富鋰層狀複合正極xLi2MnO3‧(1-x)Li(Ni1/3Mn1/3Co1/3)O2擁有非常高電容量(>250 mAh/g)與獨特的氧離子氧化還原而備受矚目，其氧離子氧化還原反應是由複合正極中之Li2MnO3觸發，但富鋰層狀複合正極在實際應用上仍受限於氧氣釋放、低循環電容量維持率、較差的高速充放電表現，這些問題主要可歸咎於富鋰正極在循環後發生的不可逆相變化。本研究旨在了解Li2MnO3第一次充放電的特殊電化學反應與後續相變化的關聯，並藉由臨場X光繞射分析研究氧氣釋放行為與後續造成電化學表現衰退的相變化之間的關聯性。
由缺陷化學的觀點可知氧氣釋放後形成的氧空缺會影響結構穩定性，本研究以高價數鎢離子摻雜來提升Li2MnO3之導電率、電化學表現與結構穩定性，藉由電荷補償效應摻雜6+價鎢離子，能使部分錳離子由4+還原為3+，經鎢摻雜後Li2MnO3之導電率能提高兩個數量級，電容量也由181 mAh/g提升至217 mAh/g。TEM結果顯示第一圈充放電後，表面區域結構由原先層狀結構變成層狀與尖晶石之混和結構；20圈循環後相轉變成Mn3O4-type尖晶石結構。S/TEM HAADF影像觀察到錳離子遷移到鄰近的鋰離子層位置。臨場X光繞射分析觀察到由氧空缺引起之局部晶格坍塌現象。
綜合上述結果來解釋Li2MnO3的衰退機制，第一次充電氧氣釋放形成的氧空缺引起局部晶格坍塌，在晶格坍塌狀態下錳離子會遷移到鄰近的鋰離子層位置並阻礙後續鋰離子遷入，造成不可逆地結構變化，此不可逆地結構相變化會逐漸由表面區域擴展至內部，造成嚴重的電化學表現劣化。藉由調整電位視窗來抑制氧空缺生成，富鋰層狀複合正極第一圈庫倫效率能由77.5%提高至89.2%，50圈電容量維持率由64.9%提高至82.2%。同時藉由鎢摻雜能使富鋰層狀正極第一次放電電容量由256 mAh/g提高到272 mAh/g，200圈1C充放電測試下電容量維持率能由32.9%提高至59.5%，這些結果可歸功於鎢摻雜對結構穩定性的改善，使更多鋰離子能進行可逆式嵌入回Li2MnO3正極。

Li-rich layered composite oxide xLi2MnO3‧(1-x)Li(Ni1/3Mn1/3Co1/3)O2 has received much attention for its astounding high capacity (>250 mAh/g) among several cathode materials due to the participation of oxygen anionic redox. It is widely recognized that Li2MnO3 integrated is essential to trigger the anionic redox reactions. However, the practical application of Li-rich layered oxide hindered by O2 evolution, low cyclic retention, and poor rate performance. Irreversible phase transformation of Li-rich layered oxide during cycling is responsible for these issues. In this study, the main objective is to examine Li2MnO3 based on its electrochemical behavior, defect chemistry with doping effect, and phase transformation after charging. In situ X-ray diffraction and scanning/transmission electron microscopy (S/TEM) were also utilized for analyzing and understanding the correlation between O2 evolution from anionic redox and the deleterious electrochemical performance due to irreversible phase transformation.
In the defect prospective, oxygen vacancies formed from O2 evolution impact the structural stability. Tungsten was chosen as dopant to improve electrical conductivity, electrochemical performance, and structure stability of Li2MnO3 as well as Li-rich composite cathode. Li2Mn1-xWxO3 (x=0, 0.03, 0.045, 0.06, 0.10) cathode materials were sucessfully synthesized by sol-gel method. The increace of electrical conductivity of Li2Mn1-xWxO3 (x>0.045) has been improved up to 2 orders of magnitude due to presence of Mn3+ charge-compensated by W6+. Charge-transfer resistance in EIS is effectively suppressed due to the enhanced electrical conductivity and Li diffusion. The charge-discharge capacities of Li2MnO3 are also enhanced from 181 mAh/g to 217 mAh/g by tungsten doping. TEM results showed that the structure at surface region begins to phase transform to mixed layered and spinel-like structure after first cycle. The structure after 20 cycles eventually phase transformed to Mn3O4-type spinel. Mn migration was determined by S/TEM HAADF image. Vacancy-induced local lattice collapse was indicated by in situ Xray diffraction.
Based on these results, it is realized that the degradation of Li2MnO3 begins from oxygen vacancy formation by O2 evolution during first charge. The oxygen vacancy-induced lattice collapse enables manganese ions to migrate into adjacent lithium layers. The Mn migration hinder the lithium-ion intercalation and cause irreversible structural and chemical evolution. The deleterious structural transformation gradually expands from surface into bulk, causing the degradation of electrochemical performance. The reduction of upper cut-off voltage was adopted to suppress the formation of oxygen vacancies in Li-rich composite layered oxide. As a result, the improvement on first coulombic efficiency is from 77.5% to 89.2% and cyclic retention from 64.9% to 82.2% during 50 cycles. In addition, the first discharge capacity of 0.5Li2MnO3‧0.5Li(Ni1/3Co1/3Mn1/3)O2 through tungsten doping increase from 256 mAh/g to 272 mAh/g. The cyclic retention after 200 cycles increases from 32.9% of pristine to 59.5% at 1C, which is attributed to better structure stability caused by tungsten doping for more lithium ions intercalation.

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