Discussion and Analysis of Polymer Lithium Ion Battery Technology

1 lithium ion battery structure features

The positive and negative active materials of lithium-ion batteries are embedded compounds. Li+ is removed from the positive electrode during charging and inserted into the negative electrode through the electrolyte. On the contrary, the charging and discharging process of the battery is actually Li+ embedded in the back and forth between the two electrodes. The process of taking out, so this battery is also called "Rocking Chair Battery" (abbreviated as RCB). The reaction diagram and basic reaction formula are as follows:

2. Polymer lithium ion battery technology

2.1 Performance characteristics of polymer lithium ion battery

A polymer lithium ion battery refers to a lithium ion battery in which an electrolyte uses a solid polymer electrolyte (SPE). The battery is formed by compacting a positive electrode current collector, a positive electrode film, a polymer electrolyte membrane, a negative electrode membrane, and a negative electrode current collector, and is sealed with an aluminum-plastic composite film, and the edges thereof are heat-sealed to obtain a polymer lithium ion battery. Since the electrolyte membrane is solid, there is no leakage problem, and the degree of freedom in battery design is large, and it can be connected in series or in parallel or in a bipolar structure as needed.

The polymer lithium ion battery has the following characteristics: 1 shape flexibility; 2 higher mass specific energy (3 times MH-Ni battery); 3 electrochemical stability window width up to 5V; 4 perfect safety and reliability; 5 longer cycle life, less capacity loss; 6 volume utilization rate; 7 a wide range of applications.

Its working performance indicators are as follows: working voltage: 3.8V; specific energy: 130Wh/kg, 246Wh/L; cycle life: >300; self-discharge: <0.1%/month; working temperature: 253-328K; charging speed: 1h 80% capacity; 3h to 100% capacity; environmental factors: non-toxic.

2.2 cathode material

The characteristics and price of a lithium-ion battery are closely related to its positive electrode material. In general, the positive electrode material should satisfy: (1) electrochemical compatibility with the electrolyte solution in the required range of charge and discharge potential; (2) mild Electrode process kinetics; (3) highly reversible; (4) good stability in air in full lithium state, with the development of lithium-ion batteries, research work on high-performance, low-cost cathode materials is constantly underway. At present, the research mainly focuses on lithium transition metal oxides such as lithium cobalt oxide lithium nickel oxide and lithium manganese oxide [1] (see Table 1).

Table 1 Comparison of three main cathode materials for lithium ion batteries

lithium cobalt oxide (LiCoO2) belongs to the α-NaFeO2 type structure and has a two-dimensional layered structure, which is suitable for deintercalation of lithium ions. Because of its simple preparation process, stable performance, high specific capacity and good cycle performance, most of the current commercial lithium-ion batteries use LiCoO2 as the positive electrode material. The synthesis methods mainly include high-temperature solid phase synthesis method and low-temperature solid phase synthesis method, as well as soft chemical methods such as oxalic acid precipitation method, sol-gel method, cold-heat method and organic mixing method.

Lithium nickel oxide (LiNiO2) is a rock salt type structural compound with good high temperature stability. Because of its low self-discharge rate, low requirements on electrolytes, no pollution to the environment, relatively abundant resources and reasonable price, it is a promising cathode material for lithium cobalt oxide. At present, LiNiO2 is mainly synthesized by solid phase 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 ratio of LiNiO2 is easily decomposed into Li1-xNi1+xO2 under high temperature conditions. The excess nickel ions are in the lithium layer between the NiO2 planes, which hinders the diffusion of lithium ions. 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 the traditional positive electrode material. At present, spinel type LixMn2O4 is widely used which has a three-dimensional tunnel structure and is more suitable for deintercalation of lithium ions. Lithium manganese oxide is rich in raw materials, low in cost, non-polluting, overcharge-resistant and heat-safe. It has relatively low requirements for battery safety protection devices and is considered to be the most promising cathode material for lithium-ion batteries. The Mn dissolution, the Jahn-Teller effect, and the decomposition of the electrolyte are considered to be the most important causes of lithium ion battery capacity loss resulting in lithium manganese oxide as a positive electrode material.

2.3 solid polymer electrolyte.

A solid material that conducts current by ions is generally referred to as a solid electrolyte, and includes three types of a crystalline electrolyte, a glass electrolyte, and a polymer electrolyte, wherein the solid polymer electrolyte (SPE) has a light weight, easy film formation, good viscoelasticity, and the like advantages, 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 surface of the electrode, reduce the reactivity of the electrolyte and the electrode, improve the specific energy of the battery, and make the battery resistant. Pressure, impact resistance, low production cost and easy processing.

A conventional solid polymer electrolyte (SPE) consists of a polymer and a lithium salt, which is an electrolyte system in which a lithium salt is dissolved in a polymer. Generally, a polymer having a polar group such as oxygen, nitrogen or sulfur which can coordinate with Li+ in a molecular chain can be used to form such a system, such as polyethylene oxide (PEO), polyoxypropylene, or polyoxyheterocycle, butane, polyethyleneimine, poly(N-propyl-1 aziridine), polysulfide, and the like. Li+ as a hard acid tends to interact with a hard base, so the solubility of a lithium salt in a polymer containing nitrogen and sulfur polar groups is smaller than that of a polymer having an oxygen-containing polar group, and electrical conductivity (σ) Very low and has no practical meaning; the conformation of PEO molecules is more favorable than other polyether molecules to form multiple coordination with cations, can dissolve more lithium salts, and exhibit good electrical conductivity, so PEO+lithium salt system becomes SPE The earliest and most widely studied system.

However, the conventional solid polymer electrolyte (SPE) has a σ room temperature of usually less than 10-4 S·cm-1. To meet the requirements of a lithium ion battery, the addition of a polymer/salt system can promote the dissociation of the lithium salt and increase the free volume of the system. A gelling agent that scores and lowers its glass transition temperature (Tg) gives a gel SPE with a σ room temperature greater than 10-3 S·cm-1. Plasticizers are generally organic solvents having a high dielectric constant, low volatility, miscibility to the polymer/salt complex, and stability to the electrode for example, ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate, N-methylpyrrolidone, sulfolane, γ-butyrolactone and the like. Commonly used lithium salts are LiPF6, LiN (SO2CF3) and the like.

The factors affecting the conductivity of the polymer were discussed by means of XRD, DSC and AC impedance.

(1) Effect of lithium salt concentration on electrical conductivity

When the concentration of the lithium salt is low, the conductivity of the polymer electrolyte is relatively low, only on the order of 10-8. In the process of increasing lithium salt concentration, the conductivity increases with the increase of the concentration of the carrier ions; and when the concentration of the salt continues to increase, the high ion concentration leads to the interaction between the ions. The enhancement causes the mobility of the carrier ions to decrease, resulting in a decrease in conductivity.

(2) Relationship between plasticizer concentration and Tg

As the plasticizer increases, the glass transition temperature of the polymer electrolyte gradually decreases, which accelerates the segmental movement of the polymer electrolyte at room temperature, and thus its electrical conductivity increases. Although the increase in the concentration of the plasticizer greatly increases the electrical conductivity of the polymer electrolyte, it also reduces the self-supporting film forming property and mechanical strength of the polymer electrolyte membrane. If the prepolymer, the plasticizer and the lithium salt are blended, the polymerization reaction is initiated by light or heat, and the gel SPE having a network structure is formed by chemical bonding so that obtained SPE not only has good mechanical properties but also inhibits the polymer. Crystallization increases the amount of plasticizer in the SPE and results in a high σ SPE.

2.4 anode material

The capacity of a lithium-ion battery depends to a large extent on the lithium insertion amount of the negative electrode. The negative electrode material should satisfy the following requirements: (1) the electrode potential change during lithium deintercalation is small and close to metallic lithium; (2) has a higher ratio Capacity; (3) higher charge and discharge efficiency; (4) higher diffusion rate in the interior and surface Li+ of the electrode material; (5) higher structural, chemical and thermal stability; (6) low cost 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.

Generally, the method for preparing the anode material is as follows: 1 heating the soft carbon at a certain high temperature to obtain highly graphitized carbon; 2 decomposing the crosslinked resin having a special structure at a high temperature to obtain a hard carbon; 3 preparing the high temperature thermal decomposition organic substance and high polymer Containing hydrogen carbon.

The difficulty to overcome in the carbon anode material is the problem of capacity cycle attenuation, that is, irreversible capacity loss due to the formation of a solid electrolyte phase interface (SEI). Therefore, the preparation of high purity and regular microstructured carbon anode materials is a development direction.

The mechanism of various metal oxides is similar to that of the positive electrode materials, and the main research direction is to obtain metal oxides of novel structures or composite structures.

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