Patent Application: US-201113337182-A

Abstract:
a secondary lithium battery having an anode comprising graphene nanosheets doped with a doping element selected from the group consisting of nitrogen , boron , sulfur , phosphorous and combinations thereof . the secondary lithium battery and the anode provide capacity and other performance without degradation during long term charge and discharge cycling .

Description:
graphene oxide was first prepared by the modified hummers method 5 as described below . graphite powder ( 1 g ) was mixed with concentrated h 2 so 4 ( 23 ml ), and stirred at room temperature for 0 . 5 h . nano 3 ( 0 . 5 g ) was added into the mixture and left overnight . then , the reaction vessel was immersed in an ice water bath , and kmno 4 ( 3 g ) was added slowly and stirred for 2 h . subsequently , the mixture was stirred at 35 ± 3 ° c . for 3 h . after the dilution with di water ( 46 ml ), 30 % h 2 o 2 was added to the mixture , and the color of mixture changed into brilliant yellow along with violent bubbling . finally , the mixture was filtered and washed with hcl aqueous solution to remove residual metal ions , then washed with di water until ph = 7 . the slurry was dried in air . the resulting solid ( graphene oxide ) was inserted into a quartz tube in an argon atmosphere . the quartz tube was quickly inserted into a preheated furnace at 1050 ° c . for 30 s . this treatment is used to promote exfoliation of the graphene sheets and reduce the oxygen moieties 6 . the nh 3 annealing process was used to synthesize n - gns 5 . gns samples were put in a quartz boat in the center of a tube furnace . after flowing the mixture of ar and nh 3 ( v / v , 9 : 1 ) for about 30 minutes , the tube furnace was heated up to 900 ° c . for 10 min . then the tube furnace was cooled down in an ar atmosphere . the samples were taken out of the tube reactor after the furnace temperature was below 50 ° c . the morphologies for gns and n - gns were performed by a field emission scanning electron microscope ( sem ) ( hitachi s - 4800 ) and transmission electron microscopy ( tem ) ( philips cm10 ). raman spectra were conducted using a raman microspectrometer at room temperature with green laser as the exciting radiation equipped with an optical microscope . cyclic voltammetry tests were performed on chi electrochemistry workstation at a scan rate of 0 . 1 mv s − 1 over a potential range of 0 . 01 to 3 . 0 v ( vs . li / li + ). charge - discharge characteristics were tested galvanostatically between 0 . 01 and 3 . 0v ( vs . li / li + ) at room temperature using an arbin bt - 2000 battery test system . working electrodes were prepared by slurry casting on a cu foil as current collector . the slurry contains the active material ( gns or n - gns ) ( 90 wt % on dry solids basis ) and a polyvinylidene fluoride binder ( 10 wt % on dry solids basis ) in n - methylpyrrolidinone ( nmp ) solvent . the electrodes were dried in a vacuum at 110 ° c . overnight . the electrolyte was composed of 1 m lipf 6 salt dissolved in ethylene carbonate ( ec ): diethyl carbonate ( dec ): ethyl methyl carbonate ( emc ) at 1 : 1 : 1 volume ratio . lithium foil was used as a counter electrode . cr - 2325 - type coin cells were assembled in a glove box under dry argon atmosphere ( moister and oxygen concentration & lt ; 1 ppm ). anode material nitrogen doped graphene nanosheets ( n - gns ) of use in the practice of the invention is shown in fig1 b and 1 d along with its precursor material graphene nanosheets ( gns ) in fig1 a and 1 c . x - ray photoelectron spectra ( xps ) was employed to analyze the compositional change of graphene before and after nitrogen doping . based on xps spectra , the estimated composition of n - gns is 95 . 5 % c , 1 . 2 % n , and 3 . 3 % o , whereas its precursor gns has 96 . 8 % c , 0 % n , and 3 . 2 % o . the n 1 s signal splits into two peaks at 399 . 02 and 402 . 10 ev . they correspond to two types of doping nitrogen , i . e . pyridinic nitrogen and graphitic nitrogen 7 . the area percentage of pyridine - like and graphite - like nitrogen is 75 . 4 and 24 . 6 , respectively . it shows that the graphite - like nitrogen structure is more abundant than the pyridine - like nitrogen structure that has lower degree of crystalline perfection 8 . during the formation of pyridinic nitrogen , carbon vacancies are formed within a predominantly hexagonal graphene network 9 . these vacancies increase the li + storage sites , resulting in the capacity increase of n - gns anode 10 . fig2 ( a ) and ( b ) show the first two cycles charge ( de - intercalation )/ discharge ( intercalation ) profiles of gns anode and n - gns anode , respectively , in the voltage range of 0 . 01 - 3 . 0 v vs . li + / li at current density of 100 mag − 1 . both curves of gns and n - gns anodes present a similar li intercalation / de - intercalation profile . the presence of the plateau at about 0 . 6 v is assigned to the formation of a solid - electrolyte - interphase ( sei ) film 11 , 12 . the capacity of the potential region lower than 0 . 5 v is due to li + intercalation into the graphene layers 13 . they show no distinct potential plateaus , different from that of well crystalline graphite anode . the absence of a potential plateau suggests electrochemically and geometrically nonequivalent li sites . in the first cycle of gns anode , the discharge and charge capacities are 786 and 401 mahg − 1 , respectively , whereas for n - gns anode , the corresponding capacities increase to 1327 and 495 mahg − 1 , respectively . a higher reversible capacity was obtained for n - gns although accompanying a higher irreversible capacity . the cyclic voltammetry ( cv ) profiles of gns and n - gns anodes were recorded in voltage range of 0 . 01 - 3 . 0 v vs li / li + at a sweep rate of 0 . 1 mvs − 1 . both gns and n - gns exhibit similar cv behaviors , highlighting nitrogen doping graphene nanosheets have no obvious influence on initial li intercalation / de - intercalation . the first reduction in the discharge sweep of both electrodes in fig2 c and 2 d gives a prominent peak locating at about 0 . 6 v due to the formation of sei films on the anodes . this peak disappears during subsequent discharge , which is attributed to the isolation of the anode from electrolyte by the dense sei film formed in first discharge . evidently , the peak related to sei film is larger for n - gns than that of gns . it is in good agreement with the higher irreversible capacity found in n - gns anode in first charge and discharge . n - gns has higher specific surface area than gns , 599 m 2 g − 1 versus 456 m 2 g − 1 , and therefore it consumes more lithium in sei formation leading to higher irreversible capacity in first charge and discharge cycle . the cycling stability of gns and n - gns anodes is presented in fig3 . gns shows regular cycling performance , i . e . gradual declining in specific capacity with charge / discharge cycles . the discharge capacity after 100 cycles dropped from 407 mahg − 1 to 269 mahg − 1 , a retention of 66 %. for n - gns , a higher discharge capacity of 454 mahg − 1 was observed in the second cycle . after a slight decline in first 17 cycles , the capacity increases during further cycling and reached 684 mahg − 1 after 500 cycles . after testing 100 cycles in cells , the gns and n - gns anodes were harvested and washed for tem observation . the results were presented in fig4 . both images still show the graphene nanosheets entangled with each other resembling crumpled paper . it is obvious that n - gns can better keep the original structure during charge and discharge process in comparison to gns . raman spectra of gns and n - gns before and after charge / discharge cycles are given in fig5 . all spectra show two obvious peaks , the g band with e 2g symmetry at around 1595 cm − 1 and the d band with a 1g symmetry at about 1310 cm − 1 that are originated from the raman active in - plane tangential stretching mode of carbon - carbon bonds in highly oriented pyrolytic graphite and disorder induced features due to the finite particle size effect or lattice distortion , respectively . clearly , there is a big difference in the raman spectra of n - gns before and after 100 cycles whereas no significant difference was found for that of gns . in order to compare the structural changes before and after 100 cycles , a critical factor ( i d / i g ), the integrated intensity ratio of d band and g band was obtained as a measure of the disorder degree of graphene . i d / i g value of gns is 5 . 6 and 6 . 2 before and after 100 cycles , respectively . however , i d / i g value of n - gns increased from 5 . 8 to 8 . 8 after cycling . for n - gns , clearly , the intensity growing of the disorder - induced d - band at 1310 cm − 1 ( relative to the g band ) indicates that li + intercalation / de - intercalaton into graphene sheets upon cycling brings about a more noticeable change in the degree of long - range ordering in the hexagonal lattice than that of gns . n - gns have more and more disordered structures , such as defects , with charge / discharge process . these defect sites in n - gns provide more li + storage electrochemical active location . therefore , the capacity of n - gns increases with cycles . fig6 shows generally as 10 a secondary lithium battery having a plurality of cathodes 12 separated from anodes 14 by separators 16 formed of a porous material comprising a non - aqueous electrolyte , within housing 18 and having a top cap 20 . anode 22 has an aluminum current collector 24 coated with active material coating 26 comprising m - gns and a binder material ( fig7 ). alternative secondary lithium batteries may be prepared wherein separator 16 is a lithium ion conducting polymer as in a secondary lithium polymer battery , or a solid , lithium ion conducting inorganic or organic material as in a solid secondary lithium battery . d . y . pan , s . wang , b . zhao , m . h . wu , h . j . zhang , y . wang , z . jiao , chem . mater . 2009 , 21 , 3136 . g . x . wang , x . p . shen , j . yao , j . s . park , carbon 2009 , 47 , 2049 . d . y . zhang , g . y . zhang , s . liu , e . g . wang , q . wang , h . li , x . j . huang , appl . phys . lett . 2001 , 79 , 3500 . y . p . wu , c . y . jiang , c . r . wan , s . b . fang , y . y . jiang , j appl polym sci . 2000 , 77 , 1735 . x . r . wang , x . l . li , l . zhang , y . yoon , p . k . weber , h . l . wang , j . guo , h . j . dai , science 2009 , 324 , 768 . m . j . mcallister , j . l . li , d . h . adamson , h . c . schniepp , a . a . abdala , j . liu , m . herrera - 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