具有NASICON結(jié)構(gòu)的Na3V2(PO4)3 (NVP)離子電導(dǎo)率高、體積變形小、理論比能量高(400 Wh/kg)和電化學(xué)穩(wěn)定性高，是鈉離子電池正極的候選材料之一[1~3] 但是，NVP較低的本征電子電導(dǎo)率使其倍率性能和循環(huán)性能不高 降低顆粒尺寸、摻雜和碳包覆，可提高其導(dǎo)電性[4]

為了制備小尺寸的NVP顆粒，可用溶膠–凝膠法[5]、水熱法[6]、噴霧干燥法[7,8]和溶液燃燒合成(Solution combustion synthesis，SCS)[9~12]等液相法，調(diào)節(jié)工藝參數(shù)可控制其形貌和尺寸 SCS是一種簡單、快速、高效的液相制備方法 Wang等[9]用SCS合成的碳包覆NVP鈉離子電池正極材料，在0.1 C下的初始充放電比容量分別為111和101 mAh·g–1，循環(huán)50圈后的容量保持率為95% 但是，SCS的反應(yīng)過程難以控制且產(chǎn)物易發(fā)生團(tuán)聚

超聲技術(shù)是一種能強(qiáng)化化學(xué)反應(yīng)和化工過程的物理手段，可用于輔助制備納米材料[13~15] 超聲的力學(xué)效應(yīng)和空化效應(yīng)不僅能促進(jìn)反應(yīng)的進(jìn)行，還能有效阻止顆粒的團(tuán)聚和長大[16] 將超聲技術(shù)引入SCS過程，可合成碳包覆NVP[17] 超聲的引入使用傳統(tǒng)SCS合成的塊狀團(tuán)聚體轉(zhuǎn)變成富含孔洞的3D網(wǎng)絡(luò)結(jié)構(gòu)，還能降低NVP的晶粒尺寸(<20 nm) 這種材料在0.1 C下的比容量可提高到117 mAh·g–1，在2 C下的比容量為85 mAh·g–1，在0.2 C下循環(huán)120次后保持初始容量的94% 但是在SCS過程中生成的碳為無定形碳，不能將NVP顆粒完全包覆也不能在NVP顆粒之間建立連續(xù)的三維導(dǎo)電網(wǎng)絡(luò)結(jié)構(gòu)

石墨烯是一種比表面積較大的納米碳材料，具有良好的導(dǎo)電性和優(yōu)良的機(jī)械性能，是很有前途的導(dǎo)電碳材料[18~22] 將大表面的石墨烯摻入NVP中可將NVP顆粒聯(lián)結(jié)起來構(gòu)建連續(xù)的電子三維通道從而提高NVP的導(dǎo)電性 為了進(jìn)一步提高超聲輔助溶液燃燒合成的NVP的導(dǎo)電性從而提高其倍率性能和循環(huán)性能，本文用超聲輔助溶液燃燒法合成硬碳和石墨烯雙層碳包覆的NVP復(fù)合材料，研究添加石墨烯對其組織結(jié)構(gòu)和電化學(xué)性能的影響

1 實(shí)驗(yàn)方法1.1 雙層碳包覆NVP的制備

用Hummers法[23,24]制備氧化石墨烯(GO)納米片 雙層碳包覆NVP的制備：將2.55 g硝酸鈉(NaNO3)、3.45 g的磷酸二氫氨(NH4H2PO4)、2.34 g偏礬酸銨(NH4VO3)和8.45 g檸檬酸(C6H8O7)溶解在去離子水，得到100 mL澄清的前驅(qū)體溶液 在前驅(qū)體溶液中加入GO(添加量分別為0.0%、2.5%、5.0%)并超聲2 h，然后將裝有前驅(qū)體溶液的坩堝放到溫度為500℃的電阻爐內(nèi)，將超聲桿的頂端浸入前驅(qū)體溶液中并開啟超聲 在超聲場和溫度場的作用下溶液沸騰、蒸發(fā)、濃縮和燃燒，得到蓬松的前驅(qū)體粉末 將前驅(qū)體粉末在Ar+H2(H2的體積比為5%)氣氛中熱處理，在800℃保溫4 h后得到碳包覆NVP復(fù)合材料

1.2 樣品性能的表征

用X' Pert PRO型X射線衍射儀表征材料的物相，Cu靶，Kα 射線源(λ=0.15418 nm)，電壓和電流分別為40 kV和40 mA，掃描速度為10 (°)/min，掃描范圍2θ為10°~60° 用LabRAM HR800型拉曼光譜分析儀測試Raman譜(使用波長為532 nm的激光，波數(shù)范圍為1000~2000 cm–1) 用Nova NanoSEM 450掃描電鏡和Tecnai G2 F30透射電子顯微鏡觀察樣品的微觀形貌、顆粒尺寸和碳包覆 用3H-2000PM1型比表面積分析儀測試粉末的N2吸附-脫附性能

按質(zhì)量比8:1:1的比例將活性物質(zhì)碳包覆Na3V2(PO4)3、科琴黑和聚偏氟乙烯(PVDF)混合后研磨使之均勻，加入適量的N-甲基吡咯烷酮(NMP)繼續(xù)研磨后得到混合均勻的漿料 將漿料均勻地涂覆在鋁箔表面在80℃干燥12 h，然后用壓片機(jī)壓實(shí)并切成直徑為8 mm的小圓片作為正極 在充滿Ar氣的手套箱內(nèi)組裝CR2032型扣式電池(水和氧的含量均小于1×10–6，體積分?jǐn)?shù))，其中對電極和參考電極為金屬鈉片，隔膜為玻璃纖維，電解液為含有1 mol·L–1 NaClO4的EC+PC溶液(其中EC、PC的體積比為1:1)

在室溫下(25℃)測試電池的電化學(xué)性能，包括電化學(xué)阻抗(使用CHI600D型電化學(xué)工作站)、恒流充放電性能(使用CT2001A型Land電池測試系統(tǒng)) 對于Na3V2(PO4)3/C/GO電極，充放電電壓區(qū)間為2.3~3.9V，以1C電流密度測試充放電循環(huán)性能，測試周期為300圈 在1、2、5、10 C的電流密度(1C=117 mAh·g–1)下測試倍率性能，在每種電流密度下循環(huán)5圈

2 結(jié)果和討論2.1 NVP的組織結(jié)構(gòu)

圖1給出了GO添加量不同的碳包覆NVP的XRD譜 可以看出，GO添加量不同的樣品其衍射峰位和相對強(qiáng)度相似，衍射峰與NVP的JCPDS No: 53-0018相符合，表明三種樣品均為結(jié)晶性良好的NVP 在XRD譜中未觀察到GO的衍射峰，表明GO以納米片無序堆疊的形式存在[25] 可用Scherrer公式

D=Kλ/Bcosθ

計(jì)算粉末的平均晶粒尺寸，其中D為樣品的晶粒垂直于晶面方向的平均厚度(nm)，K為Scherrer常數(shù)(0.89)，B為試樣寬化(實(shí)測樣品最強(qiáng)衍射峰對應(yīng)的半峰寬扣除儀器固有寬化)，θ為最強(qiáng)衍射峰對應(yīng)的衍射角，λ為X-射線波長0.15418 nm 以(116)晶面計(jì)算的結(jié)果表明，添加GO含量分別為0.0%、2.5%和5.0%的NVP其晶粒尺寸分別為16.6 nm、16.9 nm、17.0 nm，相差不大

圖1



圖1碳包覆NVP的XRD譜

Fig.1XRD patterns of carbon-coated NVP

將前驅(qū)體溶液加熱至沸點(diǎn)(500℃)，沸騰時(shí)溶劑揮發(fā)，溶液濃縮，濃縮到一定程度溶質(zhì)開始分解 達(dá)到點(diǎn)燃時(shí)溶液燃燒發(fā)生氧化還原反應(yīng)，其中硝酸鹽為氧化劑，檸檬酸為還原劑 三種實(shí)驗(yàn)現(xiàn)象相似 結(jié)合XRD譜的結(jié)果表明，GO的加入未改變?nèi)紵铣煞磻?yīng)

圖2給出了自制GO和GO添加量不同的NVP的拉曼譜 可以看出，在三個(gè)NVP樣品的譜里都出現(xiàn)了碳的兩個(gè)特征峰，分別位于1300 cm–1(D峰)和1600 cm–1(G峰)附近，與無定形碳和石墨化碳對應(yīng) 未加入GO的樣品其G峰并不明顯，表明在SCS過程中生成的碳主要為無定形態(tài) 隨著GO添加量的增加G峰變得顯著，表明出現(xiàn)了石墨烯 根據(jù)峰面積計(jì)算出比值ID/IG，得到GO含量為0.0%、2.5%和5.0%三個(gè)樣品的ID/IG值分別為2.1、1.5和1.3，表明C的有序度提高了

圖2



圖2碳包覆NVP的拉曼譜

Fig.2Raman spectra of carbon-coated NVP

圖3給出了自制石墨烯納米片的TEM照片 可見納米片的尺寸約為幾個(gè)微米到十幾個(gè)微米，厚度約為幾個(gè)納米 圖4給出了GO添加量不同的NVP的SEM照片 可以看出，三種產(chǎn)物均由團(tuán)聚顆粒組成，每個(gè)團(tuán)聚顆粒都具有均勻的三維孔結(jié)構(gòu)，孔徑為1~5 μm 但是從圖4右上角的插圖可見，NVP團(tuán)聚顆粒的次級結(jié)構(gòu)不同 不添加GO的NVP團(tuán)聚顆粒具有珊瑚結(jié)構(gòu)，因?yàn)榇渭夘w粒相互聯(lián)結(jié)形成珊瑚結(jié)構(gòu)；GO添加量為2.5%時(shí)珊瑚結(jié)構(gòu)變少，而分散的顆粒增多；隨著GO的添加量增加到5.0%珊瑚結(jié)構(gòu)基本消失，團(tuán)聚顆粒由分散的小顆粒和一些片狀顆粒組成，分別是NVP顆粒和石墨烯

圖3



圖3石墨烯納米片的TEM照片

Fig.3TEM image of graphene nanosheets

圖4



圖4GO添加量不同的碳包覆NVP的SEM照片

Fig.4SEM images of carbon-coated NVP (a) 0.0%; (b) 2.5%; (c) 5.0%

用HRTEM進(jìn)一步觀察碳包覆NVP的顯微結(jié)構(gòu)，結(jié)果在圖5中給出 從圖5a可見，不添加GO時(shí)NVP顆粒相互聯(lián)結(jié)，宏觀上表現(xiàn)出珊瑚結(jié)構(gòu) 圖5d中的高倍TEM照片清楚地顯示出晶格條紋，晶格間距為0.217 nm，對應(yīng)NVP的(306)晶面 同時(shí)，在晶粒表面還觀察到了厚度為1~2 nm的無定形碳薄層 其原因是，在溶液燃燒過程中以C6H8O7作為燃燒反應(yīng)的還原劑和各金屬離子的絡(luò)合劑，在燃燒過程中富余的C6H8O7分解后在NVP表面形成了C包覆層 由圖5b可見，GO添加量為2.5%時(shí)大部分NVP顆粒不再彼此聯(lián)結(jié) 從圖5e中的高倍TEM照片可觀察到晶粒表面包覆著雙層碳，內(nèi)層是厚度為1~2 nm的無定形碳，外層是厚度為2~3 nm的GO 由圖5c可見，隨著GO的添加量增加到5.0% NVP顆粒趨近于球形，平均顆粒尺寸約為25 nm，分散在層片狀GO上 圖5f表明，NVP晶粒表面仍然具有雙層碳結(jié)構(gòu)，內(nèi)層是厚度為1~2 nm的無定形碳，外層GO的厚度為4~5 nm 由此可見，GO含量的提高使GO層的厚度增大

圖5



圖5GO添加量不同的NVP的HRTEM圖

Fig.5HRTEM images of NVP with different GO contents (a, d) 0.0%; (b, e) 2.5%; (c, f) 5.0%

在超聲輔助溶液燃燒過程中，前驅(qū)體溶液內(nèi)產(chǎn)生了大量的、均勻分布的空化泡 超聲引起的力學(xué)效應(yīng)，有助于小氣泡在流體介質(zhì)中的均勻分散 因此，燃燒后合成的產(chǎn)物具有均勻的蜂窩結(jié)構(gòu)，燃燒產(chǎn)物熱處理后這些蜂窩結(jié)構(gòu)形成均勻的三維網(wǎng)絡(luò)結(jié)構(gòu)[17] 另外，石墨烯具有大的比表面積，為NVP晶核的形成提供大量的形核點(diǎn)并抑制NVP晶粒的長大 因此，石墨烯的加入可阻止顆粒的團(tuán)聚，使顆粒均勻分布

圖6給出了GO添加量不同的碳包覆NVP的N2吸-脫附等溫曲線及孔徑分布 由圖6可見，隨著GO添加量的增加產(chǎn)物的比表面積增大，分別為3.05、5.49、8.74 m2·g–1 孔徑分布表明，GO添加量增加至5%，產(chǎn)物中尺寸為~2nm的微孔和~30nm的介孔的體積明顯增多 其原因，一方面，超聲產(chǎn)生的空化效應(yīng)有助于介孔和微孔的形成；另一方面，石墨烯的比表面積大且易卷曲，也有助于增大產(chǎn)物表面積和形成更多的介孔和微孔

圖6



圖6GO添加量不同的NVP的N2吸/脫附等溫曲線(內(nèi)嵌為孔徑分布圖)

Fig.6Nitrogen adsorption/desorption isotherms of NVP with different GO contents. Inset: the pore-size distribution plot calculated by the BJH method in the adsorption branch isotherm

2.2 NVP的電化學(xué)性能

圖7給出了不同GO添加量的NVP的電化學(xué)阻抗奈奎斯特圖 每個(gè)奈奎斯特圖都由一個(gè)位于中高頻區(qū)的半圓和一條位于低頻區(qū)的斜線組成 半圓表示電解質(zhì)和電極之間的電荷轉(zhuǎn)移電阻(Rct)，斜線表示鈉離子擴(kuò)散引起的Warburg電阻 根據(jù)圖7中的插圖的等效電路可以得到GO含量為0.0%、2.5%和5.0%的NVP其Rct分別為848、485和201 Ω·cm2 GO為5.0%的NVP阻抗最低，因?yàn)樽銐虻腉O使分散的NVP顆粒彼此連接，為電子傳輸構(gòu)建起三維通道，有助于提高NVP的動(dòng)力學(xué)特性

圖7



圖7GO添加量不同的NVP的電化學(xué)阻抗奈奎斯特圖

Fig.7Electrochemical impedance spectroscopy with different GO contents

圖8a給出了GO添加量不同的NVP在1 C電流密度下的充放電曲線，電壓窗口為2.3~3.9 V 可以看出，三種電極材料的充/放電曲線在3.4V左右均有一個(gè)平臺，對應(yīng)V3+/4+的轉(zhuǎn)變 圖8b給出了碳包覆NVP充放電的循環(huán)性能圖(1C的電流密度) GO含量為0.0%的NVP樣品其初始比容量為99 mAh·g–1，循環(huán)120圈后比容量衰減到23 mAh·g–1 GO含量為2.5%的NVP樣品其初始比容量為104 mAh·g–1，循環(huán)240圈后比容量衰減到70 mAh·g–1，容量保持率為67% GO含量為5.0%的樣品其初始比容量為117 mAh·g–1，與NVP的理論比容量接近 循環(huán)300圈后，比容量仍高達(dá)93 mAh·g–1，容量保持率為79% 圖8c給出了GO含量為5.0%的NVP的庫倫效率曲線，可見其首效為83%，5次循環(huán)后庫倫效率接近100% 這表明，NVP的結(jié)構(gòu)穩(wěn)定，硬碳和石墨烯所構(gòu)成的三維導(dǎo)電網(wǎng)絡(luò)有利于NVP的氧化還原反應(yīng)，從而增強(qiáng)其電化學(xué)循環(huán)穩(wěn)定性 圖8d給出了石墨烯添加量不同的NVP的倍率性能 在各個(gè)倍率下，GO添加量為5.0%的NVP的比容量最優(yōu) GO含量為5.0%的樣品在不同倍率下的首次放電比容量分別為117 mAh·g–1(1 C)、112 mAh·g–1(2 C)、108 mAh·g–1(5 C)、100 mAh·g–1(10 C)，相應(yīng)倍率下的比容量保持率分別為98%、95%、88% 這些結(jié)果表明，該樣品作為正極材料有很好的倍率性能，特別是在大倍率下電池依然具有較高的比容量

圖8



圖8GO添加量不同的NVP的電化學(xué)性能

Fig.8Electrochemical performance of NVP with different GO contents (a) charge-discharge profiles at 1 C; (b) cycling performance at 1 C; (c) coulombic efficiency of NVP (GO: 5.0%) at 1 C; (d) rate capability at various current rates from 1 C to 10 C

比較上述的電化學(xué)性能，GO添加量為5.0%的NVP具有良好的電化學(xué)性能 其原因是：(1)超聲輔助SCS過程及石墨烯的協(xié)同作用有助于高分散NVP納米顆粒的生成，納米尺寸的NVP顆粒有較小的Na+的遷移路徑，有利于Na+的高速脫嵌 (2)在SCS過程中生成的無定形碳包覆在NVP顆粒表面，促進(jìn)了電子轉(zhuǎn)移并能阻止NVP和電解液直接接觸，有利于提高界面穩(wěn)定性 (3)大片石墨烯連接納米NVP顆粒，為電子傳輸構(gòu)建起三維通道 因此，無定形碳和石墨烯的共同作用使NVP的導(dǎo)電性提高

3 結(jié)論

采用超聲輔助溶液燃燒合成技術(shù)可制備雙層碳包覆NVP復(fù)合材料，其顆粒表面由內(nèi)向外包覆著無定形碳和石墨烯 添加石墨烯有助于形成豐富的多級孔結(jié)構(gòu)，并提高NVP顆粒的分散性和降低其尺寸 石墨烯添加量為5.0%的GO和硬碳為NVP顆粒的電子傳輸構(gòu)建起了三維通道，使材料的電化學(xué)性能提高 在1 C倍率下充放電，NVP的初始比容量有117 mAh·g-1，循環(huán)300圈后容量保持率為79%，在10 C倍率下放電比容量高達(dá)100 mAh·g-1

參考文獻(xiàn)

View Option 原文順序文獻(xiàn)年度倒序文中引用次數(shù)倒序被引期刊影響因子

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2013