Preface Alkaline zinc-manganese batteries, as a replacement for ordinary zinc-manganese batteries, are already in mass production in China [1]. As for the negative electrode, zinc paste discharge, is it a macroscopic reaction where the entire battery participates in the reaction or does it proceed in a specific sequence? How can we fully utilize its discharge behavior to improve battery performance? The authors explored this question using LR20 and LR14 batteries. [b]1 Experiment[/b] a. LR20 batteries (including high-mercury, low-mercury, and mercury-free types) produced under completely identical process conditions were discharged according to different discharge regimes. The batteries were then dissected to observe the reaction at the negative electrode. b. Two batteries with identical positive and negative electrodes, whose structures are shown in Figure 1, were compared in terms of discharge performance. c. The electrical performance of low-mercury batteries, differing only in zinc powder particle size, was compared. [img=200,162]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dc/0001/image1/t2801.gif[/img] Figure 1. Structures of LR14 batteries with different positive and negative electrode areas . 2. Results and Discussion 2.1 Reaction of Zinc Paste under Different Discharge Regimes 2.1.1 Reaction of Negative Electrode in LR20 Batteries under 1Ω Constant Resistance Continuous Discharge to Different Termination Voltages. By dissecting LR20 batteries that had undergone 1Ω continuous discharge with termination voltages of 1.2V, 1.0V, and 0.75V, the reaction of the negative electrode zinc paste was observed as shown in Figure 2: the higher the termination voltage, the more similar the zinc paste closer to the current collector is to the zinc paste that has not been discharged; the closer to the positive electrode, the more complete the reaction of the zinc paste (no metallic zinc particles are visible, and a light blue product is observed in the early stage of discharge). The lower the termination voltage, the thicker the zinc paste layer that has completely reacted near the positive electrode. As the discharge proceeds, some zinc paste appears dry. [img=238,147]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dc/0001/image1/t2802.gif[/img] 1 Negative electrode current collector 2 Unreacted zinc paste 3 Positive electrode 4 Fully reacted zinc paste 5 Partially reacted zinc paste Figure 2 Negative electrode reaction of LR20 battery when 1Ω is continuously discharged to different termination voltages 2.1.2 Negative electrode reaction of LR20 battery when 10Ω is intermittently discharged (4h/d) to different termination voltages By dissecting LR20 batteries that were intermittently discharged (4h/d) to 1.34V, 1.24V, and 0.9V, the negative electrode zinc paste showed the reaction shown in Figure 3: similar to the result of 1Ω discharge. This indicates that the macroscopic reaction sequence of the negative electrode of alkaline zinc-manganese battery during discharge is: from the zinc paste near the positive electrode to the area around the negative electrode current collector. [img=224,156]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dc/0001/image1/t2803.gif[/img] Figure 3 Schematic diagram of the negative electrode profile of LR20 battery when discharged at 10Ω intervals (4h/d) to different termination voltages. The macroscopic reaction sequence of zinc paste discharge is determined by the liquid-solid two-phase porous electrode. The specific conductivity of metallic zinc is much higher than that of KOH electrolyte, and the concentration gradient is caused by the consumption of OH- in the negative electrode reaction, which leads to different polarizations in different parts of the zinc paste during discharge. The higher the polarization current density, the smaller the electrolyte conductivity and the actual surface area of ​​the electrode, and the more uneven the polarization [2]. When the LR20 battery was continuously discharged at 1Ω constant resistance to 0.75V, some zinc paste was still very similar to that of the undischarged zinc paste. However, when it was intermittently discharged at 10Ω (4h/d) to 1.24V, such zinc paste was no longer present. This was due to the difference in polarization current density. During the discharge process, the thickness growth rate of the partially reactive zinc paste layer was much greater than that of the fully reactive zinc paste layer. The possible mechanism is that although the reaction rate of the zinc paste near the fully reactive layer was slow due to the difference in polarization, the formation of zinc oxide on the surface of the zinc particles during the discharge process increased the contact resistance between the zinc particles, thus making the polarization more uniform and pushing the reaction layer inward. During the high-current discharge of the cylindrical alkaline zinc-manganese battery, the entire negative electrode cylindrical reaction surface was macroscopically non-uniformly polarized, resulting in preferential reaction of the zinc paste near the positive electrode. 2.2 Effect of different positive and negative electrode areas on battery performance The discharge performance of the LR14 battery shown in Figure 1 is shown in Tables 1 and 2. The discharge curves are shown in Figures 4 and 5. Table 1. Continuous Discharge of LR14 Batteries with Different Positive and Negative Electrode Areas at 1Ω [table][tr][td][font=SimSun] Number [/font][/td][td][font=SimSun][size=3]Positive and Negative Electrode Area/cm²[sup] 2 [/sup][/size][/font][/td][td][font=SimSun][size=3]Theoretical Capacity of Positive Electrode/Ah[/size][/font][/td][td][font=SimSun][size=3]Continuous Discharge Time/min[/size][/font][/td][/tr][tr][td][font=SimSun] A [/font][/td][td][font=SimSun] 13.3 [/font][/td][td][font=SimSun] 8.15 [/font] 8.58 100.7 B 16.2 8.15 9.92 136.3 [img= 290,168 ] http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dc/0001/image1/t2804.gif [/img ] Figure 4 Table 2. LR14 Battery 1Ω Continuous Discharge Curves with Different Positive and Negative Electrode Areas. LR14 Battery 20Ω Intermittent Discharge (4h/d) with Different Positive and Negative Electrode Areas. [table][tr][td][font=SimSun] Number [/font][/td][td][font=SimSun][size=3]Positive and Negative Electrode Area/cm²[sup] 2 [/sup][/size][/font][/td][td][font=SimSun][size=3]Theoretical Capacity of Positive Electrode/Ah[/size][/font][/td][td][font=SimSun][size=3]Intermittent Discharge Time/h[/size][/font][/td][/tr][tr][td][font=SimSun] A [/font][/td][td][font=SimSun] 12.2 [/font] 8.15 8.8 98.3 B 15.8 8.15 9.3 115.7 [img=204,90]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dc/0001/image1/t2901.gif[/img] Figure 5. Discharge curves of LR14 batteries with different positive and negative electrode areas at 20Ω. As shown in Table 1, the theoretical capacity of battery B's negative electrode is only 15.6% higher than that of battery A's negative electrode, while its 1Ω discharge capacity increases by 38.5%. One reason for this is that the corresponding area of ​​its positive and negative electrodes increases by 21.8%. Meanwhile, as shown in Table 2, the capacity of battery B's negative electrode is only 6.3% higher than that of battery A's negative electrode, while its discharge capacity increases by 17.3%, but the corresponding area of ​​its positive and negative electrodes increases by 29.5%. The discharge curves show that under heavy load discharge conditions, the size of the corresponding area of ​​the positive and negative electrodes affects the discharge time of the high-voltage and low-voltage sections of the battery; under light load conditions, it mainly affects the discharge time of the low-voltage section. Because the negative electrode exhibits a certain reaction sequence on a macroscopic scale, the factors affecting the reaction area no longer solely depend on the amount of reactants. A larger corresponding area between the positive and negative electrodes results in a larger reaction area, smaller negative electrode polarization, and a correspondingly smaller ohmic internal resistance as the reaction proceeds. Therefore, the corresponding area between the positive and negative electrodes has a significant impact on electrical performance (especially under high-current discharge conditions). 2.3 Effect of Negative Electrodes Mixed with Different Particle Sizes on Battery Performance The performance of LR20 batteries with only different zinc powder particle sizes but identical processes are shown in Tables 3-4. Discharge curves are shown in Figures 6 and 7. Table 3. Continuous Discharge Performance of LR20 Batteries with Different Zinc Powder Particle Sizes at 1Ω [table][tr][td][font=SimSun] Number [/font][/td][td][font=SimSun][size=3]Zinc Powder Particle Size／μm[/size][/font][/td][td][font=SimSun][size=3]Theoretical Capacity of Positive Electrode／Ah[/size][/font][/td][td][font=SimSun][size=3]Theoretical Capacity of Negative Electrode／Ah[/size][/font][/td][td][font=SimSun][size=3]Discharge Time／min[/size][/font][/td][/tr][tr][td][font=SimSun] 1 [/font][/td][td][font=SimSun] 40～75 [/font][/td][td][font=SimSun] 18.2 [/font][/td][td][font=SimSun] 16.2 409 2 75～280 18.2 16.2 290 3 280 ～ 500 18.2 16.2 242 [/font][/td][/tr][/table] Note: Termination voltage 0.75V [img=206,100]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dc/0001/image1/t2902.gif[/img] Figure 6 1Ω continuous discharge curves of LR20 batteries with different negative electrode zinc powder particle sizes Table 4 Effect of zinc powder particle size on the 10Ω intermittent discharge (4h/d) capacity of LR20 batteries [table][tr][td][font=SimSun] Number [/font] [/td][td][font=SimSun][size=3]Zinc powder particle size / μm[/size][/font] [/td][td][font=SimSun][size=3]Theoretical positive electrode capacity / Ah[/size][/font] Theoretical negative electrode capacity (Ah) Discharge time (h) 1 40 ～ 75 18.2 16.6 110 2 75～ 280 18.2 16.6 99 3 280 ～500 18.2 16.6 91 Note: Termination voltage 0.75V [img= 192,90 ] http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dc/0001/image1/t2903.gif [/img][align=left]Figure 7 The effect of zinc powder particle size on the discharge performance of LR20 batteries at 10Ω is shown in Tables 3 and 4. Zinc powder particle size has a significant impact on the discharge performance of alkaline zinc-manganese batteries, especially under heavy load conditions. Figures 6 and 7 show that zinc powder of different particle sizes mainly affects the low-voltage period during battery discharge. The larger the zinc powder particle size, the faster the voltage drop at the end of discharge. This is because passivation occurs in the zinc paste at the end of discharge [3]: as the reaction proceeds, the negative electrode reaction area becomes smaller, and passivation may first occur in the outer reaction layer, then propagate deeper into the reaction layer, leading to a voltage drop at the end of discharge. Although smaller zinc powder particle size delays the occurrence of zinc paste passivation, which is beneficial to improving electrical performance, smaller zinc powder particle size also increases its specific surface area, activity, and the amount of zinc powder gas evolution [3], which poses considerable difficulties, especially for the mercury-free transformation of alkaline zinc-manganese batteries. The size of the zinc particle size mainly affects the speed of passivation at the end of discharge. Therefore, different adhesives and formulation processes can be selected in combination with suitable zinc powder to delay negative electrode passivation, without necessarily requiring the use of fine zinc powder. 3. Conclusions a. The macroscopic reaction sequence of negative electrode discharge in alkaline zinc-manganese batteries is: gradually proceeding from near the positive electrode to near the negative electrode current collector. This is due to the different polarizations of different parts of the porous electrode during discharge. b. Increasing the corresponding area of ​​the positive and negative electrodes can significantly improve the discharge performance of alkaline zinc-manganese batteries, especially the high-current discharge performance. c. The coarseness of the zinc powder mainly affects the speed at which negative electrode passivation occurs in the later stages of battery discharge. One reason for passivation is that the negative electrode discharges in a certain sequence, resulting in a reduction in the reaction area. Thanks to Professor Wang Lizhen of Zhengzhou University of Light Industry for his help with this paper. References : 1. China Battery Industry Association. Press Conference on the Second Domestic and International Test Results of Alkaline Zinc-Manganese Batteries. Battery Industry, 1998(2): 61. 2. Lü Mingxiang. Chemical Power Sources. Tianjin: Tianjin University Press, 1992: 42. 3. Chen Yongxin. Research on Mercury-Free Alkaline Manganese Batteries. Battery, 1997(5): 196.