vs time relationship diagram at a specific rest-discharge-rest stage (c) Voltage vs τ1/2 relationship diagram, τ is the discharge time (d) At a specific rest-charge- Voltage vs time relationship in the stationary phase Figure 4. Impedance technology Figure 5a shows the Nyquist diagram and the corresponding equivalent circuit (Randles circuit), where Rint is the inherent resistance of the CR2032 battery system, Cdl is the electric double layer capacitance, and Rct is the charge transfer resistance. , ZW is the Weber impedance, which is related to semi-infinite linear diffusion and is inversely proportional to the square root of frequency. The semicircle part of the high-frequency area in Figure 5a starts from Rint, and its radius is half of Rct. When an extra semicircle appears, one more capacitor should be considered in the equivalent circuit. When the frequency is low enough, the double layer capacitance can be considered to be in a closed state and it can be moved out of the equivalent circuit. The impedance equation at this time can be simplified to:

　　This is why the ideal Nyquist diagram has a 45° curve in the low frequency part. By fitting the linear relationship between Zre (real part) or Zim (imaginary part) and ω-1/2 (ω is the angular frequency), σ can be obtained and the diffusion coefficient can be calculated according to the following equation.

　　It can be obtained by integrating the open circuit voltage of Li+/Na+ at different concentrations (x), and A here is also the electrochemical area, not the geometric area of the electrode material or the specific surface area of the active material. In the actual CR2032 battery system, the electrode surface is rough, pseudocapacitance exists, and the actual diffusion of ions cannot be linearly diffused in one dimension, causing arcs instead of semicircles to appear in the high-frequency region, and the straight lines in the low-frequency region are not 45° (Figure 5c) . In this case, the electric double layer capacitor is replaced by a constant phase element.

　　Figure 5 (a) Randles circuit model (blue) and its corresponding electrochemical impedance spectrum (red) (b) Zre (real part) or Zim (imaginary part) and ω-1/2 (ω is the angular frequency) (c) Deviated Randles circuit model (blue) and its corresponding electrochemical impedance spectrum (red). Summary This work introduces basic electrochemical concepts related to CR2032 battery research, including testing of potential, current, and impedance. technology. For practical reasons, a large number of electrochemical equations are cited in the article, which are very meaningful for understanding and analyzing data. More importantly, the author discusses common testing techniques and analysis methods in CR2032 battery research, emphasizes the usage conditions of corresponding electrochemical equations, and points out the shortcomings of electrochemical analysis in current CR2032 battery research. This research result not only provides researchers with non-electrochemistry background with an introductory learning opportunity to systematically master electrochemical theory, but also points out potential errors for researchers who already have electrochemistry background knowledge, and improves everyone's understanding of electrochemistry knowledge. understanding and understanding, providing theoretical guidance for CR2032 battery design, research and optimization.

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