CR2450 battery

　　Exploration of key factors in CR2450 battery research and development

　　With the tense global energy situation and the increasing awareness of human environmental protection, scientific research on power batteries is being carried out around the world. At present, the research and development focus of power batteries is mainly focused on lithium-ion batteries, proton exchange membrane fuel cells and other types, and the application form in automobiles is mainly hybrid vehicles. The research and development speed and maturity of power batteries have a crucial impact on the commercialization of electric vehicles in the market. For this reason, the research and development of CR2450 battery systems has always attracted the attention of governments and scientific research institutions of various countries. The research and development of power cells has gone through two typical stages. First, there was the development boom of fuel cells, from the initial proposal of the fuel cell concept, to the reduction of the platinum loading of the fuel cell catalyst, to the updated breakthroughs in the fuel cell manufacturing process. So far, proton exchange membrane fuel cells have taken the lead and become the mainstream of fuel cells. As a result, fuel cell research has entered a state of steady development. Secondly, the development boom of lithium-ion batteries in electric vehicles has always been no less than that of fuel cells. After experiencing the selection of lithium materials, lithium-ion batteries have many new and positive developments in terms of battery safety, material cost, etc. breakthroughs, these highlights have firmly attracted the attention of electric vehicle developers. Whether it is proton exchange membrane fuel cells or lithium-ion batteries, which have many advantages in terms of current research and development, they are typical representatives of power batteries. They have common characteristics of power batteries, among which the ability to achieve large current discharge is an important factor of power batteries. feature. Large current discharge can be understood as the discharge current density can reach 200mA/cm2, or even higher. Compared with the current low-current batteries that have been maturely used in the market, power batteries need to have the characteristics of working stably in the high-current discharge state. Therefore, from a research and development perspective, we pay more attention to the high-current state. , the battery is the fundamental technology required to achieve the set functions.

　　For batteries with mature markets for small current applications, battery research and development mainly focuses on capacity, storage, sealing and other indicators. The main features of this type of battery are that it can achieve low-current discharge, has a stable discharge platform, has a long storage time, and has good sealing properties. At the same time, we have noticed that because the discharge current is small, the ohmic loss caused by internal resistance is very small. The impact of ohmic loss on battery performance is very small, and the impact on the battery is not very obvious. However, for power batteries, high current is a typical characteristic of discharge. Under the premise of high current discharge, ensuring battery capacity and power is an important parameter that needs to be improved when the CR2450 battery is used in electric vehicles. Under high current conditions, the influence of internal resistance is more prominent. For example, when fuel cells, nickel hydride batteries, lithium-ion batteries, zinc-air batteries, etc. are used as power batteries, they encounter thermal management problems to varying degrees. The heat generation problem of the battery during operation is one of the important factors affecting the battery efficiency. First, most of the heat generated by the battery is the heat released by the energy consumed by the internal resistance. It can be seen that the investigation of the internal resistance of the CR2450 battery has become one of the important issues that must be solved to truly put the CR2450 battery into practical use.

　　Looking back at the development history of power batteries, especially taking typical fuel cells as an example, both solid oxide fuel cells and the currently promising proton exchange membrane fuel cells have gone through a research and development process with the theme of catalysts. The choice is still to reduce the amount of catalyst. This process embodies the efforts of scientific researchers in developing power fuel cells. As the catalyst is generally selected as platinum and the platinum loading continues to decrease, the focus of fuel cell research and development has also focused on the mass transfer rate in the high current discharge state. After trial and research, we found that for current densities lower than 200mA /cm2 CR2450 battery, mass transfer is not a bottleneck problem. Mass transfer factors such as the supply of oxygen and the diffusion of product and reactant concentrations will not become the decisive factor restricting the discharge performance of the battery when the current density does not reach a very high state. However, after more in-depth research on the battery, it was found that , The internal resistance problem of the battery should be the technical bottleneck of the CR2450 battery at the current stage. The following verifies that battery internal resistance is an important factor affecting battery discharge performance from both experimental and theoretical derivation.

　　1Materials and methods

　　In order to simplify the experimental system, the experiments on battery internal resistance in this article are all based on the horizontal zinc air CR2450 battery system. The characteristic of this battery system is that the gas diffusion electrode is used horizontally, with one side of the electrode facing the air and the other side facing the zinc electrode.

　　The zinc electrode is composed of zinc paste and copper sheet current collector (Shanghai Zhiying Nonferrous Metals Co., Ltd., thickness 0.2mm). The zinc paste is composed of zinc powder (Shenzhen Zhongjin Lingnan Technology Co., Ltd., IBC-2) and 33% concentration potassium hydroxide. A viscous paste composed of solution (produced by Wuxi Zhanzhan Chemical Reagent Co., Ltd., analytical grade) and sodium polyacrylate (produced by Sinopharm Chemical Reagent Co., Ltd., solid content greater than 40%). The conventional manufacturing method of gas diffusion electrodes can be briefly described as follows: cutting strip-shaped nickel foam (Changsha Liyuan New Materials Co., Ltd., specification: 90mm×2.0mm) into a sheet with a width of 35mm×35mm as the substrate of the electrode. Then, activated carbon (produced by Dongguan Jingmao Carbon Co., Ltd., model pA-1), graphite (Shanghai Colloidal Chemical Plant, model F-2), acetylene black (produced by Jiaozuo Xinda Chemical Co., Ltd., 50% compression), self-modified Manganese dioxide and polytetrafluoroethylene emulsion (produced by Shanghai Buaze Industry and Trade Co., Ltd., with a concentration of 30%) are uniformly mixed in a certain proportion as catalytic substance A. In addition, the above four substances (without manganese dioxide) are mixed according to the The other proportion is evenly mixed to make catalyst B. The production of the electrode can be described as applying catalyst B evenly on the side of the foamed nickel facing the air. Similarly, apply catalyst A on the side of the foamed nickel facing the electrolyte. The standard of application is to catalyze The substance is filled into the micropores of the nickel foam, and the surface of the nickel foam is visually inspected to see that it is smooth, uniform and flat. After application, spray alcohol on both surfaces of the electrode to disperse the polytetrafluoroethylene emulsion, then place the prepared nickel foam electrode in the oven, bake at 150°C for 30 minutes, and take it out after cooling.

　　1.1 Preparation of film-coated and non-film-coated electrodes

　　The typical feature of horizontal zinc air power batteries is that air diffuses through the air surface of the electrode into the reaction area and participates in the electrochemical reaction. We made two types of gas diffusion electrodes, with the air surface coated with a polytetrafluoroethylene film and without a film, to examine the characteristics of air participating in electrochemical reactions through different diffusion modes and pathways on the electrode surface.

　　After we take out the electrode made according to the above description from the oven, take one electrode and apply a layer of 0.1mm thick polytetrafluoroethylene film (Shanghai Huahuaye Ming Fluoro Plastics Co., Ltd., thickness) on the side facing the air. 0.1mm), roll it on a roller press, and control the thickness of the electrode sheet after rolling to 0.6mm. The other electrode is not coated with PTFE film, but is also rolled according to the law to ensure the flatness and density of the nickel foam electrode, and the thickness is also controlled at 0.6mm. The film-coated and uncoated electrodes were assembled in horizontal zinc aerodynamic batteries. The assembly method of the battery can be described as follows: use copper sheet to make the battery tank with a size of 50mm×50mm×3mm, and select an area size of 35mm×35mm (the actual effective area of the gas diffusion electrode is 3.5cm×3.5cm). The surrounding area is Organic glass strips surround the reaction tank, and 15g of zinc paste is spread in the tank. The thickness of the zinc paste is kept equal to the height of the battery tank, and then a layer of alkaline separator (Zhejiang Purui Technology Co., Ltd., model: A), use 33% potassium hydroxide liquid to rehydrate the surface of the separator (the dosage is 1.5mL), then lay the gas diffusion electrode flat on the separator, cover it with a self-made breathable press sheet, and set the breathable press sheet toward the side of the electrode Use a small pressing piece to ensure that the positive and negative electrodes can fully contact through the separator, and then clamp the pressing piece to the bottom of the battery tank to fix the gas diffusion electrode and the battery shell into one body. At this time, lead out the wires from the electrode sheet and the copper battery tank at the same time , experimental testing can be carried out. Specific assembly methods can also be found in the literature.

　　1.2 Design experimental conditions and methods

　　We conducted a discharge test on the two assembled batteries under the same environmental conditions. The experimental environmental conditions were room temperature 25°C and relative humidity 65%. The zinc-air battery was discharged open in the natural environment. The test content is mainly divided into two parts. On the one hand, it is the constant current discharge characteristics of the experimental battery. Both batteries are discharged under a 1A constant current state to obtain the discharge polarization curves of the two electrodes. On the other hand, two electrodes with different diffusion modes were remade to test their voltammetry curves at the beginning of discharge and after 40 minutes of discharge to obtain the difference in apparent internal resistance between the two electrodes with different gas diffusion modes.

　　2Results and analysis

　　2.1 The influence of gas diffusion mode on electrode discharge

　　Assemble the prepared electrodes into a zinc air CR2450 battery according to the method described in Section 1.1, and test the discharge characteristics. Figure 1 is the constant current discharge polarization curve of the film-coated electrode and the non-coated electrode. Figure 2 is the voltammetry curve of the two electrodes. Curves 1 and 3 respectively represent the initial installation of the film-coated electrode and after 40 minutes of discharge (at 1A Constant current discharge) volt-ampere curve, curves 2 and 4 represent the volt-ampere curves of the membrane-free electrode after initial installation and 40 minutes of discharge respectively. It is found from the discharge results that there is not much difference in the initial discharge of the two electrodes. Even though Figure 1 shows that the constant current discharge time of the two electrodes is different, that is, the difference in discharge capacity, it can also be seen that there is not much difference between the two electrodes in the early stage of discharge. In the later stages of discharge, because the membrane-less electrode lacks the protection of a waterproof and breathable membrane, the electrolyte will penetrate through the porous structure of the electrode and form "water droplets" on the air surface of the electrode, that is, the electrolyte seepage phenomenon. This phenomenon is harmful to Air diffusion participates in the reaction and produces adverse effects. However, simply from the perspective of gas diffusion in the early stage of battery discharge, it can be concluded that direct diffusion of air through the porous surface of the electrode and diffusion through the pTFE membrane to participate in the reaction have little impact on the reaction results. It can also be understood that the diffusion transmission of gas Method is not the main issue restricting the efficiency of electrodes or even batteries.

　　In addition, the discharge polarization curve under the zinc air CR2450 battery system can usually be divided into three stages: activation polarization zone, ohmic polarization zone and concentration polarization zone. In the ohmic polarization zone, voltage and current show a linear relationship, and the straight line The slope of can basically represent the apparent internal resistance of the battery. We linearly fitted the data in the ohmic polarization section of the discharge of the two electrodes to obtain the apparent resistance of the electrodes. The apparent resistances corresponding to the initial installation of the film-coated electrode and after 40 minutes of discharge were 0.26W and 0.37W respectively. The apparent resistance of the non-film-coated electrode The apparent resistances at initial installation and 40min discharge are 0.27W and 0.40W respectively. It can be seen that there is not much difference in the apparent internal resistance between the two electrodes at the beginning of discharge after production. However, after a period of use, the electrode without membrane is affected by the leakage of electrolyte, resulting in a rapid increase in apparent resistance. At the same time, we It can be compared that the energy loss caused by resistance is the square of the resistance value. It can be seen that as the apparent internal resistance value increases in the later stage of battery discharge, the energy loss value is very considerable, especially in the high current discharge state. The results are even more obvious.

　　2.2 Theoretical analysis

　　For zinc air power batteries, we explore the impact of battery internal resistance on discharge from the perspective of theoretical analysis. We know that the diffusion coefficient of oxygen in water is 10-9m2/s, and the diffusion coefficient of oxygen in air is 10-6m2/s. In view of these two known parameters, we consider two limit situations. In the zinc-air battery system, we consider two critical states with oxygen diffusion coefficients of 10-6m2/s and 10-9m2/s. Figure 3 is a typical gas diffusion electrode model for horizontal applications, that is, one side of the electrode faces the air and the other side faces the zinc paste (containing electrolyte), and the electrode thickness is 0.6mm. We assume that the air surface of the electrode is entirely oxygen, the molar concentration is CO2, and the oxygen concentration of the liquid surface of the electrode is C'O2. When the oxygen diffusion coefficient is the limit value 10-6m2/s and 10-9m2/s, examine the change of the limit current density value. The formula for calculating the limiting current density is as shown in Equation (1).

　　In the formula: C'O2=9.378mol/L; d=0.6mm; D=10-6~10-9m2/s; F=96500C/mol; n=4 (1molO2 consumes 4mol electrons).

　　CO2 is the molar concentration of oxygen in the air, calculated as:

　　1.429 (density) × 1000 × 0.21 (volume percentage) / 32 × (molecular weight) = 9.3778125 mol/L.

　　When the oxygen diffusion coefficient is 10-6m2/s, substitute it into the formula and get JL=6033mA/cm2. When the oxygen diffusion coefficient is 10-9m2/s, substitute it into the formula and get JL=0.6033mA/cm2.

　　From this theoretical calculation, we can see that under the ideal oxygen diffusion effect, the current density of the electrode can reach 103mA/cm2, but when the oxygen supply is severely insufficient, the current density of the electrode can only be 10-1mA/cm2. In actual experiments, the current density value we usually obtain is between 100 and 200mA/cm2, which is an order of magnitude different from the ideal value. It can be seen that the power shortage caused by the diffusion of oxygen is not the dominant problem. Moreover, it can also be seen that there is still a lot of room for improving battery current density.

　　3Conclusion

　　The development of power batteries has experienced a rapid process in the past ten years. Through continuous efforts, scientific researchers have gradually overcome several important factors that restrict the maturity of power batteries.

　　This article illustrates through typical experiments that the decrease in discharge performance caused by electrode mass transfer is not the dominant factor compared to the internal resistance of the electrode. In view of the current density of electrodes used in power batteries, the discharge current density is usually between 100 and 200mA/cm2, or even higher than 200mA/cm2. At such a current density level, the mass transfer effect of the electrode should not be the decisive factor restricting the electrode performance. . Due to the change in internal resistance caused by the electrode, the difference in the battery discharge effect can be clearly found during the discharge process. In the experiment, we used small-sized electrodes. If the electrodes and batteries are enlarged accordingly, it can be realized on power vehicle equipment. application, then this energy loss due to changes in internal resistance will have a considerable impact on the battery system. Correspondingly, a series of battery management issues such as thermal management and energy consumption issues need to be added. This is also true It is an important issue that must be solved before power batteries enter the practical stage. Therefore, in the process of improving various indicators of power batteries, paying more attention to the internal resistance factor should be an important method for the development of power batteries to be more complete.

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