Lithium-ion batteries need to contain several basic materials: positively active substance, negatively active substance, isolation membrane, electrolyte. Now, just to give you a quick overview, what does this stuff do?
It is not difficult to understand the positive and negative electrodes. In order for the charge to move, you need the potential difference between the positive and negative materials. So what is the active material? As we know, batteries actually convert electrical energy and chemical energy into each other to store and release energy. To do that, you need a material that is “easy” to react with, one that is easy to oxidize and reduce, and one that can convert energy, so we need an “active material” to be the positive and negative electrodes of the battery.
As mentioned above, lithium is our preferred material for battery production, so why not use lithium metal as the active material for the electrode? Isn’t that the maximum energy density?
What are electrolytes used for? Generally speaking, it is the “water” in the swimming pool, so that lithium ions can swim freely, so the higher the ionic conductivity, the smaller the resistance swimming, the smaller the electrical conductivity insulation, the better the chemical stability, the better the thermal stability security, giving a broad potential window. Based on these principles, after a long time of engineering exploration, people found high purity organic solvents, lithium electrolyte salt and necessary additives and other raw materials, under certain conditions, according to a certain proportion of electrolyte preparation. Organic solvents PC propylene carbonate, EC vinyl carbonate, DMC dimethyl carbonate, DEC diethyl carbonate, EMC acid ethyl ester and other materials. Lithium electrolyte contains LiPF6, LiBF4 and other substances.
Lithium, in the form of metal, is so “lively” that naughty children are often disobedient and prone to sabotage. Early research into lithium-ion batteries focused on lithium metal or its alloys as negative electrodes, but safety concerns forced them to find better alternatives. In recent years, with the pursuit of energy density, there has been a trend of “whole blood resurrection” in this direction of research, which we will discuss later.
In order to achieve the chemical stability of the energy storage and release process, that is, the safety and long life of the battery charge and discharge cycle, we need an electrode material that is reactive when needed and stable when needed. After long-term research and exploration, several lithium metal oxides have been found, such as lithium cobalt oxide, lithium titanate, lithium iron phosphate, lithium manganate, nickel cobalt manganese ternary materials, which can be used as the positive or negative electrode of the battery active substances, to solve the above problems. As shown in the figure, the olivine structure of lithium iron phosphate is also a very stable cathode material structure. During the charge and discharge process, the lithium ions are stripped away without causing lattice collapse. Metal lithium-ion batteries also exist, but compared with lithium-ion batteries, the development of the technology will eventually have to serve the market.
Of course, while solving the stability problem, it also brought serious “side effect”, that is, the proportion of lithium as the energy carrier decreased significantly, and the energy density decreased by a majority of orders of magnitude.
The anode usually chooses graphite or other carbon materials as the active material, which also conforms to the above principle. The requirement is a good energy carrier, relatively stable, relatively rich reserves, easy to large-scale production. Carbon is a relatively optimal solution. Of course, this is not the only solution, and the negative material has been extensively studied in the next section.
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