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    Several factors affecting the nimh battery performance - why limited life
    Publish:Sino Energy Corporation    Time:2016-09-20

    There are many factors that can influence the properties of nickel metal hydride batteries, including positive/negative plate of the base material, the kinds of hydrogen storage alloys, particle degree of the active material, the type and quantity of additives, as well as the production process, the electrolyte, the diaphragm, into many aspects, such as. Below is additive (Co), electrolyte, diaphragm and into a process affect the performance of the battery of this a few aspects are briefly discussed.


    A positive add CoO effect the performance of the electrode


    Cobalt is added to the Ni (OH) 2 electrode, mainly form the high conductivity of CoOOH, in the process of charging activation stage, was oxidized into CoOOH, so as to improve the electrical conductivity of pole piece, due to the irreversible reaction, thus adding Co, with no contribution for the capacity of the electrode.


    Cobalt is added in the Ni (OH) 2 electrode can increase its proton conductivity and electrical conductivity, so as to improve the utilization of positive active material, improve the charge and discharge performance and increase the oxygen evolution potential, thus reduce charging voltage to improve charging efficiency. But excess cobalt adding not only lead to battery costs increase, will also reduce the discharge voltage.


    Research has shown that 10% is added in the nickel hydroxide electrode Co (OH) 2 can decrease the active substances of diffusion resistance and increase the depth of electrode discharge (DOD). Data show: cobalt powder content on FaPaoNie the influence of electrode given as follows:


    Cobalt content of Ni (OH) 2 electrode quality specific capacity (unit: mAh/g)


    Cobalt content of Ni (OH) 2 electrode specific storage volume effects (unit: mAh/ml)


    On the surface of partial oxidation of CoO showed the best activity, surface without preoxidation CoO (i.e., S - CoO) are quite active, but due to their instability in air, depth of the contact with oxygen oxidation and lower the activity.


    As surface oxidation degree of deepening, the activity of CoO gradually decreased, but before the 20% decline is not obvious, the oxidation degree of more than 20%, a sharp drop in the activity. This is because the surface of high state of Co3O4 too much and affect the CoO in into the shift.


    The influence of adding amount on the anode utilization: add a tiny amount of (2 wt %) surface without preoxidation CoO can obtain higher utilization rate of the positive active material, within the scope of 5 wt % to 10 wt % to get the best effect. After the addition amount is higher than 10 wt %, battery capacity decline instead, this is because the amount is too high, reduced the filling amount of active material, the battery capacity is unlikely to improve, but also increases the positive production costs.


    The battery for the amount of cobalt large current discharge performance: the influence of the addition of cobalt in improving battery large current discharge performance has the very good effect, join the more quantity, large current discharge performance, the better, but to join the quantity too much, the more costs also rose, and battery capacity decline, suitable ratio of 5 to 10 wt % wt %.


    Cobalt during electrical activation (charge) for the first time, due to the oxidation of Co (OH) 2 potential than low oxidation potential of Ni (OH) 2, this leads to the Ni (OH) 2 into NiOOH before forming a stable CoOOH, greatly reduce the contact resistance between the particles, both also greatly improve the electrical conductivity of the particles and matrix. If the end of the discharge voltage is not significantly lower than 1.0 V, subsequent reaction is CoOOH no longer participate in battery, so that the cathode is obtained corresponding to provide the total charge of the charge in advance. If subsequent discharge make available capacity of the anode has run out, but because of the charge in advance, the cathode is still discharge reserves, to some extent, it can avoid the battery at the end of the cathode of hydrogen evolution, and ensure the efficiency of hydrogen compound.


    Cobalt additives above has some advantages, but its also have disadvantage such as micro cause short circuit and self-discharge increases, some people think that the reasons:


    Cobalt compounds in the anode dissolve cobalt complex formed in strong alkali, after it moved to the diaphragm, the diaphragm molecular oxidation, reduction of itself of cobalt and deposition on the diaphragm, cobalt complex but also through the diaphragm to the cathode, when negative charge reduction of cobalt and deposited. Is deposited on the diaphragm of the cobalt accumulated to a certain number can be formed through the diaphragm thin "cobalt bridge", electron conduction, micro short circuit, the initial damage later developed into a complete short circuit, so that the battery failure, is deposited on the cathode of cobalt if not generate "cobalt bridge", also may be due to the cathode surface grew up in cutting-edge cobalt during the charge and discharge point discharge and cause the short circuit. The cathode in Mn dissolution can also lead to micro short circuit, cobalt and manganese dissolution will accelerate dissolution and alloy oxidation. This kind of micro short circuit is an important reason for the battery self-discharge. Battery liquid injection after rapid sealing and sealing part immediately after filling the electricity can be reduced likelihood of developing micro short circuit, namely first CoOOH cobalt can be converted to.


    Affect the performance of battery electrolyte


    Electrolyte as an important part of the battery, its composition, concentration, quantity and the amount and type of impurities will be crucial influence on the performance of the battery. It directly affect the capacity of the battery capacity, internal resistance, cycle life, properties, such as internal pressure.


    Found by comparison and electrolyte generally USES the KOH solution about 6 mol/l (or in a NaOH instead of KOH), of course also have to join a small amount of other ingredients in electrolyte such as LiOH, but for some impurities such as carbonate, chloride, sulfide are demanding.


    Of positive and negative battery can only produce electrochemical reaction in the electrolyte. For a seal battery products, the space is certain. If the electrolyte is too much, will cause the seal tone room space is smaller and make a battery in the process of charging and discharging internal pressure rise; Electrolyte, on the other hand, too much jam diaphragm hole, to prevent the transmission of oxygen, adverse hydrogen compound quickly, also can make the battery internal pressure rise and may cause pole piece of oxide sheet was blunted capacity fell, the internal pressure may increase the risk of battery leakage, alkali, make the battery failure. But if the electrolyte is too little, will make the pole piece cannot be fully impregnated into the electrolyte, electrochemical reaction and incomplete or some parts of the pole piece cannot produce electrochemical reaction, makes the battery capacity can not meet the design requirements, internal resistance, cycle life.


    Mainly using KOH electrolyte is not usually NaOH, the main reason lies in KOH than conductivity is higher than NaOH, and add a small amount of LiOH in KOH solution in order to improve the discharge capacity of the battery. It is now necessary to explore the impurities in the electrolyte and LiOH effect the performance of the battery:


    In the process of charge and discharge for a long time, Ni (OH) 2 particles will gradually become coarse, make it difficult to recharge. Reason is the temperature too high, electrolyte concentration and the presence of metal impurity. When joining LiOH, adsorption around the active material particles, it can prevent particles increase, to keep its highly fragmented. But to join LiOH shoulds not be too much, otherwise it will affect electrical activation process. Generally think that iron will reduce oxygen overvoltage, make the battery efficiency drops, carbonate generates the film in the surface of electrode, make the battery internal resistance increases; Sulfide can form dendritic products, causing the battery short circuit; Silicate can make electrode capacity loss; Chloride electrode corrosion caused by; While organic compound can increase the self-discharge effect and side effects, etc. So you must control the impurity content in an electrolyte.


    Finally, also must pay attention to the electrolyte concentration, in order to reduce concentration resistance.


    Why the battery (circulation) in the process of storage and use of internal resistance increases and discharge capacity decrease and charging efficiency to reduce?


    I think the reason is various:


    First, additives Co in the storage and use process toward the deep diffusion of the pole piece or migration, which results in the decrease of Co content on the surface of the plate, so as to make the contact resistance on the surface of the sheet increases performance for internal resistance (up), so as to reduce the charging efficiency and oxygen evolution potential, eventually led to the decrease of the discharge capacity.


    Second, in the process of circulation, pole piece by electrolytic corrosion, lead to loose, fall off the pole piece powder or bad contact (between particles and particles, particles and substrate) result in higher internal resistance, and excessive charge/discharge damage cause pole piece.


    Thirdly, may be due to the plate, the diaphragm of the electrolyte drain and sucked out, due to the electrochemical reaction is always started from the surface to deep development, then the resulting electrochemical reaction is incomplete, led to the decrease of the discharge capacity; And due to the lack of electrolyte causes increased internal resistance (concentration resistance and higher ionic conduction resistance/migration resistance), higher charging voltage, discharge voltage drop.


    Fourthly, may be due to the moisture in the electrolyte after cycle or stored for a period of time, in some form of it is not clear, such as water of crystallization and fettered by van der Waals force and hydrogen bond force, and cannot participate in the electrochemical reaction (i.e., increase the concentration of the electrolyte), causes in the process of electrochemical ionic conduction is difficult, the internal resistance increases, the charging potential rise, discharge voltage drop, eventually led to the decrease of the discharge capacity.


    Finally, also may be due to the battery in the process of circulation or store, the electrolyte is redistributed, diffusion and penetration into the pole piece of the deep, which decreased the electrolyte on the surface of the electrode, the electrochemical reaction is always started from the surface to deep development, then so incomplete electrochemical reaction in which a series of problems.


    Battery, of course, in the process of using excessive charge/discharge, the cell pressure, hydrogen/oxygen at the same time of leaking out of the electrolyte, thus making electrolyte dried up, also is one of the important reasons.


    Anatomy of the open storage and used batteries, will find inside the battery plate and diaphragm paper drying (visual), may be one reason for the above or the outcome of combined action of several factors.


    Three diaphragm effect the performance of the battery


    Diaphragm as battery isolation between positive and negative electrode plate, first of all it must have good electrical insulation, next, because it in the electrolyte in the wet state, it must have the good alkali resistance; And should have good air permeability, etc. So we should choose in relatively wide temperature range (55 ℃ -- 85 ℃) maintain electronic stability, volume stability and chemical stability, high resistance for the electronic, low resistance, for the ion to gas diffusion as far as possible thin isolation plate.


    The stand or fall of diaphragm performance largely affect the battery cycle life and self-discharge.


    Diaphragm drying up in the process of circulation is the main cause of early battery performance decline. The diaphragm liquid alkali absorption quantity, capacity and permeability are the key factors affecting the service life of the cycle of the battery. Diaphragm hydrophilic ensures good and liquid alkali uptake ability; And hydrophobic can improve the permeability of the diaphragm. Diaphragm is do the following factors:


    Changes such as: 1) the diaphragm itself properties of fluid velocity and fluid absorption capacity variation;


    2) sheet was incurred during the process of charging and discharging expansion will diaphragm electrolyte in the extrusion and sucked out;


    3) electrode surface activity and gas composition, from the positive pole of the battery overcharge oxygen to compound out quickly, causing the battery internal pressure rise, after reaching certain pressure from the relief valve pressure and electrolyte loss.


    The self-discharge of the battery can also be related to the diaphragm. Some people think that: nickel electrode active material in the nimh batteries react with hydrogen is a major cause of MH - Ni battery self-discharge (micro short circuit is also one of the reasons) : NiOOH + 1/2 h2 - > Ni (OH) 2 of them after hydrogen is due to the charging stand, composite hydrogen storage alloy release part of the hydrogen atoms, so we need to have better air permeability diaphragm plate, the permeability of here does not mean that can appear a gas but to fully assist with permeability of hydrogen or oxygen ions. Battery charge or not charge can decrease the leakage rate, at present a lot of manufacturers let stand for 30 days after the battery is full of electricity electricity rate can be more than 70% (under normal state).


    Diaphragm paper, of course, in addition to the above conditions, also should have sufficient mechanical strength and toughness, in order to make sure the battery not fracture in winding and pole piece of inflation.


    Currently there are many battery enterprises to adopt PP material of the diaphragm paper instead of nylon material of the diaphragm paper, use effect is said to be good.


    SiRe and electrical activation effect on the performance of the battery


    The mechanism of thermal activation? As well as the thermal activation time, different temperature effect on the performance of the battery? The mechanism of electrical activation and how? As well as the electrical activation time, the current different effect on the performance of the battery? Following a brief discussion on these issues.


    Sealing into process of nickel metal hydride battery charge at the beginning of the activation and big ratio when internal pressure is too high, cause battery leakage, climb capacity drops, shorten the service life, safety performance is poor, and turned to for a long time. Nimh batteries of the seal (i.e., thermal activation), heat treatment can improve its performance, especially the improvement of the internal pressure. Its essential reason is:


    In the process of heat treatment, the cathode of the hydrogen storage alloy surface under the action of strong alkaline electrolyte, segregation quickly a large number of nickel atoms that form rich nickel layer, the family of nickel atoms dispersed homogeneously in other porous metal oxide and hydroxide or its hydrate, in the family of nickel atoms under the catalysis of overcharge when the anode diffusion of oxygen to the cathode surface, and react with the hydrogen in the hydrogen storage alloy, to combine to form water, can improve the YangJi elimination of hydrogen storage alloy, reduce the battery internal pressure. In addition, when the heat treatment can reduce the surface tension of liquid electrolyte, contribute to the uniform distribution of electrolyte, is advantageous to the electrochemical reaction of uniform.


    Thermal activation time, temperature of different impact on battery performance is different also, time is too short to reach the expected effect; Time is too long, a waste of time, the efficiency is too low. Low temperature reaction speed is too slow, the temperature is too high may cause short circuit batteries, sheet expansion, affect battery performance. Generally with 50-80 ℃ advisable, 2-8 hours more appropriate.


    Early electrical activation process, the first reaction is CoO + OH - = CoOOH this reaction is irreversible reaction, thus greatly enhance the conductivity of positive plate (because of Ni (OH) 2 basic also poor conductivity of conductive and NiOOH), so as to reduce the battery internal resistance and charging voltage, improve the efficiency of charging and discharging capacity. So can let the negative charge in advance, with charging reserves. And electrical activation in the late just to charge and discharge electrodes that Ni (OH) 2 between NiOOH and convert back and forth, back and forth through this kind of transformation (crystal type conversion), in the plate surface continuously produce fresh surface, makes the electrochemical reaction to proceed. In the electrical activation of late, as long as the battery voltage is not lower than 0.8 V, cobalt will not participate in the reaction. In general, to improve the efficiency of into three charge/discharge cycle for good, charge/discharge current should be small gradually become much better. (turn)

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