New liquid metal battery with high energy density

Liquid metal battery is a new type of electrochemical energy storage device with low cost, long life and easy scale, and has broad application prospects in various scale energy storage fields.

Its all-liquid liquid metal battery structure makes it have excellent electrode structure stability and fast electrode reaction kinetics characteristics. The current low voltage of liquid metal battery affects the energy density of liquid metal batteries, so the development of high-voltage cathode materials is imminent.

Sb-based cathode materials have high lithiation potential and lithium intercalation capacity, and are currently the most promising anode materials for liquid metal battery. Due to the high melting point of Sb (630 oC), alloy components (Sn, Pb, etc.) are usually introduced in the application process to prepare Sb alloy electrodes to reduce the melting point of electrodes and achieve long-term stable operation of Sb-based electrodes.

However, components such as Sn and Pb only play a role in lowering the melting point of the material during the discharge process and do not contribute to the capacity. The introduction of a large amount of them seriously affects the specific capacity of the Sb-based material, thereby reducing the energy density of the liquid metal battery.

Electrochemical activity analysis of Sb-Zn electrode

A liquid metal battery was assembled with Sb30Zn70 as the positive electrode, metal Li as the negative electrode, and LiF-LiCl-LiBr (22:31:47 mol%) molten salt as the electrolyte for electrochemical performance tests. Molten salt energy storage is also a good way of electrochemical ways and it is widely adopted in energy storage solutions nowadays. The operating temperature was 550 °C and the electrochemical window was 0.3- 1.5V.

Li||Sb30Zn70 liquid metal battery charge and discharge curve has two pairs of charge and discharge voltage platforms located at 1.08/1.01 and 0.83/0.76 V, respectively, while the reported Sb-based materials (Sb-Sn, Sb-Pb) only have a pair of voltage platforms, Located at 0.91/0.79 V, attributed to the formation and decomposition of Li3Sb.

Obviously, the Sb30Zn70 cathode increases the high-voltage lithium storage process, which significantly improves the discharge voltage of the liquid metal battery. At deep discharge, the Zn component exhibits good lithium storage performance in the form of a voltage ramp.

In order to analyze the generation mechanism of the high-voltage platform and analyze the electrochemical behavior of Li||Sb-Zn liquid metal battery, XRD analysis was carried out on the Sb-Zn positive electrodes at different lithiation depths, and the phase evolution process in the discharge stage was studied. The formation of LiZnSb phase during discharge endows the Li||Sb30Zn70 liquid metal battery with a high lithiation potential (1.1 V).

As the discharge proceeds, the LiZnSb phase gradually transforms into the Li3Sb phase at a discharge voltage of 0.8 V until the LiZnSb phase is completely consumed. During deep lithiation, the characteristics of dual active components enable Li||Sb30Zn70 liquid metal batteries to continue to store lithium and cause Li-Zn alloying.

This excellent characteristic effectively improves the utilization rate of Sb-Zn positive electrodes and improves the electrode discharge At the same time, the voltage increases the specific capacity characteristics of the electrode, thus endowing the electrode with a higher energy density.

Kinetic process of Sb-Zn electrode reaction

SEM and EDS analysis were carried out on the cross-sections of 15% and 50% DOD positive electrodes. When discharged to 50% DOD, the intermetallic compound is still a layer, and the three elements of O, Sb, and Zn are non-uniformly distributed in the intermetallic compound layer, indicating that LiZnSb phase and Li3Sb phase are dispersedly distributed in the intermetallic compound layer.

In addition, the diffuse distribution of metal Zn was detected in the intermetallic compound layer, which was generated from the phase evolution process from LiZnSb to Li3Sb based on the analysis of the electrode reaction process.

Since metal Zn has a low melting point, is liquid at operating temperature, and has good electrical conductivity and lithium storage capacity, lithium ions enter the intermetallic compound layer after gaining electrons at the positive electrode-electrolyte interface, which is expected to be alloyed with Zn melt.

Li-Zn non-equilibrium melt is formed, which acts as a rapid diffusion channel for lithium and promotes the diffusion of Li to the depth of the intermetallic compound, which in turn leads to the dispersed distribution of the various substances of the intermetallic compound.

During the discharge process, Li ions first undergo a charge transfer process and subsequently alloy with the Sb-Zn alloy anode to form the LiZnSb phase. Due to the moderate density of LiZnSb, it is between the positive electrode and the molten salt electrolyte. When the discharge enters the second voltage plateau, the LiZnSb phase gradually transforms into the Li3Sb phase, accompanied by regeneration of the Zn melt during the process.

As the discharge continues, the regenerated Zn melt is dispersed in the intermetallic compound layer, which promotes the transport of electrons and lithium, accelerates the electrode reaction kinetics, and endows the Sb-Zn cathode with excellent rate performance.

Electrochemical properties

The Li||Sb30Zn70 liquid metal battery exhibits excellent electrochemical performance, with a positive electrode material specific capacity of 423.76 mAh g-1 and an energy density of 290.6 Wh kg-1, surpassing all reported Sb-based material systems.

The liquid metal battery exhibits ultra-high rate characteristics: when the current density increases from 100 to 1000 mA cm-2, the capacity retention rate of the liquid metal battery is as high as 93%, and the voltage polarization is only 21.9%. After 160 cycles at 200 mA cm-2, the capacity decay rate is 0.09%/cycle, showing excellent long-cycle performance.

Summary

The characteristics of dual active components of Sb-Zn electrodes and the formation and phase evolution mechanism of LiZnSb ternary intermetallic compounds can not only effectively improve the discharge voltage and specific capacity of liquid metal battery, thereby improving energy density, but also help to achieve excellent electrode reactions.

The energy density and rate characteristics of Li||Sb-Zn liquid metal batteries are significantly better than the reported Sb-based liquid metal battery systems. This work provides a new idea for the design of liquid metal battery cathode materials, provides a theoretical basis and technical support for the development of high specific energy liquid metal battery, and promotes the development of liquid metal battery technology.