The so-called silica gel battery is to replace the sulfuric acid electrolyte in lead-acid battery with silica gel electrolyte. Its working principle is still similar to that of lead-acid battery
Silica gel electrolyte is a polymer formed by mixing SiO2 gel and a certain concentration of sulfuric acid in a certain proportion. It is a porous, multi-channel polymer, in which H2O and H2SO4 are adsorbed to form a solid electrolyte. Among them, SiO2 gel is produced by mixing sodium silicate and sulfuric acid.
The first is the formation of tetrahedral silicic acid Si (OH) 4. Silicic acid cannot exist stably in solution, and it will undergo polymerization to produce SiO2. SiO2 is in the form of gel in aqueous solution, so it is called silica gel. It has the following characteristics: the diameter of gel particles is about 10-6~10-4mm. It is a charged particle with active surface. The smaller the structure is, the greater the surface activity is. Therefore, it has strong adsorption, and the surface can adsorb water molecules to form the so-called hydration layer. However, if its skeleton grows longer and larger, its surface activity will gradually disappear, and even become a general silica. Therefore, we should try to control its particle size and make it produce highly active grains. At present, silicone colloids with ring structure can be made. The free oxygen on the periphery is protected by light groups to prevent the further combination of silicone and oxygen. This silicone colloid has thixotropy like other colloids, that is, when the silicone is stirred or vibrated, the internal structure is temporarily damaged, but when the external stirring or vibration stops, it returns to its original state, which is its thixotropy. If the conditions are not well controlled, the silica gel will quickly lose water and grow into coarse particles, losing thixotropic ability. Therefore, it is necessary to control the chemical environment in order to get the required silica gel.
The influence of silica gel electrolyte on battery performance is as follows.
(1) After using silica gel electrolyte, it has no effect on the working principle of the battery, and its reaction still conforms to the bipolar sulfation theory, that is
(2) The influence on the capacity of the battery there are generally two kinds of so-called battery capacity: one is called theoretical capacity, which is the capacity calculated according to Faraday’s law according to the amount of active substances, and it is also the capacity when all active substances are used for charging and discharging; The other is called actual capacity, that is, under certain charging and discharging conditions, the actual power consumed or discharged by the battery during charging varies with the charging and discharging. For various reasons, the actual capacity is always less than the theoretical capacity.
① Charging capacity the amount of electricity consumed by the battery during charging is called charging capacity Q charge. When charging with a constant current, its charging capacity is equal to the product of charging current value I charge and charging time t charge, and its unit is ampere hour, that is
If the charging current is not a fixed value, its capacity is equal to the sum of the product of each current value and the relevant time.
② Discharge capacity the discharge capacity of a battery refers to the amount of electricity actually discharged under a given working condition, that is, under a given temperature, discharge current intensity and termination voltage. When the battery discharges with a constant current value, its capacity Q discharge is equal to the product of discharge current value I discharge and discharge time t discharge, and its unit is ampere hour, that is
If the discharge current is not a fixed value, its capacity is equal to the sum of the product of each current value and the relevant time.
The discharge capacity of the battery is not a fixed value. There are many factors that affect the discharge capacity. It is not only related to the structure of the battery itself and the amount of effective substances in the electrode plate, but also related to the concentration of electrolyte, temperature and impurities contained, discharge conditions (temperature, discharge current), various process conditions in the manufacture of the electrode plate, etc.
When the electrolyte sulfuric acid in lead-acid battery is replaced with silica gel electrolyte, its capacity will be the same as that of lead-acid battery from the macro perspective, but the study found that after the use of silica gel electrolyte, the negative capacity of the battery will increase and the positive capacity will decrease. A comparative test was carried out, and the capacity ratio of sulfuric acid electrolyte to silica gel electrolyte battery is obtained as follows:
It can be seen from this that after using silica gel electrolyte in the battery, the utilization rate of positive active material decreases, while the utilization rate of negative active material increases, and the reason remains to be further studied.
(3) Influence on battery self discharge the so-called self discharge refers to the useless consumption of battery capacity when the external circuit is disconnected. The size of self discharge is usually expressed by the percentage of capacity reduction per unit time, that is
Where Q1 – initial capacity;
Q2 – Final capacity;
T – battery laytime (day and night).
There are many factors that cause the battery self discharge, such as the automatic dissolution of sponge lead on the negative plate, the automatic reduction of lead dioxide on the positive plate, and the mixing of harmful impurities in the electrolyte, which can cause its self discharge. It is necessary to study the causes and influencing factors of battery self discharge and explore the measures to reduce self discharge for improving the performance of battery.
① Self discharge of battery positive pole
a. Spontaneous reduction of PbO2 to PbSO4
b. The positive active material PbO2 contacts with the grid material (Pb Sb alloy) to cause self discharge
② Self discharge of battery cathode
a. The self discharge of the negative electrode is mainly determined by the automatic dissolution reaction of spongy lead.
Because the overpotential of hydrogen on lead is large, if there are no external impurities in the electrode and electrolyte, the automatic dissolution rate of lead (i.e. the self discharge of negative plates) is very slow, but when there is brocade in the grid alloy, the overpotential of hydrogen decreases by 0.5V, so the self discharge rate is greatly accelerated.
b. If oxygen is dissolved in the electrolyte, it can also promote the self discharge of the negative plate.
Because oxygen is easy to be reduced on lead, the rate of reaction is actually controlled by the diffusion rate of oxygen. If there is a separator (especially microporous rubber separator) that can inhibit the diffusion rate of antimony and oxygen to the negative electrode, the self discharge rate of the negative electrode plate will be significantly reduced.
The above is the case of self discharge (excluding self discharge caused by other impurities), but after using silica gel as electrolyte, the self discharge of the battery will be reduced, because silica gel electrolyte has such a characteristic that it hinders the diffusion of water generated during the reduction of the positive plate. Because silicate particles have hydration, the diffusion rate of water decreases. If the diffusion rate of water decreases, it will affect the equilibrium potential of PbO2 electrode. According to Nernst formula of electrode potential:
The difficulty of water diffusion means that H2O increases, making the reaction formula move in the opposite direction.
That is, the spontaneous reduction of the positive electrode slows down, and of course, the released O2 is also reduced. Less O2 is released, which reduces the probability of sponge oxidation, that is
Therefore, after replacing sulfuric acid electrolyte with silica gel electrolyte, the self discharge of battery is reduced.
(4) Influence on the water loss characteristics of the battery. Silica gel electrolyte has a good ability to resist the sulfation of the polar plate. Therefore, the capacity of a charged battery with good silica gel electrolyte can easily return to the normal level when it is recharged after a long time (more than one year). In contrast, after the acid battery of the same plate is charged and placed for a long time, the capacity cannot be restored to the rated value when it is recharged. This is due to the serious sulfation of the plate in the acid solution. Therefore, silica gel electrolyte can play a protective role on the electrode plate in the discharge state.
It can be seen from this that the silica gel battery has the characteristics of less maintenance workload. A large number of studies have shown that in conventional acid battery, when charging, the peak value of oxygen released by the positive plate is at the ratio of charging ampere hour to discharging ampere hour equal to 0.94; The peak value of oxygen released by the negative electrode is where the ratio of charge ampere hour to discharge ampere hour is equal to 1.0, that is, when the battery is charged, oxygen and hydrogen are not released in a stoichiometric manner, but oxygen is released first. Based on this, an “oxygen cycle” can be designed. Its principle is to let the newly generated oxygen on the positive electrode diffuse to the negative electrode and react with sponge lead,
(5) As mentioned earlier, the battery generates PbSO4 in the discharge state. If these PbSO4 can be well reduced to PbO2 and Pb during charging, the battery life will be longer. In fact, there is always a part of PbSO4 sinking or becoming irreducible PbSO4, which is the so-called sulfation, thus shortening the life of the battery.
When the battery discharges at a high current, due to the high current density and uneven, the plate will be bent (such as the battery used for starting in the car, whose discharge current is 3 times the rated capacity when starting and discharging). Therefore, after analysis, most of these batteries have the phenomenon of plate bending, which is also one of the reasons for shortening the battery life.
In addition, the sinking of PbSO4 often causes internal short circuit of the battery, shortens the service life of the battery, and the shedding of active substances also affects the service life. These problems are troublesome and difficult to solve in lead-acid batteries. However, these problems have been greatly improved in silica gel battery, because the colloid of silica gel battery is a whole, evenly distributed between the electrode plate and the separator. Unlike H2SO4 electrolyte, the electrolyte will produce delamination (the relative density of H2SO4 above is small, and the relative density of H2SO4 below is large). In addition, the generated PbSO4 will also be prevented from sinking by the colloid, which greatly reduces the short circuit phenomenon inside the sulfated battery of PbSO4. At the same time, because the silica gel is evenly filled between the electrode plate and the separator, it strengthens the mechanical strength of the electrode plate, thus reducing the bending of the electrode plate and the shedding of active substances. These factors improve the discharge performance of the battery and prolong the service life of the battery. Therefore, when used with new energy, the service life of silicone battery can reach 3-5 years. The service life of silicone battery used for starting in automobile is about 1.5 years, which is longer than that of lead-acid battery.
Therefore, silica gel batteries not only have less maintenance work, but also have improved performance,
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