VisBlue - What Is a Redox Flow Battery?
What kind of batteries exist?
Overall, two different groups of batteries exist. These are primary and secondary batteries.
Primary batteries are either known as disposable or single-use batteries and as their names suggest these can only be used once. The reason for this is that the materials inside the battery change in an irreversible way during its discharge.
So, a primary battery should be thrown away after use as they are non-rechargeable. A, AA, AAA, C, and D batteries belong to the primary category and their overall cell potential is generally 1.5 V.
Secondary batteries, however, are so-called rechargeable batteries that can be discharged and recharged again and again. The discharge and recharge happen through an electric current, where a reverse current then helps to restore the electrons to their original composition.
So as a general rule, secondary batteries can be reused again and again (although these also have a restricted lifespan).
Now that we know the two general battery groups, let’s move on to the different battery types.
Examples of primary/single-use batteries
A common primary battery is the dry cell, also known as a zinc-carbon battery.
In this battery type zinc acts as the anode and carbon acts as the cathode, where the electrolyte is composed of a salt-base. This battery type degrades over time, as it starts to leak due to an oxidation of the zinc. This process is irreversible, which means that, as the degradation advances, the battery slowly dies.
Zinc-carbon batteries are often used in flashlights, clocks, and radios. This battery type typically lasts for three to five years.
Another well-known single-use battery is the alkaline battery.
The anode side of this battery type uses zinc, where the cathode side uses manganese-dioxide. Using an alkaline electrolyte, predominantly potassium hydroxide or ammonium chloride, alkaline batteries were invented as a replacement of the dry cell. Alkaline batteries are also up to five times more efficient than its predecessor zinc-carbon.
You may recognise alkaline batteries as the ones you put into your remote control, flashlight, clock, wristwatch, electric key, or the like. Just like the dry cell, alkaline batteries are prone to leakages over time. This inevitably means that, just like in zinc-carbon batteries, an irreversible change occurs within the alkaline battery, leaving it to degrade and slowly die. Some alkaline batteries are rechargeable; however, the majority of them aren’t.
In general, alkaline batteries last up to eight times longer than the conventional dry cell. Most alkaline batteries last five to ten years.
Silver-oxide is another example of a disposable battery.
This battery type uses zinc as the anode and silver-oxide as the cathode, as well as an alkaline electrolyte, usually being potassium hydroxide or sodium hydroxide.
With a lifetime of three to seven years and their small size silver-oxide batteries are often used in wristwatches. Silver-oxide batteries are commonly recognised as “button cells”, which refers to their relatively small size and rounded edges.
Examples of secondary/rechargeable batteries
Flow batteries are similar in structure to well-known battery types, such as lithium-ion and lead-acid, but differ in one major way, as the electrolyte in flow batteries is not stored around the cell of the electrodes of the battery, but rather outside the cell.
This means that flow batteries are often confined to large, stationary applications, but also that the battery type has a major advantage in its power output (kW) and capacity (kWh) being completely separated.
Flow batteries come in different forms, when looking at the material used for the anode and cathode. Among some of the different varieties are polysulfide-bromide, zinc-bromine, uranium, iron-chromium, etc.
The most widely and commercially-used flow battery is the all-vanadium. This type uses vanadium as both the anode and cathode, where sulphuric acid acts as the electrolyte. This is also the one we specialise in here at VisBlue.
In addition to the anode and cathode, flow batteries are comprised of proton exchange membranes, and carbon-based cells. During charge, the VO2+ ions in the positive cell are converted into VO2+ ions, as electrons are removed from the cathode side of the battery. This process is referred to as oxidation. Correspondingly, electrons are introduced in the anode side of the battery, where V3+ ions are converted into V2+. This process is referred to as reduction.
This entire process of oxidation and reduction is reversed during discharge. The name “redox” in redox flow batteries stems from the oxidation and reduction process and is a contraction of these two terms.
Flow batteries have four different oxidation stages that, in colour, reveal the charge or discharge state of the battery: V5+, V4+, V3+, and V2+. V5+ is a yellow liquid that indicates that a fully charged state, V4+ is a somewhat charged state and is a blueish colour, V3+ refers to the discharging of the battery and is recognisable by a green hue, and V2+ is a violet shade that represents the battery in a completely discharged state.
Flow batteries excel in their ability to discharge to zero without affecting the performance and charge/discharge ability of the battery. Flow batteries are also superior when it comes to lifetime, as this battery type generally lasts for 20+ years.
Li-ion batteries, along with lead-acid batteries, are arguably the most commonly known secondary batteries.
The former often uses either carbon or graphite as the anode and for the cathode it uses one of the following three: lithium cobalt oxide, lithium iron phosphate, or lithium manganese oxide.
Generally, the electrolyte in Li-ion batteries consists of anion salts containing lithium and organic carbonates, such as ethylene carbonate or diethyl carbonate.
During discharge, lithium ions move from the anode to the cathode and during charging, this process is reversed.
Li-ion batteries are often used in portable electronics such as cameras, mobile phones, and laptops, but can also be used for large stationary storage applications. Li-ion batteries are also widely used in the Power-to-X industry to power different electric vehicles.
A general lifetime of Li-ion batteries is difficult to state, as the quality of this battery type differs greatly, but as a rule of thumb Li-ion last for seven to twelve years.
The lead-acid battery is the first rechargeable battery ever developed with its origin dating back to 1859.
This battery type uses metallic lead for the anode, lead dioxide for the cathode, and the electrolyte consists of sulphuric acid. When the battery charges, the acid reacts with the lead in both the anode and cathode, where it produces lead sulphate. This process is reversed during discharge.
The production and break down of lead sulphate produce small, but potent, explosions of energy that we can harness as energy – and use e.g. to start our cars. Due to their size, lead-acid batteries are often used in vehicles and for backup-supply for e.g. data centres.
The lifetime of lead-acid batteries is dependent on the temperature in which they are stored. As a general rule, hotter operating temperatures (35°C and up) cut lead-acid batteries’ lifetime in half. In operating conditions of room temperature (20-25°C), which is their optimal operating temperature, lead-acid batteries often last for two to five years.
Nickel-cadmium (NiCd) and nickel-metal hydride (NiMH)
NiCd and NiMH batteries are similar in structure and use but are different from each other in several other ways.
NiCd batteries use cadmium as the anode, nickel oxide-hydroxide as the cathode, a separator, and potassium hydroxide as the electrolyte.
This battery type is often used in e.g. power tools, medical equipment, and toys.
NiMH batteries use a hydrogen ion as the anode, nickel hydroxide as the cathode, a separator, and potassium hydroxide as the electrolyte.
This battery type is often found in e.g. medical equipment, electrical appliances, and automotive batteries.
Then what is the real difference between these two battery types?
NiMH batteries are in general superior to NiCd batteries. NiMH batteries excel in three major ways, the first being that they have more capacity, which means that this battery type can power appliances for longer periods of time. The second difference is a better memory effect, which means that NiMH batteries can achieve a full charge for longer periods than NiCd batteries. NiMH batteries are also generally more environmental than NiCd batteries in terms of material use and recyclability.
NiCd batteries have a lifetime of one to three years, whereas NiMH batteries last for three to five years.
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