High-density Graphene For Electrochemical Energy Storage

Volumetric performance,a much more reliable and precise parameter for evaluating the electrochemical performance of electrochemical energy storage(EES)devices compared with gravimetric performance,has aroused increasing attention in recent years as the rapid development of miniaturized,portable and wearable electronic devices as well as electric vehicles.Carbon-based electrode materials have advanced the fast development of EES devices while been limited by their poor volumetric performance due to their porous structure and the resulting low packing density.Graphene as a promising electrode material has a theory capacitance of 550 F/g as a capacitive electrode material and 744 mAh/g as a lithium ion storage electrode material.Generally,adding porosity can significantly boost the gravimetric capacitance of graphene and improve the rate capability,but result a poor volumetric performance due to the dramatically decreasing packing density.On the contrary,removing the inter-plane pores of graphene,would result an aggregation thus tarnish all the expected advantages of graphene.Recently,it is still a big challenge for preparation of graphene with both high volumetric and gravimetric performance as well as an excellent rate capability.This article introduce a simple,multifunctional molten salt treatment for the preparation of high-density graphene with high electrochemical energy storage performance.The results are summarized as follows:1.Starting with expanded graphene,a simple,multifunctional molten sodium amide treatment has been illustrated for the preparation of densified,nitrogen(N)-doped and nanoperforated graphene(DNPG).The molten sodium amide can condense the expanded graphene,lead to nitrogen doping and,what is more important,create moderate 3-5 nm in-plane nanopores on graphene.Moreover,the pore size and the nitrogen content could be adjusted through control the oxygen of EG and NaNH2 dose as well as the temperature and the time of NaNH2 treatment.2.The electrochemical performance of EG and DNPG has been measured in a three electrode system with a potassium hydroxide electrolyte.The DNPG electrode reaches a packing density(p)of 1.2 g/cm3.Moreover,hybrid lithium ion capacitors have been assembled to evoluate the electrochemical performance of EG and DNPG in organic electrolyte.It was found that at a current density of 1 A/g,DNPG exhibits 435 F/g and 522 F/cm3,in 6 M KOH aqueous electrolyte,which is much higher than EG(253 F/g,96 F/cm3).Although the structure was densified,the in-plane nanopores could serve as ion access shortcuts to boost the ions transfer thus exhibits better rate capability and lower resistance for ions transfer than that of EG.In addition,thanks to the wide potential window(3 V)for DNPG electrode,the hybrid capacitor exhibited a gravimetric and volumetric energy density of 618 Wh/kg and 740 Wh/L,respectively,which is even outperforms commercial LiFePO4.3.A efficient molten potassium thioacetate(CH3COSK)treatment of expanded graphene(EG)has been presented for the preparation of densified and sulfur-doped graphene(DSG)with high lithium storage capacity,improved first-cycle reversible capacity and enhanced cycling stability.The DSG electrode reaches a packing density of as high as 1.1 g/cm3,which is almost three times that of EG.The DSG electrode delivers an initial reversible volumetric capacity of 2035 and 480 mAh/cm3 at a charge/discharge current density of 0.1 and 1 A/g,respectively,and 443 mAh/cm3 after 500 cycles at 1 A/g.In addition,the relationship of gravimetric capacity and packing density of the electrode has been discussed when taking the electrolyte into account.It was found that with the decrease of the packing density,not only the volumetric capacity decreases linearly,the gravimetric capacity of the electrode,when both the active material and the electrolyte residing in the pores are considered,also decreases fast.