| 摘要: | 我們成功的達成以非揮發性、具阻燃性、高電化學及高熱穩定性的電解質應用於高能高電容量之鋰離子二次電池。室溫離子液體被用於鋰離子電池是由於其獨特的性質，如:低蒸氣壓、在高極性溶劑有高溶解度等，最重要的是因其有高電位窗及高熱穩定性。然而，它的高黏滯性以及相對較低的導電度阻礙了整體鋰電池的效能。另一發展二十年以上的方法，即是使用高分子電解質來增加鹽類的解離及離子的傳遞運動；然而，高分子本身的特性─自由空間過小、高鏈段黏滯性及低介電常數，因而阻礙了這個離子傳導機制。
我們的研究揭示了一個增加高分子電解質導電度的可能性:使用二氧化鈦奈米管來提升鹽類的解離度，並且提供離子傳遞通道，在沒有使用任何揮發性電解質的情形下，使其離子導電度提升，並且接近10-3 S/cm。在目前的實驗中，我們發展出一新穎的電解質系統，利用離子液體與高介電常數的聚偏二氟乙烯－共-六氟丙烯，並且加入微量二氧化鈦奈米管來製備無機物複合離子液體高分子電解質。
在此系統中，離子液體與鋰鹽完全解離，並且藉由二氧化鈦奈米管的加入增加其運動性。因此，鋰離子導電度在室溫(25℃)下可達到 S/cm，並且在80℃下達到 S/cm。此高離子導電度的高分子電解質系統適用於高電壓的範圍，並且具有高電、熱穩定性，適用於高電壓正極材料的高能鋰電池、鋰/硫電池、鋰/空氣電池，或其他使用高電容負極材料鋰電池。由於沒有固態電解質介面形成，因此高循環壽命是可以被預期的一項結果。且因為電解質無揮發性，因此其安全性可以獲得相對的保證。製成薄膜的特性即暗示著此高分子電解質可應用於可撓式電池。
本實驗之高分子電解質離子導電度隨溫度而增加，市售電解液的導電度隨著溫度的上升並無太大增加，維持在大約10-3 S/cm，表示離子傳導時具有低活化能。高分子電解質中的二氧化鈦含量影響離子導電度；二氧化鈦奈米管增加至5重量百分比(相對於溶劑之重量百分比)，離子導電度隨之上升，但隨著含量增加至7-9重量百分比而迅速下降。此最佳比例改進離子導電度是因為二氧化鈦奈米管幫助鹽類解離，使鋰離子在傳遞過程中更容易移動，並提供了更多的離子傳輸通道；而導電度的迅速衰退是因為二氧化鈦奈米管在過多的情形下產生自聚。在離子液體的情形中，我們可以藉由阿瑞尼士圖中發現斜率的迅速下滑，同時也暗示著我們，相較於揮發性電解液，離子液體的傳輸展現較高的活化能。
而在使用不同組成的離子液體/高分子複合電解質系統的電池效能，展現的電容量接近於使用揮發性電解液(155 mAh/g)，而介面電阻也隨著離子液體組成增加而減少；在10P20B的例子中，我們以0.05C的速率下充放電可達到147 mAh/g的電容量。然而，此薄膜由於離子液體的量過多而導致太軟因而失去其強度。
;The success in reaching the goals of high power high capacity lithium battery requires the use of electrolytes with sufficiently high ion conductivity that are nonvolatile, retardant to flame, highly electrochemical and highly thermally stable. Room temperature ionic liquid (RTIL) has been explored for this purpose due to its unique properties such as: low vapor pressure, high solubility in polar solvent, and most importantly, high electrochemical and thermal stable window.
However, it’s high viscosity, and relatively low ion conductivity has hampered the full cell performance. Another approach widely explored over the past 20 year is the polymer electrolytes where enhancing salt dissociation and ion hopping motion are common goals that have been hampered largely by polymer inherent properties such as smaller free volume, higher chain viscosity, and low dielectric constant.
Our previous works disclosed a composite polymer electrolyte using TiO2 nanotube to induce higher salt dissociation, and to provide a direct ion conducting pathway which shows ion conductivity approaching the order of 10-3 S/cm without the use of any volatile electrolyte.
In present paper, we disclosed novel electrolyte systems which composed of ionic liquids and a high dielectric polymer PVdF with the addition of small amount of TiO2 nanotube. In this system, the ionic liquids and the lithium salt are completely dissociated and the mobility is raised in presence of TiO2 nanotube. As results, the lithium ion conductivity reached a value of S/cm (25C) and S/cm at 80C. This high ion conducting polymer electrolyte system is tolerating to high voltage, is highly thermally stable and chemical stable. Suitable applications can be found in high energy lithium battery using high voltage cathode, in Li/S, Li/Air or other systems using high capacity anode. Since there is no SEI formation, longer life cycle is expected. As the electrolyte is nonvolatile, higher safety can be guaranteed. Film forming property implies this electrolyte is suitable to be applied to thin film (flexible) battery.
Here is the result that increases of ion conductivity with increasing temperature. Commercial electrolyte displayed a value in the order of 10-3 S/cm throughout the temperature measured, with a less temperature dependant property, reflecting lower activation energy in ion transport. In the IL/PVdF composite system, samples 10P10B and 10P20B representing 10 part (by weight) and 20 part of ionic liquids in 10 part of PVdF, which displayed higher ion conductivity than that of commercial liquid electrolyte throughout all temperature range measured.
Furthermore, the addition of TiO2 has impacted ion conductivity. With initial increase of TiO2 nanotube to 5wt% (wt% relative to the amount of solvent), conductivity is raised, but it dropped quickly with further addition to 7% or 9 wt%. The improvement in ion conductivity at some optimized TiNT is related to the fact that it assisted in dissociating the salt which allows for more movable ion, and provided a more directed ion conducting channel. And the deteriorated performance at high TiNT content is possibly due to aggregation of inorganic moiety. In these cases with ionic liquids, a steeper slope in the arrheneous plot is found, indicating the ion transport is experiencing higher activation energy process compared to that in the volatile liquid electrolytes.
Moreover, we displayed the battery performance using the IL/PVdF composite electrolytes with different IL composition. The capacity is close to that using volatile electrolyte (155mAh/g), and is found to increase continuously, and the interface resistance decrease with increasing of IL content. A capacity value of 147 mAh/g can be realized at 0.05C rate in sample 10P20B. However, the membrane becomes soft and lost it free-standing property when IL exceeds this composition. |
