楊偉　薛建軍　陳勝洲　胡新發(fā)　夏信德　林維明

( 1.華南理工大學(xué) 化學(xué)與化工學(xué)院, 廣東 廣州510640；2.廣州鵬輝能源科技股份有限公司, 廣東 廣州511483；3.廣州大學(xué) 化學(xué)化工學(xué)院, 廣東 廣州510006)

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Mg2+摻雜球形LiFePO4/C的制備及電池性能*

楊偉1薛建軍2陳勝洲3胡新發(fā)2夏信德2林維明1

( 1.華南理工大學(xué) 化學(xué)與化工學(xué)院, 廣東 廣州510640；2.廣州鵬輝能源科技股份有限公司, 廣東 廣州511483；3.廣州大學(xué) 化學(xué)化工學(xué)院, 廣東 廣州510006)

以檸檬酸為碳源、草酸亞鐵為鐵源、乙酸鎂為鎂源,采用噴霧干燥-碳熱還原法制備了一系列鎂離子摻雜磷酸鐵鋰(LiMgxFe1-xPO4/C)材料,并研究了鎂離子對(duì)噴霧干燥-碳熱還原法制備的球形磷酸鐵鋰材料結(jié)構(gòu)和電化學(xué)性能的影響.結(jié)果表明，實(shí)驗(yàn)制得的LiMgxFe1-xPO4/C材料具有規(guī)則的球形空心結(jié)構(gòu)和高的比容量；當(dāng)Mg2+含量較小時(shí)其對(duì)磷酸鐵鋰晶體結(jié)構(gòu)不產(chǎn)生影響,其中LiMg0.04Fe0.96PO4/C具有最好的充放電性能,在0.2C、0.5C、1.0C、2.0C、5.0C倍率下首次放電比容量分別為149.0、145.6、141.3、132.6、123.3 mAh/g,1.0C倍率下充放電循環(huán)100周后容量保持率大于97.3%.

Mg2+摻雜；球形磷酸鐵鋰；噴霧干燥-碳熱還原；鋰離子電池

磷酸鐵鋰(LiFePO4)作為鋰離子電池正極材料,具有安全性高、充放電循環(huán)壽命長(zhǎng)、原料來(lái)源廣泛、綠色環(huán)保、成本低廉等優(yōu)點(diǎn),在電動(dòng)汽車和固定儲(chǔ)能基站等領(lǐng)域有著廣闊的市場(chǎng)前景[1- 3].但LiFePO4正極材料的電子導(dǎo)電率小(小于10-9s/cm)及離子擴(kuò)散系數(shù)低(10-16～10-14cm2/s)[4- 5],大大限制了LiFePO4的廣泛應(yīng)用.人們嘗試通過(guò)多種方法來(lái)改善LiFePO4的性能,如制備形狀規(guī)則均一的LiFePO4納米顆粒、減小材料的顆粒尺寸以縮短Li+的傳輸距離、降低傳質(zhì)阻抗[6- 8]，或者在磷酸鐵鋰材料顆粒表面及顆粒之間添加碳材料、納米金屬顆粒等導(dǎo)電劑形成導(dǎo)電結(jié)構(gòu)以提高材料的導(dǎo)電性能[9- 10].Chung等[11]發(fā)現(xiàn)在晶體結(jié)構(gòu)中摻雜金屬離子可以顯著提高LiFePO4電導(dǎo)率后,人們圍繞金屬離子摻雜改性LiFePO4展開了大量的研究[12- 14].研究發(fā)現(xiàn)Mg2+摻雜能夠加快Li+在LiFePO4中的擴(kuò)散速率,提高材料的電子導(dǎo)電率,使電子在材料中的傳遞更穩(wěn)定[15].噴霧干燥法制備的球形LiFePO4具有化學(xué)組成容易調(diào)節(jié)、前驅(qū)體易于在分子水平上的混合、材料形貌和結(jié)構(gòu)可控、振實(shí)密度高等優(yōu)勢(shì)，且電極制備工藝簡(jiǎn)單,受到了研究者的廣泛關(guān)注[16- 18].因此，文中采用噴霧干燥-碳熱還原法制備了一系列球形鎂離子摻雜磷酸鐵鋰(LiMgxFe1-xPO4/C)正極材料,并且考察不同鎂摻雜量對(duì)磷酸鐵鋰材料結(jié)構(gòu)和電化學(xué)性能的影響，以制備高性能的磷酸鐵鋰正極材料.

1　實(shí)驗(yàn)

1.1材料制備

按摩爾比n(Li)∶n(M)∶n(P)=1∶1∶1(n(M)為Mg和Fe元素的摩爾量總和)分別稱取LiOH、FeC2O4·2H2O、Mg(CH3COO)2·4H2O和NH4H2PO4,溶于二次去離子水中,配制1 mol/L(按Mgx+Fe1-x總濃度計(jì)算，x=0.00，0.02，0.04，0.08，0.10)溶液,再加入檸檬酸攪拌30 min形成均一的前驅(qū)體溶液.將所得前驅(qū)體溶液采用YC- 015實(shí)驗(yàn)室噴霧干燥器進(jìn)行噴霧干燥(進(jìn)風(fēng)溫度200 ℃,進(jìn)料速度5 mL/min),得到的粉末樣品在高純Ar氣保護(hù)下750 ℃高溫焙燒12 h,制得鎂離子摻雜磷酸鐵鋰正極材料,分別標(biāo)記為L(zhǎng)iFePO4/C、LiMg0.02Fe0.98PO4/C、LiMg0.04-Fe0.96PO4/C、LiMg0.08Fe0.92PO4/C、LiMg0.10Fe0.90PO4/C.1.2材料的物理表征方法

采用北京普析通用公司的XD-3型X射線衍射分析儀(Cu Kα,36 kV,20 mA)對(duì)樣品進(jìn)行晶相分析.采用日本Hitachi S- 4800場(chǎng)發(fā)射掃描電子顯微鏡觀察LiMgxFe1-xPO4/C的表面形貌.

1.3電池組裝與測(cè)試

首先將制得的LiMgxFe1-xPO4/C正極材料、乙炔黑、石墨、聚偏氟乙烯(PVDF)以90∶3∶2∶5的質(zhì)量比混合,以N-甲基吡咯烷酮(NMP)為溶劑,攪拌成固含量為45%的均一正極漿料,并將漿料涂布在20 μm厚的鋁箔上,經(jīng)過(guò)輥壓、裁切后在真空干燥箱中120 ℃干燥12 h,制得0.14 mm厚的正極片.然后將人造石墨、乙炔黑、羧甲基纖維素鈉、丁苯橡膠以94∶2∶1.5∶2.5的質(zhì)量比混合,以去離子水為溶劑,攪拌成固含量為45%的均一負(fù)極漿料,并將漿料涂布在10 μm厚的銅箔上,經(jīng)過(guò)輥壓、裁切后在真空干燥箱中105 ℃干燥12 h,制得0.11 mm厚的負(fù)極片.最后以1 mol/L的LiPF6溶于碳酸乙烯酯(EC)+碳酸二甲酯(DMC)+炭酸甲乙酯(EMC)(體積比為1∶1∶1)的混和液為電解液,Celgard2400聚丙烯微孔膜為隔膜,在手套箱中高純氬氣氣氛下(水含量和氧氣含量均小于10-5)卷繞、封裝制成052545型軟包電池.

采用VoltaLab PGZ301型電化學(xué)工作站對(duì)未充電的新鮮電池進(jìn)行交流阻抗測(cè)試,掃描頻率范圍為10-2～105Hz,擾動(dòng)電壓為10 mV.采用新威電池測(cè)試系統(tǒng)測(cè)試電池充放電性能,充放電制度:以0.1C倍率恒流充電至3.65 V,轉(zhuǎn)恒壓充電(截止電流0.05 C),靜置30 min后以不同倍率恒流放電至2.50 V.

2　結(jié)果與討論

2.1材料的物理表征結(jié)果

圖1所示為噴霧干燥制備的鎂離子摻雜磷酸鐵鋰正極材料的X射線衍射(XRD)譜圖和標(biāo)準(zhǔn)圖譜(JCPDS No.40-1499).為了研究鎂離子摻雜對(duì)磷酸鐵鋰材料晶體結(jié)構(gòu)的影響,測(cè)試得到了LiFePO4/C、LiMg0.02Fe0.98PO4/C、LiMg0.04Fe0.96PO4/C、LiMg0.08Fe0.92-PO4/C、LiMg0.10Fe0.90PO4/C的XRD譜圖(見圖1).從圖可知,與標(biāo)準(zhǔn)圖譜相比,樣品LiMg0.02Fe0.98PO4/C、LiMg0.04Fe0.96PO4/C沒(méi)有含鎂雜質(zhì)峰出現(xiàn),LiMg0.08-Fe0.92PO4/C、LiMg0.10Fe0.90PO4/C樣品出現(xiàn)MgH2P2O7雜質(zhì)峰,說(shuō)明鎂離子摻雜量較少時(shí),Mg2+均勻分散在LiFePO4材料的晶格中,不改變其晶相結(jié)構(gòu),而當(dāng)Mg2+摻雜量過(guò)多時(shí)，純相橄欖石結(jié)構(gòu)遭到破壞,LiMg0.10Fe0.90PO4/C的衍射峰強(qiáng)度明顯減弱.

圖1噴霧干燥制備的不同Mg2+摻雜量LiMgxFe1-xPO4/C的XRD譜圖

Fig.1XRD patterns of LiMgxFe1-xPO4/C with different Mg2+contents prepared via spray drying

圖2所示為噴霧干燥制備的鎂離子摻雜磷酸鐵鋰正極材料放大5 000倍的掃描電鏡(SEM)照片.從圖中可以看出,由于在噴霧干燥過(guò)程中液滴內(nèi)部的水分受熱后快速蒸發(fā)產(chǎn)生的水蒸氣使液滴膨脹,實(shí)驗(yàn)制備的LiMgxFe1-xPO4/C形成了二次顆粒構(gòu)成的空心球形結(jié)構(gòu),直徑為2～8 μm.LiFePO4/C、LiMg0.02Fe0.98PO4/C、LiMg0.04Fe0.96PO4/C的球形結(jié)構(gòu)較為均一、完整,LiMg0.08Fe0.92PO4/C的球形均一性較差,LiMg0.10Fe0.90PO4/C部分空心球破碎,可以看到半球殼狀碎片,由此證明合成的材料具有空心球形結(jié)構(gòu).這可能是因?yàn)镸g2+的加入引起前驅(qū)體溶液黏度或pH值變化,從而導(dǎo)致生成的球形結(jié)構(gòu)遭到破壞.

2.2LiMgxFe1-xPO4/C材料在不同倍率下的電化學(xué)性能

圖3給出了噴霧干燥制備的LiMgxFe1-xPO4/C材料在0.1 C倍率下的首次充放電曲線.從圖中可以看出,LiMgxFe1-xPO4/C材料的充放電比容量隨著Mg元素含量的增加而增加,但Mg含量過(guò)大時(shí),材料比容量迅速降低.這可能是由于Mg2+調(diào)節(jié)了磷酸鐵鋰的晶格結(jié)構(gòu),提高了材料的導(dǎo)電性,而當(dāng)Mg2+過(guò)多時(shí),破壞了磷酸鐵鋰的晶格結(jié)構(gòu),改變了Li+在固相結(jié)構(gòu)中的擴(kuò)散路徑,不利于Li+嵌入和脫出;另外,磷酸鐵鋰中出現(xiàn)MgH2P2O7雜相,無(wú)電化學(xué)活性,導(dǎo)致LiMg0.08Fe0.92PO4/C、LiMg0.10Fe0.90PO4/C的放電比容量較低.

(a)LiFePO4/C

(b)LiMg0.02Fe0.98PO4/C

(c)LiMg0.04Fe0.96PO4/C

(d)LiMg0.08Fe0.92PO4/C

(e)LiMg0.10Fe0.90PO4/C

圖2噴霧干燥制備的不同LiMgxFe1-xPO4/C樣品的SEM 照片

Fig.2SEM images of different LiMgxFe1-xPO4/C samples prepared via spray drying

圖3噴霧干燥制備的不同LiMgxFe1-xPO4/C材料的0.1C倍率首次充放電曲線

Fig.3Initial charge and discharge curves at 0.1 C of different LiMgxFe1-xPO4/C samples prepared via spray drying

圖4為L(zhǎng)iMgxFe1-xPO4/C材料在1.0 C倍率下的放電循環(huán)曲線.由圖可知,在1.0 C倍率下放電時(shí),LiMgxFe1-xPO4/C(x=0.00，0.02，0.04，0.08，0.10)材料的首次放電比容量分別為130.8、134.2、141.3、126.3、119.8 mAh/g,充放電循環(huán)100周后容量無(wú)明顯衰減.LiMg0.04Fe0.96PO4/C材料的首次放電比容量最高,且循環(huán)性能較好,充放電循環(huán)100周后容量保持率達(dá)97.3%.LiMg0.10Fe0.90PO4/C容量衰減嚴(yán)重，充放電循環(huán)100周后容量保持率為93.2%.

圖4噴霧干燥制備的不同LiMgxFe1-xPO4/C材料在1.0 C倍率下的放電循環(huán)曲線

Fig.4Discharge cycling curves at 1.0 C of different LiMgxFe1-x-PO4/C samples prepared via spray drying

圖5是LiMgxFe1-xPO4/C(x=0.00，0.02，0.04，0.08，0.10)材料在不同倍率(0.2 C、0.5 C、1.0 C、2.0 C、5.0 C)下的放電循環(huán)曲線.由圖5可知,所有的LiMgxFe1-xPO4/C樣品的放電比容量均隨放電倍率的增加而減小,不摻雜Mg2+的純LiFePO4/C的放電比容量下降最快,摻雜Mg2+的LiMgxFe1-xPO4/C材料的放電比容量則下降較慢,說(shuō)明Mg2+摻雜有利于提高LiFePO4材料的電子導(dǎo)電率,有效改善高倍率放電性能.LiMg0.04Fe0.96PO4/C材料的放電比容量最高,在不同倍率(0.2 C、0.5 C、1.0 C、2.0 C、5.0 C)下分別為149.0、145.6、141.3、132.6、123.3 mAh/g,其克容量隨放電倍率的增加而下降的趨勢(shì)較弱,而且依次經(jīng)過(guò)0.2 C、0.5 C、1.0 C、2.0 C、5.0 C各循環(huán)10周后,再在0.2 C倍率下放電仍能保持初始容量的98.1%,LiMg0.04Fe0.96PO4/C材料具有很好的嵌鋰/脫鋰活性和穩(wěn)定性[18].

圖5噴霧干燥制備的不同LiMgxFe1-xPO4/C材料在不同倍率下的放電循環(huán)曲線

Fig.5Discharge cycling curves of different LiMgxFe1-xPO4/C samples prepared via spray drying at different rates

圖6為L(zhǎng)iMgxFe1-xPO4/C(x=0.00，0.02，0.04，0.08，0.10)材料的電化學(xué)交流阻抗(EIS)譜圖.高頻區(qū)半圓弧反映電荷轉(zhuǎn)移阻抗Rct,低頻區(qū)的斜線部分反映Li+在材料及電解液中的Warburg擴(kuò)散阻抗.采用Zview對(duì)數(shù)據(jù)進(jìn)行擬合,LiMgxFe1-xPO4/C(x=0.00，0.02，0.04，0.08，0.10)的電荷轉(zhuǎn)移阻抗依次分別為38.1、31.6、19.2、41.0、47.9 Ω,隨Mg2+摻雜量的增加呈先減小后增加的趨勢(shì),LiMg0.04Fe0.96PO4/C具有最低的電荷轉(zhuǎn)移阻抗,這可能是材料具有較好的倍率性能的原因.

圖6噴霧干燥制備的不同LiMgxFe1-xPO4/C材料的交流阻抗譜圖

Fig.6EIS spectra of different LiMgxFe1-xPO4/C samples prepared via spray drying

3　結(jié)論

采用噴霧干燥-碳熱還原法制備了一系列直徑約為2～8 μm的空心球形結(jié)構(gòu)Mg2+摻雜磷酸鐵鋰(LiMgxFe1-xPO4/C)材料,當(dāng)Mg2+含量較小時(shí)其對(duì)磷酸鐵鋰晶體結(jié)構(gòu)不產(chǎn)生影響,Mg2+摻雜能夠顯著提高噴霧干燥-碳熱還原法制備的LiFePO4/C高倍率放電性能,其中LiMg0.04Fe0.96PO4/C具有最好的充放電性能,在0.2 C、0.5 C、1.0 C、2.0 C、5.0 C倍率下首次放電比容量分別為149.0、145.6、141.3、132.6、123.3 mAh/g,1.0C倍率下充放電循環(huán)100周后容量保持率大于97.3%.后期研究中應(yīng)進(jìn)一步考察LiMgxFe1x-PO4/C材料在低溫下的充放電循環(huán)性能.

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Preparation and Battery Performance of Spherical LiFePO4/C Doped with Mg2+

YANGWei1XUEJian-jun2CHENSheng-zhou3HUXin-fa2XIAXin-de2LINWei-ming1

(1.School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640,Guangdong,China;2.Guangzhou Great Power Energy & Technology Co., Ltd., Guangzhou 511483, Guangdong, China;3.School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, Guangdong, China)

Spherical LiFePO4/C doped with Mg2+(namely LiMgxFe1-xPO4/C) was prepared by means of the spray drying-carbonthermal reduction, with citric acid, ferrous oxalate and magnesium acetate as the carbon source, the iron source and the magnesium source, respectively. Then, the effects of Mg2+doping on the structure and electrochemical performance of the prepared LiFePO4/C material were investigated. The results indicate that (1) LiMgxFe1-xPO4/C possesses a hollow ball-shaped structure and a high specific capacity; (2) the crystal structure of lithium iron phosphate keeps unchanged at a low Mg2+dosage; and (3) LiMg0.04Fe0.96PO4/C possesses the best charge/discharge performance. For instance, its initial discharge capacities are 149.0, 145.6, 141.3, 132.6 and 123.3 mAh/g at the rates of 0.2C, 0.5C, 1.0C, 2.0C and 5.0 C, respectively; and the capacity retention ratio of LiMg0.04Fe0.96PO4/C at 1.0C keeps more than 97.3% after 100 charge-discharge cycles.

Mg2+doping; spherical lithium iron phosphate; spray drying-carbothermal reduction; lithium ion battery

2015- 08- 24

國(guó)家自然科學(xué)基金資助項(xiàng)目(21376056,21463030);廣東省戰(zhàn)略性新興產(chǎn)業(yè)發(fā)展專項(xiàng)資金新能源汽車產(chǎn)學(xué)研項(xiàng)目(2011- 1579)

楊偉(1982-),男,博士,助理研究員,主要從事應(yīng)用電化學(xué)與新能源材料研究．E-mail:wyang608@163.com

1000- 565X(2016)06- 0009- 05

TM 910.4

10.3969/j.issn.1000-565X.2016.06.002

Foundation items: Supported by the National Natural Science Foundation of China(21376056,21463030)