CN1274962A

CN1274962A - Overcharge protection method for secondary lithium ion battery

Info

Publication number: CN1274962A
Application number: CN00117177A
Authority: CN
Inventors: 周震涛; 严燕
Original assignee: South China University of Technology SCUT
Current assignee: Chaozhou Zhenglong Battery Industry Co., Ltd.
Priority date: 2000-06-15
Filing date: 2000-06-15
Publication date: 2000-11-29
Anticipated expiration: 2020-06-15
Also published as: CN1112739C

Abstract

The overcharge protection method is adding redox pair imidazolyl sodium or dimethyl bromobenzene as overcharge protectant into electrolyte of the battery until reaching its saturated concentration. During its normal operation, the battery will not be negatively affected by the refox pair. In case of overcharge, the battery will be protected effectively by the redox pair in a relatively wide current density range.

Description

Overcharge protection method for lithium ion secondary battery

The present invention relates to a secondary battery, and more particularly, to a method of using and maintaining a secondary battery.

Since the lithium ion secondary battery adopts a non-aqueous electrolyte solution system as an electrolyte, the lithium ion secondary battery cannot provide overcharge protection for the battery by utilizing the reversible reaction of decomposition and reduction of an aqueous solution under a certain voltage. It is known that overcharge of a secondary battery can bring about very adverse effects on battery performance, and when a lithium ion secondary battery is overcharged, if protective measures are not taken, irreversible reduction of lithium ions in the lithiumion secondary battery at a battery negative electrode is easily caused, so that the charge-discharge cycle efficiency of the battery is reduced; when the charging voltage reaches the decomposition voltage of the battery electrolyte, the decomposition of the electrolyte at the positive electrode generates gas to increase the internal pressure of the battery and heat accumulation inside the battery to lose stability, thereby seriously affecting the cycle life and safety of the battery and even leading to complete failure of the battery. Therefore, it is important to find suitable and effective overcharge protection for lithium ion batteries.

In the prior art, overcharge protection of lithium ion secondary batteries is mainly achieved by a physical integrated circuit protection method and a chemical redox couple method. The integrated circuit method is reliable, fast and widely applied, but has complex structure, needs plastic outer package and has higher price. The existing redox couple method mainly adopts several types of substances as overcharge protection agents, the first type of substances are lithium halides such as lithium bromide and lithium iodide redox couple, and the working principle is as follows: when the battery is in an overcharged state, the halogen ions are oxidized into simple substances at the positive electrode, and the generated simple substances directly act with the lithium ions on the negative electrode to generate initial substance lithium halide; the second class of materials is metallocenes and their derivatives. Although the two compounds are used as overcharge protective agents of batteries, have low price, good solubility and stability in organic solution, but have low oxidation-reduction potential and are only suitable for some low-voltage lithium secondary battery systems, such as Li/TiS₂And is not suitable for high-voltage lithium ion secondary battery systems.

The ideal redox couple should have the following conditions:

the oxidation or reduction reaction only occurs at the positive electrode or the negative electrode of the battery, other side reactions do not occur, and the electricity pair in the oxidation state and the reduction state has no negative influence on the performance of the battery.

-has a suitable redox potential. The redox potential should be slightly higher than the normal charge cut-off voltage (4.2V) of the lithium ion battery to allow proper overcharge and ensure sufficient charge of the battery during charging, but should be lower than the oxidative decomposition voltage of the lithium ion battery electrolyte (the decomposition voltage of propylene carbonate/ethylene glycol dimethyl ether (PC/DME) is 4.6V, and the decomposition voltage of ethyl carbonate/dimethyl carbonate (EC/DMC) is 5.1V) to prevent oxidative decomposition of the battery electrolyte during overcharge of the battery.

The lithium ion battery has good solubility in an organic electrolyte solution and a high enough diffusion coefficient, and can provide overcharge protection for the battery in a large current range.

Good electrochemical reversibility, providing long-term protection over the lifetime of the cell.

Good stability over the entire temperature range of use of the battery.

The invention aims to provide an overcharge protection method of a lithium ion secondary battery, which selects redox couple imidazole sodium or dimethyl bromobenzene as an overcharge protection agent to realize chemical overcharge protection of the lithium ion secondary battery.

The overcharge protection method of the lithium ion secondary battery comprises the following steps: and (3) adding redox couple imidazole sodium or dimethyl bromobenzene serving as an overcharge protective agent of the battery into the electrolyte of the battery until the saturated concentration of the redox couple is reached.

Redox couples have different solubilities in different electrolytes of lithium ion batteries, such as: 1MLiClO of the two redox pairs of imidazole sodium or dimethyl bromobenzene in the lithium ion battery₄The solubilities in + PC/DME (1: 1) (i.e., prepared from 1 mole of lithium perchlorate dissolved in 1 liter of a mixture of propylene carbonate and ethylene glycol dimethyl ether in a volume ratio of 1: 1) electrolyte were: 0.281 mol/l, 0.245 mol/l; they were in electrolyte 1MLiPF₆The solubilities in + EC/DMC (1: 1) (i.e., prepared from 1 mole of lithium hexafluorophosphate dissolved in 1 liter of a mixture of ethylene carbonate and dimethyl carbonate in a volume ratio of 1: 1) were: 0.119 mol/L and 0.543 mol/L.

The structure of imidazole sodium NaTAZ (1, 2, 4-Triazole, sodium salt) is

Dimethyl bromobenzene BrC₆H₃(CH₃)₂(Dimethylromobenzene) has the structure

The working process or principle of the overcharge protection of the present invention:

the peroxide reduction pair imidazole sodium or dimethyl bromobenzene is directly dissolved in the electrolyte solution of the battery, and the pair does not participate in any chemical or electrochemical process during the normal charging process; and upon overcharge of the battery, i.e., when the charge voltage of the battery exceeds the cut-off voltage for normal charge of the battery, and reaches the reaction potential of the pair of electricity, the pair of electricity starts to react.

It undergoes anoxidation reaction at the positive electrode of the battery:

oxidation product X⁺The reduction reaction occurs by diffusion of the battery electrolyte to the battery cathode:

the reduction product X will again diffuse back through the cell electrolyte to the positive electrode of the cell to regenerate the oxidation reaction, and so on, repeating cyclically. The redox cycling reactions of the couples thus occurring within the cell will lock the charging voltage of the cell below the redox potential of the couple, thereby preventing overcharging of the cell.

Compared with the prior art, the invention has the following advantages:

1. the redox potential of the existing redox couple currently used as a battery overcharge protecting agent is mostly less than 4V, so that it is difficult to meet the requirement of overcharge protection of high-voltage system lithium ion secondary batteries. Two redox couples of dimethyl bromobenzene and imidazole sodium are used in electrolyte 1MLiClO₄The redox potentials in + PC/DME (1: 1) were 4.24V and 4.34V, respectively, in electrolyte 1MLiPF₆The redox potentials in + EC/DMC (1: 1) were 4.29V and 4.31V, respectively, which was sufficient to protect the lithium ion secondary battery from overcharge.

2. The two redox couples have good solubility in the battery electrolyte, such as 1MLiClO₄The diffusion coefficients in + PC/DME (1: 1) are: 8.63X 10^-7cm/s，5.32×10^-7cm/s, can provide overcharge protection for the cell over a wide range of current densities.

3. Both redox couples can provide effective overcharge protection for lithium ion secondary batteries in theevent of overcharge.

4. The addition of the two electricity pairs has no negative influence on the charge and discharge performance of the battery basically.

5. The addition of the two pairs of electricity also has no negative effect on the storage performance of the battery.

FIG. 1 shows Li/PC + DME + LiClO after one week of storage₄/LiCoO₂The discharge curve of the battery was simulated.

FIG. 2 is Li/EC + DMC + LiPF after one week of standing₆/C₆The discharge curve of the battery was simulated.

The present invention will be further described with reference to examples, drawings and the like.

Example 1

Overcharge protection of 18650 type lithium ion secondary battery with a capacity of 1250 mAh.

0.1279 g of redox couple imidazole sodium was added to 5ml of electrolyte 1MLiPF₆+ EC/DMC (1: 1), to reach the saturation concentration of 0.119 mol/L of imidazole sodium in the electrolyte, to make the electrolyte into 18650 type lithium ion secondary battery with imidazole sodium overcharge protection, called battery No. 1.

The same electrode material, electrolyte and diaphragm as those of battery No. 1 were used, but no redox couple was present in the electrolyte of the battery, and battery No. 2 was produced.

Comparing the discharge performance of batteries No. 1 and No. 2 under normal charge and discharge conditions, the addition of the redox couple can not have obvious negative influence on the charge and discharge performance of the battery.

Comparingthe discharge performance of

batteries

1 and 2 under overcharge conditions, it was found that the addition of a redox couple was effective in providing overcharge protection to the batteries.

Example 2

With 0.5ml of 1MLiClO without addition of a redox couple₄The + PC/DME (1: 1) electrolyte was used as a No. 3 positive electrode mock cell. 0.01279 g of imidazole sodium and 0.01568 g of dimethyl bromobenzene are respectively added into electrolyte 1MLiClO of the positive electrode simulation battery₄+ PC/DME (1: 1) (0.5ml) to reach their saturation concentration in the electrolyte of 0.281, respectivelyAnd then, respectively preparing a No. 4 imidazole sodium overcharge protection positive electrode simulation battery and a No. 5 dimethyl bromobenzene overcharge protection positive electrode simulation battery by using the electrolyte and adopting the same material as the No. 3 battery.

Table 1 shows the results of the experiment on the charge and discharge performance of the batteries No. 3, 4 and 5 under overcharge conditions, and table 2 shows the results of the experiment on the charge and discharge performance of the batteries No. 3, 4 and 5 under normal conditions.

Charge and discharge behaviors of batteries Nos. 13, 4 and 5 in overcharge conditions

(charging and discharging voltage interval: 3.5-4.3V, unit: mAh/g)

As can be seen from table 1, the specific discharge capacity and the charge-discharge efficiency of each battery were significantly reduced in the case of overcharge. However, the discharge specific capacity and the charge-discharge efficiency of the batteries 4 and 5 with the redox couple added in the electrolyte are obviously improved compared with the batteries 3 without theredox couple. In the 4 th charge-discharge cycle, the charge-discharge efficiency of the No. 4 and No. 5 batteries is basically greater than 90 percent and is similar to the charge-discharge performance of the No. 3 battery under the normal charge-discharge condition; and the No. 3 battery without electric pair protection not only has the charge-discharge efficiency of 65.5% but also has the discharge specific capacity reduced by nearly 50% in the 4 th charge-discharge cycle. Since comparative experiments can substantially exclude the influence of other factors, the performance of the batteries of No. 4 and 5 under the condition of overcharge is maintained due to the action of redox couples in the batteries, namely, the redox couples in the electrolyte can realize effective overcharge protection for the batteries.

Charge and discharge behaviors of batteries nos. 23, 4 and 5 under normal charge and discharge conditions

(charging and discharging interval: 3.5-4.15V, unit: mAh/g)

From the experimental results in table 2, it can be seen that after the batteries 3, 4 and 5 are subjected to charge-discharge cycles for 4 times, the specific discharge capacity of the simulated batteries 4 and 5 with the pairs of redox electrodes added to the electrolyte is reduced by less than 2%, and is almost within the allowable range of error, so that the addition of two redox pairs has no influence on the charge-discharge performance of the battery positive electrode.

In fig. 1, curve 1 is a discharge curve of a No. 4 positive electrode simulation battery with sodium imidazolium added to electrolyte after being left for a week; curve 2 is the discharge curve of the No. 5 positive electrode simulated battery with the electrolyte added with the dimethyl bromobenzene after being placed for a week; curve 3 is the discharge curve of the No. 3 positive electrode simulated battery without the addition of the couple to the electrolyte after being left for a week. As can be seen from fig. 1, after the redox couple is added, the discharge performance of the battery after a period of storage is not substantially abnormal, i.e., the addition of the two couples does not have a significant negative effect on the storage performance of the battery. Example 3

0.5ml of 1MLiPF without addition of a redox couple₆The + EC/DEC (1: 1) electrolyte was used as a No. 6 negative electrode cell. 0.005415 g of imidazole sodium and 0.03475 g of dimethyl bromobenzene, respectively, were added to 0.5ml of 1M LiPF₆In + EC/DMC (1: 1) electrolyte, in order to reach the saturation concentration of redox couple in the electrolyte of 0.119 mol/L and 0.543 mol/L, make No. 7, No. 8 with redox couple protective negative pole simulation battery separately with above-mentioned electrolyte and material identical to No. 6 battery. Table 3 shows the experimental results of the charge and discharge performance of the No. 6, 7, and 8 negative electrode simulation batteries under normal charge and discharge conditions.

Charging and discharging performance under normal charging and discharging conditions of No. 36, 7 and 8 negative electrode simulation batteries

(unit: mAh/g, charge-discharge interval 0.01-1.5V)

Table 3 compares the charge and discharge performance results of the simulated batteries with and without redox couples for the 6, 7 and 8 anodes in the electrolyte. The experimental result shows that under the condition of normal charge and discharge of the battery, after 4 times of charge and discharge cycles, the difference of the discharge specific capacities of the No. 6, 7 and 8 batteries is less than 1%, and the addition of the redox couple can be considered to have no influence on the charge and discharge performance of the battery cathode within the allowable range of errors.

In fig. 2, curve 1 is a discharge curve of a No. 7 negative electrode simulation battery in which sodium imidazolium is added to an electrolyte after being left for a week; curve 2 is the discharge curve of a No. 8 negative electrode simulation battery with dimethyl bromobenzene added into the electrolyte after the battery is placed for a week; curve 3 is the discharge curve of the No. 6 negative electrode simulated battery without the addition of the couple to the electrolyte after being left for a week. As can be seen from fig. 2, after the redox couple is added, the discharge performance of the battery after a period of storage is not substantially abnormal, i.e., the addition of the two couples does not have a significant negative effect on the storage performance of the battery.

Claims

1. The overcharge protecting method for lithium ion secondary cell features that redox couple sodium imidazole or dimethyl bromobenzene is added into the electrolyte of cell until reaching the saturated concentration of redox couple as overcharge protecting agent.