原位聚合三維陶瓷骨架增強全固態鋰電池電解質

顏一垣, 鞠江偉, 于美燕, 陳守剛, 崔光磊

原位聚合三維陶瓷骨架增強全固態鋰電池電解質

顏一垣1, 鞠江偉2, 于美燕1, 陳守剛1, 崔光磊2

(1. 中國海洋大學 材料科學與工程學院, 青島 266100; 2. 中國科學院 青島生物能源與過程研究所, 青島 266101)

有機/無機復合電解質被認為是全固態鋰電池中最具潛力的固態電解質之一, 但由于無機填料易團聚, 通過提高無機填料含量來改善復合電解質的電導率難有成效。此外, 在全固態鋰電池中, 電解質和電極之間松散的固–固接觸造成過大的界面阻抗, 限制了全固態鋰電池的性能。本研究采用固相法合成具有Li+連續傳輸通道的自支撐三維多孔Li6.4Al0.1La3Zr1.7Ta0.3O12骨架, 并利用原位聚合的方法構筑一體化電解質/電極固–固界面。此策略指導合成的復合電解質的室溫電導率可達1.9×10?4S·cm?1。同時, 一體化的界面使得Li-Li對稱電池的界面阻抗從1540 Ω·cm2降低至449 Ω·cm2, 因此4.3 V(. Li+/Li)的LiCoO2|Li全固態鋰電池展現出良好的電化學性能。

固態復合電解質; 原位聚合; 多孔骨架; 全固態電池

為彌補單一固態電解質的缺陷, 結合二者的優勢, 將SIE與SPE進行復合是一種行之有效的方法。SPE中引入零維SIE顆粒或者一維SIE纖維能夠降低SPE基體的結晶度或玻璃化轉變溫度[13-14], 一般可將SPE的電導率提升一個數量級[15]。但是零維或一維SIE填料的含量過高會產生團聚, 導致電導率降低[16-17]。并且, 高電導率的SIE填料或是被SPE相孤立或是被有機/無機界面相孤立, 阻礙了Li+在高電導率SIE相中的快速傳導。不同于將SIE填料分散于SPE中, 相反, 將SPE澆注于多孔SIE骨架中, 即三維SIE填料中, 可得到具有連續SIE相的有機/無機復合電解質[17-18]。這種結構不僅能有效避免SIE填料的團聚, 還能為Li+的快速傳輸提供連續通道, 大幅提升電導率[19]。

圖1 (a)非原位聚合策略和(b)原位聚合策略制備的ASLB內部結構示意圖

1 實驗方法

固相反應法合成LLZTO粉末: 將Al2O3、ZrO2、LiOH、Ta2O5及La2O3按照Li6.4Al0.1La3Zr1.7Ta0.3O12的化學計量比稱量后倒入球磨罐中。以異丙醇為球磨介質, 并于350 r/min的速率球磨10 h。之后, 將球磨所得漿料置于60 ℃烘箱中干燥, 并于1000 ℃的空氣氛圍中燒結5 h, 再將燒結所得塊體進行研磨即可得到LLZTO粉末。

將上述所得LLZTO粉末與造孔劑石墨粉以質量比3.5 : 1.5均勻混合后, 在12 MPa的壓力下壓制成片。再將其置于氧化鎂瓷舟中, 并用LLZTO粉末包覆, 在1150 ℃的空氣氛圍中燒結5 h, 得到p-LLZTO。直接將上述所得LLZTO粉末于12 MPa壓力下壓成片, 置于氧化鎂瓷舟中, 并用LLZTO粉末包裹, 于1150 ℃的空氣氛圍中燒結5 h, 可得致密LLZTO樣品。

為證明原位聚合對ASLB的積極作用, 本工作對原位聚合及非原位聚合的LiCoO2|Li ASLB性能進行對比。原位聚合LiCoO2|Li ASLB的制備: 將p-LLZTO或纖維素隔膜置于鋰片上, 滴加100 μL的PEGMEA前驅體溶液, 放置正極片, 在手套箱中完成電池的組裝后, 移入60 ℃烘箱中, 加熱完成原位聚合。非原位聚合LiCoO2|Li ASLB的制備: 先將p-LLZTO置于聚四氟乙烯板上, 然后滴加100 μL PEGMEA前驅體溶液, 在60 ℃加熱使PEGMEA完成聚合。將3D composite從聚四氟乙烯板上取下并置于鋰片和正極片之間, 完成非原位電池組裝。

2 結果與討論

2.1 p-LLZTO的表征

2.2 3D composite的表征

首先使用傅里葉紅外光譜(Fourier transform infrared spectrometer, FT-IR)分析PEGMEA的聚合程度。在60 ℃加熱24 h后, 如圖3(a)所示, 位于1620 cm?1附近的C=C峰完全消失, 而其它官能團, 如C=O或者C?O?C的峰則仍保留, 證明在該條件下PEGMEA能夠完全聚合, 轉化為P(PEGMEA)。而3D composite中的P(PEGMEA)與純P(PEGMEA)的紅外光譜幾乎相同, 說明LLZTO不影響PEGMEA的聚合且不與PEGMEA反應。為進一步分析P(PEGMEA)的分子結構, 又對PEGMEA及P(PEGMEA)進行核磁共振測試(Nuclear magnetic resonance, NMR)測試。在PEGMEA的氫譜(圖3(b))中, 與C=C相連氫原子的峰分別位于6.4、6.2和6.0。在60 ℃加熱24 h后, 這些峰完全消失, 同時在2.5~1.5的位置上出現了若干峰。這歸結于C=C雙鍵被打開, 同樣證明PEGMEA完成了聚合。

圖2 (a)標準LLZO及本實驗制備的LLZTO粉末和p-LLZTO的XRD圖譜; (b) p-LLZTO的截面SEM照片; (c)p-LLZTO的孔徑分布曲線; (d)致密LLZTO和p-LLZTO的室溫阻抗圖譜(插圖: 局部放大的致密LLZTO阻抗譜)

圖3 (a)PEGMEA、P(PEGMEA)和3D composite中P(PEGMEA)的紅外圖譜; (b)PEGMEA和3D composite中P(PEGMEA)的核磁共振氫譜及相關結構式(溶劑為氘代N,N-二甲基甲酰胺); (c)60 ℃條件下steel|3D composite|steel電池歐姆阻抗與加熱時間關系曲線, 插圖為有/無p-LLZTOP的PEGMEA在小瓶中60 ℃加熱24 h后的照片; (d)P(PEGMEA)和3D composite的電導率與溫度的關系曲線; (e)3D composite的截面SEM照片及元素分布圖

表1 不同固態電解質的室溫電導率

a: ethylene oxide(–CH2–CH2–O–); b: lithium bis(trifluoromethanesulfonyl)imide); c: Li1.4Al0.4Ti1.6(PO4)3; d: Li0.35La0.55TiO3

2.3 3D composite與Li的相容性

對3D composite與Li的相容性進行測試以確定其能否應用于ASLB[12]。本實驗對Li|3D composite|Li進行EIS和恒流極化測試, 并以Li|P(PEGMEA)|Li和Li|LLZTO|Li作為對照。圖4(a~f)為基于不同電解質的電池在熱處理前后的EIS圖譜。加熱前Li|PEGMEA|Li的EIS圖譜由一個半圓和一條斜線組成, 半圓與實軸的交點為歐姆阻抗, 而半圓的跨度為界面阻抗[29-30]。Li|3D composite|Li的EIS圖譜中存在兩個半圓, 這可能是固態p-LLZTO所致。在60 ℃加熱24 h后, Li|P(PEGMEA)|Li電池的EIS圖譜同樣出現兩個半圓, 高頻區半圓的跨度為歐姆阻抗, 而中頻區半圓的跨度為界面阻抗[31]。

根據阻抗圖譜的擬合結果, 上述電池熱處理前后歐姆阻抗及界面阻抗的變化總結于圖4(g, h)中。加熱前, Li|3D composite|Li的歐姆阻抗為2736 Ω·cm, 僅為Li|PEGMEA|Li歐姆阻抗8550 Ω·cm的三分之一。加熱后, Li|3D composite|Li的歐姆阻抗增至5996 Ω·cm,而Li|P(PEGMEA)|Li的歐姆阻抗陡增至216743 Ω·cm。同時, 加熱后Li|3D composite|Li的界面阻抗從 115 Ω·cm2增加至449 Ω·cm2, 不到Li|P(PEGMEA)|Li界面阻抗956 Ω·cm2的二分之一。3D composite與P(PEGMEA)歐姆阻抗和界面阻抗的明顯差異, 說明p-LLZTO在提升電導率上的巨大作用。圖4(b, e)為Li|LLZTO|Li熱處理前后的EIS圖譜, 其歐姆阻抗和界面阻抗幾乎沒有變化, 分別約為8360 Ω·cm和1540 Ω·cm2, 表明LLZTO對鋰金屬穩定, 無化學反應發生, 與文獻[3]報道相符。而3D composite的界面阻抗僅為LLZTO的三分之一說明原位聚合策略能夠有效降低界面阻抗。

圖4 熱處理(a~c)前(d~f)后基于(a, d)PEGMEA、(b, e)LLZTO和(c, f)3D composite的Li-Li對稱電池的EIS圖譜; 基于不同電解質的Li-Li電池處理前后(g)歐姆阻抗和(h)界面阻抗對比; (i)P(PEGMEA)和3D composite的Li-Li電池室溫下的直流恒流循環曲線(上插圖為LLZTO的Li-Li電池室溫下的直流恒流循環曲線, 下插圖為3D composite的Li-Li電池的局部放大極化曲線, 電流密度為0.1 mA·cm?2)

P(PEGMEA)、LLZTO和3D composite與鋰金屬的相容性通過室溫下的直流極化進行測試。圖4(i)中, 電流密度為0.1 mA·cm?2時, Li|3D composite|Li的極化電壓僅為0.12 V且能夠穩定循環超過200 h, 說明3D composite與鋰金屬具有良好的相容性。作為對比的P(PEGMEA)在0.1 mA·cm?2的電流密度下, 不到90 h其極化電壓從0.33 V增至0.95 V。這應當歸因于P(PEGMEA)低的室溫電導率(3.6×10?6S·cm?1)。而致密LLZTO在電流密度為0.1 mA·cm?2時, 其電池循環不到20 h就出現短路, 與文獻[32]報道相符。這是由于LLZTO與金屬鋰接觸不緊密, 局部產生較大的電流密度, 造成鋰的不均勻沉積并使鋰枝晶沿LLZTO中的晶界生長, 最終導致電池短路。相較之下, 利用原位聚合得到的3D composite與金屬鋰之間接觸緊密, 電場分布均勻, 因此3D composite能夠有效抑制鋰枝晶的生長[33-34]。上述測試結果說明3D composite與鋰金屬有很好的相容性, 能夠應用于ASLB。

2.4 3D composite在ASLB中的應用

以LiCoO2|Li ASLB對3D composite的性能進行測試。為探究原位聚合策略對ASLB的積極作用, 即圖1中利用原位聚合形成一體化界面, 本工作對原位聚合及非原位聚合的LiCoO2|Li ASLBs性能進行對比。LiCoO2|Li ASLB工作電壓范圍為3.0~4.3 V (Li+/Li), 工作溫度60 ℃。原位聚合LiCoO2|3D composite|Li ASLB在0.1(1=140 mAh·g?1)電流密度下, 首圈放電比容量為144 mAh·g?1, 首圈庫侖效率為94%(圖5(a, b))。電流密度為0.1、0.3、0.5時, 其放電比容量分別為144、138和129 mAh·g?1。在0.1循環90圈后, 其容量保持率為88%。對于原位聚合LiCoO2|P(PEGMEA)|Li ASLB, 即使在60 ℃下, 首圈放電比容量也僅為123 mAh·g?1, 且在40圈以內迅速衰減至10 mAh·g?1。而非原位聚合的LiCoO2|3D composite|Li ASLB的性能則更差, 首圈放電比容量僅為62 mAh·g?1, 在15圈左右便失效。

圖5 (a)原位聚合LiCoO2|3D composite|Li、原位聚合LiCoO2|P(PEGMEA)|Li和非原位聚合LiCoO2|3D composite|Li ASLBs的循環性能; (b)原位聚合LiCoO2|3D composite|Li、原位聚合LiCoO2|P(PEGMEA)|Li和非原位聚合LiCoO2|3D composite|Li ASLBs的充放電曲線; (c)原位聚合和(d)非原位聚合LiCoO2|3D composite|Li ASLBs拆解后的LiCoO2/3D composite界面的截面SEM照片

分析上述ASLBs的失效機理, 首先, 低的離子電導率是造成原位聚合LiCoO2|P(PEGMEA)|Li ASLB性能不佳的主要原因, 即使在60 ℃下, P(PEGMEA)的電導率也僅為1.85×10?5S·cm?1, 這嚴重限制了Li+的快速輸運。對于非原位聚合LiCoO2|3D composite |Li ASLB, 雖然3D composite具有較高的電導率, 但電極與電解質之間存在約30 μm的間隙, 不連續的界面接觸嚴重阻礙了Li+在電解質–電極之間的傳輸, 造成電池性能的快速衰減。如圖5(c, d)所示, 通過原位聚合制備的LiCoO2|3D composite|Li具有一體化的電解質/電極界面, 確保Li+在界面處順利地傳輸。上述三種ASLBs的性能對比證明了高的電導率和一體化的電解質–電極界面是獲得高性能ASLB的必要條件, 而本工作中通過原位聚合的策略成功實現了高電導率3D composite與電極之間的一體化界面的構建。

3 結論

綜上, 本工作以石墨粉為造孔劑通過高溫燒結成功制備自支撐三維多孔Li6.4Al0.1La3Zr1.7Ta0.3O12骨架。將聚乙二醇甲基醚丙烯酸酯澆注于多孔Li6.4Al0.1La3Zr1.7Ta0.3O12中, 聚合后得到三維有機無機復合電解質。連續的Li6.4Al0.1La3Zr1.7Ta0.3O12相能夠為Li+的快速傳輸提供通道, 并將聚合后的聚乙二醇甲基醚丙烯酸酯的室溫電導率提升53倍, 達到1.9×10?4S·cm?1。更重要的是, 原位聚合的聚乙二醇甲基醚丙烯酸酯能夠在接觸不良的三維有機無機復合電解質和電極之間形成一體化界面, 有效地將電池界面阻抗從1542 Ω·cm2降低至449 Ω·cm2。最后, 原位聚合三維有機無機復合電解質被成功應用于LiCoO2|Li全固態鋰電池。本工作為制備與高電壓正極、鋰負極有良好化學機械相容性的高電導率有機/無機復合電解質提供了有價值的參考。

補充材料

本文相關補充材料可登陸https://doi.org/ 10.15541/ jim20200152查看。

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Polymerization Integrating 3D Ceramic Framework in All Solid-state Lithium Battery

YAN Yiyuan1, JU Jiangwei2, YU Meiyan1, CHEN Shougang1, CUI Guanglei2

(1. School of Materials Science and Engineering, Ocean University of China, Qingdao 266100, China; 2. Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China)

Organic/inorganic composites have been considered as promising electrolyte candidates in all solid-state lithium batteries. Aiming at improving the conductivity significantly by increasing the frequently-used 0D or 1D ceramic nano-fillers to high content is unsuccessful due to the particle tendency to agglomeration. What's worse, the loose contact between the solid electrolyte and solid electrodes is much of a serious barrier to the performance and thus to the application of all solid-state lithium batteries. Herein, self-supported 3D porous Li6.4Al0.1La3Zr1.7Ta0.3O12frameworks are employed to provide percolated fast Li+conductive pathway whilepolymerization of poly(ethylene glycol) methyl ether acrylate can integrate the loose solid-solid interface and reduce the interfacial resistance efficiently. Inspiringly, the Li+conductivity of the composite exhibits 1.9×10?4S·cm?1at room temperature. The interfacial resistance in Li-Li batteries decreases significantly from 1540 to 449 Ω·cm2, rendering good capacity and cyclability of the 4.3 V (. Li+/Li) LiCoO2|Li all solid-state lithium battery.

solid composite electrolyte;polymerization; porous framework; all solid-state battery

TQ174

A

1000-324X(2020)12-1357-08

10.15541/jim20200152

2020-03-23;

2020-05-11

國家自然科學基金(51902325) National Natural Science Foundation of China(51902325)

顏一垣(1994–), 男, 碩士研究生. E-mail: yanyiyuan94@163.com

YAN Yiyuan(1994–), male, Master candidate. E-mail: yanyiyuan94@163.com

陳守剛, 教授. E-mail: sgchen@ouc.edu.cn; 崔光磊, 研究員. E-mail: cuigl@qibebt.ac.cn

CHEN Shougang, professor. E-mail: sgchen@ouc.edu.cn; CUI Guanglei, professor. E-mail: cuigl@qibebt.ac.cn