稀有金属(英文版) 2020,39(12),1457-1463
Development of a LiFePO4-based high power lithium secondary battery for HEVs applications
Long-Zheng Deng Feng Wu Xu-Guang Gao Wei-ping Wu
Beijing Key Laboratory of Environmental Science and Engineering,School of Chemical Engineering and the Environment,Beijing Institute of Technology
National & Local United Engineering Laboratory for Power Battery,Department of Chemistry,Institute of Functional Material Chemistry,Northeast Normal University
作者简介:*Feng Wu,e-mail:wufeng863@vip.sina.com;
收稿日期:9 September 2013
基金:financially supported by the State Basic Research Development Program of China (No.2009CB220100);the Ministry of Science and Technology (MOST) of China,US-China Collaboration on Cutting-Edge Technology Development of Electric Vehicles(No.2010DFA72760);the National Natural Science Foundation of China (Nos.50901009 and 51271029);the Fundamental Research Funds for the Central Universities (No.12QNJJ013);
Development of a LiFePO4-based high power lithium secondary battery for HEVs applications
Long-Zheng Deng Feng Wu Xu-Guang Gao Wei-ping Wu
Beijing Key Laboratory of Environmental Science and Engineering,School of Chemical Engineering and the Environment,Beijing Institute of Technology
National & Local United Engineering Laboratory for Power Battery,Department of Chemistry,Institute of Functional Material Chemistry,Northeast Normal University
Abstract:
A LiFeP04-type lithium secondary battery cell of 8 Ah capacity with a high energy density and power density was developed for hybrid electric vehicle(HEV)applications by optimizing the key raw materials and process design.The 8 Ah class LiFePO4 cell with an energy density of 77.2 Wh·kg-1 exhibits a power density of2818 W·kg-1 at 50 % SOC(state of charge).The battery shows good cyclic capability with the capacity retention of81.1 % after 1,870 cycles at 5 C charge and 10 C discharge rates.It is demonstrated that the cells have an excellent balance of high-power,high-energy,low temperature,and long-life performance for meeting the requirements of HEV.
Keyword:
Li-ion battery; Lithium iron phosphate; High-power; Long-life; Hybrid electric vehicle;
Received: 9 September 2013
1 Introduction
The worldwide demand for clean,low-fuel-consuming transport is promoting the development of safe,high energy and power electrochemical storage and conversion systems
[
1,
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.Lithium ion batteries (LIBs) are nowadays considered as the best technology for these applications
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.So,research and development of electric vehicle (EV) and hybrid electric vehicle(HEV) are an effective way for energy conservation and emissions reduction.However,as the key components of HEV,power battery performance is an important factor that determines the automobile performance.Meanwhile,safety now represents the main challenge holding up the lithium-ion technology from wide application in HEVs.In all types of LIBs,lithium iron phosphate (LiFePO4)-based LIBs with the advantages of low cost,good safety,environmental friendly,and long cycle life were investigated intensively
[
5,
6]
.Many countries invested heavily in researching and developing this technology.
The objective of this work is to develop HEV used LiFePO4-type lithium ion secondary batteries in order to improve the energy density and the power density of the battery and optimize the performance using selected key raw materials including cathode materials,anode materials,membrane,electrolyte,etc.In this paper,it was reported the various performances of as-prepared LiFePO4-based8 Ah cell including rate charging,rate discharge,pulse discharge,cycle performance,etc.
2 Experimental
2.1 Key raw materials
Nano-sized LiFePO4 cathode powder was prepared by solid state method.The primary particle size was smaller than 100 nm.The secondary particle size distribution(PSD) of the cathode powder is about 2-3μm.The artificial graphite (AG) with easy processing,large fluid absorption rate,and high tap density was used as anode materials.The average PSD of the anode powder was about8-12μm,and the tap density was 1.26 g·cm3.The monolayer membrane separator was prepared by wet process with poly ethylene (PE) as raw material.The thickness was20μm and the porosity ratio was about 47%.The organic electrolyte consisted of a mixture of ethylene carbonate(EC),dimethyl carbonate,ethylmethylcarbonate,vinylene carbonate (VC),and propylene carbonate (PC) solvent containing lithium hexafluorophosphate (LiPF6) of1.3 mol·L-1.The conductivity of the positive and negative electrode was improved by adding a small amount of vapor grown carbon fiber (VGCF),thereby improving the rate discharge performance of the battery.
2.2 Electrode preparation
The cathode consisted of 89 wt%carbon-coated LiFePO4,2 wt%VGCF,3 wt%carbon black (KS-6),and 6 wt%polyvinylidene fluoride on Al foil current collector.The negative electrode consisted of 1.5 wt%carboxymethyl cellulose,2 wt%VGCF,2 wt%graphite (SFG-6),92 wt%AG(MCMB),and 2.5 wt%styrene butadiene rubber on Cu foil current collector.After coating,the positive electrode plate and the negative electrode plate were rolled and then slitted.These electrodes were dried under vacuum at 110℃for 48 h.
2.3 Pouch cell preparation
After vacuum drying,the electrodes were laminated and welded with ultrasonic spot.The stacks were sealed with aluminum film.Then they were placed in a vacuum oven at80℃for 48 h.After the injection process,the cell should be finished by the battery precharging,the formatting,and the grading.The formation processes were as follows:first,the cell was charged at 400 mA (0.05C) for 3 h,and then charged to 4.0 V at 1,600 mA (0.2C);second,the cell was discharged to 2.0 V at 0.02C,and then the cell was fully charged at 1,600 mA (0.2C);third,the fully charged cell was aged at 45℃for 24 h;finally,the cell was exhausted and tested.The test process was measured at 25℃with the charge current of 8,000 mA upto 3.85 V,after CC-CV charging the current down to 800 mA.Then the cell was held for 10 min,discharged to 2.0 V,and cut off at8,000 mAh.Then the cell was fully charged at 8,000 mA.
2.4 Characterization
The prepared active materials were characterized for their structural information by X-ray diffraction (XRD,Rigaku P/max 2200VPC using a Cu Kαradiation source withλ=0.15406 nm).XRD data were obtained at 2θfrom 10°to 90°,with a step size of 0.02°and a constant counting time of 10 s.JSM-5600LV (JEOL LTD) scanning electron microscopy (SEM) was applied to observe the particle size and morphology of the samples.
Charge-discharge performances were measured using a Neware battery testing system (5 V,300 A).The charge-discharge capacities were recorded between 2.00and 3.65 V with different rate currents (1C=8.0 A).The test was carried out at 20℃in the absence of special note.
3 Results and discussion
3.1 Selection of key raw materials
Figure 1 shows the XRD patterns of the LiFePO4/C powder materials.In the XRD spectra,all diffraction peaks can be indexed to space group Pnma
[
7]
.The diffraction peaks are detected in the spectra,meaning that the synthesized sample is of single phase without impurities.
The typical SEM image in Fig.2a shows that the LiFePO4/C sample consists of secondary microparticles with diameters of 2-5μm.SEM observation at a higher magnification (Fig.2b) reveals that each microparticle is actually a random aggregate of primary LiFePO4/C nanoparticles that are covered by carbon film,and the electron conductivity of the material is improved by carbon coating
[
8,
9,
10]
.It is generally attributed to the fact that the nanoscale electrode materials can shorten the ionic conduction path,thereby to improve the capacity and rate performance
[
11,
12,
13]
.But the monodisperse nanoscale material is not easy to process as electrodes for the enhancement of energy density
[
13]
.So,the high crystalline cathode powder material with secondary microparticles of 2-3μm and nanoscale primary particles was chosen to make the cathode electrodes in order to provide a greater energy density.At the same time,there are many micropores among the secondary particles,which is an advantage for the sufficient wetting and the uniform dispersion of the electrolyte in the particles.So both the conductivity and electrode processing are guaranteed.For anode electrodes,graphite is pided into natural graphite and AG.The reversible capacity of natural graphite is higher,but needs to be modified,otherwise electrolyte solvent is easy to be embedded during the charge-discharge process
[
14,
15,
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.So the high quality AG with shaping and processing was chosen as anode.It is not easy to react with the electrolyte.The processing performance is good.Also it has good absorption ability of electrolyte.The tap density of AG is1.26 g·cm-3 with the mean particle size of 10μm.
Fig.1 XRD patterns of prepared LiFePO4/C
Fig.2 SEM images of a,b LiFePO4/C and c,d MCMB with different magnifications
For high power battery,the ion transportation is fast,which requires high electrolyte conductivity,and less volatile solvent.So the high salt electrolyte ratio of1.3 mol·L-1 LiPF6 in the electrolyte was chosen in contrast to the general electrolyte concentration of 1.0 mol·L-1.The solid electrolyte interphase is improved by adding appropriate amount of VC additives
[
17,
18]
.It can occur reduction reaction better than EC,because VC can produce less gas compared with the conventional liquid electrolyte during the high reduction potential.Besides,the form of film is dense,and it is beneficial to the rate performance of the battery.Adding the proper amount of PC is also helpful to increase the conductivity of electrolyte.Although the viscosity of PC is high,it has high conductivity.Proper film-forming additive was chosen,the good film can be formed before the co-embedding of lithium ion and PC in the graphite due to that the PC is very easy to intercalate graphite reactions
[
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.
The PP/PE/PP membrane is chosen in most of power batteries for its good safety.But its porosity rate is low,which affects the transportation of lithium ion.In this experiment,the single-layer PE film was used,and its thickness is 20μm and it has high porosity rate of 47%.The shut down temperature of the membrane is 128℃after special surface treatment,while the normal is 132℃.Moreover,the melting temperature is as high as 180℃,and it has better safety performance.
Fig.3 SEM images of a negative electrode and b positive electrode
Fig.4 Discharge curves of cell at 0.2C,0.5C,and 1.0C rate
Carbon black and graphite are usually used as conductive agent in the common batteries.But it only belongs to the contact of point and point between active material and conductive agent.At large discharge current,the battery performance is affected.So VGCF conductive agent was used both in the negative electrode and the positive electrode for improving the conductivity of electrode.From the SEM image in Fig.3a,b,it can be clearly seen that the VGCF distributes among the particles of the active material.
3.2 Process technology of cell
Optimizing the battery technology is also very important on the basis of choosing these key raw materials in the development of high power lithium ion power battery.Some key controlling processes and parameters,including the dispersible uniformity of all raw materials and suitable coated surface density of the electrode etc.,are all key steps of the design and manufacture of batteries.If the slurry coating is too thick,the migration path of lithium ions is too long,thus the high rate discharge performance of battery will be affected.Otherwise,if the coating density is too thin,although large current charge and discharge capacity of the battery is improved,the energy density of the battery would be reduced,which will be non-economical.Based on many adjustments,the surface density of the positive electrode is adjusted to 240 g·m-2,and the negative value is110 g·m-2.
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Table 1 Related performance data of cells
All cells being charged to 4.0 V at 1C rate (CC),and then charged to 0.1C at constant voltage of 4.0 V(CV),then held for 10 min.After that.discharged to 2.0 V at 1C current.All tested at 25℃
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Table 2 Charge performance data of cells
Fig.5 Charge curves of cell at different rates
Fig.6 Discharge curves of cell at different rates
The press density of the electrode also has a great influence on the battery power property.If the press density is too large,it will reduce the electrolyte absorbing value;thus,the ionic conductivity of electrode is affected.It will have negative effect on the power capability of battery.On the contrary,if the press density is too small,the electronic conductivity is not good,the power density of the battery is also affected.Owing to the practic al condition of equipment,the cathode press density is 2.13 g·cm-3,and the negative press density is 1.45 g·cm-3.Thus,the power density and energy density of battery both improve at the same time.
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Table 3 Discharge performance data of cell
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Table 4 Power property data of cell
Fig.7 Power property of cell at different
3.3 Battery performance of cell
The discharge characteristics for a single cell are shown in Fig.4.The discharge voltage curve is very stable over3.2 V.The discharge capacity of the cell is 9276,9115,and8858 mAh at 0.2C,0.5C,and 1.0C rate,respectively.
According to the requirement of the high power battery,a total of 6 cells with 8 Ah is developed through carefully selecting key raw materials and its corresponding battery process and structure design.The related performance data of these cells are shown in Table 1.
The fast charge properties are shown in Fig.5.It can be found that the cell can afford fast charge rate of over 3C(20 min).Even charged at 5C rate,the battery still has good charge platform.So charge capability of the cell can meet the requirements of HEV.The detailed charge data are listed in Table 2.
From Table 2,it can be found that the capacity of constant current (CC) decreases gradually with the increase of the charge current due to the electrochemical polarization,and the capacities of the constant voltage increase.The cell can afford 5C charge rate in case of emergency.
The discharge properties are shown in Fig.6.The battery displays excellent electrochemical performance at different discharge rates.Even discharged at 10C rate,the battery still has good voltage platform and the ratio of 20C/1C is 92.29%.So,the discharge capability of the cell can meet the requirements of HEV.The detailed discharge data are listed in Table 3.
The power performance of the cell is vital to provide the sufficient energy for starting,acceleration,and highspeed operation of the car.The power property of the cell at different SOCs is shown in Fig.7.The cell can exhibit an output power density of 2,818 W·kg-1 at 50%SOC.So,the power density of the cell can meet the requirements of HEV.The detailed power data are listed in Table 4.
The high rate continuous charge-discharge capability is shown in Fig.8.A continuous discharge upto 80 A is possible,though the discharge voltage is around 0.5 V lower than that of 8 A (Fig.5).After 1870 cycles at5C charge and 10C discharge with cut-off voltage3.8-2.0 V,a good cyclic property is observed.The capacity ratio of its initial capacity is 81.1%after 1,870cycles.At higher charge rate of 10C and discharge rate of 10C,the fading is larger and the capacity retention ratio is 85.7%after the 700th cycle.At 3C/20C,the capacity retention ratio is 88.1%after the 700th cycle.The specific data are listed in Table 5.The fluctuation of cyclic curve is related to the temperature change.The fading could be related to structural variations or charge transfer in host structure or contact losses between the conductive binder and the active material particles,resulting from volume variations during lithium extraction.
Fig.8 Cyclic capability of cell at different charges and discharges:a 5C charge and 10C discharge,b 3C charge and 20C discharge,and c 10C charge and 10C discharge
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Table 5 Cyclic performance data of cell
Cell being charged to 4.0 V at 5C rate (CC),then charged to 0.1 C (CV),then held for 10 min.and then discharged to 2.0 V at 10C rate;tested at(25±10)℃
4 Conclusion
A LiFePO4-based lithium secondary batteries cell of 8 Ah capacity was developed for HEV applications by selecting cathode,anode,electrolytes,and the membrane materials as well as the conductive additive and process design.The average energy density reaches 77.2 Wh·kg-1,and the internal resistance on average is less than 1.1 mΩ.The large current charge and discharge of the battery performance are very excellent due to the small internal resistance.The charge capacity of 20C is92.29%of that of the 1C rate.Meanwhile,the cell has a power density of 2,818 W·kg-1 at 50%SOC.After 1,870 cycles,the capacity retention is still 81.1%at 5C charge and 10C discharge rate.The power property of the lithium ion battery is also very good,and can meet the requirements of HEV.
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