Electrochemical Insertion of Sodium into Carbon

Electrochemical Insertion of Sodium into Carbon Marca M. Doeff,* Yanping Ma,** Steven J. Visco, and Lutgard C. De Jonghe Materials Sciences Division, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720 ABSTRACT Electrochemical insertion of sodium ions into carbon using solid polymer electrolytes or organic liquid electrolytes is described. Cells with the configuration Na/P(EO)sNaCF3SOJCP(EO) = polyethylene oxide) or Na/liquid electrolyte/C were galvanostatically discharged, charged, and cycled. The extent of insertion into C (Le., x in Na§ was found to be a strong function of the type and particle size of the carbon used, and the reversibility of the process was highly dependent upon the type of electrolyte used. The possibility of designing a sodium ion rocking chair cell is discussed, and a first-generation example, using a petroleum coke anode, polymer electrolyte, and sodium cobalt bronze cathode is described. Rocking chair batteries, in which both the anode and cathode are intercalation materials, have recently been commercialized. Because the anodes are commonly inexpensive carbons such as petroleum coke or graphite, reductive intercalation of lithium into these materials is now the subject of intense scrutiny] Similar sodium insertion reactions into carbons have been observed, 2 but have not yet been exploited for use in batteries. We now describe a preliminary study of these insertion reactions and discuss the possibility of developing a sodium ion cell analogous to the wellknown lithium ion systems. carbon powder proceeded the furthest, to NaCre. This suggests that the inserted sodium may, at least in part, be associated with sites on the surfaces of the particles rather than being truly intercalated between layers in the disordered carbons, Le., supercapacitor behavior is exhibited. 7 2.5 Petroleum Coke, as received 2 Graph~e 1.5 Experimental Conoco petroleum coke, Shawinigan black, and JohnsonMatthey microcrystalline graphite were either ground in an attritor mill or used as received after heat-treatment. Polymer electrolytes of composition P(EO)sNaCF3SO3(PEO = polyethyleneoxide) and composite cathodes containing the carbon of interest, PEO, and NaCF3SO3were made as described previously.3 Electrodes for use in cells with liquid electrolytes consisted of carbon and ethylene propylene diene monomer (EPDM) binder (2% by weight) and were vacuum dried prior to use. Battery-grade solvents from Mitsubishi Petrochemical Company were stored in an inert atmosphere glove box (02 < 1 ppm) and used as supplied. Sodium was purified as described previously? , ~ ~ Geuom Plret r oCoke u n d ..... O.5 0 id.5 O% 10~ I 20% I 30% I 40% I I 50% 60% % to NaCI2 7O% I 80% t 90"1o Fig. 1. Discharges (50 ttA/cm 2) of Na/P(EO)sNaCF3SO3/C cells heated to 86~ The spikes in the profiles are due to periodic current interrupts to assess the cell polarization. Results and Discussion Cyclic voltammetry experiments on cells with graphite and petroleum coke working electrodes and P(EO)sNaCF3SO3polymer electrolytes have revealed a broad feature well positive of the sodium plating potential that is chemically reversible.4 This indicates that faradaic sodium insertion processes into petroleum coke, and significantly, into graphite occur (both positive2 and negative~ results have been reported previously for this material). To assess the extent of reaction (x in NaCx) and thus the suitability of these materials for anodes in rocking chair batteries, cells with carbon electrodes, polymer electrolytes, and sodium anodes were galvanostatically discharged at 50 i~A/cm2. Figure 1 shows results for various carbon samples. The cell potentials dropped rapidly upon passage of current, and a cutoff of 0.03 V was used to prevent interference from sodium plating (expected to occur at slightly below 0 V in operating sodium cells). The extent of sodium insertion was calculated from the time and amount of current passed. Complexes of approximate compositions NaC3oand NaC~5were formed for petroleum coke and Shawinigan black, respectively, but the maximum for graphite was only about NaCTo,in contrast to lithium into graphite (LiC6) and petroleum coke (LiC~2).True reductive intercalation in Li/graphite cells occurs only below about 0.4 V 6; thus the corresponding process in sodium cells is expected to occur only below 0.1 V. (The standard reduction potentials for Li+/Li and Na+/ Na are -3.0 and -2.7 V, respectively.) In other words, most of the reductive insertion process is expected to occur below the sodium plating potential in Na/graphite cells, and thus cannot be observed. For disordered carbons, insertion of sodium occurs at a higher voltage than graphite, and can proceed farther. Figure 1 also shows that the extent of reaction depends very strongly upon the particle size in disordered carbons. Petroleum coke used as received (average particle size, 70 Fm) does not insert sodium ions appreciably, but forms compounds of approximate stoichiometry NaCaoafter it has been ground to 1 i~m. Insertion into Shawinigan black, a very fine (submicron particle size) * ElectrochemicalSocietyActive Member. ** Electrochemical Society StudentMember. charge = 0.025 m A / c m~ 2.5 discharge 1,6.7=0.05 m A / c m2 discharge 2,4,5 = 0.1 m A / c m2 2 discharge 3 = 0,2 m A / c m~ 1.5 i 0.5 0.2 0.4 0.6 0.8 x in Na,C~ 3.5 2.5 >C 2 1.5 1 6 05 0 -0.5 i 0% 10% 20% p , 30% 40% 50% % to NaC,2 Fig. 2. (a, top) Cycling of an Na/P(EO)sNaCF3SO3/(ground) petroleum coke cell at 0.05 to 0.2 mA/cm = (discharge) and 0.025 mA/cm 2 (charge). Temperature = 86~ The gray line indicates the first cycle, and the dashed line, the third discharge. The voltage instabilities evident in cycles 6 and 7 are probably due to dendrite formation. (b, bottom) First cycle of an Na/DME, NaClO4/(ground) petroleum coke cell at 50 ~A/cm 2. Temperature = 21~ The spikes in the profiles are due to current interrupts to estimate the cell polarization. J. Electrochem. Soc., Vol. 140, No. 12, December 1993 9 The Electrochemical Society, Inc. L169 Downloaded 21 Jul 2009 to 131.243.15.129. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jsp J. Electrochem. Soc., Vol. 140, No. 12, December 1993 9 The Electrochemical Society, Inc. L170 Figure 2a shows several cycles of an Na/P(EO)sNaTf/petroleum coke cell, discharged at various rates. Approximately 20% of the sodium ions originally inserted into petroleum coke cannot be removed upon the first charge; however, no further losses are seen on subsequent cycles. This is similar to the situation with lithium and petroleum coke, and has been attributed to an irreversible reaction of alkali metal ions and formation of a protective layer on the carbon electrode. 6 In Na/petroleum coke cells, insertion was reversible (except for the loss noted above) when either PEO or dimethoxyethane-based electrolytes (Fig. 2b) were used. When propylene carbonate, ethylene carbonate, ~/-butyrolactone, or mixtures of these solvents were used instead, the cells could not be recharged. Apparently, passivation occurs only in the presence of ether-containing electrolytes. Interestingly, Li/PEO/graphite and Li/ PEO/petroleum coke cells also show evidence of continuous decomposition (irreversibility) upon intercalation, rather than passivation. This may reflect the greater extent of insertion of lithium ions into carbon and the fact that more strongly reducing compounds are formed. Irreversibility was also seen in the Na/PEO/Shawinigan black cells and may be occurring for the same reasons. An unoptimized rocking chair cell consisting of a petroleum coke anode, P(EO)sNaCF3SO3separator, and Nao.6CoO2cathode was assembled and cycled (Fig. 3) at moderate rates. Only half the theoretical capacity of the cathode can be utilized in this configuration (Nao.6CoO2is in the halfway discharged state and exhibits a sloping discharge profile vs. Na), but the results clearly illustrate the viability of such a system. Polyorganodisulfides 8 may be a better choice for cathodes, because they exhibit flat discharge profiles vs. Na and have extremely high capacities. Optimization of a sodium ion rocking chair cell requires not only proper choice of cathode, 4 3.5 3 2.5 2 1.5 1 0.5 0. l m A / c m 2 0 0% 10% " ~ ] i i i i 20% 30% 40% 50% 60% % Utihzation in Cathode Fig. 3. First cycle of an Na0.6CoO2/P(EO)sNaCF3SO~/(ground) petroleum coke cell, 0.1 m ~ c m ~ discharge, 0.26 m N c m 2 charge. Temperature = 100~ The spikes in the profiles are due to current interrupts to estimate cell polarization, anode, and electrolyte but also careful balancing of the electrode capacities to compensate for the initial loss of ions upon charge. It is now apparent, however, that the concept of the rocking chair battery need not be limited to lithium ion systems alone. Conclusions Carbon undergoes reversible electrochemical insertion of sodium ions, a process that may be exploited in sodium ion rocking chair batteries. A maximum composition of NaC15 has been achieved, and reversible insertion to NaC24for petroleum coke in cells with ether-based electrolytes has already been demonstrated. An unoptimized system with a petroleum coke anode, polyethylene oxide separator, and sodium cobalt bronze cathode could be charged and discharged successfully, and suggestions for improved performance have been given. Acknowledgments This work was supported by the Assistant Secretary for Conservation and Renewable Energy, Office of Transportation Technologies, Electric & Hybrid Propulsion Division of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098. Manuscript received Aug. 26, 1993. Lawrence Berkeley Laboratory assisted in meeting the publication costs of this article. REFERENCES 1. (a) J. R. Dahn, A. K. Sleigh, H. Shi, J. N. Reimers, Q. Zhong, and B. M. Way, Electrochim. Acta, 38, 1179 (1993); (b) Z. X. Shu, R. S. McMillan, and J. J. Murray, This Journal, 140, 922 (1993). 2. (a) P. Ge and M. Fouletier, Solid State Ionics, 28-30, 1172 (1988); (b) Y Maeda and S. Harada, Syn. Metals, 31, 389 (1989); (c)J. G. 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