Lithium ion Battery Successful Practice

Article ID :042727104(2004)0420549210Lithium 2ion B attery :Successf ul Practiceof the Intercalation TheoryXI NG Xue 2kun(Chemical Engineering Depart ment ,Case Western Reserve U niversity ,Cleveland ,O H 44106,USA )Abstract :Li 2ion battery system is a successful example for putting the intercalation theory into practice.An overview on the principle ,cell chemistry ,components ,performance characteristics and advances of the Li ion battery is presented.K eyw ords :Li ion battery ;Li ion polymer battery ;rechargeable battery ;intercalation ;non aqueous electrolytesC LC number :O 646.54 Document code :A1　IntroductionThe Lithium 2ion (Li 2ion )battery is a large 2scale 2commercialized rechargeable battery system of the high 2est energy density to date.The first commercial Li 2ion battery was introduced into market by Sony in 1991.Through a continuous improvement in the battery performance and the cost reduction ,Li 2ion batteries have been undergoing a rapid growth in its production volume and in its application fields since then.The produc 2tion volume of various Li 2ion batteries approaches about 1250million cells with a total revenue of about US ＄3.8billion worldwide in 20031.In addition to its high energy density and specific energy ,the Li 2ion cell has a number of other advantages ,such as long charge 2discharge cycling life ,wide operational temperatures and reasonable rate performance.All these make the Li 2ion battery an attractive power source for many applica 2tions ,especially for various portable electronic devices such as cellular phones ,notebook PC ’s ,PDA ’s ,digital cameras and audio 2video systems.The Li 2ion batteries are replacing some traditional applications of the Nick 2el 2Cadmium (Ni 2Cd )and Nickel 2Metal Hydride (Ni 2MH )batteries ,and this trend is expected to continue in the future.The Li 2ion cell has several marked characteristics that differentiate it from most of other rechargeable cell systems.First of all ,the Li 2ion cell is a “high 2operation 2voltage ”system with an average operation voltage of3.6—3.7V ,which is about 3times of that of Ni 2Cd or Ni 2MH cells.The higher operation voltage becomes practically possible due to the employment of selected non 2aqueous electrolytes ,which have a much wider elec 2trochemical window compared with its aqueous counterpart.For the latter ,the electrochemical window is rather narrow because it is governed by the water decomposition reaction.Secondly ,there is no metallic lithi 2um being used in the Li 2ion cell ,which thus avoids difficulties in the cell cycling life and the cell safety usually associated with the formation of the lithium dendrites on the metallic Li anode during charge 2discharge cycles.Thirdly ,the cell reaction of the Li 2ion cell is not a traditional redox reaction but rather an intercalation/dein 2tercalation process of Li ions in the anode and the cathode active materials.R eceived d ate :2004202210Biography :Xing Xue 2kun (1964—),male ,PH.D ,under su pervision of Professor WU Hao 2qing ,Adjunct professor.第43卷　第4期2004年8月复旦学报(自然科学版)Journal of Fudan University (Natural Science )Vol.43No.4Aug.2004Although the commercial Li2ion batteries have entered the market for just about12years,the basic con2 cept and research work related to the development of the Li2ion cell can be traced back to the early1970’s. Whittingham and coworkers2did pioneer work on the mechanism of the intercalation reaction,and various insertion compounds such as TiS2,MoS2,V6O13,etc.were studied and tested as cathode active materials in non2aqueous electrolytes.Steele3suggested using two intercalation compounds,graphite and TiS2,as elec2 trodes for battery applications as early as in1973.G oodenough’s group4studied properties of various lithiat2 ed transition metal oxides(Li x MO2,M=Ni,Co,Mn)and proposed to use these compounds as the candi2 dates of the cathode active material in early1980’s.Around the same period of time,in China,Wu and his group reported the results related to Li intercalation thermodynamics and kinetics through a series of studies on a number of metal oxides cathode materials.The initial concept of the Li2ion cell with two intercalation elec2 trodes in a non2aqueous solution was also proposed and studied by Murphy and coworkers.After an intensive close2door research and development work,Sony successfully introduced the first commercial Li2ion cell to the market based on a carbon anode and a LiCoO2cathode in1991.2　Cell components and cell chemistry2.1　C athode material and cathode reactionLayered structural LiCoO2is the cathode active material being used for most of the commercial Li2ion bat2 teries today.This compound was introduced for the cathode active material by G oodenough et al.in19804. LiCoO2has a typical layered structure,in which the oxide ions form a cubic2close2packed array with close2 packed(111)octahedral2site planes alternately filled with Li+and Co3+ions.LiCoO2has a good electronic conductivity mainly contributed by the availability of the holes of large O22p character.On the other hand, Li+ions in the structure is in part movable,that is,Li+ions can be reversibly extracted from and reinserted to the LiCoO2structure,as shown in Eq.(1)below:LiCoO2chargedischargeLi12x CoO2+x Li++x e-.(1) It is noticed that the reversibility of the reaction(1)is available only in a range of0≤x≤0.5.In the case of x>0.5,oxygen evaluation may take place and the crystal structure may be damaged accordingly.As well established in the literature,there are three plateaus on the voltage2composition curve associated with Eq.(1),indicating the occurrence of three distinct phase transitions as x varies from1to0.5in Li x CoO2during the deintercalation of lithium.The limitation of x=0.5implies that the reversible specific capacity of LiCoO2is137mAh/g.Within the limitation of the compositions,the voltage accompanied with Li+intercalation/deintercalation process is around4V(versus Li),a range much higher than what is shown by cathode materials in all aqueous batteries,which makes the Li ion battery a power system of high energy density.2.2　Anode material and anode reactionTo date,all commercial Li2ion batteries use carbonaceous materials as the anode active material.Among various carbonaceous materials,graphite and disordered carbons have been dominantly employed in Li2ion bat2 teries.Graphite has a typical layered structure which consists of stacked graphene sheets with an ABAB... sequence along the C2axis.The graphene sheets are bonded together by van der Waals forces with an inter2 sheet gap of about0.3354nm,which provides a space and functions as a host for enabling Li ion intercalation to take place.It has been found that the Li ion intercalation/deintercalation in/out graphite structure during charge/discharge processes are reversible,and the reaction scheme can be described as follows: 055复旦学报(自然科学版) 第43卷6C +x Li ++x e -chargedischarge Li x C 6.(2)From Equation 2,a theoretical specific capacity (372mAh/g at x =1)of the graphite can be derived.One important character of the Li ion intercalation/deintercalation reaction in graphite is that the reaction takes place for its most part at potentials close to the metallic lithium potential in the given electrolytes.This char 2acter makes graphite an anode having an operational voltage close to that of the metallic lithium while avoiding cycling difficulty associated with the latter due mainly to the formation of lithium dendrites formed in the cell charging process.The good reversibility and unique potential character make graphite a good substitute of the metallic lithium and an ideal anode material for Li 2ion cells which can provide both good charge/discharge cy 2cling life and a reasonably high energy density.The mechanism of lithium electrochemical intercalation into graphite has been substantially studied.The intercalation process is characterized with the staging phenomena ,and the lithium 2graphite intercalation com 2pounds have a staging structure.This means that the lithium intercalate layers undergo a successive distribu 2tion of stages with respect to graphite host layers following the progress of the intercalation.The stage struc 2ture changes from a higher to a lower stage during the charge (lithium intercalation ),and an opposite order is followed during discharge (lithium deintercalation ).There is always some irreversible capacity loss during the 1st charge/discharge steps (Li intercalation/deintercalation )of a Li ion cell.The irreversible capacity loss is attributed to the formation of a solid elec 2trolyte interface (SEI )on the graphite surface at the initial stage of the lithium intercalation process (11).Theformation of the SEI is crucial to maintain the stability of the graphite anode for the long term charge/dis 2charge cycles.2.3　Overall cell reactionThe overall cell reaction of the Li 2ion cell can thus be described as follows :6C +LiCoO 2charge discharge Li x C 6+Li 12x CoO 2.(3)This cell reaction is the base of the high average operational voltage of 3.6—3.7V for the Li 2ion cell ,which is substantially higher than any other rechargeable battery systems available so far.2.4　Electrolyte and SEI layerThe Li 2ion cell uses a non 2aqueous electrolyte as the ion 2conductive medium that consists of a Li salt and a combination of two or more organic solvents.The majority of commercially available Li 2ion batteries use a mixture of solvents consisting of a cyclic alkylene carbonate (typically ,such as ethylene carbonate EC ,and propylene carbonate PC )and a linear alkyl carbonate (typically ,such as diethyl carbonate DEC ,dimethyl car 2bonate DMC ,and ethyl methyl carbonate EMC ).Two properties ,among others ,of the solvent are crucial to its application to the Li 2ion cell.The first one is its electrochemical window ,which should be wide enough to meet the requirement of the high operation voltage of the Li 2ion cell.The second one is the ionic conductivity of the electrolyte ,which should be large enough to meet the requirement of the rate capability of the Li 2ion cell.The electrochemical window of a solvent can be determined from its oxidation and reduction potentials.G enerally ,organic solvents undergo oxidation at its oxidation potential with CO 2and other small organic compounds as the oxidation products.On the other hand ,organic solvents undergo reduction if the potential is low enough to approach its reduction potential.A comprehensive summary on the liquid electrolytes used in Li 2ion cells has been presented recently.The solvent reduction takes place usually at the anode ,and it is closely related to the formation of the SEI on the surface of the anode.The SEI is an electronic insulator but an ionic conductor ,which covers the active 155第4期 邢雪坤:锂离子电池:嵌入理论的成功实践255复旦学报(自然科学版) 第43卷surface of the anode and functions as a passivation layer to stabilize the anode active material(e.g.,blocking the solvent co2intercalation into the anode structure)for sustaining a long term charge2discharge cycling life. The mechanism of the SEI formation at the anode has been studied in depth in recent years.It has been gen2 erally agreed that the SEI formation results from complex reactions involving solvents,salt,atmospheric im2 purities(e.g.O2,N2,CO2,H2O)and additives if any.It has been established that(CH2CO2Li)2is a major surface species formed on carbon electrodes for EC2based ually,the SEI involves other reaction products such as LiF,Li2CO3,Li2O,LiOH,etc.Aurbach4has provided a comprehensive summary for the formation and the composition of the SEI in various electrolytes.The SEI formation and its properties(e.g., its adhesive property on the carbon surface,its forming potential and film smoothness,etc.)are crucial to maintain the long term stability of graphite electrodes in Li intercalation/deintercalation processes.The quali2 ty of the SEI is dependent on many factors including solvents,salts,impurities,polarization current and tem2 perature,and additives,the latter has specially attracted a substantial effort in recent years to improve cell performance(see below).2.5　Electrolyte additives and f unctional electrolytesThe first major effort to bring additives into the electrolytes of the Li2ion cell is driven by overcoming the problem of the PC2based electrolytes,which failed to be used for the Li2ion cells with the graphite anode due to a large irreversible capacity loss at the first charge/discharge step,and a poor charge2discharge cycling life).One argument is to relate this failure to the co2intercalate of PC solvent molecules into graphite structure together with solvated Li ions5.This co2intercalation process may lead to exfoliation of graphite structure.In addition,co2intercalated PC molecules may undergo reduction within the graphite structure and its reduction products could block Li ion to entry into the graphite lattice.As a result of these effects,the battery cycling life is adversely affected.Aurbach and coworkers6have proposed a different mechanism,and they attribute the failure of the graphite electrode in the PC2based electrolytes to the failure in forming a smooth,cohesive SEI layer on the graphite surface due mainly to the structure hindrance caused by the methyl group on PC molecules.Furthermore,the formation of propylene gas during PC reduction could cause higher internal pres2 sure within graphite crevices and split the particles,which could loss electrical contact with the current collec2 tor due to isolation by surface films.It has been found that the addition of some cyclic ethers,such as122Crown4(12Cr4),to PC2based elec2 trolytes can significantly improve the reversibility of graphite electrodes.Crown ether is a strong complex a2 gent for Li ions,the preferential formation of the Li+2Crown ether complexes shields PC out when the Li ions approach the electrode.A series of new additives to the Li ion battery electrolytes have been developed in recent years.Depending on their own structure and chemical properties,additives can play important roles to improve Li ion battery performance,such as cycling life,safety performance and others.More discussion about this issue will be found in7.2below.2.6　SeparatorLike other battery systems,the Li2ion cell needs a separator to electronically separate its cathode and an2 ode in order to avoid any electrical short circuit between them,while keeping good ionic pathway through the electrolyte absorbed in the pore structure of the separator.An adequate separator material needs to meet the following requirements among others:(1)good electronic insulator;(2)high degree of porous structure to be able to absorb enough electrolytes ensuring a good ionic conductance;(3)high chemical resistance to the elec2 trolyte and good electrochemical stability to the cell operation conditions;(4)adequate mechanical and physi2cal strength ;and (5)good wetability to the electrolyte used.To date ,the most commonly used separator ma 2terials for Li 2ion cells are porous polyethylene (PE )single 2layer and polypropylene/polyethylene/polypropy 2lene (PP/PE/PP )multiple 2layer films (Celgard type ).These separators have a thermal shut 2down function ,that is ,the PE porous film will melt down partly when the temperature approaches its melting point (130—140℃,typically ).This partly 2melt 2down process leads to pore 2close while maintaining film integrating ,which results in maximizing internal resistance of the cell and minimizing the current and thus avoiding con 2tinuous increase in the cell temperature.This function is specially important to protect the Li 2ion cell from overcharge and short circuit ,because any abnormal current can be automatically cut off due to the rapid in 2crease in the cell resistance.3　Li 2ion cell constructionThe first generation of the Li 2ion battery from Sony is of the cylindrical structure ,which is known as model 18650(cell diameter :18mm ,cell heights :65mm ).This model is being manufactured on a very large scale and is being widely employed in notebook PC and other electronic devices.With rapid growth of the pro 2duction volume of cellular phones ,PDA ,and other thin ,light ,portable electronic devices ,Li 2ion batteries have been more and more manufactured in a thin prismatic format to meet market needs.The prismatic cell usually has a rectangular and/or square shaped metal case (stainless steel or aluminum )and ,to some extent ,a porch envelop made of aluminum/polymer laminate.The latter ,which is initially employed for the Li 2ion Polymer battery (LiP )and is extended late to the so 2called Advanced Li 2ion Batteries (ALB ),has a lighter weight and thus improves cell ’s specific energy.Both cylindrical and prismatic cells use the wound core structure ,which is usually called “jelly roll ”7.Prior to making jelly rolls ,the anode and cathode films are prepared by coating the corresponding slurry con 2sisting of selected active material ,conductive material ,binder and solvent on thin metal strips (copper foil for the anode ,aluminum foil for the cathode ,res pectively ),followed by drying ,calendaring and slitting opera 2tions.Then an anode strip ,a cathode strip and separator strips with adequate length ,width and thickness are combined together and wound into a tight coil ,“jelly roll ”,via a winding machine.It is crucial to maintain close contact without void or gap between electrodes and separators in order to ensure a good performance of Li 2ion cells fabricated.The jelly roll fabricated is then inserted into a metal case of adequate dimension and shape ,which functions not only as a container to housing electrodes and electrolyte ,and to provide a hermeti 2cal seal from atmosphere ,but also to provide an internal pressure to further closely hold electrodes and separa 2tors together.After thorough vacuum 2heat dry to remove water ,the case with the jelly roll is then filled with a selected electrolyte solution of adequate amount.The electrolyte filling process is usually carried out with a precision pump under vacuum conditions ,and it is operated in a dry 2room or a dry 2box to minimize moisture absorption.The final step is to seal the cell case by compression of a polymer gasket located between the cell case and the case cap.The sealed cell can then enter a first charging step ,namely ,the cell formation process.4　Li 2ion cell performance characteristics4.1　Charging characteristicsThe most common charging mode of Li 2ion cells is the “constant 2current/constant 2voltage ”(CCCV )method ,by which the cell is charged first at a selected constant current (e.g.0.5,1C or others ),once the cell voltage approaches a predetermined value (typically ,4.1—4.2V ),the charging continues at this con 2stant voltage until the charging current decreases to a predetermined cut 2off value (e.g.0.05,0.1C or oth 2355第4期 邢雪坤:锂离子电池:嵌入理论的成功实践455复旦学报(自然科学版) 第43卷ers).Unlike Ni2Cd cells,Li2ion cells usually have a near100%charging efficiency at normal conditions, overcharging is not necessary and actually it should be avoided.The charging voltage at the constant2voltage stage is crucial to ensure a good performance of Li2ion cells.If the charging voltage is too low,the cell design capacity may be not fully used due to a lesser amount of Li extracted from the cathode at a lower cell charging voltage.On the other hand,if the charging voltage is too high,it may cause structure damage of the cathode active material due to over2extracting Li.Besides,it may also cause metallic lithium deposition at the surface of the anode due to excess lithium over the intercalation capacity of the anode.All these may adversely impact the battery cycling life and may also induce safety concerns.4.2　Discharging CharacteristicsLi2ion cells can be discharged under different modes depending on user’s needs.The constant current dis2 charging is one of the most commonly used,by which the Li2ion cell undergoes discharge at a selected current ranged from a few tenth C to a few C depending on applications.The pulse current discharging(such as GSM operation mode for cellular phones,etc.)is another mode widely employed in mobile phone applications. There are other discharging modes in applications,such as the constant power discharging,which is typically used in notebook PC’s.4.3　E nergy density and specif ic energyLi2ion batteries belong to battery systems of high energy and have a substantially higher energy density and specific energy compared with other common rechargeable batteries such as lead2acid,Ni2Cd,Ni2MH, etc.The energy density of Li2ion batteries are related to battery size,and it generally increases with increas2 ing battery size.At the present,the single Li2ion cell of small2medium size used for normal cellular phone ap2 plications has a typical energy density around350—450Wh/L,and a specific energy around170—200 Wh/kg.4.4　R ate capabilityLi2ion batteries have a reasonable rate capability which can meet the power requirement of various portable electronic devices.On the other hand,because Li2ion cells use non2aqueous electrolyte and the cell re2 action involves Li ion solid diffusion process,its overall rate capability and power density/specific power are usually lower than its aqueous counterpart such as Ni2Cd cells.The rate capability of Li2ion batteries can be improved through cell designing such as reducing the electrode thickness,selecting adequate electrode active materials,separators and electrolytes,etc.4.5　Charging/discharging cycling lifeLi2ion batteries are known to have a　long charge2discharge cycling life because the cell reactions do not involve large structure change of active materials at both anode and cathode.At the normal charging/discharg2 ing conditions,Li2ion batteries can be cycled at100%DOD(depth of discharging)over500cycles still re2 maining over80%of the initial capacity.Cell cycling life is strongly dependent on cell design,active materi2 als,electrolyte,etc..Manufacturing conditions,such as electrode coating quality,battery case sealing quali2 ty,moisture control,etc.,also play an important role in impacting the cell cycling life.4.6　Storage life,self2discharging and temperature effectIn addition to its long cycling life,Li2ion batteries have low self2discharging rate and long storage life.In addition,compared with Ni2Cd batteries,Li ion batteries do not have memory effect.Although Li2ion cells can be stored at various states of charge(SOC),it is recommended to store it at around half2charged state, because fully2charged state may promote some oxidative side reactions associated with the electrolyte.During the long term storage,Li2ion cells expect to loss a portion of its original capacity due to the self2discharging.Although the major part of this capacity loss can be restored after a few charging 2discharging cycles ,there is always some portion of the capacity loss which can ’t be totally recovered due to side reactions taking place during the storage.This “irreversible capacity loss ”is determined by the time length of the storage and ,more importantly ,the temperature of the storage.A noticeably higher self 2discharging rate is observed at the ele 2vated temperature.It should be mentioned that the self 2discharging rate is also impacted by the state of charge (SOC )of batteries ,and batteries at the fully charged state usually show higher self 2discharging rate.Li 2ion batteries can be discharged in a wide temperature range ,typically ,-20to 60℃.The discharg 2ing capacity usually decreases with decreasing temperature.The battery performance at low temperatures is to a large extent determined by the electrolyte employed.Some batteries are designed for applications at ex 2tremely low temperatures (e.g.40℃or lower ),which need specially 2designed electrolytes to meet the re 2quirement of the applications.5　Safety issues related to Li 2ion batteriesLi 2ion batteries use flammable organic electrolytes and work at a voltage range significantly higher than that of other aqueous rechargeable batteries ,thus safety becomes an important issue in its applications.Vari 2ous inadequate operations such as over 2charging ,over 2discharging ,hard short circuit ,abnormal high tempera 2ture environment ,impact ,etc.may cause batteries to catch fire or explosion.There are several industrial standards of safety tests for Li 2ion batteries ,such as UL1640,IEC and others ,which regulate Li 2ion battery safety requirements.These standards are widely employed by Li 2ion battery manufacturers and users to evalu 2ate battery safety characteristics.To ensure the battery safety in applications ,various safety devices including thermal control devices such as positive temperature coefficient switches (PTC ),and electronic IC protection circuits ,are widely employed for Li 2ion batteries and battery packs.With an adequate combination of various safety devices ,one can protect Li 2ion batteries from over 2charge ,over 2discharge ,hard short circuit ,impact and other safety concerns during applications.6　Li 2ion Polymer Batteries (LiP )Commercial Li 2ion batteries use liquid non 2aqueous electrolytes.One of the developments in this area is to use polymer electrolytes ing polymer electrolytes provides several advantages over its liquid coun 2terpart ,such as avoiding electrolyte leakage ,enhancing cell form factor ,improving cell specific energy and safety via using light 2weight metal 2plastic laminates as battery package material ,etc..Li 2ion polymer batter 2ies (LiP )have been studied and developed by many institutes and battery manufacturers worldwide.The most practical systems of Li 2ion polymer batteries to date are those with “gelled polymer electrolyte ”.The gelled polymer electrolyte usually consists of a polymer matrix and controlled amount of liquid electrolyte absorbed in the polymer network.In general ,there is no visible free liquid electrolyte in the gelled polymer electrolyte.Based on the type of polymers commonly used for the polymer matrix ,the gelled polymer electrolytes can be categorized as :“PEO 2based ”,“PVDF/HFP 2based ”,“PAN 2based ”and others.The PEO 2based gelled poly 2mer electrolyte is the first one receiving attention in the development of the rechargeable batteries with metal 2lic Li as the anode.The attention was then extended to other gelled polymers such as poly (acrylonitrile )(PAN ),poly (methylmetacrylate )(PMMA )and poly (vinylidene fluoride )(PVDF ).A common character of these gelled polymer electrolytes is the direct trapping of the liquid electrolyte in the polymer network to form a gel 2type membrane.In general ,the gel 2type membrane can be prepared by dissolving selected amount of a polymer and a Li salt in an adequate solvent or solvent 2mixture ,followed by homogenization ,heat lamination555第4期 邢雪坤:锂离子电池:嵌入理论的成功实践655复旦学报(自然科学版) 第43卷and other necessary processes.The obtained gel2electrolyte membrane in such a way has a conductivity around 10-3s/cm or higher at room temperature.Recently,the gelled electrolytes have undergone various modifications in their composition and process2 ing,and they have been successfully employed for manufacturing commercial Li2ion polymer batteries.One of the new developments in this field is to mix certain amount of a monomer and an initiator with a selected liquid electrolyte,which is then injected into a cell container with inserted jelly2roll,followed by polymerization un2 der heating or radiation.The advantage of this type of Li2ion polymer batteries is that it has basically the same manufacturing process as the regular liquid Li2ion batteries.It thus provides a practical means to manufacture Li2ion polymer batteries on a large production scale.A different type of gel2electrolyte Li2ion batteries has been developed by Bellcore(now Telecordia)8. The Bellcore plastic Li2ion battery technology has two major characteristics.First,this technology uses a spe2 cial porous polymer separator which consists of a PVDF2HFP copolymer matrix mixed with a fine inorganic filler such as fumed SiO2,and a liquid electrolyte is thoroughly absorbed in the polymer matrix.The PVDF2 HFP copolymer matrix has a porous structure and its porosity is controlled by first adding a plasticizer(e.g. dibutyl phthalate,DBP)during the polymer/filler/solvent mixing process,and then extracting the plasticizer out from the formed film to form porous structure in a controllable way.The second major character of the Bellcore technology is that the electrodes(anode and cathode,which both use PVDF2HFP as the binder)and separator are fussed together by heat lamination or heat pressing process.This special character makes this technology advantageous to batteries with thin format and stacking structure.This character also helps im2 prove battery cycling life and long term storage performance.7　New developments in the Li2ion battery field7.1　Development of ne w cathode active materialsAlthough a most widely employed cathode active material,LiCoO2possesses several disadvantages,such as,relatively expensive(due to the limited natural source of cobalt element),toxicity of cobalt,and limited specific capacity and thermal stability at elevated temperatures,etc.Therefore,considerable efforts have been devoted to the development of new cathode active materials other than LiCoO2.Among several candidates of new cathode materials,the layered LiN i O2has a higher specific capacity(e.g.≈180mAh/g)and more natu2 ral sources,but this material has disadvantages such as lower thermal stability at elevated temperatures and the resulted safety concern,less synthesis easiness,etc.,which should be resolved prior to its practical appli2 cations.Various modifications to this compound,such as LiNi12x Co x O2,LiNi12x2y Co x M y O2(M:other metals) and others,have been substantially studied9,10.Another promising candidate of the cathode active material for the Li2ion cell is the Mn2based spinal(LiMn2O4)and doped2spinal(Li1+x Mn22x2y N y O4)compounds. Main advantages of the spinal compounds are its low2cost,non2toxicity and better safety performance at ele2 vated temperatures.Two of the main problems to be overcome for the spinal compounds are its relatively poor2 er cycling life at elevated temperatures and its relatively lower specific capacity.Most recently,two new materials have attracted great interest.One is the layered structural LiMn22x M x O2(M:other metals,e.g.Ni)compounds,the structure of which is stabilized with the dopant of other metals.This group of materials have a higher specific capacity(≈200mAh/g)and an operational voltage similar to that of LiCoO2,they are still at a developing stage at this moment.Another material is the Olivien struc2 tured LiFePO4compound.This material is cheap,stable at elevated temperatures and non2toxic,it has a spe2。