What Are Solid-State Batteries?

As early as May this year, many media outlets have been hyping up the 6 billion solid-state battery project.

Afterwards, some solid-state battery companies have announced that they will industrialize all-solid-state batteries in 2027. Various press conferences have followed to announce how high the energy density and cycle performance can be. One can often hear the energy density of a single cell of a certain company of 700+Wh/kg.

So, what exactly is a solid-state battery? What level has the market reached now? After communicating with many friends around me, I found that the first concept that is easy to confuse with solid-state batteries is semi-solid and full solid.

It can be said that almost all the so-called solid-state batteries currently circulating on the market are semi-solid-state batteries, that is, a solid-liquid mixture. However, after disassembling many solid-state batteries, it was found that the so-called solid-liquid mixture is actually difficult to see, and is almost the same as the state of liquid batteries. Even through some precise characterizations, it is difficult to find clues.

There are two representative Chinese companies: Qingtao and Weilan. Qingtao's main system is lithium iron phosphate (of course they also have a ternary system), and Weilan is represented by ternary (of course they also have iron lithium). The former is mainly ceramic coating technology, and the latter is advertised as in-situ curing technology (the emphasis is on publicity). It is said that Qingtao currently has 380Wh/kg cells in circulation, and Weilan is currently selling 350Wh/kg cells with a capacity of 110Ah.

What about all-solid-state batteries? All-solid-state batteries are mainly divided into oxides, polymers and sulfides (of course, there are also halides). Judging from the current development status of the overall leading companies, the entire technology route is sulfide. The so-called sulfide solid-state battery is actually a mixture of positive and negative electrode materials with electrolytes and binder conductive agents to form a positive electrode (of course there are dry and wet methods), and then the electrolyte and a small amount of binder are mixed to form a film (of course there are dry and wet methods). If it is a wet method, the sulfide system is very sensitive to the solvent system, and of course a special binder is required. Finally, the positive and negative electrodes and the electrolyte membrane are stacked layer by layer to form an all-solid-state battery cell. Each of these processes has a gap that blocks the industrialization of all-solid-state batteries.

The second thing that is easy to confuse is solid-state battery = high safety and high energy density

This is a big misunderstanding of most people, including those in the industry. They think that all-solid-state batteries have high energy density. Many people outside the industry often pin their hopes on all-solid-state batteries, often saying "there will be no liquid when all-solid-state batteries come out". In fact, this is not the case. To understand this logic, we must first start from the concept of energy density: energy density = energy/weight, and energy is determined by the material itself, so the energy density of the battery cell is determined by the material system of the battery.

Iron-lithium batteries are currently 180Wh/kg. Since ternary batteries are divided into many systems, their energy density is basically in the range of 240-360 or even 380Wh/kg (more than 285Wh/kg requires silicon-based materials). Of course, the lithium cobalt oxide system is basically more than 200 energy densities. Now many energy density propaganda on the market has reached 450, 500, 600 or even 700Wh/kg or more. Basically, the negative electrode material is lithium metal or no negative electrode. This is the overall state of energy density. The positive and negative electrode materials of all-solid-state batteries have not been separated from the liquid raw materials. Therefore, the energy density of all-solid-state batteries will not be higher than that of liquid batteries.

The high value that everyone talks about is actually based on the expectation that all-solid-state batteries can use lithium metal negative electrodes to achieve high energy density of the battery cell after solving the safety problem, but this difficulty is no less than solving the safety problem of liquid lithium metal batteries. Therefore, it is untenable to say that the energy density of solid-state batteries is low. On the contrary, from the actual development status, the energy density of all-solid-state batteries will be lower. The first one comes from the application of high-energy system materials, the second one comes from the proportion of active materials, the third one comes from the thickness of the electrolyte membrane, and the fourth one comes from the problem that everyone is not paying much attention to at present. The operation of all-solid-state batteries requires a high-pressure clamp. The clamp will increase the weight of the electrical equipment during actual use, thereby reducing the advantage of the energy density of the battery cell to a certain extent.

Overall, all-solid-state batteries have greatly improved safety (there are actual tests), but as a sulfide solid-state battery with soft-pack packaging material, sulfide itself is a material with great safety risks. Secondly, the safety improvement of all-solid-state batteries is also limited. It is not inherently safe. To a certain extent, it can still trigger thermal runaway of the battery.

The above is some relatively macroscopic understanding of the current solid-state batteries, including all-solid-state and semi-solid-state. Of course, in the long run, all-solid-state is still optimistic. From the current situation, the difficulty of solving the safety problems of high-energy liquid batteries is not necessarily lower than the difficulty of developing a new generation of high-safety all-solid-state batteries. I believe that with the joint efforts of the upstream and downstream industrial chains, we can break through the status quo and realize the revolution.

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