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Why is the electric car short in the winter

- Aug 16, 2018 -

 Let me first say why winter is not enough.

Most of the electric cars on the market today are lithium batteries, so let’s take a look at what happened in the winter.

Have to start with the principle. The main types of lithium batteries used in electric vehicles, lithium iron phosphate, ternary lithium and lithium manganate are the three mainstream lithium batteries, and the negative graphite materials are the main ones. Their basic reflection principle is similar, and they are all "rocking chair" electrochemical energy storage processes.

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As shown in FIG. During the charging process, due to the external voltage of the battery, the electrons in the vicinity of the positive current collector move to the negative electrode under the electric field. After reaching the negative electrode, they combine with the lithium ions in the negative electrode material to form a local electrical neutral storage in the graphite gap. The surface of the negative electrode that consumes a part of lithium ions has a low lithium ion concentration, and a difference in ion concentration is formed between the positive electrode and the negative electrode. Under the concentration drive, the lithium ions in the positive electrode material move from the inside of the material to the surface of the positive electrode, and along the electrolyte, pass through the separator to the surface of the negative electrode; further, under the action of potential, pass through the SEI film and deep into the negative electrode material. The diffusion occurs, and the electrons coming from the external circuit meet, and the local display is electrically neutral and stays inside the negative electrode material. The discharge process is just the opposite. After the circuit containing the load is closed, the discharge process begins with electrons flowing out of the negative current collector and through the external circuit to the positive electrode; finally, lithium ions are embedded in the positive electrode material and combined with the electrons coming from the external circuit.


The negative graphite is a layered structure, and the manner in which lithium ions are intercalated and extracted is not much different in different types of lithium ions. Different cathode materials have different lattice structures, and lithium ions diffuse in and out during charge and discharge, and the process is slightly different.


Resistance and power of electrochemical processes


Resistance, as can be seen from the electrochemical process described in the previous section, during the discharge process, lithium ions want to come from the negative electrode to the positive electrode, and some resistance must be overcome to overcome some resistance. These resistances include: diffusion from the negative electrode structure to overcome the negative SEI film impedance; diffusion along the electrolyte needs to overcome the electrolyte conduction impedance; across the diaphragm between the positive and negative electrodes, it is necessary to overcome the impedance of the diaphragm; from the electrolyte into the positive electrode, the need Overcome the positive SEI film (the structure of this film is not particularly noticeable) and the internal diffusion resistance of the material.


So where does the power of lithium ions overcome these resistances come from? On the one hand, the potential difference between the positive and negative materials, the greater the potential difference between the positive electrode material and the negative electrode material, the higher the open circuit voltage exhibited by the battery, the more energy the battery stores, and this property is also the basic power that the battery can discharge; On the one hand, the difference in ion concentration at different positions in the electrolyte drives the ions to move from the high concentration position to the low concentration position, so-called concentration drive.

Effect of low temperature on electrochemical processes


In this way, as long as we know how low temperature affects these resistances and dynamics, we can understand how the effect of low temperature on the performance of lithium batteries works.


The active material of the positive electrode material, the lower the temperature, the worse the activity, and the lower the potential; the more difficult the diffusion of the positive electrode lithium ion in the internal passage of the material, the increase in impedance;


The SEI film on the surface of the negative electrode is a passivation film formed when the electrolyte contacts the negative electrode material for the first time. Its presence protects the negative electrode material from further corrosion by the electrolyte, and at the same time allows lithium ions to enter and escape. When the temperature is lowered, lithium ions also become difficult to pass through the SEI film, which is manifested by an increase in impedance;


The activity of the electrolyte also deteriorates at low temperatures, and the ability of ions to diffuse in the electrolyte is lowered. The moving rate of charged ions, the macroscopic performance is the magnitude of the current value. Recall the definition of current: the amount of electricity flowing through any section of the conductor per unit time. The relationship between the rate of charge movement and the current is linked, and the low temperature reduces the ability of the electrolyte to pass current. The obstacle to the movement of charge is represented by the loop impedance. As the temperature drops, the electrolyte impedance rises.


On the whole, in the lithium battery system, the charge movement is not smooth, which is manifested by a decrease in potential and an increase in impedance. The potential or the open circuit voltage of the battery has a clear correspondence with the energy contained in the battery at a certain temperature, and the potential drop indicates a decrease in the electric energy in the battery.


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