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Lithium-ion capacitors with 2D Nb<sub>2</sub>CT<sub>x</sub> (MXene) – carbon nanotube electrodes
Byeon, Ayeong,Glushenkov, Alexey M.,Anasori, Babak,Urbankowski, Patrick,Li, Jingwen,Byles, Bryan W.,Blake, Brian,Van Aken, Katherine L.,Kota, Sankalp,Pomerantseva, Ekaterina,Lee, Jae W.,Chen, Ying,Gog Elsevier 2016 Journal of Power Sources Vol.326 No.-
<P><B>Abstract</B></P> <P>There is a growing interest to hybrid energy storage devices, such as lithium-ion capacitors, in which battery-type electrodes are combined with capacitor-type ones. It is anticipated that the energy density (either gravimetric or volumetric) of lithium-ion capacitors is improved if pseudocapacitive or fast insertion materials are used instead of conventional activated carbon (AC) in the capacitor-type electrode. MXenes, a new family of two-dimensional transition metal carbides, demonstrate metallic conductivity and fast charge-discharge behavior that make them suitable for this application. In this study, we move beyond single electrodes, half-cell studies and demonstrate three types of hybrid cells using Nb<SUB>2</SUB>CT<SUB>x</SUB>–carbon nanotube (CNT) films. It is shown that lithiated graphite/Nb<SUB>2</SUB>CT<SUB>x</SUB>-CNT, Nb<SUB>2</SUB>CT<SUB>x</SUB>-CNT/LiFePO<SUB>4</SUB> and lithiated Nb<SUB>2</SUB>CT<SUB>x</SUB>-CNT/Nb<SUB>2</SUB>CT<SUB>x</SUB>-CNT cells are all able to operate within 3 V voltage windows and deliver capacities of 43, 24 and 36 mAh/g (per total weight of two electrodes), respectively. Moreover, the polarity of the electrodes can be reversed in the symmetric Nb<SUB>2</SUB>CT<SUB>x</SUB>-CNT cells from providing a positive potential between 0 and 3 V to a negative one from −3 to 0 V. It is shown that the volumetric energy density (50–70 Wh/L) of our first-generation devices with MXene electrodes exceeds that of a lithium titanate/AC capacitor.</P> <P><B>Highlights</B></P> <P> <UL> <LI> 3 types of lithium-ion capacitors using Nb<SUB>2</SUB>CT<SUB>x</SUB>-CNT as one electrode were tested. </LI> <LI> The highest volumetric energy density of 50–70 Wh/L was achieved. </LI> <LI> Energy density of symmetric cell exceeds that of lithium titanate/activated carbon. </LI> <LI> The lithiated graphite/Nb<SUB>2</SUB>CT<SUB>x</SUB>-CNT shows the highest gravimetric performance. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>