Analysis of lithium battery

Ideal lithium ion battery, except lithium ion in between the positive and negative electrode embedded and out, do not occur other side reactions, and do not appear lithium ion irreversible consumption. The actual lithium ion battery has side reactions all the time. There are also irreversible consumption, such as electrolyte decomposition, active material dissolution, lithium metal deposition, but the degree varies. The actual battery system, any side reaction that can produce or consume lithium ions or electrons in each cycle,

Can lead to changes in battery capacity balance. Once the capacity balance of the battery is changed, the change is irreversible and can be accumulated through multiple cycles, which has a serious impact on the battery performance.

The dissolution of Mn in spinel LiMn2O4 is the main reason for the reversible capacity attenuation of LiMn2O4. Generally, there are two explanations for the dissolution mechanism of Mn: REDOX mechanism and ion exchange mechanism. The REDOX mechanism refers to the high concentration of Mn3+ at the end of discharge, and the Mn+ on the LiMn2O4 surface will undergo a dismutation reaction: 2Mn3+ (solid) Mn4+ (solid) +Mn2+ (liquid) dismutation reaction produces the divalent manganese ions dissolved in the electrolyte. Ion exchange mechanism refers to the exchange of Li+ and H+ on the surface of spinel to form HMn2O4 with no electrochemical activity.

Xia etc. Research has shown that capacity loss caused by the dissolution of manganese accounted for the proportion of the whole battery capacity loss with the rising of temperature are increased (by 23% of room temperature increases to 55 ℃, 34%).

The phase change of the positive material

[15] there are two types of phase transitions in lithium-ion batteries:

One is the phase transition of the electrode material when the lithium ion is normally released. The other is the phase transition of the electrode material during over - charge or over - discharge.

For the first kind of phase transition, it is generally believed that the normal delinking reaction of lithium ions is always accompanied by the change of the molar volume of the host structure, and at the same time, stress is generated in the material, which leads to the change of the host lattice. These changes reduce the electrochemical contact between particles and between particles and electrodes.

The second type of phase transition is jahn-teller effect. Jahn-teller effect means that the expansion and contraction of the structure are caused by repeated insertion and detachment of lithium ions, and the oxygen octahedron deviates from the spherical symmetry and becomes a deformed octahedron configuration. The irreversible transformation of spinel structure caused by jahn-teller effect is also one of the main reasons for the capacity attenuation of LiMn2O4. In the deep discharge, the average valence of Mn was lower than 3.5v, and the structure of spinel changed from cubic phase to tetragonal phase. The tetrahedral phase is characterized by low symmetry and strong disorder, which reduces the degree of reversibility of lithium ions, and manifests as the attenuation of the reversible capacity of anode materials.

But the electrolyte reduction

[15] the electrolyte commonly used in lithium ion batteries mainly consists of various organic carbonates

(e.g. PC, EC, DMC, DEC, etc.) as well as lithium salts (e.g. LiPF6, LiClO4, LiAsF6, etc.)

Of electrolytes. Under the condition of charging, the electrolyte is unstable to the carbon electrode, so the reduction reaction will occur. Electrolyte reduction consumes electrolytes and solvents, and has a negative impact on battery capacity and cycle life. The resulting gas will increase the internal pressure of the battery and pose a threat to the safety of the system.

Of the amount of loss caused by charging

[15] deposition of anode lithium:

Overcharging occurs when lithium ions deposit on the surface of the negative active material. Lithium ion deposition on the one hand

On the other hand, the deposited lithium metal can easily react with the solvent or salt molecules in the electrolyte.

The formation of Li2CO3, LiF or other substances can block the electrode hole, eventually leading to loss of capacity and loss of life.

Electrolyte oxidation:

The electrolyte commonly used in lithium ion batteries is easy to decompose into insoluble products such as Li2CO3 when overcharged, which blocks the pole hole and produces gas, which will also lead to loss of capacity and potential safety hazards.

Positive oxygen defect: high voltage positive LiMn2O4

There is a tendency to lose oxygen in the air, which leads to oxygen defects and thus to loss of capacity.

5] self-discharge

The capacity loss caused by self-discharge of lithium ion battery is mostly reversible, only a small part is irreversible. Cause is not

The main causes of reversible self-discharge are: loss of lithium ions (formation of insoluble substances such as Li2CO3); Electrolyte oxygen product plug

Plug electrode micro hole, resulting in increased internal resistance

Generally, the no-load voltage of lithium battery is considered to be used up when it is below 3.0v (the specific value depends on the threshold value of the battery protection board, such as as low as 2.8v or 3.2v). Most lithium batteries cannot discharge the no-load voltage below 3.2v, or excessive discharge will damage the battery (generally, lithium batteries in the market are basically used with a protective plate, so excessive discharge will lead to the protection plate can not detect the battery, thus unable to charge the battery).

4.2 V is the highest limit of the battery voltage, it is generally believed to the no-load voltage of lithium-ion batteries charged to 4.2 V is considered the electricity, the battery charging process, the battery voltage rise gradually in 3.7 V to 4.2 V, lithium batteries cannot be no-load voltage to over 4.2 V, otherwise will damage the battery, this is the place where special lithium-ion batteries, generally speaking, 18650 lithium-ion battery has the following advantages.

1. Wide range of use

Notebook computer, walkie-talkie, portable DVD, instruments and meters, audio equipment, model aircraft, toys, cameras, digital cameras and other electronic equipment.

2, series

The 18650 lithium battery pack can be combined in series or in parallel.

3. Low internal resistance

The internal resistance of the polymer batteries is less than liquid batteries, domestic the internal resistance of the polymer batteries can even do below 35 m Ω, greatly reduced the battery power consumption, extend the standby time of mobile phone, can reach with international level. This kind of polymer lithium battery which supports large discharge current is the ideal choice of remote control model and becomes the most promising alternative to ni-mh battery.

The page contains the contents of the machine translation.