Patent Application
20130273431
The present novel technology relates generally to electrochemistry and, more particularly, to graphene-vanadium oxide aerogel composites as cathodes for Li-ion batteries.
The need for alternative fuel sources has grown over the last twenty years, due in part, to the rise of oil prices. As world population continues to grow so will the need to develop more efficient vehicles that require fewer natural resources. Since the late 1990s, advancements in battery technologies have driven the possibility of ending a vehicle's need for liquid fuel. These advancements still face strong opposition due to their high cost, limited range, and weight parity issues in comparison to the traditional oil based fuel. In addition the traditional Li-ion batteries typically have short cycle lives and show significant degradation issues with age. These limitations have shown to be a hindrance to the development of the alternative fuel sources.
Additional problems in the field of Li-ion batteries have become more apparent as the search continues for an alternative fuel source. The driving range of electric vehicles (EVs) and the fuel economy of hybrid electric vehicles (HEVs) are limited by the specific energy of Li-ion batteries (LIBs). Current state-of-the-art LIBs can achieve only 250 Wh/kg with prismatic cell configuration. The specific energy of LIB cells are limited by both the anode (e.g. graphite, 372 mAh/g) and the cathode (e.g. LiNiO2, 180 mAh/g). Rechargeable lithium metal electrodes have remained a major challenge for high specific energy anodes for decades due to internal-shorting caused by dendrite formation.
Alternative materials, such as V2O5, have been studied as a replacement for the current cathode materials due to its high specific capacity, but V2O5 has not been considered practical for use in batteries due to performance issues arising from its low electronic conductivity. The challenge is to improve e− conductivity, not only the interparticle but also intraparticle. The major technical risks associated with this improvement must address (1) the cycle life of the rechargeable Li metal anode, which is targeted to at least 1000 cycles, and (2) the capacity and cycle life of a nanostructured V2O5/graphene composite. The present novel technology addresses this need.
For the purposes of promoting an understanding of the principles of the claimed technology and presenting its currently understood best mode of operation, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the claimed technology is thereby intended, with such alterations and further modifications in the illustrated device and such further applications of the principles of the claimed technology as illustrated therein being contemplated as would normally occur to one skilled in the art to which the claimed technology relates.
Referring now to
Internal shorting is typically an issue with conventional Li metal electrodes due to dendrite formation. The novel cathode material 5 allows control over the direction of dendrite growth to overcome the cells internal short circuiting. Dendrites 15 initially grow toward each other in the through-plane direction (simultaneously starting from the Li metal surface and from the separator surface), contact each other, stop growth in the through-plane direction but continue growth in the in-plane direction, and eventually form a Li metal layer that prevents the internal-shorting. A carbon layer 20 with immobilized Li+-ions as seeds on the separator surface 25 is formed. These Li+-ions induce the Li dendrites 15 to grow on the coated separator surface 25, and eventually the dendrites 15 cancel each other out.
In one embodiment (not shown), a coin cell consisting of Li metal anode and a LiFePO4 cathode may reach seven hundred twenty five cycles with eighty percent initial capacity by coating a carbon layer or sheet on the surface of the separator, which does not require extraneous changes to the configuration or components of the current batteries and does not incur any significant additional use of resources.
In another embodiment, the cathode material 5 improves e− conductivity (via both the interparticles and intraparticles) by wrapping a carbon sheet 50, typically a single-atomic-layer-thick graphene sheet 50 around a V2O5 substrate 55, typically a nanorod and/or nanofiber 55, which effectively improves the interparticle e− conductivity. To improve the intraparticle e− conductivity, the graphene sheet 50 may wrap around the basic building blocks of the V2O5 aerogel 60. In one example, the nanorod 55 may be about 1 nm wide and about 100 nm long. For a V2O5 nanofiber and/or nanorod 55 synthesized through electrospinning, the nanorod 55 diameter may be reduced to the nanoscale (from the micro scale), so that the graphene wrap 70 may enhance the interparticle e− conductivity. The intraparticle e− conductivity may be improved by introducing the graphene oxide sheet 50 into a sub-nanometer unit of the V2O5 aerogel 60 and incorporating it into the precursor solution for electrospinning. The graphene sheet wrap 70 may increase the e− conductivity because graphene has a high e− conductivity and high mechanical strength, which can constrain the volume expansion of the V2O5 nanorod and/or nanofiber 55 when the Li+-ion is inserted. Enhancing the e− conductivity of the V2O5 is desired for maintaining rate performance while constraining the expansion is desired for maintaining cycle life. One way of wrapping the graphene sheet 50 around the V2O5 nanorod and/or fiber 55 is to anchor the graphene sheet 50 onto the surface of the nanorod and/or fiber 55, which requires the establishment of an interaction or weak bond between the graphene sheet 50 and the V2O5 nanorod and/or fiber 55.
In one embodiment, a graphene oxide solution 80 is used, which possesses a considerable number of hydroxyl and epoxide functional groups on both surfaces of the graphene sheet 50 and also includes carboxyl groups, mostly at the sheet edges 52. These partial charges/lone unpaired e− of the surface groups may easily establish weak bonds with charges on the surface of the V2O5 through a Columbic force, causing the graphene sheet 50 to wrap around the nanorod 55 and/or fiber 55.
To this end, various approaches may be taken, including: wrapping the graphene sheets 50 (synthesized through the sol-gel process 301) around the V2O5 aerogel nanorod and/or nanofiber 55 wrapping graphene sheets 50 (synthesized through electrospinning 201) around the V2O5 nanorods and/or fibers 55 or the like.
In one example, the graphene oxide solution 60 is first prepared 300 using a modified Hummer's method, or the like. HVO3 is formed 305 by the acidification of a sodium vanadate solution (by passing it down an acidified ion-exchange column). GO and HVO3 are mixed 310, aged 315 for two weeks, and freeze-dried under a vacuum 320. The gels or composite solutions are calcined 325 at a temperature of at least three hundred degrees Celsius in nitrogen for two hours. The working electrodes are prepared 335 by mixing the graphene-V2O5 composite 85, carbon black and Polyvinyl difluoride with the weight ratio of 8:1:1. The discharge and charge measurements are conducted 340 with a commercially available battery tester.
The addition of graphene may increase the electrochemical performance of the V2O5 aerogel 60. For as-prepared 2 wt. % graphene-V2O5 aerogels, 3 wt. % graphene-V2O5 aerogels, and 4 wt. % graphene-V2O5 aerogels, the materials deliver discharge capacities of approximately 182, 304, and 189 mAh−1, respectively. Importantly, when 1 wt. % graphene has been loaded into the V2O5 aerogel, the initial capacity is as high as 380 mAhg−1, as compared to the bare V2O5 aerogel with 166 mAhg−1.
As shown in
The 1 wt. % graphene-V2O5 aerogels typically have high cycling stability. The initial discharge capacities of the composite are 307 mAhg−1, respectively, when cycled between 2V to 3.8V at 1 C rate. The three hundredth discharge capacities decreased to 150 mAhg−1, while the capacity retention is still as high as fifty percent for the composite. The electrochemical performance may be attributed to the incorporation of the graphene sheet 50 in the V2O5 nanostructure, which significantly enhances the conductivity and stability of the V2O5 aerogel 60, consequently enhancing the capacity, rate performance, and cycle life.
In some embodiments, the graphene-V2O5 aerogel composites 85 are synthesized via a sol-gel process 301, and its electrochemical properties may be investigated for Li-ion intercalation applications. The electrochemical analysis shows that 1 wt. % graphene-V2O5 aerogels delivers a high initial capacity of 380 mAhg−1. Compared with bare V2O5 aerogels, the incorporation of graphene increases the capacity, rate performance, and cycle life of the composites.
The driving range of electric vehicles (EVs) and the fuel economy of hybrid electric vehicles (HEVs) are limited by the specific energy of Li-ion batteries (LIBs). Current state-of-the-art LIBs can achieve only 250 Wh/kg with prismatic cell configuration. The specific energy of LIB cells is limited by both the anode (e.g. graphite, 372 mAh/g) and the cathode (e.g. LiNiO2, 180 mAh/g). We propose to develop a at least 1000 Wh/kg Li/V2O5 battery system, consisting of a Li metal anode and a V2O5 cathode with theoretical specific capacities of 3860 mAh/g and 442 mAh/g, respectively. The theoretical specific energy of this Li/V2O5 cell is 1326 Wh/kg. Two major technical challenges are (1) a rechargeable Lithium anode, and (2) a high specific energy and high rate V2O5 cathode. The deliverable of this project is a Li/V2O5 battery system with at least 1000 Wh/kg and 1000 cycles at 1 C rate and room temperature demonstrated in the pouch cell configuration.
Rechargeable lithium metal electrodes have remained a major challenge for high specific energy anodes for decades due to internal-shorting caused by dendrite formation. The approach we are taking to overcome this challenge is to control the direction of the dendrite growth so the dendrites initially grow toward each other in the through-plane direction (simultaneously starting from the Li metal surface and from the separator surface), touch each other, stop growth in through-plane direction but start growth in an in-plane direction, eventually form a Li metal layer that prevents the internal shorting. The key to this approach is to form a carbon layer with immobilized Li+ ions as seeds on the separator surface. These Li+ ions induce the Li dendrites to grow on the coated separator surface, and eventually the dendrites cancel each other out. Using such an approach, a coin cell consisting of a Li metal anode and a LiFePO4 cathode has reached seven hundred and twenty five cycles with 80% initial capacity. The technology uses a simple approach by coating a special carbon layer on the surface of the separator, which does not require changes to the configuration or components of current batteries and does not incur any significant additional cost. In the proposed project, we will focus on increasing cycle life and reducing capacity loss with a target of at least 1000 cycles and 80% initial capacity.
V2O5 has been studied as a cathode material for decades because of its high specific capacity, but has not been used in practical batteries due to its poor rate performance caused by its low electronic conductivity. The challenge is to improve e− conductivity not only the interparticle but also intraparticle. The concept we propose is to wrap a single-atomic-layer-thick graphene sheet around the V2O5 nanorod and/or nanofiber, which will improve the interparticle e− conductivity. To improve the intraparticle e− conductivity, the graphene sheet wrap around the basic building blocks of the V2O5 aerogel: the nanorod (e.g. 1 nm wide and 100 nm long). For a V2O5 nanofiber synthesized through electrospinning, the diameter needs to be reduced to the nanoscale so that the graphene wrap can enhance the interparticle e− conductivity. The intraparticle e− conductivity can be improved by introducing the graphene oxide sheet into a sub-nanometer unit of the V2O5 aerogel and incorporating it into the precursor solution for electrospinning. The graphene sheet wrap can increase the e− conductivity because graphene has a high e− conductivity and high mechanical strength, which can constrain the volume expansion of the V2O5 nanorod/fiber when the Li+ ion is inserted. Enhancing the e− conductivity of the V2O5 is critical for rate performance, while constraining the volume expansion is critical for cycle life. The key to wrapping the graphene sheet around the V2O5 nanorod and/or fiber is to anchor the graphene sheet onto the surface of the nanorod and/or fiber, which requires the establishment of an interaction or weak bond between the graphene sheet and the V2O5 nanorod and/or fiber. We use graphene oxide, which possesses a considerable number of hydroxyl and epoxide functional groups on both surfaces of each sheet and also includes carboxyl groups, mostly at the sheet edges. These partial charges/lone unpaired e− of surface groups can easily establish weak bonds with charges on the surface of the V2O5 through a Columbic force, causing the graphene sheet to wrap around the nanorod and/or fiber. Two approaches will be taken: (1) wrap graphene sheets (synthesized through the sol-gel process) around the V2O5 aerogel nanorod, and (2) wrap graphene sheets (synthesized through electrospinning) around the V2O5 nanofiber/rods.
Successful improvement of the e− conductivity of the V2O5 nanorod (aerogel) as demonstrated by a two-fold (2×) performance improvement with the addition of 1 wt. % graphene: from 166 mAh/g to 380 mAh/g (498 Wh/kg to 1140 Wh/kg) at 1/20 C and a three-fold (3×) improvement for nanofiber (electrospinning) with the addition of 2 wt. % graphene: from 116 mAh/g to 350 mAh/g (348 Wh/kg to 1050 Wh/kg) at 1/20 C The nanorod (aerogel) with 1 wt. % graphene shows excellent rate performance as shown in
While the claimed technology has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character. It is understood that the embodiments have been shown and described in the foregoing specification in satisfaction of the best mode and enablement requirements. It is understood that one of ordinary skill in the art could readily make a nigh-infinite number of insubstantial changes and modifications to the above-described embodiments and that it would be impractical to attempt to describe all such embodiment variations in the present specification. Accordingly, it is understood that all changes and modifications that come within the spirit of the claimed technology are desired to be protected.
This application claims priority to copending U.S. provisional patent application Ser. No. 61/623318, filed on Apr. 12, 2012, and to U.S. provisional patent application Ser. No. 61/707245, filed on Sep. 28, 2012.
| Number | Date | Country | |
|---|---|---|---|
| 61623318 | Apr 2012 | US | |
| 61707245 | Sep 2012 | US |
