Preparation of 3D Frame Material for Lithium Metal Battery Anode Based on Waste Lithium-ion Battery Anode Graphite

. With the increase of waste lithium-ion batteries (WLIBs), the recycling of WLIBs has been paid more attention. Lithium metal battery (LMB) has extremely high theoretical specific capacity. However, the decline of cycle performance and the danger of short circuit caused by lithium dendrite formation and dead lithium accumulation are still the most thorny problems for the anode of LMB. Herein, the high value utilization of WLIBs anode graphite was explored, and it was prepared as a 3D frame material for LMB. Liquid-phase reduced graphite oxide nanosheets (lrGO) were prepared from AG, then, ZnO nanoparticles were loaded on lrGO. SEM, EDX and XRD were used to characterize lrGO and lrGO-ZnO , and the electrochemical tests were carried out. The results showed that lrGO and lrGO-ZnO maintained excellent cycle stability. Under the constant current density of 1mA cm -2 , the stable cycle of lrGO-ZnO was 700 cycles, and the charge-discharge platform potential difference maintained at 22.5 mV after 200th cycle. The unique 3D structure of lrGO increased the electrode reaction area and suppressed the volume expansion of Li. The loading of ZnO significantly improved the lithiophilicity and cycle stability..


Introduction
Lithium-ion batteries (LIBs) are currently the most popular and widely used secondary batteries in the world because of their high operating voltage, high specific energy, small self-discharge, and long cycle life [1].In recent years, with the widespread popularity of electric vehicles, mobiles, laptops and other electronic products, the production and application of LIBs are rapidly increasing.However, improper disposal of a large number of waste lithium-ion batteries (WLIBs) will lead to serious waste of resources and environmental pollution, which has aroused people's concerns [2].WLIBs are known as "urban mines" because they contain a variety of rare metals and resources, such as lithium, nickel, cobalt and graphite, which have high development and utilization value [3].Paying attention to the recycling of WLIBs anode graphite will help to reduce the exploitation of natural graphite resources, thus promoting environmental protection and the benign development of LIBs industry [4].
Lithium metal batteries (LMBs) are a secondary battery system composed of lithium metal as the anode material, with extremely high theoretical specific capacity (3860 mAh g -1 ).LMBs are considered the direction of the next generation of high-capacity secondary batteries.However, lithium metal as anode materials directly will lead to many problems, the most significant of which is the formation and growth of lithium dendrites [5].The formation and growth of dendrites will damage the solid electrolyte interphase (SEI), leading to dead lithium accumulation, which will reduce the coulomb efficiency (CE), and will also lead to piercing the separator, leading to a short circuit of the battery, resulting in a potential safety hazard [6].In order to avoid the problem caused by dendrite growth, it is necessary to modify the anode of LMBs to suppress dendritic growth and dead lithium accumulation, which is the direction that researchers have been working hard in recent years [7].
The 3D frame has been proved to be an effective modification method for LMBs anode.By uniformly depositing lithium metal on the surface of the 3D framework, the local current density is reduced, dendrite growth is inhibited, the volume expansion is reduced, and the cycle performance and stability of LMBs anode are improved [8].Carbon materials are light in weight, good in electrical conductivity, and modifiable in chemical structure, making them ideal 3D frame materials for LMBs anodes [9].In this study, the reduced graphite oxide nanosheets with ultra-thin lamella and porous structure were prepared from anode graphite of WLIBs, and ZnO nanoparticles were loaded on the surface of it.Through a series of material characterization and electrochemical tests, the method of preparing 3D frame materials for LMBs anode from anode graphite of WLIBs was explored.

Preparation of lrGO and lrGO-ZnO
The anode material powder was separated from spent anodes of WLIBs, Copper foil was removed, and then heated the powder at 600 ℃ to remove impurities to obtain purified anode graphite powder.Anode graphite powder was oxidized to graphite oxide by KMnO 4 in the mixture of concentrated H 2 SO 4 and concentrated H 3 PO 4 with a volume ratio of 8:1 by improved Hummers method [10].Graphite oxide nanosheets (GO) were obtained by ultrasonic treatment of graphite oxide at 40 kHz.GO was reduced by ascorbic acid under weak alkaline condition, and liquid-phase reduced graphite oxide nanosheets (lrGO) were obtained.Then lrGO was mixed with zinc acetate solution with concentration of 0.1 molL -1 , filtered, and heated to 400 ℃ for 1 h to obtain zinc oxide modified liquid-phase reduced graphite oxide nanosheets (lrGO-ZnO).The type of waste battery is 18650 WLIBs, and the reagents used are all analytical pure reagents purchased from Beijing Tongguang Fine Chemical Co. Ltd.

Materials characterization
The micro-morphology and surface element distribution of lrGO and and lrGO-ZnO were analyzed by scanning electron microscopy (SEM, Hitachi S-4800, Japan) equipped with an energy dispersive X-ray detector (EDX, Oxford JEOLJSM-5600LV, UK).The crystal structure of the sample was analyzed by X-ray diffraction (XRD, Rigaku UltimaIV, Japan), with a scanning angle 2θ in range of 5 ° to 80 °, Scanning speed of 10 °/ min.

Electrochemical tests
The electrode for electrochemical tests was obtained by active material, conductive carbon and binder on the current collector at a mass ratio of 8:1:1, in which the active substances are lrGO and lrGO-ZnO, and the current collector is Cu foil.The Cu foil with a diameter of 1.1 cm was used as the electrode for control experiment.The electrode piece is cut into a wafer with a diameter of 11mm, and the Li metal sheet was used as the counter electrode to assemble the button cell.The electrolyte was 1 M LiTFSI and 5.0 wt% LiNO3 with DOL and DME (volume ratio 1:1) as the solvent.The galvanostatic charge-discharge cycle tests were carried out at a current density of 1mA cm -2 for a capacity of 1 mAh cm -2 , which were performed on the LAND test system (CT3002A, Wuhan, China).The SEM characterization of Fig. 1 shows that the curled ultra-thin lamellae of lrGO have been aggregated and stacked into a porous spatial structure, the thickness of lamella is between 10-20 nm, and the average spacing between lamellae is about 40 nm.The unique 3D structure of lrGO provides a sufficient contact area for the deposition of Li metal, and Li + can be transferred more smoothly inside the frame during the electrode reaction, so that the local current density is reduced, and the generation of dendrites and the accumulation of dead lithium are inhibited.After being modified by ZnO nanoparticles, the structure of lrGO substrate has not been significantly changed or destroyed, and the loose and porous spatial structure is still maintained, which proves that the modified loading of ZnO nanoparticles would not affect the unique spatial structure of lrGO.ZnO nanoparticles are uniformly loaded on the surface of lrGO, and size of these ZnO nanoparticles are less than 30 nm.The results of EDX surface element distribution mapping analysis of lrGO-ZnO are shown in Fig. 2. It can be seen that the distribution of C, O, and Zn elements on the surface of lrGO-ZnO is basically the same, and the packing density of the substrate material is also consistent, which proves that ZnO nanoparticles has achieved uniform load on lrGO substrate.The uniform loading of ZnO nanoparticles leads to a more uniform nucleation of Li metal on the surface of the frame, which makes the deposition and growth rate of Li metal on the surface consistent, thereby improving the lithiophilicity and cycle stability of the material.

Fig. 3. X-ray diffraction patterns of lrGO and lrGO-ZnO
According to the XRD characterization of lrGO and lrGO-ZnO in Fig. 3, the XRD spectra of lrGO and lrGO-ZnO both show a wide diffraction peak at 2θ = 25 °, which corresponds to the diffraction peak of (002) lattice plane of Carbon.The result indicates that lrGO has a high degree of graphitization after liquid phase reduction of ascorbic acid [11].The XRD spectrum of lrGO-ZnO show sharp diffraction peaks at 2θ= 31.8°,34.4°, 36.3°,56.6°.Compared with the PDF card of hexagonal wurtzite zinc oxide, it is proved that there is ZnO nanocrystals with hexagonal wurtzite loading on the surface of lrGO.These ZnO nanocrystals increase the lithiophilicity of the surface because they can be used as active sites to promote the nucleation and deposition of Li metal on the material surface.

Electrochemical test results
The Coulombic efficiency and stable cycle number of Cu, lrGO and lrGO-ZnO at current density of 1 mA cm -2 for a capacity of 1 mAh cm -2 are shown in Fig. 4.The failure of Cu occurs at the 160th cycle, which is due to the short circuit caused by the dendrite growth on the surface of Cu piercing the separator.For lrGO and lrGO-ZnO, the Coulombic efficiencies of the first three cycles are all low, less than 90%.The main reason is that side reactions occur on the surface of the frame to generate SEI, which makes a part of Li irreversible [12].After the 20th cycle, the Coulombic efficiency of lrGO-ZnO remains above 98%, and after the 50th cycle, the Coulombic efficiency remains stable at over 99%.The Coulombic efficiency of lrGO maintains between 95% and 100% after 5 cycles.Finally, lrGO fails after 490 cycles, while the Coulombic efficiency of lrGO-ZnO decrease steadily until it fails after 700 cycles.Compared with lrGO, lrGO-ZnO maintains higher Coulombic efficiency and longer cycle life.The main reason for this result is the different surface chemical states of lrGO and lrGO-ZnO during the cycling process.The uniformity of Li metal deposition on the rGO surface is lower than that of lrGO-ZnO, resulting in the inability to form a completely stable SEI.The unstable SEI is constantly destructed and regenerated during the cycling process, which leads to the continuous fluctuation of Coulombic efficiency of the anode.In contrast, a more stable SEI is formed on the surface of lrGO-ZnO during the cycling process, resulting in higher Coulomb efficiency, cycle stability and long cycle life.The comparation of the charge-discharge platform potential differences of three electrodes composed by Cu, lrGO, and lrGO-ZnO under different cycle numbers at a current density of 1mA cm -2 for a capacity of 1 mAh cm - 2 is shown in Fig. 5.In Fig. 5a, at the 50th cycle, the charge-discharge potential differences of Cu, lrGO, and lrGO ZnO are 71.2 mV, 36.6 mV, and 22.0 mV, respectively; In Fig. 5b, the potential differences at the 100th cycle are 69.4 mV, 42.2 mV, and 21.3 mV, respectively; In Fig. 5c, at the 200th cycle, the Cu has failed because of the short circuit caused by the dendrite growth, the potential differences of lrGO and lrGO-ZnO are 41.9 mV and 22.5 mV, respectively.The result shows that Cu has the highest charge-discharge platform potential differences and the worst cycling stability among these three materials.The highest chargedischarge platform potential differences is due to the worst lithiophilicity of Cu among the three.The worst cycle stability is due to the inability of Li metal to be deposited uniformly on the surface during cycling, resulting in the formation and growth of Li dendrites on the Cu surface.For lrGO and lrGO-ZnO, as the number of cycles increases, the charge-discharge platform potential differences of lrGO-ZnO is always lower than that of lrGO.Moreover, the charge-discharge platform potential differences of lrGO-ZnO at the 200th cycle is almost consistent with that of the 50th cycle.The result shows that the lrGO-ZnO on the electrode maintains structural stability during the Li metal depositionstripping reaction of cycling process, which is due to the 3D frame structure inhibiting the volume expansion.Therefore, lrGO-ZnO performs extremely high cyclic stability.

Conclusion
In this study, lrGO with ultra-thin layers and porous structure is successfully prepared by using spent anode graphite of waste lithium-ion batteries as raw material, and uniform loading of ZnO on the surface of lrGO has been achieved.Electrochemical test results show that both lrGO and lrGO-ZnO have better cycle stability than Cu, because the 3D frame enables larger reaction area while suppressing volume expansion.Compared with lrGO, lrGO-ZnO maintains higher cycle stability, cycle life, Coulombic efficiency and lower charge-discharge platform potential difference under constant current cycle test.This is due to the fact that the surface of lrGO-ZnO is loaded with ZnO nanoparticles to enhance lithiophilicity of the surface, which makes the Li deposition more uniform and forms a more stable SEI, thereby achieving extremely high cycle stability.This study demonstrates the feasibility of preparing 3D frame materials for lithium metal battery anode from waste lithium ion battery anode graphite, which not only achieves the high value utilization of waste anode graphite, but also provides a new idea for the preparation of 3D frame material for LMBs anode with better performance.

Fig. 2 .
Fig. 2. EDX images of mapping elements of C, O, and Zn of lrGO-ZnO

Fig. 4 .
Fig. 4. Coulombic efficiency-cycle number diagram of Cu, lrGO and lrGO-ZnO at current density of 1 mA cm -2 for a capacity of 1 mAh cm -2