Cathode materials are a crucial component of lithium-ion batteries, and must meet requirements such as high capacity, strong stability, and low toxicity. Compared with other cathode materials, LiFePO4 electrode material has many advantages, such as high theoretical specific capacity, stable operating voltage, structural stability, good cycle performance, low raw material cost, and environmental friendliness. Therefore, this material is a relatively ideal cathode material and has been selected as one of the important cathode materials for power lithium-ion batteries.
Many researchers have studied the mechanism of accelerated performance degradation of lithium batteries (LIBs) at low temperatures, believing that the deposition of active lithium and the catalytic growth of the solid electrolyte interface (SEI) are key factors. These factors lead to a decrease in ionic conductivity and electron migration rate in the electrolyte, resulting in reduced LIB capacity and power, and sometimes even battery performance failure. The low-temperature operating environment of LIBs mainly occurs in winter and in high-latitude, high-altitude regions, where the low temperatures affect LIB performance and lifespan, and can even cause extremely serious safety problems.
Low temperatures reduce the rate of lithium intercalation in graphite, making it easier for metallic lithium to precipitate on the negative electrode surface, forming lithium dendrites that puncture the separator and cause internal short circuits in the battery. Therefore, improving the low-temperature performance of LiFePO4 batteries is of great significance for promoting the use of electric vehicles in cold regions. This article summarizes methods to improve the low-temperature performance of LiFePO4 batteries from the following four aspects:
1) Heat generation via pulsed current;
2) High-quality SEI membranes were prepared using electrolyte additives;
3) Interfacial conductivity of surface-coated modified LiFePO4 materials;
4) Bulk conductivity of ion-doped modified LiFePO4 materials.
I. Rapid Heating of Low-Temperature Batteries with Pulse Current
During LIB charging, the movement and polarization of ions in the electrolyte induce heat generation within 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 remains constant while its intensity or voltage changes periodically over time. To rapidly and safely raise the battery temperature at low temperatures, DeJongh et al. theoretically simulated how pulsed current heats LIBs using a circuit model and verified the simulation results through experimental testing with commercial LIBs. The difference in heat generation between continuous charging and pulse charging is shown in Figure 1. As can be seen from Figure 1, a microsecond pulse duration can promote the generation of more heat in the lithium battery.
Figure 1 shows the heat generated by pulse and continuous charging modes.
Zhao et al. investigated the application of pulsed current in the excitation of LiFePO4/MCNB batteries. Their study found that after the pulsed current excitation ended, the battery surface temperature rose from -10℃ to 3℃, and compared with the traditional charging mode, the total charging time was reduced by 36 min (23.4%). At the same discharge rate, the capacity increased by 7.1%. Therefore, this charging mode is beneficial for the rapid charging of low-temperature LiFePO4 batteries.
Zhu et al. investigated the effects of pulsed current heating on the low-temperature battery life (state of health) of LiFePO4 power lithium batteries. They studied the effects of pulsed 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 heat accumulation and temperature rise in LIBs. Furthermore, after 240 heating cycles (each cycle equal to 1800 s of pulsed heating at -20°C), they assessed the state of health (SOH) of LIBs after pulsed current heating by studying battery capacity retention and electrochemical impedance spectroscopy. They also investigated the changes in the surface morphology of the battery anode using SEM and EDS. The results indicated that pulsed current heating did not increase lithium ion deposition on the anode surface; therefore, pulsed heating did not exacerbate the capacity decay and lithium dendrite growth risks associated with lithium deposition.
Figure 2 shows the change in battery temperature over time during lithium battery charging with pulse currents of 30 Hz (a) and 1 Hz (b) at different current intensities and voltage ranges.
II. Electrolyte-modified SEI membrane to reduce charge transfer resistance at the electrolyte-electrode interface
The low-temperature performance of lithium-ion batteries is closely related to the ion mobility within the battery, and the SEI film on the electrode material surface is a key factor affecting lithium-ion mobility. Liao et al. investigated the effect of a carbonate-based electrolyte (1 mol/L LiPF6/EC+DMC+DEC+EMC, volume ratio 1:1:1:3) on the low-temperature performance of commercial LiFePO4 lithium-ion batteries. At operating temperatures below -20℃, the battery's electrochemical performance significantly decreased. Electrochemical impedance spectroscopy (EIS) tests showed that increased charge transfer resistance and reduced lithium-ion diffusion capacity were important factors contributing to the performance degradation. Therefore, it is hoped that modifying the electrolyte can improve the reactivity of the electrolyte-electrode interface, thereby improving the low-temperature performance of LiFePO4 batteries.
Figure 3(a) EIS of LiFePO4 electrode at different temperatures; (b) Equivalent circuit model fitted to LiFePO4 EIS.
To find an effective electrolyte system for improving the low-temperature electrochemical performance of LiFePO4 batteries, Zhang et al. attempted to add a LiBF4-LiBOB mixed salt to the electrolyte, which improved the low-temperature cycling performance of LiFePO4 batteries. It is noteworthy that optimized performance was only achieved when the molar fraction of LiBOB in the mixed salt was less than 10%. Zhou et al. dissolved LiPF4(C2O4) (LiFOP) in propylene carbonate (PC) as the electrolyte for LiFePO4/C batteries and compared it with the commonly used LiPF6-EC electrolyte system. Experiments showed that the first-cycle discharge capacity of the LIBs decreased significantly when the batteries were cycled at low temperatures; simultaneously, EIS data indicated that the LiFOP/PC electrolyte improved the low-temperature cycling performance of the LIBs by reducing their internal impedance.
Li et al. investigated the electrochemical performance of two lithium difluoro(oxalate)borate (LiODFB) electrolyte systems: LiODFB-DMS and LiODFB-SL/DMS, and compared their electrochemical performance with the commonly used LiPF6-EC/DMC electrolyte. They found that LiODFB-SL/DMS and LiODFB-SL/DES electrolytes can improve the low-temperature cycle stability and rate performance of LiFePO4 batteries. EIS studies revealed that LiODFB electrolyte facilitates the formation of a lower interfacial impedance SEI film, promoting ion diffusion and charge movement, thereby improving the low-temperature cycle performance of LiFePO4 batteries. Therefore, a suitable electrolyte composition can help reduce charge transfer resistance and increase the diffusion rate of lithium ions at the electrode material interface, thus effectively improving the low-temperature performance of LIBs.
Electrolyte additives are also an effective method to regulate the composition and structure of the SEI film, 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 temperatures. They found that adding 2% FEC by volume to the electrolyte resulted in higher discharge capacity and rate performance of the LiFePO4 battery at low temperatures. SEM and XPS showed the formation of the SEI. EIS results indicated that adding FEC to the electrolyte effectively reduced the impedance of the LiFePO4 battery at low temperatures. Therefore, the improvement in battery performance was attributed to the increase in the ionic conductivity of the SEI film and the reduction in the polarization of the LiFePO4 electrode. Wu et al. used XPS to analyze the SEI film and further investigated the relevant mechanism. They found that when FEC participated in the interfacial film formation, the decomposition of LiPF6 and carbonate solvents was weakened, and the content of LixPOyFz and carbonate substances produced by solvent decomposition was reduced, thereby generating a low-impedance, dense SEI film on the LiFePO4 surface. As shown in Figure 4, the CV curve of LiFePO4 after the addition of FEC shows that the oxidation/reduction peaks converge, 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 improving the electrochemical performance of the LiFePO4 electrode.
Figure 4 shows the cyclic voltammetry of the LiFePO4 battery at -20℃ in electrolytes containing 0% and 10% FEC by volume fraction.
In addition, Liao et al.'s research also found that adding butyl sulfonyl lactone (BS) to the electrolyte has a similar effect, namely, forming a thinner SEI film with lower impedance, which increases the migration rate of lithium ions through the SEI film. Therefore, the addition of BS significantly improves the capacity and rate performance of LiFePO4 batteries at low temperatures.
III. Surface coating with a conductive layer reduces the surface resistivity of LiFePO4 materials.
One of the key reasons for the performance degradation of lithium batteries at low temperatures is the increased impedance and decreased ion diffusion rate at the electrode interface. Coating the LiFePO4 surface with a conductive layer can effectively reduce the contact resistance between electrode materials, thereby improving the diffusion rate of ions entering and exiting LiFePO4 at low temperatures. As shown in Figure 5, Wu et al. coated LiFePO4 (LFP@C/CNT) with two carbonaceous materials (amorphous carbon and carbon nanotubes). The modified LFP@C/CNT exhibited excellent low-temperature performance, with a capacity retention of approximately 71.4% during discharge at -25°C. EIS analysis revealed that this performance improvement primarily stemmed from the reduced impedance of the LiFePO4 electrode material.
Figure 5. HRTEM image (a), structural schematic (b), and SEM image of LFP@C/CNT nanocomposite material.
Among numerous coating materials, metal or metal oxide nanoparticles have attracted considerable attention from researchers due to their excellent conductivity and simple preparation methods. Yao et al. investigated the effect of CeO2 coating on the performance of LiFePO4/C batteries. In the experiment, CeO2 particles were uniformly distributed on the surface of LiFePO4. At low temperatures, the intercalation/deintercalation ability of lithium ions in the CeO2-modified LiFePO4 electrode material and the electrode kinetics were significantly improved. This was attributed to the improved contact between the electrode material and the current collector, as well as between the particles, and the increased charge transfer at the LiFePO4-electrolyte interface. These factors reduced electrode polarization.
Similarly, Jin et al. utilized the excellent conductivity of V₂O₃ to coat the surface of LiFePO₄ and tested the electrochemical performance of the coated samples. Studies on lithium ions showed that the highly conductive V₂O₃ layer significantly promoted lithium ion transport in the LiFePO₄ electrode. Therefore, the V₂O₃-modified LiFePO₄/C battery exhibited excellent electrochemical performance at low temperatures, as shown in Figure 6.
Figure 6 shows the cycling performance of LiFePO4 with different V2O3 content at low temperatures.
Lin et al. coated Sn nanoparticles onto the surface of LiFePO4 material using a simple electrodeposition (ED) process and systematically investigated the effect of the Sn coating on the electrochemical performance of LiFePO4/C batteries. SEM and EIS analyses showed that the Sn coating improved the contact between LiFePO4 particles, resulting in lower charge transfer resistance and higher lithium diffusion rate at low temperatures. Therefore, the Sn coating improved the specific capacity, cycle performance, and rate performance of LiFePO4/C batteries at low temperatures.
Furthermore, Tang et al. coated the surface of LiFePO4 electrode material with aluminum-doped zinc oxide (AZO) as a conductive material. Electrochemical test results showed that AZO coating can also significantly improve the rate performance and low-temperature performance of LiFePO4, because the conductive AZO coating increases the conductivity of the LiFePO4 material.
IV. Bulk doping reduces the bulk resistivity of LiFePO4 electrode materials
Ion doping can create vacancies in the olivine lattice structure of LiFePO4, promoting the diffusion rate of lithium ions in the material and thus improving the electrochemical activity of LiFePO4 batteries.
Zhang et al. synthesized a lanthanum and magnesium-doped Li0.99La0.01Fe0.9Mg0.1PO4/graphite aerogel composite electrode material by solution impregnation. The material exhibited excellent electrochemical performance at low temperatures. Electrochemical impedance spectroscopy results showed that this excellence was mainly attributed to the improved electronic conductivity of the material due to ion doping and graphite aerogel coating.
Huang et al. prepared Mg and F co-doped LiFe0.92Mg0.08(PO4)0.99F0.03 electrode materials via a simple solid-state reaction. Structural and morphological characterization results showed that Mg and F could be uniformly doped into the LiFePO4 lattice without altering the electrode material's structure and particle size. Compared to undoped LiFePO4 materials and Mg- or F-doped LiFePO4 materials, the co-doped LiFePO4 exhibited the best electrochemical performance at low temperatures. EIS results indicated that Mg and F co-doping increased the electron transfer rate and ion conduction rate, partly because the Mg-O bond length is shorter than the Fe-O bond, resulting in a wider lithium-ion diffusion channel and improved ionic conductivity of LiFePO4.
Wang et al. synthesized samarium-doped LiFe1-xSmxPO4/C composite materials via liquid-phase precipitation reaction. The results showed that a small amount of Sm3+ ion doping could reduce polarization overpotential and charge transfer resistance, thereby improving the low-temperature electrochemical performance of LiFePO4. Cai et al. prepared Ti3SiC2-doped LiFePO4 electrode materials via suspension mixing. Their research found that Ti3SiC2 doping effectively improved the lithium-ion transfer rate at the LiFePO4 electrode material interface at low temperatures. Therefore, Ti3SiC2-doped LiFePO4 exhibited excellent rate performance and cycle stability at low temperatures. Ma et al. prepared Li3V2(PO4)3-doped LiFePO4 electrode materials (LFP-LVP). EIS results showed that the LFP-LVP electrode material had lower charge transfer resistance, and accelerated charge transfer improved the low-temperature electrochemical performance of the LiFePO4/C battery.
V. Conclusion and Outlook
This article briefly outlines four methods to improve the low-temperature performance of lithium iron phosphate (LiFePO4) batteries: pulsed current heating; electrolyte modification of the surface SEI film; surface coating to improve the surface conductivity of LiFePO4 materials; and bulk ion doping to enhance the conductivity of LiFePO4 materials. At low temperatures, the increased interfacial resistance and the SEI film growth induced by lithium deposition in LiFePO4 batteries are significant reasons for the decline in battery performance. Therefore, the key to improving their low-temperature performance lies in safe, stable, and rapid heating or reducing impedance.
Pulsed current can accelerate the movement of charges in the electrolyte, generating heat and causing LIBs to heat up rapidly. Using a low-impedance electrolyte system or film-forming additives is beneficial for forming a dense, ultrathin, and highly ionicly conductive SEI film, improving the reaction resistance at the LiFePO4 electrode-electrolyte interface and reducing the negative impact of slowed ion diffusion caused by low temperatures. There are two main ways to modify LiFePO4 materials: surface coating and ion doping. Surface coating of LiFePO4 electrode materials helps improve the surface conductivity of the electrode material and reduce contact resistance; while ion doping helps to form vacancies and valence changes in the crystal structure, widening ion diffusion channels and promoting the mobility of lithium ions and electrons in the material. Therefore, based on the above analysis, the key to improving the low-temperature performance of lithium iron phosphate batteries lies in reducing the internal impedance of the battery.
