全固態(tài)鋰離子電池正極與石榴石型固體電解質(zhì)界面的研究進(jìn)展

李棟, 雷超, 賴華, 劉小林, 姚文俐, 梁彤祥, 鐘盛文

全固態(tài)鋰離子電池正極與石榴石型固體電解質(zhì)界面的研究進(jìn)展

李棟1,2, 雷超1,2, 賴華3, 劉小林1,2, 姚文俐1,2, 梁彤祥1, 鐘盛文1,2

(江西理工大學(xué) 1. 材料科學(xué)與工程學(xué)院; 2. 江西省動力電池及材料重點(diǎn)實(shí)驗(yàn)室; 3. 資源環(huán)境與工程學(xué)院, 贛州 341000)

全固態(tài)鋰離子電池具有高安全性、高能量密度、寬使用溫度范圍以及長使用壽命等優(yōu)勢, 在動力電池汽車和大規(guī)模儲能電網(wǎng)領(lǐng)域具有廣闊的應(yīng)用前景。作為全固態(tài)電池的重要組成部分, 無機(jī)固體電解質(zhì)尤其是石榴石型固態(tài)電解質(zhì)在室溫下鋰離子電導(dǎo)率可達(dá)10–3S·cm–1, 且對金屬鋰相對穩(wěn)定, 在全固態(tài)電池的應(yīng)用中具有明顯的優(yōu)勢。然而正極與石榴石型固體電解質(zhì)間接觸性能以及界面的穩(wěn)定性差, 使得電池表現(xiàn)出高的界面阻抗、低的庫倫效率和差的循環(huán)性能。本文以全固態(tài)鋰離子電池正極與石榴石型固體電解質(zhì)界面為研究對象, 分析了正極/固體電解質(zhì)的界面特性以及界面研究中存在的問題, 綜述了正極復(fù)合、界面處理工藝、界面層引入等界面調(diào)控和改性的方法, 闡述了優(yōu)化正極與石榴石型固體電解質(zhì)界面結(jié)構(gòu), 改善界面潤濕性的解決思路, 提出了未來全固態(tài)鋰離子電池發(fā)展中有待進(jìn)一步改進(jìn)的關(guān)鍵問題, 為探索全固態(tài)鋰離子電池的實(shí)際應(yīng)用提供了借鑒。

無機(jī)固體電解質(zhì); 復(fù)合電解質(zhì); 界面潤濕性; 界面阻抗; 界面改性; 綜述

全固態(tài)鋰離子電池在解決傳統(tǒng)鋰離子電池使用溫度范圍窄、能量密度低、使用壽命短、安全等級低等關(guān)鍵問題的同時, 有望大幅降低電池制造成本、有效改善電池的安全性問題, 在動力電池和大容量新型儲能方面具有廣闊的應(yīng)用前景[1-8]。

固體電解質(zhì)作為全固態(tài)鋰離子電池的重要組成部分, 其性能的優(yōu)劣很大程度上制約著全固態(tài)電池實(shí)際應(yīng)用的發(fā)展[9-12]。固體電解質(zhì)分為無機(jī)固體電解質(zhì)和聚合物固體電解質(zhì)。相對于聚合物固體電解質(zhì)容易結(jié)晶、適用溫度范圍窄以及力學(xué)性能提升難的問題, 無機(jī)固體電解質(zhì)能在寬的溫度范圍內(nèi)保持良好的化學(xué)穩(wěn)定性、電化學(xué)穩(wěn)定性、力學(xué)性能和更高的安全特性[13-16]。圖1為室溫下, 不同結(jié)構(gòu)類型無機(jī)固體電解質(zhì)的離子電導(dǎo)率, 從圖中可以看出類Lisicon型以及Argyrodite型的固體電解質(zhì)具有與液體電解質(zhì)相近的電導(dǎo)率[17], 但這些含硫化合物存在對水分比較敏感、化學(xué)穩(wěn)定性欠佳、易揮發(fā)、難以合成所需計(jì)量化合物[18]和電壓窗口窄[19]等問題, 在全固態(tài)電池的應(yīng)用中受到較大的限制。具有高電導(dǎo)率的石榴石型電解質(zhì), 如Li7La3Zr2O12[20-23](LLZO)以及元素?fù)诫s后的LLZO室溫電導(dǎo)率可達(dá)10–3S·cm–1數(shù)量級[24], 與金屬Li有相對穩(wěn)定的界面, 且在空氣中也有著良好的化學(xué)穩(wěn)定性, 因而石榴石型電解質(zhì)在全固態(tài)電池研究和開發(fā)中有著良好的應(yīng)用前景。目前, 電極與固體電解質(zhì)間的高界面阻抗使得全固態(tài)鋰離子電池的容量、倍率和循環(huán)性能都不理想。全固態(tài)電池體系的界面主要包括正極/固體電解質(zhì)界面和負(fù)極/固體電解質(zhì)界面[25-30], 本文以正極與LLZO石榴石型固體電解質(zhì)界面為研究對象, 從正極/石榴石型固體電解質(zhì)的界面特性、界面研究中存在的問題、以及界面改性等方面進(jìn)行概述。

圖1 室溫下不同類型固體電解質(zhì)的電導(dǎo)率[17]

1 正極/石榴石型固體電解質(zhì)界面特性以及研究中存在的問題

1.1 正極/電解質(zhì)界面接觸性能以及循環(huán)過程中的結(jié)構(gòu)穩(wěn)定性

石榴石型固體電解質(zhì)具有高的彈性模量[31], 導(dǎo)致電解質(zhì)/正極接觸面積較小, 并在界面處形成一定的結(jié)構(gòu)缺陷(如: 空隙[32]、裂紋[33]), 制約了鋰離子在界面上的傳輸速率, 形成了高的界面阻抗。另外, 電池在充放電循環(huán)過程中, 電極材料與固體電解質(zhì)自身結(jié)構(gòu)的變化和界面區(qū)域第三相的形成, 導(dǎo)致界面處產(chǎn)生一定的結(jié)構(gòu)應(yīng)力, 隨著循環(huán)的進(jìn)行, 應(yīng)力累加使界面斷裂、分離, 最終導(dǎo)致全固態(tài)電池失效[34-35]。

1.2 正極與電解質(zhì)界面的化學(xué)穩(wěn)定性

正極與電解質(zhì)界面的化學(xué)穩(wěn)定性影響著全固態(tài)電池體系的結(jié)構(gòu)穩(wěn)定性、電化學(xué)性能和使用壽命。正極與電解質(zhì)界面的化學(xué)穩(wěn)定性主要受制于電解質(zhì)自身的化學(xué)穩(wěn)定性以及與正極材料的化學(xué)匹配性兩個方面:

①在潮濕的空氣中, LLZO表面層與水分發(fā)生Li+/H+交換, 生成氫氧化鋰, 并進(jìn)一步與CO2反應(yīng), 形成Li2CO3高阻抗相[36-44]; 另外, 潮濕空氣中的H2O和CO2均可誘導(dǎo)LLZO發(fā)生四方–立方相變, 在材料體相形成混合相, 抑制Li+的傳輸[44]。

②在全固態(tài)電池的制備和使用過程中, 正極材料與LLZO電解質(zhì)差的化學(xué)相容性, 會在界面上形成高阻抗第三相[45-47]。Kim等[46]利用脈沖激光沉積(PLD)工藝在Li7La3Zr2O12上沉積一層LiCoO2, 發(fā)現(xiàn)Co、La和Zr元素在LiCoO2/Li7La3Zr2O12界面處發(fā)生擴(kuò)散, 生成了La2CoO4高阻相。清華大學(xué)南策文課題組[48]在正極與固體電解質(zhì)片共加熱優(yōu)化界面結(jié)構(gòu)的過程中, 發(fā)現(xiàn)電解質(zhì)Li6.75La3Zr1.75Ta0.25O12與正極材料LiNi0.33Co0.33Mn0.33O2(NCM)和LiCoO2(LCO)有著良好的高溫穩(wěn)定性, 而與LiMn2O4和LiFePO4(LFP)的相容性則相對較差。Miara等[49]研究發(fā)現(xiàn)Li6.6La3Zr1.6Ta0.4O12(LLZTO)在600 ℃時與正極材料Li2NiMn3O8、Li2FeMn3O8以及LiCoMnO4均在高溫下反應(yīng)生成新相。這些新相的存在增加了固態(tài)電池中復(fù)合正極的界面阻抗, 是界面穩(wěn)定性差的另一個主要原因。

1.3 正極/電解質(zhì)界面的電化學(xué)穩(wěn)定性

充放電倍率、使用電壓范圍和循環(huán)壽命是衡量電池性能優(yōu)劣的重要指標(biāo)。工作電壓和充放電倍率分別從材料熱力學(xué)和鋰離子擴(kuò)散動力學(xué)兩個方面影響正極與電解質(zhì)之間的界面穩(wěn)定性。高充放電倍率會引起負(fù)極極片上鋰枝晶的生長, 導(dǎo)致全固態(tài)電池的短路, 而對于正極與電解質(zhì)界面性能的影響則相對較小。工作電壓對于正極/電解質(zhì)界面穩(wěn)定性的影響主要表現(xiàn)為: 隨著電池充電電壓的升高, 正極/電解質(zhì)的相間電勢增大, 正極與電解質(zhì)界面層區(qū)域的荷電粒子微觀結(jié)構(gòu)發(fā)生重排, 使界面結(jié)構(gòu)變得不穩(wěn)定。從熱力學(xué)角度分析, 兩種物質(zhì)混合過程勢必會引起體系吉布斯自由能降低, 正極與固體電解質(zhì)形成界面相的過程同樣會在一定程度上降低該體系的能量, 吉布斯自由能降低得越多, 形成界面相的驅(qū)動力就會越大, 界面越不穩(wěn)定。通過對不同電壓正極與固體電解質(zhì)界面相形成能的計(jì)算, 能夠有效預(yù)測界面的電化學(xué)穩(wěn)定性, 如: Miara等[50]采用第一性原理對LCO、LiMnO2(LMO)和LFP陰極與電解質(zhì)LLZO, Li7La3Ta2O12(LLTO)在不同電壓范圍內(nèi)的界面反應(yīng)能量進(jìn)行計(jì)算(圖2), 結(jié)果表明: LLZO|LCO界面最為穩(wěn)定, 而LLZO|LFP界面最易反應(yīng)形成其它相。對于界面電化學(xué)穩(wěn)定性也可直接組裝全固態(tài)電池, 測試其在不同電流、電壓下的界面結(jié)構(gòu), 研究其界面的電化學(xué)穩(wěn)定性[51-52]。

圖2 0到5 V(vs. Li), 電解質(zhì)/正極界面相形成能[藍(lán)色, LCO; 紅色, LMO; 綠色, LFP; 粗線, LLZO; 細(xì)線, LLTO], 計(jì)算得到的材料內(nèi)在穩(wěn)定窗口如圖中底部行線所示[50]

2 正極/LLZO電解質(zhì)界面改性

針對正極/LLZO電解質(zhì)界面存在的問題, 改性的方法有: 正極復(fù)合、界面處理工藝優(yōu)化、界面層引入以及電解質(zhì)復(fù)合等。

2.1 復(fù)合正極

區(qū)別于傳統(tǒng)鋰離子電池正極, 在全固態(tài)電池的正極組成中引入高電導(dǎo)率的石榴石型固體電解質(zhì)粉、Li(CF3SO2)2N(LiTFSI)[53]、Li3BO3[54]、In2O5Sn[54]、丁二腈(SCN)[55]、In2(1–x)Sn2xO3[56]等形成復(fù)合正極, 在提升正極電導(dǎo)率的同時, 能夠改善正極與石榴石型固體電解質(zhì)片之間的界面相容性、降低界面阻抗。青島大學(xué)郭向欣課題組[53]將碳包覆的LFP與LiTFSI、聚偏氟乙烯(PVDF)為復(fù)合正極漿料涂覆在固體電解質(zhì)一側(cè)形成了緊密接觸的正極/LLZTO界面(圖3), LiTFSI與LFP的質(zhì)量百分比為75％時獲得了最佳的電化學(xué)性能。電池首次放電比容量由未改性的～5 mAh·g–1提升為150 mAh·g–1[53]。該課題組[55]還使用具有優(yōu)異可塑性和擴(kuò)散性的SCN構(gòu)造LFP:KB:PVDF:LiTFSI-SCN柔性復(fù)合正極, 改善了正極/電解質(zhì)界面接觸性能, 縮短了鋰離子在界面處的遷移距離, 降低了電池的界面電阻。添加7.5mol%的SCN改性后, 電池首次放電比容量由93.8 mAh·g–1提升至149.8 mAh·g–1, 100次循環(huán)后仍然保持143.5 mAh·g–1的容量。清華大學(xué)南策文課題組[56]將Li[Ni0.5Co0.2Mn0.3]O2, Li3BO3, In2(1–x)Sn2xO3(質(zhì)量比為54 : 27 : 9)復(fù)合正極漿料通過流延成型在LLZO-Ta電解質(zhì)上, 退火處理后有效改善了復(fù)合陰極和電解質(zhì)之間界面接觸性能。寧波材料所韓偉強(qiáng)課題組[57]將56wt%納米固體電解質(zhì)粉加入多孔結(jié)構(gòu)的正極材料中(圖4)改善正極與石榴石電解質(zhì)的界面接觸。組裝的Li/LLZO/LFP電池在室溫下展現(xiàn)出良好的循環(huán)性能和優(yōu)異的倍率性能, 首次放電比容量高達(dá)160.4 mAh·g–1, 循環(huán)100次后仍能保持136.8 mAh·g–1。Wakayama等[58-59]以聚苯乙烯-4-乙烯吡啶共聚物(PS-P4VP)作為基底材料, 再與LCO和10.6wt%的LLZO前驅(qū)體粉末混合, 蒸發(fā)溶劑后高溫煅燒, 制備了LiCoO2/Li7La3Zr2O12納米復(fù)合材料, 該納米復(fù)合材料具有大的界面表面積, 很好地改善了復(fù)合正極與電解質(zhì)片界面的接觸性能。改善后的LCO/LLZO/Li全固態(tài)電池在48 ℃下首次放電比容量為134 mAh·g–1, 100次循環(huán)后容量保持率為98%左右。

圖3 (a)復(fù)合陰極/LLZTO電解質(zhì)界面的SEM照片; (b) LLZTO電解質(zhì)表面的SEM照片; 在(c)二次電子和(d)背散射電子下的復(fù)合陰極的SEM照片[53]

在活性物質(zhì)中引入高電導(dǎo)率物質(zhì)形成復(fù)合正極能夠有效改善界面的潤濕性, 降低界面阻抗[60], 但復(fù)合正極材料中活性物質(zhì)所占整個正極質(zhì)量的比例偏低, 不利于全固態(tài)電池容量和能量密度的提升。

2.2 界面處理工藝優(yōu)化

由于正極與石榴石型電解質(zhì)固–固接觸面積小, 潤濕性差, 在界面處易形成氣孔、裂紋等缺陷導(dǎo)致高的界面阻抗。通過界面處理工藝, 如: 電解質(zhì)片上原位生長電極層[56]、脈沖激光沉積(PLD)[46,61]、溶膠–凝膠法[62]、共燒結(jié)[47,52,56]等方法能有效改善正極與石榴石型電解質(zhì)界面結(jié)構(gòu), 降低界面阻抗。Park等[52]將LCO與LLZO共燒, 發(fā)現(xiàn)溫度高于500 ℃時, 界面區(qū)域元素的相互擴(kuò)散形成了界面第三相。為了抑制共燒過程中界面第三相的形成, Ohta等[47]將Li6.8(La2.95, Ca0.05)(Zr1.75, Nb0.25)O12電解質(zhì)粉、Li3BO3和Al2O3組成的混合物在10 MPa下壓制成片, 空氣氣氛下, 790 ℃保溫40 h獲得了高致密的固體電解質(zhì)片。該電解質(zhì)片與LCO共燒結(jié)在改善界面物理接觸性能的同時, 消除了界面之間的副反應(yīng), 但組裝成的全固態(tài)電池仍然存在高的界面阻抗, 其首次放電比容量為78 mAh·g–1, 由于電池極化較大, 導(dǎo)致電池電位逐漸下降直至失效。為避免高溫處理(>500 ℃)過程中界面元素的擴(kuò)散[63], Ohta等[61]采用PLD工藝構(gòu)建LiCoO2/Li6.75La3Zr1.75Nb0.25O12界面, 以鋰為負(fù)極, 制備了全固態(tài)電池。室溫下, 測得LiCoO2/ Li6.75La3Zr1.75Nb0.25O12的界面阻抗約為170 Ω·cm2, 電池的首次放電比容量為129 mAh·g–1 [61], 100次循環(huán)后放電比容量為127 mAh·g–1, 容量保持率約為98%, 循環(huán)后正極與固體電解質(zhì)的界面處沒有明顯的元素擴(kuò)散現(xiàn)象, 表現(xiàn)出良好的結(jié)構(gòu)穩(wěn)定性和電化學(xué)穩(wěn)定性。

圖4 ASSLB的合成過程示意圖[57]

(a) Microscale LLZO particles; (b) Nanoscale LLZO particles; (c) Nanoscale LLZO slurry; (d) Cathode layer of LFP; (e) LLZO film; (f) All-solid-state battery of Li/LLZO/LFP

優(yōu)化界面的處理工藝中, PLD能夠精確控制沉積層的厚度, 強(qiáng)化界面接觸效果, 有效降低界面阻抗, 優(yōu)化電池性能, 但成本較高, 且不適于大規(guī)模生產(chǎn)應(yīng)用。共燒結(jié)、原位生長電極層等難以控制界面層的組成、結(jié)構(gòu)和形貌, 為獲得接觸良好、結(jié)構(gòu)穩(wěn)定的低阻抗界面, 簡單、低成本、易操控的界面處理工藝有待進(jìn)一步開發(fā)。

2.3 界面層的引入

在正極和電解質(zhì)界面之間引入過渡層(如Li3BO3[52,64]、Nb[65]、Ta[65]、Li4Ti5O12[54,66-67]、Li2SiO3[68]、Li3PO4[69]), 形成兩個新的界面, 能夠較好地改善原有正極/電解質(zhì)的界面相容性, 抑制界面高阻相的生成, 提高界面的結(jié)構(gòu)穩(wěn)定, 降低界面電阻。Kato等[65]利用PLD技術(shù)在LCO/LLZO界面處引入一層~10 nm的Nb層(圖5), 在600 ℃的O2氣氛下熱處理2 h形成Li-Nb-O無定形界面層, 使得LCO/LLZO界面電阻從2600 Ω·cm2降低到150 Ω·cm2, 改性后的全固態(tài)電池首次放電比容量由未改性的100 mAh·g–1提升至140 mAh·g–1。Li-Nb-O無定形界面層在降低界面阻抗方面起到三種混合作用: 抑制非鋰元素的相互擴(kuò)散和界面相的形成; 消除LLZO和LCO界面原子尺度上的缺陷; Li-Nb-O無定形層具有較高的Li+電導(dǎo)率, 便于Li+在界面處的快速移動。清華大學(xué)南策文課題組[56]在Li[Ni0.5Co0.2Mn0.3]O2表面涂覆一層Li-Ti-O, 700 ℃熱處理后表面原位形成了 Li[Ti0.1Mn0.9]2O4尖晶石, 提升了界面鋰離子的傳輸速率, 降低了界面阻抗, 電池放電比容量由未改性的112.7 mAh·g-1提高到123.3 mAh·g–1。另外, 將聚偏氟乙烯–六氟丙烯(PVDF-HFP)作為固體電解質(zhì)和固體電極之間的夾層, 構(gòu)建成石榴石/PVDF-HFP/電極結(jié)構(gòu)[70], 使石榴石電解質(zhì)與正極的界面阻抗由6.5×104Ω·cm2降至248 Ω·cm2, 也起到了良好的界面改性效果。

圖5 (a)界面引入Nb層改性前后的LLZO/LiCoO2界面示意圖; (b) Nb層改性后LLZO/LiCoO2界面的截面-HAADF-STEM照片, 虛框?yàn)樵胤植紙D[65]

界面層的引入, 本質(zhì)上改變了原有的正極/電解質(zhì)界面結(jié)構(gòu), 形成了正極/界面層/電解質(zhì)的三明治結(jié)構(gòu), 起到兩種較好的效果: 第一, 為正極與電解質(zhì)界面提供了緩沖區(qū), 改善界面的相容性; 第二, 能夠抑制和引導(dǎo)正極、電解質(zhì)之間的元素互擴(kuò)散, 改善界面層的鋰離子擴(kuò)散速率, 降低界面阻抗。但目前對于引入界面層的成分和結(jié)構(gòu)的選擇、界面層與正極和電解質(zhì)的兼容性有待深入理解和系統(tǒng)性地研究。

2.4 復(fù)合型固體電解質(zhì)

2.4.1 LLZO-聚合物復(fù)合型

將聚合物(如: 聚氧化乙烯(PEO)[71-72]、PVDF[73]、PVDF-HFP[74-75]、聚丙烯腈(PAN)[76]、聚碳酸丙烯酯(PPC)[76-77])與石榴石型固體電解質(zhì)復(fù)合形成柔性復(fù)合型電解質(zhì), “軟化”電解質(zhì)界面, 增大正極與固體電解質(zhì)的接觸面積, 有效降低了界面阻抗。郭向欣課題組[71]將LLZTO納米顆粒作為填料分散在PEO中, 形成40 μm厚的柔性復(fù)合電解質(zhì)膜, 增加了正極/電解質(zhì)的界面接觸面積, 有效降低了界面阻抗; 但隨著聚合物不可避免的分解, PEO: LLZTO膜結(jié)構(gòu)遭到破壞, 電池結(jié)構(gòu)穩(wěn)定性惡化。清華大學(xué)南策文課題組[72]將PEO同時引入到正極與固體電解質(zhì)中, 制備出了具有良好柔韌性的PEO: In2O5Sn:LFP復(fù)合正極與PEO:Al-LLZTO復(fù)合固體電解質(zhì), 在復(fù)合正極與復(fù)合電解質(zhì)間形成了無孔隙的致密界面, 如圖6所示。以鋰為負(fù)極, 組裝成的全固態(tài)電池具有155 mAh·g–1的高放電比容量, 在受到惡劣條件(反復(fù)彎曲, 甚至在放電過程中切割成小塊)時仍能正常工作, 表現(xiàn)出優(yōu)良的電化學(xué)性能和物理穩(wěn)定性。采用PVDF制備的石榴石型復(fù)合 電解質(zhì)也能夠有效改善正極與電解質(zhì)之間的界 面結(jié)構(gòu)和界面接觸性能[73], 如: 由PVDF基體和Li6.75La3Zr1.75Ta0.25O12填料組成的柔性復(fù)合膜, 在環(huán)境溫度下表現(xiàn)出優(yōu)異的電化學(xué)性能、機(jī)械性能和良好的熱穩(wěn)定性。與PVDF相比, PVDF-HFP具有電子吸附效應(yīng)官能團(tuán)和低結(jié)晶度, 呈現(xiàn)出更高的電導(dǎo)率。中科院北京納米能源與系統(tǒng)研究所孫春文課題組[74]將Li7La3Zr2O12作為填料與PVDF-HFP復(fù)合, 制備出的復(fù)合電解質(zhì)膜與正極材料具有良好的界面接觸性能和低的固/固界面阻抗, 能夠卷繞、彎曲, 表現(xiàn)出優(yōu)越的機(jī)械性能。郭向欣課題組[76]嘗試用PPC與5wt%的Li6.75La3Zr1.75Ta0.25O12電解質(zhì)復(fù)合(PPCL-SPE), 制得的復(fù)合型電解質(zhì)表現(xiàn)出良好的機(jī)械強(qiáng)度(6.8 MPa)、優(yōu)異的柔韌性以及在20 ℃下良好的倍率(5)性能, 對稱Li/PPCL-SPE/Li電池以0.1 mA?cm–2電流密度循環(huán)1000 h后也沒有發(fā)現(xiàn)鋰枝晶形成?；诖藦?fù)合電解質(zhì)的LFP/PPCL-SPE/Li全固態(tài)電池在1倍率下循環(huán)200次后仍具有95%的容量保持率, 采用Li4Ti5O12負(fù)極的LFP/PPCL-SPE/Li4Ti5O12在1倍率下首次放電比容量為110 mAh·g–1, 循環(huán)800次后容量保持率為95%。隨后該課題組[77]采用PPC/(16wt%) LLZTO/LiTFSI復(fù)合電解質(zhì)、Si負(fù)極、LFP正極組裝的全電池在室溫下, 0.1循環(huán)100次后, 容量保持率為82.6%。

圖6 含15wt%聚合物的LFP復(fù)合陰極與LLZTO復(fù)合電解質(zhì)的界面SEM照片[72]

2.4.2 LLZO-液態(tài)電解質(zhì)復(fù)合型

LLZO-液態(tài)電解質(zhì)復(fù)合電解質(zhì)中包含少量離子液體或者液體電解質(zhì), 其中液體電解質(zhì)充當(dāng)潤濕劑, 通過增強(qiáng)正極與LLZO型電解質(zhì)界面浸潤性來改善界面接觸性能。離子液體與液體電解質(zhì)不同的是, 通過離子液體浸潤的正極與LLZO型電解質(zhì), 仍然表現(xiàn)出固態(tài)電解質(zhì)的狀態(tài)。為進(jìn)一步改善全固態(tài)電池的性能, 郭向欣課題組[78]在LLZO與PEO復(fù)合電解質(zhì)中加入離子液體(IL)潤濕界面(PEO/LLZTO@ IL), 獲得了具有低界面阻抗的復(fù)合膜, 表現(xiàn)出較好的電化學(xué)穩(wěn)定性和高容量保持率, 組成的LFP/PEO/ LLZTO@IL/Li固態(tài)電池(圖 7)首次放電比容量達(dá)133.2 mAh·g–1。北京大學(xué)潘鋒課題組[79]將含有Li+的離子液體封裝于金屬有機(jī)框架(MOF)中, 形成離子導(dǎo)體(LIM), 再將LLZO粉與20wt％ LIM簡單混合后, 獲得了具有5.2 V寬電化學(xué)窗口、離子電導(dǎo)率為1.0×10–4S·cm–1的復(fù)合電解質(zhì)。研究表明: 將LIM離子導(dǎo)體引入正極時, LIM通過其三維開放的晶體結(jié)構(gòu)為原子尺度上的LLZO和正極顆粒提供了豐富的接觸點(diǎn), 在電池內(nèi)部建立高效的Li+傳輸網(wǎng)絡(luò), 顯著降低了全固態(tài)電池的界面阻抗。組裝后的電池具有良好的倍率性能和循環(huán)穩(wěn)定性, 循環(huán)150次后容量保持率為97%。

在固體電解質(zhì)中引入用于傳統(tǒng)鋰離子電池體系中的少量液態(tài)電解液, 形成固–液混合型的電解質(zhì), 在改善固體電解質(zhì)/電極界面相容性的同時, 也可以有效降低界面阻抗, 改善全固體電池的電化學(xué)性能。本課題組[80]采用Li7La3Zr1.5Ta0.5O12和常規(guī)碳酸鹽基液態(tài)電解液組成復(fù)合電解質(zhì), 加入少量的-BuLi, 制備出的LiFePO4/復(fù)合電解質(zhì)/L全固態(tài)電池, 以100 μA·cm–2循環(huán)200次后, 容量保持率約為87%, 繼續(xù)以200 μA·cm–2循環(huán)200次后的容量保持率為99% (以200 μA·cm–2下的首次放電容量為基準(zhǔn))。

復(fù)合電解質(zhì)在改善與正極材料界面潤濕性、降低界面阻抗的同時, 具備良好的柔韌性和機(jī)械加工性能。目前, 采用復(fù)合電解質(zhì)組裝的全固態(tài)電池, 在使用過程中為避免復(fù)合電解質(zhì)產(chǎn)生結(jié)構(gòu)退化, 多數(shù)情況下對電池的使用溫度和電壓有一定的要求, 限制了全固態(tài)電池性能的發(fā)揮, 因而, 電化學(xué)循環(huán)過程中復(fù)合電解質(zhì)結(jié)構(gòu)的穩(wěn)定性還需更進(jìn)一步的改善。

3 結(jié)論與展望

石榴石型固態(tài)電解質(zhì)具有高的離子電導(dǎo)率, 良好的化學(xué)穩(wěn)定性以及寬的電化學(xué)窗口, 在全固態(tài)電池中的應(yīng)用前景廣闊。然而, 固態(tài)電解質(zhì)與電極之間差的界面潤濕性能、高阻抗界面相以及界面結(jié)構(gòu)應(yīng)力使得石榴石型全固態(tài)電池的放電比容量低、倍率性能差、循環(huán)壽命短, 成為制約石榴石型全固態(tài)電池應(yīng)用的瓶頸。目前改性的方法有: 在活性材料中引入高電導(dǎo)率物質(zhì)形成復(fù)合正極、優(yōu)化界面處理工藝、引入界面過渡層、制備石榴石復(fù)合電解質(zhì)等。界面改性研究已取得了顯著的成果, 全固態(tài)電池性能見表1, 但仍有很多關(guān)鍵性的問題有待解決:

圖7 PEO/LLZTO@IL膜合成和全固態(tài)電池示意圖[78]

表1 以Li7La3Zr2O12石榴石型固體電解質(zhì)組裝的全固態(tài)電池性能

1) 不同種類活性材料與石榴石型固體電解質(zhì)界面潤濕性差的微觀機(jī)制還不清楚。

2) 復(fù)合正極材料中, 活性物質(zhì)僅占復(fù)合正極質(zhì)量的50%左右, 比例偏低, 不利于全固態(tài)電池容量和能量密度的提升。在不降低電池電導(dǎo)率的同時增加正極活性物質(zhì)的含量是全固態(tài)電池應(yīng)用的關(guān)鍵之一。

3) 對于界面層成分和結(jié)構(gòu)的選擇以及界面層與正極、電解質(zhì)的界面相容性缺乏深入理解和系統(tǒng)性的研究。

4) 實(shí)際應(yīng)用中, 固體電解質(zhì)片薄且韌性較差。引入聚合物形成的柔性復(fù)合電解質(zhì)膜能夠大幅改善石榴石型固體電解質(zhì)的柔韌性, 但聚合物導(dǎo)電性能較差, 且?；瘻囟雀呷菀孜鼍? 因而對于電池的使用溫度有一定的要求, 限制了全固態(tài)電池的應(yīng)用環(huán)境。

盡管以石榴石型固體電解質(zhì)組裝成的全固態(tài)鋰電池存在著諸多問題, 但隨著正極與石榴石固體電解質(zhì)界面局域結(jié)構(gòu)、鋰離子輸運(yùn)機(jī)制等研究的不斷深入, 具有寬電壓窗口、高能量密度、高安全性的石榴石型全固態(tài)鋰電池的應(yīng)用前景將更加廣闊。

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Recent Advancements in Interface between Cathode and Garnet Solid Electrolyte for All Solid State Li-ion Batteries

LI Dong1,2, LEI Chao1,2, LAI Hua3, LIU Xiao-Lin1,2, YAO Wen-Li1,2, LIANG Tong-Xiang1, ZHONG Sheng-Wen1,2

(1. School of Materials Science and Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, China; 2. Jiangxi Key Laboratory of Power Battery and Materials, Jiangxi University of Science and Technology, Ganzhou 341000, China; 3. School of Resources and Environmental Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, China)

All-solid-state lithium battery (ASSLB) with inorganic solid state electrolytes is one of promising candidates for electric vehicles and large-scale smart grids for storage of alternative energy resources due to their benefits in safety, energy density, operable temperature range, and longer cycle life. As the key component in ASSLB, inorganic lithium-ion-based solid-state electrolytes (SSEs), especially the garnet-type solid electrolytes that own ionic conductivities in the order of 10–3S·cm–1at room temperature and are relative safeLi metal, have obvious advantages in ASSLB. However, interfacial instability and their poor solid?solid contact between garnet and cathode result in high interfacial resistance, low efficiency, and poor cycle performance. Based on these understandings and analyses of interface characteristics and issues, this work presents a brief review on modification of interface, covering composite cathode, composite electrolyte, interface engineering, and interface layer．Some approaches of improving interface wettability and future research directions of ASSLB are given as well, which endeavor to realize the practical applications of ASSLB.

inorganic solid state electrolyte; composite electrolyte; interfacial wettability; interfacial impendence; interface modification; review

TM911

A

1000-324X(2019)07-0694-09

10.15541/jim20180512

2018-10-31;

2019-01-15

國家自然科學(xué)基金(51874151); 江西省教育廳一般項(xiàng)目(GJJ170510); 江西省科技支撐計(jì)劃項(xiàng)目(20151BBE50106)National Natural Science Foundation of China (51874151); General Program by Jiangxi Provincial Department of Education (GJJ170510); Science and Technology Support Project of Jiangxi Province (20151BBE50106)

李棟(1982–), 男, 博士, 講師. E-mail: libehave@jxust.edu.cn