近年來，科學家致力發展高電壓的正極材料和無陽極鋰金屬電池，以提升電池的使用電容量，而傳統電解液已經無法負擔新型電池系統的運作。因為傳統電解液含有過多的游離溶劑，以至於無法負荷高電位的環境，以及容易沉積富含有機化合物的固態電解液介面，導致電容量和循環壽命會急速下降。科學家極力發展新型的電解液來匹配新穎的電池系統，其中的高濃度電解液是一個解方，但是高濃度電解液有黏度過高的問題，這會讓電解液不易潤濕隔離膜，而形成多餘的介面問題，影響電池整體的循環效率。
本次研究為開發一款新型局部高濃度電解液，此電解液以LiPF6為主要鹽類，並且以ethylene carbonate(EC)/ethyl methyl carbonate(EMC)3:7(v:v)為主要溶劑，調配3MLiPF6-EC/EMC3:7(v:v)，並以1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether(TTE) 為稀釋劑，TTE的添加量為整體電解液體積量的50v %。此款電解液通稱-BC-1.5M-EC/EMC/TTE3:7:10(v:v:v)，此電解液對於隔離膜的接觸角為28.5°優於傳統電解液的接觸角為48.19°，可以證明新型電解液對於隔離膜的親和力優於傳統電解液，接著新型電解液在Cu‖LiNi0.8Mn0.1Co0.1O2的無陽極鋰金屬電池中，第1圈的庫倫效率為91.87 %，20圈的平均庫倫效率為94.52 %，第20圈的電容量保持率為37.21 %，其整體效能優於傳統電解液的表現。在Li‖ LiNi0.8Mn0.1Co0.1O2的電池中，第一圈的庫倫效率為91.67 %，其優於傳統電解液的90.98 %，且在高電壓的環境中，在正極材料表面會形成穩定的介面而且電解液本身的氧化電位較高，則沒有任何分解反應的發生。接著在SEM、XPS、介面阻抗分析皆有不錯的表現。接下來，為了提升局部高濃度電解液的電化學表現，探究添加劑對於局部高濃度電解液的影響，劑量從0.5wt %、1wt %、1.5wt %和2wt %進行探討。添加LiDFOB之後，對於電池的正極材料具有良好的影響性，在Li‖ LiNi0.8Mn0.1Co0.1O2的電池中，其10圈的平均庫倫效率為100 %，優於BC-1.5M-EC/EMC/TTE(3:7:10 v:v:v)的99.8 %，而在Cu‖ LiNi0.8Mn0.1Co0.1O2的全電池中，其第20圈的電容量保持率為41 %優於BC-1.5M-EC/EMC/TTE(3:7:10 v:v:v)的37.21 %。由此可知，添加LiDFOB可以改變SEI層的組成，使無陽極鋰金屬電池呈現更好的循環壽命以及電容量的維持率。

We are constantly searching for lithium-ion batteries with higher capacity. High voltage cathode, such as NMC811 (LiNi0.8Mn0.1Co0.1O2) paired with an anode-free cell design, can be a potential solution for this challenge. However, traditional electrolytes have a low potential window range and form a weak and more organic solid-electrolyte interface because of more free solvents. It cannot fulfill the corresponding requirements. Though highly concentrated electrolyte has been reported, the concentrated electrolyte is usually highly viscous and will not quickly soak the separator. It also results in new interfacial issues and poor cycling performance.
Here, we report a locally concentrated electrolyte, 3 M LiPF6 in ethylene carbonate/ethyl methyl carbonate (3:7 v:v ratio) diluted with 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE). Different percentages of TTE were applied to optimize the electrolyte formulation. The addition of 50v %TTE can give the best performance, which is stable between 2.5 V~4.5 V. It performs good cycle life in the Li∥NMC811 half-cell with the first coulombic efficiency of 93 %, and average coulombic efficiency of 99.6 % comparing with traditional electrolyte, namely 1 M LiPF6 EC/EMC (1:1 v:v ratio) whose first coulombic efficiency is 85.9 %, and average coulombic efficiency of 98.7 % at the current density of 0.2mA/cm. In the anode-free full cell (Cu∥NMC811), the first coulombic efficiency is 91.9 %, and the average coulomb efficiency is 94.5 % after the 20th cycle. The capacity retention is 37.2 % after the 20th cycle, which is better than the traditional electrolyte with 4.26 %. The separator (Celgard 2325) affinity for the electrolyte has been improved by the new formulation with decreased contact angle from 48.19° to 28.51°. It can be attributed to decreasing the amount of the free solvent and improving the composition of the solid-electrolyte interface.
For improving the performance of locally concentrated electrolytes, this research is about the various amount of lithium difluoro(oxalate)borate (LiDFOB) to modify the locally concentrated electrolyte. In the LiNi0.8Mn0.1Co0.1O2‖Li cell, the coulombic efficiency of 50 cycles is 99.9 % that is better then the BC-1.5M that is 99.6 %, and then, in the LiNi0.8Mn0.1Co0.1O2 ‖Cu half cell, the areal capacity retention at 20th cycle is 41.1 % that is better then BC-1.5M that is 37.2 %. according to the results, LiDFOB can change the composition of SEI layer to raise the cycle life.

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