The composition and principle of lithium ion battery

The composition and principle of lithium ion battery

According to different classification methods, there are many types of lithium-ion batteries: ①According to the different electrolytes used in lithium batteries, they can be divided into all solid-state lithium-ion batteries, polymer lithium-ion batteries and liquid lithium-ion batteries; ②According to temperature, they can be classified Divided into high temperature lithium ion battery and normal temperature lithium ion battery; ③According to the shape classification, generally can be divided into cylindrical, square, button and thin plate shape. The cylindrical battery model has five digits: the first two digits are the diameter, and the last three digits are the height. The prismatic battery model has six digits: two digits are used to indicate the thickness, width and height. Lithium-ion secondary battery is a new type of lithium-ion concentration difference battery developed on the basis of lithium metal battery. It is mainly composed of positive electrode, negative electrode, electrolyte, separator, positive and negative current collector, and shell.

The positive electrode active material generally chooses a lithium storage material that has a higher redox potential [>3V (vs.Li﹢/Li)] and can stably exist in the air and can provide a lithium source. Currently, there are mainly layered lithium cobalt oxide, Spinel-type lithium manganate, nickel cobalt lithium manganate ternary materials, lithium-rich materials and new types of polyanion materials, such as phosphate materials, silicate materials, fluorophosphate materials, and fluorosulfate materials. The ideal cathode material of a lithium-ion battery should have the following characteristics.

① There is a large reversible Gibbs free energy in the reaction with lithium ions, which can reduce the energy loss due to polarization and can ensure a high electrochemical capacity; in addition, the discharge reaction should have Larger negative Gibbs free energy changes make the output voltage of the battery high.
②Lithium ions have a large diffusion coefficient, which can reduce the energy loss due to polarization, and can also ensure faster charge and discharge to obtain high power density; in addition, the molecular weight of the intercalation compound should be as small as possible And allows a large amount of reversible intercalation and deintercalation of lithium to obtain a high specific capacity.
③In the process of lithium intercalation/deintercalation, the change of the main structure and its redox potential with the amount of deintercalated lithium should be as small as possible to obtain good cycle performance and a stable output voltage platform.
④The material has good discharge voltage stability, and should have good chemical stability in the entire potential range, and it should not react with the electrolyte, which is conducive to the wide application of lithium-ion batteries.

There are many factors that affect the electrochemical performance of the cathode material. In addition to its own structural factors, there are mainly the following points.

① Crystallinity. The product structure is well developed, that is, the crystallinity is high, which is beneficial to the stability of the structure and the diffusion of Li﹢, and the electrochemical performance of the material is good; otherwise, the electrochemical performance is poor.
②The stoichiometric deviation. In the material preparation process, the difference of condition control is prone to stoichiometric deviation, which affects the electrochemical performance of the material. For example, Li, NiO, electrode materials, due to the large diffusion coefficient of Li﹢ in LiNiO, any dislocations in its layered structure will affect the electrochemical performance of the material. LiNiO is usually synthesized by solid-phase reaction. Due to the different control conditions during the preparation process, it is easy to be non-stoichiometric. When nickel is excessive, LiNiO phase will appear, and the excess nickel will occupy the position that Li﹢ may occupy, thereby affecting Electrochemical properties such as the specific capacity of the material.
③ Particle size and distribution. Lithium-ion battery electrode sheets are thin films with a certain thickness, and the membrane structure is required to be uniform and continuous. The positive electrode of the battery includes the positive electrode active material interface (flat and only molecular layer thickness, except for the original composition material outside the surface without other substances) and the positive electrode active material electrolyte interface (sub-micron interface reactant layer interface ). If the particle size of the material is too large, the specific surface area is small, and the powder has relatively poor adsorptivity. It is difficult for the positive electrode active material to adsorb each other at the interface of the positive electrode active material, and it is difficult to form a uniform and continuous film structure, which is easy to cause electrode sheets. Defects such as cracks appear on the surface, reducing the service life of the battery. In addition, the electrolyte has poor wettability to the cathode material, the interface resistance increases, the diffusion coefficient of Li﹢ in the electrolyte decreases, and the battery capacity decreases. If the particle size of the powder of the active material is too small (nano-level), the specific surface area is too large, the powder is very easy to agglomerate, the active material of the electrode sheet is locally distributed unevenly, and the battery performance is reduced; at the same time, the powder is too fine and it is easy to cause surface defects , Induce battery polarization and reduce the electrochemical performance of the positive electrode. Therefore, the ideal cathode material powder particle size should be controlled at the micron level and the distribution should be narrow to ensure a more ideal specific surface area, thereby improving its electrode activity.
④The uniformity of the structure and composition of the material. If the structure and composition of the material are not uniform, the active material of the electrode sheet will be locally distributed unevenly, and the electrochemical performance of the battery will be reduced.

At present, the successful commercialization of lithium-ion batteries is mainly attributed to the use of lithium intercalation compounds instead of metal lithium negative electrodes. Anode materials are usually selected from materials with low lithium insertion potential and close to metal lithium potential, which can be divided into carbon materials and non-carbon materials. Carbon materials include graphitized carbon (natural graphite, artificial graphite, modified graphite), amorphous carbon, fuller spheres (ene), and carbon nanotubes. Non-carbon materials mainly include transition metal oxides, nitrogen-based, sulfur-based, phosphorus-based, silicon-based, tin-based, titanium-based and other new alloy materials. The ideal negative electrode material is mainly used as the main body of lithium storage. It realizes the insertion and extraction of lithium ions during the charging and discharging process. It is an important part of the lithium ion battery. Its performance directly affects the electrochemical performance of the lithium ion battery. The anode material of ion battery should meet the following requirements:

① The oxidation-reduction potential (relative to metal lithium) during lithium ion insertion is sufficiently low to ensure that the battery has a higher output voltage;
②As much as possible, reversibly deintercalate lithium ions in the positive and negative active materials to ensure a large reversible specific capacity;
③In the process of reversible deintercalation of lithium ions, the matrix structure of the negative electrode active material hardly changes or changes very little, ensuring that the battery has good cycle stability;
④With the continuous insertion of lithium ions, the potential of the negative electrode material should remain unchanged or change very little to ensure that the battery has a stable charging and discharging voltage platform to meet the needs of practical applications;
⑤It has high ionic and electronic conductivity, reduces the influence of the increase of charge and discharge rate on the reversibility of lithium ion insertion and extraction, reduces the degree of polarization, and improves the high rate performance;
⑥The surface structure is stable, forming a protective solid electrolyte membrane in the electrolyte, reducing unnecessary side reactions;
⑦With a large lithium ion diffusion coefficient, it can realize rapid charge and discharge;
⑧ Rich resources, low price, environmentally friendly, etc.

The electrolyte is a mixed solution of an organic solvent and an electrolyte that does not decompose under high voltage. The electrolyte provides a medium for lithium ion transportation, and usually has higher ion conductivity, thermal stability, safety and compatibility, and is generally a fluorine-containing lithium salt organic solution with lower lattice energy. Among them, the electrolyte salt mainly includes LiPF, LiClO, LiBF, LiCFSO, LiAsF, and other lithium salts, and LiPF is generally used as a conductive salt. Organic solvents often use alkyl carbonates such as propylene carbonate (PC), chloroethylene carbonate (CEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and diethyl carbonate (DEC) or these的mixed solvents. Lithium-ion battery separators are generally microporous membranes made of high-molecular polyolefin resins, which mainly isolate the positive and negative electrodes, preventing electrons from passing through the internal circuit of the battery, but allowing ions to pass freely. Since the diaphragm itself is insulated from ions and electrons, adding a diaphragm between the positive and negative electrodes will reduce the ionic conductivity between the electrodes. Therefore, the porosity of the diaphragm should be as high as possible and the thickness should be as thin as possible to reduce the internal resistance of the battery. Therefore, the separator adopts ion-permeable polyolefin microporous membranes, such as polyethylene (PE), polypropylene (PP) or their composite membranes, especially Celgard 2300 (PP/PE/PP three-layer microporous membranes produced by Celgard). Hole diaphragm) not only has a higher melting point, can play a thermal protection role, but also has a higher puncture resistance.

Lithium-ion battery is actually a new type of secondary battery in which Li﹢ is repeatedly inserted and extracted between the anode and cathode electrodes, and is a lithium-ion concentration difference battery. In the charged state, the positive electrode of the battery reacts to produce lithium ions and electrons. The electrons, that is, negative charge, migrate from the positive electrode to the negative electrode of the battery through the external circuit, forming a current flowing from the negative electrode to the positive electrode. At the same time, the lithium ions produced by the positive electrode reaction pass through the electrolyte inside the battery, migrate to the negative electrode area through the separator, and are embedded in the micropores of the negative electrode active material, and combine with the electrons from the external circuit to generate LiC, which forms in the battery. The current flowing from the positive pole to the negative pole and the same size as the external circuit will eventually form a complete closed loop; the discharge process is just the opposite. During charging, the more lithium ions inserted in the negative electrode, the higher the charge capacity; when the battery is discharged, the lithium ions inserted between the active material layers of the negative electrode escape and migrate to the positive electrode, and the more lithium ions return to the positive electrode. The higher the discharge capacity. In the normal charging and discharging process, Li﹢ will not destroy its lattice parameters and chemical structure in the process of insertion and extraction. Therefore, in theory, the process of charging and discharging lithium-ion batteries is a highly reversible chemical reaction and physical conduction process, so lithium-ion batteries are often called rocking-chair batteries. Moreover, there is no deposition and dissolution process of lithium metal during charging and discharging, which avoids the formation of lithium dendrites, and greatly improves the safety and cycle life of the battery. This is also the advantage of lithium ion batteries and replacement of lithium metal secondary batteries. root cause.

When the lithium battery is charged, Li﹢ is extracted from the positive electrode LiFePO, and is inserted into the negative electrode through the electrolyte, so that the positive electrode becomes a lithium-poor state and the negative electrode is in a lithium-rich state. An electron is released at the same time, the positive electrode undergoes an oxidation reaction, and Fe changes from +2 valence to +3 valence. The liberated Li﹢ is embedded in graphite through the diaphragm to form an intercalation compound of LiC, and the negative electrode undergoes a reduction reaction; on the contrary, when Li﹢ is released from graphite and re-embedded in FePO, Fe drops from +3 to +2 At the same time, electrons flow out from the negative electrode and flow to the positive electrode through the external circuit to maintain charge balance.

It can be seen from the above that the core of a lithium-ion battery is mainly positive and negative materials, which directly determines the working voltage and cycle performance of the lithium battery.