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Reviving rechargeable lithium metal batteries enabling next-generation

Reviving rechargeable lithium metal batteries:enabling next-generation high-energy and high-power cells ?

Aruna Zhamu,ab Guorong Chen,a Chenguang Liu,*a David Neff,a Qing Fang,a Zhenning Yu,b Wei Xiong,b Yanbo Wang,b Xiqing Wang a and Bor Z.Jang *a

Received 15th October 2011,Accepted 24th November 2011DOI:10.1039/c2ee02911a

Herein reported is a fundamentally new strategy for reviving rechargeable lithium (Li)metal batteries and enabling the emergence of next-generation safe batteries featuring a graphene-supported Li metal anode,including the highly promising Li–sulfur,Li–air,and Li–graphene cells with exceptionally high energy or power densities.All the Li metal anode-based batteries suffer from a high propensity to form Li dendrites (tree-like structures)at the anode upon repeated discharges/charges.A dendrite could eventually penetrate through the separator to reach the cathode,causing internal short-circuiting and even explosion,the main reason for the battery industry to abandon rechargeable lithium metal

batteries in the early 1990s.By implementing graphene sheets to increase the anode surface areas,one can signi?cantly reduce the anode current density,thereby dramatically prolonging the dendrite

initiation time and decreasing the growth rate of a dendrite,if ever initiated,possibly by a factor of up to 1010and 105,respectively.

Introduction

Today’s most favorite energy storage devices—lithium-ion batteries—actually evolved from rechargeable ‘‘lithium metal batteries’’using lithium (Li)metal as the anode and a Li inter-calation compound as the cathode.Li metal is an ideal anode material due to its light weight (the lightest metal),high elec-tronegativity (à3.04V vs.the standard hydrogen electrode),and high theoretical capacity (3860mA h g à1).1–3Based on these outstanding properties,lithium metal batteries were proposed 40years ago as an ideal system for high energy-density applications.During the mid-1980s,several prototypes of rechargeable Li metal batteries were developed.A notable example was a battery composed of a Li metal anode and a molybdenum sul?de cathode,developed by MOLI Energy,Inc.(Canada).This and several other batteries from different manufacturers were aban-doned due to a series of safety problems 4,5caused by sharply uneven Li growth (formation of Li dendrites)as the metal was re-plated during each subsequent recharge cycle.As the number of cycles increases,these dendritic or tree-like Li structures could eventually traverse the separator to reach the cathode,causing internal short-circuiting.Since early 1990s,only a limited number of companies have continued to produce rechargeable

lithium metal cells,and some being based on polymer or gel electrolytes.

To overcome these safety issues,several alternative approaches were proposed in which either the electrolyte or the anode was modi?ed.The ?rst approach involved replacing Li metal by graphite (another Li insertion material)as the anode.6–8The operation of such a battery involves shuttling Li ions between two Li insertion compounds,hence the name ‘‘Li-ion battery.’’Presumably because of the presence of Li in its ionic rather than metallic state,Li-ion batteries are inherently safer than Li-metal batteries.1The second approach entailed replacing the liquid electrolyte by a dry polymer electrolyte,leading to the Li solid polymer electrolyte (Li-SPE)batteries.9However,Li-SPE has seen very limited applications since it typically requires an operating temperature of up to 80 C.10

The past two decades have witnessed a continuous improve-ment in Li-ion batteries in terms of energy density,rate capa-bility,and safety,and somehow the signi?cantly higher energy density Li metal batteries have been largely overlooked.However,the use of graphite-based anodes in Li-ion batteries has several signi?cant drawbacks:low speci?c capacity (theoretical capacity of 372mA h g à1as opposed to 3860mA h g à1for Li metal),long Li intercalation time (e.g.low solid-state diffusion coef?cients of Li in and out of graphite and inorganic oxide particles)requiring long recharge times (e.g.7hours for electric vehicle batteries),inability to deliver high pulse power (power density (1kW kg à1),and necessity to use pre-lithiated cathodes (e.g.lithium cobalt oxide),thereby limiting the choice of avail-able cathode materials.Further,these commonly used cathodes have a relatively low speci?c capacity (typically <200mA h g à1).

Nanotek Instruments,Inc.,1240McCook Ave.,Dayton,Ohio,45404,USA.E-mail:Bor.Jang@https://www.doczj.com/doc/8114123935.html,;chenguang.liu@https://www.doczj.com/doc/8114123935.html, b

Angstron Materials,Inc.,1240McCook Ave.,Dayton,Ohio,45404,USA ?Electronic supplementary information (ESI)available.See DOI:10.1039/c2ee02911a

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P u b l i s h e d o n 12 D e c e m b e r 2011. D o w n l o a d e d b y S u z h o u I n s t i t u t e o f N a n o -T e c h a n d N a n o -B i o n i c s , C h i n e s e A c a d e m y o f S c i e n c e s o n 28/11/2015 08:44:56.

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These factors have contributed to the two major shortcomings of today’s Li-ion batteries—a low energy density (typically 150–180W h kg cell à1)and low power density (typically <1kW kg à1).10

After all,the highest energy density can be achieved by rechargeable batteries that utilize a lithium metal anode,provided that a solution to the safety problem can be formulated.These cells include (a)the traditional Li metal battery having a Li insertion compound in the cathode,(b)the Li–air or Li–O 2cell that uses oxygen as a cathode instead of lithium metal oxide (and Li metal as an anode instead of graphite),(c)the Li–sulfur cell,and (d)the new Li–graphene cell developed by us.

The Li–O 2battery is possibly the highest energy density elec-trochemical cell that can be con?gured today.The Li–O 2cell has a theoretic energy density of 5.2kW h kg à1when oxygen mass is accounted for.11A well con?gured Li–O 2battery can achieve an energy density of 3kW h kg à1,15–20times greater than those of Li-ion batteries.12However,current Li–O 2batteries still suffer from poor energy ef?ciency,poor cycle ef?ciency,and dendrite formation issues.13In the Li–S cell,elemental sulfur (S)as a cathode material exhibits a high theoretical Li storage capacity of 1672mA h g à1.With a Li metal anode,the Li–S battery has a theoretical energy density of $1600W h kg à1.14Despite its great potential,the practical realization of the Li–S battery has been hindered by several obstacles,such as low utilization of active material,high internal resistance,self-discharge,and rapid capacity fading on cycling.These technical barriers are due to the poor electrical conductivity of elemental sulfur,the high solu-bility of lithium polysul?des in organic electrolyte,the formation of inactivated Li 2S,and the formation of Li dendrites on the anode.14–16

Despite great efforts worldwide,dendrite formation remains the single most critical scienti?c and technological barrier against widespread implementation of all kinds of high energy density batteries having a Li metal anode.

We have discovered a highly dendrite-resistant,nano-graphene-enabled Li metal cell con?guration that exhibits a high energy and/or high power density.Each cell consists of a gra-phene surface-supported Li metal anode and a cathode con-taining either graphene itself or a graphene-enhanced Li insertion compound (e.g.vanadium oxide)as a cathode active material.Graphene is a single-atom thick layer of sp 2carbon atoms arranged in a honeycomb-like lattice.17,18Graphene can be readily prepared from graphite,activated carbon,graphite ?bers,carbon black,and meso-phase carbon beads.19Single-layer gra-phene and its slightly oxidized version (GO)can have a speci?c surface area (SSA)as high as 2670m 2g à1.19,20It is this high surface area that dramatically reduces the effective electrode current density ,which in turn signi?cantly reduces or eliminates the possibility of Li dendrite formation (further explained later).Although our research group pioneered the use of graphene as an electrode active material for the supercapacitor 21,22and the Li-ion battery anode,23these and other previous studies 24–28did not use graphene as part of an anode current collector in a Li metal cell to address the dendrite issue.

Experimental procedure

Preparation of Li–graphene cells :oxidized graphene or graphene oxide (GO)was prepared with a modi?ed Hummers’method

that involved exposing the starting graphitic materials (natural graphite,arti?cial graphite,meso-phase carbon,and carbon ?bers)to a mixture of sulfuric acid (98weight%),sodium nitrate,and potassium permanganate at a weight ratio of 4:1:0.1for 72hours.The resulting GO was then thoroughly rinsed with water to obtain GO suspension,which was followed by two different routes of material preparation.

One route involved subjecting the GO suspension to ultra-sonication to obtain isolated graphene oxide sheets suspended in water.After drying the ultrasonicated suspension in an aero-solizing oven,the resulting ?ower-shape aggregates of GO sheets were thermally reduced at 500 C for 2hours in nitrogen to signi?cantly improve the electrical conductivity (Electrode N).The other route involved drying the GO suspension to obtain the graphite intercalation compound (GIC)or GO powder.The GIC or GO powder was then thermally exfoliated at 1050 C for 45seconds to obtain exfoliated graphite or graphite worms.Exfoliated graphite (EG)from arti?cial graphite was used to make an electrode.In addition,exfoliated graphite worms from meso-phase carbon and carbon ?bers were subsequently sub-jected to ultrasonication to separate or isolate oxidized graphene sheets to prepare two electrodes (Electrode-M and -C,respectively).

Each electrode,composed of 85%graphene,5%Super-P (acetylene black-based conductive additive),and 10%PTFE,was mixed in a mortar to form a slurry.The slurry was coated on Al foil for use as a cathode.For use as an extension of an anode current collector,the mixture paste was coated to a sheet of Cu foil.Alternatively,in 4of the cells prepared,no copper foil was used,where the graphene layer itself was the current collector.The thickness of the electrode was typically around 150–300m m,but an additional series of samples with thicknesses of approxi-mately 20–150m m was prepared to evaluate the effect of elec-trode size on the power and energy densities of the resulting Li–graphene cells.The electrode was dried in a vacuum oven at 120

C for 12hours before use.

Prelithiation of some graphene layers as a negative electrode was conducted by plating Li metal onto graphene sheet surfaces in an electrochemical cell containing exactly the same electrolyte as utilized in the Li–graphene cells (1M LiPF 6/EC +DMC).The weight ratio of EC to DMC in the electrolyte is 1:1.Both coin-size and pouch cells were assembled in a glove box with a good control of both moisture and oxygen levels.

Graphene sheets of various morphologies (Fig.1)were made into highly porous electrodes,having large amounts of electro-lyte-accessible surfaces.The graphene electrode was used at the anode as part of an extended current collector for two types of Li-metal cells investigated:(a)a Li–graphene cell which contains graphene as the only cathode active material and (b)a Li–vanadium oxide cell containing graphene sheets as a property modi?er for the vanadium oxide cathode.

In the present study,the testing current density imposed on the cells is typically in the range of 1–225A g à1for Li–graphene cells and 0.01–1A g à1for Li–vanadium cells.With a graphene speci?c surface area (SSA)of typically 300–900m 2g à1implemented as an anode current collector,the local current density imposed upon the anode would be in the range from (0.01A g à1/900m 2g à1?1.1?10à9A cm à2)to 7.5?10à5A cm à2.In contrast,the Cu current collector used in the conventional Li metal cell has a SSA

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of 0.089m 2g à1;the local anode current density would be in the range of 1.1?10à5to 0.25A cm à2.This implies that the presence of graphene sheets at the anode side could signi?cantly reduce the local current density by at least 4orders of magnitude.The resulting Li–graphene cells were monitored for 1000–5000cycles under high-rate conditions,but the Li–vanadium oxide cells for 100–500cells under lower-rate conditions to look for any

dendrite formation.Selected cells were intentionally stopped after hundreds or thousands of cycles for SEM examination of the disassembled electrodes.

Results and discussion

Two types of Li metal cells are herein reported as examples to illustrate the effectiveness of implementing high-SSA graphene sheets as an extended anode current collector for improved dendrite resistance:the Li–graphene cell as a high-power cell and the Li–vanadium oxide as a high-energy-density cell.When a Li–graphene cell is made,a porous,nano-structured graphene layer is inserted as a current collector in lieu of,or in addition to,copper foil.This graphene electrode can be pre-loaded with lithium in one of the following ways:(a)coating graphene sheets with lithium via electro-deposition;(b)mixing surface-passivated lithium metal particles with graphene sheets;or (c)implementing strips of Li foil between the graphene layer and the separator layer.A graphene electrode layer,but with no pre-loaded lithium,is used as a cathode active material,which is optionally bonded to an aluminium current collector.

The pre-loaded Li is ionized during the ?rst discharge,supplying a large amount of lithium ions,which migrate to the nano-structured cathode through a liquid electrolyte,entering the pores and reaching the graphene surfaces in the interior of the cathode without undergoing solid-state intercalation (Fig.2a,lower portion).Li ions are adsorbed onto the graphene planes,or captured by the defect sites or functional groups on graphene surfaces/edges at the cathode.There are several mecha-nisms 25,29–37by which Li ions can be captured by graphene (Fig.2a,right portion),which are further discussed in the ESI?.When the cell is re-charged,a massive ?ux of lithium ions are quickly released from the large amounts of cathode surfaces,migrating into the anode zone.The large surface areas of gra-phene sheets,acting as an extended anode current collector,enable concurrent and high-rate deposition of lithium ions under a low current density condition.

In other words,the operation of a Li–graphene cell involves rapid exchange of massive Li ions between the graphene surfaces of an anode current collector and the graphene surfaces at the cathode.Since the graphene surfaces at both electrodes are in direct contact with liquid electrolyte,this con?guration completely obviates the need for Li ions to undergo solid-state intercalation into or out of an anode or cathode.Such an extremely rapid,totally intercalation-free operation enables a high-power cell capable of fast charging and discharging.37In several cells,an incredibly fast discharge or charge time of 0.4–0.6seconds was achieved (e.g.Fig.2b),which is comparable to that of electric double layer (EDL)supercapacitors.As shown in Fig.2c,a power density as high as 187kW kg cell à1was achieved at an energy density of 24W h kg cell à1.This power density is one order of magnitude higher than that of conventional EDL supercapacitors that are noted for their high power densities (but with an energy density of typically only 5W h kg cell à1).This exceptional power density is 2–3orders of magnitude higher than those (typically 0.1–1.0kW kg cell à1)of conventional lithium-ion batteries.Furthermore,the Li–graphene cells can exhibit a high energy density of 320W h kg cell à1,signi?cantly greater than that of a lithium-ion battery.These data have clearly

demonstrated

Fig.1Rich morphologies of graphene sheets (before electrode fabri-cation):(a)SEM image of highly exfoliated graphite composed of interconnected graphene sheets forming a highly porous 3-D network of electron-conducting paths.(b)Curve-shaped graphene sheets obtained by thermally reducing graphene oxide sheets in a ?uidized bed furnace (curved shapes prevent re-stacking of graphene sheets during electrode preparation,thereby preserving meso-scaled pores between sheets in an electrode that are accessible to liquid electrolyte).(c)‘‘Graphene ?owers’’prepared by aerosolizing graphene oxide suspension into a vertical furnace (?ower shapes maintain a desirable meso-porous structure,yet being amenable to compact packing into an electrode with a good tap density).

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Fig.2(a)The upper portion shows the structure of a Li–graphene cell when it is made,containing an optional anode current collector and a graphene layer acting as an extended current collector,a porous separator,a liquid electrolyte,and a porous graphene nano-structure acting as a cathode active material.The lower left portion shows the structure of this cell after its ?rst discharge.The right portion shows 4plausible mechanisms with which graphene surfaces can capture Li ions at the cathode.(b)Charge–discharge curves of select Li–graphene cells,indicating that the cell can be discharged in seconds or <1second.(c)Ragone plot of various Li–graphene cells (high power cells)and Li–vanadium oxide cells (high energy cells).Blue data points are Electrode M-based Li–graphene cell (LG-M,80m m thick cathode from meso-carbon);red points for a cathode of 100m m thickness;black points of 150m m;violet points are from LG-EG;olive points are from LG-C;and green points are from LG-N cell.Data points in a box are for Li–vanadium oxide cells (NLVO and LVO).(d)The speci?c capacity of various graphene-based cathode active materials obtained via cyclic voltammetry and/or charge–discharge curves over a voltage range of 1.5V to 4.5V vs.Li/Li +.(e)Cycling performance of a Li–graphene cell.The data were collected at 0.3C rate.

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that the Li–graphene cells are a new class of energy storage cells by itself,distinct from both conventional supercapacitors and lithium-ion batteries.If proven as highly dendrite resistant,the Li–graphene cells can be the next generation of energy storage cells for a wide range of applications.

The amount of deposited/implemented Li metal in the anode can be easily adjusted between 10%and 97%by weight according to the battery design requirement.For instance,an electro-deposited layer of 1nm thick Li (density ?0.53g cm à3)is equiv-alent to 40%by weight of Li (assuming a graphene thickness of 0.34nm and a density of 2.25g cm à3).A 30nm thick coating would mean 95%by weight of Li,and the Li–graphene composite anode would have a speci?c capacity of 3860?0.95?3667mA h g à1.The speci?c capacity of various graphene-based cathode active materials was obtained via cyclic voltammetry and/or charge–discharge curves over a voltage range of 1.5V to 4.5V vs.Li/Li +.These data,plotted as a function of the speci?c surface area in Fig.2d,indicate that the capacity is essentially proportional to the surface area,pointing to a theoretical speci?c capacity of approximately 930mA h g à1if all the surface area of 2670m 2g à1is accessible to the electrolyte.Currently,most of our graphene cathodes typically exhibit a speci?c capacity of 150–350mA h g à1,leading to a cell-level energy density of 180–320W h kg à1based on the actual weight of a pouch cell (Fig.2c).The cycling performance for a Li–graphene cell is shown in Fig.2e,indicating good long-term stability.

The issue of Li dendrite formation has been subjected to intensive investigation over the past 20years.4,38Although many approaches have been proposed,none has been able to overcome this challenge.However,all the studies have consistently identi-?ed the electrode current density as the most critical factor that dictates the dendrite formation propensity.4,38–41For instance,Rosso et al.39have observed that the dendritic growth is initiated at a time t i :

t i ?p D (eC o /2J )2[(m a +m a )/m a ]2

(1)

In eqn (1),J ?effective electrode current density,D ?ambipolar diffusion coef?cient,e ?electronic charge,C o ?initial concen-tration of ions,and m a and m a are the anionic and cationic mobilities.This equation suggests that the dendrite initiation time is inversely proportional to J 2.If the Cu metal foil anode having a speci?c surface area of 0.089m 2g à1(ref.41)in a conventional Li metal cell is now replaced by a graphene-based electrode having a speci?c surface area of 890m 2g à1(readily achievable),the effective current density J (units ?mA cm à2)would be reduced by a factor of 105.Hence,the dendrite initia-tion time would be extended by a factor of 1010.Putting this in perspective,an original dendrite starting time of 20hours at 0.1mA cm à2for a conventional Li-polymer cell of Rosso et al.would now become 2.0?1011hours.

Further,Monroe and Newman 40developed a comprehensive model to predict the tip growth rate (y tip )of a dendrite,once initiated:

y tip ?(J n V )/F

(2)

where J n ?effective current density normal to the dendrite tip,V ?molar volume of lithium,and F ?Faraday’s constant.This

equation indicates that the dendrite tip growth rate would be slowed down if the current density is reduced.An original dendrite tip propagation speed of 100nm s à1would now be reduced to 10à3nm s à1,requiring 108seconds (27777hours)to penetrate through a 100m m thick separator to cause internal short-circuiting.

These considerations provide an effective strategy to suppress the formation of dendrites in all Li metal batteries having a gra-phene-supported anode current collector.Careful examination of the graphene anodes of 30Li–graphene cells (listed in Table S1?)after a large number of cycles (some >2000cycles)con-ducted at high gravimetric current densities (typically 1–225A g à1)has not revealed any feature that would be considered a remnant of a dendrite (representative SEM images of pre-and post-testing electrodes are shown in Fig.S3?).Practically speaking,one could not inspect every possible spot on an elec-trode surface using a scanning electron microscope (SEM)and,hence,could not claim that there was absolutely no dendrite in these cells.However,we have interrupted cycling tests of 30cells (after 1000–2000cycles without failure)for the purpose of searching for dendrite (no trace of dendrite was found),and additional 20cells have been progressing for >2000cycles without a failure.In contrast,some early failures were observed with control cells featuring Cu foil as the anode current collector.These observations,coupled with the theoretical predictions (eqn (1)and (2)),provide a strong support for the approach of using graphene-enabled electrodes for dendrite suppression.

Great dendrite resistance is also observed with the Li–vana-dium oxide cell (L–VO)that makes use of the same type of graphene-supported anode con?guration,but a graphene-enhanced lithium vanadium oxide as a cathode active material.Most of the current Li-ion cathode materials exhibit a speci?c capacity signi?cantly lower than 200mA h g à1(e.g.,<150mA h g à1for LiCoO 2).One exception is vanadium oxide-based mate-rials (e.g.Li x V 2O 5and Li 1+x V 3O 8)that exhibit exceptional speci?c capacity due to their ability to incorporate more than one lithium ion per vanadium atom (theoretical capacity of 437mA h g à1).2,42–47

The speci?c capacity and speci?c energy of a Li–VO cell with a graphene-supported anode and a graphene-modi?ed Li 1+x V 3O 8cathode (Fig.3b),and those of a corresponding Li–VO cell containing a graphene-free Li 1+x V 3O 8cathode (Fig.3a)are plotted as a function of the cycle number in Fig.3c and d,for two discharge rates (0.5C and 2.5C).The presence of graphene during the preparation of Li 1+x V 3O 8allows Li 1+x V 3O 8nano-belts to be wrapped around by graphene sheets,effectively pre-venting direct contact between vanadium oxide and electrolyte,overcoming the vanadium oxide dissolution problem.Graphene sheets also serve to maintain a 3-D network of electron-con-ducting paths when the Li 1+x V 3O 8phase is lithiated or de-lithi-ated.A total of 12Li–VO cells were inspected after 500charge/discharge cycles and no sign of any dendrite formation could be found.The power and energy density data of a graphene-modi-?ed Li 1+x V 3O 8cathode-based cell are also summarized in Fig.2c,indicating exceptional energy densities achieved at good power densities relative to current Li-ion cells.

It may be noted that overcharge of a lithium-ion cell can cause signi?cant degradation of both anode and cathode.Overcharge on the anode can cause plating of lithium on graphite particle

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surfaces,rather than lithium intercalation into the interior of graphite.48,49Plated lithium on anode graphite surfaces of a lithium-ion cell,just like on a current collector of a conven-tional lithium metal cell,can form dendrites that can grow over

time and then cause internal shorting.The proposed approach of increasing the speci?c surface area of the anode current collector to reduce the effective electrode current density can also help to reduce the tendency for dendrite nucleation and growth,provided the adverse SEI formation effect can be avoided.

There are other additional safety concerns associated with both lithium-ion and lithium-metal cells,including thermal runaway,mechanical abuse,other electrical abuse (e.g.over-discharge),and electrode–electrolyte interactions.49,50These issues must be properly addressed before rechargeable lithium-metal batteries can be reliably revived.

Conclusions

In summary,a surprisingly simple yet effective approach has been discovered to address the dendrite formation issue associ-ated with all rechargeable batteries that use lithium metal or alloy as an anode active material.This approach of implementing graphene to dramatically increase the anode surface area,thereby reducing the effective current density,has great potential to enable the revival of rechargeable lithium metal batteries (Li–VO was used as an example),and the emergence of next-generation safe lithium rechargeable batteries,such as ultrahigh-power Li–graphene cells.The same dendrite-suppressing approach is expected to be applicable to both Li–air and Li–S cells as well.

Acknowledgements

The support from NSF SBIR-STTR Program grant (Program Manager:Dr Grace Wang)is gratefully acknowledged.

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