Applications of Graphene Materials in Lithium-ion Batteries

. In recent years, the escalating global energy crisis and the ever-increasing concern over environmental pollution have stimulated extensive research efforts towards exploring new green energy sources and accelerating the transformation and upgrading of traditional energy structures. Lithium-ion batteries (LIBs), known for their exceptional properties including high energy density, excellent discharge performance, long lifespan, non-toxicity, and environmental friendliness, have been regarded as the backbone of new energy systems. Additionally, graphene, a two-dimensional novel nanomaterial, has recently gained attention as a kind of electrode material exhibiting great promise for utilization in LIBs due to exceptional performance attributes. Notably, significant evidence from current studies has revealed that the integration of graphene into LIBs leads to a substantial improvement in their electrochemical performance. However, advancements in the performance during cycling and the capacity for charging of LIBs remain imperative. Therefore, the investigation of materials based on graphene for enhancing the performance of LIBs holds substantial importance. This paper has various aspects related to the utilization of graphene and graphene-based composites materials in electrodes in LIBs. A comprehensive analysis is conducted on the current literature and recent progress in the respective field, including the modification of composites formed by integrating metal oxides with graphene, the usage of graphene as a conductive agent, etc. Furthermore, this paper provides an outlook on the future enhancements and developments concerning the utilization of graphene in LIBs. The findings and analyses presented in this study hope to contribute novel insights for researchers and encourage the development of LIBs with larger capacity and longer-lasting operational capabilities


Introduction
The energy crisis encompasses various issues in areas such as societal domains, technological research, and economic advancement.These challenges include energy shortages leading to tight energy supplies and surging energy prices [1].Against the backdrop of global environmental pollution and the energy crisis, there is an urgent need to expedite the transformation of conventional energy structures.Research efforts towards the development of new energy sources have become increasingly crucial for achieving global sustainable development.
Lithium-ion batteries (LIBs), as the backbone of modern energy storage technology, possess considerable market potential and are experiencing steady growth.In 2022, the worldwide market for LIBs recorded a valuation of $48.19 billion.Over the next decade, it is anticipated to demonstrate a compound annual growth rate (CAGR) of 18.9% [2].LIBs find extensive applications in diverse industries such as mobile communications (the digital 3C field), new energy vehicles, etc [3].As of 2018, around 50% of LIBs sales were utilized in electric vehicles and buses.Moreover, the cost of LIBs has plummeted by over ninety percent since their commercialization two decades ago [2].However, the performance and production cost of lithium-ion electrode materials represent bottlenecks that hinder their further development in terms of high energy and low cost, significantly influencing the performance indices of the final products.Each electrode material for LIBs possesses its own strengths and limitations, necessitating ongoing research in this area to advance energy storage technology [4].
Graphene, characterized by its single-layered carbon atom arrangement forming a honeycomb lattice structure, has gained significant prominence in scientific discourse since its first successful isolation and characterization in 2004.Carbon atoms in graphene are connected by σ bonds, and each carbon atom consists of an π orbital and an extranuclear electron to form a delocalized large π bond.With considerable specific surface area, exceptional electrical conductivity, and robust chemical stability, graphene has emerged as the optimal choice for electrode materials in LIBs.Marappan Sathish et al. synthesized graphene nanocomposites with anchored magnetite (Fe3O4) nanoparticles through hydrothermal method.Three distinct weight ratios of Fe3O4 and graphene nanosheets (GNSs) were utilized in the fabrication process.Experimental investigations demonstrated a substantial improvement in the capacity for storing lithium-ion and the ability to maintain reversible capacity at high rates for the nanocomposite electrodes containing Fe3O4/GNS (40: 60) compared to the individual electrodes of pure GNS and Fe3O4 nanoparticles.This enhancement can be attributed to the utilization of highly conductive graphene nanosheets (GNS) as a supportive material [5].
Within the context of the energy crisis, this paper paper concentrates on the utilization of various graphene materials in LIBs, encompassing their utilization as cathode materials, anode materials, and conductive agents.The paper also identifies existing challenges in large-scale production and implementation, providing a comprehensive summary and outlining future research directions for the use of graphene materials in LIBs.

LIBs
LIBs operate through a reversible electrochemical process, involving the intercalation and deintercalation of lithium ions.While charging, lithium ions are produced at the cathode and travel through the separator and electrolyte to the anode.In contrast, during discharging, the extraction of lithium ions from the anode and their subsequent transport back to the cathode will happen [6].In a typical graphite-LiCoO2 LIB configuration (Fig. 1, where M=Co), the anode consists of graphite-coated copper foil separated by a porous insulating separator, while the cathode comprises an aluminum collector coated with LiCoO2.Both the electrodes, along with the separator, are immersed in the electrolyte.The charging chemical reaction is represented in equation ( 1)-( 4).
(1) (2) (3) (4) where, x represents the extent of lithium intercalation.The process of recharging involves reversing the aforementioned reaction.The lithium ions will be embedded in the graphite sheets.The capacity and performance characteristics of a LIB are tied to the quantity and kinetics of lithium ion intercalation and deintercalation processes within the cathode material, as indicated in the aforementioned electrode reaction equation.The total rate as well as amount of lithium ions that can be reversibly cycled between the cathode and anode materials significantly impact the capacity and overall performance of the LIB [6].It is critical to optimize the intercalation and deintercalation kinetics of lithium ions in both electrodes materials to enhance the battery's capacity and performance.

Graphene materials in cathode
The cathode material is a vital component of LIBs, accounting for a significant proportion of their weight distribution (approximately 40% of the total weight), while its production cost is more than twice that of the anode material.The cathode material directly impacts various electrochemical parameters in LIBs such as the energy density, rate capability, and working voltage.Currently, commonly used cathode materials for LIBs are generally poor electronic conductors.This includes various materials such as lithium cobalt oxide (LCO, LiCoO2), lithium iron phosphate (LFP, LiFePO4), lithium manganate (LMO, LiMn2O4), as well as ternary materials (NCM, LiNixCoyMn1−x−yO2).Due to structural reasons, the cathode materials often exhibit limited conductivity, which severely restricts their electrochemical performance, specifically under high charge and discharge rates [7].Research findings indicate that the utilization of graphene in conjunction with transition metal oxides, such as lithium, leads to enhanced electrochemical performance of the materials.by expansive graphene sheets.(iv) Large amounts of active material particles are anchored to the outer layer of a expansive graphene sheet (most commonly used).(v) Sandwich-like structures formed by active material particles sandwiched between two layers of graphene sheets.(vi) Alternating layered structures of active material particles and graphene sheets.

Conductive additives
To improve the utilization efficiency of active materials, enhance the conductivity, and optimize the capacity, rate performance, and cycling stability of the LIBs, it is common to incorporate materials with excellent conductivity, low density, and stable structural and chemical properties into the active materials.These materials are known as "conductive additives".The conductive additives can play a role in collecting micro current between the active materials, between the active materials and the current collector, thereby improving the migration rate of lithium ions within the electrode materials and enhancing the efficiency of the electrode's charge-discharge processes.
As shown in Table 1, carbon-based conductive additives are the mainstream types of conductive additives used in LIBs.Novel conductive additives such as vapor grown carbon fiber (VGCT), carbon nanotubes (CNT) and graphene nanoparticles (GN) possess a different conductive network compared to traditional additives like conductive carbon black and conductive graphite.This allows for a reduction in the amount of conductive additives added to the battery while increasing the content of active materials.Among them, graphene material possesses excellent strength and toughness (the tensile strength is estimated at 130 GPa, while theoretical values for Young's modulus indicate a stiffness of 1.0 TPa), high thermal conductivity (highest among carbon materials), outstanding electronic properties (room temperature charge carrier mobility of approximately 15,000 cm 2 /(V•s)), the capacity to undergo the adsorption and desorption of diverse atoms and molecules, and exceptional optical characteristics.It finds significant applications in the fields of new energy, coatings, composite materials, biomedical materials, and electronic communication, among others.Graphene materials, due to its unique two-dimensional high surface area (2630 m 2 /g), are utilized in the cathode materials of LIBs to enhance electronic transport capabilities and the industrialization of graphene materials finds its earliest applications in conductive additives.Graphene, due to its high electronic conductivity and low impedance, can serve as a conductive additive in LIBs to improve cycling performance and enhance the lithium-ion intercalation rate.Fig. 2b has depicted that the free movement of electrons within the graphene crystalline phase, combined with the ultra-thin particle size and "plane-point" contact mode, allows graphene to form a more efficient conductive network.Therefore, graphene as a conductive additive exhibits noticeable advantages in terms of discharge and conductivity compared to traditional additives under laboratory conditions.However, in practical processes, the planar structure of graphene within the electrode introduces resistance to lithium-ion transport, particularly at high current rates, where this effect becomes more prominent.Fig. 2b illustrates the lithium-ion movement pathways within the mechanically mixed electrode structures based on carbon black (left) and graphene (right), reflecting the corresponding "resistance effect " mentioned earlier and highlighting the potential drawbacks of graphene's layered structure in terms of lithium-ion flow.Researches have shown that the influence of graphene on ion movements is intricately linked to the structural characteristics of graphene, electrode thickness, and the variation in particle sizes of active materials.In addition, the stratified arrangement of graphene's atomic layers has a certain impact on the dispersion of conductive additives during electrode preparation, and currently, there is no particularly effective solution method [7].

Binary conductive additives
In practical applications, a combination of two different conductive additives is commonly used to form a binary conductive additives, which enables the construction of a synergistic conductive network at different hierarchical levels of the electrode, thereby significantly improving the performance of LIBs.According to the research of Su et al., they investigated the synergistic conductive mechanism of GN/CB binary conductive additives in LFP and LCO cathode systems.The experiments revealed that the binary conductive additives can effectively reduce polarization and substantially decrease the required amount of graphene.The "plane-point" interaction mode between GN and the "point-point" interaction mode between CB demonstrate remarkable synergistic effects, facilitating the concurrent formation of extensive and localized conductive networks [7].Different from previous literatures that reported the preparation of binary conductive additives through physical mixing in the electrode fabrication process, Li et al. chemically combined GN and CB to create hybrid materials before electrode preparation.Experimental studies had shown that the GN/CB binary conductive additives are more stable, not only improving the dispersion problem of graphene conductive agents but also enhancing the electronic conductivity, cycling stability, and energy density while reducing costs [8].

Composite of graphene and cathode materials
, 01010 (2023) In the application of LIB cathodes, graphene materials are commonly combined with polyanions.Polyanion cathode materials can be classified as a group of compounds characterized by the presence of a series of tetrahedral anion units, denoted as (XO4) n− or their derivatives [9].These tetrahedral or octahedral structures are interconnected through strong covalent bonds to form a three-dimensional network structure with voids.This imparts distinct crystallographic structures and properties to polyanion cathode materials, differentiating them from metal oxide cathode materials like LiCoO2.Notably, they exhibit stable framework structures, flexible charge-discharge potentials, and relatively low electronic conductivity.Researches indicate that the challenge of inadequate electronic conductivity can be effectively mitigated through the integration of graphene materials with polyanions.
The interaction between nanocarbon and phosphate is significantly influenced by factors such as graphene type, integration method, and graphene concentration, which ultimately govern the performance and properties of the composite materials [10].Different kinds of graphene integrated into LiFePO4 show various electrochemical performance.In comparison to reduced graphene oxide (rGO), direct deposition or stripping methods of graphene exhibit superior electrical conductivity and fewer inherent drawbacks.Such advantageous characteristics can effectively reduce the load threshold value in various applications [11].In addition to its combination with phosphates, graphene materials can also be combined with silicates.Zhu et al. synthesized and tested three-dimensional macroporous graphenebased Li2FeSiO4 composites (3D-G/Li2FeSiO4/C) as anode materials for LIBs using a templated-assembly method.The performance of these composites was compared with anode materials without graphene (Li2FeSiO4/C) and anode materials with two-dimensional graphene sheets (2D-G/Li2FeSiO4/C).Experimental studies revealed that the 3D-G/Li2FeSiO4/C composite exhibited significantly improved electrochemical properties, including discharge capacities and cyclability, compared to the other two composite structures.This can be attributed to the unique porous structure of the 3D-G/Li2FeSiO4/C composite, which provides a larger contact surface area, allowing electrolyte ions to freely diffuse within the conductive network.Additionally, the production method for these composites is more common and easily achievable.

Graphene materials in anode
When graphene is directly used as a negative electrode material, the specific capacity of LIBs is 540mAh/g.However, after multiple cycles of use, the capacity will severely decay and this will significantly reduce the battery lifespan.Guo et al. have developed graphene nanosheets intended for usage as anode materials, attaining a reversible capacity of 672 mAh/g initially.Even after undergoing 30 cycles, the reversible capacity maintains a steady level of 502 mAh/g.In recent years, researchers have been focusing on the design of anode materials comprising graphene with doping, graphene with pores, and nanocomposites combining graphene and electrochemically active substances, aiming to develop LIBs with enhanced electrochemical capabilities [12].
Jiang et al. developed graphene with pores by subjecting graphite oxide to hydrothermal reaction and subsequent strong alkaline etching.The discharge capacities of the resulting material were found to be 2207 mAh/g, 220 mAh/g, and 147 mAh/g at discharge rates of 0.05 C, 5 C, and 10 C, respectively.Even after undergoing a 10 C discharge rate followed by 40 cycles at a discharge rate of 0.5 C, the specific capacity remained high at 672 mAh/g.Compared to pristine graphene layers, the graphene with pores showed an increased specific surface area and a heightened density of flaws, resulting in a larger quantity of active sites for lithium storage [13].
Graphene can also be used as anode materials for LIBs when combined with metal oxides or alloy materials such as tin-based or silicon-based oxides [12].Taking TiO2/graphene composites as an example, TiO2 is considered a highly promising anode material due to its safety, natural abundance, high capacity, and structural stability.Li et al. employed a high-temperature heating-quenching method to fabricate three-dimensional porous graphene with a large surface area and excellent conductivity.The resulting three-dimensional porous graphene/TiO2 composite demonstrated a remarkable capacity of 120 mAh/g even after 100 cycles, surpassing the pure TiO2 capacity by 4.8 times.This significant enhancement effectively improved the electrochemical characteristics of electrode materials for LIBs.

Conclusion
With the rapid development of the new energy vehicle industry, the advancement of LIBs with elevated energy density and power density, rapid charge-discharge capability, and enhanced safety has become an urgent need.Possessing substantial specific surface area, exceedingly high electrical conductivity, commendable mechanical properties, as well as excellent chemical and thermodynamic stability, graphene in LIBs has demonstrated superior performance in improving electrode conductivity and shows great prospects.In summary, the application of graphene materials in LIBs has many advantages and meets the current demand for higher energy density and power density in LIBs development.The use of graphene-based composite conductive agents instead of traditional carbon black and carbon nanotubes has become an effective approach to enhance the performance of LIBs, gaining widespread recognition in the industry and carrying significant market potential, albeit with certain limitations.In the future, key areas of research for the mass application and production of graphene materials in LIBs will include cutting-edge techniques for the preparation of graphene materials, tailored development options, and appropriate dispersion technologies for conductive agents.

Fig. 2 .
Fig. 2. Structural models and schematic diagrams of lithiumion movement pathways (https://www.nature.com/articles/nmat4170/figures/4)Asshown in Fig.2a, based on the microscopic interaction between graphene sheets and the active material particles, the following six combination modes can be identified: (i) Serving as a protective casing, the graphene sheets encapsulate the individual or a small number of active material particles.(ii) Active material particles and graphene are mixed mechanically.(iii) Large amounts of active material particles are wrapped

Table 1 .
Comparison of various kinds of commonly used conductive additives.