Catlcatl uses its commercial lithium iron phosphate ion battery to explore its reasons for storage capacity loss in an electric space, 60 ° C. Mechanism of battery capacity attenuation from battery and pole level system by physical characterization and electrochemical performance evaluation. I.
Experimental process experiments using CATL production of square phosphate ion battery with 86AH. The battery is a positive electrode material in LifePO4, graphite is a negative electrode material, using a polyethylene separator and a LiPF6 electrolyte. Select 20 batteries close to the same batch and electrical performance to store, test the electrical performance of the battery.
. 100% SOC battery 60 ° C is stored in a press between 2.50 to 3.
65V, a discharge of 0.5C magnification – charging cycle. Then the full rechargeable battery is stored at 60 ° C.
Such repeated, recording the capacity attenuation process of the battery. During each capacity test, the DC internal resistance (DCR) of the battery 5C / 30S is tested..
Take the battery through different storage times and in a fully discharged state, disassembled in an AR gas glove box.. Use field emission scanning electron microscope to observe the polar morphology, use a specific surface analyzer to test the specific surface area.
In the glove box, the electrode piece is sealed with a transparent tape, and the electrode material is analyzed using X-ray diffractometer.. The polar piece after the dissolution of the battery is the working electrode, the lithium sheet is the counter electrode, and is equipped into a CR2032 buckle battery, and the electrochemical properties of the yin and inferior plate.
. Electrochemical impedance spectrum of buckle battery with electrochemical workstation. Analysis of the elemental content of the electrode sheet using an inductive coupling plasma emission spectrometer.
Second, the results discussed 1. Battery performance analysis Figure 1 is battery capacity attenuation and charge and discharge performance. With the extension of storage time, battery capacity gradually decays.
When the storage time reaches 575d, the battery capacity attenuation is 85.8% of the initial capacity..
The battery is charged and discharged at 0.02 C, and the medium battery voltage curve contains lithium ions embedded from the plurality of platforms caused by the graphite, indicating that the 0.02c magnification has been supplied to the graphite structure in the graphite structure during the process of lithium ion.
It is sufficient. , Effectively eliminate the effects of polarization on cycles. Figure 1 Battery capacity attenuation and charge and discharging performance are compared with 0.
5 magnification, the charge and discharge ratio is reduced to 0.02c, which can only increase the capacity retention ratio of storage 181 and 575d batteries to 0.8% and 1.
4%.. Therefore, the battery capacity attenuation caused by long-term high temperature storage is an irreversible capacity attenuation.
. In addition, it is displayed that the amplitude of the battery’s DC internal resistance increases and is not significant, which also shows that the internal polarization of the battery is not an important cause of the calendar storage battery capacity irreversible attenuation..
2. Battery Capacity Attenuation Mechanism Analysis To analyze the source of battery capacity, the battery is charged to 100% SOC or discharged to 100% DOD after 1C magnification..
Analysis of the dismantled pole to examine the effects of high temperature storage on the structure, elemental composition and electrochemical properties of the yin and inferior active material. The immersion analysis of different high temperature storage time battery cathode slides in 100% DOD XRD map. Compared with the XRD standard spectrum of LifePO4 and FEPO4, all diffraction peaks of the polar slide correspond, no miscellaneous phase.
Figure 2 XRD spectrum of the battery cathode of different storage times High temperature memory rear electrode sheet electrochemical properties reduce different storage times at 100% SOC, in which the electrode is used as the working electrode Battery, charge and discharge test with 0.1C magnification. The first discharge ratio of the cathode active substance of different storage time batteries is higher than 155 mAh / g, and the specific capacity of the cathode active substance without the storage battery is close to the storage of the LIFEPO4 structure without obvious damage.
. The constant voltage charge of the buckle battery in Figure 3 (c) is slightly added, but the total amount of charging is still close to the specific capacity of the cathode active substance without the storage battery..
The polarization of the battery cathode after 575D is increased, but the lithium storage capacity of the cathode material is not affected, and the electrolyte decomposition product deposition in the stored procedure may be related. Fig. 3 is a buckle battery in which the charge and discharge curve of a buckle battery is assembled by an indoor electrode of an unsolved battery is from 181 and 575d, respectively, with 335.
6 and 327.1 mAh / g, respectively, respectively. The buckle battery of the stored battery anode is inveracted to be 0.
8% and 3.0%, indicating that the high temperature storage of the lithium graphite is also very small..
For battery safety perspective, the total amount of anode in the whole battery usually exceeds 10% of the total cathode total capacity, so the anode irreversible capacity attenuation caused by high temperature storage does not affect the capacity of the whole battery.. Storage 181 and 575D The anode is the first charge ratio capacity of the unstoppable amount of 90.
4% and 84.5% of the first charge ratio of the anode, respectively, and the capacity retention rate of the actual battery is close..
Therefore, the important reason for battery capacity attenuation is the loss of active lithium ions in all batteries.. In summary, the high temperature storage will not significantly affect the deintercalation of the LIFEPO4 and graphite electrodes.
. 100% DOD high temperature storage battery The cathode of a pell is presence, the cause of the amount of lithium ion capable of receiving the anode is not a significant change in the ability to delatically change the active electrode material, but due to the battery in the battery. The number of ions become less.
The active lithium ion in the battery is consumed by the electrode / electrolyte interface of the electrode / electrolyte interface, and the root cause of the active lithium ion loss helps to deepen awareness of the mechanism of storage capacity loss.. Polar micropatological analysis of the LifePO4 particles in the cathode in the cathode, the particle size is about 200 nm; after 181D storage, the void size between the LIFEPO4 particles is not significantly changed; after 575D storage, the gap between the particles is significantly reduced.
. In the graphite anode, as the storage time has increased, the amount of side reactive product is also changed [Fig. 4 (d), (e), (f)].
The sub-reactive product in the high temperature stored procedure is deposited in the pole, and the morphology of the pole is changed.. In order to characterize the influence of the sub-reaction to the aforementioned active lithium ion loss, the Li content in the yin and male element is further analyzed to study the root cause of the active lithium ion loss.
. Figure 4 Battery pole morphology table 1 is an ICP-OES test result of 100% SOC battery yin anode. The change in Li content in the cathode is not obvious.
The LI content of the anode is also maintained at the same level, so the total amount of the intensity of the yin and elder pole LI in different storage time batteries is substantially unchanged.. Table 1 Different storage time batteries (100% SOC) polar element content Since the 100% SOC battery cathode sheet contains very low, the loss of active lithium ion is important to deposit in the anode.
In the 100% SOC high temperature storage, the anode is in a state in which the potium is in a state in which the potential is very low, and the electrolyte is easily reacted at its surface, and lithium ions are consumed, and lithium-containing side reactive products.. In order to determine the composition of the soluble lithium surface of the anode, the dismantling of the 100% DOD battery is titrated, and the results are shown in Table 2.
Table 2100% DOD battery Anode soluble lithium constituts the anode surface in a carbonate morphology, which is incremented as the storage time is extended (see Table 2), indicating that the battery storage process produces a large number of inorganic lithium salt components.. The inorganic salt is an important product of the solvent reduction reaction, which is caused by a large amount of decomposition of electrolyte during battery storage.
. Electrode Reaction Dynamics Electrochemical Exhaust Spectroscopy (see Figure 5), although the cathode RCT increases with high temperature storage time [Fig. 5 (a)], but the cathode RCT is smaller, the internal resistance of the battery is also small.
. Anode EIS [Fig. 5 (b)] RSEi is not obvious with the storage time, but the RCT is prolonged with the storage time.
. Due to the deposition of the electrolyte sub-reaction product during high temperature storage, the anode ratio surface area decreases with storage time, and the anode specific surface area of 0, 181 and 575d battery is 3.42, 2.
97 and 1.84cm2 / g..
The anode ratio surface area decreases the electrochemical reaction activity that occurs on the surface of the anode, resulting in an increase in charge transfer resistance RCT on the surface of the anode / electrolyte. Fig. 5 is described in the electrochemical impedance spectrum of the buckle battery.
During the high temperature storage process, the lithium state anode is in a low potential state, and the electrolyte reduction reaction consumes active lithium ions, and finally generates an inorganic lithium salt; high temperature added electrolysis Liquid reduction reaction rate, enabling a large amount of lithium ion (Figure 6). Further, the anode side reactive product deposits, the SEI film is thickened, resulting in deterioration of electrode kinetic performance..
Figure 6, the storage capacity attenuation machine is shown. 3. Battery high temperature storage performance Improved due to capacity loss in the battery high temperature storage process important lithium ion loss caused by side reactions from the surface of the anode, Since adding SEI film thermally stabilizing additives ( ASR) can enhance the high temperature stability of the SEI film, reduce the side reactivity of the surface of the anode, reduce the active lithium ion loss.
Figure 7 Different electrolyte battery storage curves and SEI membrane thermostability infrastructure add 1% ASR can effectively improve the high temperature storage life of the battery. After adding 1% ASR, the 575D capacity retention ratio increased from 85.8% to 87.
5% [Figure 7 (a)]. DCR Rolling Rate is significantly lower than that of the base electrolyte, and the content of the anode soluble lithium-containing compound has also decreased (Table 3). DSC analysis is performed on 100% SOC battery anode [Figs.
7 (b)], heat absorption peaks below 100 ° C for residual solvent. Table 3 Before the anode soluble lithium 100% DOD battery, the anode soluble lithium is added, and the anode 90 ° C begins to exotherm, which is decomposed for the anode surface SEI; after adding ASR, the decomposition temperature is increased to 101 ° C..
After adding ASR, the thermal stability of SEI is significantly improved, and the active lithium ion loss can be effectively reduced, and the battery storage life can be improved.. Third, the final conclusion analyzes the electrochemical properties, polar physics and electrochemical properties of commercialized phosphate ion battery high temperature storage, and found that the battery capacity loss in high temperature storage is important from an anode reduction electrolyte in low potential.
, Resulting in active lithium ion loss. The sub-reactive product of the anode reduction electrolyte is deposited in an anode, and the inorganic component in the deposit hinders lithium ion diffusion, so that the anode reaction kinetics decreases..
By adding the SEI membrane thermostability in the electrolyte to effectively improve the thermal stability of the SEI film, reduce the reduction reaction of the electrolyte, reduce the active lithium ion consumption, and improve high temperature storage life.