Last week, Stanford University researchers unveiled a new aluminum-ion battery chemistry with the unique ability to charge or discharge in less than a minute.

The battery’s incredibly fast charging and discharging times are not its only breakthrough. It is also the first aluminum-based battery to achieve an operating voltage sufficient for common applications and last longer than a few hundred charge-discharge cycles. In other words, it’s the first aluminum-ion battery to really work.

At the same time, the new battery is not without its limitations. There are a number of reasons why we probably won’t see it in our smart phones or electric vehicles anytime soon.

This post will introduce the new aluminum-ion battery technology, and then examine its key performance metrics, and how they affect its potential applications.

What’s Inside the Aluminum-Ion Battery?

To store energy, a battery requires two materials with an electrochemical voltage difference between them and an electrolyte that impedes the flow of electrons but enables the flow of ions between the two materials.

The aluminum-ion battery introduced last week uses simple aluminum metal in its negative side (anode) and a specialized three-dimensional graphite foam in its positive side (cathode). The positive and negative sides of the battery are separated by a liquid electrolyte of 1-ethyl-3-methylimidazolium chloride and anhydrous aluminum chloride. This electrolyte was selected because it contains mobile AlCl4- ions, which are exchanged between the two sides of the battery as it charges and discharges.

This schematic shows how the aluminum-ion battery shuttles AlCl4- ions from its graphite cathode to its aluminum anode in order to produce a discharge current (Source: Lin et al., 2015)

To test the viability of their proposed battery cell, the Stanford researchers constructed an experimental cell, and then charged and discharged it at various current rates to determine: 1) how much energy the cell can store, 2) how quickly the cell can charge or discharge, and 3) how many times the cell can be repeatedly charged and discharged.

How much energy can it store?

The amount of energy a battery can store is determined by two factors: the inherent voltage difference between its positive and negative sides, and the amount of charge the battery materials can store in the form of ions and electrons.

The voltage difference between the two sides of the aluminum-ion battery is approximately 2-2.5 volts, depending on the battery’s state of charge. This is less than the typical voltage of a lithium-ion battery, which varies from approximately 3.5-4 volts. This means about twice as many aluminum-ion battery cells would have to be placed in series to match the voltage of a comparable lithium-ion battery pack.

The aluminum-ion battery can store about 70 ampere-hours of charge per kilogram of battery material. This is approximately one half a lithium-ion battery’s charge capacity, which ranges from 120-160 ampere-hours per kilogram.

Put together, the aluminum-ion battery’s lower voltage and lower charge capacity give it about one quarter the energy density of a typical lithium-ion battery (about 40 Watt-hours per kilogram versus about 160 Watt-hours per kilogram for lithium-ion). Thus, powering your smart phone, laptop, or electric vehicle with an aluminum-ion battery would require a battery that weighs about four times the weight of a comparable lithium-ion battery.

How Much Electric Power Can It Produce?

Energy storage capacity is one important battery metric, but it isn’t the only one. Another crucial metric is a battery’s power capacity, or how quickly it can safely and reliably charge and discharge.

How quickly a battery can charge or discharge is determined by how quickly its materials can undergo an electrochemical reaction, and how quickly ions can diffuse inside the battery cell itself.

The Stanford researchers specifically designed their aluminum-ion battery to charge and discharge quickly. To speed up the motion of ions inside the negative side of the battery, they developed a unique three-dimensional graphite foam cathode with the internal gaps and surface area required to enable very fast ion movement.

Stanford's aluminum-ion battery uses a unique three-dimensional graphite foam to speed up the movement of ions inside the battery, and unlock its unprecedented charging and discharging times. (Source: Lin et al., 2015)

This specialized cathode enables the aluminum-ion battery to charge and discharge at unprecedented rates. Researchers tested discharging and charging the battery at rates corresponding to a full charge or discharge in less than one minute. They found the battery could charge within a minute and then discharge over periods ranging from 48 seconds to 1.5 hours without suffering major capacity or efficiency losses.

The aluminum-ion battery’s fast charging and discharging times give it a decisive advantage over conventional lithium-ion batteries. On a mass basis, a hypothetical one-kilogram aluminum-ion battery could produce approximately 3,000 watts of power—enough to power about two to three typical residential homes, albeit for only a minute or less. On the other hand, a typical one-kilogram lithium-ion battery could only produce about 200-300 watts of power—about a tenth the power capacity of Stanford’s aluminum-ion battery.

How Long Does It Last?

The aluminum-ion battery’s unique three-dimensional graphite foam cathode doesn’t just unlock the ability to charge and discharge quickly; it also enables the battery to charge and discharge thousands of times over without suffering significant material degradation and capacity loss.

The Stanford researchers tested how long their battery lasts under different conditions by charging it at a fast one-minute rate, and then discharging it at the same one-minute rate thousands of times over. Across over 7,500 of these fast charge-discharge cycles, the researchers observed essentially no fade in the battery’s capacity.

This figure shows the measured charging capacity, discharging capacity, and coulombic efficiency of the aluminum-ion battery across 7,500 charge-discharge cycles. The fact that both charging and discharging capacity do not decline over the 7,500 cycles shown indicates that cycling does not cause the battery capacity to fade. The fact that coulombic efficiency is always close to 100 percent indicates most units of charge stored in the battery come back when the battery is discharged, and aren't consumed by side reactions or other processes. (Source: Lin et al., 2015)

This stands in contrast with a lithium-ion battery, which can typically only deliver 1,000-3,000 charge-discharge cycles before its capacity fades significantly. Thus, there is potential for the aluminum-ion battery to last much longer than conventional lithium-ion batteries.

At the same time, the Stanford researchers have not shown how their battery stands up to the effects of time, so it is unclear if the aluminum-ion battery can last long enough to fulfill electric grid applications. Because each charging or discharging process tested only took one minute to complete, the 7,500 charge-discharge cycles demonstrated correspond to an operating period of only a few of weeks. If there are other passive reactions that cause the battery to fade over longer time periods, than the aluminum-ion battery might not last the years required by grid applications.

What Might It Be Used For?

Based on the performance specifications identified above, Stanford’s aluminum-ion battery will be useful for applications that require very fast charging and discharging times and the capability to charge and discharge thousands of times without suffering capacity loss. The battery won’t be useful in applications that require energy density, because it’s energy density is only about a quarter of existing lithium-ion batteries.

Thus, you shouldn’t expect to be using Stanford’s aluminum-ion battery in your smartphone, tablet, or electric vehicle anytime soon. While the battery might allow you to charge your smartphone or electric vehicle in under a minute, it would significantly increase the weight of your phone or vehicle.

However, there is a chance you will see the aluminum-ion battery deployed on the grid one day. One application that might be a perfect fit for Stanford’s aluminum-ion battery is providing balancing and reserve power to the electric grid in order to maintain the balance between total electricity supply and total electric demand. This application requires high-power batteries with the capability to charge and discharge many times without failing. If Stanford’s aluminum-ion battery can be constructed at a sufficiently low cost in the future, it might be used to provide this service on the grid.

 

References: Lin et al., 2015; Kurzweil, 2015, Dunn et al., 2011.