By: Akanksha Urade (Ph.D. Scholar at IIT Roorkee and Graphene & 2D Materials Science Writer)
Imagine having a smartphone that could recharge in only ten minutes and last more than a week. Unfortunately, it may appear impossible with existing lithium-ion battery (LIB) technology, but breakthroughs in graphene batteries are bringing these possibilities to life. Graphene batteries have recently made significant advances, but there are still obstacles to overcome before they can be used in practical applications. This article will explore how a defect-free form of graphene (large (>50 µm lateral flake size), thin and nearly defect free (LTDF) graphene flakes) could aid in realizing the full potential of next-generation batteries.
How does graphene improve a battery?
LIBs are the power source of choice for modern mobility. LIB performance, however, is limited in two ways: First, the capacity of a battery is restricted by the number of lithium (Li) ions that can be packed into the anode or cathode. Second, the charging rate of a battery is restricted by the rate at which Li ions may move from the electrolyte to the anode. To drive future advancements in battery capacity, we require new advanced materials that can not only significantly increase performance qualities but also allow for the transition to new battery types.
In today’s LIBs, graphite is employed as an anode material, however, it has limitations. Because of its small surface area of 10 m2 /g, graphite can only store one Li atom for every six carbon atoms. Unlike graphite, graphene (< 10 atomic layers of carbon) has a surface area of 2630 m 2 /g, which allows it to hold Li ions on both faces of the sheet, including its edges, substantially improving the battery’s ability to store energy. Graphene has enormous potential to hold charges with less degradation through long-term cyclic stability. Because of its high surface area and superior electron transfer capability, using graphene as a conductive additive, a supporting substrate or a composite ingredient (e.g., with Si) can significantly improve the diffusion and transmission of ions and electrons in cathodes.
In addition, graphene can serve as an effective electron transport pathway, lowering the internal resistance of LIBs and increasing their power output. Graphene’s superior mechanical properties improve the durability of electrode materials, resulting in increased rate capability and cycle stability. Adding graphene to Li-ion batteries addresses low electrical conductivity and poor ion diffusion that leads to capacity fading due to continuous charge-discharge. It appears that dozens of companies are at some stage of using graphene for different types of batteries, including Li-sulfur, graphene-aluminum hybrid, polymer battery and non-flammable graphene battery.
Which graphene material is optimal for energy storage?
Reduced graphene oxide (rGO) is the most common form of graphene for electrodes in electrochemical energy storage applications. This is most likely because rGO is widely available and has a conductivity that is higher than graphene oxide (GO) and activated carbon. Although rGO is frequently employed at the lab scale, there are several issues that arise when rGO is produced in large quantities. The principal challenges are quality and price due to problematic chemical heterogeneity, batch-to-batch reproducible consistency and inevitable damage to the sp2 structure owing to carbon-oxygen bonding. This damage cannot be fully repaired by the reduction process, which further lowers the conductivity of rGO. There is also the question of environmental damage due to acid waste from the oxidation process.
A promising solution is LTDF graphene manufactured by a non-oxidative method. LTDF has high conductivity, purity and structural integrity in contrast to rGO. This is particularly important for batteries, as oxygen-related impurities impair electrical conductivity and battery performance. Moreover, LTDF has significantly higher surface area compared to Li, GO and rGO, making LTDF graphene flakes several orders of magnitude more appealing for use in electrochemical energy storage than currently used materials.
One of the most challenging aspects of real-world Li metal battery applications is the formation of solid electrolyte interphase (SEI), which favors dendrite growth during charge-discharge cycles leading to internal shorting (i.e., physical risk hazards such as fire and explosion). The crucial unresolved concerns for the battery community, however, have always been whether graphene chemical and structural defects are advantageous or detrimental to Li metal battery applications. A recent study published in Advanced Energy Materials, however, shows that highly defective graphene promotes unstable SEI and harmful dendritic growth. This study reveals what researchers have speculated for some time – that LTDF graphene can suppress Li dendrite formation, which has been a long-standing challenge to Li batteries. More precisely, the lateral flake size influences graphene’s electrochemical properties because of the causal relationship between the flake size and integrity of electron path. Based on research, the larger the flake size, the better the Li-ion diffusion pathway. The smaller the flake size, the greater the reduction of ionic conductivity, creating a “barrier effect” resulting in specific capacity loss and decreased rate performance. It should be noted that this is an active area of academic study.
Commercial examples of uses of low defect graphene for energy storage
When employed as an electrode material for a supercapacitor, 0.05% of Avadain’s LTDF free graphene flakes have been reported to provided 100% stable specific capacitance, even at a higher current density of 10A/g, whereas activated carbon and rGO show a 30% decline in the specific capacitance. Avadain’s flakes also enabled faster charging/discharging, 100% depth of discharge and increased power density versus rGO and activated carbon.
In another report, scientists from the UK-based Levidian reported that their sub-micron sized graphene powder, manufactured from methane using plasma chemistry, exhibits a significant improvement in the charge-discharge rate and higher conductivity (550 S/cm) compared to activated carbon or graphene nanoplatelets (GNP-90S/cm) when used as an additive in Li-ion battery electrodes. Levidian claims increases of up to 20% and a capacity that was somewhat greater (138 mAh/g) than activated carbon (116 mAh/g) for a coin cell prototype.
Lyten, a California-based company which has attracted a number of high-profile investors, claims to have the theoretical ability to achieve an energy density of 900 Wh/kg – three times that of typical Li-ion batteries, and a continuous charge-discharge cycle of up to 1400 cycles by incorporating a graphene stack – called 3D graphene – into a cathode in their rechargeable Li-sulfur battery. One of Lyten’s patents states that it has “a cathode formed of few layer graphene (FLG) sheets defining a three-dimensional (3D) carbon-based multi-modal structure.” Not much is known about Lyten’s 3D graphene, but it appears to be a crumpled/folded sheet or stack of graphene created by cracking methane, resulting in sub-micron lateral flake size.
In each of these instances, the low defect graphene structure is responsible for varying levels of electrochemical performance.
Why has graphene not made a significant influence on Li-ion batteries?
Despite researchers having proved the performance of graphene batteries that outperform commercially available LIBs, their practical applicability is limited by a lack of effective methods for mass producing LTDF graphene flakes. Another factor is that most battery companies do not want to incur the risk of being a first mover by introducing graphene into their manufacturing process or changing their existing battery chemistries. But this reluctance could change in the next year or two if Lyten or some other company combines LTDF with their advanced batteries, providing increased electrode density, faster cycle times, as well as possessing the ability to hold the charge longer and improve the battery’s lifespan.
There are several uses for graphene in batteries.
Liu, Wei, et al. (2019) “Pristine or highly defective? Understanding the role of graphene structure for stable lithium metal plating.” Advanced Energy Materials.
Liu, Fei, et al. (2015) “Electrochemical energy storage applications of “pristine” graphene produced by non-oxidative routes.” Science China Technological Sciences.
Du, Wencheng, et al. (2019) “Pristine graphene for advanced electrochemical energy applications.” Journal of Power Sources.
Levidian (2022) LOOP Reactor. [online] levidian.com Available at https://www.levidian.com/levidian-graphene