Lithium batteries are batteries that use lithium metal as the anode. Because lithium metal has lower mass density, higher specific power and higher electrode potential, lithium batteries have higher energy density.

Lithium batteries are divided into primary lithium batteries and secondary lithium batteries. The primary lithium battery uses lithium metal as the anode and MnO₂ as the cathode; the secondary lithium battery uses lithium ion and carbon materials as the anode, and MnO₂ as the cathode. The secondary lithium battery is also called lithium ion battery. discharge capacity. Lithium-ion batteries can be divided into two categories: liquid lithium-ion batteries (LIB) and polymer lithium-ion batteries (LIP). The basic composition comparison is shown in Table 1. The positive and negative materials used in liquid lithium-ion batteries are the same as those used in polymer lithium-ion batteries, and the working principles of the batteries are basically the same. The main difference between them is the difference in electrolytes. Liquid lithium-ion batteries use liquid electrolytes; polymer lithium-ion batteries are replaced by solid polymer electrolytes, which can be solid or colloidal. At present, colloidal polymers are relatively Commonly used. The positive electrode material of lithium ion battery is lithium cobalt oxide, and the negative electrode material is carbon material. The battery realizes the charging and discharging process of the battery through the insertion and removal of lithium ions generated by the positive electrode in the negative electrode carbon material. Figure 1 shows the general structure of a lithium-ion battery.

type	electrolyte	housing/packaging	diaphragm	current collector
Liquid Lithium Ion Batteries	liquid	Aluminum/PP composite film	25μmPE	Copper and aluminum foil
Polymer Lithium Ion Batteries	colloidal polymer	stainless steel, aluminum	No diaphragm or few microns PE	Copper box and aluminum foil

Lithium-ion batteries can be used in photovoltaic systems, which have many advantages and are the more advanced rechargeable batteries. Lithium-ion batteries have high energy density. The weight of lithium-ion batteries is half that of nickel-cadmium or Jin-hydrogen batteries of the same capacity, and the volume is 40% to 50% of Jinbo and 20% to 30% of Jinhydro. The theoretical value of the specific energy of the lithium-ion battery is 570W.h/kg, and its current performance indicators are: the specific energy is 100W/kg, and the specific power is 200W/kg, so there is still great potential in the future. The single-cell voltage of lithium-ion batteries is high, and the working voltage is about 3.7V, which is equivalent to three series-connected nickel-cadmium or nickel-hydrogen batteries. The cycle life of lithium-ion batteries is high, and the current product can reach 1200 times, which increases the service life. Lithium-ion charging time is fast. Using a constant current and constant voltage charger with a rated voltage of 4.2V can fully charge the lithium-ion battery within 1~2h, and there will be no memory effect for charging and discharging. The main disadvantage of such batteries is that they are expensive.

The main components of lithium-ion batteries: battery cover, positive electrode, separator, negative electrode, organic electrolyte, battery shell, etc. The cathode material is generally made of lithium diamond oxide, which has the advantages of high voltage, stable discharge, suitable for large current discharge, high specific energy, and good cyclability. The negative electrode material is usually carbon powder intercalated with lithium. The electrolyte mainly adopts lithium salt, such as LiCiO4, LiAsF6, LiPF. etc., they are generally dissolved in aprotic organic solutions. Lithium-ion batteries use the lithium ions generated by the positive electrode to embed and migrate out of the carbon material of the negative electrode to realize the charging and discharging process of the battery. When the battery is charged, lithium ions are generated on the positive electrode of the battery, and the generated lithium ions move to the negative electrode through the electrolyte. The carbon as the negative electrode has a layered structure with many micropores, and the lithium ions reaching the negative electrode are embedded in the micropores of the carbon layer. The more lithium ions are intercalated, the higher the charging power. In the same way, when the battery is discharged, the lithium ions of the carbon in the carbon layer of the negative electrode come out and move back to the positive electrode. The more lithium ions that return to the positive electrode, the higher the discharge power.

Although lithium-ion batteries rarely have the memory effect of Jin-cadmium batteries, the capacity of lithium-ion batteries will still decrease after multiple charging and discharging. The reasons are complex and diverse. It is mainly due to the change of the structure of the positive and negative materials itself, the hole structure that accommodates lithium ions on the positive and negative electrodes will gradually collapse and block; there is also active passivation of the positive and negative materials, and side reactions will generate other stable compounds; and the positive electrode material gradually peels off . These changes reduce the number of lithium ions in the battery that are free to charge and discharge, resulting in a drop in capacity. Overcharging and discharging can also cause permanent damage to the positive and negative electrodes of lithium-ion batteries, mainly resulting in defects in the lamellar structure, so that some of the lithium ions can no longer be released. Unsuitable temperature will trigger other chemical reactions inside the lithium-ion battery to generate other compounds, so many lithium-ion batteries are provided with protective temperature-controlled diaphragms or electrolyte additives between the positive and negative electrodes. When the temperature of the battery reaches a certain level, the pores of the composite membrane are closed or the electrolyte is denatured, the internal resistance of the battery increases until the circuit is disconnected, and the battery does not heat up any more to ensure that the battery charging temperature is normal.

Among different battery solutions, lithium cobalt and lithium manganese have relatively high electrical energy efficiency, while lithium iron phosphate batteries have the highest stability. Compared with lithium diamond and lithium manganese batteries, lithium iron phosphate batteries are especially suitable for high-power discharge, because It has high stability and is less prone to explosion. In addition, due to the easy availability of iron deposits, it is a potential advantage for lithium-iron batteries to win in the future. Lithium iron phosphate batteries are referred to as LFPs, which do not contain precious metal elements such as cobalt. Moreover, raw materials such as phosphorus, lithium and iron are rich in content and low in price, so there will be no supply problems. The internal structure of the lithium iron phosphate battery is shown in Figure 2. The positive electrode of the lithium iron phosphate battery is connected by aluminum foil, and its active material is LiFePO₄ with a mormonite structure; the middle is a polymer separator, which separates the positive and negative electrodes, so that lithium ions can pass through the separator but electrons cannot pass; the negative electrode is made of graphite. Copper foil connection. When a lithium iron phosphate battery is charged, lithium ions move toward the negative electrode through the separator, and vice versa when charging.

The lithium iron phosphate battery has a single working voltage of 3.2V, a large capacity (currently up to 170A h/kg), fast charging, long life, and good stability in high temperature environments, so it has good application prospects in the future.

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