Solid state batteries are so good, why are they difficult to produce?

Before introducing solid-state batteries, it is necessary to first understand the basic composition of lithium batteries. Simply put, today's lithium batteries are mainly composed of four parts: positive electrode, negative electrode, electrolyte, and separator. According to the different electrolytes, lithium batteries can be divided into two types: liquid and solid. The lithium batteries we use today are mostly liquid electrolytes (also known as electrolytes), while the disruptive solid-state batteries of the future will actually turn liquid electrolytes into solids.

Friends familiar with batteries know that there is a very important performance indicator called energy density, and the higher this indicator, the more energy the battery can store under the same weight. The key to improving battery energy density mainly lies in using electrode materials with high specific energy and high voltage. Firstly, let's take a look at high specific energy materials. Currently, the graphite negative electrode used in liquid batteries has a theoretical specific capacity of only 372mA · h/g, while the ideal negative electrode material for lithium batteries - lithium metal - has a theoretical specific capacity of 3860mA · h/g, which is nearly 10 times higher than the two.

Since lithium metal has such a high specific capacity, why isn't it used as a negative electrode material in liquid batteries? The reason behind this is actually quite helpless. Simply put, the metal lithium negative electrode will produce lithium dendrites during the charging process. Over time, the lithium dendrites may pierce the battery separator, leading to battery short circuits and explosions. In addition, even if the battery does not experience a short circuit, the metal lithium negative electrode may react chemically with the electrolyte, continuously consuming the electrode and electrolyte, ultimately leading to a significant reduction in battery life. It is not difficult to see that the compatibility between liquid batteries and high specific energy electrode materials is not good.

Is it feasible to use high voltage electrode materials when high specific energy electrode materials cannot be used in liquid batteries? As we all know, the battery label on mobile phones and computers is' capacity ', measured in milliampere hours (mA · h), but the battery label on cars is' energy', measured in kilowatt hours (kW · h). The relationship between the two is very simple, battery energy=battery capacity x battery voltage, so as long as the voltage of a single battery is increased, the energy density of the entire battery pack will increase.

In battery design, the voltage of a single cell is determined by the voltage magnitude of the electrode material itself. For example, the well-known lithium iron phosphate battery has a positive electrode voltage of around 3.6V, while the positive electrode voltage of ternary lithium batteries is around 4.2V. From the voltage alone, it can be seen that ternary lithium batteries have an advantage in energy density.

But if we want to further increase the voltage of electrode materials, we will encounter performance bottlenecks in liquid batteries. Specifically, the theoretical upper limit of the electrolyte voltage for liquid batteries is 4.5V. Beyond this voltage, the electrolyte will decompose and produce gas, which is the phenomenon of battery bulging that many people may have encountered.

The advantages of solid-state batteries are reflected in the high specific energy and high voltage mentioned earlier. Firstly, solid-state electrolytes have high mechanical strength, which can prevent lithium dendrite piercing. Therefore, solid-state batteries can use lithium metal negative electrode materials with extremely high specific capacity, and the energy density of lithium batteries can be increased to 400W · h/kg solely through this improvement. As a reference, the current best performing liquid ternary lithium battery has an energy density of 255W · h/kg, while lithium iron phosphate has a lower energy density of only 140-160W · h/kg.

In addition, solid-state electrolytes also support higher electrode voltages, and as long as high-voltage electrode materials are properly matched, the energy density of solid-state batteries can even be increased to 600W · h/kg. In summary, solid-state batteries have unparalleled advantages in terms of energy density.

In terms of safety, liquid batteries are more prone to spontaneous combustion due to their more complex internal structure and materials that are not resistant to high temperatures. More specifically, the electrolyte layer and separator of liquid batteries begin to decompose and melt at around 80-130 ℃. As long as the temperature inside the battery reaches this level, short circuits and spontaneous combustion will occur. Therefore, liquid batteries have extremely high requirements for heat dissipation.

Pure solid state batteries are undoubtedly the most disruptive technology in the field of new energy vehicles. However, at the current level of technology, pure solid state batteries have not completely overcome the problems of low charging and discharging power, short cycle life, and high cost. Therefore, the popularization of pure solid state batteries still needs a long time. In contrast, semi-solid state batteries that can improve battery energy density while achieving a balance between lifespan and cost are more easily accepted by the market. Before the mass production of pure solid state batteries, semi-solid state batteries will be the best transition solution.