It is generally believed that the energy density limit of existing lithium-ion battery systems is 350Wh/kg. To further improve the energy density of batteries, new systems must be adopted. From the perspective of existing technologies, all-solid-state battery technology based on solid electrolytes is the most promising candidate for the next generation of battery technology. A number of outstanding scholars, including Cui Yi and Goodenough, have high hopes for all-solid-state technology.
All-solid-state batteries use solid electrolytes, which have higher mechanical strength than liquid electrolytes and can suppress the growth of lithium dendrites. Therefore, theoretically, all-solid-state batteries can achieve an energy density of over 500 Wh/kg by using Li metal anodes. However, in reality, there are still many problems to overcome in solid-state batteries, such as interface contact issues, solid-state battery manufacturing processes, and thinning of solid electrolyte films. Therefore, most solid-state batteries are still in the laboratory exploration stage.
Solid electrolytes can be broadly classified into oxides, sulfides, and organic polymers based on their composition. Among them, oxide electrolytes have been extensively studied due to their high conductivity and good environmental adaptability. Today, we will analyze the mass production technology and cost of oxide electrolytes.
First, let's compare the advantages and disadvantages of oxide, sulfide, and organic polymer solid electrolytes (as shown in the table below). Polymer electrolytes are much better in terms of processability than the other two types of electrolytes. However, polymer electrolytes have low conductivity at room temperature, which has a certain impact on the discharge capacity of the battery. Sulfide electrolytes have excellent conductivity and good processability, but they will react with moisture in the air to generate highly toxic H2S gas. Therefore, the production process must be carried out in a protective atmosphere. Oxide solid electrolytes have excellent conductivity and excellent stability in the air environment, but their processability is poor.
Solid electrolytes have lower conductivity than liquid electrolytes. Therefore, to reduce the battery's internal resistance and improve its high-current discharge capability, the solid electrolyte membrane should be made as thin as possible. The sheet resistance of a solid electrolyte can be calculated using the following formula, where L is the thickness of the electrolyte and δ is the conductivity. Taking a liquid electrolyte with a conductivity of 20 mS/cm and a thickness of 25 μm as an example, its sheet resistance is 0.125 Ω/cm². However, in reality, due to the greater tortuosity of the membrane pores used in liquid electrolytes, the sheet resistance of the electrolyte can actually reach 3.75 Ω/cm². Since solid electrolytes do not use a membrane, to achieve the same sheet resistance as liquid electrolytes, the conductivity of solid electrolytes can be lower. Taking a 10 μm solid electrolyte as an example, to achieve a similar effect to liquid electrolytes, only a conductivity of 0.27 mS/cm is needed.
The key challenge in fabricating oxide solid-state batteries is obtaining an electrolyte layer with lower porosity and higher conductivity. Sintering is a common method to achieve this goal. However, recent studies have shown that most cathode materials react with solid electrolytes at high temperatures. For example, LNMO and LLZ react above 600°C, and NCM622 reacts with LLZ above 700°C. To reduce the porosity and increase the conductivity of the solid electrolyte, the sintering temperature usually needs to reach above 1000°C. Therefore, the cathode fabrication of oxide solid-state batteries cannot be solved by simple sintering and requires more complex processes.
Oxide solid electrolyte technology is also used in solid-state fuel cells (SOFC) and solid-state capacitors (MLCC), which can provide a certain reference for the production of oxide all-solid-state batteries. The common preparation processes that can be used for oxide solid electrolyte films are shown in the table below. Among them, the vapor deposition method has too high an error probability when preparing large-size and thick (5-30um) films, so it is not practical. Plasma or flame spraying methods cannot be applied due to material stability issues. Therefore, there are only 6 methods that can be used for the production of oxide all-solid-state batteries. The figure below shows the convenience of the 6 film preparation methods in the preparation of solid electrolyte and cathode layers, as well as their reliability in solid-state battery preparation.
The following diagram illustrates two feasible production processes for oxide solid-state batteries designed by the author based on the above analysis. Figure a below shows the positive electrode supported type. First, a slurry containing positive electrode material, solid electrolyte, binder, additives, and solvent is coated onto the current collector. After drying, it is laser-cut, then calcined at low temperature, and then laser-shaped again. A solid electrolyte layer is then deposited on the electrode surface using a spray deposition method, followed by sintering at a moderate temperature (600℃). The prepared electrode and lithium metal negative electrode are then combined to form an all-solid-state battery. The advantage of this method is that it avoids high temperatures, thus preventing side reactions. Furthermore, different types of solid electrolytes can be used for the positive electrode and electrolyte layer to fully utilize their advantages. The key immature aspect of this method is the spray deposition process.
Figure b below illustrates the fabrication process of a three-layer composite solid electrolyte layer battery. First, a porous electrolyte layer is prepared, then a high-density electrolyte layer is coated. After high-temperature sintering, a positive electrode slurry is coated on one side of the porous layer to penetrate into the porous structure. Then, low-temperature sintering is performed to ensure good ionic conductivity between the positive electrode material and the electrolyte. Finally, molten metallic Li is coated on the other side of the solid electrolyte to complete the battery assembly.
Another major factor affecting the application of power lithium batteries is their production cost. Although most solid-state batteries are still in the laboratory stage and effective data for cost estimation is lacking, we can estimate their cost using SOFC fuel cell batteries, which are similar in nature (as shown in the figure below). Figure a below shows the production cost of SOFC batteries, with processing costs, including labor and sintering, accounting for 75%, while material costs account for only 25%. Since the production process of three-layer composite electrolyte layer solid-state batteries is similar to that of SOFC batteries, we can use SOFC data to predict their costs. Currently, the material cost of all-solid-state batteries is mainly controlled by oxide solid electrolyte (LLZ). The current price of LLZ is as high as $2000/kg, but with the development of solid-state battery technology, the cost of LLZ will decrease significantly. Here, we can assume that the minimum cost of LLZ can be reduced to $50/kg. Therefore, with similar battery structures, when the thickness of the positive electrode LNMO is 70µm, the cost of a single battery is $0.12. If the thickness of the positive electrode is reduced to 150µm, the cost of a single battery will increase to $0.23.
Since the majority of the production cost of solid-state batteries is the production process cost, scaling up production can effectively reduce the battery cost. As shown in Figure b below, the production process cost reaches $750-2500/kWh when producing on a small scale (10,000 units/year). However, if the production volume is expanded to 100 million units/year (10-20 GWh/year), the production process cost will drop significantly to $75-240/kWh. Therefore, the final cost of all-solid-state batteries is expected to decrease to 140-350 Wh/kg. Even so, the production process cost still accounts for more than 50%, which is still significantly higher than that of lithium-ion batteries (where the process cost is only 20-30%).
Material costs still have a significant impact on solid-state batteries. As shown in Figure c below, if the cost of LLZ electrolyte drops to $20/kg, the cost of the battery using LNMO cathode can be reduced to $180-310/kWh. If high-nickel NMC is used, the cost is expected to drop further to $120-210/kWh. The ultimate goal for all-solid-state batteries is $150/kWh, which requires a lot of optimization work.
Oxide solid electrolytes have high conductivity and good environmental stability, making them one of the best choices for solid-state battery electrolytes. However, solid electrolytes have high hardness and poor processing performance, so designing a suitable production process is more important. At the same time, the production cost of solid-state batteries is still relatively high at present. In the future, the production cost of solid-state batteries can be effectively reduced through the reduction of raw material costs and economies of scale, and it is expected to be reduced to $150/kWh.
