【摘要】：金屬鋰具有極高的理論比容量(3860 mAh/g)和最負的電極電位(-3.04 V vs.標(biāo)準(zhǔn)氫電極),是最有前景的下一代鋰電池負極材料。然而,循環(huán)過程中鋰枝晶的形成和生長以及金屬鋰與電解液的反應(yīng),導(dǎo)致鋰負極安全性和穩(wěn)定性差的問題,嚴(yán)重限制金屬鋰負極的應(yīng)用。針對以上問題,本文分別從電解液和鋰負極結(jié)構(gòu)兩方面開展改性研究工作,抑制鋰枝晶的形成,改善鋰二次電池金屬鋰負極的安全性和穩(wěn)定性。對電解液的改性方面,在碳酸酯類電解液中使用添加劑硼酸三(2,2,2-三氟乙基)酯(TTFEB)改善電解液的性質(zhì)和鋰負極界面性質(zhì)。一方面,通過TTFEB中缺電子中心硼原子與鋰鹽中陰離子的耦合作用,促進鋰鹽的解離,提高電解液的鋰離子遷移數(shù),從而提高鋰離子在電解液中的遷移能力,促進鋰電極表面濃度的均勻分布,抑制鋰枝晶的產(chǎn)生。另一方面,理論計算和電化學(xué)測試結(jié)果表明TTFEB具有較高的還原分解電位,可以優(yōu)先于電解液在負極表面分解形成富含LiF的SEI膜,從而提高SEI膜的穩(wěn)定性,抑制枝晶的產(chǎn)生。加入2%TTFEB后,鋰電極的可逆性和穩(wěn)定性顯著提升,Li|Cu電池在0.1 mA/cm2電流密度下的循環(huán)效率由90%提高至96%,Li|Li對稱電池的循環(huán)壽命提高至1000 h以上,界面阻抗更小且更為穩(wěn)定。對鋰電極在循環(huán)過程中的形貌研究發(fā)現(xiàn),TTFEB的加入明顯抑制鋰枝晶的產(chǎn)生,在SEI膜組分的研究中,證實TTFEB可以形成富含LiF的SEI膜,抑制電解液的還原分解。TTFEB在鋰二次全電池中應(yīng)用時,2%TTFEB的加入顯著提高了Li|LiFePO4電池的循環(huán)穩(wěn)定性和倍率性能,500次循環(huán)后的容量保持率和庫倫效率都明顯提升,高倍率下的容量保持率明顯提高,且TTFEB能抑制全電池中金屬鋰電極表面鋰枝晶的形成以及在循環(huán)過程中逐漸粉化。對鋰負極結(jié)構(gòu)的改性方面,采用真空熱蒸鍍的方法,在鋰電極摻入少量的In制備富鋰Li-In合金負極取代鋰電極作為鋰二次電池負極材料。通過對工藝條件的探索,可知蒸發(fā)源中In含量為20%,電源輸出功率為95 W時,富鋰Li-In合金的電化學(xué)性能最好,其循環(huán)穩(wěn)定性相比于Li電極顯著提升。在Li|LiFePO4電池中的循環(huán)壽命可由140次提高至超過300次,對稱電池界面穩(wěn)定性和循環(huán)穩(wěn)定性顯著提升,鋰溶解/沉積可逆性明顯提高。通過對電極結(jié)構(gòu)、形貌、表面膜組分的表征和分析,研究富鋰Li-In合金改善鋰電極循環(huán)穩(wěn)定性的作用機制。研究結(jié)果表明,In的加入可以抑制循環(huán)過程中鋰枝晶的形成和電極劇烈體積變化引起的粉化現(xiàn)象,同時,鋰電極與電解液的反應(yīng)活性降低,SEI膜的穩(wěn)定性提高,從而減少活性鋰的消耗,顯著提升鋰電極的循環(huán)穩(wěn)定。
[Abstract]:Lithium metal has extremely high theoretical specific capacity (3860 mAh/g) and the most negative electrode potential (-3.04 V vs.). Standard hydrogen electrode) is the most promising cathode material for the next generation of lithium batteries. However, the formation and growth of lithium dendrite and the reaction between lithium metal and electrolyte lead to the problem of poor safety and stability of lithium anode, which seriously limits the application of lithium anode. In order to improve the safety and stability of lithium anode in lithium secondary battery, the modification of electrolyte and lithium anode structure was carried out in order to restrain the formation of lithium dendrite and improve the safety and stability of lithium negative electrode. In the aspect of modification of electrolyte, the additive of triborate trisodiborate (TTFEB) was used in carbonate electrolyte to improve the properties of electrolyte and the interface property of lithium negative electrode. On the one hand, the coupling of electron deficient boron atoms in TTFEB and anions in lithium salts can promote the dissociation of lithium salts and increase the lithium-ion mobility of the electrolyte, thus enhancing the mobility of lithium ions in the electrolyte. Promote the uniform distribution of lithium electrode surface concentration and inhibit the formation of lithium dendrite. On the other hand, the theoretical calculation and electrochemical measurements show that TTFEB has a higher reduction potential and can be preferentially decomposed on the anode surface to form LiF rich SEI film, thus improving the stability of SEI film and inhibiting the dendrite formation. After the addition of 2%TTFEB, the reversibility and stability of the lithium-ion electrode significantly improved the cycle efficiency of Li Cu cell at current density of 0.1 mA/cm2 from 90% to 96%. The cycle life of Li symmetric battery was increased to more than 1000 h, and the interface impedance was smaller and more stable. It was found that the addition of TTFEB significantly inhibited the formation of lithium dendrites. In the study of the composition of SEI films, it was proved that TTFEB can form SEI films rich in LiF. When inhibiting the reduction and decomposition of electrolyte. TTFEB was used in the lithium secondary battery, the addition of TTFEB significantly improved the cycle stability, capacity retention and Coulomb efficiency of the Li LiFePO4 battery after 500 cycles. The capacity retention rate at high rate was significantly increased, and TTFEB could inhibit the formation of lithium dendrite on the surface of the metal lithium electrode and the gradual pulverization during the cycling process. In the aspect of modification of lithium negative electrode structure, lithium Li-In alloy anode electrode was prepared by vacuum thermal evaporation method and a small amount of In was added into lithium electrode to replace lithium electrode as cathode material for lithium secondary battery. Through the exploration of the process conditions, it can be seen that the electrochemical properties of the lithium-rich Li-In alloy are the best when the content of In in the evaporator is 20 and the power output power is 95 W, and the cyclic stability of the lithium-rich Li-In alloy is significantly improved than that of the Li electrode. The cycle life of Li LiFePO4 battery can be increased from 140 times to more than 300 times, the interface stability and cycle stability of symmetric cell are improved significantly, and the reversible lithium-ion dissolution / deposition is improved obviously. The mechanism of improving the cyclic stability of lithium electrode by Li-rich Li-In alloy was studied by the characterization and analysis of electrode structure, morphology and composition of surface film. The results show that the addition of in can inhibit the formation of lithium dendrite and the powder phenomenon caused by the drastic volume change of the electrode. At the same time, the reaction activity of lithium electrode with electrolyte decreases the stability of SEI film. Thus, the consumption of active lithium is reduced, and the cycle stability of lithium electrode is improved significantly.
【學(xué)位授予單位】：哈爾濱工業(yè)大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2017
【分類號】：TM912

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