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Lithium batteries are mainly composed of oxide cathode materials and graphite system anode materials. The oxide cathode materials usually contain high cost Co and Ni elements, which makes the cost of lithium batteries high. The positive and negative electrodes of the dual-ion battery are made of graphite materials, so the cost of raw materials is far lower than that of the traditional lithium battery, which has a good application prospect.

Recently, ZhanyuLi (the first author and the corresponding author) of Hebei University of Technology and others prepared a high-performance dual-ion battery with AlCl3/NaCl mixed salt as electrolyte. The battery has a capacity of 183.8mAh/g at a high current density of 1A/g, and can still reach 132mAh/g even at a super current density of 4A/g, with a cycle life of 700 times.

In the experiment, the author cut the graphite paper into 0.5 cm× 1 cm square piece, and then paste the square piece on the platinum piece as the positive and negative electrode. The molar ratio of mixed molten salt is AlCl3: NaCl=1.63:1, which is close to its lowest melting point of 108 ℃, and then the electrical performance of the battery is tested at 120 ℃.

The following figure shows the XRD spectrum of the molten salt mentioned above. From the figure, it can be seen that the important component of molten salt is NaAlCl4, and its reaction is shown in the following formula.

Since the maximum stable electrochemical window of the electrolyte is 2.4V, the author sets the charging cut-off voltage to 2.25V. From the following figure a, it can be seen that under the current density of 1A/g, the charging capacity of the battery in the 1st, 5th and 10th cycles is 256.7mAh/g, 236.9mAh/g and 206mAh/g respectively, and the discharge capacity is 183.8mAh/g, 176.5mAh/g and 171.3mAh/g respectively (subject to the positive electrode quality), and the first coulomb efficiency is 71.6%, The coulomb efficiency increased to 83.2% in the 10th cycle.

As for the dual-ion battery, the lower coulomb efficiency is a common short plate. From Figure b, we can see that the position of the oxidation peak during the charging process has hardly changed significantly, while the reduction peak during the discharge process has a significant shift during the cycle, which is mainly affected by the electrochemical polarization. It can be seen from the following figure c that the capacity of the battery is 183.8, 170.3, 165.6 and 132mAh/g at the current density of 1, 2, 3 and 4A/g, which is significantly higher than the traditional Al ion battery. At the same time, the coulomb efficiency of the battery reaches 94.1% at the high current density of 4A/g. From the cycle performance curve of the following figure f, it can be seen that after 700 cycles at a high current density of 4A/g, the specific capacity of the battery can still reach 103.3 mAh/g, and the capacity retention rate can reach 79.5%, showing excellent cycle performance.

In order to study the self-discharge mechanism of dual-ion batteries, the author conducted self-discharge tests under different cut-off voltages. The following figures a and b show the time-voltage and voltage-capacity curves when the cut-off voltage is 2.25V, respectively. From the statistical data in the following table, it can be seen that if the battery is discharged directly after charging, the discharge capacity of the battery is 190.4mAh/g, and the coulomb efficiency is 74.1%, but if the battery is discharged after 1000s, The battery voltage will be reduced to 2.08V, the discharge capacity will also be reduced to 158.9mAh/g, and the coulomb efficiency will also be reduced to 64.8%, which may be caused by the decomposition of molten salt electrolyte (AlCl3=Al+1.5Cl2 (g) and NaAlCl4=Al+1.5Cl2 (g)+NaCl). If we control the charging cut-off voltage at 1.95V, the voltage after 1000s of shelving will only decrease by 0.02V. From the voltage curve, we can also see that the voltage decreases first and then increases. This is important because the graphite electrode adsorbs the anion and cation in the electrolyte during the quiescence process, and the specific reaction is C+n+AlCl− 4→ Cn[AlCl4]andC+n+Al2Cl− 7→ Cn [Al2Cl7], due to the existence of self-charging, the discharge capacity of the battery increased by 49mAh/g, and the coulomb efficiency reached 218.9%. The same phenomenon was also observed at the cut-off voltage of 1.85V. During the quiescent process, the voltage first decreased and then increased, and finally stabilized at 1.93V. The capacity increased by 46.9mAh/g, and the coulomb efficiency reached 252.7%.

In order to analyze the reaction mechanism of the battery, the author analyzed the graphite electrode with a high-resolution transmission electron microscope. From the structure of the positive electrode in the figure a-c, it can be seen that the interlayer spacing of the graphite material has increased from 0.334 nm to 0.41 nm, which is mainly due to AlCl&Minus; 4 and Al2Cl− 7 Caused by insertion and detachment. From the negative electrode picture of d-f below, we can see that there are many black spots on the graphite surface, which is mainly caused by the deposition and decomposition of Al on the negative electrode surface.

In this dual-ion battery, metal Al is mainly deposited on the surface of the negative electrode, so the layered structure of graphite has not changed significantly. However, on the side of the positive electrode of graphite, due to the insertion and removal of anions, the carbon layer peels off, resulting in a structure similar to graphene.

In order to further analyze the reaction mechanism of the battery, the author used XPS tool to analyze the positive and negative electrodes after different cycles. From the following figure a, it can be seen that with the increase of the number of cycles, Cl2p gradually shifts to the direction of low binding energy, and the strength is significantly increased. This is mainly caused by the fact that some of the aluminum chloride anions cannot be removed after being embedded in the graphite material and accumulate in the graphite, This is why the coulomb efficiency of dual-ion batteries is low. From Figure b, we can see that the binding energy of Al2p does not shift, but with the increase of the number of cycles, the strength increases significantly. From the following figure c, we can also observe a significant increase in the content of Al in the negative electrode.

In the figure below, the author used EIS and CV tools to analyze the battery. From the figure below a, it can be seen that the CV curve of the battery has hardly changed during the first to third cycles. As can be seen from Figure b below, at different rates, the battery can have three pairs of redox peaks, of which 1/1 peak is mainly due to the adsorption and desorption of ions on the electrode surface. At the speed of 1mV/s, the pseudo-capacitance contribution capacity accounts for 39.6% of the total capacity. With the increase of scanning speed, the pseudo-capacitance capacity proportion gradually increases, and at the speed of 5mV/s, the proportion can reach 53.8%. The 2/2 and 3/3 peaks are mainly from lCl− 4 and Al2Cl− 7.

It can be seen from the following figure c that the AC impedance data of the battery is mainly composed of the semicircle of the charge exchange impedance in the high frequency region and the diffusion curve in the low frequency region. The data can be fitted using the equivalent circuit shown in the following figure e. According to the fitting results, the charge exchange impedance of the battery before the cycle is 0.4Ω, Add to 1.2Ω after circulation;, It is still at a very low level, which also ensures the good rate performance of the battery. At the same time, we do not observe the battery interface impedance from the figure, which indicates that there is no SEI film formed on the electrode surface due to the high temperature.

In addition, we can also calculate the diffusion coefficient according to the diffusion curve in the low frequency region. The calculation formula is as follows: R is the ideal gas constant, T is the absolute temperature, A is the electrode area, n is the charge exchange quantity, F is the Faraday constant, C is the concentration,σ Is Warburg coefficient,σ It can be calculated from the slope of the curve in the following figure d. the calculation shows that the solid phase diffusion coefficient of anions before circulation is 1.12× 10− 11cm2/s, 9.23&times after cycling; 10− 12cm2/s, which is in the same order as the solid phase diffusion coefficient of lithium battery, so that the battery has good electrochemical performance.

ZhanyuLi prepared a dual-ion battery using low temperature AlCl3/NaCl molten salt. At a high current density of 1A/g, the specific capacity of the positive electrode can still reach 183.8mAh/g, and the capacity retention rate can reach 79.5% after 700 cycles, which has excellent electrochemical performance.

The following documents are important references in this article. The article is only used for the introduction and review of relevant scientific works, as well as classroom teaching and scientific research, and cannot be used for commercial purposes. If you have any copyright issues, please feel free to contact us.

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