JP3216835B2 - How to charge a lithium secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for charging a lithium secondary battery, and more particularly, to an improvement in a negative electrode thereof.

[0002]

2. Description of the Related Art LiClO ₄ , LiBF ₄ , and organic solvents such as propylene carbonate, γ-butyrolactone, dimethoxyethane, tetrahydrofuran, and dioxolane are used.
Lithium batteries that combine an electrolyte obtained by dissolving a solute such as LiAsF ₆ , LiPF ₆ , or LiCF ₃ SO ₃ with a negative electrode using lithium metal as an active material have a high energy density, and thus include electronic watches and cameras. Widely used for small electronic devices. Then, development as a secondary battery of this kind has been actively performed.

[0003] One of the issues in the development of this lithium secondary battery is to improve the charge and discharge efficiency of the negative electrode. That is, the amount that can be eluted in the next discharging process is increased to almost 100% of the amount of lithium deposited on the negative electrode in the charging process.
However, at present, only a charge / discharge efficiency of 90 to 95% is obtained. The reason for the low charge / discharge efficiency of the negative electrode is considered to be that needle-like dendrites (dendritic crystals) formed on the negative electrode at the time of charging are released from the electrode plate, or that the dendrites are separated by uneven dissolution at the time of discharging. I have.

Therefore, in order to avoid the formation of needle-like dendrites during such charging, it has been studied to perform charging under relatively mild conditions (Progress in Batterie).
s and Solar Cells, Vol. 2, p. 54, 1979). The content is that even if the potential of the negative electrode during charging is maintained at a potential no higher than -50 mV vs. Li / Li ⁺ , the total amount of lithium deposited on the negative electrode is 5600 layers per unit area of the negative electrode (1 C / cm ² ) Is the limit. In addition, the potential at the time of deposition was -50 mV vs. Li / Li
⁺ It is said that the current density should be reduced in order to keep the potential at a higher value than that of the current source. At present, no technique has been developed for suppressing an overvoltage in a high current density region.

[0005]

As described above, -50 mV v
Keeping the potential nobler than s. Li / Li ⁺ reduces the charging current density, making it difficult to keep the charging time within 8 hours (overnight), which is generally accepted as the charging time. Also, in order to construct a battery that can extract high energy density as an actual product, about 56,000 layers per unit area (about 10 C / c
m ² ) lithium must be deposited on the negative electrode during charging. For this reason, in the conventional technology, it is about
This means that only 1/10 of the energy density can be used. As described above, during charging, the potential of the negative electrode was set to -50 mV vs. L
Although the formation of dendrite is suppressed by maintaining the potential at a value higher than i / Li ^+, there have been problems that the charging time is prolonged and a high energy density cannot be obtained.

According to the present invention, in order to solve the above-mentioned problems of the prior art, even if the current density or the amount of charged electricity per electrode area is increased, the formation of a dendrite on the negative electrode is small and a negative electrode having high negative electrode charging / discharging efficiency is obtained. Accordingly, it is an object to provide a highly reliable lithium secondary battery having a high energy density and a long charge / discharge life.

[0007]

In order to achieve the above object, a method for charging a lithium secondary battery according to the present invention comprises the steps of:
In a non-aqueous electrolyte and a battery using lithium metal as a negative electrode, at the start of charging, the potential of the negative electrode is -35 mV vs. Li / Li ⁺
At least 100 atomic layer of lithium per unit area of the negative electrode is deposited on the negative electrode while maintaining the noble potential.

In the above configuration, it is preferable that the potential of the negative electrode maintained at the start of charging be a potential nobler than -20 mV vs. Li / Li ⁺ .

[0009]

According to the configuration of the present invention described above, the potential of the negative electrode at the start of charging is extremely gentle compared to the conventional deposition potential.
-35 mV vs. Li / Li ⁺
By depositing at least 100 atomic layer equivalent of lithium on the negative electrode per unit area of the negative electrode, the formation of dendrites on the negative electrode is small and the charge / discharge efficiency of the negative electrode is high even if the current density or the amount of charge per electrode area is increased. By obtaining the negative electrode, a highly reliable lithium secondary battery having a high energy density and a long charge / discharge life can be obtained. This is attributable to the fact that various causes of dendrite generation were examined and found to be related to the surface state of the lithium anode. That is, dendrites are likely to be formed in the case where the places where the negative electrode lithium can be deposited by being covered with the passivation film are dispersed in the form of spots, or when there are many edges that cause ubiquitous precipitation due to severe irregularities.

For this reason, the deposition potential on the negative electrode is -35 mV v
When the potential is maintained at s. Li / Li ⁺ , the deposited lithium does not become acicular, but spreads and deposits in a plane along the negative electrode surface so as to cover the edge of the passivation film or unevenness. Then, the entire negative electrode is covered with active lithium at a deposition amount of 100 atomic layers per unit area of the negative electrode.

Once such a totally active negative electrode is obtained, the overvoltage of the negative electrode does not increase remarkably even if the charging current density is increased, and the conventionally preferred -50 mV vs. Li /
It can be kept at a noble potential higher than Li ⁺ . Therefore, lithium ions are uniformly deposited on the entire electrode from the electrolyte, and the formation of acicular dendrites is suppressed. Even if the amount of charged electricity is further increased, dense deposition is performed relatively uniformly. Therefore, the dissolution of deposited lithium by subsequent discharge does not become nonuniform, and most of the lithium efficiently participates in the electrode reaction.

Here, in order to uniformly cover the entire negative electrode, a deposition amount of 100 atomic layers per unit area of the negative electrode is required. However, since the amount of impurities such as moisture contained in the electrolyte is consumed to fill the unevenness of the electrode or to consume the impurities such as water contained in the electrolyte. It is considered that the whole was covered for the first time by the amount of the precipitation.

[0013] The above effect is achieved by setting the potential of the negative electrode to -35 mV v
s. Li / Li ⁺ is obtained by depositing at least 100 atomic layers per unit area while maintaining a noble potential of at least
During the atomic layer analysis, it is desirable to keep the potential at a noble potential of -20 mV vs. Li / Li ⁺ or higher.

[0014]

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. Example 1 A working electrode, a counter electrode, and a reference electrode were produced by press-bonding lithium sheets to nickel expanded metal to which nickel leads were attached. Incorporate these into a glass cell,
LiC to propylene carbonate (PC) as electrolyte
A solution in which 10 ₄ was dissolved at a concentration of 1 M / l was used. All tests were performed under an argon atmosphere.

The working electrode (corresponding to the negative electrode of the battery) is -30 mV vs.
Maintaining a constant potential of Li / Li ⁺ , 200 atomic layers of lithium per unit area were pre-deposited on the working electrode. Then, 0.
After deposition for 1 hour at a current density of 5 mA / cm ² (1.8 C / cm ² , 1
When the potential of the working electrode was measured at -25 mV vs. Li / Li ⁺ . FIG. 1 shows the result of measuring the potential of the working electrode by depositing the same test at each current density for 1 hour.
Are plotted.

As a comparative example, the same test as in the example was carried out except that the working electrode was not preliminarily deposited, and the potential at each current density was measured. As is apparent from FIG. 1, in the embodiment of the present invention, the potential of the working electrode is maintained more nominally than -50 mV vs. Li / Li ⁺ even when the deposition is performed at a current density of 1.0 mA / cm ² . In contrast, in the comparative example, in order to keep it more precious than -50 mV vs. Li / Li ^+, it can be performed only with a current density of about 0.4 mA / cm ^{2 at} the maximum. That is, according to the present invention, deposition at a high current density is possible. Furthermore, the total amount of precipitation per unit area of the working electrode is about 4
Potential is -5 even if it increases to C / cm ² (deposition at about 1 mA / cm ² )
It was confirmed that the potential was higher than 0 mV vs. Li / Li ⁺ , and the potential at which dendrite was hardly formed.

Example 2 An electrolyte was prepared using various solvents in addition to propylene carbonate used for the electrolyte in Example 1, and the same test as in Example 1 was performed to measure the potential of the working electrode. Table 1 shows the deposition current density at -50 mV vs. Li / Li ⁺ for each electrolyte. From Table 1, it is clear that the pre-precipitated one can be deposited at a high current density in any of the electrolytes.

[0018]

[Table 1]

Example 3 Using the glass cell used in Example 1, depositing (corresponding to charging) and dissolving (discharging) lithium on the working electrode (100 mAh / cm ² of lithium) at various constant current densities. Cycle) was repeated. The charge / discharge capacity in each cycle is 5 mAh / cm ² . Here, as an embodiment of the present invention, when performing deposition in each cycle, the potential of the working electrode at the time of charging was set to -3.
Lithium was deposited for 200 atomic layers per unit area while maintaining 0 mV vs. Li / Li ⁺ prior to the deposition at the above constant current density. In addition, a comparative example in which the charge and discharge cycle was performed in the same manner except that the deposition at the preliminary constant potential was not performed was performed.

FIG. 2 is a plot of charge / discharge cycle life at each current density. Here, the cycle life was defined as the end point of the number of cycles when the potential of the working electrode after discharge exceeded 1000 mV vs. Li / Li ⁺ . As is clear from this, it can be seen that the electrode subjected to the preliminary deposition has an excellent cycle life even when the current density is increased. This is shown in FIG.
As can be seen, the potential of the working electrode is -5 even when charged at a high current density.
0 mV vs. Li / Li ⁺ , which makes it possible to form a uniform and dense precipitate with suppressed formation of dendrite.

Example 4 A glass cell similar to that of Example 1 was assembled, and after changing the potential in the preliminary deposition on the working electrode and the amount of deposition per unit area, a charge / discharge cycle was performed at a constant current density. The initial lithium amount at the working electrode is 100 mAh / cm ² , the current density after pre-deposition is 0.4 mA / cm ² , and the charge / discharge capacity in each cycle is 5 mAh / cm ² .

Table 2 summarizes the charge / discharge cycle life obtained in the same manner as in Example 3 using various pre-deposition conditions.

[0023]

[Table 2]

According to Table 2, the potential of preliminary deposition on the working electrode was -35.
It can be seen that the charge / discharge cycle life is good if the potential is more noble than mV vs. Li / Li ⁺ and the deposition amount is 100 atomic layers or more per unit area. In particular, when pre-deposition is performed at a noble potential of -25 mV vs. Li / Li ⁺ or more, the cycle life is significantly increased, and the results of pre-deposition at an extremely low overvoltage of -5 mV vs. Li / Li ⁺ It can be understood that they are comparable.

[0025]

As described above, if the charging method of the present invention is applied to a lithium secondary battery, the potential of the negative electrode can be reduced to -50 mV vs. a potential nobler than Li / Li ⁺ even when charged at a high current density. , And a secondary battery with high charge / discharge efficiency with less formation of dendrite can be obtained. Further, since the amount of lithium deposited per unit area of the negative electrode can be increased, a lithium secondary battery capable of obtaining a large energy density in each charge / discharge cycle even if the electrode plate area is small can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram plotting the potential of a working electrode (negative electrode) with respect to each current density when the charging method according to one example of the present invention and the charging method according to the comparative example are used.

FIG. 2 is a diagram plotting the cycle life of a working electrode (negative electrode) with respect to each current density when the charging method according to one embodiment of the present invention and the comparative example are used.

Continuation of front page (58) Fields investigated (Int.Cl. ⁷ , DB name) H01M 10/44 H01M 10/40 H02J ^7/ 00-7/12

Claims (2)

(57) [Claims] 1. A battery using a positive electrode, a non-aqueous electrolyte, and a lithium metal as a negative electrode, wherein at the start of charging, the negative electrode is kept at a potential no less than −35 mV vs. Li / Li ^+. A method for charging a lithium secondary battery, comprising depositing 100 atomic layers of lithium per unit area of the negative electrode on the negative electrode. 2. The potential of the negative electrode maintained at the start of charging is -20 mV v
2. The method for charging a lithium secondary battery according to claim 1, wherein the potential is higher than s. Li / Li ⁺ .