王 楠，燕紹九，彭思侃，陳 翔，戴圣龍

(中國航發(fā)北京航空材料研究院 石墨烯及應(yīng)用研究中心，北京 100095)

3D打印石墨烯制備技術(shù)及其在儲能領(lǐng)域的應(yīng)用研究進(jìn)展

王 楠，燕紹九，彭思侃，陳 翔，戴圣龍

(中國航發(fā)北京航空材料研究院 石墨烯及應(yīng)用研究中心，北京 100095)

石墨烯優(yōu)異的力學(xué)和物理性能使其成為理想的儲能材料。因結(jié)構(gòu)精確可控，易實現(xiàn)規(guī)?；苽?，3D打印石墨烯材料有望在儲能領(lǐng)域得到廣泛應(yīng)用。本文全面綜述了3D打印石墨烯制備技術(shù)及其在儲能領(lǐng)域的應(yīng)用研究進(jìn)展。石墨烯墨水的黏度和可打印性是實現(xiàn)石墨烯3D打印的制約因素。實現(xiàn)工藝簡單、濃度可控、無黏結(jié)劑石墨烯墨水的規(guī)?；蛴⒊蔀?D打印石墨烯制備技術(shù)未來的研究熱點。石墨烯超級電容器、鋰硫電池、鋰離子電池等儲能元件一體化打印成型是3D打印石墨烯在儲能領(lǐng)域應(yīng)用的發(fā)展方向。

3D打印；石墨烯；儲能；超級電容器；鋰離子電池

石墨烯納米片是從石墨材料中剝離出來、由碳原子組成的單層原子厚度的二維晶體，6個碳原子以sp2雜化方式構(gòu)成蜂窩狀晶格[1,2]。石墨烯展現(xiàn)出超高的強(qiáng)度，優(yōu)異的熱導(dǎo)率、透光率以及柔性輕質(zhì)特性[3]。此外，石墨烯具有巨大的比表面積和超高的電導(dǎo)率，其理論比表面積>2500m2·g-1[4]，電子遷移率高達(dá)200000cm2·V-1·s-1[5]。憑借獨特的物理、化學(xué)性能，石墨烯在納米電子、傳感器、復(fù)合材料、生物支架，尤其是儲能元件方面展現(xiàn)出廣闊的應(yīng)用前景[6-12]。

然而，研究發(fā)現(xiàn)二維石墨烯傾向于團(tuán)聚或堆疊以降低表面能，這極大削弱了石墨烯的優(yōu)異性能[13]。但三維石墨烯同時具備多孔結(jié)構(gòu)和石墨烯優(yōu)異的固有特性，這不僅為電荷儲存提供額外離子可接觸表面，同時有利于離子在其中傳輸[14]。從而使得由三維石墨烯構(gòu)成的器件有望在儲能領(lǐng)域表現(xiàn)更為突出。三維石墨烯制備技術(shù)存在的最大挑戰(zhàn)是在保留石墨烯固有優(yōu)異性能的同時，實現(xiàn)結(jié)構(gòu)可設(shè)計的規(guī)?；苽鋄15]。目前三維石墨烯的制備方法主要包括模板法和非模板法(表1)。模板法包括CVD法[16-20]、水熱/溶劑熱合成法[21,22]、電沉積法[18]、溶膠-凝膠合成法[23]。模板法可控制三維石墨烯內(nèi)部孔徑尺寸，但很難實現(xiàn)規(guī)模化制備。同時模板法制備出的三維石墨烯的力學(xué)性能通常并不理想，表現(xiàn)出可壓縮性差的特點[15]。非模板法包括真空抽濾法[24]、水熱凝膠合成法[25]、真空離心蒸發(fā)法[26]。非模板法雖能實現(xiàn)規(guī)模化制備，但三維石墨烯的形狀仍然存在隨機(jī)性。因此，以上三維石墨烯制備方法都無法同時滿足形狀精確可設(shè)計和可規(guī)?；苽涞囊?。3D打印技術(shù)具有打印結(jié)構(gòu)可設(shè)計，易實現(xiàn)快速、規(guī)?；圃斓膬?yōu)點。將3D打印技術(shù)應(yīng)用到三維石墨烯制備中，有望解決上述問題。

表1 三維石墨烯材料主要制備方法Table 1 Main methods to synthesize three dimensional graphene materials

3D打印技術(shù)，又稱增材制造(additive manufacturing)技術(shù)。根據(jù)美國材料與試驗協(xié)會(ASTM)公布的定義，增材制造是一種與傳統(tǒng)的材料加工方法截然相反的、基于三維CAD 模型數(shù)據(jù)、通過增加材料逐層制造的方式[30]?，F(xiàn)有3D打印技術(shù)種類眾多，主要包括熔融沉積成型(Fused Deposition Modeling，F(xiàn)DM)、直接噴墨打印技術(shù)(Direct Ink Writing，DIW)、立體平板印刷(Stereolithography，SLA)和選擇性激光熔化技術(shù)(Selective Laser Melting，SLM)等。同時，3D打印材料是3D打印技術(shù)發(fā)展的重要“物質(zhì)基礎(chǔ)”，決定3D打印技術(shù)能否有更廣泛的應(yīng)用。不同3D打印材料的特點不同，但打印材料的力學(xué)性能、加工性能、耐熱性、耐腐蝕性、化學(xué)穩(wěn)定性通常是制約3D打印技術(shù)發(fā)展的因素。目前，3D打印材料主要包括熱塑性塑料、光硬化樹脂、陶瓷材料、橡膠材料、金屬材料以及石墨烯等。3D打印技術(shù)及打印材料歸納于表2。

表2 3D打印技術(shù)及打印材料Table 2 3D printing technologies and materials

以熱塑性材料/石墨烯混合物作為打印材料的熔融沉積成型3D打印技術(shù)可被視為石墨烯3D打印技術(shù)的雛形。隨后，有研究者采用直接噴墨打印技術(shù)，以氧化石墨烯(GO)/聚合物或氧化石墨烯為墨水，通過逐層堆疊方式制備出多種結(jié)構(gòu)三維石墨烯。直接噴墨打印技術(shù)是最常用的石墨烯3D打印技術(shù)，該技術(shù)是將墨水裝入3D打印機(jī)噴墨打印腔室，對腔室內(nèi)的墨水施加一定壓力，墨水被從腔室前端噴頭擠出，層層堆疊，同時在三維空間移動噴頭即可打印出經(jīng)結(jié)構(gòu)設(shè)計的三維石墨烯。石墨烯3D打印技術(shù)不僅能實現(xiàn)三維石墨烯的規(guī)?；苽?，還能精確控制三維石墨烯的形狀，使其在儲能領(lǐng)域的工程應(yīng)用成為可能。隨著石墨烯3D打印技術(shù)的不斷進(jìn)步，今后有望實現(xiàn)儲能元件的整體、快速及規(guī)?；a(chǎn)。

1 3D打印石墨烯研究進(jìn)展

3D打印石墨烯研究以解決墨水可打印性的研究歷程為主線。起初，研究者們嘗試將石墨烯添加到熱塑性聚合物中以提高其導(dǎo)電性。隨著石墨烯3D打印技術(shù)的發(fā)展，直接噴墨打印技術(shù)成為石墨烯3D打印技術(shù)主流。其中，石墨烯墨水的黏度和可打印性是該打印技術(shù)的關(guān)鍵因素。然而，因石墨烯墨水黏度不足，可打印性差，隨后的研究工作都是圍繞解決石墨烯墨水可打印性展開的。導(dǎo)電聚合物以黏結(jié)劑的形式與石墨烯混合以增加墨水黏度。另外，在加入導(dǎo)電聚合物的同時，控制墨水的pH值也能提高墨水的可打印性。隨著打印技術(shù)的改進(jìn)，納米3D打印技術(shù)可將無黏結(jié)劑的氧化石墨烯墨水打印成納米尺度三維石墨烯。同時，冷凍鑄造方法也可實現(xiàn)石墨烯3D打印,從而實現(xiàn)低濃度(<13.35mg/mL)氧化石墨烯3D打印。研究發(fā)現(xiàn)，高濃度(>13.35mg/mL)氧化石墨烯墨水具有可打印性，這為純石墨烯打印提供了基礎(chǔ)。隨后，高濃度氧化石墨烯成功實現(xiàn)3D打印。高濃度氧化石墨烯3D打印被視為較具應(yīng)用潛力的石墨烯3D打印技術(shù)。

1.1 3D打印聚合物/石墨烯復(fù)合材料

熱塑性聚合物常被用于熔融沉積成型3D打印。為提高熱塑性材料電導(dǎo)率，將適量石墨烯以導(dǎo)電添加劑形式與熱塑性材料混合，打印出聚合物/石墨烯復(fù)合材料。Wei等[35]采用熔融沉積成型打印技術(shù)，打印出丙烯腈-丁二烯-苯乙烯共聚物(ABS)/石墨烯復(fù)合材料。復(fù)合材料中石墨烯的質(zhì)量分?jǐn)?shù)最高為5.6%。為避免團(tuán)聚，采用氧化石墨烯作為復(fù)合材料導(dǎo)電添加劑的前驅(qū)體，并在混合后采用肼將氧化石墨烯還原。

3D打印材料制備過程：首先將氧化石墨烯和ABS分別溶于N-甲基吡咯烷酮(NMP)中，隨后將兩種溶液混合攪拌。氧化石墨烯經(jīng)肼還原后，放入水中以使ABS/還原氧化石墨烯復(fù)合材料與NMP分離。最后，經(jīng)真空干燥得到ABS/還原氧化石墨烯復(fù)合材料，后經(jīng)擠壓成細(xì)絲。打印過程中復(fù)合材料細(xì)絲被加熱到玻璃化溫度以上，經(jīng)噴嘴擠出，以堆疊方式進(jìn)行3D打印。研究發(fā)現(xiàn)，石墨烯的質(zhì)量分?jǐn)?shù)在5.6%以下時，復(fù)合材料可從噴嘴平滑地擠出。但當(dāng)石墨烯的質(zhì)量分?jǐn)?shù)增加至7.4%時，石墨烯發(fā)生團(tuán)聚，從而堵塞噴嘴。如采用高能量分散技術(shù)，可使石墨烯分散更均勻，石墨烯含量也可進(jìn)一步提高。電導(dǎo)率測試結(jié)果表明，復(fù)合材料的電導(dǎo)率隨石墨烯含量的增加而顯著增加。復(fù)合材料的導(dǎo)電機(jī)理可以用滲流模型[78]解釋，當(dāng)石墨烯聯(lián)結(jié)成導(dǎo)電網(wǎng)絡(luò)后，電導(dǎo)率出現(xiàn)突變。石墨烯的分散性直接影響復(fù)合材料的可打印性和其他物理性能。為此，改進(jìn)石墨烯納米片在復(fù)合材料中的分散性是后續(xù)研究工作重點。以石墨烯作為導(dǎo)電添加劑的3D打印聚合物/石墨烯復(fù)合材料受制于石墨烯難以分散的問題，石墨烯含量難以提高，嚴(yán)重影響復(fù)合材料導(dǎo)電性，同時也未充分發(fā)揮石墨烯本身優(yōu)異特性。除大功率超聲分散技術(shù)外，采用石墨烯表面化學(xué)修飾以及使用表面活性劑等方法也可解決石墨烯難以分散的問題。

除將石墨烯作為導(dǎo)電添加劑外，還希望用高石墨烯含量墨水打印三維石墨烯，以最大限度利用三維石墨烯本身高比表面積、低密度、高電導(dǎo)率等優(yōu)異性能。然而，要實現(xiàn)這一目標(biāo)，3D打印石墨烯墨水需要滿足如下要求：(1)墨水應(yīng)具有一定黏彈性，以滿足墨水持續(xù)流動性，并確保打印后能保持打印形狀；(2)墨水具有一定機(jī)械強(qiáng)度，以承受后續(xù)堆疊墨水的質(zhì)量而不發(fā)生變形，同時還能附著在前一層石墨烯上。由于石墨烯/氧化石墨烯墨水黏彈性不足等問題，純石墨烯/氧化石墨烯墨水打印在成型上還存在很多困難。研究者們通常在石墨烯/氧化石墨烯墨水中加入聚合物作為黏結(jié)劑以提高墨水黏彈性。

Jakus等[39]采用基于噴出原理的直接噴墨打印技術(shù)，制備出高石墨烯含量的石墨烯/PLG(polyester polylactide-co-glycolide)復(fù)合材料。將不同比例石墨烯與PLG在二氯甲烷(DCM)中混合，同時溶液中還加入表面活性劑2-丁氧基乙醇以及塑化劑酞酸二丁酯。均勻攪拌溶液，直到靜態(tài)剪切速率黏度達(dá)到30Pa·s左右。打印過程中石墨烯/聚合物墨水從直徑100μm噴嘴，以大于40mm/s的速率噴出，層層堆疊出網(wǎng)格結(jié)構(gòu)。力學(xué)性能測試結(jié)果表明，石墨烯含量超過20%(體積分?jǐn)?shù)，下同)后，復(fù)合材料抗拉強(qiáng)度與石墨烯含量成反比。含20%石墨烯的復(fù)合材料抗拉強(qiáng)度較純PLG材料有所提高，應(yīng)變達(dá)210%。然而含60%石墨烯的復(fù)合材料抗拉強(qiáng)度出現(xiàn)顯著下降，應(yīng)變?yōu)?1%。這一結(jié)果被解釋為，拉伸載荷最初由PLG彈性體承擔(dān)，石墨烯含量增加并超過40%后，PLG彈性體無法包覆并緊固周圍的石墨烯片，從而導(dǎo)致復(fù)合材料的強(qiáng)度和彈性模量都大幅降低。研究表明，擠壓過程中剪切力使得石墨烯片沿著流動方向重新定位和排列，導(dǎo)致打印出的復(fù)合材料具有各向異性結(jié)構(gòu)與性能。電導(dǎo)率測試結(jié)果表明，復(fù)合材料沿擠出方向的電導(dǎo)率與噴嘴直徑成反比。這一規(guī)律很可能與擠出細(xì)絲的微觀結(jié)構(gòu)有關(guān)，小口徑噴嘴在墨水?dāng)D出過程中能提供更大的切變速率，使得石墨烯片排列更加整齊。另外，經(jīng)測量復(fù)合材料電導(dǎo)率大于800S·m-1，且隨石墨烯含量的增加而增加。該研究工作利用聚合物作為黏結(jié)劑，成功實現(xiàn)石墨烯/聚合物復(fù)合材料3D打印。同時，對擠出過程中剪切應(yīng)力對復(fù)合材料力學(xué)性能和電學(xué)性能的探討為后續(xù)研究提供可借鑒的依據(jù)。然而，該研究工作仍未能實現(xiàn)純石墨烯的3D打印。

1.2 3D打印低濃度氧化石墨烯

雖然在墨水中添加黏結(jié)劑可增加墨水的黏性，保證三維石墨烯的良好成型性。然而，引入黏結(jié)劑會在一定程度上影響3D打印三維石墨烯的導(dǎo)電性。研究表明，將特殊成型工藝與直接噴墨打印技術(shù)相結(jié)合可打印出無黏結(jié)劑的三維石墨烯。Kim等[41]將直接噴墨打印技術(shù)與微打印技術(shù)相結(jié)合，成功實現(xiàn)無黏結(jié)劑石墨烯納米尺度3D打印。氧化石墨烯墨水濃度為1g·L-1，氧化石墨烯片徑尺寸為1，3，5μm。打印過程中，氧化石墨烯墨水被裝入微吸液管中，室溫下擠壓石墨烯墨水，同時利用彎液面成型原理在滴管尖端形成納米線，并進(jìn)行局部堆疊。微吸液管尖端直徑為1.3μm，石墨烯納米線直徑為150nm。石墨烯納米線直徑與墨水?dāng)D出速率直接相關(guān)，可通過控制墨水?dāng)D出速率來控制石墨烯納米線直徑。隨后，將得到的氧化石墨烯進(jìn)行熱還原或化學(xué)還原以得到還原氧化石墨烯。電導(dǎo)率測試結(jié)果表明，室溫下單根石墨烯納米線的電導(dǎo)率為11.3S·cm-1，同時以單根石墨烯納米線為導(dǎo)線連接兩個金電極組成的電路可以點燃一個LED燈。該研究工作成功實現(xiàn)石墨烯微觀尺度3D打印，同時還能精確控制納米絲線直徑。該打印技術(shù)能在微尺度上打印出多種結(jié)構(gòu)三維石墨烯器件，這些器件有望應(yīng)用在電子設(shè)備上。然而，如何將石墨烯納米線直徑控制在10nm以下仍然是一項挑戰(zhàn)。要減小納米線尺寸，3D打印過程中墨水流動性的細(xì)節(jié)還有待研究，如墨水黏度研究，噴嘴尺寸對納米線直徑的影響規(guī)律研究等。同時，該打印技術(shù)僅限于微觀尺度打印，低濃度氧化石墨烯墨水宏觀尺度打印還有待進(jìn)一步研究。

除納米尺度3D打印，Zhang等[42]也成功實現(xiàn)無黏結(jié)劑宏觀尺度石墨烯3D打印，如圖1所示。與其他通過熔化或者室溫下噴射的石墨烯3D打印方法不同，該方法通過純氧化石墨烯溶液多管嘴噴墨和冷凍鑄造技術(shù)來實現(xiàn)快速石墨烯3D打印。溶液中純水結(jié)冰后起到支撐結(jié)構(gòu)作用。隨后，將三維石墨烯放入液氮中，冷凍干燥以去除水分。最后，經(jīng)熱還原得到3D打印超輕三維石墨烯。石墨烯墨水濃度范圍為0.5～10mg·mL-1。

圖1 3D打印與冷凍鑄造法制備三維石墨烯示意圖[42](a)3D打印裝置；(b)3D打印冰支撐；(c)3D打印氧化石墨烯；(d)3D打印石墨烯冰結(jié)構(gòu)液氮處理；(e)冷凍干燥；(f)熱還原成超輕3D氣凝膠Fig.1 Schematic diagrams of three dimensional graphene prepared by integrating process of 3D printing and freeze casting[42](a)3D printing setup;(b)3D printing of ice support；(c)3D printing of GO suspension；(d)immersing printed ice structure into liquid nitrogen；(e)freeze drying；(f)thermally reduced to 3D ultralight graphene aerogels

為研究3D打印石墨烯電子傳輸行為，對-192～400℃范圍內(nèi)3D打印石墨烯電阻變化進(jìn)行測試。結(jié)果表明，電阻變化表現(xiàn)為負(fù)溫度系數(shù)，并且出現(xiàn)5個不同區(qū)域。當(dāng)實驗溫度降低到-192℃時，石墨烯電阻急劇增加了97%。當(dāng)實驗溫度在-40～60℃之間時，石墨烯電阻保持穩(wěn)定狀態(tài)。但是當(dāng)溫度升高到350℃時，其電阻降低了48%。隨后，當(dāng)溫度繼續(xù)升高到400℃時，其電阻沒有明顯變化。實驗結(jié)果表明，3D打印石墨烯電阻率和電導(dǎo)率隨溫度的變化規(guī)律為典型半導(dǎo)體行為。同時，3D打印三維石墨烯的電導(dǎo)率隨三維石墨烯密度的增加而增加。當(dāng)三維石墨烯密度從0.5mg·cm-3增加到10mg·cm-3，其電導(dǎo)率從2.2S·m-1增加到15.4S·m-1。

內(nèi)面壓縮測試和動態(tài)形變場測試結(jié)果表明，3D打印石墨烯具有高度可壓縮性。6.21mg三維石墨烯可承重100g而不發(fā)生畸變，其載重比高達(dá)16100。三維石墨烯表現(xiàn)出非線性超彈性行為和可逆超大壓縮性，并且最大應(yīng)變高達(dá)50%。同時，三維石墨烯在50%應(yīng)變下的應(yīng)力與其密度呈線性關(guān)系。三維石墨烯的楊氏模量與密度的關(guān)系可以用方程E=Aρn表示[79]，其中E為彈性模量，ρ為有效密度，A為常數(shù)，n為指數(shù)。3D打印石墨烯的楊氏模量與密度之間關(guān)系為：E∞ρ1.4，與通常由水熱法或冷凍干燥法制備出的塊體三維石墨烯指數(shù)(n=2.5)不同。方程指數(shù)減小與3D打印石墨烯獨特的宏觀中空結(jié)構(gòu)有關(guān)。同時，該獨特結(jié)構(gòu)可使其在密度低于1mg·cm-3時仍然保持高楊氏模量。該研究巧妙地將噴墨打印與冷凍鑄造技術(shù)相結(jié)合，實現(xiàn)無添加劑石墨烯3D打印。該方法可打印出不同密度三維石墨烯，甚至可低于1mg·cm-3。然而，打印過程所要求的低溫環(huán)境將限制其向規(guī)?；苽浞较虻陌l(fā)展。

1.3 3D打印高濃度氧化石墨烯

研究發(fā)現(xiàn)，低濃度氧化石墨烯溶液表現(xiàn)出類液晶性質(zhì)[80]。當(dāng)氧化石墨烯濃度超過13.35mg·mL-1[81]時，其獨特的黏彈性行為使得氧化石墨烯溶液可被用于凝膠噴射打印工藝。2015年，Zhu等[29]采用直接噴墨打印技術(shù)，以高濃度氧化石墨烯為墨水打印出具有優(yōu)異可壓縮性的三維石墨烯宏觀體。墨水流變性能可通過其表觀黏度與剪切速率以及模量與剪切應(yīng)力之間的關(guān)系進(jìn)行表征(圖2)。氧化石墨烯墨水濃度分別為20，40mg·mL-1，采用碳酸銨和R-F(間苯二酚-甲醛溶液)2種凝膠化添加劑。結(jié)果表明，氧化石墨烯濃度對墨水的流變性能影響巨大。當(dāng)濃度從20mg·mL-1增加到40mg·mL-1時，墨水表觀黏度增加近1個數(shù)量級(圖2(a))；同樣地，其儲能模量和損耗模量也分別增加1個數(shù)量級(圖2(b))。為進(jìn)一步增加石墨烯墨水黏性，改善流動性，還可向其中加入氣相二氧化硅粉末。研究表明，加入后明顯增強(qiáng)氧化石墨烯墨水的流變性能，其表觀黏度、儲能模量及損耗模量較純氧化石墨烯墨水都增加1個數(shù)量級。

打印過程中氧化石墨烯墨水從噴嘴擠出，堆疊出三維結(jié)構(gòu)，如圖3所示[43]。打印出的宏觀體被密封在玻璃瓶中，并在85℃環(huán)境下加熱，以使其凝膠化。石墨烯中的二氧化硅可通過氫氟酸刻蝕法去除。隨后，在1050℃高溫下采用超臨界二氧化碳干燥法干燥三維石墨烯宏觀體。研究發(fā)現(xiàn)，通過改變墨水中凝膠化物質(zhì)成分可調(diào)控三維石墨烯微觀結(jié)構(gòu)。3D打印得到的三維石墨烯的物理性能如表3所示[29]，三維石墨烯的物理性能可以通過改變初始氧化石墨烯溶液中添加的R-F溶液濃度加以調(diào)節(jié)。力學(xué)性能測試結(jié)果表明，3D打印所得到的三維石墨烯具有優(yōu)異的應(yīng)變記憶效果，超強(qiáng)可壓縮性，壓縮變形量可達(dá)90%。2016年，Zhu等[43]在原有高濃度氧化石墨烯墨水中摻入不同比例的石墨烯納米片，進(jìn)一步提高三維石墨烯電導(dǎo)率。另外，該三維石墨烯作為電極材料被進(jìn)一步應(yīng)用在超級電容器中。

圖2 添加與未添加二氧化硅時氧化石墨烯墨水的流變性能[29](a)剪切速率與表觀黏度的關(guān)系；(b)剪切應(yīng)力與儲能模量和損耗模量的關(guān)系Fig.2 Rheological properties of GO ink with and without silica fillers[29](a)apparent viscosity as a function of shear rate；(b)storage modulus and loss modulus as a function of shear stress

圖3 高濃度氧化石墨烯3D打印示意圖[43]Fig.3 Schematic diagrams of high concentration GO 3D printing[43]

目前，40mg·mL-1氧化石墨烯墨水已具備良好可打印性[29]。研究表明，提高氧化石墨烯墨水濃度會進(jìn)一步增強(qiáng)墨水可打印性。Fu等[44]采用高濃度氧化石墨烯(85mg·mL-1)，成功打印出鋰離子正極材料磷酸鐵鋰(LiFePO4，LFP)/還原氧化石墨烯、負(fù)極材料鈦酸鋰(Li4Ti5O12，LTO)/還原氧化石墨烯、固態(tài)電解質(zhì)及隔膜材料，實現(xiàn)鋰離子電池主要部件整體打印。正極材料打印墨水為高濃度氧化石墨烯與磷酸鐵鋰混合物。負(fù)極材料打印墨水為高濃度氧化石墨烯與鈦酸鋰混合物。聚偏氟乙烯共聯(lián)六氟丙烯(PVDF-co-HFP)與氧化鋁顆粒混合物墨水兼具電解質(zhì)及隔膜材料作用。研究表明，3種墨水表現(xiàn)出相似的剪切稀化行為，說明它們都是非牛頓流體，可被用作打印墨水。另外，3種墨水在1s-1切變速率下，表觀黏度高達(dá)102～103Pa·s。高表觀黏度保證了墨水的可打印性，也為復(fù)雜結(jié)構(gòu)打印提供了基礎(chǔ)。同時，3種墨水經(jīng)長時間靜置仍能保持穩(wěn)定的表觀黏度。墨水的儲能模量(G′)和損耗模量(G″)分別表征其彈性和黏性性能。電極材料墨水的平臺儲能模量和損耗模量在104～105Pa和103～104Pa內(nèi)。2種墨水屈服應(yīng)力都高達(dá)103Pa。墨水的高平臺模量和高屈服應(yīng)力對打印過程中墨水成型性至關(guān)重要。打印過程如圖4所示[44]。2種電極材料打印墨水分別貯存在2個注射器中，墨水通過壓力從噴嘴中擠出，層層堆疊出打印結(jié)構(gòu)。打印產(chǎn)物經(jīng)過冷凍干燥去除水分并得到三維結(jié)構(gòu)，隨后經(jīng)熱還原處理將氧化石墨烯還原。熱還原處理工序?qū)μ岣?D打印三維石墨烯電導(dǎo)率起到至關(guān)重要的作用。電導(dǎo)率測試結(jié)果表明，經(jīng)熱還原后2種電極材料的電導(dǎo)率分別為31.6，6.1S·cm-1，明顯大于未經(jīng)熱處理的電極材料的電導(dǎo)率(10-6～10-7S·cm-1)。

表3 不同配方3D打印石墨烯凝膠的物理性能[29]Table 3 Physical properties of different 3D printed graphene aerogel formulations[29]

圖4 3D打印鋰離子電池石墨烯電極示意圖[44](a)以LTO/GO為墨水3D打印電池負(fù)極；(b)以LFP/GO為墨水3D打印電池正極；(c)3D打印固態(tài)電解質(zhì)Fig.4 Schematic diagrams of 3D printing graphene based electrodes for lithium ion batteries[44](a)LTO/GO ink used to print the anode via 3D printing；(b)LFP/GO ink used to print the cathode via 3D printing；(c)3D printing of solid-state electrolyte

值得一提的是，以氧化石墨烯作為墨水的打印工藝都需要經(jīng)熱還原處理將氧化石墨烯還原成石墨烯。為簡化打印工藝，未來可在3D打印機(jī)上增加同步加熱裝置，進(jìn)而實現(xiàn)3D打印氧化石墨烯同步還原技術(shù)。另外，儲能元件各部件同時采用3D打印的高度集成化打印工藝，即一體化3D打印技術(shù)，也將為3D打印石墨烯規(guī)?；苽涞於ɑA(chǔ)。

2 3D打印石墨烯在儲能領(lǐng)域中的應(yīng)用研究

因兼具規(guī)模化制備、形狀可控以及結(jié)構(gòu)可設(shè)計的特點，3D打印技術(shù)已被用于鋰離子電池和超級電容器電極材料的制備中。Sun等[82]采用磷酸鐵鋰LiFePO4(LFP)和鈦酸鋰Li4Ti5O12(LTO)作為打印墨水，設(shè)計并打印出鋰離子電池的正負(fù)極。隨后組裝成微電池，并進(jìn)行電化學(xué)性能測試。在2.7mW·cm-2功率密度下，得到的面能量密度為9.7J·cm-2，這使其在微電子和生物醫(yī)療設(shè)備上的應(yīng)用潛力巨大。Zhao等[83]采用選擇性激光熔化3D打印技術(shù)打印出三維鈦金屬電極，并在金屬電極表面沉積聚吡咯。該電極被用作超級電容器極板，電化學(xué)測試結(jié)果表明，在37.4mA·cm-3電流密度下，電極材料體積比電容為2.4F·cm-3，體積能量密度為213.5Wh·m-3，體積功率密度為15.0kW·m-3。三維石墨烯具有大的比表面積、優(yōu)異的化學(xué)穩(wěn)定性，尤其是優(yōu)異的導(dǎo)電性，已經(jīng)被廣泛應(yīng)用于鋰離子電池、鋰硫電池、超級電容器等儲能元件中。然而隨著儲能元件應(yīng)用的發(fā)展，其對元件形狀及尺寸精度也提出更高的要求。為此，研究者采用3D打印技術(shù)制備三維石墨烯，并將其應(yīng)用到超級電容器和鋰離子電池電極上。

Nathan-Walleser等[84]以高濃度還原氧化石墨烯為3D打印墨水，采用3D微噴出打印技術(shù)制備出超級電容器電極材料。與其他打印墨水制備工藝不同的是，氧化石墨烯首先在400℃高溫下被部分還原成石墨烯，再經(jīng)高壓勻質(zhì)機(jī)加工得到均勻分散的打印墨水。同時，由于得到的還原氧化石墨烯表面仍存在含氧官能團(tuán)，使其仍然易分散于水、酒精及異丙醇等溶劑中形成打印墨水。該工藝無須向墨水中添加黏結(jié)劑和分散劑，從而避免了因黏結(jié)劑等非導(dǎo)電物質(zhì)的殘留對墨水導(dǎo)電性造成的影響。濃度為15g·L-1的石墨烯墨水電導(dǎo)率高達(dá)16S·cm-1。石墨烯墨水經(jīng)3D微噴出打印機(jī)打印成圓盤狀電極材料，并組裝成超級電容器電極。電化學(xué)測試結(jié)果表明，電容器表現(xiàn)出典型的雙電層電容特性。在全范圍掃速下，其電壓-電流曲線都幾乎保持規(guī)則的矩形形狀，即使在15V·s-1下，石墨烯電極的體積比容量仍為4F·cm-3。這表明該電極具有快速的電流響應(yīng)速率，同時電解液在電極表面發(fā)生了快速擴(kuò)散。另外，在體積能量密度4.43mWh·cm-3下，其體積功率密度為42.74kW·cm-3。

Sun等[85]采用微噴出打印技術(shù)打印出平面超級電容器微電極。石墨烯墨水濃度高達(dá)20mg·mL-1，并且墨水中未添加黏結(jié)劑和分散劑。打印過程采用微噴出打印技術(shù)制備出逐層堆疊結(jié)構(gòu)的氧化石墨烯薄膜電極。該結(jié)構(gòu)表現(xiàn)出高機(jī)械強(qiáng)度、高電導(dǎo)率以及良好的電解液側(cè)向滲透性。經(jīng)測量，4層結(jié)構(gòu)電極材料的電導(dǎo)率高達(dá)(114.0±17.7)S·cm-1。電化學(xué)測試結(jié)果表明，電容器表現(xiàn)出典型的雙電層電容特性，在0.06A·cm-3電流密度下，超級電容器的體積能量密度和體積功率密度分別為7mWh·cm-3和30mV·cm-3。為驗證該電極材料應(yīng)用于柔性電子器件的可能性，將石墨烯墨水打印在PET薄膜上并進(jìn)行電化學(xué)性能測試(圖5)，其電壓-電流曲線并未因電極的彎曲而發(fā)生變化。充放電循環(huán)過程中，初始容量為38.4F·cm-3，經(jīng)300次循環(huán)后容量下降到約20.3F·cm-3，并且在隨后的10000次循環(huán)過程中逐漸保持穩(wěn)定。

圖5 分層結(jié)構(gòu)微型超級電容器電極電化學(xué)性能測試結(jié)果[85](a)平直和彎曲狀態(tài)下電容器的CV曲線；(b)電容器體積比容量與充放電循環(huán)次數(shù)的關(guān)系Fig.5 Electrochemical performance of planar micro-supercapacitors electrodes[85](a)CVs at flat or bent states；(b)dependence of volumetric capacitance on charge-discharge cycling numbers

Zhu等[43]采用高濃度氧化石墨烯作為3D打印墨水，并將3D打印石墨烯用作超級電容器電極材料。為提高電極材料導(dǎo)電性，墨水中還添加適量石墨烯納米片。與未添加石墨烯納米片3D打印石墨烯相比，添加適量石墨烯納米片可以有效降低電極內(nèi)阻，同時電極材料仍然具有較高比表面積。3D打印石墨烯電極容量性能十分優(yōu)異，當(dāng)電流密度從0.5A·g-1增加到10A·g-1時，石墨烯電極容量保留率達(dá)到90%。如果考慮電極厚度，該性能甚至更優(yōu)異，因為3D打印石墨烯電極厚度為1mm，而其他大多數(shù)電極厚度都不到100μm。另外，該電極性能極其穩(wěn)定，經(jīng)過10000次充放電循環(huán)后容量保持率高達(dá)95.5%，高于其他研究報告中的性能(見表4)。3D打印三維石墨烯電極表現(xiàn)出來的良好性能，一方面，得益于電極中石墨烯納米片的加入，另一方面，得益于三維石墨烯中的大孔徑結(jié)構(gòu)，這有利于提高離子的傳輸速率。需要強(qiáng)調(diào)的是，由于打印過程中由管嘴造成的剪切應(yīng)力使得墨水中的石墨烯沿著噴出方向排列，從而在電極中產(chǎn)生有序的大孔徑，有助于厚電極結(jié)構(gòu)中的質(zhì)量傳輸，并有效降低傳輸過程中的內(nèi)阻。隨后，3D打印石墨烯被組裝成準(zhǔn)固態(tài)對稱超級電容器，其功率密度和能量密度的測試結(jié)果也非常優(yōu)秀。超級電容器在能量密度0.26Wh·kg-1下，具有最大功率密度4079.9W·kg-1；而在功率密度76.36W·kg-1下，具有最大能量密度0.43Wh·kg-1(能量密度和功率密度計算基于整個設(shè)備質(zhì)量)。

隨后，Liu等[100]采用電化學(xué)離子插層法對3D打印石墨烯進(jìn)行改性，并將其應(yīng)用到超級電容器上。電化學(xué)離子插層法分為兩步，包括鋰離子插層和高氯酸根離子插層。隨后高氯酸根離子插層化合物發(fā)生分解，從而在石墨烯表面功能化含氧基團(tuán)。電化學(xué)離子插層法是一種易實現(xiàn)且環(huán)境友好的石墨烯功能化方法。被用作超級電容器電極材料的功能化石墨烯能有效提高超級電容器的電化學(xué)性能。測試結(jié)果表明，在0.5A·g-1電流密度下，石墨烯電極的比容量達(dá)到124.7F·g-1，是相同電流密度下未經(jīng)功能化石墨烯電極的兩倍。當(dāng)電流密度達(dá)到10A·g-1時，石墨烯電極的比容量仍能達(dá)到101.7F·g-1。另外，該電極在200mV·s-1掃速下，經(jīng)10000次充放電循環(huán)后，其容量保持率高達(dá)97%。功能化3D打印石墨烯電極(3DG-N-Ox)與原始3D打印石墨烯電極(3DG)電化學(xué)結(jié)果對比如表5所示[100]。石墨烯電極電化學(xué)性能的顯著提高與石墨烯表面功能化后比表面積增大以及表面存在的含氧官能團(tuán)有關(guān)。該研究為3D打印石墨烯功能化提供一種簡單易行的方法，同時也是提高超級電容器電化學(xué)性能的一種有效方法。

表4 超級電容器三維石墨烯電極電容性能Table 4 Capacitive performance of three dimensional graphene electrodes for supercapacitors

表5 3D打印石墨烯與功能化3D打印石墨烯電極電化學(xué)性能[100]Table 5 Electrochemical performance of 3DG and 3DG-N-Ox[100]

Note:rate capability was from 0.5A·g-1to 10A·g-1.

3D打印石墨烯除在超級電容器上有著廣泛應(yīng)用外，F(xiàn)u等[44]應(yīng)用3D打印技術(shù)打印出帶有活性物質(zhì)的鋰離子電池正極(LiFePO4，LFP)/還原氧化石墨烯、負(fù)極(Li4Ti5O12，LTO)/還原氧化石墨烯、隔膜以及固態(tài)電解液。對半電池及全電池進(jìn)行電化學(xué)性能測試，如圖6所示，在10mA·g-1電流密度下，3D打印磷酸鐵鋰正極材料充放電比容量約為168mAh·g-1和164mAh·g-1(圖6(a))，該比容量明顯優(yōu)于表6中其他鋰離子電池正極材料。同時，第10次和第20次充放電循環(huán)對應(yīng)的比容量已接近磷酸鐵鋰?yán)碚摫热萘?70mAh·g-1。這表明石墨烯/磷酸鐵鋰正極材料具有優(yōu)異的容量性能。同樣，在10mA·g-1電流密度下，3D打印鈦酸鋰負(fù)極材料充放電比容量約為184mAh·g-1和185mAh·g-1(圖6(c))。這一數(shù)值略高于鈦酸鋰?yán)碚摫热萘?175mAh·g-1)，這可能源于還原氧化石墨烯對比容量的貢獻(xiàn)。倍率性能測試結(jié)果表明，與正極材料相比，負(fù)極材料的容量衰減更大。這可能與電極材料電導(dǎo)率有關(guān)。正負(fù)極材料電導(dǎo)率分別為31.6S·cm-1和6.1S·cm-1，負(fù)極材料電導(dǎo)率明顯小于正極材料電導(dǎo)率，較低的電導(dǎo)率會導(dǎo)致更大的容量衰減。這一問題可通過向負(fù)極材料中添加高導(dǎo)電物質(zhì)來提高材料電導(dǎo)率。另外，在50mA·g-1電流密度下，全電池初始充放電比容量分別為117mAh·g-1和91mAh·g-1。結(jié)果表明，以3D打印石墨烯為電極材料的鋰離子電池展現(xiàn)出優(yōu)異的電化學(xué)性能，這為鋰離子電池一體化打印提供了支持。

3 結(jié)束語

三維石墨烯具有多孔結(jié)構(gòu)、超大的比表面積以及超高的電導(dǎo)率，使其在儲能領(lǐng)域得到廣泛應(yīng)用。3D打印技術(shù)有望實現(xiàn)三維石墨烯結(jié)構(gòu)精確可控及規(guī)?；苽洹Ｈ欢?，石墨烯3D打印作為一種新興的三維石墨烯制備工藝，還有一些問題有待進(jìn)一步研究。石墨烯墨水的黏度及其可打印性是制約石墨烯3D打印發(fā)展的重要因素。通常，氧化石墨烯墨水因黏度不足無法實現(xiàn)有效打印，須添加聚合物作為黏結(jié)劑以提高墨水黏度，但引入聚合物會降低三維石墨烯的電導(dǎo)率。將3D打印與納米尺度3D打印技術(shù)或冷凍鑄造技術(shù)相結(jié)合可實現(xiàn)無黏結(jié)劑石墨烯墨水3D打印。但是該類打印工藝條件苛刻，難以實現(xiàn)三維石墨烯規(guī)?；苽洹Ｗ钚卵兄瞥晒Φ母邼舛妊趸?D打印技術(shù)被視為最具應(yīng)用潛力的制備方法。高濃度氧化石墨烯3D打印無需黏結(jié)劑，同時可有效提高三維石墨烯電導(dǎo)率。打印得到的三維石墨烯具有高電導(dǎo)率、輕質(zhì)以及高度可壓縮性等優(yōu)點。這些都為實現(xiàn)三維石墨烯結(jié)構(gòu)設(shè)計和規(guī)?；苽涮峁┯辛χС?。然而，目前采用高濃度氧化石墨烯墨水打印得到的三維石墨烯的比表面積還有待進(jìn)一步提高。如何實現(xiàn)工藝簡單、濃度可控的石墨烯3D打印成為目前亟待解決的問題。研制3D打印氧化石墨烯同步還原技術(shù)對簡化石墨烯3D打印制備工藝、拓展石墨烯3D打印的規(guī)?；瘧?yīng)用具有重要意義。3D打印石墨烯已被用作超級電容器和鋰離子電池的電極材料，其容量性能均得到顯著提高。除超級電容器和鋰離子電池外，3D打印石墨烯還有望應(yīng)用于鋰硫電池、燃料電池、太陽能電池等眾多儲能元件中。石墨烯儲能元件一體化打印成型是3D打印石墨烯在儲能領(lǐng)域應(yīng)用的發(fā)展方向。

圖6 3D打印石墨烯電極電化學(xué)性能[44](a)LFP/rGO半電池充放電曲線；(b)不同電流密度下LFP/rGO半電池的倍率性能曲線；(c)10mA·g-1電流密度下LTO/rGO半電池充放電曲線；(d)不同電流密度下LTO/rGO半電池的倍率性能曲線；(e)3D打印全電池循環(huán)穩(wěn)定性；(f)3D打印全電池充放電曲線Fig.6 Electrochemical performance of 3D-printed graphene electrodes[44](a)charge and discharge curves of the LFP/rGO half-cell；(b)rate curves of the LFP/rGO half-cell at various currents；(c)charge and discharge curves of the LTO/rGO half-cell at a current of 10mA·g-1；(d)rate curves of the LTO/rGO half-cell at various currents；(e)cycling stability of the 3D-printed full cell；(f)charge and discharge curves of the 3D-printed full cell

MaterialCyclingperformanceRateperformanceCurrentdensity/(mA·g-1)Initialcapacity/(mAh·g-1)CycleRemaincapacity/(mAh·g-1)Capacityretention/%Currentdensity/(mA·g-1)Capacity/(mAh·g-1)ReferenceG/TiO20.5C31410019762.720C124.0[101]G/SiO210001200100109090.920000500.0[102]rGO/SnO2-Fe2O32001556100830531000550.0[103]G/LFP/CNT0.2C168.9100≈165.5≈9820C115.8[104]G/NiO/C20010405075472.51600580.0[105]G/CNT/C-LVP20C147.5200012282.720C122[106]G/TiO2500500500082[107]G/CoFe2O45003009668000146[108]G/MnO280151560111373.41200440[109]G/MoS210067730670993200≈400[110]G/NiCo2O420013876098571600761[111]GA100256610095437800587[112]C/Si/rGO0.5C19933501913965C830[113]G(3Dprinting)1016820170[44]

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Research Progress on 3D Printed Graphene Materials Synthesis Technology and Its Application in Energy Storage Field

WANG Nan,YAN Shao-jiu,PENG Si-kan,CHEN Xiang,DAI Sheng-long

(Research Center of Graphene Applications,AECC Beijing Institute of Aeronautical Materials,Beijing 100095,China)

Graphene is an ideal material for energy storage application as its excellent mechanical and physical properties. 3D printed graphene materials will be widely applied in energy storage field for its precisely controllable structure and it is easy to realize large-scale preparation. In this paper, the progress of 3D printed graphene materials synthesis technology and its application in energy storage field were reviewed. The viscosity and printability of graphene ink are key factors for realizing graphene 3D printing. Scalable preparation of graphene ink with facile process, controllable concentration and additive free will be the research focus of graphene 3D printing technologies in the future. The integrated printing of graphene energy storage devices such as graphene supercapacitor, lithium-sulfur battery and lithium ion battery is the development direction in this area.

3D printing;graphene;energy storage;supercapacitor;lithium ion battery

10.11868/j.issn.1001-4381.2016.001102

O613.71

A

1001-4381(2017)12-0112-14

2016-09-14;

2017-05-02

燕紹九(1980-)，男，高級工程師，博士，主要從事磁性材料及石墨烯應(yīng)用方面的研究工作，聯(lián)系地址：北京市81信箱72分箱(100095)，E-mail:shaojiuyan@126.com

(本文責(zé)編：王 晶)