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The cathode material is the key part of lithium battery, which must meet the requirements of high capacity, strong stability and low toxicity. Compared with other cathode materials, LiFePO4 electrode materials have many advantages, such as high theoretical specific capacity, stable working voltage, stable structure, good cycling, low raw material cost and environmental friendliness. Therefore, this material is an ideal cathode material and is selected as one of the important cathode materials for power lithium batteries.

Many scientific researchers have studied the mechanism of accelerated degradation of LIBs at low temperatures, and believe that the deposition of active lithium and its catalytic growth of solid electrolyte interface (SEI) have led to the reduction of ionic conductivity and electron migration rate in the electrolyte, which has led to the reduction of LIBs capacity and power, and sometimes even led to battery performance failure. The low temperature working environment of LIBs mainly occurs in winter, 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 low temperature, the speed of lithium intercalation in graphite is reduced, and it is easy to precipitate metal lithium on the surface of the negative electrode to form lithium dendrites, puncture the diaphragm, and cause internal short circuit of the battery. Therefore, the way to improve the low temperature performance of LIBs is of great significance in promoting the use of electric vehicles in cold 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) Using electrolyte additives to prepare high-quality SEI films;

3) Surface coating modified LiFePO4 material interface conductivity;

4) Bulk phase conductivity of ion doped LiFePO4 material.

1、 Pulse current fast heating low-temperature battery

During the charging process of LIBs, the ion movement and polarization in the electrolyte will promote the heat generation in LIBs, which can be effectively used to improve the performance of LIBs at low temperatures. Pulse current refers to the current whose direction is constant and the current intensity or voltage changes periodically with time. In order to quickly and safely raise the battery temperature at low temperature, DeJong et al. used a circuit model to theoretically simulate how pulse current heats LIBs, and verified the simulation results through the experimental tests of commercial LIBs. The heating difference between continuous charging and pulse charging is shown in Figure 1. It can be seen from Figure 1 that the microsecond pulse time can promote more heat in the lithium battery.

Figure 1 Heat generated by pulse and continuous charging mode

Zhao et al. studied the use of pulse current to excite LiFePO4/MCNB battery, and found that after the pulse current excitation, the surface temperature of the battery increased from - 10 ℃ to 3 ℃, and compared with the traditional charging mode, the whole charging time decreased by 36 minutes (23.4%), and the capacity increased by 7.1% at the same discharge rate. Therefore, this charging mode is conducive to rapid charging of low-temperature LiFePO4 battery.

Zhu et al. studied the effect of pulse current heating mode on the life (health state) of LiFePO4 power lithium battery at low temperature. They studied the effect 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 would enhance the heat accumulation and temperature rise of LIBs. In addition, after 240 heating cycles (each cycle is equal to 1800s of pulse heating at - 20 ℃), they evaluated the state of health (SOH) of LIBs after pulse current heating by studying the battery capacity retention rate and electrochemical impedance, and studied the changes of the surface morphology of the negative electrode of the battery by SEM and EDS. The results showed that pulse current heating would not increase the deposition of lithium ions on the negative electrode surface, Therefore, pulse heating will not aggravate the risk of capacity attenuation and lithium dendrite growth brought by lithium deposition.

Fig. 2 Change of battery temperature with time when charging lithium battery with pulse current of 30Hz (a) and 1Hz (b) at different current intensity and voltage range

2、 Electrolyte modified SEI film to reduce the charge transfer resistance at the electrolyte-electrode interface

The low temperature performance of lithium battery is closely related to the ion mobility in the battery, and the SEI film on the surface of electrode material is the key link that affects the lithium ion mobility. Liao et al. studied the effect of carbonate base electrolyte (1mol/LLiPF6/EC+DMC+DEC+EMC, volume ratio 1:1:1:3) on the low temperature performance of LiFePO4 commercial lithium battery. When the operating temperature is lower than - 20 ℃, the electrochemical performance of the battery decreases significantly. The electrochemical impedance spectroscopy (EIS) test shows that the increase of charge transfer resistance and the decrease of lithium ion diffusion ability are important factors for the reduction of battery performance. Therefore, it is expected to improve the reaction activity of the electrolyte-electrode interface by changing the electrolyte, thus improving the low-temperature performance of LiFePO4 batteries.

Fig. 3 (a) EIS of LiFePO4 electrode at different temperatures; (b) Equivalent circuit model of LiFePO4EIS fitting

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

Li et al. studied the electrochemical performance of two kinds of 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. It was found that LiODFB-SL/DMS and LiODFB-SL/DES electrolytes can improve the cycle stability and magnification performance of LiFePO4 battery at low temperature. EIS study found that LiODFB electrolyte is conducive to the formation of SEI film with lower interface impedance, promoting the diffusion of ions and the movement of charges, thus improving the low-temperature cycling performance of LiFePO4 battery. Therefore, the proper composition of the electrolyte is conducive to reducing the charge transfer resistance and improving the diffusion rate of lithium ion at the interface of the electrode material, thus effectively improving the low-temperature performance of LIBs.

Electrolyte additives are also one of the effective methods to control the composition and structure of SEI film and improve the performance of LIBs. Liao et al. studied the effect of FEC on the discharge capacity and rate performance of LiFePO4 battery at low temperature. The study found that the LiFePO4 battery showed higher discharge capacity and rate performance at low temperature after adding 2% FEC in the electrolyte. SEM and XPS show the formation of SEI. EIS results show that adding FEC into the electrolyte can effectively reduce the impedance of LiFePO4 battery at low temperature, so the improvement of battery performance is attributed to the increase of ionic conductivity of SEI membrane and the reduction of LiFePO4 electrode polarization. Wu et al. analyzed the SEI film with XPS, and further studied the relevant mechanism. It was found that when FEC was involved in the interface film formation, the decomposition of LiPF6 and carbonate solvent was weakened, and the content of LixPOyFz and carbonate substances in the solvent decomposition decreased, thus forming a SEI film with low resistance and dense structure on the surface of LiFePO4. As shown in Figure 4, after adding FEC, the CV curve of LiFePO4 shows that the oxidation/reduction peak is close, indicating that adding FEC can reduce the polarization of LiFePO4 electrode. Therefore, the modified SEI promotes the migration of lithium ion at the electrode/electrolyte interface, thus improving the electrochemical performance of LiFePO4 electrode.

Fig. 4 Cyclic voltammogram of LiFePO4 battery in electrolyte containing 0% and 10% FEC at - 20 ℃

In addition, Liao et al. also found that the addition of butylsulfonolactone (BS) in the electrolyte has a similar effect, that is, to form a SEI film with thinner structure and lower impedance, and improve the migration rate of lithium ion through the SEI film. Therefore, the addition of BS significantly improved the capacity and magnification performance of LiFePO4 battery at low temperature.

3、 The surface resistance of LiFePO4 material is reduced by covering the conductive layer

At low temperature, one of the important reasons for the performance degradation of lithium batteries is the increase of impedance at the electrode interface and the reduction of ion diffusion rate. The conductive layer on the surface of LiFePO4 can effectively reduce the contact resistance between electrode materials, thus improving the diffusion rate of ions in and out of LiFePO4 at low temperature. As shown in Figure 5, Wu et al. used two kinds of carbon materials (amorphous carbon and carbon nanotubes) to coat LiFePO4 (LFP @ C/CNT). The modified LFP @ C/CNT has excellent low-temperature performance, and the capacity retention rate is about 71.4% when discharged at - 25 ℃. EIS analysis found that the improvement of this performance is mainly due to the reduction of impedance of LiFePO4 electrode material.

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

Among many coating materials, metal or metal oxide nanoparticles have attracted the attention of many researchers because of their excellent conductivity and simple preparation methods. 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. At low temperature, the intercalation/desorption ability of lithium ions in CeO2 modified LiFePO4 electrode material and electrode dynamics were significantly improved, which was attributed to the improved contact between the electrode material and the collector and the particles, as well as the addition of charge transfer in the LiFePO4 electrolyte interface, These factors reduce the electrode polarization.

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

Fig. 6 Cyclic performance of LiFePO4 coated with different V2O3 content at low temperature

Lin et al. coated Sn nanoparticles on the surface of LiFePO4 material through a simple electrodeposition (ED) process, and systematically studied the effect of Sn coating on the electrochemical performance of LiFePO4/C battery. SEM and EIS analysis shows that the 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, the Sn coating improves the specific capacity, cycle performance and multiplier performance of LiFePO4/C battery at low temperature.

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

4、 Bulk phase doping reduces bulk phase resistance of LiFePO4 electrode material

Ion doping can form vacancies in LiFePO4 olivine lattice structure, promote the diffusion rate of lithium ion in the material, and thus improve the electrochemical activity of LiFePO4 battery.

Zhang et al. synthesized Li0.99La0.01Fe0.9Mg0.1PO4/graphite aerogel composite electrode material doped with lanthanum and magnesium through the solution impregnation process. The material showed excellent electrochemical performance at low temperatures. The electrochemical impedance experiment results showed that this excellent performance was mainly due to the improvement of the electronic conductivity of the material by ion doping and graphite aerogel coating.

Huang et al. prepared LiFe0.92Mg0.08 (PO4) 0.99F0.03 electrode material co-doped with Mg and F by simple solid state reaction. The structure and morphology characterization results show that Mg and F can be uniformly doped into LiFePO4 lattice without changing the structure and particle size of the electrode material. Compared with the LiFePO4 material without ion doping and the LiFePO4 material with Mg or F single doping, the co-doped LiFePO4 material 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 Mg-O bond is shorter than that of Fe-O bond, which leads to the broadening of lithium ion diffusion channel, and improves the ionic conductivity of LiFePO4.

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

5、 Conclusion and prospect

In this paper, four methods to improve the low-temperature performance of lithium iron phosphate battery are briefly summarized: pulse current heating; Surface SEI film modified by electrolyte; Surface coating improves the surface conductivity of LiFePO4 material; Bulk ion doping improves the conductivity of LiFePO4 material. In low temperature environment, the increase of interface resistance in LiFePO4 battery and the growth of SEI film induced by lithium deposition are the important reasons for the decline of battery performance. Therefore, the key to improve its low temperature performance lies in safe, stable and rapid temperature rise or resistance reduction.

Pulse current can accelerate the movement of electric charge in the electrolyte and generate heat, so that LIBs can rapidly rise in temperature. The use of low impedance electrolyte system or film-forming additives is conducive to the formation of a dense, ultra-thin SEI film with high ionic conductivity, improving the reaction resistance of the LiFePO4 electrode-electrolyte interface, and reducing the negative impact of slow ion diffusion caused by low temperature. There are two important ways to modify LiFePO4 materials: surface coating and ion doping. The surface coating of LiFePO4 electrode material is beneficial to improve the surface conductivity of the electrode material and reduce the contact resistance; The ion doping is conducive to the formation of vacancies and valence changes in the lattice structure, broadening the ion diffusion channel, and promoting the mobility of lithium ions and electrons in the material. Therefore, based on the above analysis, the key to improve the low-temperature performance of lithium iron phosphate battery is to reduce the internal impedance of the battery.

Reference: Hu Chen et al. Brief introduction of modification methods for low temperature performance of lithium iron phosphate battery

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