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Basic knowledge of lithium battery

January 5, 2021

Basic knowledge of lithium battery

 

A battery that uses materials containing lithium as the electrode. It is a representative of modern high-performance batteries. Today, I will post some basic knowledge of lithium batteries and friends who want to know about leaving a message! The HOPPT battery will be sorted out before sharing it with everyone!

 

INTRODUCTION TO NOUNS

 

The lithium-ion battery is a kind of rechargeable battery, which mainly relies on lithium ions between the positive and negative electrodes to work. During the charging and discharging process, Li+ intercalates and deintercalates back and forth between the two electrodes: when recharging the battery, Li+ deintercalates from the positive electrode and inserts into the negative electrode through the electrolyte, and the negative electrode is in a lithium-rich state; the opposite is true during discharge.

 

Lithium-ion batteries are easily confused with the following two types of batteries:

 

(1) Lithium battery: Lithium element exists.

 

(2) Lithium-ion polymer battery: replace liquid organic solvent with a polymer.

 

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COMPONENT

 

Steel shell/aluminum shell/cylindrical/soft packaging series:

 

(1) Positive electrode-The active material is generally lithium manganese oxide or lithium cobalt oxide. Nowadays, nickel cobalt manganese oxide materials have appeared. Electric bicycles use lithium iron phosphate. The conductive current collector uses electrolytic aluminum foil with a thickness of 10-20 microns.

 

(2) Diaphragm-a special composite membrane allows ions to pass through, but it is an electronic insulator.

 

(3) Negative electrode-the active material is graphite or carbon with a similar graphite structure, and the conductive current collector uses electrolytic copper foil with a thickness of 7-15 microns.

 

(4) Organic electrolyte—carbonic acid ester solvent with lithium hexafluorophosphate dissolved, and gel electrolyte for the polymer.

 

(5) Battery case-divided into steel case (square type is rarely used now), aluminum case, nickel-plated iron case (used for cylindrical batteries), aluminum plastic film (soft packaging), etc., as well as battery caps, which are also battery Positive and negative terminals.

 

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MECHANISM OF ACTION

 

Lithium-ion batteries use carbon materials as the negative electrode and lithium-containing compounds as the positive electrode. There is no metal lithium, only lithium ions, which is a lithium-ion battery. Lithium-ion battery refers to the general term for batteries with lithium-ion intercalation compound as the cathode material. The charging and discharging process of lithium-ion batteries is the process of intercalation and deintercalation of lithium ions. The intercalation and deintercalation of lithium ions are accompanied by the intercalation and deintercalation of electrons equivalent to lithium ions (the positive electrode is usually represented by insertion or deintercalation, and the negative electrode is represented by insertion or deintercalation). In the process of charging and discharging, lithium ions are intercalated/deintercalated and intercalated/deintercalated back and forth between the positive and negative electrodes, which is vividly called "rocking chair battery."

 

WORKING STATUS AND EFFICIENCY

 

Lithium-ion batteries have high energy density and high average output voltage. The self-discharge is small, and the good battery is below 2% per month (recoverable). There is no memory effect. The wide operating temperature range is -20℃~60℃. Excellent cycle performance, fast charging and discharging, charging efficiency up to 100%, and high output power. Long-lasting. It does not contain toxic and hazardous substances and is called a green battery.

 

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RECHARGE

 

Charging is an important step for repeated use of batteries. The charging process of lithium-ion batteries is divided into constant-current fast charging and constant voltage current decreasing stages. In the constant current fast charge stage, the battery voltage gradually rises to the battery's standard voltage and then switches to the constant voltage stage under the control chip. The voltage does not rise to ensure that it will not overcharge, and the current gradually decreases as the battery power rises. To the set value, and finally complete charging. The power statistics chip can sample and calculate the battery power by recording the discharge curve. After repeated use of lithium-ion batteries, the discharge curve will change. Although lithium-ion batteries have no memory effect, improper charging and discharging will seriously affect battery performance.

 

NOTES ON CHARGING

 

Overcharge and discharge of lithium-ion batteries can cause permanent damage to the positive and negative electrodes. The excessive discharge causes the negative carbon sheet structure to collapse, and the collapse will cause the lithium ions to be unable to be inserted during the charging process; overcharging causes too much lithium ions to be embedded in the negative carbon structure, and part of the lithium ions can no longer be released.

 

The charging capacity is equal to the charging current multiplied by the charging time. When the charging control voltage is constant, the larger the charging current (, the faster the charging speed), the smaller the charging power. Excessive battery charging speed and improper termination voltage control point will also cause insufficient battery capacity. In fact, part of the electrode active material of the battery stops charging without sufficient reaction. This insufficient charging phenomenon increases with the increase in the number of cycles.

 

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DISCHARGE

 

For the first charge and discharge, if the time can be longer (generally 3 to 4 hours is enough), then the electrode can reach the highest oxidation state as much as possible (full charge). The discharge (or use) will be forced to the specified Voltage, or until the automatic shutdown, to activate the battery capacity.

 

However, in the normal use of lithium-ion batteries, this operation is not required, and it can be charged as needed at any time. It is not necessary to fully charge the battery or discharge it first. Operations like the first charge and discharge only need to be performed one to two consecutive times every three to four months.

 

CHEMICAL ANALYSIS

 

Overview: Like all chemical batteries, lithium-ion batteries are composed of a positive electrode, negative electrode, and electrolyte. The electrode materials are all lithium ions inserted (inserted)/deintercalated (deintercalated).

 

Cathode: Cathode material. As mentioned above, there are many available cathode materials, and most mainstream products currently use lithium iron phosphate.

 

 

Positive electrode reaction: Lithium ions are intercalated during discharge, and lithium ions are deintercalated during charging. When charging: LiFePO4→Li1-xFePO4+xLi+xe When discharging: Li1-xFePO4+xLi+xe→LiFePO4

 

Anode: Anode material, mostly graphite. New research has found that titanate may be a better material.

 

Negative reaction: Lithium ions are deintercalated during discharge, and lithium ions are inserted during charging. When charging: xLi+xe+6C→LixC6 When discharging: LixC6→xLi+xe+6C

 

ELECTROLYTE LIQUID

 

Solute: Lithium salts are often used, such as lithium perchlorate (LiClO4), lithium hexafluorophosphate (LiPF6), and lithium tetrafluoroborate (LiBF4). Solvent: Since the working voltage of the battery is much higher than the decomposition voltage of water, organic solvents are often used in lithium-ion batteries, such as ether, ethylene carbonate, propylene carbonate, and diethyl carbonate. Organic solvents often destroy graphite structure during charging, causing it to peel off and form a solid electrolyte membrane (solid electrolyte interphase, SEI) on its surface to cause electrode passivation. Organic solvents also bring safety problems such as flammability and explosion.

 

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THE MAIN ADVANTAGE

 

 

(1) High voltage: The single battery's working voltage is as high as 3.7-3.8V (3.2V for lithium iron phosphate), which is 3 times that of Ni-Cd and Ni-H batteries.

 

(2) Large specific energy: The actual specific energy that can be achieved at present is about 555Wh/kg; that is, the material can reach a specific capacity above 150mAh/g (3 to 4 times Ni-Cd, 2--3 times Ni-MH ), which is close to about 88% of its theoretical value.

 

(3) Long cycle life: generally, it can reach more than 500 times, even more than 1000 times, and lithium iron phosphate can reach more than 2000 times. For electrical appliances with low current discharge, the battery life will double the competitiveness of electrical appliances.

 

(4) Good safety performance: no pollution and no memory effect. As the predecessor of Li-ion, the lithium battery is short-circuited due to the easy formation of dendrites from metal lithium, which reduces its application fields: Li-ion does not contain cadmium, lead, mercury, and other elements that pollute the environment: some processes (such as sintered A major drawback of Ni-Cd batteries is the "memory effect," which severely restricts the use of batteries, but Li-ion does not have this problem at all.

 

(5) Low self-discharge: The self-discharge rate of fully-charged Li-ion at room temperature is about 2% after 1 month, much lower than 25-30% of Ni-Cd and 30-35% of Ni and MH.

 

(6) Fast charge and discharge: 1C charge for 30 minutes, the capacity can reach more than 50% of the nominal capacity, and the discharge can support high current 3C/5C high rate discharge.

 

(7) The working temperature range is high; the working temperature is -25~45°C. With the improvement of electrolyte and the positive electrode, it is expected to expand to -40~70°C.