鉭摻雜之鹵化物型固態電解質應用於鋰離子電池

Abstract

隨科技之進步，電動車與數位產品之普及，鋰離子二次電池之需求急速上升。為提升鋰離子二次電池之能量密度與安全性，鹵化物固態電解質為不可或缺之選項。其相對液態電解質可大幅提升安全性，且其匹配高電壓陰極可顯著增加鋰離子二次電池之能量密度。 本研究重點於鹵化物型固態電解質鋰銦氯(Li3InCl6)中摻雜高價數鉭(Ta)離子，以增加結構中之鋰空位(lithium vacancy)，以提升離子導電度(ionic conductivity)。故本研究使用針對鉭離子進行Li3−2xIn1−xTaxCl6系列之合成，於其中Li2.8In0.9Ta0.1Cl6作為電解質具最佳離子導電度於室溫達1.27 mS/cm，此說明鉭摻雜可增加鋰空位，並降低活化能(activation energy)。其中最優化之條件為前驅物經球磨法後並於200°C條件下真空燒結4 h，產物之固態電解質活化能僅0.293 eV，並證實其熱穩定性達410°C。本研究亦藉中子繞射與X光吸收光譜鑑定Li3−2xIn1−xTaxCl6系列，使用結構之精修證明鉭摻雜不僅可增加鋰空位，且可提升Li2之位置佔有率，提升鋰離子遷移能力，並說明過多之鉭摻雜將使結構扭曲破壞，使Li3之四面體為鋰離子之遷移瓶頸，而降低離子導電度。並藉理論計算證明鉭摻雜有助於降低二維遷移之活化能，提升離子導電度。 同時為解決陰極與固態電解質之電位不匹配，本研究使用鈮酸鋰(LiNbO3)塗層保護鎳錳酸鋰(LiNi0.5Mn1.5O4; LNMO)。本研究亦說明鈮酸鋰塗層可保護鎳錳酸鋰陰極，避免鎳錳酸鋰陰極與固態電解質發生反應。並揭示無鈮酸鋰塗層將使Li2.8In0.9Ta0.1Cl6之In3+與Ta5+還原且鎳錳酸鋰之Mn3+氧化。 最後本研究組裝Li2.8In0.9Ta0.1Cl6之鈷酸鋰(LiCoO2; LCO)全固態電池，其首次放電電容量可達135.6 mAh/g，於第50次循環之放電電容量達121.8 mAh/g，其保持率(retention rate)於50次循環後達90%。其中Li2.8In0.9Ta0.1Cl6之LNMO全固態電池首次放電電容量可達111.6 mAh/g。

As technology progresses, electric vehicles (EVs) and other products spread throughout our lives, and the demand for lithium secondary batteries increases. Halide solid-state electrolytes (HSSEs) are the best candidates for pursuing safety and energy density. Compared to liquid electrolytes (LE), HSSEs are not flammable and show high energy density. Moreover, their compatibility with high-voltage cathodes is extraordinary. This research focuses on doping high-valent Ta in HSSEs Li3InCl6 to increase the vacancy in the structure, increasing in ionic conductivity. A series of electrolyte Li3−2xIn1−xTaxCl6 was synthesized. The highest ionic conductivity was Li2.8In0.9Ta0.1Cl6, with an ionic conductivity of 1.27 mS/cm and an activation energy of 0.293 eV. The optimized process was precursors sintered at 200°C for 4 h in a vacuum. The refinements of neutron powder diffraction reveal that the dopant Ta could increase the lithium vacancy in position Li2. On the other hand, redundant dopants result in structure distortion, and the ionic conductivity decreases. Also, the theoretical calculation proves the decrease of activation energy in the 2D conduction pathway. To solve the incompatibility between the electrolyte and cathode, we use LiNbO3 (LNO) to protect the LiNi0.5Mn1.5O4 (LNMO) cathode. This research demonstrates the effect of LNO, which could avoid the reaction between the halide solid electrolyte and the cathode. The absence of LNO could lead to the oxidation of Mn3+, and the reduction of In3+ and Ta5+. In the last part of this research, the discharge capacity of the full cell using Li2.8In0.9Ta0.1Cl6 and LiCoO2 (LCO) cathode was 135.6 mAh/g and 121.8 mAh/g at the 1st and 50th cycle, respectively. The discharge capacity of the full cell using the LNMO cathode shows 111.6 mAh/g at the 1st cycle。