|Schematic of graphene/lithium iron phosphate battery. Credit: ACS, Hassoun et al. Click to enlarge.|
Researchers in Italy have developed an advanced lithium-ion battery based on a graphene nanoflake ink anode and a lithium iron phosphate cathode. By balancing the cell composition and suppressing the initial irreversible capacity of the anode in the round of few cycles, they reported an optimal specific capacity of 165 mAhg–1, of an estimated energy density of about 190 Wh kg–1 and a stable operation for more than 80 charge–discharge cycles.
In a paper published in the ACS journal Nano Letters, they observed that—to the the best of their knowledge—complete, graphene-based, lithium-ion batteries having comparable performances are rarely reported. They suggested that their results disclosed might open up new opportunities for exploiting graphene in lithium-ion battery science and development.
A critical issue of LIBs technology is the low theoretical specific capacity of conventional graphite anodes, limited to 372 mAh/g. For this reason, a large fraction of current research is focusing on alternative anode materials such as Si (4200 mAhg-1), Sn (994 mAhg−1), and SnO2 (782 mAhg−1). However, their application has been mostly limited by their poor cycling caused by large volume changes (100−300% with respect to the initial volume) during the repeated alloying and dealloying with Li ions that characterizes their electrochemical process.
Graphene, thanks to its large surface to mass ratio (SSA) exceeding 2600 m2/g, high electrical conductivity (σ), and high mechanical strength with the added value of mass production, is a promising material for electrodes in LIBs. Because single layer graphene (SLG), grown by chemical vapor deposition (CVD), has a limited capability of uptaking Li ions (5% surface coverage) due to repulsion forces between Li+ ions at both sides of the graphene layer, large efforts have been devoted to the exploitation of chemically modified graphene (CMGs) such as graphene oxide (GO) and reduced GO (RGO), both at the anode and cathode. However, although CMGs can be produced in large quantities they suffer from limited conductivity and slow diffusion of Li+ ions. To date, the best anodes with CMG have reached specific capacity of ∼1200 mAhg−1 at 100 mAg−1 current rate in half cell and ∼100 mAhg−1 at 29 mAg−1 current rate when assembled in a full battery.—Hassoun et al.
In contrast, the researchers noted, graphene nanoflakes, which are obtained by the exfoliation of pristine graphite, are an ideal yet not fully explored material platform for electrodes. Nanoflakes possess high crystallinity which helps assure fast electron transport to the electrode support; the small (<100 nm) lateral size offers a large edge to bulk ratio of carbon (C) atoms (important, since edges are considered very active sites for Li+ ions storage); and graphene nanoflake inks can be produced by liquid phase exfoliation (LPE) methods that offer a route toward mass production.
The challenge here is to properly balance such high- capacity anode materials with the counter cathode material. We demonstrate in this work that this demanding task can be successfully achieved by selecting an optimized lithium ion battery configuration based on the combination of a prelithiated graphene-based anode with a very thin lithium iron phosphate cathode. We believe that this combination provides a rare case of a full, graphene-based LIB offering very promising performances in terms of cycle life and capacity stability.—Hassoun et al.
They designed the anode by drop casting graphene nanoflake ink on a polycrystalline copper substrate. Initial testing showed that their graphene nanoflakes ensured a Li ion uptake much higher than that normally obtained with graphite.
They then electrochemically tested the Cu-supported graphene electrode in a half cell using a lithium counter electrode. They found that after an extremely high initial specific capacity (∼7500 mAhg−1), the following charge/discharge cycles evolved with an initial capacity fading but then stabilize at about 750 mAh g−1.
To suppress the high initial irreversible capacity—which would be an undesired event in a practical application—they used a pre-treatment ex situ lithiation process carried out by directly contacting a lithium metal foil, wet by the electrolyte, with the Cu-supported graphene nanoflakes electrode.
The resulting cycling response of the pretreated was 700 mAg−1; first cycle Coulombic efficiency approached 100%. The subsequent increase and stabilization of the Coulombic efficiency approaching 100% confirmed their procedure, they concluded. They found a stable reversible specific capacity of ∼650 mAh g−1 after 150 charge/discharge cycles.
To create a full battery, they coupled the graphene nanoflake anode with a lithium iron phosphate, LiFePO4cathode commonly used in commercial batteries.
The battery operated around 2.3 V with a reversible capacity of 165 mAhg−1. The achieved theoretical specific energy density is 380 Wh kg−1.
We stress that the electrochemical performances achieved by the full battery are the result of a careful electrode balance achieved in this work by properly reducing the thickness of the cathode.
… The use of ultralight high-capacity graphene nanoflakes anode estimates a practical energy density of about 190 Wh kg−1, that is, a value exceeding (∼25−60%) that of current lithium ion battery technology. Although the optimization of engineering aspects for practical industrial implications requires further work beyond the scope of the present paper, the wide possibilities for further improvement of this battery’s performances suggest that the approach here demonstrated might impact available technological solutions for lithium-ion batteries. In addition, graphene nanoflakes can be exploited in other energy storage devices such as Li−air and Li−sulfur batteries and supercapacitors.—Hassoun et al.
Jusef Hassoun, Francesco Bonaccorso, Marco Agostini, Marco Angelucci, Maria Grazia Betti, Roberto Cingolani, Mauro Gemmi, Carlo Mariani, Stefania Panero, Vittorio Pellegrini, and Bruno Scrosati (2014) “An Advanced Lithium-Ion Battery Based on a Graphene Anode and a Lithium Iron Phosphate Cathode,” Nano Letters doi: 10.1021/nl502429m