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RPI researchers develop safe, long-cycling Li-metal rechargeable battery electrode; demonstrate Li-carbon battery
27 April 2014
Researchers at Rensselaer Polytechnic Institute have developed a safe, extended cycling lithium-metal electrode for rechargeable Li-ion batteries by entrapping lithium metal within a porous graphene network (Li-PGN). The graphene “cage” prevents dendritic growth, enabling extended cycling of the electrode.
In a paper in the journal Nature Communications, the team reported that the plating of lithium metal within the interior of the porous graphene structure results in very high specific capacities in excess of 850 mAh g-1. Extended testing for over 1,000 charge/discharge cycles indicates excellent reversibility and coulombic efficiencies above 99%. The RPI team also demonstrated the use of the PGN material as a high-capacity anode, and demonstrated a full-cell configuration with a PGN anode and a lithium-metal/PGN cathode, thus creating a Li-carbon rechargeable battery.
Background. Lithium metal has a very high theoretical capacity of 3,842 mAh g-1 in rechargeable lithium batteries; however, the use of metallic lithium leads to extensive dendritic growth that short circuits the battery, leading to serious safety hazards as well as truncated battery life.
The first practical lithium (Li) battery was developed as far back as 1976 by Exxon, using TiS2 as the anode and bare lithium metal as the cathode. However, soon after its inception the viability of such a cell composition was questioned owing primarily to the use of the lithium metal–liquid electrolyte combination. In a non-aqueous electrolyte, dendritic projections tend to develop at the lithium–electrolyte interface. The dendritic growth occurs during the charge/discharge cycling process, expanding in diameter and at the same time, extending from the tip.
The tip-wise extension poses a serious challenge as extended cycling can often cause the dendrite tip to pierce through the separator, resulting in an electrical short between the anode and cathode. Moreover, the tip-wise dendritic formation process is accelerated as the current density is increased (typically ≥0.1 C, where a rate of nC is defined as charge or discharge in 1/n hours), which is attributed to a higher localized charge density at the dendrite tip. This in turn significantly limits the rate capability of such batteries.—Mukherjee et al.
To get around that problem, industry replaced lithium metal with layered lithium metal oxide and phospho-olivine cathodes that offer safer performance over extended cycling, although with significantly reduced capacities.
The RPI work. The RPI team thermally shocked graphene oxide (GO) paper, creating pores—defects—in the material measuring between a few nanometers to a few hundred nanometers. When compared with graphitic anodes (commonly LiC6: capacity ~372 mAh g-1, energy density ~180 Wh kg-1), the resulting PGN delivers capacities of ~900 mAh g-1 with an energy density of ~547 Wh kg-1.
The reason why such a porous graphene anode delivers a capacity and energy density that is almost threefold higher than conventional graphitic anodes and stable over 1,000 charge/discharge cycles remains unclear. Some studies with graphene anodes have reported achieving a capacity higher than the theoretical capacity of graphite and have generally attributed it to the formation of Li2C6, corresponding to the intercalation of lithium ions on both sides of the graphene sheets. However, a detailed understanding of this phenomenon is fundamentally lacking at this point.—Mukherjee et al.
They then looked at the interaction of elemental lithium with such defect sites in the graphene. They found that lithium ions are strongly attracted to defect sites, which creates a very high local concentration of lithium near the defects. This initiates the formation, or “plating,” of metallic lithium at the defect sites.
Compared with conventional cathodes, the PGN structure with entrapped lithium metal provides capacities of ~850 mAh g-1, with an energy density of ~637 Wh kg-1. Extended testing for over 1,000 charge/discharge cycles indicates excellent reversibility and coulombic efficiencies above 99%. Even after 1,000 cycles there was no indication of any significant dendritic structures.
So why does this Li plating for PGN not result in the formation of dendrites? … Within the interior of the electrode, Li metal plating occurs within pores that are few tens of nanometres in size … As a consequence, any dendritic structures associated with this plating will tend to remain confined within these pores due to its size constraints. However, it is noteworthy that even the exterior (outer) surface of the PGN electrode did not show any significant dendritic growth even though there is no entrapment mechanism for Li that plates onto the exterior surface of the PGN.
To understand this, we investigated the thickness of the plated Li on the exterior surface of the PGN. Our estimate for the maximum Li film thickness that plates onto the external PGN surface is ~5.8 nm … The reason why this layer is so thin is that the exterior (outer) surface area of the PGN is very small when compared with the total surface area contained within the bulk of the porous PGN electrode.
In other words, the porous PGN serves as a sponge that absorbs the majority of the Li metal into the interior of the electrode structure. The recent study by Harry et al. shows that contrary to conventional wisdom dendrites are formed in the subsurface of Li foils, and not on the exterior surfaces as was previously thought. In fact, they found that the volume of the dendrite within the electrode is of the same order as the volume protruding into the electrolyte. For these reasons, the few-nanometre-thick Li film that plates onto the external surfaces of the PGN electrode will be unable to generate significant volumes of dendrites, which is consistent with our experimental observations.—Mukherjee et al.
To demonstrate the capabilities of PGNs, they assembled a full-cell with PGN anodes and lithiated PGN (Li-PGN) cathodes, stacked to provide an ampere-hour rating of ~20 mAh. Since exclusively carbon based materials are used for the electrode construction, the only active source of lithium in the full-cell is the lithium metal that is entrapped within the pores of the PGN cathode. They used the pouch cell to power an LED device.
It should be noted that the full-cell (with PGN electrodes) … operates on the well-established principle of transfer of Li ions back and forth between the cathode and anode material. In this regard this concept is no different from a conventional Li-ion battery. What is different is that the electrode material that stores the Li is all-carbon for both the anode as well as the cathode. Therefore the electrodes in our Li-ion full-cell configuration are exclusively made from carbon (PGN) materials into which Li ions are reversibly plated and stored.—Mukherjee et al.
Along with Koratkar, co-authors of the paper are: graduate students Rahul Mukherjee, Abhay Thomas, and Eklavya Singh, and visiting scientist Osman Eksik, of the Rensselaer Department of Mechanical, Aerospace, and Nuclear Engineering; graduate student Dibakar Datta of Brown University; and postdoctoral scientist Junwen Li and Professor Vivek Shenoy of the University of Pennsylvania.
Koratkar’s graduate student and first author of the paper, Mukherjee, was a finalist in the 2014 MIT-Lemelson National Collegiate Student Prize Competition in the graduate student category and presented his research at the Massachusetts Institute of Technology earlier this month. With fellow student Eklavya Singh, Mukherjee won the “best of the best” grand prize at the spring 2014 Change the World Challenge competition at Rensselaer.
This research was supported by the National Science Foundation, as well as the John A. Clark and Edward T. Crossan Professorship at Rensselaer, and a Bob Buhrmaster ’69 grant from the Severino Center for Technological Entrepreneurship and Lally School of Management at Rensselaer.
Rahul Mukherjee, Abhay V. Thomas, Dibakar Datta, Eklavya Singh, Junwen Li, Osman Eksik, Vivek B. Shenoy & Nikhil Koratkar (2014) “Defect-induced plating of lithium metal within porous graphene networks,” Nature Communications 5, Article number: 3710 doi: 10.1038/ncomms4710
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