Lithium-ion batteries are mainly composed of positive electrodes, negative
electrodes, separators, electrolytes, and structural parts. Outside the
lithium-ion battery, electrons from the negative electrode are conducted to the
positive electrode through wires and loads, while inside the battery, the
positive and negative electrodes The two are connected through the electrolyte.
During discharge, Li+ diffuses from the negative electrode to the positive
electrode through the electrolyte and is embedded in the crystal structure of
the positive electrode. Therefore, in lithium-ion batteries, electrolyte is a
very important part and has an important impact on the performance of
lithium-ion batteries. Ideally, there should be sufficient electrolyte between
the positive and negative electrodes, and there should be sufficient Li+
concentration during the charge and discharge process, thereby reducing the
performance degradation caused by concentration polarization of the electrolyte.
However, during the actual charging and discharging process, due to factors such
as the Li+ diffusion speed, a Li+ concentration gradient will occur in the
positive and negative electrodes, and the Li+ concentration fluctuates with
charging and discharging. Due to reasons such as structural design and
production process, the electrolyte will be unevenly distributed inside the
battery core. Especially during the charging process, as the electrode expands,
some dry areas will be formed inside the battery core. The dry area The
existence of SOC leads to the reduction of active materials that can participate
in the charge and discharge reactions, causing local SoC unevenness in the
battery, which leads to accelerated local aging in the battery. M.J. Mühlbauer,
in his study of the impact of aging of lithium-ion batteries on Li distribution,
found that during the charge and discharge process, both the positive and
negative electrodes expand to a certain extent, resulting in a certain degree of
volume expansion and contraction in the battery core. It will repeatedly inhale
and spit out the electrolyte just like breathing, so at different times, the
infiltration of the electrolyte in the battery cell also changes in real time
(as shown in the figure below).
Limited by technical means, in the past we lacked an intuitive
understanding of the behavior of the electrolyte inside the lithium-ion battery
during the charge and discharge process. It was more like studying a black box.
We proposed various theories to speculate on the behavior. In order to study the
behavioral characteristics of electrolytes in lithium-ion batteries more vividly
and intuitively, Toshiro Yamanaka and others from Kyoto University in Japan [2]
used Raman spectroscopy tools to study laminated square lithium-ion batteries.
The biggest feature of this research is Real-time observation of the
distribution of electrolyte and changes in ion concentration in the electrolyte
during the charge and discharge process is achieved.
In the experiment, Toshiro Yamanaka used square laminated batteries as the
research object. The electrolyte used EC and DEC solvents, and LiClO4 as the
electrolyte salt. In order to be able to observe the behavior of the electrolyte
inside the battery core in real time, Toshiro Yamanaka used laminated lithium
ion Eight optical fibers were introduced inside the battery as Raman spectrum
detectors to study the infiltration of the electrolyte in the battery and the
changes in ion concentration. The arrangement of the eight optical fibers in the
battery is shown in Figure c below.
The figure below shows the changing trends of different ion concentrations
detected by the No. 7 optical fiber detector (cell edge) during the charging and
discharging process. From the results, we can see that the concentrations of
EC-Li+ and ClO4- during the charging process It shows an upward trend, while it
shows a downward trend during the discharge process. It shows that with the
charging process, Li+ escapes from the positive electrode and enters the
electrolyte, causing the Li+ concentration in the electrolyte to rise.
The figure below shows the concentration change of the intensity of
EC-Li+/EC detected by 8 optical fibers during the charge and discharge process.
As can be seen from the figure below, the intensity change trend of EC-Li+/EC in
different parts of the battery is also different. The same, for example, at the
No. 7 optical fiber (edge of the battery core), the intensity of EC-Li+/EC
increases with battery charging and decreases with discharge, while the
detection result of No. 2 optical fiber (middle of the battery core) is exactly
the opposite, 5 The intensity changes of EC-Li+/EC at fiber No. 1 and optical
fiber No. 6 (left side of the battery core) are relatively small, while the
intensity of EC-Li+/EC at the position of optical fiber No. 1 (center of the
battery core) changes relatively strongly in the first three times, and then
changes It begins to weaken. The No. 8 optical fiber (outside the battery core)
detector is outside the battery core, so the concentration of Li+ does not
change with charging and discharging. It shows that during the charging and
discharging process of lithium-ion batteries, there are also differences in Li+
concentration changes between different locations inside the cell. Toshiro
Yamanaka believes that this is mainly due to the unevenness of the battery
structure, such as the uneven distance between electrodes and Uneven porosity of
the separator. Due to the expansion and contraction of the electrode volume
caused by the embedding of Li+ into the positive and negative electrode
materials, the pressure exerted by the electrode on the separator will be
uneven, causing the porosity in the separator to change, and the ion migration
impedance of the separator will also change. , causing differences in the
concentrations of EC-Li+/EC in each part.
However, after 30 cycles, the change trend of Li+ concentration from fiber
optic detector No. 1 to fiber optic detector No. 7 became consistent. The
concentration increased during charging and decreased during discharge. Toshiro
Yamanaka believes that this is mainly because after 30 cycles After several
times, the structure and composition of the SEI film of the negative electrode
gradually stabilized, resulting in the ionic impedance between various parts of
the battery core becoming consistent. However, the change trend of each fiber
optic detector is also different. For example, the change in ion concentration
at the No. 3 and No. 4 fiber optic detectors during the charge and discharge
process is relatively small. The change in ion concentration at the No. 2 fiber
optic detector gradually becomes smaller, while The change in ion concentration
at the No. 5 detector position gradually increases. This shows that there is
still a large inhomogeneity inside the lithium-ion battery at this time. For
example, the change in ion concentration at position 4 is relatively small,
indicating that the reaction rate here during the charge and discharge process
is slow, causing uneven local SoC, thus Negatively affects battery
performance.
During the study of changes in Li+ concentration, Toshiro Yamanaka also
found that the distribution of the electrolyte also changed with charging and
discharging. The figure below shows the changing trend of the intensity of the
solvent EC at different locations during the charge and discharge process. The
intensity change of EC mainly reflects the amount of electrolyte there. As the
volume of the battery core changes, the distribution of the electrolyte in the
battery core also changes. are changing. For example, at positions No. 1 and 2,
the concentration of EC experienced two rapid drops and slow recoveries. At the
position of detector No. 6, there was a rapid drop and a rapid rise in EC
concentration. These changes all reflect the internal structure of the
lithium-ion battery. During the charging and discharging process, due to the
volume expansion of the electrode, the electrolyte dries and rewets in parts of
the battery core.
Toshiro Yamanaka's work provides a new idea for studying the behavioral
characteristics of electrolyte during the charging and discharging process of
lithium-ion batteries. It also revealed that in addition to the Li+
concentration gradient in the direction perpendicular to the pole piece, in the
direction parallel to the pole piece, affected by the uneven structure of the
cell, the ion impedance is uneven, so that the Li+ concentration also has a
certain unevenness. At the same time, due to the inconsistency of electrode
volume changes during the charge and discharge process, the electrolyte in the
electrode part also has a process of drying and rewetting.
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