溫書劍，張熠霄，陳 陽，宋春陽，崔曉莉

(復旦大學 材料科學系，上海 200433)

鋰離子電池負極材料鈦酸鋰研究進展＊

溫書劍，張熠霄，陳 陽，宋春陽，崔曉莉

(復旦大學 材料科學系，上海 200433)

鋰離子電池被廣泛應用于移動電子設備，在電動汽車和各類儲能系統(tǒng)有良好的應用前景，是未來最具發(fā)展前途的儲能電池之一。作為一種鋰離子電池負極材料，鈦酸鋰因具有高的脫嵌鋰平臺電位，優(yōu)異的循環(huán)性能，突出的熱穩(wěn)定性和安全特性而備受重視?？偨Y了鈦酸鋰負極材料在結構形貌和電化學性能方面的研究進展，對其微納米化、表面改性和離子摻雜等方面新的研究成果進行了概述。微納米化可以賦予材料更大的表面積，有助于鋰離子遷移，電極材料與電解液可以更好的接觸，產(chǎn)生更大的鋰離子遷移電流，有利于提升鈦酸鋰材料倍率性能；表面改性手段主要以碳包覆、金屬-鈦酸鋰復合材料和形成新表面相為主，通過這些手段可以改善材料導電性和提高電池的循環(huán)性能；離子摻雜可使部分Ti4+轉變?yōu)門i3+，提高鈦酸鋰材料的電子導電性。對鈦酸鋰作為鋰離子電池負極材料未來的發(fā)展方向進行了展望。

鋰離子電池；鈦酸鋰；負極材料

0 引 言

伴隨能源危機和人們對于環(huán)境問題的日益關注，尋找可再生綠色能源成為科學研究的重要發(fā)展方向。作為清潔能源器件，鋰離子電池具有體積小、重量輕、使用壽命長等優(yōu)點，廣泛應用于便攜式電子產(chǎn)品，也是理想的電動汽車動力電池之一[1]。

鋰離子電池研究的關鍵是電極材料的選取。碳材料由于其儲量豐富，循環(huán)穩(wěn)定性好，性價比高，是商業(yè)鋰離子電池中最廣泛應用的負極材料[2]。但兩大缺點制約著碳材料在鋰離子電池市場上的進一步發(fā)展：一是在充放電過程中，鋰離子脫嵌會引起碳電極材料的體積膨脹與收縮，長時間的使用可能使電極材料晶體結構發(fā)生坍塌，進而引起電池容量降低[3-4]；二是碳電極在脫嵌鋰過程中電位接近金屬鋰，當電池過充時在表面容易形成鋰枝晶，引起電池內(nèi)部短路，因而具有潛在的安全隱患[5-6]。

因此，尋找安全可靠的負極材料勢在必行。各類新型負極材料包括鈦酸鋰(Li4Ti5O12)、 Si- Li4Ti5O12復合材料[7]、硅[8-10]、多種過渡金屬氧化物(如TiO2[11-17]、Co3O4[18]、MnO[19]、Fe2O3[20]、Fe3O4[21]及Cr2O3[22])等受到了廣泛的關注。其中鈦酸鋰是最具發(fā)展前景的鋰離子電池負極材料之一[23]。與碳材料相比，鈦酸鋰脫嵌鋰平臺電位較高(1.55 V vs Li/Li+)，可避免鋰枝晶的產(chǎn)生，保障了電池的安全性；其理論比容量為175 mAh/g，具有平穩(wěn)的放電平臺，容量利用率較高；被稱為“零應變”材料，充放電過程中無明顯體積變化，能夠避免電極材料因反復脹縮而導致的結構破壞，具有穩(wěn)定的循環(huán)性能[24-26]。除此之外，鈦酸鋰具有高熱穩(wěn)定性[27-28 ]。

然而，鈦酸鋰材料由于其Ti4+的3d電子層中沒有電子[29-30]，致使其自身導電性差，影響了其高倍率性能。鋰離子嵌入鈦酸鋰材料包含3個過程：(1)溶劑化鋰離子在電解液中的擴散；(2)鈦酸鋰電極和電解液交界面上發(fā)生電荷轉移反應；(3)鋰離子在鈦酸鋰電極內(nèi)的固相擴散[30]。因此，研究者們通常采用2類方法提高鈦酸鋰材料的倍率性能：一是通過設計納米結構減小鋰離子在鈦酸鋰電極中的固相擴散距離[6]；二是通過表面改性如包覆碳等，通過元素摻雜增強離子擴散和電子傳導速率，從而加快電荷轉移反應的速度[6]。本文將對以上方法進行討論，并對鈦酸鋰材料未來的發(fā)展方向進行展望。

1 鈦酸鋰材料的結構與性能

鈦酸鋰是由金屬鋰和過渡金屬鈦復合的氧化物，為白色晶體，立方密堆積尖晶石結構，具有鋰離子的三維擴散通道，固有電子電導率為10-9S/cm。鈦酸鋰的晶格常數(shù)a為0.826 nm，晶體中O2-占據(jù)32e位置，Li+中的3/4占據(jù)8a位置，余下的Li+與Ti4+共同占據(jù)八面體的16d 位置[24]。鈦酸鋰的結構式為Li[Li1/3Ti5/3]O4，圖1為其晶體結構示意圖[23]，式(1)為鋰離子脫嵌過程的相變表達式。

(1)

式(1)中，鈦酸鋰晶格內(nèi)Li+的脫嵌表現(xiàn)為兩相過程。Li+在外界電流的作用下嵌入鈦酸鋰晶格，與四面體8a位置的Li+一同占據(jù)16c位置，成為淡藍色巖鹽相的Li7Ti5O12。嵌鋰過程中Ti4+/Ti3+的變價提高了電子導電性能[31]。在充放電過程中，四面體8a位置和八面體16c位置的Li+可以相互轉換，鈦酸鋰的晶格常數(shù)不會受到影響，所以這種變化是高度可逆的。因此，這種兩相可逆轉變的方式有利于實現(xiàn)鈦酸鋰材料的充放電循環(huán)。

圖1 尖晶石結構和巖鹽型結構的Li4Ti5O12(黃色四面體代表Li+,綠色八面體代表錯位的Li+與Ti4+)[23]

Fig 1 Spinel and rock-salt Li4Ti5O12structures(yellow tetrahedra represent lithium, and green octahedra represent disordered lithium and titanium[23]

2 鈦酸鋰顆粒的形貌與尺寸調控

相比于傳統(tǒng)微尺度級別鈦酸鋰材料，新型納米鈦酸鋰材料因為其出色的倍率性能更加受到人們關注。納米尺寸的形態(tài)賦予材料更大的表面積，縮短鋰離子遷移距離，電極材料與電解液可以更好的接觸，電解液中可以產(chǎn)生更大的鋰離子遷移電流，有利于鈦酸鋰材料倍率性能的提升[32-34]。更深層的原理可以從結構上分析得出，與微米結構材料相比，納米結構的鈦酸鋰材料在充放電過程中增加了單相和固溶態(tài)脫嵌鋰離子的數(shù)量[35]。Borghols等[36]研究發(fā)現(xiàn)，固溶體區(qū)域中8a與16c的共存促進了鋰離子的遷移速率與電導性，對于電池性能的提升做出了主要的貢獻。

鈦酸鋰材料的納米化研究主要集中在制備納米顆粒、具有孔洞結構的顆粒和具有微米-納米混合結構的顆粒。

2.1 納米顆粒

傳統(tǒng)微米級別鈦酸鋰(800 nm)由TiO2與鋰鹽通過固相反應合成，這種方法較難對產(chǎn)物的形態(tài)與尺寸進行調控，因而限制了鈦酸鋰材料的倍率性能[37-50]。目前，研究人員嘗試利用多種方法制備納米級別鈦酸鋰，例如軟化學法與熔融鹽法。這些方法使得設計與制備納米尺寸的鈦酸鋰材料成為可能。

Ren等[37]采用固相合成法制備Li4Ti5O12/C納米復合物，粒徑可控制在50～200 nm，其中碳層厚度為2～4 nm。在0.2與10 C倍率下，產(chǎn)物比容量分別為172和141 mAh/g，且在10 C倍率下200次循環(huán)后比容量保持95.7%。Liu等[38]制備的Li4Ti5O12/石墨復合材料平均粒徑為200～500 nm。100次循環(huán)后，在1和3 C倍率下，產(chǎn)物在1～2 V窗口比容量為144和96.2 mAh/g，高于純Li4Ti5O12的108和75.4 mAh/g。Han等[39]采用高能球磨法獲得粒徑在200 nm以下的3組Li4Ti5O12納米球，在0.1 C倍率下比容量為171，174和167 mAh/g，較普通球磨樣品性能有明顯提升。圖2為Li4Ti5O12納米球在不同球磨介質尺寸下的掃描電子顯微鏡圖[39]。

2.2 孔洞結構

另一種可以減小鋰離子遷移距離并有利于電解液在電極中滲透的方法是制備具有孔洞結構的納米薄層鈦酸鋰。納米顆粒相互連接形成的納米薄層通過減小擴散距離極大提高了鋰離子的運輸速率，孔洞結構產(chǎn)生的孔隙可使電解液與電極材料形成有效接觸[51]。

Yu等[52]將納米SiO2與鈦酸丁酯作為前驅體, 采用溶膠-凝膠法合成具有核殼結構的SiO2-TiO2。在此基礎上，將產(chǎn)物放入LiOH溶液中，除去SiO2后形成了具有孔洞結構的Li4Ti5O12。鈦酸鋰殼層厚度為50～200 nm，可由反應物鈦酸丁酯加入量進行控制。在5和10 C下，產(chǎn)物的比容量為128和115 mAh/g，在20 C的高倍率下，產(chǎn)物依然保持104 mAh/g的可逆比容量。He等[53]以碳作為前驅體制得的Li4Ti5O12同樣具有孔洞結構，產(chǎn)物顆粒尺寸為1 μm，平均壁厚60 nm，在10 C倍率下的放電比容量為100 mAh/g，在2 C下連續(xù)充放電200次，放電比容量變化不大，保持在150 mAh/g左右。Shao等[54]以單分散聚苯乙烯球作為模板，采用溶膠-凝膠法制備出含孔洞結構的納米Li4Ti5O12?？锥幢诤?0 nm，孔徑為100 nm。在10 C倍率下，產(chǎn)物在0.5～3 V窗口比容量為102 mAh/g。Zhou 等[55]在500～800 ℃范圍煅燒水熱產(chǎn)物制備直徑2 μm的孔洞微球結構Li4Ti5O12(圖3)。實驗發(fā)現(xiàn)，700 ℃煅燒生成產(chǎn)物性能最佳，在1，2, 3, 5, 10, 15和20 C倍率下，比容量分別為157.9, 155.3, 154.1, 150.3, 144.5, 137.8和131.3 mAh/g(圖4)。

圖2 Li4Ti5O12納米顆粒經(jīng)歷24 h普通球磨，與尺寸為0.05,0.10, 0.45 mm的球磨介質高能球磨3 h后，在800 ℃下熱處理3 h的SEM圖[39]

Fig 2 SEM images of the Li4Ti5O12particles heat-treated at 800 ℃ for 3 h after 24 h ball milling, and 3 h high-energy milling using different sizes milling media of 0.05, 0.10, 0.45 mm[39]

圖3 不同溫度下煅燒制得的Li4Ti5O12樣品SEM圖[55]

2.3 微米-納米混合結構

如前所述，納米尺寸與孔洞結構有效減小鋰離子擴散距離與增加電解液接觸面積，提高鈦酸鋰的倍率性能。然而，隨著表面積的增加，振實密度降低且不可逆容量增加，導致電池的體積能量密度降低。為改善這一特點，制備微米-納米混合結構材料，同時利用兩種尺寸材料的優(yōu)點，是獲得高性能鈦酸鋰鋰離子電池的一種新思路。

介孔微球體鈦酸鋰材料符合這樣的設計構思。Tang等[56]制備的單分散性納米粒子通過堆積生成200～400 nm的Li4Ti5O12顆粒，孔間隙為5～10 nm，可保存納米粒子的優(yōu)良特性；納米粒子堆積的二次微球可保證高振實密度。

圖4 1.0～2.5 V電壓下700 ℃煅燒制得的Li4Ti5O12樣品在不同倍率下的恒流充放電曲線和比容量[55]

Fig 4 Galvanostatic charge-discharge curves and rate performance of the hierarchical Li4Ti5-O12spheres calcinated at 700 ℃ between 1.0 and 2.5 V at different current densities[55]

這種材料在30 C倍率下具有114 mAh/g的高比容量，且以20 C倍率200次循環(huán)后仍可保持125 mAh/g的比容量，表現(xiàn)出良好的循環(huán)特性，與單一形貌的鈦酸鋰納米棒[57]相比具有更好的性能。

圖5為鈦酸鋰微球制備過程中的掃描電子顯微鏡圖[56]。與之類似，通過水熱法合成的多孔層狀高結晶鈦酸鋰微球同樣具備優(yōu)良的倍率和循環(huán)性能[58]。

盡管目前在納米鈦酸鋰材料的制備上取得了巨大進步，其實際應用仍面臨一些挑戰(zhàn)。首先，低溫方法難以使納米尺寸與最佳結晶度匹配；熔融鹽法可使兩者有效結合，但形成的鈦酸鋰純度不高；其次，納米粒子的團聚和納米顆粒間接觸的排斥力增強，限制了材料的倍率性能；此外，針對簡易低組裝密度制作方式的改良尚待進一步研究。

3 鈦酸鋰材料的表面改性

針對鈦酸鋰材料的低導電性，通過碳材料、金屬材料復合和有機無機復合物材料來對其進行表面改性是一種很好的解決方案。

3.1 碳包覆

碳包覆是改善Li4Ti5O12電化學性能的一種重要方法[59]。在鈦酸鋰充放電過程中，綜合考慮電子電導率和離子擴散速率，對于實現(xiàn)最佳倍率性能是至關重要的。電子電導率是材料傳遞電子的能力，而離子擴散速率反映了鋰離子在材料中遷移速率的大小。通過將含碳物質加熱分解，導電碳可以分散或包覆于顆粒表面。

圖5 水合氧化鈦球體、退火前鈦酸鋰微球和500 ℃退火1 h后鈦酸鋰微球 的SEM圖[56]

Fig 5 SEM images of hydrous titanium oxide spheres, as-synthesized Li4Ti5O12spheres before annealing and spinel Li4Ti5O12spheres after annealing at 500 ℃ for 1 h[56]

在制備鈦酸鋰材料過程中，導電碳的添加可以使反應前驅體更加緊密、均勻的混合，使材料導電性和電池的能量密度得到提升。各類碳材料如聚乙炔黑[60-61]、海藻酸[62]、蔗糖[63-64]、聚乙烯醇[65]、檸檬酸[66-67]、瀝青[68]、葡萄糖[69]、炭黑[70]、聚苯胺[71]以及石墨烯[72-76]等均運用于Li4Ti5O12的包覆。Guo等[63]以蔗糖作為碳源，分別在TiO2與Li2CO3反應前進行碳包覆，離心煅燒后得到C含量5%的C-Li4Ti5O12復合物。在電化學性能測試中前驅體TiO2預先進行碳包覆的產(chǎn)物性能表現(xiàn)最好，在0.2，5與30 C下比容量分別為171，150和82 mAh/g。Jung等[68]采用瀝青作為碳源進行碳包覆，將TiCl4與尿素和硫酸銨混合后在120 ℃ 加熱24 h，冷卻干燥后在400 ℃下煅燒4 h得到TiO2前驅體，然后與Li2CO3和瀝青混合，在900 ℃氬氣環(huán)境中煅燒20 h制得表面包覆整齊碳層的鈦酸鋰材料，在1與50 C倍率下充放電，比容量可達163和142 mAh/g。

另一方面，碳包覆層的存在在一定程度上阻礙了鋰離子的脫嵌。對此，解決方案之一是包覆具有納米結構的鈦酸鋰，諸如納米桿、孔洞球體和納米粒子等。尺寸上的減小便于鋰離子進行晶格擴散，也有助于電極與電解液的接觸。Luo等[69]以葡萄糖作為碳源，采用水熱法制備碳包覆鈦酸鋰納米棒，碳包覆層厚度為1～3 nm。實驗結果表明，產(chǎn)物在10 C倍率下放電比容量為92.7 mAh/g。圖6為合成過程中的TiO2納米管前驅體、Li4Ti5O12和Li4Ti5O12/C的SEM圖[69]。

圖6 TiO2納米管的FESEM圖(內(nèi)部為TEM圖)、Li4Ti5O12納米顆粒和Li4Ti5O12/C納米棒的SEM圖[69]

Fig 6 Typical SEM images of TiO2nanotubes (inset shows the TEM image of TiO2nanotubes), Li4Ti5O12nanoparticles and Li4Ti5O12/C nanorods[69]

碳包覆可以提高鈦酸鋰材料表面的電導率，碳膜的形成可以有效抑制納米粒子團聚，從而提升了電池的循環(huán)穩(wěn)定性[77-78]。鈦酸鋰材料中鋰離子的擴散主要包括體相擴散、液相擴散和包覆層擴散。材料本身的固有電導率是制約其性能的主要因素，碳包覆對于改善材料本身的導電性具有一定的局限，因此對鈦酸鋰材料進行金屬元素的修飾是提高材料電導率的重要研究方向。

3.2 鈦酸鋰-金屬復合材料

除了碳包覆及與碳材料的復合外，鈦酸鋰材料也可以通過引入金屬元素形成Li4Ti5O12/M (M=Au、Cu、Ag等)新材料。盡管失去了部分比容量，但新復合材料的導電性能得到明顯提高，這與離子摻雜情形類似，唯一的區(qū)別是金屬陽離子無法進入晶格結構，無法與鋰離子一同參與電化學反應[79-85]。表1總結了不同金屬材料和實驗制備方法對鈦酸鋰-金屬復合材料性能的影響。

3.3 通過形成新表面相的改性

除了以上介紹的幾種導電材料外，合成新的導電相是提高材料導電性的另一種手段[86-91]。例如Park等[86]研究發(fā)現(xiàn)表面氮化可以使鈦酸鋰表面原子與氮原子形成化學鍵，在鈦酸鋰表面附著一層致密導電層。當鈦酸鋰處于熱氨氣流中時，會產(chǎn)生氮化鈦與碳酸鋰位于表面、鈦酸鋰占據(jù)主體的核-殼結構，且合成過程中晶格常數(shù)不會變化?；旌蟽r態(tài)中間相Li4+δTi5O12和導電層TiN的引入極大提高了鈦酸鋰的電導性能。正因如此，氮改性的鈦酸鋰在10 C倍率下可以保持165 mAh/g的容量。圖7為氮改性前后Li4Ti5O12的SEM與TEM圖。

表1 不同金屬復合材料種類和制備方法對鈦酸鋰-金屬復合材料性能的影響

Table 1 The effects of different types of metal and preparation process on the performance of Li4Ti5O12-metal composite

鈦酸鋰-金屬復合材料制備方法與形貌特征電化學性能參考文獻Au@Li4Ti5O12固相研磨法,顆粒比表面積為50.35m2/g,鋰離子擴散系數(shù)7.32×10-10cm2/S5C倍率下比容量169mAh/g,100次循環(huán)后容量保持率為91.1%[79]n-Ag/Li4Ti5O12通過固相研磨法在Li4Ti5O12晶粒上沉積2～10nm厚的銀顆粒層4%wtn-Ag/Li4Ti5O12在10C倍率下比容量為206.06mAh/g,50次循環(huán)后容量保持96.51%[80]Li4Ti5O12納米片/Ag水熱法合成10nm厚Li4Ti5O12單晶層,分散平均直徑為5.8nm的銀30C條件下比容量為140.1mAh/g,高于復合前的126.4mAh/g[81]Ag/Li4Ti5O12納米纖維采用靜電紡絲法合成納米纖維平均直徑50～200nm,長度約10μm1C倍率比容量為163.5mAh/g,200次循環(huán)后160mAh/g[82]Ag/Li4Ti5O12通過化學鍍方法在顆粒表面沉積Ag在1,10,60C倍率下比容量分別為170,88,46mAh/g[83]Ag/Li4Ti5O12超聲輔助法固相合成,顆粒直徑約500nm2.5～1.0V電壓區(qū)間,10C倍率下比容量為140mAh/g,容量保持率76.3%[84]Li4Ti5O12/Cu納米粒子水熱法合成,顆粒直徑100～200nm在1,2,5,10和20C倍率下比容量分別為167,155,148,133和117mAh/g[85]

圖7 原始Li4Ti5O12與氮改性后Li4Ti5O12的FESEM、TEM與高分辨透射電子顯微鏡圖[86]

Fig 7 SEM, TEM and high resolution-TEM images of pristine Li4Ti5O12nanofibers and nitridated Li4Ti5O12nanofibers, respectively[86]

此外，Gao等[87]還通過氮化的修飾手段得到Li4Ti5O12/TiN復合物。產(chǎn)物在3 C倍率下首次放電比容量為130.8 mAh/g，是Li4Ti5O12的2.6倍；0.1 C下循環(huán)100次后，仍有97%的容量保持率。Zhao等[88]將1-乙基3-甲基二氰銨的離子液體作為前驅體，可以產(chǎn)生高溫分解的含碳非晶形層與金屬性的導電TiNx界面，形成三維導電結構。這比孔狀碳包覆不含氮元素的鈦酸鋰具備更好的倍率性能，體現(xiàn)出鈦-氮-碳表面相在提高電導性與鈦酸鋰表面穩(wěn)定性上的優(yōu)勢。

通常，合成新導電相可以使導電層與電極材料之間更好接觸。值得注意的是，放電容量會因鈦酸鋰表面的相反應而減小，因此，在一定程度內(nèi)的表面改性可以使材料在可接受的容量減小范圍內(nèi)獲得優(yōu)良的導電性能。

4 離子摻雜的鈦酸鋰材料

離子摻雜是提高鈦酸鋰材料電子導電性的重要方法之一[92]，這種方法使部分Ti4+轉變?yōu)門i3+，產(chǎn)生Ti3+/Ti4+混合物作為電荷補償，增加了電子的集中度[93-97]。多種陰陽離子都是可以選擇摻雜的對象，諸如Ta5+[98]、Ca2+[99]、Zr4+[100]、V5+[101]、 Mg2+[102]、La3+[103]、Zn2+[104]、Ru4+[105]、Ni2+[106]、Sc3+[107]、Gd3+[108-109]、W6+[110-111]、Nb5+[112]、Na+[113-114]、K+[115]、Ce3+[116-117]、Al3+[118]、Nd3+[119]、Cu2+[120]、Sr2+[121]、F-[122]、Br-[123]等，離子摻雜在Li+, Ti4+和O2-的空位處，相關結構特性、離子價態(tài)和導電性能被廣泛研究。摻雜后的復合物除了電子導電性提高外，摻雜離子在鈦酸鋰的晶格結構的引入會產(chǎn)生晶格扭曲，進而影響電池比容量和循環(huán)性能。

Guo等[98]對鈦酸鋰中Ta5+摻雜進行了研究。Ta5+的嵌入增加了鈦酸鋰的晶格常數(shù)，加速鋰離子的遷移速率。摻雜后部分Ta5+替換Ti4+進入晶格。作為電荷補償，Li4Ti5O12材料中部分Ti4+轉化為Ti3+，而Ti3+的存在會提高材料整體的電子導電性，從而使材料的電化學性質得到提升。在對電導率的測試中，摻雜后的Li4Ti4.995Ta0.005O12在10 C倍率下比容量為95.1 mAh/g，與未摻雜的鈦酸鋰50.4 mAh/g的比容量相比提高了88.7%。Zhang等[99]以Li2CO3、TiO2和CaO作為前驅體，采用固相反應制備了Ca2+摻雜的Li4-xCaxTi5O12(x= 0, 0.05, 0.1, 0.15, 0.2)，平均粒徑為1～2 μm。測試發(fā)現(xiàn)當x=0.1時Li3.9Ca0.1Ti5-O12性能最佳，在倍率為1，5和10 C時100次循環(huán)后，比容量分別為162.4，148.8和138.7 mAh/g。

摻雜離子會部分進入鈦酸鋰晶格中，因此推測摻雜可能會影響材料的形貌和顆粒尺寸[124]。Nithya等[100]采用熔融鹽法制備Zr4+摻雜的鈦酸鋰，其粒徑可降低至1 μm，減緩團聚作用，有利于倍率性能的提高。其摻雜樣品在0.01 C倍率下放電容量為325 mAh/g。

一些離子的摻雜會通過陽極放電至0 V以改變鈦酸鋰的電化學性能，這類深放電陽極有利于拓寬電化學反應窗口，增加鋰離子電池的比容量[125-126]。Yang等[101]發(fā)現(xiàn)釩元素摻雜形成的Li4Ti4.95V0.05O12/C。由于V5+的引入降低了電極極化，在0.5～2.5 V的電位窗口與0.1 C倍率下，0，5%，10%與15%的V5+摻雜產(chǎn)物的放電容量分別為158.7，173.2，179.8和181.3 mAh/g。這是因為釩離子與氧離子的結合可以保持局部的對稱性，使得材料尖晶石的結構保持穩(wěn)定。圖8為原始Li4Ti5O12，加入呋喃樹脂后的Li4Ti5O12和V5+摻雜后Li4Ti5O12的SEM圖。

圖8 原始Li4Ti5O12,含5%呋喃樹脂的Li4Ti5O12/C和10%V摻雜含5%呋喃樹脂的Li4Ti5O12/C的SEM圖[101]

Fig 8 SEM images of pristine Li4Ti5O12, Li4Ti5O12/C with 5wt% Furan resin and 10%V-doped Li4Ti5O12/C with 5wt% Furan resin[101]

在O位摻雜F-，Br-等負離子，同樣可以增加Ti3+的含量，提高Li4Ti5O12的電子導電性，改善材料的電化學性能。比如，Wang等[123]通過在O位上摻雜Br-合成Li4Ti5O11.7Br0.3，在經(jīng)歷100次循環(huán)后在10和20 C下比容量為138和104 mAh/g，與未摻雜Li4Ti5O12相比提高了4～7倍。

在離子摻雜中，引入摻雜離子對于鈦酸鋰材料結構、界面性質、基體和相應離子分布的影響有待深入研究。其中需使用諸如電子順磁共振(EPR)、微拉曼光譜儀(micro-Raman spectroscopy)和核磁共振(NMR)等新型設備，它們在深刻認識材料結構和電化學性能之間的關系中起到了重要的作用[127-129]。

5 結 語

鈦酸鋰作為鋰離子電池負極材料具有結構穩(wěn)定、安全性高和循環(huán)穩(wěn)定等優(yōu)點，在動力儲能電池方面具有良好的應用前景。目前，國內(nèi)外研究人員對鈦酸鋰進行了深入的研究，針對鈦酸鋰固有的電子電導率低，大倍率性能差的缺陷，采用減小尺寸、表面改性和離子摻雜等手段改善鈦酸鋰的電化學特性。這也是未來鈦酸鋰材料的發(fā)展方向，其中的諸多方面值得進一步探討研究：深入研究Li4Ti5O12不同形貌與尺寸對于性能的影響；選用更多過渡金屬離子以及非金屬離子對Li4Ti5O12進行摻雜改性，嘗試使用兩種或多種離子進行雙摻雜或共摻雜手段降低Li4Ti5O12高充放電平臺，以改善Li4Ti5O12電化學性能；基于Li4Ti5O12自身優(yōu)異的循環(huán)穩(wěn)定性能，嘗試引入其它高容量材料以制備兼具兩者優(yōu)點的復合材料等。

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The recent development of lithium titanate as anode materials for lithium-ion batteries

WEN Shujian, ZHANG Yixiao, CHEN Yang, SONG Chunyang, CUI Xiaoli

(Department of Materials Science, Fudan University, Shanghai 200433, China)

Lithium-ion batteries are one of the most promising battery systems to be widely used in portable electronics, electric vehicles, and energy storage systems. Lithium titanate (Li4Ti5O12) has been intensively investigated as an important anode material for lithium-ion batteries due to its high potential of around 1.55 V (vs. Li/Li+) during charge and discharge, excellent cycling stability, and high thermal stability and safety. This paper reviews the recent advances in structure and electrochemical performance of lithium titanate involving on new preparation methods of micro/macro particle, surface modification and ion doping. The micro/macro particles can provide greater surface area and shorten the migration distance for Li+. The better contact between the electrode and electrolyte produces benefits transportation of Li+, which improves the cycling performance of Li4Ti5O12. The major methods of surface modification are carbon coating, forming Li4Ti5O12/metal composites and modification by new surface phase. Such methods aim to increase the conductivity and improve the cycling performance of Li4Ti5O12. Doping ions increases the electron concentration and electronic conductivity since the partial Ti4+transform to Ti3+. The future development of lithium titanate as anode materials in lithium-ion batteries is also prospected in this review.

lithium-ion battery; lithium titanate; anode materials

1001-9731(2016)12-12038-12

2016-03-21

2017-07-11 通訊作者：崔曉莉，E-mail: xiaolicui@fudan.edu.cn

溫書劍 (1994-)，男，北京人，從事鋰離子電池負極材料研究。

TB34

A

10.3969/j.issn.1001-9731.2016.12.007