402030 polymer battery

　　Lithium cobalt oxide (LiCoO2) belongs to the α-NaFeO2 type structure and has a two-dimensional layered structure, which is suitable for the deintercalation of lithium ions. Due to its relatively simple preparation process, stable performance, high specific capacity, and good cycle performance, most commercial lithium-ion batteries currently use LiCoO2 as the cathode material. The synthesis methods mainly include high-temperature solid-phase synthesis and low-temperature solid-phase synthesis, as well as soft chemical methods such as oxalic acid precipitation, sol-gel method, cold and heat method, and organic mixing method. Lithium nickel oxide (LiNiO2) is a rock salt structure compound with good high temperature stability. Due to its low self-discharge rate, low requirement for electrolyte, no environmental pollution, relatively abundant resources and affordable price, it is a promising positive electrode material to replace lithium cobalt oxide. At present, LiNiO2 is mainly synthesized by solid state reaction of Ni(NO3)2, Ni(OH)2, NiCO3, NiOOH and LiOH, LiNO3 and LiCO3. The synthesis of LiNiO2 is more difficult than LiCoO2. The main reason is that the stoichiometric LiNiO2 is easily decomposed into Li1-xNi1+xO2 under high temperature conditions, and the excess nickel ions are in the lithium layer between the NiO2 planes, hindering the diffusion of lithium ions. It will affect the electrochemical activity of the material, and because Ni3+ is more difficult to obtain than Co3+, the synthesis must be carried out in an oxygen atmosphere [2]. Lithium manganese oxide is a modification of traditional positive electrode materials. At present, spinel LixMn2O4 is more widely used. It has a three-dimensional tunnel structure and is more suitable for lithium ion deintercalation. Lithium manganese oxide has rich raw materials, low cost, no pollution, better overcharge resistance and thermal safety, and relatively low requirements for battery safety protection devices. It is considered to be the most promising lithium-ion battery cathode material. Mn dissolution, Jahn-Teller effect and electrolyte decomposition are considered to be the most important reasons for the capacity loss of lithium-ion batteries with lithium manganese oxide as the cathode material. 2.3 Solid polymer electrolyte Solid materials that conduct current with ions are usually called solid electrolytes, which include three types of crystal electrolytes, glass electrolytes and polymer electrolytes, among which solid polymer electrolytes (SpE) are light in weight and easy to form films , Good viscoelasticity, etc., can be used in batteries, sensors, electrochromic displays and capacitors. The use of SpE in lithium-ion batteries can eliminate the problem of liquid electrolyte leakage, replace the separator in the battery, inhibit the generation of dendrites on the electrode surface, reduce the reactivity of the electrolyte and the electrode, increase the specific energy of the battery, and make the battery durable. It has the advantages of high pressure, impact resistance, low production cost and easy processing. Conventional solid polymer electrolytes (SpE) are composed of polymers and lithium salts, which are electrolyte systems formed by dissolving lithium salts in polymers. Generally, polymers containing polar groups such as oxygen, nitrogen, and sulfur on the molecular chain that can coordinate with Li+ can be used to form such systems, such as: polyethylene oxide (pEO), polypropylene oxide, polyoxyheterocycle Butane, polyethyleneimine, poly(N-propyl-1 aziridine), polyalkylene sulfide, etc. As a hard acid, Li+ tends to interact with hard bases, so the solubility of lithium salts in polymers containing nitrogen and sulfur polar groups is smaller than that in polymers containing oxygen polar groups, and the conductivity (σ) It is very low and has no practical significance; the conformation of pEO molecules is more conducive to forming multiple coordination with cations than other polyether molecules, which can dissolve more lithium salts and show good electrical conductivity, so the pEO+lithium salt system becomes SpE The earliest and most extensively studied system. However, the σ room temperature of conventional solid polymer electrolytes (SpE) is usually less than 10-4S cm-1. In order to meet the requirements of lithium-ion batteries, adding lithium salts to the polymer/salt system can promote the dissociation of lithium salts and increase the free volume of the system. Fraction and reduce its glass transition temperature (Tg) of the plasticizer, can get σ room temperature greater than 10-3S·cm-1 gel SpE. Plasticizers are typically organic solvents with a high dielectric constant, low volatility, miscibility with the polymer/salt complex, and stability with respect to the electrode. Such as ethylene carbonate (EC), propylene carbonate (pC), dimethyl carbonate, N-methylpyrrolidone, sulfolane, γ-butyrolactone, etc. Commonly used lithium salts are LipF6, LiN (SO2CF3) and so on. Using XRD, DSC and AC impedance test methods, the factors affecting the conductivity of polymers were discussed preliminarily. (1) Effect of lithium salt concentration on conductivity When the concentration of lithium salt is low, the conductivity of the polymer electrolyte is relatively low, only on the order of 10-8. In the process of gradually increasing the concentration of lithium salt, due to the increase of the concentration of carrier ions, the conductivity also increases; and when the concentration of salt continues to increase, the high ion concentration leads to the interaction force between ions Enhanced, the mobility of the carrier ions decreases, resulting in a decrease in conductivity. (2) The relationship between plasticizer concentration and Tg With the increase of plasticizer, the glass transition temperature of the polymer electrolyte gradually decreases, which accelerates the chain segment movement of the polymer electrolyte at room temperature, so its conductivity also increases with increase. Although the increase of the plasticizer concentration greatly improves the conductivity of the polymer electrolyte, it also reduces the self-supporting film formation and mechanical strength of the polymer electrolyte membrane. If the prepolymer, plasticizer and lithium salt are blended, the polymerization reaction is initiated by light or heat, and a gel SpE with a network structure is formed through chemical bonds. The obtained SpE not only has good mechanical properties, but also inhibits the polymer Crystallization increases the content of plasticizer in SpE, and high σ SpE can be obtained. 2.4 Anode material The capacity of a lithium-ion battery depends largely on the amount of lithium intercalation in the anode, and the anode material should meet the following requirements: (1) The electrode potential changes little during the lithium intercalation process and is close to metallic lithium; High specific capacity; (3) high charge and discharge efficiency; (4) Li+ has a high diffusion rate inside and on the surface of the electrode material; (5) high structural, chemical and thermal stability; (6) low price and easy preparation. At present, research work on anode materials for lithium-ion batteries mainly focuses on carbon materials and other metal oxides with special structures.