Aluminum Ion Batteries for Solar PV Systems
Batteries are an invaluable component of solar PV systems since they provide a storage solution for solar power that’s intermittently produced by solar panels.
For a while, lithium-ion batteries have been dominating the energy storage systems for solar, but today there’re cheaper and more reliable batteries that you can also use.
One such type of alternative battery for solar PV systems is aluminum ion battery which will be our main focus for this article.
We’ll examine what these batteries are, how they work, their characteristics, pros, and cons so you can know if they’re suitable for your solar PV system or not.
Let’s get started.
What is an Aluminum Ion Battery?
An aluminum ion (Al-ion) battery is a high-valent ion battery that leverages aluminum ions to store and provide power on demand.
Aluminum ions are transferred between the electrodes along with electrons, thus facilitating the charging and discharging processes.
Aluminum is the most abundant metal on earth. Aluminum ions, on the other hand, have one of the highest theoretical volumes of charge carriers.
These characteristics suggest aluminum's potential for developing reliable energy storage systems for solar PVs.
More importantly, Al-ion batteries are less expensive and have higher energy storage capacity than their counterparts, so they merit being looked into.
Components of Aluminum Ion Batteries
Aluminum ion batteries have aluminum metal as the positive electrode. Typically, pure aluminum is used to ensure that the electrochemical reactions proceed as expected.
Aluminum ion batteries can’t function properly if the metal anode is impure or has been contaminated by the electrolyte.
The most commonly used cathode material in aluminum ion batteries is graphite. Graphite is known to be a stable carbon-based material with excellent electric conductivity.
Anthraquinone (a nanostructured carbon-based molecule) can also serve as a cathode for aluminum ion batteries.
Scientists from the Chalmers University of Technology and Slovenia's National Institute of Chemistry, who developed aluminum ion batteries using anthraquinone as the cathode, found that this material supports the storage of aluminum ions from the electrolyte.
These improvements were found to increase the energy density of the batteries substantially.
This is the medium in which the aluminum ions are transferred from one electrode to another. Two main types of electrolytes can be used for these batteries: liquid electrolytes and solid electrolytes.
Liquid electrolytes are unstable, highly corrosive, and hence are not preferred for Al-ion batteries.
Solid electrolytes exhibit better ion conduction, low corrosivity, and are stable; hence are the perfect electrolyte for aluminum ion batteries.
Solid electrolytes may be more expensive and relatively complex to process, but their benefits for aluminum ion batteries can’t be ignored.
How do Aluminum Ion Batteries Work?
Aluminum ion batteries work by moving aluminum ions from one electrode to another during charging and discharging.
During charging (usually done by connecting the battery to the output of your solar inverter), the anode, which is pure aluminum metal, ionizes, releasing the ions to the electrolyte.
Aluminum has 13 electrons, of which 3 are on its outer shell (valence electrons). During ionization, the 3 electrons are lost, leaving an aluminum ion with a charge of +3.
The electrochemical chemical equation:
Al → Al3+ + 3e−
Al3+ ions move through the electrolyte also towards the cathode while electrons released in the process travel through an external circuit through the power source.
When all the aluminum ions have moved to the positive electrode, the battery is said to be fully charged. During discharging (battery use), these charge movements are reversed.
The electrons flow back through the external circuit but this time through the load, while Al3+ migrates back to the positive electrode to rejoin the electrons and reform the metal anode.
Aluminum ions have a trivalency characteristic, so they can produce more current in one battery cycle.
However, it’s required that a stable electrolyte be present to support Al3+ migration, which occurs concurrently with electron transfer.
Characteristics of Aluminum Ion Batteries
Al3+ ion batteries boast three electrons redox properties which make them better at storing energy.
In a single cycle, aluminum ion batteries can store and release three times more energy than lithium-ion batteries.
High power density
Power density is how fast a battery can charge or discharge.
Aluminum ion batteries have a high power density of around 250-700 Wh/kg. An aluminum-ion battery is usually compared to an ultracapacitor because such high power density is uncommon in conventional energy storage systems.
Low energy density
Al-ion batteries have an energy density of around 150Wh/kg. Lithium-ion batteries can go up to 265Wh/kg, which means Al-ion will offer less than 60% of the energy density of conventional PV storage batteries.
In order to exploit the optimum energy density of aluminum ion batteries, pure aluminum must be used as the positive electrode and a solid electrolyte for stable ion transfer.
High charge and discharge cycles
Aluminum ion batteries can last for 2000 cycles without deterioration in performance.
Most conventional solar batteries have a shorter cycle life than this; hence Al-ion batteries are superior.
With 2000 cycles, you can be assured that you'll have your batteries for at least 2-3 years.
Relatively low standard potential
The standard reduction potential of aluminum is -1.676.
Al3+ + 3e− ⇌ Al(s) E° (V) -1.676
Aluminum is, therefore, a poor oxidizing agent since the aluminum ion cannot readily gain electrons (be reduced). Other metal ions such as potassium are a leap ahead in this, hence are more preferred for solar PV batteries.
Poor intercalation properties
For a battery that needs to be recycled on a daily basis– such as in the case of a solar PV system– the stability of the cathode is crucial.
One technique that can be used to achieve this is intercalation of the cathode with the charge carrier ions.
Unfortunately, the complex nature of aluminum ions makes it hard for intercalation with the material forming the cathode.
This is not the case with other batteries, such as potassium ion batteries, which are pretty easy to intercalate with materials such as graphite.
The aluminum ion battery charges faster than other commonly used PV batteries. This is due to the fact the battery can transfer 3 electrons per ion during a charge-discharge cycle.
Other charge carriers like sodium, potassium, and lithium can only move one electron per ion per cycle; hence their charging is relatively slower.
In less than 5 minutes, you can have your aluminum ion battery fully charged by your solar PV system.
In fact, Brisbane-based company Graphene Manufacturing Group claims that they developed Al-ion batteries with a charging ability 60 times greater than lithium-ion batteries.
High operation safety
Aluminum is inert and easy to handle. Since aluminum is used as the main active component of aluminum ion batteries, these benefits also apply.
As well, Al-ion batteries have a low level of flammability, so they don’t pose fire risks.
Poor ion transport kinetics
If an unstable liquid electrolyte is used for aluminum ion batteries, Al3+ exhibits poor mobility.
While this does not affect the charging speed of Al-ion batteries due to the tri-valency characteristics of aluminum ions, it definitely affects the energy density.
Therefore, material research and development are crucial for aluminum ion batteries, specifically when it comes to electrolytes.
A kg of aluminum produces up to 40kgs of CO2.
So, despite the real applications of aluminum ion batteries being relatively safer, interventions must be put in place to ensure that mass production of aluminum isn’t a threat to the environment.
Other than that, aluminum itself is non-toxic, and alkaline batteries pose no environmental hazards, provided proper disposal is ensured.
At the end of their useful life, aluminum ion batteries are easily recyclable. Nevertheless, it may be impossible to remove the by-product from aluminum ion batteries, primarily when traditional electrolytes are used.
In this case, the aluminum used in these batteries may not be used for any other purpose.
Effects of high temperatures
Aluminum ions have no maximum ampere limit; hence the efficiency and operation of aluminum ion batteries aren’t affected by high temperature.
In fact, unlike lithium-ion batteries, Al-ion storage systems have no cooling components in the battery unit.
Aluminum Ion Batteries Advantages
High power density
With a power density as high as 700Wh/kg, Al-ion batteries prove to be one of the most reliable energy storage systems for solar PV systems.
As mentioned earlier, this power density level leans more on the side of ultracapacitors, meaning that Al-ion batteries are the kind of energy storage solution which you should invest in for your solar PV system.
Wide material availability
Aluminum is the most abundant metal and the third most abundant element in the earth's crust.
This means that Al-ion batteries can be accessed globally as long as the necessary manufacturing technologies are established.
The good thing is, processing and recycling technologies for this metal are well established in many areas, so producing aluminum ion batteries is very feasible.
High volumetric capacity
Compared to other anodes, the aluminum anode is ‘special’ due to its high volumetric capacity.
However, the entire battery unit materials have to be optimized for this characteristic to be exhibited.
Since high volumetric capacity indicates how powerful a battery is, aluminum ion batteries prove to be a reliable energy storage solution for solar PV systems.
Aluminum Ion Batteries Disadvantages
Passivating oxide film formation
Aluminum is an active battery material with a high affinity for oxygen. When exposed to the air at standard conditions, aluminum is oxidized to form a solid oxide.
In the case of aluminum ion batteries, this solid film will form a thin film on the surface of the aluminum electrode. Well, the layer itself doesn’t interfere with electric conduction.
However, it creates a barrier for aluminum’s solvation, which reduces the effectiveness of aluminum as a negative electrode.
Lack of solid electrolytes
Liquid electrolytes are unstable for aluminum ions which renders aluminum ion batteries relatively ineffective for energy storage in solar PV systems.
For aluminum ion batteries to be fully effective for storing solar power, inorganic materials must be developed quickly for use as solid electrolytes.
Corrosion of the anode
This is a big challenge for the operation of aluminum ion batteries.
Aluminum metal anodes are easily corroded if an aqueous electrolyte is used in the Al-ion battery. This interferes with charge transfer and consequently lowers the efficiency of these energy storage systems.
Again this indicates how important solid electrolytes are needed for aluminum ion batteries.
What are graphene aluminum ion batteries?
Graphene aluminum ion batteries are aluminum ion batteries that use graphite as the cathode material.
Graphene aluminum ion batteries charge ultra-fast and are more reliable and sustainable than other conventional solar batteries.
Are aluminum ion batteries better than lithium-ion batteries?
In terms of energy storage capacity, operational safety, and charging speed, aluminum ion batteries may be seen as better than lithium-ion batteries.
However, some issues such as material corrosion and the formation of an aluminum oxide layer need to be addressed for aluminum ion batteries to be clearly defined as better than lithium-ion batteries.
What are aluminum-air batteries?
Aluminum air batteries are energy storage systems that leverage the reaction of aluminum and oxygen in the air to store and produce electricity.
Al-air batteries are one of the largest capacity batteries you can use for your solar PV system.
If well leveraged, these batteries, along with their Aluminum ion counterparts, can aid in the generation and distribution of solar which is the go-to source of power nowadays.
Aluminum-ion batteries are a highly promising energy storage system for solar PV systems. Currently, wide material availability, high power density, and fast charging are the main benefits of this battery technology.
There is still much to be done, though, such as determining the best solid electrolyte and electrode material for these batteries.
With some material improvements, the main hurdles of aluminum batteries, such as low energy density and poor transport kinetics, will be obsolete.
Aluminum ion batteries might not be the best option for solar PV systems right now, but they’ll improve as this battery technology advances.