From spent lithium ion battery to supercapacitors: a green and facile approach for recovering carbon-based anode materials

With the rapid development of electric vehicles and boosting lithium-ion battery production, recycling waste lithium-ion batteries has been the focus for researchers all over the world. In this study, carbon material was recovered via forming lithium graphite intercalated compound through charging and followed by high energy ball milling. The recovered carbon sample was then prepared as electrodes for symmetric supercapacitors. Electrochemical tests and material characterizations showed that the recovered carbon has a porous graphene-like sheet structure, and the specific surface area and specific capacitance can be effectively increased to 155 m²/g, which is over 10 times higher than that of the untreated carbon. The maximum specific capacitance of recovered carbon material was 41.66 F/g at a scan rate of 100 mV/s with a voltage range of 0-2V, which is 10 times higher than that of untreated carbon. The green and facile recovery method of batteries proposed in this paper provides a new idea and method for the recycling and utilization of waste lithium-ion battery anode materials.


Introduction
Lithium-ion battery as an energy storage device was widely used in consumer electronics and electric vehicles, followed by a sharp increase in its production volume.Large volumes of waste lithium-ion batteries will not only seriously damage the living environment but also cause a waste of resources, which is contrary to the current development concept of "carbon neutrality, carbon peak" [1] .Nowadays, the recycling technologies of cathode materials for waste lithium-ion batteries have been thoroughly studied, and the recovery rate can be as high as 99% [2] .However, the recycling technology of anode materials is rarely reported since it is an abundant resource with relatively lower economic value.
In recent years, researchers have paid more attention to the recycling of anode in spent lithium-ion batteries [3] .Recycling of carbon-based anode material mainly includes the graphite repair method [4,5] , mechanical-physical method [6,7] , solvent dissolution method [8][9][10] , Hetero atom doping method [11][12][13][14] , and electrolysis method [15] .Decent capacity has been reported for this recovered anode.Carbon from spent lithium-ion battery anode can also be synthesized into graphene through an electrochemical approach via the formation of a graphite intercalated compound [16][17][18][19] .This work proposes to prepare the supercapacitor's carbon material from anode via a green and facile electrochemical approach so as to solve the current problems with waste lithium-ion battery recycling.

Sample preparation
The spent lithium-ion battery used in this work adopts a waste vivo brand mobile phone battery with a nominal capacity of 2300 mAh.The spent lithium-ion battery was charged on the battery testing system at a constant current of 100 mA for a full charge to form a lithium graphite intercalated compound, followed by disassembly in a glove-box under an argon atmosphere.The disassembled anode sheets were taken out from the glove box and oxidized under ambient conditions.Then, the anode material can be easily detached from copper foil using deionized water.The anode material was placed with deionized water and zirconia balls in a PTFE container, and a high-energy ball was milled at 500 r/min for 4 hours.After repeated filtration and purification, the carbon sample was isolated, transferred to a vacuum drying box, and dried for 4 h at 80℃.The untreated carbon sample was prepared from the same wasted lithium-ion battery without charging and further treatment.All other reagents were of analytical grade and were used without further purification.
The slurry was made by weighing 0.8 g of recycled carbon material and binder (poly acrylic acid PAA) at a 9:1 ratio, adding 2 mL of deionized water, and stirring on a vacuum mixer for 15 minutes.The slurry was applied to 10 um thick copper foil with a coating thickness of 75 um and vacuum dried at 90℃ for 24 hours.The dried electrode sheet was die-cut into a disc with a diameter of 16 mm.Two symmetrical electrode discs were assembled using 1 mol/L LiPF6(EC: DMC at 1:1 vol) as an electrolyte in an inert atmosphere glove box into a 2032 coin cell-type lithium-ion supercapacitor.The specific preparation process of the material is shown in Figure 1.1Figure 1. Flow chart of the experimental procedures.

Material characterization
Sample morphology and composition were characterized by scanning electron microscope (SEM) with energy dispersive spectrometer (EDS).The structure of the recovered carbon material and untreated carbon materials was characterized by an X-ray diffractometer (XRD) and Raman Spectrometer.BET surface area was analyzed via an automatic specific surface and porosity analyzer.

Electrochemical analysis
Electrochemical properties were characterized on a potentiostat and battery test system for cyclic voltammetry and constant current charge and discharge tests.For the cyclic voltammetry test, the scanning range set in this topic is 0-2 V, and the scanning rate is 25, 50, and 100 mV/s, respectively.For constant current charge and discharge test, the current density is 1.0, 5.0, and 10.0 A/g, respectively.

Material characterization analysis
Figures 2(a) and 2(b) were recovered carbon material observed via SEM under magnification at 500 times and 30000 times, respectively.As shown in Figure 2(a), the recovered carbon were particles ranging from 5 to 20 micrometers with uneven shapes, which might need further milling or sieving for even distribution.Layered graphene structure and pores were observed on the recovered carbon surface in Figure 2(b) in accordance with He et al.'s study [16] , which was desirable for high surface area and capacitor purposes.EDS energy spectrum of the recovered carbon material demonstrated the recovered carbon material only contains carbon elements and contains no other impurities.Since the lithium graphite intercalated compound was fully oxidized, the lithium-ion and other impurities were stripped into deionized water after high-energy ball grinding, from which some soluble electrolyte ions in water can be dissolved, and then extracted and purified to obtain the carbon material with high purity.
BET surface area data are shown in Table 1.The specific surface area of recovered carbon material was 155 m 2 /g, which was over 10 times higher than that of untreated carbon.1Table 1. BET surface area for recovered and untreated carbon material.
Sample name Specific surface area (m²/g) Recovered carbon 155.428Untreated carbon 10.323 XRD spectrum showed no significant crystalline structure change for recovered carbon material and untreated carbon material.Both of the samples showed characteristic 2θ peaks at 26.38°and 54.54°, corresponding to (002) and (004) crystalline planes for graphite.
Raman spectra of recovered carbon and untreated carbon are shown in Figure 3.The two most intense D and G peaks were at 1340 cm -1 and 1577 cm -1 for untreated carbon, while that of recovered carbon were at 1342 and 1576 cm -1 .The intensity ratio I D /I G decreased from 0.92 (untreated carbon) to 0.47 (recovered carbon), indicating a lower level of defects for recovered carbon material.The asymmetric 2D peaks were 2698 cm -1 and 2703 cm -1 for untreated carbon and recovered carbon.It is suggested that the untreated carbon might be less exfoliated than recovered carbon.The results were also in accordance with the results reported [16] .

Electrochemical performance
The cyclic voltammetry (CV) performance of the recovered carbon material and the untreated carbon material is shown in Figure 4.The CV curve reflected the capacitance properties of the supercapacitor electrode material.According to Figure 4(a), the CV curve of the recovered carbon material significantly increased the area compared with the control group, with the specific capacity reaching 32.57F/g.In comparison, the untreated carbon was only 3.41 F/g, which was nearly 10 times increase.Figure 4(b) was the CV for the supercapacitors prepared from the recovered carbon material at different scan rates (25, 50, 100 mV/s).
As the scan rate increases, the curve area also gradually increases, and its specific capacity increases slightly at 25.83 F/g, 32.57F/g, and 41.66 F/g. Figure 4(c) shows the recovered carbon material and the untreated carbon at a current density of 1.0 Ag -1 .The recovered carbon showed significantly increased charge and discharge capacitance.Figure 4(d) shows the supercapacitors prepared from the recovered carbon materials at different current densities (1.0 Ag -1 , 5.0 Ag -1 , and 10.0 Ag -1 ).With the increase of the current density, the voltage drop at the top of the curve gradually decreases, indicating that the electric resistance gradually decreases.It can be seen that although the recovered carbon material has a higher specific capacitance, it is more suitable for preparing supercapacitors, which need to be improved and optimized later.

Conclusions
Through the investigation and study of recovered carbon materials from spent lithium-ion batteries, a green and facile approach for recycling anode material to prepare supercapacitor material is obtained.Material characterization shows that the recovered carbon material had a porous sheet graphene-like structure and high surface area compared with that of untreated carbon.The electrochemical performance test shows that the supercapacitors made from the recovered carbon material have improved specific capacity, with a maximum specific capacity of 41.66 F/g, which was approximately 10 times that of the control group.This shows that the charging of the battery is of great improvement for its subsequent application as a supercapacitor.This study provides a new idea for spent anode recovering and may be adopted in future life cycle assessment and management for lithium-ion batteries.

Figure 2 .
Figure 2. SEM images of treat graphite material: magnification at (a) 500 times and (b) 30000 times.

Figure 3 .
Figure 3. Raman spectra of recovered carbon material and untreated carbon material.

Figure 4 .
Figure 4. Electrochemical performance :(a) CV for recovered and untreated at 50 mVs -1 , (b) CV for recovered carbon under various scan rates, (c) GCD for recovered and untreated at 1.0 Ag -1 , and (d) GCD for recovered carbon under various current density.