Analysis of the factors affecting rate performance of LiFePO4lithium-ion batteri

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New Material Industry Innovation Center,China Baowu Steel Group Co.,Ltd.,Shanghai 201999,China

Abstract: In this paper,a water-based binder was used in LiFePO4 Li-ion batteries and the factors affecting the battery performance were analyzed.The type and amount of conductive agent and the amount of binder were found to have a significant impact on the rate performance of LiFePO4 Li-ion batteries.The impact of the two types of binders used in the test was not obvious.

Key words: water-based pulping; LiFePO4; button battery; rate performance

1 Introduction

Lithium-ion batteries,which have the highest energy of all secondary batteries,have had great success in their application in portable electronic equipment[1].As the global sales volume of vehicles increases,the environmental and fuel scarcity problems associated with traditional-fuel vehicles are becoming increas-ingly recognized.Many countries and fields of research are devoted to new-energy vehicles.The demand for and performance requirements of the batteries used in electric vehicles are both rising rapidly[2].As lithium-ion batteries attract increasing attention,their market share in powering electric vehicles continues to grow.

Lithium-ion batteries consist of the cathode materi-al,anode material,current collector,separator,elec-trolyte,and shell.The electrode materials are the key components associated with the energy and power den-sities of the batteries.Hence,electrode materials research is vital to improve the performance of lithium-ion bat-teries[3].Lithium manganate (LMO),lithium iron phos-phate (LFP),lithium nickel cobalt manganate (NCM) and lithium nickel cobalt aluminate (NCA) are the main cathode materials used at present.As a cathode material,olivine-type LiFePO4is known to have a stable structure,high safety,good performance at high temperature,good thermostability,and a long cycling life.At the same time,LFP material is cheap because of its natural abundance[4 - 5].

At present in the commercial cells industry,lithium-ion batteries mostly use polyvinylidene fluoride (PVDF) as a binder[6 -8]and n-methyl pyrrodidone (NMP) as a solvent.As a solvent,NMP is used in large quantities,is hard to recycle,is high in cost,and is environ-mentally unfriendly[9].As there is growing recognition of the importance of environmental protection and the commercial cells industry is maturing,electrode slurries that use aqueous binders and water solvents are attracting more attention[10 -12].

Using electrode slurries prepared with water as the solvent,this paper focuses on the factors that affect the rate performance of LiFePO4Li-ion batteries,such as different types and amounts of conductive agents and binders.The influence of mixing different types of conductive agents on the rate performance of LiFePO4Li-ion batteries is also explored.

2 Materials and experiments

2.1 Electrode preparation

Electrode slurries were prepared using the appropriate ratios of LiFePO4(grade M12 from Aleees),conductive agent (Li400,Li250,ECP-600JD,super P Li (SP),carbon nanotube (CNT),graphene,SDCF500 and UP-5-α),and aqueous binder (LA132,AG) in a deionized water solvent.First,the materials were weighed according to the ratio,and the LFP and conductive agent were mixed.Next,the aqueous binder was added to the mixture.Then,deionized water was slowly added while manually stirring.Then,the overhead stirrer (EUROSTAR 60 digital by IKA) was used to stir the mixture until the contents of the electrode slurry were well-distributed.The electrode slurries were spread onto a piece of 0.015-mm-thick current-col-lecting aluminum foil and dried in an oven at 80 ℃.The electrode was cut into circles with a diameter of 14 mm and moved into a glove box filled with high-purity argon.

2.2 Button cell fabrication and performance test

While in the glove box,the electrode was assembled into button cells.The CR2025 button was used as the container and a lithium sheet as the anode material.The separator was made from polypropylene.Model LB303 was chosen as the electrolyte.

Each electrode was weighed and measured in the glove box prior to assembling the button cell.The voltage of each button cell was tested after fabrication to determine whether or not the battery had a short circuit.If it did,a new battery was assembled to replace it.The areal density and theoretical capacity of each electrode were also calculated.

To allow the electrolyte to fully infiltrate the button cells,they were left for two days at room temperature after assembly.The charge-discharge system (model CT2001A from Wuhan LAND Electronic Co.,Ltd.) was used for battery formation and testing.The voltage was set to a range from 2.0-4.0 V.The button cells were charged with a constant current (CC) at the rate of 0.2 C until reaching 4.0 V,then at a constant voltage (CV) until the current decreased to 0.02 C.They were then discharged at a constant current at the rate of 0.2 C to 2.0 V at room temperature.This process was repeated three times during the battery formation process.

Rate performance test:

(1) Discharge rate performance test at 25 ℃ (room temperature)

Equipment:ESPEC charge-discharge system;

Charge:0.2 C CC-CV,4.0 V 0.02 C cut off;

Discharge:0.2 C,0.5 C,1 C,2 C,3 C,5 C,8 C,10 C,20 C CC,2.0 V cut off.

(2) Charge rate performance test at 25 ℃ (room temperature)

Equipment:ESPEC charge-discharge system;

Charge:0.2 C,0.5 C,1 C,2 C,3 C,5 C,8 C,10 C,20 C CC,4.0 V cut off;

Discharge:0.2 C CC,2.0 V cut off.

3 Results and discussion

3.1 Effect of the amount of conductive agent used

The electrode slurries,which were prepared using four different ratios of LiFePO4,ECP-600JD,and aqueous binder (LA132 from Chengdu Indigo Power Sources Co.,Ltd.) in deionized water sol-vent,were spread onto aluminum foil and made into but-ton cells.To ensure that the materials tightly adhered to the foil,the binder was also increased with in-creases in the amount of conductive agent.The rate performance of the button cells was then tested.

The results presented in Fig.1 show that the rate performance of the cells obviously became better with increases in the amount of the conductive agent used.When this amount reached a certain value,no further improvement in the rate performance was evident.

The electrode that contained no conductive agent exhibited almost no capacity,which also proves that the conductivity of LFP is very poor.When the conductive agent was added to the slurry,it enabled the formation of a conductive net between the LFP particles and improved the conductivity of the LFP electrode.With increases in the amount of conduc-tive agent used,the binder added into the slurry must also be increased to ensure that the materials will not peel off.The conductivity of the binder is poor,which is disadvantageous to the battery rate performance.In addition,as the additive was in-creased,the percentage of the cathode material de-creased,which means the specific capacity of the electrode would also decrease.To produce an effective slurry,we must control the amounts of the conductive agent and binder used to within their optimum ranges.

3.2 Effect of the type of conductive agent used

Electrode slurries were prepared using a ratio of LiFePO4∶conductive agent∶binder(LA132)=90 ∶5 ∶5.Each slurry used only one type of the eight possible conductive agents.

Fig.2 shows the rate performance curves of the LFP electrodes containing one of eight different types of conductive agents.The eight types of con-ductive agents were Li400,ECP-600JD,SP,CNT,graphene,SDCF500,Li250,and UP-5-α,as shown in Figs.2(a)-(h).

The results presented in Fig.2 show that the elec-trodes using either graphene or Li250 as the con-ductive agent had the highest specific capacities,and that using Li250 as the conductive agent showed better rate performance than that using gra-phene.The electro-chemical performance of the elec-trode using only UP-5-α or ECP-600JD or SDCF500 as the conductive agent did not perform well.The electrodes using other types of conductive agent exhibited no obvious differences,and their specific capacities and rate performance were slightly worse than that using Li250 as the conductive agent.

3.3 Effect of the type of binder used

Electrode slurries were prepared using a ratio of LiFePO4∶Li250∶GP ∶binder=50 ∶20 ∶5 ∶15.LA132 and AG were used as binders,with each slurry using only one type of aqueous binder.

The results presented in Fig.3 show that the rate performance of both electrodes was almost the same,but the specific capacity of that using LA132 as a binder was slightly better than that using AG.

3.4 Effect of the amount of binder used

Electrode slurries were prepared using four diffe-rent ratios of LiFePO4,conductive agent (Li250),and aqueous binder(LA132) in deionized water solvent.The electrode slurries were spread onto aluminum foil and made into button cells to test their rate performance.

The results presented in Fig.4 show that the rate performance of the electrodes tended to improve first and then degrade with increases in the proportion of the binder used.The function of the binder is to adhere the cathode materials together as well as firmly adhering the material to the foil.When an insufficient amount of binder is used,the cathode material on the surface tends to free itself from the electrolyte and the adhesion between the cathode material and the foil is too weak to keep the material adhered to the foil.When this is the case,the cathode material cannot adhere to the foil stably,which reduces the capacity of the battery over its cycle life.However,adding too much binder increases the internal resistance and affects the electrical performance due to the poor conductivity of the binder.Hence,choosing an appropriate binder ratio when preparing electrode slurries is very important.

3.5 Effect of using different combinations of various conductive agents

Electrode slurries were prepared using different ratios of LiFePO4,conductive agents (CNT,graphene,ECP-600JD,SDCF1000) and aqueous binder (LA132) in deionized water solvent.These electrode slurries were then spread onto aluminum foil and formed into button cells.Fig.5 shows the specific ratio of each slurry.

The results presented in Fig.5 show no obvious differences among the four different ratios of different conductive agents.These preliminary results indicate that different combinations of conductive agents have no obvious influence on the rate performance of the cells.However,there are many different types of conductive agents available and they can be mixed in different ratios.Consequently,more detailed experiments must be conducted and further research performed to confirm these findings.

4 Conclusions

This paper presented a preliminary study of the conditions that affect the rate performance of the LiFePO4button cell.The research results indicated that the type and amount of the conductive agent and the amount of the binder used had a significant influence on the rate performance of the LiFePO4button cell.

(1) With respect to the type of conductive agent used,the cell using DB (Li250) exhibited the best performance.

(2) In terms of the amount of the conductive agent used,the button cell’s performance became obviously better with increases in the amount to a certain point,after which the performance remained relatively stable.

(3) The rate performance of the electrodes tended to first improve and then decrease with increases in the proportion of the binder used.When too little binder is used,the cathode material cannot adhere stably to the foil,which decreases the capacity of the battery over its cycle life.However,adding too much binder increases the internal resistance and affects the electrical performance due to the poor conductivity of the binder.

(4) The two types of aqueous binders (LA132 and AG) tested in this research showed little difference in the relative rate performance of the LiFePO4button cell.

(5) Preliminary results showed that different combinations of various conductive agents had no obvious influence on the rate performance of the cell,but this finding requires further research.