JP3544259B2 - Negative electrode for lithium secondary battery - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a negative electrode for a lithium secondary battery, and more particularly to a negative electrode for a lithium secondary battery in which the growth of dendrite giant crystals is suppressed.
[0002]
[Prior art]
JP-A-5-47381 discloses a solid solution in which any one of silver, zinc, and magnesium is dissolved in lithium at a content of 0.01 to 15 wt% (when a large amount is added, a solid solution phase and an intermetallic compound phase are formed). It is described that the cycle life (cumulative number of charge / discharge cycles up to half the initial capacity) of the battery can be extended by using a negative electrode for a lithium secondary battery with an alloy comprising
[0003]
[Problems to be solved by the invention]
In a lithium secondary battery using lithium for the negative electrode, there was a problem that dendrite crystals grew on the negative electrode surface due to repeated dissolution and deposition of lithium during charge and discharge, penetrated the separator, and short-circuited in contact with the anode. .
According to the above publication, one of the above silver, zinc, and magnesium is dissolved in lithium in an amount of 0.01 to 15% by weight to form a solid solution phase or an alloy containing a solid solution phase and an intermetallic compound phase. It can be seen that the extension was achieved, but it was not verified whether this extension of the cycle life was due to the inhibition of the growth of the dendrite crystal, and the true cause of the extension of the cycle life was unknown.
[0004]
Further, it has been found that the addition of silver, zinc, magnesium and the like causes the following problem.
That is, the standard monopolar potential of lithium is -3.02 V, that of silver is +0.8 V, that of zinc is -0.76 V, and their ionization tendency including magnesium is considerably smaller than that of lithium. Therefore, when a lithium alloy containing them is used as a negative electrode, these additional elements are ionized as the discharge proceeds, and are temporarily dissolved in the electrolytic solution. It is deposited back on the surface, and as a result, the density of the additional element on the surface of the negative electrode is significantly increased from the original density. Further, as can be seen from the above description, the density of the additional element on the surface of the negative electrode varies depending on the amount of discharge, and becomes extremely high during charge and discharge.
[0005]
As described above, in the method according to the above publication, since the state of the lithium surface is different each time the charge / discharge cycle is repeated, stable precipitation of dendrite crystals on the negative electrode surface is not achieved, and as a result, it is sufficient to reduce the charge / discharge efficiency. Is concerned.
Also, adding a large amount of the additional element having a large atomic weight naturally lowers the capacity density of the negative electrode.
[0006]
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a negative electrode for a lithium secondary battery that can suppress the precipitation of a large dendrite crystal during charging.
In addition, the present invention has been made in view of the above circumstances, and a lithium secondary battery capable of suppressing excessive concentration of an additional element on the surface of a negative electrode by charging and discharging to suppress precipitation of a large dendrite crystal and improving a battery reaction. It is an object to provide a negative electrode for a battery.
[0007]
[Means for Solving the Problems]
According to the means of the present invention, calcium (Ca) or strontium (Sr), which is an alkaline earth metal having an electronegativity equal to that of lithium , is within a solid solution limit of 0.60 to 2.00 wt% in lithium. Since the solid solution obtained by the solid solution is adopted as a negative electrode for a lithium secondary battery, a negative electrode for a lithium secondary battery that can suppress dendrite giant crystals during charging has been realized. That is, the present inventor has found that precipitation of giant dendrite crystals can be remarkably prevented in a negative electrode for a lithium secondary battery using a solid solution in which Ca or Sr is dissolved in lithium as a negative electrode.
[0008]
The reason for this is that Ca or Sr dispersed very well in lithium due to solid solution or crystal defects due to it form an extremely large number of precipitation nuclei on the negative electrode surface during charging of the battery, whereby lithium is good over the entire negative electrode surface. This is considered to be due to dispersion and precipitation. Therefore, the entire surface of the negative electrode surface becomes a crystal growth surface, and only the thickness of the lithium layer increases due to continuation of charging, so that local formation of dendrite crystals is suppressed. Therefore, it is understood that the use of the negative electrode for a lithium secondary battery according to the present means makes it possible to favorably prevent short-circuiting between the electrodes.
[0009]
The electronegativities of Li, Ca, and Sr determined by Linus Pauling are all 1.0, and the ionization tendency (standard unipolar potential) of Li is -3.02 V and Ca is -2. 81V and Sr are very close to -2.89V, Li and Ca or Sr are adjacent groups in the periodic table, and their physicochemical properties are close to those of Ca or Sr in lithium. It seems to be related to high dispersibility or formation of a large number of precipitation nuclei during battery charging.
[0010]
In particular, the fact that the standard monopolar potentials of these three elements are close means that, for example, Ca or Sr dissolved as ions in the electrolytic solution during discharge or the like does not easily replace lithium on the surface of the negative electrode. This means that excessive deposition of the additional element on the negative electrode surface during non-charging described above is suppressed, and as a result, intermetallic compounds and the like are generated during subsequent charging on the negative electrode surface and the like. And the additive element, ie, Ca or Sr, forms a solid solution well even when charging progresses on the surface of the negative electrode. It can be seen that the growth of GaN is suppressed and a favorable maintenance of the negative electrode reaction is realized.
[0012]
If the addition concentration is less than 0.06 wt%, the precipitation density of the additional element is reduced and the probability of the generation of dendrite giant crystals increases, and if the addition concentration exceeds 2.00 wt%, the alloy containing the intermetallic compound phase may be removed. Presumably, the probability of generation of dendrite giant crystals increases, and the capacity decrease due to the decrease in the amount of lithium increases.
Furthermore, when the additional element increases and lithium and a large amount of intermetallic compound are generated by this additional element, these intermetallic compounds are inferior in dispersibility to lithium as compared with the additional element alone. Occurs. Further, there is a disadvantage in that the capacity density is reduced because the amount of effective lithium involved in charging and discharging is reduced.
[0013]
These problems are solved by this means.
[0014]
【Example】
The following five samples were prepared as negative electrodes. Sample 1 is a conventional product having a Ca addition concentration of 0 wt%, Sample 2 is an Example 1 product having a Ca addition concentration of 0.06 wt%, Sample 3 is an Example 2 product having a Ca addition concentration of 0.60 wt%, Sample 4 is the product of Example 3 with an addition concentration of Ca of 2.00 wt%, and Sample 5 is the product of Example 4 with an addition concentration of Ca of 12 wt%.
[0015]
Each sample was prepared by mixing Ca powder in the above ratio with Li powder, melting and stirring, and then cooling to produce a solid solution ingot. This solid solution ingot was rolled into a disk having a thickness of 0.4 mm and a diameter of 10 mm. It was formed into a working electrode. Li of the same shape was used as a counter electrode, these were faced in an electrolytic solution, and a DC voltage was applied to perform a deposition test. As an electrolytic solution, a solution obtained by dissolving LiClO ₄ at a concentration of 1 mol / l in propylene carbonate was used, and a current was applied at a negative electrode current density of 1.9 mA / cm ² for 80 minutes, and then the surface of each sample was observed.
[0016]
As a result, it was found that dendrite crystals were precipitated on the entire surface in Sample 1, that dendrite crystals were considerably precipitated in Samples 2 and 5, and that in Samples 3 and 4, most of the surfaces became smooth with some dendrite crystals left. Was.
FIGS. 1 to 3 show photographs of the surface states of Samples 1 to 3 after completion of the charging. 1 shows the surface of sample 1, FIG. 2 shows the surface of sample 2, and FIG.
[0017]
Next, the unevenness of the cross section of Samples 1 and 3 after completion of the charging was examined. A laser microscope was used as an inspection device.
As a result, as in the case of the above-mentioned surface observation results, although the sample 1 had large surface irregularities, the sample 3 had almost no surface irregularities. FIG. 4 is a photograph of a screen on which a cross section of Sample 1 is measured and plotted, and FIG. 5 is a photograph of a screen on which a cross section of Sample 3 is measured and plotted.
[0018]
In this example, propylene carbonate (PC) / LiClO ₄ was used as the electrolyte. However, other electrolytes such as PC + DME / 1M LiClO ₄ and PC + DME / 1M LiPF ₆ can be used, and LiMn ₂ O ₄ as the positive electrode. Materials that dope and dedope lithium can be used. Further, the shape of the battery is not limited to the button type, but may be other shapes.
[0019]
(Example 2)
The same test as above was performed using Sr, which has the same electronegativity as lithium and the same alkaline earth as Ca, instead of Ca. The results were almost the same as for Ca.
[Brief description of the drawings]
FIG. 1 is a copy of a photograph showing the particle structure of a charged surface of a negative electrode for a lithium secondary battery made of lithium having a Ca content of 0 wt%.
FIG. 2 is a copy of a photograph showing the particle structure of a charged surface of a negative electrode for a lithium secondary battery made of a lithium solid solution having a Ca content of 0.06 wt%.
FIG. 3 is a copy of a photograph showing the particle structure of the charged surface of a negative electrode for a lithium secondary battery made of a lithium solid solution having a Ca content of 0.60 wt%.
FIG. 4 is a diagram showing an oscilloscope waveform showing unevenness of a cross section of a surface portion of a negative electrode for a lithium secondary battery made of lithium having a Ca content of 0 wt% after charging.
FIG. 5 is a view showing an oscilloscope waveform showing unevenness of a cross section of a surface portion of a negative electrode for a lithium secondary battery made of lithium having a Ca content of 0.60 wt% after charging.

Claims (1)

At least the surface portion is constituted by a solid solution in which calcium or strontium , which is an alkaline earth metal having an electronegativity equal to lithium, is dissolved in lithium at a solid solution limit of 0.60 to 2.00 wt%. Characteristic negative electrode for lithium secondary batteries .

Images