Mass production technology and cost analysis of "3.7V lipo battery"

At present, many domestic power battery manufacturers have launched power battery products with a weight energy density of more than 300Wh/kg to meet the increasing mileage of electric vehicles for high energy density power batteries. However, the energy density of lithium-ion batteries based on liquid electrolytes is about to hit the ceiling. It is generally believed that the energy density upper limit of the existing lithium-ion battery system is 350Wh/kg. To further improve the energy density of batteries, a new system needs to be adopted. From the existing technology, all-solid-state battery technology based on solid electrolytes is the most promising candidate for the next generation of battery technology. A group of top scholars including Goodenough and Cui Yi 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 inhibit the growth of lithium dendrites. Therefore, in theory, all-solid-state batteries can achieve an energy density of more than 500Wh/kg by using Li metal negative electrodes. However, in fact, there are still many problems that need to be overcome in solid-state batteries, such as interface contact problems, solid-state battery production processes, and thinning of solid electrolyte membranes. Therefore, most solid-state batteries are still in the laboratory exploration stage. Solid electrolytes can be mainly divided into oxides, sulfides and organic polymers based on their composition. Oxide electrolytes have been widely 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 than the other two types of electrolytes in terms of processability, but 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 processing performance, but they will react with moisture in the air in the atmospheric environment to generate highly toxic H2S gas, so the production process needs to be carried out in a protective atmosphere. Oxide solid electrolytes have excellent conductivity and excellent stability in the air environment, but poor processing performance. Solid electrolytes have lower conductivity than liquid electrolytes. Therefore, in order to reduce the internal resistance of the battery and improve the battery's large current discharge capability, the solid electrolyte membrane needs to be made as thin as possible. The surface resistance of the solid electrolyte can be calculated by the following formula, where L is the thickness of the electrolyte and δ is the conductivity of the electrolyte. For example, a liquid electrolyte with a conductivity of 20mS/cm and a thickness of 25um has a surface resistance of 0.125Ω/cm2. However, in fact, due to the large tortuosity of the pores of the diaphragm used in the liquid electrolyte, the surface resistance of the electrolyte can actually reach 3.75Ω/cm2. Solid electrolytes do not require a diaphragm. Therefore, to achieve the same surface resistance as the electrolyte, the conductivity of the solid electrolyte can be lower. For example, a 10um solid electrolyte only requires a conductivity of 0.27mS/cm to achieve an effect similar to that of the electrolyte. The main problem faced in the preparation of 3.7V lipo battery is how to obtain an electrolyte layer with lower porosity and higher conductivity. To achieve this goal, sintering is a common method, but studies in recent years have shown that most positive electrode materials will react with solid electrolytes at high temperatures. For example, LNMO and LLZ will react above 600°C, and NCM622 materials will react with LLZ above 700°C. However, in order to reduce the porosity of the solid electrolyte and increase the conductivity, the sintering temperature usually needs to reach above 1000°C. Therefore, the preparation of the positive electrode of the oxide solid-state battery cannot be solved by simple sintering, but requires a more complex process. 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 following table. Among them, the vapor deposition method has a high probability of error when preparing large-size and thick (5-30um) films, so it is not practical, and the plasma or flame spray method 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 layers and positive electrode layers, as well as their reliability in the preparation of solid-state batteries. The figure below shows two feasible production processes of 3.7V lipo battery designed by the author based on the above analysis. Figure a below is a positive electrode support type. First, a mixed slurry of positive electrode materials, solid electrolytes, binders, additives and solvents is coated on the current collector. After drying, laser cutting is performed, and then low-temperature calcination is performed. Then, laser shaping is performed again. Then, a layer of solid electrolyte layer is deposited on the electrode surface by spray deposition, and then sintered at a medium temperature (600°C). The prepared pole piece can be combined with a metal lithium negative electrode to prepare an all-solid-state battery. The advantage of this method is that high temperature is not required, thus avoiding the occurrence of side reactions. At the same time, different solid electrolyte types can be used for the positive electrode and electrolyte layers to give full play to their advantages. The main immature link of this method is the spray deposition process. Figure b below shows the preparation process of a three-layer composite solid electrolyte layer battery. First, a porous structure electrolyte layer is prepared, and then a high-density electrolyte layer is applied. After high-temperature sintering, the positive electrode slurry is applied on one side of the porous layer to allow it 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, the molten metal Li is applied on the other side of the solid electrolyte to complete the battery assembly. Another major factor affecting the application of power batteries is their production cost. Although most solid-state batteries are still in the laboratory stage and there is a lack of effective data on cost estimates, we can estimate it through SOFC fuel cells that are close to them (as shown in the figure below). Figure a below shows the production cost of SOFC batteries. The processing cost, including labor and sintering, accounts for 75%, while the material cost is only 25%. Since the production process of three-layer composite electrolyte solid-state batteries is similar to that of SOFC batteries, we can use SOFC data to predict their costs. At present, the material cost of all-solid-state batteries is mainly controlled by the oxide solid electrolyte LLZ. The current price of LLZ is as high as 2000$/kg, but with the development of solid-state battery technology, the cost of LLZ will drop significantly. It can be assumed here that the minimum cost of LLZ can be reduced to 50$/kg. Therefore, when the battery structure is similar, when the thickness of the positive electrode LNMO is 70um, the cost of a single battery is 0.12$. If the thickness of the positive electrode is reduced to 150um, the cost of a single battery will increase to 0.23$. Since most of the production costs of solid-state batteries are production process costs, expanding the scale of production can effectively reduce the cost of batteries. From Figure b below, we can see that in small-scale production (10,000 units/year), the production process cost will reach 750-2500$/kWh, but if the production capacity is expanded to 100 million units/year (10-20GWh/year), the production process cost will drop significantly to 75-240$/kWh, so the final cost of all-solid-state batteries is expected to drop to 140-350Wh/kg. However, even so, the production process cost still accounts for more than 50%, which is still significantly higher than that of lithium-ion batteries (the process cost is only 20-30%). Material costs still have an important impact on solid-state batteries. From Figure c below, we can see that if the cost of LLZ electrolyte drops to 20$/kg, the cost of the battery can be reduced to 180-310$/kWh when using LNMO positive electrode. If high-nickel NMC is used, the cost is expected to drop further to 120-210$/kWh. The ultimate goal of all-solid-state batteries is 150$/kWh, and a lot of optimization work is still needed. 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 it is even more important to design a suitable production process. At the same time, at this stage, the production cost of solid-state batteries is still relatively high. The subsequent reduction in raw material costs and economies of scale can effectively reduce the production cost of solid-state batteries, which is expected to drop to 150$/kWh.

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