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Backed by the world's thickest wallet, a group of the world's smartest brains are struggling with batteries.
Even if you are a sure advocate of electric vehicles, it is impossible to say that the battery technology that exists on the market is enough to support the full popularity of pure electric vehicles - who does not recognize who is not objective. Really 1000km? 10 minutes full? Low temperature does not decay? Ultra-long cycle life? Half the cost? We want everything, we need everything.
Lithium batteries are not new in principle, the first commercial lithium-ion batteries were born as early as 1991; but advanced vehicle power batteries still have room for development, and the emergence of practical pure electric vehicles is only seven or eight years ago. So in recent years, with the influx of capital, the battery circle has a new term after another...
Final Fantasy: Solid State Batteries
To talk about the future of batteries, solid state must be how to get around. Over the years, the sales of electric vehicles have soared, and we will think that the solid state will be better; the electric car endurance is embarrassing, and we will also say that the solid state is coming. Coupled with Toyota's strategy of not trusting existing lithium batteries and betting on solid-state batteries, it has made solid-state batteries a hot word.
What is the "solid" solid-state battery? What is the current battery that is not solid?? Why does this solid state make the battery better? And why can't we use solid state so far?
Common cylindrical batteries, in fact, the inside is rolled up, a layer of positive electrode and a negative electrode sandwiched with a layer of electrolyte
The so-called solid-state battery says that the electrolyte is solid rather than liquid. At present, all the lithium batteries for cars on the market, although the outside looks like a solid iron box, the inside is actually electrolyte without exception - but the liquid is "fixed" better by various structural designs, and will not be like mineral water.
Simplifying the above diagram is actually like this
If your junior high school physics has not been returned to the physical education teacher, you should at least remember: the battery is composed of positive electrode, negative electrode, electrolyte.
Outside the battery, the negative electrode loses electrons flowing along the wire to the positive electrode, because the electrons are negatively charged, so the current flows from the positive electrode to the negative electrode; inside the battery, the cations that have lost electrons on the negative electrode flow to the positive electrode in the electrolyte, and the anions that get the electrons at the positive electrode flow to the negative electrode. When we use electricity, we only use the part outside the battery, and the current flows through the wires and electrical appliances, and the luminescence and heat output are contributed.
To put it bluntly, it is a large electron ion running away scene, the outside is the electron running off one, the inside is the lithium ion running off one. Then if the internal ion flow is not "smooth" enough, the external current cannot be "smooth", which is manifested as poor charging and discharging performance. Obviously, liquids are the most convenient ions to swim around in, even if you don't understand chemistry, you should have seen a chemistry teacher draw a variety of solutions.
Switching the electrolyte to a solid state, it is also clear that it is more difficult for ions to flow between them: what used to be from the liquid end to the liquid now needs to pass through a solid substance. This is also the biggest difficulty encountered by the current solid-state battery, the high-current charging and discharging capacity is insufficient, and high-power discharge and high-power charging are indispensable for future electric vehicles.
But the temptation also comes from solid electrolytes.
The electrolyte of traditional lithium batteries occupies a considerable part of the weight, while the solid-state electrolyte can be thinner and lighter, thereby improving the energy density of the battery. The electrolyte containing lithium compounds is already flammable, and the possible leakage of liquids further increases the risk, requiring the positive and negative electrodes to be separated internally and protected by a shell externally, which is also an increase in weight and volume. In liquid electrolytes, it is difficult to use lithium metal electrodes to increase energy density.
In other words, the electrolyte changes from liquid to solid, and the energy density and safety of the battery may be significantly improved. However, at present, in terms of fast charge and fast release, cycle life, preparation cost, etc., there are still deficiencies in solid-state batteries that have not been completely solved. Early products may be in the market in these years, but mature solid-state batteries that can be commercialized on a large scale will almost certainly have to wait until 2025 or even further.
Far left
The so-called solid-state battery of WEILAI ET7 has actually been marked in its own publicity as "in situ cured solid-liquid mixed electrolyte". The so-called in situ curing, to put it bluntly, is that the electrolyte is not solid at the beginning, after coating on the electrode after some measures or reactions (specific unknown), the electrolyte forms a part of the solid substance in situ. Because there is still some liquid state left after the end of this process, it is called solid-liquid electrolyte.
So the energy density of this semi-solid-state battery, the number given by Weilai is 360Wh/kg (single body/cell energy density). This is certainly a bit higher than the current conventional non-solid-state batteries (currently up to about 300Wh/kg), but it is not as far away as our fantasies about solid-state batteries, especially considering that their positive and negative electrode materials also use more cutting-edge technology.
Is it fixed? Solid, but not fully fixed. The good thing is that users can indeed enjoy the current first-class battery energy density as soon as possible; the regret is mainly in the industry technology perspective, which cannot fully explain that WEILAI has mastered enough future all-solid-state battery technology. Of course, the second point is not very sufficient, and the first should not be obscured at the same time.
Reinventing the past: lithium metal anode
One of the advantages of solid-state batteries is that lithium metal electrodes are more likely to be used. Just last month, SES, a startup that has been delving into lithium metal batteries, released a lithium metal battery called Apollo at its inaugural SES Battery World event, with a single energy density of 417Wh/kg, and this is only the first generation of lithium metal batteries.
The so-called lithium metal battery, lithium metal refers to the negative electrode material, and the current commonly used anode material is graphite, the mainstream development trend is to incorporate silicon, that is, silicon carbon anode (Weilai solid state is also used). The negative electrode material provides electrons (discharge) externally, and lithium ions are isolated from the internal electrolyte, so how many lithium ions can be "stored" by the negative electrode will determine the upper limit of the battery energy density.
This "storage" capacity of silicon is more than ten times higher than that of carbon, and naturally becomes a better choice. However, it is not feasible to use the silicon anode directly, because the volume changes too much when the silicon anode is charged and discharged, and the expansion rate can reach 300%, while graphite is only 10%. Therefore, the current trend is to add silicon as much as possible to the graphite anode - that is, the silicon doped in the so-called "silicon-doped lithium supplement" of Zhiji L7.
Since the role of the negative electrode is to provide lithium ions, why do we have to find another way to find carbon and silicon that "stores" lithium ions, rather than directly using lithium metals that "bring" lithium ions? Compared with carbon and silicon, the lithium metal negative electrode only needs a thin piece, which greatly reduces the weight and volume of the negative electrode, which is an important source of high energy density for lithium metal batteries.
Compared with traditional anode materials, the lithium metal anode only needs a thin layer; the negative electrode is the anode
In fact, lithium metal batteries in a broad sense have long existed, but the lithium metal batteries in the past are not rechargeable batteries at all, and can only be used once. This is because lithium metal as a negative electrode is good, but when charging lithium ions need to return to the negative electrode, the traditional liquid electrolyte, lithium ions will return to the negative electrode on the surface of the precipitation of lithium metal, this precipitation will grow the so-called dendrites over time.
Over time, dendrites may grow to a certain extent to puncture the diaphragm between the positive and negative electrodes, may also puncture the battery shell to cause dangerous electrolyte leakage, and may also directly grow in contact with the cathode material to cause a short circuit. Therefore, in the past, if lithium metal was used as the negative electrode, it must be a disposable or short-life battery, and it was necessary to pull down after using it to ignore the dendrite problem.
Dendrites are a risk factor
The reason why lithium metal batteries that reappear in the jianghu today can be ranked among the power batteries for automobiles is also due to the rise and maturity of solid-state electrolyte technology. Precipitated lithium "grows wildly" in the electrolyte to become annoying dendrites, so if the electrolyte is solid, can't it block sharp dendrites?
SES's lithium metal batteries use a partial solid-state hybrid electrolyte, and one of the advantages of solid-state batteries also includes easier activation of the lithium metal anode. The two can be said to promote each other and even coexist in the relationship, the solid electrolyte makes the lithium metal negative electrode available, and the lithium metal negative electrode makes the solid electrolyte more advantageous.
Lithium metal anode technology is still in the early stage, and the degree of solution of the dendrite problem by electrolyte solidification still needs time and experience to verify. The current large-scale mass production time is also expected to be after 2025. Car companies that are more optimistic about lithium metal batteries are mainly GM and Hyundai, which have participated in several rounds of financing for SES.
Continuous evolution: high nickel and cobalt-free
Compared to the electrolyte and the negative electrode, the "advanced play" of the positive electrode is slightly monotonic. For ternary lithium cathode materials, the main theme is to increase the nickel content, reduce or even remove cobalt. This trend has continued from NCM (nickel: cobalt: manganese) 523 to 622 to 811: the proportion of nickel has increased to 80%, and the cobalt content has been reduced again and again.
Nickel is a direct factor in increasing energy density, and the content determines the reversible lithium capacity of the positive electrode. But the proportion of nickel is of course not to say that the increase is improved, too much nickel will lead to the positive electrode of the cation mix, nickel ions and lithium ions occupy each other's position, reducing the cycle performance and life of the battery; high nickel will exacerbate the self-heating phenomenon at high temperature of the battery, and lead to the internal temperature and pressure of the battery is more likely to rise, so that safety is affected.
The role of cobalt is to help the battery improve cycle life. When lithium ions can reversibly enter and exit the cathode material, that is, charge and discharge, cobalt can help the layered molecular structure in the cathode to remain stable; but at the same time, the higher the cobalt content, the lower the reversible lithium capacity of the cathode, which is manifested as a decrease in energy density.
Notice the position of the nickel-cobalt-manganese element on the left
Because cobalt is a toxic metal that accompanies a great deal of inhumanity in the mining process, it has been protested by environmental and human rights groups around the world. At the same time, the price of cobalt is also very expensive due to the difficulty of mining. Therefore, both morally and costly, battery manufacturers have enough incentives to reduce cobalt consumption.
After years of technical iteration, Tesla's cobalt in its ternary lithium battery has dropped to 3%, and the next step will be reduced to 1% until it finally achieves cobalt-free. To be low cobalt-free, other ways must be found to replace the stabilizing role of cobalt in the molecular layer structure of cathode materials.
In China, Nest Energy, which was born out of the Great Wall, was the first to mass-produce cobalt-free this year, and its current NMx battery removed the cobalt element, and the energy density of the monomer remained at a high level of 240Wh/kg. A common practice of cobalt-free is to improve the molecular layer structure by doping cations and nano-network coating to replace cobalt elements to play a stabilizing role. The SEMI-solid-state battery that has been talking about Weilai also mentions the nanoscale clad ultra-high nickel cathode (low cobalt).
170Wh/kg is the encapsulation energy density
The positive electrode, the negative electrode, and the electrolyte are only the vehicle battery technology at the cell level, and in addition to the battery, the whole package technology is also the key to improving the overall performance of the battery.
Like what Tesla is doing, replacing the 2170-size battery cell with a larger 4680 specification, reducing structural redundancy in a CTC manner, and implementing module-free CTP at the same time are all optimizations at the package level. It can be seen that even if the radical technical effect of the battery cell is not sought, there is still enough room for excavation at the whole package level. But at the same time, the optimization of the whole package is inseparable from the progress of the battery cell level, if it is not confident enough in the stability of the battery cell, like CTP and CTC can not be talked about.
If you are confident enough in the speed of advances in battery technology, you may be able to start (zuò) to (mèng) a pure electric vehicle with solid electrolyte, lithium metal anode, high nickel cathode, and CTC battery pack.