Comparison of advantages and disadvantages between iron-lithium battery and lithium polymer battery.

The performance of lithium-ion batteries depends mainly on the structure and performance of the internal materials of the batteries used. The internal materials of these batteries include negative electrode materials, electrolytes, diaphragm and positive electrode materials. Among them, the selection and quality of positive and negative electrode materials directly determine the performance and price of lithium ion batteries. Therefore, the research of cheap and high-performance positive and negative materials has always been the focus of the development of lithium ion battery industry. Negative materials generally use carbon materials, and the current development is relatively mature. The development of positive materials has become an important factor that restricts the further improvement of the performance and price of lithium ion batteries. In the current commercialized lithium ion batteries, the cost of the cathode material accounts for about 40 % of the total battery cost. The reduction of the price of the cathode material directly determines the reduction of the price of the lithium ion battery. This is particularly true for lithium-ion power cells. For example, a small lithium-ion battery for a mobile phone requires only about 5 grams of positive material, while a lithium-ion power battery for a bus may require up to 500 kilograms of positive material.

Measuring the quality of lithium ion battery positive electrode materials can be roughly evaluated from the following aspects:(1) Positive electrode materials should have a higher Redox potential, so that the battery has a higher output voltage; (2) Lithium ions can be reversibly embedded and de-embedded in a large number of cathode materials to allow the battery to have a high capacity; (3) In the process of lithium-ion embedding / de-embedding, the structure of the positive electrode material should not change or change slightly as much as possible to ensure good cycle performance of the battery; (4) The change of the positive Redox potential in the embedding / de-embedding process of lithium ions should be as small as possible so that the voltage of the battery does not change significantly to ensure that the battery is charged and discharged smoothly; (5) The positive electrode material should have a high conductivity so that the battery can be charged and discharged with large currents; (6) The positive pole does not react with electrolytes, etc.; (7) Lithium ions should have a large diffusion coefficient in the electrode material to facilitate rapid battery charging and discharge; The price is cheap and free of pollution to the environment.

Lithium-ion battery positive electrode materials are generally lithium oxides. More studies have been conducted on LiCoO2, LiNiO2, LiMn2O4, LiFePO4, and vanadium oxides. Conductive polymer cathode materials have also aroused great interest.

1, LiCoO2

LiCoO2 with layered structure is basically used as a positive material in the current commercial lithium ion battery. Its theoretical capacity is 274mAh/g, and its actual capacity is about 140mAh/g. It is also reported that the actual capacity has reached 155mAh/g. The main advantages of this positive material are: higher operating voltage(average operating voltage is 3.7 V), stable charging and discharging voltage, suitable for large current charging and discharging, higher energy than, better circulation performance, high conductivity, simple production process, easy to prepare and so on. The main disadvantages are: expensive, poor resistance to overcharging, and circulation performance needs to be further improved.

2, LiNiO2

LiNiO2 for lithium ion battery positive electrode materials has a layered structure similar to LiCoO2. Its theoretical capacity is 274mAh/g, and its actual capacity has reached 190mAh/g to 210mAh/g. The operating voltage range is 2.5 to 4.2 V. The main advantages of this positive material are: low self-discharge rate, no pollution, good compatibility with a variety of electrolytes, and cheaper than LiCoO2. However, LiNiO2 has fatal disadvantages: LiNiO2's preparation conditions are very harsh, which brings considerable difficulties to the commercial production of LiNiO2; The thermal stability of LiNiO2 is poor. Compared with LiCoO2 and LiMn2O4 positive electrode materials under the same conditions, LiNiO2 has the lowest thermal decomposition temperature(about 200 °C) and the highest heat release, which brings great safety risks to the battery; LiNiO2 is prone to structural changes in the process of charging and discharging, resulting in poor cycle performance of the battery. These shortcomings make LiNiO2 a considerable way to go as a positive material for lithium ion batteries.

3, LiMn2O4

LiMn2O4 used in lithium ion battery positive electrode materials has a spinel structure. Its theoretical capacity is 148 mAh/g, and its actual capacity is 90 to 120 mAh/g. The operating voltage range is 3 to 4V. The main advantages of this positive material are: rich in manganese resources, cheap, high safety, and relatively easy to prepare. The disadvantage is that the theoretical capacity is not high; The material will slowly dissolve in the electrolyte, that is, its compatibility with the electrolyte is not good; In the process of deep charging and discharging, the material is prone to lattice deformation, resulting in rapid decay of battery capacity, especially when used at higher temperatures. In order to overcome the above shortcomings, a layered structure of trivalent manganese oxide LiMnO2 has been newly developed in recent years. The theoretical capacity of this cathode material is 286 mAh/g, and the actual capacity is about 200 mAh/g. The operating voltage range is 3 to 4.5 V. Although compared with the spinel structure LiMn2O4, LiMnO2 has a large increase in theoretical capacity and actual capacity, there is still a problem of structural instability during charging and discharging. During the process of charging and discharging, the crystal structure changes repeatedly between the layered structure and the spinel structure, causing repeated expansion and contraction of the electrode volume, resulting in deterioration of the battery cycle performance. Moreover, LiMn O2 also has dissolution problems at higher operating temperatures. The solution to these problems is to doped and surface modified LiMnO2. Good progress has been made.

4, LiFePO4

The material has olivine crystal structure and is one of the most popular lithium ion battery cathode materials studied in recent years. Its theoretical capacity is 170mAh/g, and its actual capacity is as high as 110mAh/g without doping modification. By surface modification of LiFePO4, its actual capacity can be as high as 165 mAh/g, which is already very close to the theoretical capacity. The operating voltage range is about 3.4 V. Compared with the positive materials described above, LiFePO4 is highly stable, safer *, more environmentally friendly and inexpensive. The main disadvantages of LiFePO4 are low theoretical capacity and low room temperature conductivity. For the above reasons, LiFePO4 has a very good application prospect in large lithium-ion batteries. However, to demonstrate strong market competitiveness in the entire lithium ion battery field, LiFePO4 faces the following disadvantages:(1) Low-cost competition from LiMn2O4, LiMnO2, and LiNiMO2 positive materials; (2) People may give preference to more suitable specific battery materials in different application fields; (3) LiFePO4 has a low battery capacity; (4) In the high-tech field, people may be more concerned about not the cost but the performance, such as the application of mobile phones and laptops; (5) LiFePO4 urgently needs to improve its conductivity when discharging at a depth of 1C speed to increase its specific capacity. (6) In terms of safety, LiCoO2 represents the safety standards of the current industry, and the safety of LiNiO2 has also been greatly improved. Only LiFePO4 shows higher safety performance, especially in electric vehicles and other applications. In order to ensure its full competitive advantage in security. The following table compares the properties of different lithium-ion battery cathode materials.

A comparison of the battery properties produced by several materials is as follows

Battery component lithium iron phosphate battery lithium cobalt battery lithium manganese battery lithium cobalt nickel battery

C-LiFePO 4LiCoO2LiMn2O4Li(NiCo) O2

Safety and environmental protection require the best safety, and most environmentally friendly requirements very poor stability, very unsafe is acceptable stability very poor, very unsafe

Best acceptable number of cycles

Energy density acceptable, acceptable, optimal.

The most economical and acceptable cost of long-term use

The temperature tolerance is excellent(-40 °C ~ 70 °C can still be used normally) is higher than 55 °C or lower than -20 °C, the decline is higher than 50 °C, and the rapid decline is higher than 55 °C or lower than -20 °C.

Although there are many types of positive electrode materials that can theoretically be used as lithium-ion batteries, the most widely used positive electrode material in commercially-produced lithium-ion batteries is still LiCoO2. Although the layered structure of LiNiO2 has a higher specific capacity than LiCoO2, due to the structural changes and safety problems caused by its thermal decomposition reaction, there is a considerable distance between the direct use of LiNiO2 as a positive material. However, replacing the more secure LiNi1-xCoxO2 with Co may be an important development direction in the future. The spinel structure LiMn2O4 and the layered structure LiMnO2 are considered to be one of the market competitive positive candidates due to their rich raw material resources, obvious price advantages and high safety performance. But the problem of structural instability in the process of charging and discharging will be an important research topic in the future. The actual discharge capacity of LiFePO4 with olivine structure has reached about 95 % of the theoretical capacity, and it has the advantages of low price, high safety, stable structure, and no environmental pollution. It is considered to be an ideal cathode material in large lithium ion batteries.

The P-O bond in the lithium iron phosphate crystal is stable and difficult to decompose. Even at high temperatures or overcharges, it will not collapse and heat like lithium cobalt, or form a strong oxidizing substance, so it has good safety. Some reports pointed out that in the actual operation, a small number of samples were found to burn in acupuncture or short-circuit experiments, but no explosion occurred. In the over-charging experiment, high voltage, who was several times more than his own discharge voltage, was used for charging. There is still an explosion. In spite of this, its overcharge safety has greatly improved compared to ordinary liquid electrolytic lithium cobalt acid batteries.

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