In the final analysis, energy storage technology is a demand-oriented technology, and its evaluation index system covers energy index, power index and so on. With different application scenarios, the demand type and weight of energy storage technology for indicators are also different in lithium battery industry.
Among various energy storage technologies, secondary batteries (electrochemically rechargeable batteries) are a very critical component. Its application range is very large, and the connection ability with renewable energy power is also very strong in lithium battery industry. In terms of the applicable energy and power range, a variety of secondary batteries cover the technical needs of most energy storage applications, but the lithium battery industry is more popular.
Germination of lithium
During the electrochemical reaction, lithium battery industry needs the movement of carriers (ions or ion groups) to provide the charge balance of the whole circuit: the electrode material, electrolyte and electrode-electrolyte interface conduct charge carriers, and the electrode, current collector and external circuit conduct electronic. Different materials have different bulk conductance, as well as interface conductance.
The higher the ion diffusion coefficient/electronic conductivity corresponding to lithium battery industry material, the higher the upper limit of the rate capability of the lithium ion battery cell. If the secondary lithium battery that can be applied on a large scale has a low cost, it is best to use a relatively high abundance of elements, and the basic material system is cheap and the lithium battery industry production process is simple. Obviously, this element is lithium.
Lithium battery cathode: the development of shortages
Cathode is a part of lithium battery industry and lithium cobaltate was the first commercially available layered oxide cathode. It has a relatively high theoretical specific capacity (274mAh/g, and capacity density), low self-discharge rate, and high cut-off voltage (and is increasing to close to 4.5V under continuous optimization). At the same time, cobalt is expensive, and lithium cobalt oxide is relatively stable in its delithiation state, and its thermal stability and high-rate cycle life are worrying.
Lithium nickelate and lithium cobaltate have the same crystal structure, and nickel is cheaper than cobalt. However, the trivalent nickel in lithium nickelate is not stable enough due to the J-T effect, and part of the divalent nickel in lithium nickelate will be mixed with lithium ions (may occur during material synthesis and delithiation, and cobalt inhibits this).
During cycling, lithium nickelate undergoes a high degree of irreversible phase transition, which affects the performance-lifetime performance. Moreover, the thermal stability of lithium nickelate is not as good as that of lithium cobaltate. This is also an important part in lithium battery industry.
A small amount of cobalt and aluminum doped NCA cathode is an improvement on lithium nickelate. The overall electrochemical performance and thermal stability are better, but the lack of structural stability under high pressure may release oxygen. The influence of capacity and voltage changes on the cycle life of Li-rich manganese-based cathodes during material cycling is the main problem and affects magnification performance.
Based on this, various modification methods are also required to control the composition of the main element of the lithium-rich manganese-based cathode materials and synthesize suitable particle morphology and structure. Such as bulk doping, surface coating, surface treatment, etc., the purpose includes inhibiting oxygen evolution, inhibiting electrode-electrolyte side reactions, and improving electrical conductivity.
Lithium battery anode
Lithium-graphite intercalation compounds have been synthesized as early as the 1950s. After matching with a suitable electrolyte, the formation process enables the formation of an SEI film on the surface of the graphite negative electrode, which can reversibly intercalate/deintercalate lithium ions. Although the specific capacity of 372mAh/g is not outstanding among negative electrode materials, the comprehensive properties of graphite are balanced.
Therefore, artificial graphite/natural graphite has become the representative of carbon materials in lithium storage anode, and even the representative of lithium storage anode. And it is also widely used in lithium battery industry at present.
In addition, some special types of graphite materials, such as partial graphene, highly oriented pyrolysis graphite, etc., actually have a higher specific lithium storage capacity. The lithium storage potential of the graphite negative electrode is very low, and the SEI film formed on its surface stabilizes the entire system during lithium battery industry formation. The lithium mass transfer of SEI film includes two parts: interfacial mass transfer and bulk mass transfer.
The silicon-based anode material also shows a very obvious intrinsic volume change during the lithium intercalation process, which affects the battery cycle. Therefore, alleviating the circulation volume change of silicon-based anode materials is a problem that must be solved in all research work. On this basis, silicon-based anodes have derived technical routes such as elemental silicon-carbon (and low-dimensional silicon materials) anodes, silicon oxide-carbon anodes, and silicon alloys.
The first two (generally referred to as silicon-carbon anodes) are Subdivision technology route with strong practicability. In short, in order to improve the actual specific capacity, rate performance (especially fast charging capability) and cycle life of silicon-based anode materials to meet the growing demand for high-performance power batteries, in addition to the control of the synthesis/modification methods of silicon/silicon oxide itself (Mechanical grinding or silane decomposition to prepare a silicon-based substrate with a suitable particle size, and then carbon composite and coating, pre-lithium, and surface structure strengthening by different means), and various components such as electrolyte, conductive agent, and adhesive are also used. Further adaptation is required. Both of the two anode are widely used in lithium battery industry.
Lithium battery electrolyte
The solvent in the electrolyte is self-insulating electronically and is used to dissolve the lithium salt in lithium battery industry. The basic requirement of the electrolyte solvent system is to have a certain polarity (high dielectric constant) to dissolve the lithium salt. Wide electrochemical window (the electrochemical window of the electrolyte is mainly reflected in the electrochemical window of the solvent), resistant to positive oxidation and negative reduction; low viscosity, easy to wet the electrode and improve low temperature performance; heat resistance.
Additional functions of the solvent, such as improving Li-ion solvation properties, synergistic formation, stabilizing solid electrolyte membranes (SEI), assisting in flame retardancy, etc., also depend on the role of solvent additives. Solvent additives include conventional chain/cyclic esters (such as vinylene carbonate VC), fluorinated chain/cyclic/amino esters (such as fluoroethylene carbonate FEC), sulfates (such as vinyl sulfate DTD) , vinyl sulfite ES), sulfones, nitriles, phosphorus-based additives, silicon-based additives, ethers, heterocyclic compounds, etc.
As an electrolyte, solid electrolytes, like electrolytes, should consider performance requirements such as ionic conduction, electronic insulation, and good physical contact with electrodes, as well as the need for large-scale promotion with low comprehensive costs. The lithium conduction mechanism of polymer solid electrolytes is quite different from that of electrolytes.
Lithium ions usually migrate in the amorphous region of polymers, including the ion migration formed by the local movement of lithium ions with the polymer molecular segments, and the ion migration formed by lithium ions within or between polymer chains, usually in the form of complexes or in the form of combination and decomplexation.
Therefore, reducing the glass transition temperature of the polymer, expanding the amorphous region (plasticizing), etc., are the main means to optimize the performance of polymer solid electrolytes. Polymer solid electrolytes generally require doping with lithium salts to obtain lithium ion conductivity. They are all very important in lithium battery industry.
Polymer solid electrolytes include polyethers, monoionic polymers, and the like. Some polyionic liquids-lithium salts also behave in a gel state (called ionic gel). Polymer solid electrolytes have lower density, better physical contact with electrodes, and are generally easier to process.
Pre-lithiation and conductive agent
Pre-lithiation is mainly to deal with the lithium consumption of the SEI film on the negative electrode surface, and the application of negative electrode lithium supplementary agents is the most common negative electrode lithium supplementation method. In lithium battery industry, the first method that can be thought of is to add reduced lithium powder to the negative electrode.
Given that lithium has a capacity of up to 3860mAh/g, a small amount of addition can usually achieve the effect of lithium supplementation. Similar to reducing lithium powder, directly contacting the negative electrode with lithium metal (lithium foil, etc.) can also play a pre-lithiation effect. At this time, the degree of pre-lithiation is relatively difficult to control, and the relevant technical requirements of lithium foil are also high.
Lithium-related alloys (alloys formed from lithium and metals of the fourth main group, etc.) are also used for lithium supplementation: they have a low voltage to lithium, a high capacity, and may be slightly more chemically stable than lithium. For example, lithium-silicon alloys coated with artificial SEI can be stable to dry air (that is, unstable to humid air). Lithium-related alloys can be added as powders or as foils for contact lithium supplementation.
In lithium battery industry, electrode materials, electrolytes and electrode-electrolyte interfaces conduct carriers, and electrodes, current collectors, and external circuits conduct electrons. Efficient transport of lithium ions requires high corresponding bulk phase, interfacial ionic conductance, and sufficient contact between electrolyte and electrode.
The effective transport of electrons requires high corresponding bulk phase and interfacial electronic conductance, so the application of conductive agents in electrode materials to improve interfacial electron transport properties has become an important measure to optimize lithium battery industry performance. Compared with conventional carbon black, carbon nanotubes can significantly improve the electronic conductivity of lithium battery industry materials.
Of course, a certain degree of compounding of the two, especially the compounding of carbon tubes and high-end carbon black, is of positive significance. When carbon tubes with higher aspect ratios dominate, the electronic conductance of the lithium battery industry material is higher.
But this does not mean an infinite increase in the amount and proportion of high aspect ratio carbon tubes. The overall performance and cost of the lithium battery need to be considered. In the foreseeable future, carbon tubes are still the representative of high-end conductive agents, and depending on their cost control, they will gradually penetrate to the middle and low end.