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▲ 左圖為瀝青、石墨烯和鋰金屬組成的電池陽極。（Source：萊斯大學）
來自素有南方哈佛之稱的美國萊斯大學團隊開發出以瀝青製成的多孔性碳陽極，讓鋰金屬電池充電速度比目前的商業鋰離子電池快10～20 倍，一首歌的時間就能充飽電！此外，從瀝青衍生的碳可防止鋰電池最害怕的樹枝狀結晶（dendrite）形成，研究人員稱，這項研究將使瀝青材料在高功率快速充放電裝置中大放異彩，成為鋰電池材料界的明日之星。
我們知道鋰金屬由於其高比容量（specific capacity）和低化學電位特性而成為電池市場寵兒，但以純金屬鋰為電極的電池卻有一個致命傷：隨著充放電次數增加，電池內部會生成越來越多樹枝狀結晶，導致鋰電池短路而產生爆炸風險。為了改善這問題，1990 年代後鋰離子電池的陽極便多捨棄金屬鋰而改用嵌鋰的石墨，在一定程度上減少了樹突結晶的形成，但也因此出現另一個困擾：鋰電池效能大幅降低，儲電能力僅剩下以金屬鋰為鋰電池電極的十分之一。
不過，由萊斯大學材料科學與奈米工程學教授詹姆斯托爾（James Tour）領導開發的鋰電池陽極一次解決了上述兩大問題。在實驗室中，研究人員使用瀝青製成多孔性碳陽極，尤其是未處理過的高錳礦來為鋰鍍層的廉價材料，實驗結果發現，電池產生的大電流密度（註1）為20 mA/cm2 ，經過500 次充放電循環後，仍維持出色穩定性。
至於電池充電速度更是大幅提升10～20 倍，詹姆斯托爾表示，他們可以在5 分鐘內將電池電量從零充到滿，若是其他電池，一般都需要2 小時或更長時間才能充飽。研究人員再將瀝青碳陽極和硫化碳陰極結合進行完整測試後，發現電池的功率密度（註2）達1,322 W/Kg，能量密度達到943 Wh/Kg，作為比較，以鋰鈷氧化物為正極、碳為負極的鋰離子電池，其能量密度為370 Wh/kg。顯然，對需要高功率密度的快速充放電裝置而言，瀝青材料彰顯了它的特殊地位。
2013 年時，詹姆斯托爾實驗室曾經開發以石墨烯碳奈米管為陽極材料的鋰離子電池，這類陽極也可以阻止樹突結晶形成，不過相比之下，雖然兩者電池容量相差無幾，但瀝青複合材料的製造過程卻更簡單，成本也更低廉，它不需要電子束沉積、化學氣相沉積等步驟。這項研究已發表在《美國化學學會》期刊。
A touch of asphalt may be the secret to high-capacity lithium metal batteries that charge 10 to 20 times faster than commercial lithium-ion batteries, according to Rice University scientists.
The Rice lab of chemist James Tour developed anodes comprising porous carbon made from asphalt that showed exceptional stability after more than 500 charge-discharge cycles. A high-current density of 20 milliamps per square centimeter demonstrated the material’s promise for use in rapid charge and discharge devices that require high-power density. The finding is reported in the American Chemical Society journal ACS Nano.
“The capacity of these batteries is enormous, but what is equally remarkable is that we can bring them from zero charge to full charge in five minutes, rather than the typical two hours or more needed with other batteries,” Tour said.
The Tour lab previously used a derivative of asphalt — specifically, untreated gilsonite, the same type used for the battery — to capture greenhouse gases from natural gas. This time, the researchers mixed asphalt with conductive graphene nanoribbons and coated the composite with lithium metal through electrochemical deposition.
The lab combined the anode with a sulfurized-carbon cathode to make full batteries for testing. The batteries showed a high-power density of 1,322 watts per kilogram and high-energy density of 943 watt-hours per kilogram.
Testing revealed another significant benefit: The carbon mitigated the formation of lithium dendrites. These mossy deposits invade a battery’s electrolyte. If they extend far enough, they short-circuit the anode and cathode and can cause the battery to fail, catch fire or explode. But the asphalt-derived carbon prevents any dendrite formation.
An earlier project by the lab found that an anode of graphene and carbon nanotubes also prevented the formation of dendrites. Tour said the new composite is simpler.
“While the capacity between the former and this new battery is similar, approaching the theoretical limit of lithium metal, the new asphalt-derived carbon can take up more lithium metal per unit area, and it is much simpler and cheaper to make,” he said. “There is no chemical vapor deposition step, no e-beam deposition step and no need to grow nanotubes from graphene, so manufacturing is greatly simplified.”
Rice graduate student Tuo Wang is lead author of the paper. Co-authors are Rice postdoctoral researcher Rodrigo Villegas Salvatierra, former postdoctoral researcher Almaz Jalilov, now an assistant professor at King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia, and former Rice research scientist Jian Tian, now a professor at Wuhan University, China. Tour is the T.T. and W.F. Chao Chair in Chemistry as well as a professor of computer science and of materials science and nanoengineering at Rice.
The Air Force Office of Scientific Research, EMD-Merck and Prince Energy supported the research.
Source:http://news.rice.edu/2017/10/02/asphalt-helps-lithium-batteries-charge-faster-2/
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