The structure of lead-acid battery mainly has four parts: positive electrode and negative electrode, electrolyte, separator, battery tank.
(1) The positive electrode and the negative electrode are made of lead-antimony alloy as the skeleton, which is tightly coated with lead paste. After chemical treatment, the positive and negative plates form their respective active materials. The active material of the positive electrode is PbO2, and the negative electrode is PbO2. The active material is sponge lead. During the flow process, the negative electrode is oxidized and the positive electrode is reduced. The negative plate is generally dark gray, and the positive plate is dark brown.
(2) Electrolyte The electrolyte of lead-acid battery is dilute sulfuric acid solution; the electrolyte of silica gel battery is a certain concentration of sulfuric acid with a certain concentration of silica gel, that is, a soft solid silica gel electrolyte. The electrolyte of alkaline batteries is potassium hydroxide solution with a concentration of 22% to 40%.
(3) Separator separators include water separators, glass fiber separators, microporous rubber separators, plastic separators, etc. The function of the separators is to minimize the distance between the positive and negative electrodes without short-circuiting each other; the separators It can prevent the bending and deformation of the electrode plate, and prevent the active material from falling off. To play these roles, the separator is required to have a high degree of porosity, acid resistance, not easy to deform, good insulation performance, and good hydrophilicity and sufficient. Mechanical strength.
(4) The battery slot is the battery shell, which is an integral structure. The shell is divided into three or six grids that are not connected to each other by the partition wall; there are protruding auxiliary bars at the bottom to place the electrode plate group; The gap is used to stack the active material falling off the plate to prevent the plate from short circuit, and the shell material is generally made of rubber or engineering plastics.
The working principle of the battery includes the discharging process and the charging process.
(1) During the discharge process, the negative plate lead plate is affected by two aspects in the electrolyte. On the one hand, it has a tendency to dissolve in the electrolyte, so a small amount of lead enters the solution to generate Pb2+ (oxidized) and negatively charged on the plate; On the one hand, since Pb2+ has a positive charge, the plate has a negative charge, and the positive and negative charges attract each other. At this time, the Pb2+ ions tend to be deposited on the plate. When the two are in dynamic equilibrium, the negative plate has a negative potential relative to the electrolyte, and its electrode potential is about -0.1V.
The reaction between Pb2+ and the dissociation in the electrolyte produces PbSO4, and the solubility of PbSO4 is very small, so it is precipitated from the solution after being formed and attached to the electrode. The reaction formula is (1):
The reaction of the positive electrode is that in general, the amount of PbO2 dissolved in H2SO4 is very small, but under the condition of passing current, the amount of dissolution will increase slightly, so a small amount of PbO2 enters the electrolyte and interacts with H2O during discharge. Pb(OH)4 is generated; however, it is unstable, and it is quickly electrolyzed into Pb4+ and OH-. Pb4+ is deposited on the positive plate, so that the positive plate has a positive potential. When it reaches dynamic equilibrium, its electrode potential is about +2.0V . When Pb4+ is deposited on the positive plate, the 2 electrons from the external circuit are captured by Pb4+, and the generated Pb2+ reacts with the electrolyte to become PbSO4. These PbSO4 are adsorbed on the positive plate in solid form , so the reaction on the positive electrode is (2):
So the total reaction of the discharge process is (3):
The discharge process is the process of converting chemical energy into electrical energy. At this time, the active material PbO2 of the positive electrode becomes PbSO4, and the active material sponge lead of the negative electrode changes to PbSO4. The H2SO4 molecules in the electrolyte continue to decrease, and the H2O molecules are gradually consumed, and the H2O molecules increase accordingly. The relative density of the electrolyte decreases. A typical battery discharge curve is shown in Figure 4.
(2) Charging process The charging process is to convert electrical energy into chemical energy. During charging, PbSO4 on the negative plate enters the solution and dissociates into Pb2+. The H2O in the electrolyte dissociates into H+ and OH-. On the negative electrode, Pb2+ on the negative electrode plate gets two electrons at this time and is reduced to Pb (precipitated in a spongy solid state), at this time, the H+ in the electrolyte moves to the negative electrode and combines with H2SO4 near the negative electrode. The negative reaction is (5):
On the positive electrode, the Pb2+ on the positive plate is oxidized under the action of the external power supply, loses two electrons and becomes Pb4+, which combines with OH- to form Pb(OH)4, which is then decomposed into PbO2 and H2O, and the ions move to the positive electrode Combined with H+ to generate H2SO4, the reaction formula is (6):
Therefore, the overall reaction formula of the charging process is (7):
It can be seen from the above formula that during the charging process, the effective substances on the positive and negative plates gradually recover, and the specific gravity of the electrolyte H2SO4 gradually increases, so the degree of charging can also be judged from the value of the increased specific gravity. Most of the reactions are PbO2 and spongy Pb. If charging is continued, water will be decomposed, the positive electrode will release O2, and the negative electrode will release H2.
The charging curve of the battery is shown in Figure 8.
Read more: How are solar cells structured and how they work?