How to improve the low temperature performance of lithium iron phosphate battery?

Compared with other cathode materials, LiFePO4 electrode materials have many advantages, such as higher theoretical specific capacity, stable working voltage, stable structure, good cyclability, low cost of raw materials and environmental friendliness. Therefore, this material is an ideal positive electrode material and is selected as one of the main positive electrode materials for power batteries.

Many researchers have studied the mechanism of the accelerated performance degradation of LIBs at low temperature, and it is believed that the deposition of active lithium and its catalytically grown solid-state electrolyte interface (SEI) lead to the decrease of ionic conductivity and the decrease of electron mobility in the electrolyte. drop, which leads to a reduction in the capacity and power of LIBs and sometimes even battery performance failures. The low temperature working environment of LIBs mainly occurs in winter and high latitude and high altitude areas, where the low temperature environment will affect the performance and life of LIBs, and even cause extremely serious safety problems.

Affected by the low temperature, the rate of lithium intercalation in graphite is reduced, and metal lithium is easily precipitated on the surface of the negative electrode to form lithium dendrites, which pierce the diaphragm and cause an internal short circuit in the battery. Therefore, methods to improve the low-temperature performance of LIBs are of great significance for promoting the use of electric vehicles in alpine regions. This paper summarizes the methods to improve the low temperature performance of LiFePO4 batteries from the following four aspects:

1) Pulse current generates heat;

2) Use electrolyte additives to prepare high-quality SEI films;

3) Interface conductivity of surface coating modified LiFePO4 material;

4) Bulk conductivity of ion-doped modified LiFePO4 material.

1. Rapid heating of low temperature batteries by pulse current

During the charging process of LIBs, the movement and polarization of ions in the electrolyte will promote the generation of heat inside the LIBs. This heat generation mechanism can be effectively used to improve the performance of LIBs at low temperatures. Pulse current refers to a current whose direction does not change and whose current intensity or voltage changes periodically with time. To rapidly and safely raise the battery temperature at low temperatures, De Jongh et al. used a circuit model to theoretically simulate how a pulsed current heats up LIBs, and verified the simulation results through experimental testing of commercial LIBs. The difference in heat generation between continuous charging and pulsed charging is shown in Figure 1. As can be seen from Figure 1, the microsecond pulse time can promote more heat generation in the lithium battery.

Figure 1 Heat generated by pulsed and continuous charging modes

Zhao et al. studied the excitation effect of pulse current on LiFePO4/MCNB batteries. The study found that after the pulse current excitation, the surface temperature of the battery increased from -10 °C to 3 °C, and compared with the traditional charging mode, the entire charging time was reduced by 36min ( 23.4%), the capacity increased by 7.1% at the same discharge rate, therefore, this charging mode is favorable for fast charging of low-temperature LiFePO4 batteries.

Zhu et al. studied the effect of pulse current heating on the low-temperature battery life (health state) of LiFePO4 power lithium-ion batteries. They studied the effects of pulse current frequency, current intensity and voltage range on battery temperature, as shown in Figure 2. The results showed that higher current intensity, lower frequency and wider voltage range enhanced the heat accumulation and temperature rise of LIBs. Furthermore, after 240 heating cycles (each cycle equal to 1800 s of pulsed heating at -20°C), they evaluated the state of health (SOH) of LIBs after pulsed current heating by studying the battery capacity retention and electrochemical impedance, and by SEM and EDS studied the changes of the surface morphology of the negative electrode of the battery. The results showed that the pulse current heating does not increase the deposition of lithium ions on the negative electrode surface, so the pulse heating will not exacerbate the risk of capacity decay and lithium dendrite growth caused by lithium deposition.

Fig.2 Variation of battery temperature with time when the lithium battery is charged by pulse current with frequency of 30Hz(a) and 1Hz(b) with different current intensity and voltage range

2. Electrolyte modification of SEI membrane to reduce the charge transfer resistance at the electrolyte-electrode interface

The low temperature performance of lithium ion batteries is closely related to the ion mobility in the battery, and the SEI film on the surface of the electrode material is the key link affecting the lithium ion mobility. Liao et al. studied the effect of carbonate-based electrolyte (1 mol/L LiPF6/EC+DMC+DEC+EMC, with a volume ratio of 1:1:1:3) on the low-temperature performance of LiFePO4 commercial lithium batteries. When the operating temperature is lower than -20 °C, the electrochemical performance of the battery decreases significantly. Electrochemical impedance spectroscopy (EIS) tests show that the increase in charge transfer resistance and the decrease in lithium ion diffusion capacity are the main factors for the degradation of battery performance. Therefore, it is expected to improve the low-temperature performance of LiFePO4 batteries by changing the electrolyte to enhance the reactivity of the electrolyte-electrode interface.

Fig. 3 (a) EIS of LiFePO4 electrode at different temperatures;

(b) Equivalent circuit model fitted by LiFePO4 EIS

In order to find an electrolyte system that can effectively improve the low-temperature electrochemical performance of LiFePO4 batteries, Zhang et al. tried adding LiBF4-LiBOB mixed salts to the electrolyte to improve the low-temperature cycling performance of LiFePO4 batteries. Notably, the optimized performance was achieved only when the molar fraction of LiBOB in the mixed salt was less than 10%. Zhou et al. dissolved LiPF4(C2O4)(LiFOP) into propylene carbonate (PC) as an electrolyte for LiFePO4/C batteries and compared it with the commonly used LiPF6-EC electrolyte system. It was found that the first cycle discharge capacity of LIBs decreased significantly when the battery was cycled at low temperature; meanwhile, the EIS data indicated that the LiFOP/PC electrolyte improved the low-temperature cycling performance of LIBs by reducing the internal impedance of LIBs.

Li et al. studied the electrochemical performance of two lithium difluoro(oxalate)borate (LiODFB) electrolyte systems: LiODFB-DMS and LiODFB-SL/DMS, and compared the electrochemical performance with the commonly used LiPF6-EC/DMC electrolyte, and found that LiODFB-SL/DMS and LiODFB-SL/DES electrolytes can improve the cycling stability and rate capability of LiFePO4 batteries at low temperature. EIS study found that LiODFB electrolyte is conducive to the formation of SEI film with lower interfacial impedance, which promotes the diffusion of ions and the movement of charges, thereby improving the low-temperature cycling performance of LiFePO4 batteries. Therefore, a suitable electrolyte composition is beneficial to reduce the charge transfer resistance and increase the diffusion rate of lithium ions at the electrode material interface, thereby effectively improving the low temperature performance of LIBs.

Electrolyte additives are also one of the effective ways to control the composition and structure of SEI films, thereby improving the performance of LIBs. Liao et al. studied the effect of FEC on the discharge capacity and rate performance of LiFePO4 batteries at low temperature. The study found that after adding 2% FEC to the electrolyte, LiFePO4 batteries showed higher discharge capacity and rate performance at low temperature. SEM and XPS showed the formation of SEI, and EIS results showed that the addition of FEC to the electrolyte can effectively reduce the impedance of LiFePO4 batteries at low temperature, so the improvement of battery performance is attributed to the increase of ionic conductivity of SEI film and the polarization of LiFePO4 electrode. reduce. Wu et al. used XPS to analyze the SEI film and further studied the related mechanism. They found that when FEC participated in the interface film formation, the decomposition of LiPF6 and carbonate solvent was weakened, and the content of LixPOyFz and carbonate substances produced by solvent decomposition decreased. Thereby, the SEI film with low resistance and dense structure is formed on the surface of LiFePO4. As shown in Fig. 4, after adding FEC, the CV curves of LiFePO4 show that the oxidation/reduction peaks are close together, indicating that the addition of FEC can reduce the polarization of the LiFePO4 electrode. Therefore, the modified SEI promotes the migration of lithium ions at the electrode/electrolyte interface, thereby enhancing the electrochemical performance of LiFePO4 electrodes.

Fig.4 Cyclic voltammograms of LiFePO4 cells in electrolytes containing 0% and 10% FEC at -20°C

In addition, Liao et al. also found that the addition of butyl sultone (BS) to the electrolyte has a similar effect, that is, to form an SEI film with a thinner structure and lower impedance, and improve the migration rate of lithium ions when passing through the SEI film. Therefore, , the addition of BS significantly improves the capacity and rate performance of LiFePO4 batteries at low temperature.

3. Surface coating conductive layer to reduce the surface resistance of LiFePO4 material

One of the important reasons for the degradation of lithium battery performance in low temperature environment is the increase of impedance at the electrode interface and the decrease of ion diffusion rate. LiFePO4 surface coating conductive layer can effectively reduce the contact resistance between electrode materials, thereby improving the diffusion rate of ions in and out of LiFePO4 at low temperature. As shown in Fig. 5, Wu et al. used two carbonaceous materials (amorphous carbon and carbon nanotubes) to coat LiFePO4 (LFP@C/CNT), and the modified LFP@C/CNT had excellent low-temperature performance. The capacity retention rate is about 71.4% when discharged at -25°C. EIS analysis found that this improvement in performance is mainly due to the reduced impedance of the LiFePO4 electrode material.

Fig.5 HRTEM image (a), structural diagram (b) and SEM image of LFP@C/CNT nanocomposite

Among many coating materials, metal or metal oxide nanoparticles have attracted the attention of many researchers due to their excellent electrical conductivity and simple preparation method. Yao et al. studied the effect of CeO2 coating on the performance of LiFePO4/C battery. In the experiment, CeO2 particles were uniformly distributed on the surface of LiFePO4. The kinetics are significantly improved, which is attributed to the improved contact between the electrode material and the current collector as well as the particles, as well as the increased charge transfer in the LiFePO4-electrolyte interface, which reduces the electrode polarization.

Similarly, Jin et al. took advantage of the good electrical conductivity of V2O3 to coat the surface of LiFePO4, and tested the electrochemical properties of the coated samples. The study of lithium ions shows that the V2O3 layer with good conductivity can significantly promote the lithium ion transport in the LiFePO4 electrode, and thus the V2O3 modified LiFePO4/C battery exhibits excellent electrochemical performance in low temperature environment, as shown in Figure 6. Show.

Fig.6 Cycling performance of LiFePO4 coated with different contents of V2O3 at low temperature

Lin et al. coated Sn nanoparticles on the surface of LiFePO4 material by a simple electrodeposition (ED) process, and systematically studied the effect of Sn coating on the electrochemical performance of LiFePO4/C cells. SEM and EIS analysis show that Sn coating improves the contact between LiFePO4 particles, and the material has lower charge transfer resistance and higher lithium diffusion rate at low temperature, therefore, Sn coating improves LiFePO4/C battery at low temperature specific capacity, cycle performance and rate performance under

In addition, Tang et al. used aluminum-doped zinc oxide (AZO) as a conductive material to coat the surface of LiFePO4 electrode material. The electrochemical test results show that the AZO coating can also greatly improve the rate capability and low temperature performance of LiFePO4, which is due to the conductive AZO coating increasing the electrical conductivity of the LiFePO4 material.

Fourth, bulk doping reduces the bulk resistance of LiFePO4 electrode materials

Ion doping can form vacancies in the LiFePO4 olivine lattice structure, which promotes the diffusion rate of lithium ions in the material, thereby enhancing the electrochemical activity of LiFePO4 batteries. Zhang et al. synthesized lanthanum and magnesium doped Li0.99La0.01Fe0.9Mg0.1PO4/graphite aerogel composite electrode material by solution impregnation process, which showed excellent electrochemical performance at low temperature, and the results of electrochemical impedance experiments It is shown that this superiority is mainly attributed to the enhanced electronic conductivity of the material by ion doping and graphite aerogel coating.

Huang et al. prepared Mg and F co-doped LiFe0.92Mg0.08(PO4)0.99F0.03 electrode material by a simple solid-state reaction. The results of structure and morphology characterization showed that Mg and F could be uniformly doped into LiFePO4 crystals. in the grid without changing the structure and particle size of the electrode material. Compared with the non-ion-doped LiFePO4 material and the Mg or F single-doped LiFePO4 material, the co-doped LiFePO4 at low temperature has the best electrochemical performance. The EIS results show that the co-doping of Mg and F increases the electron transfer rate and ion conduction rate, one of the reasons is that the length of the Mg-O bond is shorter than that of the Fe-O bond, which leads to the broadening of the lithium ion diffusion channel and improves the LiFePO4 ionic conductivity.

Wang et al. synthesized samarium-doped LiFe1-xSmxPO4/C composites by liquid-phase precipitation. The results show that a small amount of Sm3+ ion doping can reduce the polarization overpotential and charge transfer resistance, thereby improving the low-temperature electrochemical performance of LiFePO4. Cai et al. prepared Ti3SiC2-doped LiFePO4 electrode materials by a suspension mixing method. The study found that Ti3SiC2 doping can effectively improve the transfer rate of lithium ions at the interface of LiFePO4 electrode material at low temperature. Therefore, Ti3SiC2-doped LiFePO4 shows excellent performance at low temperature. rate performance and cycle stability. Li3V2(PO4)3 doped LiFePO4 electrode material (LFP-LVP) was prepared by Ma et al. The EIS results showed that the LFP-LVP electrode material had lower charge transfer resistance, and the acceleration of charge transfer improved the low-temperature electrical performance of LiFePO4/C batteries. chemical properties.