This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 1945--1947 1945 Cite this: Chem. Commun., 2013, 49, 1945 Improving the electrochemical performance of organic Li-ion battery electrodes† Ste ´ven Renault, Daniel Brandell, Torbjo ¨rn Gustafsson and Kristina Edstro ¨m* Dilithium benzenediacrylate was prepared and investigated as an example of a readily available organic electrode material for lithium-ion batteries. Its poor conductive properties were overcome by a method of carbon-coating in the liquid state, resulting in enhanced cycling performance, displaying a reversible capacity of 180 mA h g 1 . In the field of chemical energy storage, lithium-ion batteries (LIBs) currently constitute the dominant chemistries for portable electronic devices, since they display both design flexibility and high energy density. However, the standard electrode materials in LIBs are typically inorganic lithium metal oxides (e.g. , LiFePO 4 , LiCoO 2 , LiMn 2 O 4 , Li 4 Ti 5 O 12 , etc. ) prepared from finite and non-renewable mineral resources. The growing large-scale production of LIBs makes their environmental impact and recycling a problem that must be addressed in the future. 1,2 During the last few years, organic electrodes have emerged as a promising alternative due to the fact that they are potentially environmental friendly, cheap, abundant and recyclable materials, if derived from biomass via ecofriendly processes with minimum energy consumption. 3 Specifically, molecules containing carbonyls have attracted attention due to their redox-active nature and abundance in natural-product-based compounds. Structures such as quinones, 4–8 imidates, 8–11 anhydrides 12,13 or lithium carboxylates 14,15 have been successfully used in prototype rechargeable LIBs. However, organic electrodes suffer from different drawbacks when compared to inorganic materials. The most well known are poor energy density, poor conductivity and high solubility in electro- lytes (especially carbonate-based). For the latter issue, the resulting capacity fading 16 can be tackled with various strategies such as grafting on insoluble particles 17,18 or employing a quasi-solid cell design, 19,20 but to the best of our knowledge, no systematic studies have so far been done to improve the conductivity of insulating or poorly conductive organic electrodes. A common way to increase the conductivity of any electrode material is by the addition of a conductive additive ( e.g. , carbon). Adding a large proportion of such a conductive additive will result in lower energy density in the cell and should therefore, if possible, be avoided. Another strategy is improving the surface contacts between the electrode material and the conductive additive by a mixing procedure. Classical solid state methods are grinding with mortar and pestle and ball-milling (even if a solvent can be used as a dispersant) and are moderately efficient. However, given the tendency of organic molecules to solubilise in organic solvents, a liquid–solid mixing might be possible under proper conditions, thus further improving the carbon coating of an organic electrode material and thereby enhancing its conductivity. Taking advantage of the supposedly perfect dispersion while in solution, a solubilised organic material might more easily penetrate into the porous structure of the conductive additive and increase their surface contact when dried, as compared to classical solid state methods. Therefore, we have investigated the effect of carbon coating an organic electrode material in the liquid state. The selection of a proper poor electron conductor material as a model compound is essential here. Given the inherent tendency of conjugated dilithium dicarboxylates to have a low conductivity that might limit their performance, 21 dilithium trans–trans benzene diacrylate 1 has been selected as a model compound (Fig. 1). Dilithium dicarboxylates are also insoluble in carbonate type solvents but their ionic nature makes them generally soluble in aqueous systems. Dilithium benzenediacrylate 1 has previously been described as a UV absorber and photoreactant, 22,23 but its usage for energy storage has never been reported. Its molecular weight of 230.07 g mol 1 corresponds to a theoretical capacity of 116.5 mA h g 1 per lithium for an expected uptake of 2 lithium atoms per formula unit. Two samples were prepared: the first (A) using a classical synthesis routine with Fig. 1 Chemical structure of dilithium trans–trans benzenediacrylate 1, dilithium trans–trans muconate 2 and dilithium terephthalate 3. Dept. of Chemistry – Ångstro¨m Laboratory, La ¨gerhyddsva¨gen 1, Polacksbacken, 751 21 Uppsala, Sweden. E-mail: [email protected]; Fax: +46 18 51 35 48; Tel: +46 18 471 37 13 † Electronic supplementary information (ESI) available: 1 H NMR, 13 C NMR and IR spectra of dilithium benzenediacrylate 1. See DOI: 10.1039/c3cc39065a Received 20th November 2012, Accepted 21st January 2013 DOI: 10.1039/c3cc39065a www.rsc.org/chemcomm ChemComm COMMUNICATION Downloaded by Monash University on 21 March 2013 Published on 22 January 2013 on http://pubs.rsc.org | doi:10.1039/C3CC39065A View Article Online View Journal | View Issue
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This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 1945--1947 1945
Cite this: Chem. Commun.,2013,49, 1945
Improving the electrochemical performance of organicLi-ion battery electrodes†
Steven Renault, Daniel Brandell, Torbjorn Gustafsson and Kristina Edstrom*
Dilithium benzenediacrylate was prepared and investigated as
an example of a readily available organic electrode material for
lithium-ion batteries. Its poor conductive properties were overcome
by a method of carbon-coating in the liquid state, resulting in enhanced
cycling performance, displaying a reversible capacity of 180 mA h g�1.
In the field of chemical energy storage, lithium-ion batteries (LIBs)currently constitute the dominant chemistries for portable electronicdevices, since they display both design flexibility and high energydensity. However, the standard electrode materials in LIBs aretypically inorganic lithium metal oxides (e.g., LiFePO4, LiCoO2,LiMn2O4, Li4Ti5O12, etc.) prepared from finite and non-renewablemineral resources. The growing large-scale production of LIBsmakes their environmental impact and recycling a problem thatmust be addressed in the future.1,2 During the last few years, organicelectrodes have emerged as a promising alternative due to the factthat they are potentially environmental friendly, cheap, abundantand recyclable materials, if derived from biomass via ecofriendlyprocesses with minimum energy consumption.3 Specifically,molecules containing carbonyls have attracted attention due totheir redox-active nature and abundance in natural-product-basedcompounds. Structures such as quinones,4–8 imidates,8–11
anhydrides12,13 or lithium carboxylates14,15 have been successfullyused in prototype rechargeable LIBs.
However, organic electrodes suffer from different drawbackswhen compared to inorganic materials. The most well known arepoor energy density, poor conductivity and high solubility in electro-lytes (especially carbonate-based). For the latter issue, the resultingcapacity fading16 can be tackled with various strategies such asgrafting on insoluble particles17,18 or employing a quasi-solid celldesign,19,20 but to the best of our knowledge, no systematic studieshave so far been done to improve the conductivity of insulating orpoorly conductive organic electrodes. A common way to increase theconductivity of any electrode material is by the addition of a
conductive additive (e.g., carbon). Adding a large proportion of sucha conductive additive will result in lower energy density in the celland should therefore, if possible, be avoided. Another strategy isimproving the surface contacts between the electrode material andthe conductive additive by a mixing procedure. Classical solid statemethods are grinding with mortar and pestle and ball-milling (evenif a solvent can be used as a dispersant) and are moderately efficient.However, given the tendency of organic molecules to solubilise inorganic solvents, a liquid–solid mixing might be possible underproper conditions, thus further improving the carbon coatingof an organic electrode material and thereby enhancing itsconductivity. Taking advantage of the supposedly perfectdispersion while in solution, a solubilised organic materialmight more easily penetrate into the porous structure of theconductive additive and increase their surface contact whendried, as compared to classical solid state methods.
Therefore, we have investigated the effect of carbon coating anorganic electrode material in the liquid state. The selection of aproper poor electron conductor material as a model compound isessential here. Given the inherent tendency of conjugated dilithiumdicarboxylates to have a low conductivity that might limit theirperformance,21 dilithium trans–trans benzene diacrylate 1 has beenselected as a model compound (Fig. 1). Dilithium dicarboxylates arealso insoluble in carbonate type solvents but their ionic naturemakes them generally soluble in aqueous systems. Dilithiumbenzenediacrylate 1 has previously been described as a UV absorberand photoreactant,22,23 but its usage for energy storage has neverbeen reported. Its molecular weight of 230.07 g mol�1 correspondsto a theoretical capacity of 116.5 mA h g�1 per lithium for anexpected uptake of 2 lithium atoms per formula unit. Two sampleswere prepared: the first (A) using a classical synthesis routine with
Fig. 1 Chemical structure of dilithium trans–trans benzenediacrylate 1, dilithiumtrans–trans muconate 2 and dilithium terephthalate 3.
Dept. of Chemistry – Ångstrom Laboratory, Lagerhyddsvagen 1, Polacksbacken,
1946 Chem. Commun., 2013, 49, 1945--1947 This journal is c The Royal Society of Chemistry 2013
isolation of the organic material followed by a carbon-coating in thesolid state, and the second (B) with an in situ carbon-coating duringsynthesis.‡
Method A: The preparation of dilithium benzenediacrylate 1 isstraightforward: benzenediacrylate acid 4 is stirred with a stoichio-metric equivalent of lithium carbonate at 50 1C for 2 days in asolution of water and ethanol (1 : 1, v/v) (Scheme 1). Although animmediate bubbling is observed corresponding to CO2 evolutionand reaction progress, the extended reaction time assures acomplete conversion of the reactants. Under these conditions, nosolid particles can be observed in the reaction solution, thusindicating that the resulting lithium carboxylate is completelydissolved. After drying, dilithium benzenediacrylate 1 is obtainedas a crystalline white solid with an excellent yield of 98%. Both IR, 1Hand 13C NMR data indicate the successful synthesis of dilithiumbenzenediacrylate 1. The white colour is usually an indication of abroad band gap and a corresponding low conductivity. The activematter is then mixed with carbon super porous (SP, 33% in totalweight) and subjected to a conventional ball-milling procedure.
Method B: In a similar approach, the same proportion of carbonadditive is added directly during synthesis (Scheme 1). Carbon SPcan be considered as chemically inert in this synthesis and is justdispersed in the reaction solution. The same preparation procedurehas been used but no other conductive additive has been addedafterwards. It should be noted that a moderate quantity of whiteuncoated particles is obtained during the drying sequence that arepresumably obtained through a simultaneous decantation of thecarbon phase/dissolution of the lithium salt, thus indicating that theresulting obtained material is not homogeneously coated.
The electrochemical behaviour of 1 has been obtained fromSwagelok-type half-cells using Li metal disc as negative electrode anda glass fibre separator soaked with a 1 M LiTFSI solution in DMC aselectrolyte. Fig. 2 shows the first discharge/charge curve for dilithiumbenzenediacrylate 1 cycled vs. Li at a rate of one Li+ exchanged in10 h (i.e., C/10 for the monolithiated compound or C/20 for thedilithiated compound). The novel lithium carboxylate displays anefficient reversible electrochemical activity, characterized by a singleplateau with an average value of 1.2 V for a two electrons process(Scheme 2), weakly polarized and occurring at a more reducingpotential as compared to dilithium trans–trans muconate 2, but at amore oxidizing potential as compared to dilithium terephthalate 3(Fig. 1).14 The extended conjugation compared to these standardmaterials allows a good stabilization of the as-produced radical bymesomeric effects and avoids the formation of a biradical (oftenunstable) for the fully lithiated compound. However, significantdifferences can be observed for the two samples (Fig. 2). For thesample prepared according to procedure A, the first discharge curvereveals an uptake of 1.74 Li per formula unit (B203 mA h g�1) andthe removal of only 0.67 Li in the subsequent charging process,leading to a practical reversible capacity of B80 mA h g�1 with asignificant capacity fade after 20 cycles. The sample prepared withmethod B, on the other hand, shows enhanced performance with
less irreversible capacity loss. The first discharge curve reveals theuptake of 2.08 Li per formula unit (B246 mA h g�1), indicating acontribution from a SEI layer formation. The first charge shows theremoval of 1.53 Li per formula unit and a practical reversiblecapacity of B170 mA h g�1. Both samples were examined by SEM(Fig. 3). Sample A revealed a non-homogeneous distribution ofcarbon SP and dilithium benzenediacrylate 1 which is characterizedby crystalline particles in a 2 to 10 mm size range of irregular shapeswith flat surfaces. A noticeable proportion of carbon is present asagglomerates, leaving some surfaces of the electroactive materialpoorly coated. For sample B, larger particle size was observed; mostexceeding 20 mm. However, a majority of their surface is completelycoated with carbon in accordance with their electrochemical perfor-mance. This establishes a proof of the successful strategy of carbon-coating in the liquid state. This strategy could well be extended toany organic compound that can be solubilised, thereby including abroad scope of LIB electrode material. Nevertheless, the overall largeparticle size obtained for both samples can be attributed to an
Scheme 1 Synthesis of dilithium benzenediacrylate 1.
Fig. 2 Electrochemical behaviour of a Li half cell with dilithium benzenediacrylate(1) with preparation method A (a) or B (b) as counter electrode and cycledgalvanostatically between 0.9 and 3 V at a rate of 1 Li+ per 10 h in 1 M LiTFSI/DMC electrolyte. Inset: corresponding capacity retention curve.
This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 1945--1947 1947
aggregation phenomenon during the solvent evaporation in thepreparation procedure. This constitutes a drawback in this strategythat will be needed to be tackled and leaves room for furtherimprovement.
In order to explore the cyclability of this LIB electrodematerial, a cell has been prepared using sample preparationmethod B and has been cycled at a rate of one Li+ exchanged in1 h (C/2 for the dilithiated compound). As seen in Fig. 4, 100cycles can be achieved with only a moderate loss of capacityappearing after the second cycle. Although a noticeable drop of50% of the capacity of sample B is obtained when the C-rate isincreased 10 times, it is worth noting that this result is stillhigher than for sample A at C/20 (Fig. 2A).
In summary, dilithium benzenediacrylate 1, a new material forLIBs, was obtained using straight-forward chemical synthesis and
showed promising electrochemical performance. Its poor conductivitycan be overcome by using a carbon-coating method in a liquidmedium, thereby taking advantage of its solubility in specific solvents.This method for increasing the conductivity can in theory be general-ized to a broad range of soluble organic electro-active material andmight help to improve their performance for their utilization in thenext generation of rechargeable batteries. Further investigations onthis carbon-coating for a better understanding of its mechanism andeffect (including XRD, TEM pictures) will be reported in a forthcomingpublication.
This work has been supported by the Swedish ResearchCouncil and STandUp for Energy.
Notes and references‡ Solvents and reagents were purchased from Aldrich or Alfa Aesar and wereused as received. 1H and 13C NMR spectra were recorded at 400 MHz and100 MHz on a JEOL ECP-400 spectrometer, at room temperature, respectively.Chemical shifts (d) were expressed in parts per million (ppm) relative toresidual D2O or an internal standard. Infrared spectra were recorded with aPerkin Elmer Spectrum One FT-IR spectrometer in the 650–4000 cm�1
frequency range equipped with an attenuated total reflectance probe (ATR).The positive electrode was prepared without a binder by mixing organiccompounds with 33% carbon SP (in total mass) in the solid state (method A)or in the liquid state (method B). Mechanical mixing was carried out on aRestch for 1 hour. The powder and 2 balls (diameter 20 mm) were stowed in amilling container (55 mL). Electrochemical performances were tested vs.lithium in Swageloks-type cells using Li metal disc as negative electrode anda fibreglass separator soaked with a molar LiTFSI solution in DMC as theelectrolyte. Cells were cycled in galvanostatic mode using an Arbin BT-2043system. The morphology of the samples was observed using a high resolutionscanning electron microscope (HRSEM LEO 1550).
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Fig. 3 SEM of the mixture of dilithium benzenediacrylate 1 and carbon SP(33 wt%) for sample A (A) and B (B). Different magnifications were used.
Fig. 4 Capacity retention curve of a Li half cell using dilithium benzenediacrylate (1)prepared according to method B and cycled galvanostatically between 0.9 and 2 V ata rate of 1 Li+ per h in 1 M LiTFSI/DMC electrolyte.
