Phone has now become a daily necessities for everyone, presumably we have experience, with the increase in the use of mobile phones, mobile phone standby time will become shorter and shorter, had a full day to meet the needs of use, and now need every day Two charge Caixing, until the end, two charge one day can not solve the problem, which is what we often say cell phone batteries die, we generally call the 'life of lithium-ion battery decline.' What is the reason for the lithium-ion battery Life expectancy reduction? Before answering this question, we must first familiar with a concept, what is the life of lithium-ion battery? In general, we will lithium-ion battery capacity during the cycle down to 80% of the initial capacity The number of cycles, defined as the life of lithium-ion battery.Mobile phones are often need to replace the electronic products, so the general battery manufacturers will design the life of the phone lithium-ion battery at about 500 times, which means that if we charge the phone once a day , After about a year and a half or so, the capacity of our cell phone battery will drop to about 80% of the initial capacity, which Which is why we feel more and more mobile phone power shortage.Of course, this is not to say that we try not to charge less charge less, but the need for scientific and reasonable charging, at the end of the article, Xiao Bianhui to introduce Some tips on how to charge the phone, I hope everyone can help.
Here we come to understand the following, why lithium-ion batteries in the process of using the decline? First of all, we must be clear that in the lithium-ion battery, in addition to the normal lithium ion intercalation and deintercalation of positive and negative reactions, There will be many side effects, such as SEI film formation and growth, electrolyte decomposition, binder decomposition, positive and negative active material rupture and so on, will result in lithium-ion battery capacity decline.Although the lithium-ion battery decay There are a lot of factors, but can be summed up in three categories: 1) Lithium loss, lithium-ion battery is a closed system, the internal substance is constant, SEI film formation, destruction, negative analysis of lithium will consume only Some Li resources; 2) loss of cathode active material, lithium-ion batteries often occur in the process of particle crushing of the cathode material, binder decomposition and the cathode material crystal structure changes and other factors, which will lead to a part of the cathode active material loss of embedded Lithium and other capabilities; 3) loss of negative active material, such as the negative active material shedding, binder decomposition and other factors, will lead to some negative active material Connected to the conductive particles lose a conductive network, making it impossible to lithium, resulting in a lithium ion battery capacity loss.
Although we made the above assumptions about the possible mechanism of decay of lithium-ion battery and made the relevant models, there is a lack of corresponding detection means and the lack of relevant experimental evidence support.With a square lithium-ion battery as an example, The cell was fully charged at a small C / 25 magnification for dissection and found that one of the cells had a significant imbalance of lithium embedded in the negative plate, as shown in the figure above, but in fact the capacity of the same batch of cells Only a difference of 0.2%, but only a piece of uneven anode battery but showed the highest capacity, so the traditional screening means it is difficult to distinguish the battery with this defect, but the uneven distribution of negative pole pieces of lithium will lead to lithium-ion Long-term battery performance degradation.
Recently, Christoph R. Birkl, University of Oxford, UK, proposed a method for detecting a lithium-ion battery by using the open circuit voltage of the battery.The open circuit voltage of the lithium-ion battery is the potential difference between the positive electrode and the negative electrode, which reflects the thermodynamic characteristics of the lithium- So we can provide us with a wealth of information on the positive and negative.Christoph R. Birkl using button cells on the lithium-ion battery decay caused by the three modes of battery open-circuit voltage was tested and found that by opening the battery Voltage curve detection, you can identify the pattern of cell degradation caused by Christoph R. Birkl can be said that the work for the management of lithium-ion battery system made a breakthrough contribution.
Christoph R. Birkl developed a coin cell using electrodes removed from a commercial square battery and simulated three decay patterns for lithium-ion batteries: 1) Lithium loss; 2) Negative active material loss; 3) Positive active material Loss. In order to ensure the accuracy of the experiment, all experimental cells need to first be stable in the incubator 3h, in order to achieve thermal equilibrium in the test measured two kinds of voltage, one is measured during charging and discharging voltage, which also Times called fake voltage, fake voltage can be used to help determine the decay mode lithium-ion battery, while the real lithium-ion battery can determine the decay mode lithium-ion battery is the open circuit voltage.
First of all, Christoph R. Birkl calculated the theoretical capacity of the coin cell according to the area of the button cell, and then simulated the lithium-lithium ion loss by charging and decharging the positive pole piece. By cutting the negative pole piece, To simulate the loss of negative active material and simulate the loss of positive active material for positive plate cutting, in order to analyze the test results, Christoph R. Birkl used the histogram to establish a physical model, as shown in the figure above, where the left is Negative SoC state, the right side of the positive SoC state, the model for the normal model of the battery can be seen from the figure in normal battery design, the general design of the negative electrode capacity is slightly higher, which is generally called positive Negative redundancy, appropriate redundancy can ensure that the lithium-ion battery cycle performance, but also in the case of overcharge lithium-ion battery can accommodate enough lithium to ensure that lithium is not precipitated to ensure battery safety.
In the previous section, we mainly introduced the possible causes for the decay of Li-ion batteries and Christoph R. Birkl's button-on dummy cells, based on these possible factors, to simulate: 1) lithium loss; 2) loss of positive active material ; 3) Loss of Negative Electrode Materials, a brief introduction to the physical model of lithium-ion batteries established by Christoph R. Birkl. Next, we will show how to use this physical model to open the lithium-ion battery to different factors that lead to the decay of lithium-ion batteries The impact of voltage is analyzed.
First of all, let's review the physical model established by Christoph R. Birkl. As shown in the figure above, the model contains two left and right rectangular columns, the left column shows the SoC state of the negative electrode and the right side shows the positive SoC Status, 0% -100% The two lines represent the lower and upper limits for the use of lithium-ion batteries, respectively.
Note that for the different SoC states of the batteries we refer to, we refer to the theoretical capacity of the reference cell and include the lost lithium, that is, if the positive electrode has lost 30% of Li, And is irreversible, although at this moment the lowest SOC state of the battery becomes 30%, the range of the battery SoC also becomes 30% -100%.
First of all, let's look at the case of 30% lithium loss at the positive side. In the negative 0% SoC state, the positive electrode has reached 30% SoC state. This is reflected in the open circuit voltage. Under the same SoC condition, the open circuit voltage of the battery drops significantly. In the control cell, the open circuit voltage of the 0% SoC cell dropped to 2.7V, but the lithium loss cell reached 2.7V at 30% SoC and also charged the cell to 4.2V. The SoC state of the cell was higher than normal The battery is 2% higher, mainly because the anode potential is higher than the normal cell in the same SoC state, which means that comparing a cell's discharge curve, The decay is faster and the cut-off voltage is reached earlier, with a high probability of lithium loss.
Next, let's look at the coexistence of negative active material loss and lithium loss. This is usually the case because the active material particles of the negative electrode are broken and lose contact with the conductive network or current collector, resulting in a decrease in the amount of active material capable of participating in the reaction , Making the current density increases will also increase the rate of decline of the battery button cell simulation is the case of 30% loss of negative active material and lithium battery discharge curve compared with normal cells from the curve point of view, In the high SoC range, the battery voltage curve almost completely coincides with that of a normal cell, but in the low SoC range, the potential of the negative electrode rapidly rises due to excessive delithiation of the negative electrode, causing the voltage of the battery to drop rapidly, resulting in a faster battery That is to say, comparing the discharge curve of a battery, the voltage curve has no significant change in the previous period compared to the initial state of the battery, but the voltage drops rapidly and is faster in the low SoC range The cut-off voltage is reached, indicating that the cell's decay is highly probable due to part of the negative active and the Into.
Here we use the model analysis, only the negative case of the loss of active material, the coin cell to simulate the loss of 30% of the negative active material, but without loss of Li, which will lead to the negative Li acceptable cathode less than the positive The available lithium (positive 12% more than the negative), reflected in the battery open-circuit voltage is the battery in the low-SoC state, the voltage curve and the normal lithium-ion battery almost no difference, but in the higher SoC, Battery voltage rises rapidly, mainly because the negative transition lithium intercalation caused by the decline in negative potential, if the battery is charged to 4.2V, will lead to precipitation of lithium dendrites on the negative surface, which is the most dangerous situation, may lead to lithium In other words, if the voltage curve of the lithium-ion battery is almost coincident with the initial voltage curve of the battery in the initial stage (low SoC stage), the voltage curve The voltage at the end of the curve increases rapidly and the capacity of the battery decreases. It is highly probable that the negative active material will cause the loss of capacity.
Below we analyze some of the positive active material loss, accompanied by lithium loss, which may be due to some of the cathode active material particles rupture, and the conductive network connection and other reasons, due to the availability of lithium-ion active material to reduce , Which led to the remaining positive active material in the process of lithium deintercalation faster, the positive potential in the same SoC state was significantly higher than the control normal battery, the battery open-circuit voltage is the same SoC state, the cathode active material loss of the battery voltage is significantly higher than the normal battery, and quickly reach the charge cut-off voltage.
Finally, the analysis of the part of the positive active material loss, there is no loss of lithium, the situation is generally de-embedded cathode active material particles broken off, and other factors, this situation for lithium-ion battery capacity of low SOC section The influence is larger because the active material capable of participating in the reaction is reduced, the Li that can participate in the reaction is not reduced, the discharging cut-off voltage is reached earlier in the discharging process of the battery, the reaction is in the voltage curve, the battery is in the high SoC segment, the voltage curve is close to, with the SoC state decreases, the positive electrode active material loss of the battery voltage is significantly lower than the normal battery, and in higher SoC state (about 20%) reached the cut-off voltage.
We use the physics model established by Christoph R. Birkl to analyze the decay mechanism of different types of lithium-ion batteries and familiarize themselves with the different effects of different lithium-ion battery failure modes on the open-circuit voltage of lithium-ion batteries. Next, we will According to the above physical model, a mathematical model is established for the lithium ion battery, and the different failure modes of the lithium ion battery are deduced according to the open circuit voltage of the lithium ion battery.
Since different decay modes have little influence on the open circuit voltage of lithium-ion batteries, we need to accurately fit the open-circuit voltage of the lithium-ion batteries to improve the accuracy of judging the decay mode of lithium-ion batteries.
In general, the capacity of an electrode material can be described by the occupancy x of the available lattice sites in the active material, ranging from 0 to 1. In multiphase composites, the value of x can be determined from the open circuit voltage (Eoc) calculation, the formula shown below
Where N is the number of phases in the material, ΔXi is the contribution of the ith phase, Eo, i is the energy of the lattice in phase i, ai is the energy estimate of the interaction between the embedded ions, and e is The charge of the element, k is Boltzmann's constant, and T is the absolute temperature.
The parameterization process of open-circuit voltage and battery capacity is divided into two steps. The first part uses the false process voltage measured during charging and discharging to fit the open-circuit voltage OCV of the battery. The fitting result is very close to the actual test result. The positive The root mean square error is only 7mV, the root mean square error of the negative is only 12mV, the whole battery voltage RMS error is not 3mV. The second step is to simulate the open-circuit voltage of the electrode, open-circuit voltage fits mainly based on the battery The voltage is equal to the voltage difference between the positive and negative electrodes to obtain the positive and negative electrode voltage.
The decay mode of lithium-ion battery can be analyzed by fitting the open-circuit voltage of decaying lithium-ion battery and comparing the fitted voltage curve with the reference voltage curve.The decay model of lithium-ion battery is designed to estimate three Important parameters: 1) Lithium loss; 2) Loss of cathode active material; 3) Loss of anode active material. The modeling process will not be detailed due to space limitations. Interested readers can refer to our references Let's take a look at it to fit the result.
According to the OCV curve of the open-circuit voltage fitted by the process voltage measured during charging and discharging, the battery loss decay model was used for calculation and analysis. Christoph R. Birkl calculated the lithium loss (LLI), the loss of positive active material (LAMPE) and the negative electrode It is noteworthy that the amount of lithium loss calculated here includes both the lithium consumed for the formation and growth of the SEI film and the lithium element contained in the lost positive and negative active materials Since in actuality , The loss of positive and negative active materials may contain different levels of lithium, it is difficult to distinguish between different factors due to the loss of lithium, lithium loss here includes a variety of factors caused by lithium loss.In order to verify the effectiveness of the model Sex, set up three special batteries, battery information as shown in the following table.
The graph below shows the fitting results. The voltage and voltage curves for the reference cell are shown on the left. The histogram on the right represents three types of cells: 1) lithium loss, 2) loss of positive active material and 3) negative Loss of active material. The red part represents the actual loss, the yellow part represents the result calculated according to the model, and the figure shows that the two are very close. In all three cases, the model can accurately judge lithium Ion battery decay mode.
In order to verify the validity of this model, several different decay modes were validated, first of all with only 25% lithium loss. The fitting results are shown in the following figure. The mean square error of the voltage fit is only 6.7mV, The model analysis is very close to the actual result with only a small error.
Then the 36% lithium insertion loss of negative electrode, the analysis results shown in the following figure, the model accurately determine the battery's main decay mode is embedded lithium negative active material loss, the error is only about 4%, detected positive Loss of active material is due to the positive edge of the cutting in a certain arc, resulting in part of the active material does not participate in the reaction, resulting in a certain error.
Let's look at the combination of two decay rates, which include 25% lithium loss and 13% lithium-containing positive active material loss, so in this case the total lithium loss is 38% The results of the fit to the failure mode are shown. From the figure it can be seen that the model accurately analyzes the total amount of lithium loss, which is only slightly higher than the actual situation and is also the edge of the positive electrode Effect
The model established by Christoph R. Birkl established the OCV curve of the open circuit voltage of the battery and the electrode based on the process voltage collected during charging and discharging by the open-circuit voltage fitting. The open circuit voltage curve and the voltage curve of the reference battery The comparative analysis can accurately determine the mode of decay of lithium-ion battery and determine the proportion of the three modes in the decay of lithium-ion battery, which is supported by the experimental data. For example, if a large amount of negative active material is lost and lithium is not lost, the situation of negative electrode lithium precipitation tends to occur, causing an internal short circuit. The positive electrode Active material and lithium loss at the same time, it is prone to the positive charge and discharge in the process of potential is too high, resulting in reduced stability of the positive active material, which are very dangerous failure mode, the need for timely replacement of these batteries.
