Comments on “An ultrafast rechargeable aluminum ion battery”

(Suzhou Youlion Battery Inc., Suzhou 215500, Jiangsu, China)

Naturepublished an article “An ultrafast rechargeable aluminum ion battery” On April 6, 2015.The authors used many new materials to compose the cell, such as three-dimensional graphitic-foam as the cathode, an ionic liquid electrolyte.The experimental cell has shown well-defined discharge voltage plateaus near 2 V.The cell is mechanically bendable and foldable without affecting its operation.In addition, the experimental cell exhibited a degree of tolerance to electrical and mechanical abuse such as drilling.It is a step towards a practical use of aluminum ion battery.However, there are many more steps to go and we come with a few comments on the article as in the following:

(1) The paper above described the specific capacity in the abstract (in boldface) as “The cell exhibits well-defined discharge voltage plateaus near 2 V, a specific capacity of about 70 mA·h/g and ….”.It gives reader an impression that the cell has a specific capacity 70 mA·h/g.In the next text on the same page, it described as “…specific capacity of 60~66 mA·h/g (based on graphitic cathode mass)….”.There are misleading words on the specific capacity.One cannot use cathode specific capacity to imply the cell specific capacity.It would be more professional to say “The cell exhibits well-defined discharge voltage plateaus near 2 V, the cathode possess a specific capacity of about 70 mA·h/g and ….”.This can be very different as we will explain below.

(2) In the view of the paper, the electrolyte is also a cathode reactant, so the mass of the electrolyte must be included in the calculation of cathode specific capacity.As indicated in the in equation (1) and (2) on Page 3:

For cathode Cn, it must react with AlCl4-from electrolyte to complete the cathode reaction.The heavy part of electrolyte is also seen in the extended data Fig.4 as “the minimum amount of electrolyte could be 0.02 mL/mg of cathode”.In METHODS page, the Authors indicated as they “lowered the electrolyte amount to about 0.02 mL/mg of cathode material…”.This equals 20 mL of electrolyte per gram of cathode.The density of [EMIm]Cl is 1.112 g/mL (MSDS of 1-ethyl-3-methylimidazolium chloride), so for every gram of cathode material, the reaction need minimum amount of 20 mL×1.112 g/mL= 22.24 g electrolyte.The total cathode reaction mass should be 1 + 22.24 =23.24 g.The specific capacity should be calculated using the total mass not only the small part of graphite mass.If using the total reaction mass, the specific capacity would be only 70 mA·h/23.24 g = 3 mA·h/g and the as-mentioned highest discharge rate (5000 mA/g) would be only 215 mA/g of active material.According to this calculation and the battery making practice, this system cannot be an acceptable battery in specific capacity.

However, the authors indicated that further decrease in the electrolyte ratio (electrolyte/cathode) is possible through battery engineering on METHODS page.We therefore estimated a theoretical amount of electrolyte using the reaction equations provided in the article and further estimate a theoretical battery energydensity.

Again, use equation (1) and (2) proposed as in the paper, we add cat-ions in the electrolyte for an overall battery reaction as below

Wherenis the molar ratio of carbon atoms to intercalated anions in the graphite.Based on the specific capacity of 70 mA·h/g (0.07A·h/g) given in the paper, we can calculate the molar ration.Moles of 1 g of carbon can be calculated as 1 g/(12 g/mol); and moles of Cn[AlCl4]molecules is calculated as the following

where 96485(C/mol) is the Faraday Constant.The equation (3) can be re-written as:

The molecular weights of all reactants of the proposed Al-ion battery system is:

We can work out the theoretical specific capacity as:

With the 2 V battery voltage, the specific energy density is 28.77 mA·h/g×2 V=57.54 mW·h/g; or 57.54 W·h/kg.This energy density is for cathode reaction only, not include the weights of anode, separator and current collector, etc.Therefore, the overall battery would have even lower energy density than the estimated 57.54 W·h/kg.

Is it an “ultrafast battery”?

The authors ignored the heavy weight of the cathode reaction component-electrolyte to use only the weight of graphitic cathode, resulted a conclusion of“ultrafast battery”.They confused a battery with one component of a battery.A fast battery is the battery with its all components to carry on high specific current flow.It may be a fast reaction cathode but not necessary a fast battery.The title clearly indicate this is a “ultrafast battery”, but in the text, only the mass of graphitic cathode current density (mA/g) was discussed and no detailed data is given for overall battery value.Because the nature of the said Al ion battery, the specific battery current density in the METHODES page is not fast at all if one takes the weights of electrolyte, anode and other components into account.

It is believed that the authors calculated the current density based on the pouch cell on METHODS page with about 3 mg of cathode, about 2 mL of electrolyte and about 70 mg Al foil as anode.If we use the same ratio, for 1 g of cathode, we need 741 g of electrolyte and 23.33 g Al foil anode, we can work out the current density as:5000 mA/(1 + 741 + 23.33)g =6.5 mA/g for the Al-ion battery.For an experimental cell, it may not fare to scale up using the same factor (1g/~3 mg =333 times) for electrolyte, but we could be able to use the “minimum amount of 0.02 mL per mg cathode” (equivalent of 20 mL per gram of cathode) in Fig.4, extended data.The 20 mL electrolyte weight is 22.24 g as we calculated earlier.The current density for this Al-ion battery using minimum amount of electrolyte is then 5000 mA/(1 + 22.24 + 23.33)g = 107 mA/g.This current density is less than the current density of a commercial laptop Li-ion battery at 2 C rate.For example, a 2.6 A·h 18650 cell is about 45 g; at its 2 C rate, the current is 5.2 A, so the current density is 2600 mA×2/45 g = 115 mA/g.

In conclusion, we found the paper has misleading words in cell specific capacity; the whole cell has low energy density.Considering a battery that includes all components of cathode, anode, electrolyte, separator and so on, the “ultrafast” merit of the titled battery is also not true in practice.