Alkaline Manganese Batteries

Alkaline batteries are the most common form of primary non-rechargeable cell used in a large number of electronic devices. These are cheaper than lithium ion cells and have a higher output voltage of 3 volts.

The alkaline battery consists of a zinc container and a manganese dioxide cathode. The electrolyte is potassium hydroxide.

Cathode

The cathode is the region of an electrical cell where positive electric current enters from the electrolyte. The cathode is made of either an electrochemically active or inert material such as manganese dioxide.

It is important for the cathode to be an active material because it is where all the energy of the electrochemical reaction occurs. It is also necessary for the cathode to be conductive to allow electrons to flow from the negative terminal to the positive one.

There are many different types of cathodes available, ranging from solid oxide to polycrystalline. Some are coated with alkali metals, such as barium and strontium, to enhance their performance in high-drain applications. Others are made of pure materials, such as platinum.

These are generally chosen because of their excellent activity and long life against iodide redox couples in the electrolyte. Alternatively, they can be made of other materials such as carbon, lithium or niobium.

However, despite the availability of these options, there remains a need for an improved cathode composition that results in increased battery performance while permitting manufacturing in a continuous production environment. In particular, there is a need for an improvement in the ability to charge these cells, especially when they are containing additives and binders that reduce the amount of active component available for the electrochemical reaction.

Accordingly, the present invention aims to provide a rechargeable alkaline battery cell with an improved cathode composition. This improvement enables the cell to be charged at a lower voltage than is commonly used for conventional cells. This lowering of the charging voltage allows more of the cell’s capacity to be utilized in the charging process.

This is accomplished by implementing an improved charging method for rechargeable alkaline batteries that employs a hydrophobic binder in place of a conductive material. The hydrophobic binder is a polyethylene powder that does not participate in the electrochemical reaction and is therefore not conductive to the battery’s electrode materials, reducing the available ‘active’ component for the electrochemical reaction.

The use of the hydrophobic binder instead of a conductive material is an attractive approach because it can improve the performance of the cell by increasing its cumulative discharge capacity, cycle life and discharge current. Furthermore, it is relatively inexpensive to implement and can be added to any type of rechargeable alkaline battery at a relatively low cost per unit of energy.

Anode

The anode of the alkaline manganese cell is the part of the battery that derivates energy from the chemical reaction between zinc metal and manganese dioxide. It also serves as a current collector, and is surrounded by an electrolyte that contains a non-acidic potassium hydroxide solution.

As with all batteries, an alkaline cell must have a good quality anode and cathode to provide the correct performance. The anode is made from pure zinc, either in powder form or as a porous electrode. It is usually coated with a plastic-made gasket to reduce leakage. The cathode is normally manganese dioxide, a solid powder that is compacted into an inner cylinder of the cell. It is surrounded by the electrolyte and separated from the anode with a separator.

An alkaline battery consists of a cylindrical steel can filled with an electrolyte containing manganese dioxide in the outermost internal cathode region and zinc in the center-most internal anode region. The cell is enclosed by a cover and is sealed off from the environment with aluminum foil, a plastic film or cardboard to protect the battery against corrosion.

Increasing commercial demands for better cell performance in applications requiring high power are putting pressure on battery suppliers to develop a more reliable way of improving the service life of conventional primary alkaline cells particularly for cells used in modern electronic devices such as cellular phones, digital cameras and toys, flash units, remote control toys, camcorders and high intensity lamps, which typically require operation at power requirements between about 0.5 and 2 Watts.

Applicant has discovered that the performance of alkaline cells with a higher electrolyte loading in the anode (reduced zinc bulk density in the anode) can be improved, especially under high power application when electrically conductive powders are added to the anode material. Adding about 1 weight percent tin powder to the anode composition of cells with higher electrolyte loading in the anode, as in Example 4, gives such cells better performance in all high power continuous, intermittent and pulsed tests than the same cell without the tin additive.

Electrolyte

The electrolyte in an alkaline manganese cell is a basic potassium hydroxide solution. This is alkaline manganese cell similar to the electrolyte used in a Leclanche cell, but it has a higher purity and activity. The battery uses a high concentration of manganese dioxide as the cathode and zinc powder as the anode.

Unlike an acidic battery, the electrolyte of an alkaline cell remains unchanged during charge and discharge. This is an advantage for high-energy applications.

A typical cell contains a cylindrical drum filled with the anode and cathode mixtures. The anode is made from a dispersion of zinc powder in a gel containing the electrolyte and the cathode is formed from manganese dioxide. The anode and cathode are sealed together with a plastic gasket to prevent leakage and reduce the risk of oxidation.

To prevent corrosion of the zinc electrode during discharge, an elemental bromine ion complex is liberated from manganese dioxide during charge. This ion complex is then circulated in the electrolyte for preventing self-discharge. The circulating aqueous electrolyte also facilitates good morphology of the zinc electrode.

An alkaline cell can be found in many household appliances and other products that require a constant power source for intermittent high current bursts of electricity. The cells also provide a longer lasting and more reliable power supply than lithium ion batteries.

Although lithium ion batteries are more widely used in recent years, the alkaline manganese battery is still widely available and can be found in a wide variety of devices. These include torches, flashlights, electronic circuit designs and general devices where the cost of lithium ion cells may be an issue.

The alkaline battery chemistry is dominant in the primary battery market. It accounts for around 65 percent of the world’s primary battery production.

This chemistry was developed by Dr Ernst Waldemar Jungner in 1899, as a way of improving the reliability of a battery that could be used for large power storage applications. The alkaline battery was the first battery to use an alkali as its electrolyte instead of an acidic one.

In an alkaline cell, the zinc oxide at the anode reacts with the aqueous electrolyte to form zinc hydroxide. As the battery is discharged, the resulting zinc oxide slowly dissolves and the anode becomes “dead”. The manganese dioxide at the cathode reacts with the anode’s remaining zinc to form manganese oxide and then manganese hydroxide. This is a much faster process than the chemical reaction between zinc and water that leads to zincate, so it can be more efficient and provide better power.

Separator

As the name suggests the separator is an integral part of an alkaline manganese cell. It plays an important role in keeping the cathode and anode contacts separate, while also allowing the movement of charged ions between the electrodes.

The battery’s separator is made from a non-woven layer of cellulose or a synthetic polymer which conducts ions and remains stable in the electrolyte. These ionically active separation materials alkaline manganese cell have been found to improve the performance of alkaline batteries.

In particular, a patented sodium super ion conducting ceramic (NaSICON) separator has been demonstrated to reduce soluble zinc crossover to the anode by up to 122% over commercial membranes and has a long cycle lifetime in 30% NaOH. Moreover, its high ion selectivity enables it to function at lower pHs than commercially available membranes.

These ionically selective separators were fabricated by coating a hydrolyzable polymeric ester on an absorber, which was a flexible, fibrous substrate that is resistant to strong alkali and oxidation. The coated electrolyte absorber is then immersed in an alkaline electrolyte and the resulting mixture of the ionically active coating and the absorbed electrolyte forms a salt and an alcohol that is stable at room temperature and has high resistance to electrode ion transfer.

This ionically active separation material was used in an alkaline manganese dioxide battery. It was shown to reduce the amount of soluble zinc crossover from the anode by up to 122%, which in turn improved battery life and increased the power density. It also exhibited a low thickness that was stable in these highly alkaline electrolytes for months.

Another type of battery separator that can be used in an alkaline manganese cell is a mat. These mats are made from ultrafine polybenzimidazole fibers which provide a better alternative to the use of asbestos.

Compared with the asbestos separations, these mats are more durable and have a high level of alkali resistance. They are also highly flexible and able to prevent the cracking that is caused by the growth of dendrites on the electrode.

These alkaline manganese cell separations are useful in many applications where the cost and weight of lithium ion batteries may be a problem. They are widely used in toys, torches and other electronic circuit designs that don’t require the charge capacity of a lithium ion battery.