Conductivity of traditional lithium ion batteries

Lithium polymer is better. Compared with lead-acid batteries with the same capacity, the weight and volume of lead-acid batteries are 1/3~ 1/4. Most of them are used in mobile phones, notebook computers, PDA personal digital assistants, video cameras, digital cameras, electric cars and so on.

Structural characteristics of 1 lithium ion battery

The positive and negative active materials of lithium ion batteries are intercalation compounds. When charging, Li+ escapes from the positive electrode and is inserted into the negative electrode through the electrolyte. On the contrary, when discharging, the charging and discharging process of the battery is actually a process in which Li+ is inserted and taken out between two electrodes, so this battery is also called "rocking chair battery" (RCB for short). The schematic reaction diagram and basic reaction formula are as follows:

2. Polymer lithium ion battery technology

2. Performance characteristics of1polymer lithium-ion battery

Polymer lithium-ion battery refers to lithium-ion battery with solid polymer electrolyte (SPE) as electrolyte. The battery is formed by pressing and compounding a positive current collector, a positive film, a polymer electrolyte film, a negative film and a negative current collector, and is wrapped with an aluminum-plastic composite film, and the edge of the aluminum-plastic composite film is hot-melt sealed to obtain a polymer lithium-ion battery. Because the electrolyte membrane is solid, there is no leakage problem, and the battery design has great freedom, and it can be connected in series and parallel or adopt bipolar structure as needed.

Polymer lithium-ion battery has the following characteristics: ① flexible molding; ② Higher specific mass energy (3 times that of MH-Ni battery); ③ The electrochemical stability window is wide, up to 5V; ④ Perfect safety and reliability; ⑤ Longer cycle life and less capacity loss; ⑥ High volume utilization rate; ⑦ Wide application range.

Its performance indicators are as follows: working voltage: 3.8 V; Specific energy: 130Wh/kg, 246wh/l; Cycle life: > 300; Self-discharge: < 0. 1%/ month; Working temperature: 253-328k；; ; Charging speed: 1h reaches 80% capacity; 3 hours to reach 100% capacity; Environmental factors: non-toxic.

2.2 cathode material

The characteristics and price of lithium-ion batteries are closely related to their cathode materials. Generally speaking, the cathode material should meet the following requirements: (1) It has electrochemical compatibility with electrolyte solution within the required charging and discharging potential range; ⑵ Mild electrode process dynamics; (3) High reversibility; ⑷ It has good stability in air in the state of full lithium. With the development of lithium-ion batteries, the research on high-performance and low-cost cathode materials is also going on. 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). Lithium cobaltate (LiCoO2) belongs to α-NaFeO2 structure, which has a two-dimensional layered structure and is suitable for lithium ion deintercalation. Because of its simple preparation process, stable performance, high specific capacity and good cycle performance, most commercial lithium-ion batteries now use LiCoO2 as cathode material. Its synthesis methods mainly include high-temperature solid-state synthesis and low-temperature solid-state synthesis, as well as oxalic acid precipitation method, sol-gel method, cold-heat method, organic mixing method and other soft chemical methods.

Lithium nickel oxide (LiNiO2 _ 2) is a kind of compound with rock salt structure and good high temperature stability. Because of its low self-discharge rate, low requirement for electrolyte, no environmental pollution, relatively rich resources and reasonable price, it is a promising cathode material to replace lithium cobaltate. At present, LiNiO2 _ 2 is mainly synthesized by the solid-state reaction of Ni (NO3) _ 2, Ni (OH) _ 2, Nico _ 3, Ni(OH)2 and LiOH, Lino _ 3 and LiCO3. The synthesis of LiNiO2 is more difficult than that of LiCoO2. The main reason is that stoichiometric LiNiO2 is easily decomposed into Li 1-xNi 1+xO2 at high temperature. Excessive nickel ions in the lithium layer between NiO2 planes hinder the diffusion of lithium ions and affect the electrochemical activity of the materials. At the same time, because Ni3+ is more difficult to obtain than Co3+, the synthesis must be carried out in oxygen atmosphere.

Lithium manganese oxide is a modification of traditional cathode materials. At present, spinel LixMn2O4 is widely used, which 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, good overcharge resistance and thermal safety, and relatively low requirements for battery safety protection devices. It is considered to be the most promising cathode material for lithium ion batteries. The dissolution of manganese, Jahn-Teller effect and the decomposition of electrolyte are considered to be the main reasons for the capacity loss of lithium-ion batteries with lithium-manganese oxides as cathode materials.

2.3 Solid polymer electrolyte

Solid materials that conduct current through ions are usually called solid electrolytes, including crystal electrolytes, glass electrolytes and polymer electrolytes. Solid polymer electrolyte (SPE) has the advantages of light weight, easy film formation and good viscoelasticity, and can be used in batteries, sensors, electrochromic displays and capacitors. The application of SPE in lithium-ion battery can eliminate the leakage of liquid electrolyte, replace the diaphragm in the battery, inhibit the generation of dendrites on the electrode surface, reduce the reactivity between electrolyte and electrode, improve the specific energy of the battery, and make the battery have the advantages of voltage resistance, impact resistance, low production cost and easy processing.

Conventional solid polymer electrolyte (SPE) is composed of polymer and lithium salt, which is an electrolyte system formed by dissolving lithium salt in polymer. Generally, polymers containing polar groups such as oxygen, nitrogen and sulfur that can coordinate with Li+ in the molecular chain can be used to form such a system, such as polyethylene oxide (PEO), polypropylene oxide, polyoxyoxetane, polyethyleneimine, poly (n-propyl-1 aziridine) and polythiophene. As a hard acid, Li+ is easy to interact with hard base, so the solubility of lithium salt in polymers containing nitrogen and sulfur polar groups is less than that in polymers containing oxygen polar groups, and the conductivity (σ) is very low, which is of no practical significance. Compared with other polyether molecules, the conformation of PEO molecule is more conducive to the formation of multiple coordination with cations, which can dissolve more lithium salts and show good conductivity. Therefore, PEO+ lithium salt system has become the earliest and most widely studied system in SPE.

However, the σ room temperature of conventional solid polymer electrolyte (SPE) is usually less than10-4 s cm-1. In order to meet the requirements of lithium ion batteries, by adding a plasticizer which can promote the dissociation of lithium salt, increase the free volume fraction of the system and reduce its glass transition temperature, the σ room temperature can be greater than10-3s cm-1. Plasticizers are usually organic solvents with high dielectric constant, low volatility, miscibility with polymer/salt complexes and stability with electrodes. Such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate, N- methylpyrrolidone, sulfolane, γ -butyrolactone and so on. Commonly used lithium salts are LiPF6, LiN(SO2CF3) and so on.

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

⑴ Influence of lithium salt concentration on conductivity

When the concentration of lithium salt is low, the conductivity of polymer electrolyte is relatively low, only 10-8 order of magnitude. In the process of increasing lithium salt concentration, the conductivity increases with the increase of current-carrying ion concentration. However, when the concentration of salt continues to increase, the high ion concentration leads to the enhancement of the interaction force between ions, which reduces the mobility of current-carrying ions and leads to the decrease of conductivity.

⑵ Relationship between plasticizer concentration and Tg

With the increase of plasticizer, the glass transition temperature of polymer electrolyte gradually decreases, which accelerates the segment movement of polymer electrolyte at room temperature, so its conductivity also increases. Although the increase of plasticizer concentration greatly improves the conductivity of polymer electrolyte, it also reduces the self-supporting film-forming and mechanical strength of polymer electrolyte membrane. If the prepolymer, plasticizer and lithium salt are mixed, the polymerization reaction is initiated by light or heat, and the gel SPE with network structure is formed by chemical bonds, the obtained SPE not only has good mechanical properties, but also inhibits polymer crystallization, improves the content of plasticizer in SPE, and can obtain SPE with high σ.

2.4 cathode materials

The capacity of a lithium-ion battery depends largely on the amount of lithium embedded in the negative electrode, and its negative electrode material should meet the following requirements: (1) The electrode potential changes little during lithium insertion, which is close to metallic lithium; (2) High specific capacity; (3) High charge and discharge efficiency; ⑷ Li+ has a high diffusion rate both inside and on the surface of electrode materials; 5) High structural, chemical and thermal stability; [6] Low price and easy preparation. At present, the research work of anode materials for lithium ion batteries mainly focuses on carbon materials and other metal oxides with special structures.

The general methods for preparing anode materials include: ① heating soft carbon at a certain high temperature to obtain highly graphitized carbon; ② The crosslinked resin with special structure decomposes at high temperature to obtain hard carbon; ③ Preparation of hydrogen-containing carbon by pyrolysis of organic matter and polymer at high temperature.

The difficulty that carbon anode materials have to overcome is the problem of cyclic capacity attenuation, that is, irreversible capacity loss due to the formation of solid electrolyte interface (SEI). Therefore, it is a development direction to prepare carbon anode materials with high purity and regular microstructure.

A true polymer lithium-ion battery refers to a lithium-ion battery with all-solid electrolyte formed only by polymer and salt. Wright was first obtained by directly mixing PEO and lithium salt, but the conductivity of this all-solid electrolyte, especially at room temperature, is difficult to meet the requirements and has not been widely used.

Then it was found that the ionic conductivity of solid electrolyte with plasticizer at room temperature increased by one order of magnitude, which basically met the requirements of use, but there was still a gap of two orders of magnitude compared with liquid electrolyte.

At present, the so-called lithium polymers can be roughly divided into two categories: one is the soft packaging of pure liquid lithium-ion batteries (the manufacturers of such batteries include most polymer battery companies in China), and the other is gel lithium-ion batteries. Gel-type lithium-ion battery is equivalent to colloidal electrolyte with a large amount of plasticizer added, and there is no flowable liquid.

Gel electrolytes can be divided into two categories: one is Bellcore technology of ATL, and the other is "in-situ gel technology" represented by Japanese companies. Here is a brief introduction:

Bellcore technology uses PVDF-HFP*** polymer to form its own film through a certain process, and then winds it. After liquid injection, PVDF absorbs water and expands to form colloidal electrolyte.

In-situ gel technology uses general PP/PE diaphragm, and polymer monomer and initiator are added to electrolyte. After liquid injection, the battery needs to initiate polymer monomer polymerization to form gel at a certain temperature.

Lithium-ion battery uses liquid electrolyte, while polymer lithium-ion battery is replaced by solid polymer electrolyte, which can be "dry" and ... In addition, polymer lithium-ion battery is superior to lithium-ion battery in working voltage, charge-discharge cycle life and so on.