Solid-state batteries are the only way forward in the post-lithium era

As the global electric vehicle wave sweeps across the world, more and more news about solid-state batteries are coming out: from Fisker's announcement of developing a solid-state battery that can travel 500 kilometers after charging for one minute, to BMW's cooperation with SolidPower to develop solid-state batteries for the next generation of electric vehicles, to Toyota's announcement that it will realize the practical application of all-solid-state batteries by 2025. As a representative of the next generation of battery technology, solid-state batteries have attracted great attention from the market.

▌Traditional liquid lithium battery will not be the end of power battery technology

Traditional power battery systems will be unable to meet the energy density requirements in 10 years.

As we all know, power batteries directly correspond to the cost-effectiveness of new energy vehicle products, and energy density is a key indicator of power batteries.

my country's electric vehicle market is undergoing a transition from "policy-driven" to "policy-assisted". The policy orientation for improving the energy density of the lithium battery industry has been clear. Subsidies are directly linked to energy density and the threshold is constantly increasing.

"Made in China 2025" issued by the Ministry of Industry and Information Technology states: "By 2025 and 2030, the energy density of my country's power battery cells must reach 400Wh/kg and 500Wh/kg respectively." The indicators correspond to 2-3 times the current average level of 170Wh/kg for passenger car power battery cells.

In order to clarify the concept of 400-500Wh/kg for the energy density of power batteries, we have sorted out the iterative path of lithium-ion battery technology. my country is in the process of developing from the second generation to the third generation of lithium batteries.

In terms of the selection of positive electrode materials, my country has shifted from lithium iron phosphate to ternary materials, and is gradually developing towards high-nickel ternary materials. The current industrialization of negative electrode materials is still concentrated on graphite materials, and in the future it will also transition to silicon-carbon negative electrodes.

It is estimated that the weight energy density limit of the liquid lithium-ion power battery currently used, which is composed of high-voltage layered transition metal oxides and graphite as positive and negative active materials, is about 280Wh/kg.

After introducing silicon-based alloys to replace pure graphite as negative electrode materials, the energy density of lithium-ion batteries is expected to reach more than 300Wh/kg, with an upper limit of about 400Wh/kg.

Safety issues are related to the healthy development of the industry and are difficult to completely eradicate

The flammable liquid organic electrolyte is the culprit behind the spontaneous combustion of batteries. The sales of new energy vehicles have been increasing year by year, but the number of safety accidents has increased. Among them, spontaneous combustion of batteries accounts for 31% of the causes of accidents. The cause of spontaneous combustion is that after an internal or external short circuit occurs in the lithium battery, the battery releases a large amount of heat in a short period of time, and the temperature rises sharply, leading to thermal runaway. The flammable liquid electrolyte will ignite at high temperatures, eventually causing the battery to catch fire or explode.

Faced with the two great challenges of energy and safety, where will the next generation of lithium batteries come from? Looking back at the development history of electric vehicle battery technology, from early lead-acid batteries, to nickel-metal hydride batteries promoted by Japanese companies such as Toyota, to lithium-ion batteries used by Tesla roasters in 2008, traditional liquid lithium-ion batteries have dominated the power battery market for a decade.

In the future, the contradiction between energy and safety demands and traditional lithium battery technology will become increasingly prominent. Among the next generation of lithium battery technologies, solid-state batteries have received the highest attention, prompting companies around the world to take positions in advance.

Why must it be a solid-state battery?

No burning, eliminate safety hazards

Solid-state batteries are lithium-ion batteries that use solid electrolytes. In terms of working principle, there is no difference between solid-state lithium batteries and traditional lithium batteries: traditional liquid lithium batteries are called "rocking chair batteries", with the two ends of the rocking chair being the positive and negative poles of the battery, and the middle being a liquid electrolyte. Lithium ions migrate in the electrolyte to shuttle between the positive and negative poles to achieve charging and discharging, while the electrolyte of solid-state batteries is solid, which is equivalent to the place where lithium ions migrate being transferred to the solid electrolyte. Solid electrolyte is the core of solid-state batteries.

Solid electrolytes are non-flammable, greatly improving battery safety. Compared with traditional lithium batteries, the most prominent advantage of all-solid-state batteries is safety. Solid-state batteries are non-flammable, high-temperature resistant, non-corrosive, and non-volatile, avoiding electrolyte leakage and electrode short circuits in traditional lithium-ion batteries, reducing the battery pack's sensitivity to temperature, and eliminating safety hazards.

At the same time, the insulating properties of the solid electrolyte enable it to effectively isolate the positive and negative electrodes of the battery, avoiding short circuits caused by contact between the positive and negative electrodes while also acting as a diaphragm.

Compatible with high-capacity positive and negative electrodes + lightweight battery system, promoting a big leap in energy density

(1) Wider electrochemical window, easier to carry high voltage cathode materials

To increase the capacity of the positive electrode material, it is necessary to charge it to a high voltage in order to release more lithium. Currently, the electrolyte solution for lithium cobalt oxide can be charged to 4.45V, and the ternary material can be charged to 4.35V. If it is charged to a higher voltage, the liquid electrolyte will be oxidized and an irreversible phase change will occur on the positive electrode surface. The promotion of the ternary 811 battery is currently restricted by the high-voltage electrolyte.

Solid electrolytes have a wider electrochemical window, up to 5V, and are more suitable for high-voltage electrode materials. With the continuous upgrading of positive electrode materials, solid electrolytes can be better adapted, which is conducive to improving the energy density of battery systems.

(2) Compatible with lithium metal anode, increasing the upper limit of energy density

The characteristics of high capacity and high voltage make lithium metal the "ultimate negative electrode" after graphite and silicon negative electrodes. In order to achieve higher energy density goals, battery systems with lithium metal as negative electrodes have become an inevitable choice. Because:

The gram capacity of lithium metal is 3860mAh/g, which is about 10 times that of graphite (372mAh/g).

Metallic lithium is the material with the lowest electrochemical potential in nature, at -3.04 V. At the same time, it is itself a lithium source, and the positive electrode material selection is wider. It can be an embedded compound containing or not containing lithium, or sulfur or sulfide or even air, which corresponds to lithium-sulfur and lithium-air batteries with higher energy density, respectively, and the theoretical energy density is close to 10 times that of current batteries.

Lithium metal anode is difficult to achieve in current traditional liquid battery systems. The research on lithium metal batteries can be traced back to the 1960s, and it was successfully developed and applied to primary batteries in the 1970s.

In the field of rechargeable batteries, there are a series of technical problems with metallic lithium negative electrodes in liquid batteries, and there is still a lack of effective solutions. For example, there are many side reactions at the interface between metallic lithium and liquid electrolyte, the SEI film is unevenly distributed and unstable, resulting in poor cycle life, and the uneven deposition and dissolution of metallic lithium leads to the uneven formation of lithium dendrites and pores.

Solid electrolytes are highly expected by the scientific community to solve the application problem of lithium metal anodes. Researchers place their hopes on the use of solid electrolytes to solve the application problem of lithium metal anodes. The main idea is to avoid the side reactions that continue to occur in liquid electrolytes, and at the same time use the mechanical and electrical properties of solid electrolytes to inhibit the formation of lithium dendrites.

In addition, since the solid electrolyte separates the positive electrode from the negative electrode material, there will be no short-circuit effect of lithium dendrites piercing the diaphragm. In short, solid electrolytes have better compatibility with lithium metal negative electrodes, and lithium metal materials will be used first on solid-state battery platforms.

Reduce system weight and further improve energy density

The weight reduction of the solid-state battery system further improves the energy density. The power battery system needs to produce monomers first, and then assemble the monomers in series after the monomer packaging is completed. If the monomers are connected in series first, it will cause a short circuit between the positive and negative poles and self-discharge. The solid-state battery cell does not contain liquid inside, so it can be connected in series and parallel before assembly, which reduces the material used for the assembly shell and greatly simplifies the PACK design.