Generally, battery internal resistance is divided into ohmic internal
resistance and polarization internal resistance. Ohmic internal resistance
consists of electrode material, electrolyte, diaphragm resistance and contact
resistance of various parts. Polarization internal resistance refers to the
resistance caused by polarization during electrochemical reactions, including
electrochemical polarization internal resistance and concentration polarization
internal resistance. The ohmic internal resistance of the battery is determined
by the total conductivity of the battery, and the polarization internal
resistance of the battery is determined by the solid-phase diffusion coefficient
of lithium ions in the electrode active material.
1. Ohmic internal resistance
Ohmic internal resistance is mainly divided into three parts, one is ionic
resistance, the other is electronic resistance, and the third is contact
resistance. We hope that the internal resistance of the lithium battery will be
smaller and smaller, so we need to take specific measures to reduce the ohmic
internal resistance for these three items.
1. Ionic impedance
Lithium battery ion impedance refers to the resistance to the transfer of
lithium ions inside the battery. In lithium batteries, lithium ion migration
speed and electron conduction speed play an equally important role. Ion
resistance is mainly affected by positive and negative electrode materials,
separators and electrolytes. To reduce ionic impedance, you need to do the
following:
① Ensure that the positive and negative electrode materials and electrolyte
have good wettability.
It is necessary to select an appropriate compaction density when designing
the pole piece. If the compaction density is too high, the electrolyte will not
be easily infiltrated and the ionic impedance will increase. For the negative
electrode plate, if the SEI film formed on the surface of the active material
during the first charge and discharge is too thick, it will also increase the
ion resistance. At this time, the battery formation process needs to be adjusted
to solve the problem.
②The influence of electrolyte
The electrolyte must have appropriate concentration, viscosity and
conductivity. When the viscosity of the electrolyte is too high, it is not
conducive to the infiltration between it and the positive and negative active
materials. At the same time, the electrolyte also needs a lower concentration,
and too high a concentration is also detrimental to its flow and infiltration.
The conductivity of the electrolyte is the most important factor affecting ion
impedance, which determines the migration of ions.
③The influence of diaphragm on ion impedance
The main influencing factors of the separator on ion impedance include:
electrolyte distribution in the separator, separator area, thickness, pore size,
porosity and tortuosity coefficient, etc. For ceramic diaphragms, it is also
necessary to prevent ceramic particles from blocking the pores of the diaphragm,
which is not conducive to the passage of ions. While ensuring that the
electrolyte fully infiltrates the separator, no excess electrolyte can remain in
it, which reduces the use efficiency of the electrolyte.
2. Electronic impedance
There are many factors affecting electronic impedance, which can be
improved from aspects such as materials and processes.
①Positive and negative plates
The main factors that affect the electronic impedance of the positive and
negative electrode plates include: the contact between the active material and
the current collector, the factors of the active material itself, the plate
parameters, etc. The active material must be in full contact with the current
collector surface, which can be considered from the current collector copper
foil, aluminum foil base material, and the adhesion of the positive and negative
electrode slurries. The porosity of the active material itself, by-products on
the particle surface, uneven mixing with the conductive agent, etc. will all
cause changes in electronic impedance. Plate parameters such as active material
density are too small and the particle gaps are large, which is not conducive to
electron conduction.
②Diaphragm
The main factors that affect the electronic impedance of the separator are:
separator thickness, porosity, and by-products during the charge and discharge
process. The first two are easy to understand. After the battery core is
disassembled, it is often found that the separator is stained with a thick layer
of brown substance, which includes graphite anode and its reaction by-products,
which will cause the separator pores to be clogged and reduce the battery
life.
③Current collector base material
The material, thickness, width of the current collector and its contact
with the tab all affect the electronic impedance. The current collector needs to
choose a base material that is not oxidized and passivated, otherwise the
impedance will be affected. Poor welding between the copper aluminum foil and
the tabs will also affect the electronic impedance.
3. Contact resistance
Contact resistance is formed between the contact between the copper
aluminum foil and the active material, and it is necessary to focus on the
adhesion of the positive and negative electrode slurries.
2. Polarization internal resistance
When current passes through an electrode, the phenomenon that the electrode
potential deviates from the equilibrium electrode potential is called the
polarization of the electrode. Polarization includes ohmic polarization,
electrochemical polarization and concentration polarization, as shown in Figure
1. Polarization resistance refers to the internal resistance caused by the
polarization of the positive and negative electrodes of the battery during
electrochemical reactions. It can reflect the internal consistency of the
battery, but is not suitable for production due to the influence of operations
and methods. The polarization internal resistance is not a constant and changes
with time during the charge and discharge process. This is because the
composition of the active material, the concentration and temperature of the
electrolyte are constantly changing. Ohmic internal resistance obeys Ohm's law.
Polarization internal resistance increases with the increase of current density,
but the relationship is not linear. It often increases linearly with the
logarithmic increase in current density.
Generally speaking, the DC internal resistance of a battery is equal to the
sum of polarization internal resistance and ohmic internal resistance. The
measurement of DC internal resistance is of great significance. There are many
factors that affect polarization internal resistance, such as charge and
discharge rate, ambient temperature, SOC status, electrolyte concentration, etc.
Here is an example of the effect of temperature on the internal resistance of
lithium iron phosphate batteries. If you need relevant literature, you can send
a private message to the editor, as shown in the figure below:
3. Battery internal resistance measurement methods currently used in the
industry
In industrial applications, accurate measurement of battery internal
resistance is carried out through special equipment. Currently, there are two
main battery internal resistance measurement methods used in the industry:
1. DC discharge internal resistance measurement method
According to the physical formula R=U/I, the test equipment forces the
battery to pass through a large constant DC current in a short period of time
(usually 2 to 3 seconds) (currently, a large current of 40A to 80A is generally
used), and the battery is measured at this time. The voltage at both ends, and
the current battery internal resistance is calculated according to the
formula.
This measurement method has high accuracy. If controlled properly, the
measurement accuracy error can be controlled within 0.1%. However, this method
has obvious shortcomings:
(1) Only large-capacity batteries or accumulators can be measured.
Small-capacity batteries cannot load a large current of 40A to 80A within 2 to 3
seconds;
(2) When the battery passes a large current, the electrodes inside the
battery will be polarized, resulting in polarized internal resistance.
Therefore, the measurement time must be very short, otherwise the measured
internal resistance value will have a large error;
(3) Large current flowing through the battery will cause certain damage to
the electrodes inside the battery.
2. AC voltage drop internal resistance measurement method
Because the battery is actually equivalent to an active resistor, we apply
a fixed frequency and fixed current to the battery (currently generally using
1kHz frequency, 50mA small current), then sample its voltage, and undergo a
series of processes such as rectification and filtering. Finally, the internal
resistance value of the battery is calculated through the operational amplifier
circuit. The battery measurement time of the AC voltage drop internal resistance
measurement method is very short, generally around 100 milliseconds.
The accuracy of this measurement method is also good, and the measurement
accuracy error is generally between 1% and 2%.
Advantages and disadvantages of this method:
(1) Almost all batteries, including small-capacity batteries, can be
measured using the AC voltage drop internal resistance measurement method. This
method is generally used to measure the internal resistance of laptop battery
cells.
(2) The measurement accuracy of the AC voltage drop measurement method is
likely to be affected by ripple current, and there is also the possibility of
harmonic current interference. This is a test for the anti-interference ability
of the measuring instrument circuit.
(3) Using this method to measure will not cause much damage to the battery
itself.
(4) The measurement accuracy of the AC voltage drop measurement method is
not as good as the DC discharge internal resistance measurement method.
So what are the low-temperature characteristics of the NCM ternary cathode
material commonly used in lithium batteries? There are many models of
nickel-cobalt-manganese ternary cathode materials, including
NCM111/NCM523/NCM622/NCM721/NCM811, etc. They are all layered structures. The
layered structure not only has unparalleled rate performance of one-dimensional
lithium ion diffusion channels, but also has the structural stability of
three-dimensional channels. The earliest commercial cathode material was layered
lithium cobalt oxide. Since then, it has been gradually doped and modified to
evolve into what it is today. Commonly used NCM ternary materials.
Xie Xiaohua and others used LiCoO2/MCMB as the research object and tested
its low-temperature charge and discharge characteristics. The results show that
as the temperature decreases, its discharge platform drops from 3.762V (0℃) to
3.207V (–30℃); its total battery capacity also drops sharply from 78.98mA·h (0℃)
to 68.55mA·h (–30°C). Similar phenomena are also seen in research reports by
Chen Jitao et al.
Ternary materials are popular for their high capacity and low cost, and
research on their low-temperature properties is also in the ascendant. Smart et
al. studied the electrochemical properties of ternary lithium-rich materials
(Li1+x(Co1/3Ni1/3Mn1/3)1-xO2) in low-temperature electrolytes and found that the
capacities of low-temperature electrolytes with different compositions increased
as the temperature decreased. Attenuation, and the lower the temperature, the
more serious the trend of capacity attenuation. For example, for
1.0mol/LLiPF6/EC:EMC (20:80), the discharge capacity can reach 52% of room
temperature at 0.2C and –40°C, but only 28% of room temperature at –50°C.
Not only the performance of the battery cathode material has a great impact
on the low-temperature performance of lithium batteries, but the intrinsic
impact of the negative electrode material is also very important. At the same
time, it is also important to study the matching low-temperature electrolyte. In
short, in order to ensure the low-temperature performance of lithium-ion
batteries, the following points need to be done:
(1) Form a thin and dense SEI film;
(2) Ensure that Li+ has a large diffusion coefficient in the active
material;
(3) The electrolyte has high ionic conductivity at low temperatures.
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