Research progress of liquid metal battery

Liquid metal battery is a new type of electrochemical energy storage technology, which consists of a low-density negative electrode, a high-temperature molten salt electrolyte in the middle layer and a high-density positive electrode. Release and storage have the advantages of battery high energy density, outstanding rate characteristics and cycle performance, low cost, and easy scale preparation.

It is especially suitable for large-scale energy storage in the power grid, and has the potential to realize the technical economy of energy storage in terms of cost and life. Although this technology is still in the laboratory research stage, it has initially shown the advantages of low cost and long life.

Basic properties of liquid metal battery

Basic principles

The liquid metal battery is composed of three layers of liquid substances, including a low-density negative electrode in the upper layer, a high-density positive electrode in the lower layer, and a molten salt electrolyte that separates the positive and negative electrodes. The selection of metal electrode materials

Liquid metal battery follows the following three principles: (1) Considering the operating temperature of the battery, the melting point of the metal electrode should be higher than 25 °C, and the boiling point should be less than 1 000 °C; (2) The electronic conductivity of the metal electrode should be The ionic conductivity is higher than that of typical molten salt electrolytes (>1 S/cm); (3) non-toxic, easy to obtain and stable.

Advantages and disadvantages

Due to its unique composition, liquid metal battery have the advantages of high power characteristics, low cost, and long life expectancy. The specific performance is as follows:

• High power characteristics: During the working process of the liquid metal battery, both the electrode and the electrolyte are in liquid state, and the charge migrates rapidly in the electrode material body and the electrode-electrolyte interface during charging and discharging. Compared with traditional lithium metal battery, there is no problem of slow de-intercalation of ions at solid-state electrodes and electrode-electrolyte interfaces; ions have a fast migration rate (over 3 S/cm) in high-conductivity molten salt electrolytes. Based on the above two reasons, liquid metal battery have excellent rate characteristics during charging and discharging, can achieve fast response, and can be used in fields such as grid voltage regulation and frequency regulation.
• Low-cost potential: Liquid metal battery use cheap metals with abundant resources in the earth's crust as electrode materials, and the key materials have been screened for cost, which greatly reduces the material cost; moreover, the electrode and electrolyte are automatically layered during the operation of the device. Compared with traditional batteries, the device assembly process is simple and the cost is low. Although the research and development of liquid metal battery is not yet mature, the low material cost and process cost make it have low cost expectations and the potential for large-scale application.
• Long life expectancy: The metal negative electrode continues to form and disappear during the charging and discharging process of liquid metal battery, which is not affected by the decay mechanism of electrode cycle life and has the potential for long life, which is also an important technology for large-scale energy storage technology in the future. one of the indicators. Based on the above three points, when the liquid metal battery is mature, it will have a certain competitiveness in the grid energy storage market in the future. Nonetheless, liquid metal battery still have some drawbacks that limit their application in mobile facilities.

For example, the operating temperature of liquid metal battery is generally higher than 200 °C, the battery equilibrium potential is generally lower than 1.0 V, the active components of the battery are highly corrosive, and the dissolution of individual metal electrodes in the electrolyte leads to higher self-discharge rates for individual systems. . These factors work together to reduce the application scenarios of liquid metal battery, which are mainly suitable for large-scale energy storage, voltage regulation and frequency modulation in stationary applications.

Electrode materials suitable for large-scale energy storage

Energy conversion efficiency

Cost of key electrode materials

After multi-directional screening of the discharge equilibrium voltage, energy conversion efficiency and cost of the electrode material system, at present, the equilibrium potential of Sb-based electrodes, especially Li-Sb, Ca-Sb and Ba-Sb electrodes, is generally > 0.7 V , the energy conversion efficiency is more than 80%, and the cost of electrode materials is less than $100/kWh, making it an ideal electrode material for liquid metal battery.

Research status of liquid metal battery devices

The prototype of the liquid metal battery is an all-liquid electrolytic cell, and its original concept can be traced back to a three-layer liquid Hoopes electrochemical cell, that is, an electrolytic cell in which liquid aluminum and copper alloys are used as the negative and positive electrodes, respectively, and molten AlF3-NaF-BaF2 is used as the electrolyte. Today, the liquid metal battery research team led by MIT in the United States has gradually simplified the composition of liquid metal battery devices.

At this stage, liquid metal battery devices for laboratory research are basically composed of stainless steel shells, insulating ceramics, negative electrode current collectors and other components. The positive and negative electrodes are separated by an electrolyte, and the device development and assembly are simple and easy. Based on the above reasons, scientists have invented a liquid metal battery and a liquid metal battery kilowatt-level module. The positive and negative electrodes of the liquid metal battery are drawn from the side wall of the battery case.

The kilowatt-level module is composed of a number of battery modules connected in series. The battery module is composed of the inner core of the module and its outer insulation layer. The inner core of the battery module is composed of a heating separator and a number of liquid. The metal battery is composed of repeated units stacked and stacked, and all the liquid metal battery in the inner core of the battery module are connected in series in sequence.

The battery structure of the present invention facilitates single-layer connection between batteries, minimizes the connection distance between batteries, saves space, minimizes the connection resistance between batteries, and minimizes energy loss inside the battery module. Improve the energy conversion efficiency of battery modules. The invention fills the blank of the design of the kilowatt-level module of the liquid metal battery, and can greatly promote the commercialization process of the liquid metal battery.

Conclusion

As a new type of energy storage technology with low-cost and long-life potential, liquid metal battery have made great progress in the research of key material systems and device development in recent years. However, its research is still in the laboratory stage, and it is still some distance away from large-scale applications. Future research on materials, devices and systems can focus on the following aspects:

• New material system. There are many kinds of electrode material systems for liquid metal battery. In order to meet the multi-faceted needs of platform pressure, efficiency and cost, the diversified screening of positive and negative electrode materials is the general trend. Therefore, on the basis of existing electrode materials, it is an important direction of electrode material research to comprehensively evaluate the performance of each electrode material system by introducing other suitable metals between the positive and negative electrodes.
• High temperature corrosion. High temperature corrosion is a key factor affecting device stability. Unlike commercial room temperature batteries, liquid metal battery operate at high temperatures for a long time. The high operating temperature and high electrode reactivity make the key components of the device (such as current collectors, battery casings, insulating materials, and encapsulation materials) extremely vulnerable to electrodes. Corrosion of materials and molten salt electrolytes. In order to ensure that the device operates at high temperature for a long time, the problem of high temperature corrosion must be solved.
• Packaging process. High-sealed devices are also a key factor affecting device stability. The thermal expansion coefficients of various parts of the liquid metal battery are different during high temperature operation. During the high temperature process, it is very likely that the components in the air will enter due to improper packaging, resulting in corrosion of the device and side reactions, affecting its stability. Therefore, the development of suitable packaging processes is of great significance for the development of long-life devices and the commercialization of liquid metal battery.
• Heat management. Liquid metal battery need to operate at high temperatures, and the heat source needs to be stable during operation, and the heat management system needs to ensure uniform and stable heat to reduce heat loss, which brings great challenges to heat management. Therefore, for future applications, the thermal management problem of liquid metal battery devices must be solved.

From the perspective of battery system design, the liquid metal battery adopts an all-liquid structure different from ordinary batteries, and uses cheap metals and inorganic salts as electrodes and electrolytes. Its structure is simple, easy to scale up, long operating life, and low energy storage price. It is expected to have good prospects in large-scale energy storage applications.

Related article: lithium ion battery electrolyte, sulfide all-solid-state battery, high-Ni