Exploration of key factors in power battery research and development

With the tense global energy situation and the improvement of human environmental protection awareness, the research and development of power batteries is in full swing around the world. At present, the research and development hotspots of power batteries are mainly concentrated in lithium-ion batteries, proton exchange membrane fuel cells and other types, and the application forms in automobiles are mainly hybrid vehicles. The development speed and maturity of power batteries have a vital impact on the commercialization process of electric vehicles in the market. For this reason, the research and development of power battery systems has always been concerned by governments and scientific research institutions of various countries. The research and development of power batteries has gone through two typical stages. First, there is the development boom of fuel cells, from the initial proposal of the concept of fuel cells, to the reduction of the platinum loading of fuel cell catalysts, to the update and breakthrough of fuel cell manufacturing technology, so that at present, proton exchange membrane fuel cells are leading the way and becoming the mainstream of fuel cells. As a result, the research on fuel cells 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 the selection of lithium materials, lithium-ion batteries have made many new and positive breakthroughs in battery safety and material costs. These highlights firmly attract the attention of electric vehicle developers. Whether it is the proton exchange membrane fuel cell or lithium-ion battery, which currently has more advantages in terms of research and development, they are typical representatives of power batteries. They have the general characteristics of power batteries, among which the ability to achieve large current discharge is an important feature of power batteries. Large current discharge can be understood as a discharge current density of up to 200 mA/cm2, or even higher. Compared with the small current batteries that have been maturely applied in the market, power batteries must have the characteristics of being able to work stably under large current discharge. Therefore, from the perspective of research and development, we are more concerned with the fundamental technology that the battery needs to ensure in order to achieve the set function under large current conditions.

For batteries with small current applications in mature markets, battery research and development focuses mainly on indicators such as capacity, storage, and sealing. The main features of this type of battery are that it can achieve small current discharge, stable discharge platform, long storage time, and good sealing. At the same time, we noticed that because the discharge current is small, the ohmic loss caused by internal resistance is very small, and the impact of ohmic loss on battery efficiency accounts for a small proportion, and the impact on the battery is not very obvious. However, for power batteries, large current is a typical feature of discharge. Under the premise of large current discharge, ensuring battery capacity and power is an important parameter that needs to be improved when power batteries are used in electric vehicles. Under high current conditions, the impact of internal resistance is more prominent. For example, when fuel cells, nickel-hydrogen batteries, lithium-ion batteries, zinc-air batteries, etc. are used as power batteries, they encounter thermal management problems to varying degrees. The heat generation of batteries during operation is one of the important factors affecting the working efficiency of batteries. The heat generated by the battery is mostly the heat released by the internal resistance consuming energy. It can be seen that the investigation of the internal resistance of power batteries has become one of the important issues that must be solved for power batteries to truly enter 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 popular proton exchange membrane fuel cells have undergone a research and development process with catalysts as the theme. Whether it is the selection of catalysts or the reduction of catalyst dosage, this process embodies the hard work of scientific researchers in developing power fuel cells. With the catalyst generally selected as platinum and the continuous reduction of platinum loading, the focus of fuel cell research and development has also been on the mass transfer rate problem under high current discharge state. After trial and research, we found that for power batteries with current density below 200 mA/cm2, mass transfer is not a bottleneck problem. Mass transfer factors such as oxygen supply and diffusion of product reactant concentration will not become the decisive factors restricting the battery discharge efficiency when the current density does not reach a very high state. After a more in-depth study of the battery, it was found that the internal resistance of the battery should be the technical bottleneck of the power battery at the current stage. The following verifies from both experimental and theoretical derivation that the internal resistance of the battery is an important factor affecting the battery discharge efficiency.

1 Materials and methods

In order to simplify the experimental system, the battery internal resistance experiments in this paper are all established in a horizontal zinc-air power 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 Company, thickness 0.2mm). The zinc paste is a viscous paste composed of zinc powder (Shenzhen Zhongjin Lingnan Technology Co., Ltd., IBC-2), 33% concentration potassium hydroxide solution (produced by Wuxi Zhanwang Chemical Reagent Co., Ltd., analytical grade) and sodium polyacrylate (produced by Sinopharm Chemical Reagent Co., Ltd., solid content greater than 40%). The method for making a commonly used gas diffusion electrode can be briefly described as follows: a strip of nickel foam (Changsha Liyuan New Materials Co., Ltd., with a specification of 90 mm×2.0 mm) is cut into a sheet with a length and width of 35 mm×35 mm as the substrate of the electrode, and then activated carbon (produced by Dongguan Jingmao Carbon Co., Ltd., model PA-1), graphite (Shanghai Colloid Chemical Plant, model F-2), acetylene black (produced by Jiaozuo Xinda Chemical Co., Ltd., 50% compression), homemade 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 catalyst A, and the above four substances (without manganese dioxide) are uniformly mixed in another proportion to make catalyst B. The preparation of the electrode can be described as applying catalyst B evenly and forcefully on the side of the nickel foam facing the air, and similarly applying catalyst A on the side of the nickel foam facing the electrolyte. The standard of application is that the catalytic substance fills the micropores of the nickel foam, and the surface of the nickel foam is visually smooth, uniform and flat. After coating, spray alcohol on both surfaces of the electrode to disperse the polytetrafluoroethylene emulsion, then place the prepared nickel foam electrode in an oven, bake at 150°C for 30 min, and take it out after cooling.

1.1 Making electrodes with and without film

The typical characteristic of horizontal zinc-air power battery is that air diffuses into the reaction area through the air surface of the electrode and participates in the electrochemical reaction. We made two types of gas diffusion electrodes, one with a polytetrafluoroethylene membrane and the other without a membrane, to investigate the characteristics of air participating in the electrochemical reaction through different diffusion modes and pathways through the electrode surface.

After we took the electrodes made in the above-described manner out of the oven, we took one electrode, coated a 0.1 mm thick polytetrafluoroethylene film (Shanghai Huahua Yeming Fluoroplastic Co., Ltd., thickness 0.1 mm) on the side facing the air, and rolled it on a roller press, controlling the thickness of the electrode sheet to be 0.6 mm after rolling. The other electrode was not coated with polytetrafluoroethylene film, and was also rolled in accordance with the same method to ensure the flatness and density of the nickel foam electrode, and the thickness was also controlled at 0.6 mm. The coated and uncoated electrodes were assembled in horizontal zinc-air power batteries. The assembly method of the battery can be described as follows: a battery tank with a size of 50 mm×50 mm×3 mm is made of copper sheet, in which an area of ​​35 mm×35 mm is selected (the actual effective area of ​​the gas diffusion electrode is 3.5 cm×3.5 cm), and a reaction tank is formed by surrounding it with organic glass strips. 15 g of zinc paste is spread in the tank, and the thickness of the zinc paste is kept equal to the height of the battery tank. Then a layer of alkaline diaphragm (Zhejiang Purui Technology Co., Ltd., model A) is spread on the zinc paste, and the surface of the diaphragm is replenished with 33% potassium hydroxide liquid (the amount is 1.5 mL), and then the gas diffusion electrode is spread on the diaphragm, and covered with a self-made breathable pressing sheet. A small pressing block is set on the side of the breathable pressing sheet facing the electrode to ensure that the positive and negative electrodes can fully contact through the diaphragm, and then the pressing sheet is clamped with the bottom of the battery tank to fix the gas diffusion electrode and the battery shell into one. At this time, the electrode sheet and the copper battery tank are simultaneously led out of the wires, and the experimental test can be carried out. The specific assembly method can also be found in the literature.

1.2 Design of experimental conditions and methods

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

2 Results and Analysis

2.1 Effect of gas diffusion mode on electrode discharge

According to the method described in Section 1.1, the prepared electrodes were assembled into a zinc-air power battery and the discharge characteristics were tested. Figure 1 is the constant current discharge polarization curve of the film-coated electrode and the non-film-coated electrode, and Figure 2 is the volt-ampere curve of the two electrodes, where curves 1 and 3 represent the volt-ampere curves of the film-coated electrode after initial installation and 40 minutes of discharge (1 A constant current discharge), and curves 2 and 4 represent the volt-ampere curves of the non-film-coated electrode after initial installation and 40 minutes of discharge. From the discharge results, it was found that the initial discharge difference between the two electrodes was not large. 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 the difference between the two electrodes in the initial discharge is not large. In the later stage of discharge, because the non-film-coated electrode lacks the protection of a waterproof and breathable membrane, the electrolyte will penetrate the porous structure of the electrode and form "water beads" on the air surface of the electrode, that is, the electrolyte seepage phenomenon, which has an adverse effect on air diffusion and participation in the reaction. However, simply from the perspective of gas diffusion in the initial stage of battery discharge, it can be concluded that the direct diffusion of air through the porous surface of the electrode and the diffusion through the PTFE membrane to participate in the reaction have little effect on the reaction results. It can also be understood that the diffusion and transmission method of gas is not the main problem restricting the efficiency of electrodes and even batteries.

In addition, the discharge polarization curve of the zinc-air power battery system can usually be divided into three stages: activation polarization zone, ohmic polarization zone and concentration polarization zone. In the ohmic polarization zone, the voltage and current show a linear relationship, and the slope of the straight line can basically represent the apparent internal resistance of the battery. We linearly fit the data of the ohmic polarization section of the two electrodes to obtain the apparent resistance of the electrode. The apparent resistance of the electrode after the initial installation and 40 min discharge is 0.26 W and 0.37 W respectively, and the apparent resistance of the electrode without the membrane is 0.27 W and 0.40 W respectively. It can be seen that the apparent internal resistance of the two electrodes is not much different at the beginning of the discharge after the production is completed, but after a period of use, the electrode without the membrane is affected by the electrolyte seepage, resulting in a relatively rapid increase in the apparent resistance. At the same time, we can compare that the energy loss caused by the resistance is the square of the resistance value. It can be seen that with the increase of the apparent internal resistance value of the battery in the later stage of discharge, the energy loss value is very considerable, especially in the high current discharge state, the result is more obvious.

2.2 Theoretical Analysis

For zinc-air power batteries, we explore the effect of battery internal resistance on discharge from the perspective of theoretical analysis. We know that the diffusion coefficient of oxygen in water is 10-9 m2/s, while the diffusion coefficient of oxygen in air is 10-6 m2/s. In view of these two known parameters, we consider two extreme cases. In the zinc-air battery system, we consider the oxygen diffusion coefficient to be 10-6 m2/s and 10-9 m2/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 0.6 mm. We assume that the air side of the electrode is full of oxygen with a molar concentration of CO2, and the oxygen concentration on the liquid surface of the electrode is C'O2. When the oxygen diffusion coefficient is the limit value of 10-6 m2/s and 10-9 m2/s, the change of its limiting current density value is examined. The formula for calculating the limiting current density is shown in formula (1).

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

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

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

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

From the theoretical calculation, we can see that under the most ideal oxygen diffusion effect, the current density of the electrode can reach 103mA/cm2, while when the oxygen supply is seriously insufficient, the current density of the electrode is only 10-1 mA/cm2. In actual experiments, the current density values ​​we usually get are between 100 and 200 mA/cm2, which is 1 order of magnitude away from the ideal value. It can be seen that the power shortage caused by oxygen diffusion is not the dominant problem, and it can also be seen that there is still a lot of room to improve the battery current density.

3 Conclusion

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

This paper uses typical experiments to illustrate that the discharge performance degradation caused by electrode mass transfer is not the dominant factor compared to the electrode internal resistance. Given that the discharge current density of the electrodes used in current power batteries is usually between 100 and 200 mA/cm2, or even higher than 200 mA/cm2, at this current density value 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 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 to realize their application in power vehicle equipment, then this energy loss caused by the change in internal resistance will have a considerable impact on the battery system, and accordingly, a series of battery management issues such as thermal management and energy consumption issues need to be added. This is also an important problem that must be solved before the power battery enters the practical stage. Therefore, in the process of improving various indicators of the power battery, paying more attention to the internal resistance factor should be an important method for the development of a more perfect power battery.