Recent+developments+in+cathode+materials+for+lithium+ion+batteries
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Journal of Power
Sources
j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /j p o w s o u
r
Recent developments in cathode materials for lithium ion batteries
Jeffrey W.Fergus ?
Auburn University,Materials Research and Education Center,275Wilmore Laboratories,Auburn,AL 36849,United States
a r t i c l e i n f o Article history:
Received 29June 2009
Received in revised form 30August 2009Accepted 31August 2009Available online xxx Keywords:
Lithium ion batteries Cathodes Manganese Nickel Cobalt
a b s t r a c t
One of the challenges for improving the performance of lithium ion batteries to meet increasingly demanding requirements for energy storage is the development of suitable cathode materials.Cath-ode materials must be able to accept and release lithium ions repeatedly (for recharging)and quickly (for high current).Transition metal oxides based on the ?-NaFeO 2,spinel and olivine structures have shown promise,but improvements are needed to reduce cost and extend effective lifetime.In this paper,recent developments in cathode materials for lithium ion batteries are reviewed.This includes comparison of the performance characteristics of the promising cathode materials and approaches for improving their performances.
? 2009 Elsevier B.V. All rights reserved.
1.Introduction
The development of improved battery technology is critical for advancements in a variety of applications ranging from hybrid electric vehicles to consumer electronics [1,2],and improved bat-tery performance depends on the development of materials for the various battery components [3–6].Most lithium ion batteries use organic solvents as the electrolyte,the most common being LiFP 6,which has a low electrical resistance [7],and is typically mixed with carbonates.Solid electrolytes,including polymers [8]and inor-ganic compounds [9,10],are used for solid state batteries,which have advantages in terms of miniaturization and durability.The most common anode materials are carbon-based compounds and lithium-containing alloys.Both approaches result in the establish-ment of a reduced lithium activity (as compared to lithium metal),which reduces reactivity with the electrolyte and improves safety,but also leads to a lower cell voltage.There are efforts in the devel-opment of improved electrolyte and anode materials,but the focus of this paper is on the cathode materials.
Cathode materials are typically oxides of transition metals,which can undergo oxidation to higher valences when lithium is removed [11,12].While oxidation of the transition metal can maintain charge neutrality in the compound,large compositional changes often lead to phase changes,so crystal structures that are stable over wide ranges of composition must be used.This struc-tural stability is a particular challenge during charging when most
?Tel.:+13348443405;fax:+13348443400.E-mail address:jwfergus@https://www.wendangku.net/doc/a59664857.html, .
(ideally all)of the lithium is removed from the cathode.During dis-charge lithium is inserted into the cathode material and electrons from the anode reduce the transition metal ions in the cathode to a lower valence.The rates of these two processes,as well as access of the lithium ions in the electrolyte to the electrode surface,control the maximum discharge current.Exchange of lithium ions with the electrolyte occurs at the electrode–electrolyte interface,so cathode performance depends critically on the electrode microstructure and morphology,as well as the inherent electrochemical properties of the cathode material.For example,there is considerable work on the use of nanostructured electrodes with high surface and inter-facial areas to improve performance [13–17].While this paper will include some discussion of general microstructural features,the focus is on the cathode materials rather than the microstructures.
2.Cathode materials
The cathode material most commonly used in lithium ion bat-teries is LiCoO 2[18].LiCoO 2forms the ?-NaFeO 2structure,which is a distorted rock-salt structure where the cations order in alter-nating (111)planes.This ordering results in a trigonal structure (R ˉ3
m )and,for LiCoO 2,planes of lithium ions through which lithia-tion and delithiation can occur [19].Although LiCoO 2is a successful cathode material,alternatives are being developed to lower cost and improve stability.Cobalt is less available,and thus more costly,than other transition metals,such as manganese,nickel and iron.In addition,LiCoO 2is not as stable as other potential electrode mate-rials and can undergo performance degradation or failure when overcharged [20–22].The increase in charging voltage can increase the cell capacity,but can also lead to more rapid decrease in capac-
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ity as the cell is cycled during recharging[23].Several reasons have been given for the degradation during cycling.One is that cobalt is dissolved in the electrolyte when the electrode is delithiated dur-ing charging[24],such that less lithium can be intercalated during discharge.Another is that the CoO2layer formed after full delithi-ation shears from the electrode surface[25],which also results in less capacity for lithium intercalation.In addition,there is a sharp change in lattice parameter with change in lithium content[26], which can lead to stresses and micro-cracking of the cathode par-ticles[27].Stoichiometric LiCoO2can be dif?cult to obtain[28],so heat treatment to control the surface phase content is needed to improve performance during cycling[29].
LiNiO2,which also forms the?-NaFeO2structure,is lower in cost and has a higher energy density(15%higher by volume,20% higher by weight)[30],but is less stable[31,32]and less ordered [33],as compared to LiCoO2.The lower degree of ordering results in nickel ions occupying sites in the lithium plane,which impedes lithiation/delithiation and also creates challenges in obtaining the appropriate composition[34].The addition of cobalt to LiNO2 increases the degree of ordering,which leads to nickel ions occu-pying sites in the nickel/cobalt plane rather than in the lithium plane[12].Thus,LiNi1?x Co x O2,typically containing mostly nickel (x～0.8),has been used to take advantage of the low cost and higher capacity of nickel relative to cobalt[35–39].LiMnO2forms a monoclinic,rather than rhombohedral,structure[40],which can transform to a layered hexagonal structure during cycling[41].The addition of nickel[42,43],or more commonly nickel and cobalt, to LiMnO2can lead to the formation of the?-NaFeO2structure [44,45].The ratio of the trigonal lattice parameters,c/a,depends on the composition,and as this ratio approaches1.633the distortion from cubic symmetry decreases,which leads to less ordering and thus more transition metal ions in the lithium ion plane[12].The most commonly used Li(Ni,Mn,Co)O2composition contains equal amounts of the three transition metals,i.e.Li(Ni1/3Mn1/3Co1/3)O2, and has high capacity[46],good rate capability[47,48]and can operate at high voltages.A higher charging voltage increases the capacity,but also leads to more rapid loss of capacity during cycling[49–51].As with LiNi1?x Co x O2,cobalt helps to reduce the amount of nickel in the lithium layer[12]and small amounts of cobalt(up to0.20–0.25)have been shown to improve capacity [51–57].Increased cobalt content can also reduce the loss in capac-ity during cycling[51,58–60].The improved performance has been attributed to cobalt increasing the conductivity[51,59]and improv-ing the structural stability[59,60]of the cathode.Although nickel in the lithium layer can be detrimental to lithium transport,it has been shown to stabilize the structure during delithiation and thus improve cycling performance[61].Li(Ni,Mn,Co)O2can be overlithi-ated,which has been shown to improve electrode performance [62,63],especially with low cobalt contents[64].There can be an oxidative loss during the?rst cycle,which is not recovered during normal cycling[65].However,deep discharging has been shown to recover this?rst-cycle loss[66].
Another promising cathode material is LiMn2O4that forms a spinel structure(Fdˉ3m),in which manganese occupies the octa-hedral sites and lithium predominantly occupies the tetrahedral sites[67].In this case,the paths for lithiation and delithiation are a3-dimensional network of channels rather than planes,as in the?-NaFeO2structure.LiMn2O4,is lower cost and safer than LiCoO2[12,68,69],but has a lower capacity as compared to the cath-ode materials that form the?-NaFeO2structure described above [46,70].One of the challenges in the use of LiMn2O4as a cathode material is that phase changes can occur during cycling[71–73].For example,LiMn2O4cathodes have been?eld tested in the DC power supply of an operating telecommunications transceiver.During this test,a relatively rapid loss of capacity occurred in the?rst few days,but the rate of capacity loss subsequently decreased [74].The initial loss has been attributed to loss of oxygen dur-ing charging[75].Capacity loss has also been observed during storage due to dissolution of manganese in the electrolyte[76], or due to changes in particle morphology or crystallinity[77,78]. Other transition metals,including iron[79]and cobalt[80–85], and have been added to LiMn2O4.The addition of iron results in an additional discharge plateau at high voltages,while cobalt improves the capacity retention during cycling by stabilizing the spinel crystal structure.However,the most common addition to LiMn2O4is nickel[86],which decreases the lattice parameter and the electrical conductivity of LiMn2O4[87].The capacity increases with increasing manganese content and a3:1Mn:Ni ratio(i.e. Mn1.5Ni0.5O4)is the most commonly used composition[88,89].The manganese and nickel cations can order on the octahedral sublat-tice,but a disordered spinel structure has been shown to have a higher capacity[90].Partial substitution of cobalt for nickel(i.e. Li[Mn1.42Ni0.42Co0.16]O4)has been used to reduce the formation of Li x Ni1?x O,which can degrade cell performance during cycling[91]. The addition of nickel to the surface of LiMn2O4through coatings, rather than as a bulk dopant,can also be effective in improving capacity retention during cycling[92–94].
Vanadium oxide forms layered compounds and vanadium can have multiple valences,so vanadium oxides have been used as electrode materials.In particular,orthorhombic V2O5[95–99]and monoclinic LiV3O8[100–104]have been used as cathode materials. These electrodes have high capacities,but relatively low voltages (typically3V or less)as compared to the compounds discussed above.
Another promising class of cathode materials are phosphates (LiMPO4)with the olivine structure(Pnma),in which phosphorous occupies tetrahedral sites,the transition metal(M)occupies octa-hedral sites and lithium forms one-dimensional chains along the [010]direction[105].The phosphate most commonly used for the cathode is LiFePO4,which delithiates to FePO4as the Fe2+is oxi-dized to Fe3+[106].Some iron ions occupy lithium sites,which results in the formation of lithium ion vacancies to maintain charge neutrality[107–109].There is a miscibility gap between FePO4and LiFePO4[110],so the delithiation occurs by growth of a two-phase front rather than a continuous change in lithium content[111–116]. The formation of a two-phase mixture establishes a?xed activ-ity,which results in a relative?at discharge pro?le(i.e.the voltage remains relatively constant during discharge)[46].Electronic con-duction in LiFePO4occurs by small polaron hopping[117]and is relatively low(10?9S cm?1for pure LiFePO4[12]).Conductivity can be improved by heat treating to increase the hole conduc-tivity[118],but the addition of a conductive phase is generally needed for satisfactory performance[13].Additives for increasing the conductivity of LiFePO4will be discussed below,but one con-ductive phase,Fe2P,can form during preparation and/or use and has been observed to improve performance[119,120],so Fe2P is some-times deliberately added in LiFePO4/Fe2P composites[121–124]. The amount of Fe2P is critical because small amounts increase con-ductivity,but larger amounts block lithium ion paths[125].
Other phosphates used for cathodes in lithium ion batteries include LiMnPO4[126–128]and LiCoPO4[128,129].LiMnPO4and LiCoPO4have higher open circuit voltages(4.1and4.8V,respec-tively)than LiFePO4(3.5V)[12],but have lower capacities.For example,the capacities of LiMnPO4and LiCoPO4prepared by microwave hydrothermal synthesis were reported to be～1/6and ～1/3,respectively,that of LiFePO4prepared by the same pro-cess[128].In addition,Mn2P4O7and Co2P4O7have been observed to form in delithiated LiMnPO4[127]and LiCoPO4[129]elec-trodes,respectively,which degrades the lifetime and can be a safety concern as oxygen is evolved during the decomposition reaction.Mixtures of phosphates,including LiMnPO4[130–132] or LiCoPO4[133,134]with LiFePO4,have been used for cathode
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materials.In such mixtures,the operating voltage increases with increasing manganese content [135,136],while capacity increases with increasing iron content [133,134,137].Although LiNiPO 4also forms the olivine structure [105]it is not typically used as a cathode material.However,nickel has been added to other phos-phate cathode materials,including simple lithium transition metal phosphates (e.g.LiFePO 4[138,139]and LiMnPO 4[140]),and com-plex compounds,(e.g.Li(Mn,Fe)PO 4[141,142]and Li(Mn,Fe,Co)PO 4[143,144]).Another phosphate used as a cathode is Li 3V 2(PO 4)3,which forms a monoclinic structure (P 21/n )[145,146].Li 3V 2(PO 4)3has a high operating voltage and good performance at high dis-charge currents [147,148].For example,plateau voltages of greater than 4V for discharge currents of 0.2–2C and greater than 3.9V for a discharge current of 10C have been reported [148].Vanadium additions have also been shown to improve the capacity of LiFePO 4,especially at high discharge currents [138,149,150].For example,at 0.1C ,the increases in capacity are relatively modest (5–15%)[138,149,150],while at 10C increases in capacity of 80%to more than 200%have been observed [149,150].The bene?cial effect of vanadium additions has been attributed to enhancing lithium dif-fusion [138,150]or reducing the energy required for nucleation of LiFePO 4in the LiFePO 4–FePO 4[149]two-phase region.
3.Cathode performance
The multitude of materials,geometries and operational vari-ables in lithium ion batteries complicates comparison of the performances of different cathode materials.Although there are a few reports in which different types of electrodes are tested in the same conditions and compared on a single plot (e.g.[47])most reports focus on a particular type of electrode material with variations in composition or microstructure.On the other hand,summaries of results from different sources (e.g.[151])may include results for different operating conditions,which compli-cates making comparisons between materials.The voltage ranges for different electrodes have been summarized [152],but this sum-mary does not include capacities.
In this paper,the performances of different cathodes will be summarized according to operating conditions,so that compar-isons can be made between results from different sources.Results for different electrodes will be included on the same plots or on different plots with the same scales to allow for direct comparison between materials.In addition,multiple results for each electrode will be included to re?ect the variations between reports,which can occur due to,for example,microstructural or morphological differ-ences.To make such comparisons,the voltage range and capacity for each electrode have been determined as shown in Fig.1.The voltage range is determined by the points of in?ection in the dis-charge curve and the capacity is the capacity at second point of in?ection.The capacities determined in this way (Figs.2–5)are lower that the corresponding maximum capacities,but this method eliminates the lower cut-off voltage as a variable and allows for comparison of results from different sources.The purpose is not to present the absolute maximum capacity,but rather to provide a comparison of the useful voltage/capacity ranges of the voltage plateau for operation.The subsequent plots (Figs.6–12),however,showing the trends in capacity with discharge current and after cycling use the maximum capacities reported in the respective articles.
The operating voltages and capacities of LiFePO 4[123,153–162]and LiCoO 2[163–167]with a charging voltage of 4.2V and dis-charge current of 1C are shown in Fig.2.The operating voltage for LiCoO 2is higher than that for LiFePO 4and LiFePO 4has a nar-rower voltage range.The narrow voltage range for LiFePO 4is a result of the formation of a two-phase mixture,rather than
a
Fig.1.Schematic discharge curve for lithium ion battery with parameters used in Figs.2–5
.
Fig.2.Plateau voltage and capacity (see Fig.1)for LiFePO 4[123,153–162]and LiCoO 2[163–167]with a charging voltage of 4.2V and discharge current of 1C .
continuous change in lithium content.Fig.3shows that a reduc-tion in the discharge current by a factor of 10(i.e.0.1C )increases the capacity of LiFePO 4[123,150,153,156,158–161,168–174].Figs.4and 5summarize the operating voltages and capacities for Li(Ni 1/3Mn 1/3Co 1/3)O 2[63,175–177],LiCoO 2[23,178,179,189],LiFePO 4[138,180]and LiMn 2O 4[181–188]at a higher charging voltage of 4.3V.LiMn 2O 4,LiCoO 2and Li(Ni 1/3Mn 1/3Co 1/3)O 2
all
Fig. 3.Plateau voltage and capacity (see Fig.1)for LiFePO 4[123,150,153,156,158–161,168–174]with a charging voltage of 4.2V and discharge current of 0.1C .
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Fig.4.Plateau voltage and capacity (see Fig.1)for Li(Ni 1/3Mn 1/3Co 1/3)O 2[175–177],LiCoO 2[23,178,179],LiFePO 4[138,180]and LiMn 2O 4[181–184]with a charging voltage of 4.3V and discharge current of 1C
.
Fig.5.Plateau voltage and capacity (see Fig.1)for Li(Ni 1/3Mn 1/3Co 1/3)O 2[63],LiMn 2O 4[183–188]and LiCoO 2[189]with a charging voltage of 4.3V and discharge current of 0.1C .
have higher operating voltages as compared to LiFePO 4.LiMn 2O 4has a similar,or higher,operating voltage as compared to LiCoO 2and Li(Ni 1/3Mn 1/3Co 1/3)O 2,but its capacity is lower.Although the number of data points is small,with a decrease in discharge current from 1to 0.1C (compare Figs.4and 5),the capacity
of
Fig. 6.Discharge capacity of LiCoO 2as a function of discharge rate
[164,165,179,190–192]
.
Fig.7.Discharge capacity of Li(Ni,Mn,Co)O 2[47,64,193–196]and LiMn 2O 4[185]as a function of discharge
rate.
Fig.8.Discharge capacity of LiFePO 4as a function of discharge rate
[115,153,197–203].
Li(Ni 1/3Mn 1/3Co 1/3)O 2increases more than that of LiCoO 2,suggest-ing that the kinetics of charge transfer and/or mass transport are slower in Li(Ni 1/3Mn 1/3Co 1/3)O 2than in LiCoO 2.
The trend with discharge current is also illustrated in Figs.6and 7,which show the capacity as a function of dis-charge current for LiCoO 2[164,165,179,190–192],Li(Ni,Mn,Co)O
2
Fig.9.Discharge capacity of LiFePO 4at high discharge currents [115,153,154,204].
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Fig.10.Capacity after 1st/50th cycle and percent change in capacity after 50cycles for LiFePO 4cathodes [156,198,205,206]
.
Fig.11.Capacity after 1st/50th cycle and percent change in capacity after 50cycles for LiCoO 2cathodes [207,208].
[47,64,193–196]and LiMn 2O 4[185].Although there is signi?cant variation among the results,the decrease in capacity with increas-ing discharge current is generally smaller for LiCoO 2than for Li(Ni,Mn,Co)O 2.Fig.8shows that the discharge rate dependence of capacity for LiFePO 4is similar to that for LiCoO 2.The result exhibit-ing a rapid decrease in capacity (symbol “×”in Fig.8)is for a cell with a polymer electrolyte,rather than a liquid LiPF 6-based elec-trolyte,so the high current performance may be limited by
the
Fig.12.Capacity after 1st/50th cycle and percent change in capacity after 50cycles for Li(Ni,Mn,Co)O 2cathodes [49,209,210].
electrolyte rather than the electrode.Fig.9shows that LiFePO 4can be used at high discharge currents.
The change in performance during cycling for LiFePO 4[156,198,205,206],LiCoO 2[207,208],Li(Ni,Mn,Co)O 2[49,209,210]are compared in Figs.10–12by plotting the capacities after the 1st and 50th cycles as a function of discharge current.In addition,the right axis is used to show the percent change in capacity dur-ing the 50cycles.The decrease in capacity of LiFePO 4(～10–20%)after cycling is much smaller than that for LiCoO 2or Li(Ni,Mn,Co)O 2(～30–40%).These data also illustrate the sharper decrease in capac-ity with increasing discharge rate for Li(Ni,Mn,Co)O 2as discussed above.
https://www.wendangku.net/doc/a59664857.html,posite cathodes
The combination of two electrode materials to form a composite electrode can be used to improve performance [211].For exam-ple,the addition of LiFePO 4to other electrodes,including LiCoO 2[178,212],Li(Li 0.17Mn 0.58Ni 0.25)O 2[212]and Li(Ni 0.5Mn 0.3Co 0.2)O 2[213],improves capacity retention during cycling and perfor-mance at high discharge currents.Similarly,a phosphate surface treatment can improve capacity [214]or performance after cycling [215]of oxide electrodes.Monoclinic (C2/m )Li 2MnO 3acts as a lithium reserve and improves capacity retention during cycling of layered LiMO 2cathode materials,including Li(Co 1?y Ni y )O 2[216],Ni 0.8Co 0.15Zr 0.05O 2[217],LiNi 0.5Mn 0.5O 2[218],Li(Ni 1/3Mn 1/3Co 1/3)O 2[218–221]and other Li(Ni,Mn,Co)O 2compositions [218,222–224].Spinel electrodes have been com-bined with layered cathode materials,including LiCoO 2[225]and Li(Ni,Mn,Co)O 2[226,227]to expand the operating voltage range.The lithium is removed from the spinel at high voltages and then from Li(Ni,Mn,Co)O 2at lower voltages [226].
The interface between the two components is where charge and mass transfer occur and is thus important in the operation of the electrode,so mechanical activation [225]or heat treatment [227]has been used to take full advantage of composite properties.LiCoO 2and Li(Ni,Mn,Co)O 2have been combined in a composite electrode [228],in which case the improved performance was attributed to the microstructure (https://www.wendangku.net/doc/a59664857.html,bination of large and small particles)rather than to the inherent electrochemical properties of the cathode materials.This illustrates one of the challenges in com-paring electrode materials,in that the performance of the cathode,particular in a two-phase material,depends on the morphology and geometry of its constituents.
Composite cathodes can also be formed by coating particles of one cathode material with another active material rather than mix-ing separate particles.For example,the cycling and rate capability of LiMn 2O 4has been improved with a Co 3O 4coating,which is purported to form a ?uoride layer and reduce electrode degrada-tion [229].Another example is the reduction in the capacity loss during cycling of LiNiO 2with a cobalt–manganese coating,which has been attributed to suppression of a detrimental phase transi-tion [230].Vanadium compounds,including V 2O 5[231]and LiV 3O 8[223],which have high capacities,but relatively low operating volt-ages,have been used to increase the capacity of Li(Ni,Mn,Co)O 2by providing supplemental capacity at lower voltages late in the dis-charge cycle.Another lower voltage electrode that has been used in composite cathodes is Li 4Ti 5O 12.Because of its low voltage (～1.5V vs.Li/Li +),Li 4Ti 5O 12is more commonly used as an anode material,but it can be used as the cathode in low voltage cells [232–236].Li 4Ti 5O 12coatings have been shown to improve the capacity of LiCoO 2[237]and LiMn 2O 4[238]cathodes,as well as to improve the capacity retention during cycling of LiMn 1.4Cr 0.2Ni 0.4O 4[239].Sim-ilarly,coating LiCoO 2with a lithium-conducting solid electrolyte material (LiPON)has also been used to improve the capacity and
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cycling performance of the cell[240]presumably by enhancing the kinetics and mass transfer at the electrode interface.
5.Effect of doping
The performance of cathode materials can be improved by dop-ing,but the interpretation of doping effects can be complicated by the interrelations between doping and microstructure and mor-phology,since the microstructure formed can be affected by the dopant additions.Some examples in which the effects of doping on the electrochemical properties of the electrode are attributed to the effects of the dopant on the cathode microstructure or morphology rather than the effects on the material properties include cesium-doping of LiMn2O4[241],copper-doping of phosphates[140,242] and aluminum-doping of LiCoO2[189].With that caveat,the effects of dopant additions on the performance of cathode materials are discussed below.
Although iron-based oxides are not typically used as cath-ode materials,iron is used as a dopant in nickel-,manganese-, and cobalt-based cathode materials.Iron has been shown to improve the capacity of LiNiO2[243,244]and Li2MnO3[245,246], but the bene?cial effect diminishes with cycling.Iron doping has also been shown to be bene?cial to the performance of LiNi0.125Mn0.75Co0.125O2[247],but detrimental to the performance of LiNi1/3Mn1/3Co1/3O2[248].Similarly,iron impurities have been shown to decrease the capacity and increase the capacity loss dur-ing cycling of LiCoO2[249].However,the detrimental effect was attributed to clusters of iron ions,which could be eliminated by annealing the material to disperse the clusters.
Another transition metal that has been used as a dopant for cathode materials is ruthenium,which has been added as a dopant to spinel electrode materials(e.g.LiMn2O4[188]and LiMn1.5Ni0.5O4[250])and to LiFePO4[251].The bene?cial effect has been attributed to stabilizing the crystal structure as well as to contributions from the Ru4+/Ru5+redox couple.Ruthenium has also been added as RuO2to Li3V2(PO4)3[252]and improved per-formance by increasing the electrical conductivity.
Chromium forms compounds with the spinel structure and has been added to LiMn2O4[54,83]and LiMn1.5Ni0.5O4 [239,247,253–255].Chromium reduces the ordering of lithium ions in LiMn2O4,which stabilizes the single phase spinel structure[256], and has been shown to increase the capacity retention during cycling for spinel electrode materials,including LiMn2O4[54,83] and LiMn1.5Ni0.5O4[254,255]Nuclear Magnetic Resonance(NMR) analysis indicates the oxidation of Mn3+to Mn4+,and thus the asso-ciated lithium deintercalation,is not uniform,but rather occurs preferentially near the chromium dopant ions[257].Small amounts of chromium additions have also been shown to improve the per-formance of other cathode materials including Li[Mn0.5Ni0.5]O2 [258,259],Li(Ni1/3Mn1/3Co1/3)O2[260],V2O5[261]and Li3V2(PO4)3 [262].
Zinc-doping has been shown to improve the performance of Li[Mn0.4Mn0.3Co0.3]O2[263],LiFePO4[264],and,through addition of zinc to the electrolyte,LiMn2O4[265]by stabilizing the respec-tive crystal structures.However,the bene?cial effect of zinc is usually when added in a coating of ZnO(e.g.LiNi0.5Mn0.25Co0.25O2 [266],Li[Ni0.42Mn1.42Co0.16]O4[91],LiMn1.5Ni0.5O4[267,268], LiFePO4[269]),ZnMn2O4(e.g.LiMn2O4[270])or Zn3(PO4)2(e.g. LiCoO2[271])where it reduces reaction between the electrode and electrolyte.
Titanium is added as a dopant in layered structures (Li(Ni1/3Co1/3Mn1/3)O2[248],LiNi0.8Co0.2O2[272],LiNiO2[273]), spinels(LiMn2O4[274–276],LiNi0.5Mn1.5O4[277])and phos-phates(LiFePO4[278],LiMnPO4[126]).Although not bene?cial for LiNiO2[273],titanium is bene?cial when co-doped with cobalt(i.e.LiNi0.8Co0.2?x Ti x O2)[272].The improved performance is attributed to titanium stabilizing the crystal structure(e.g.LiNi0.8Co0.2O2 [272],Li(Ni1/3Mn1/3Co1/3)O2[248])or reducing dissolution of the electrode[274–276].Titanium from impurities in the precursor has been shown to occupy iron sites in LiFePO4and improve elec-trode performance[279].Titanium is also added as a TiO2coating to reduce electrode dissolution in the electrolyte[195,280],but degradation of the TiO2can lead to degradation in cell performance [180].
Zirconium has similar effects on cathode performance as tita-nium.Zirconium doping has been used to stabilize the layered crystal structure(e.g.LiCoO2[23]and LiNi0.8Co0.2O2[281])or in LiFePO4[282–284]to increase the lattice parameter.Zirconium has also been added as a ZrO2or Zr(OBu)4coating to reduce reaction of the electrolyte with cathode materials with layered (e.g.LiNi1/3Mn1/3Co1/3O2[177,210]and Li[Li1/6Ni1/6Mn1/2Co1/6]O2 [285])and spinel(LiMn2O4[183,286]and LiMn1.5Ni0.5O4[287]) structures.
Aluminum is a very commonly used dopant in cathode materials.In some cases small amounts of aluminum doping improve the capacity of electrode materials(e.g.LiCoO2[189,288], LiNi0.5Mn0.5O2[289–291]and LiFePO4[138]),but in most cases the capacity is decreased(e.g.LiCoO2[189,288],LiNi0.5Mn0.5O2 [289,292],Li(Ni1/3Mn1/3Co1/3)O2[248,260,293–295],other Li(Ni,Mn,Co)O2compositions[295–297],LiMn2O4)[83,291,298]). The decreased capacity is expected since Al3+cannot be further oxidized,so each transition metal ion replaced with aluminum represents one less oxidizable ion.The observed increases have been attributed to improved electrode kinetics,structural modi?-cations and microstructural effects.For example,the addition of aluminum to LiCoO2results in an increase in lattice parameter c [288,299],which facilitates lithiation and delithiation.Aluminum doping has also led to improvements in retention of capacity during cycling(e.g.Li(Ni,Mn,Co)O2[294,297],LiMn2O4[83,298]), and performance at high discharge currents(e.g.Li(Ni,Mn,Co)O2 [248,296],LiMn2O4[291]).However,there are also cases where cycling performance is degraded with aluminum additions(e.g. LiCoO2[23],LiNiO2[273],Li(Ni1/3Mn1/3Co1/3)O2[260,293]).Alu-minum is commonly used as a co-dopant with cobalt in LiNiO2 (i.e.LiNi0.8Co0.15Al0.05O2)for improved stability[211,300–303]. Aluminum can also be added as an alumina coating and has been shown to improve the capacity(e.g.LiCoO2[304],Li(Ni,Mn,Co)O2 [52,91],LiMn1.5Ni0.42Zn0.08O4[267],LiMn2O4[184]),capacity retention during cycling(e.g.LiCoO2[304,305],LiNi0.8Co0.2O2 [306],Li(Mn,Ni,Co)O2[91,286,307],LiMn1.5Ni0.42Zn0.08O4[267], LiMn2O4[286])and performance at high discharge currents [91,267].The improvements are attributed to improved charge transfer kinetics and improved stability with the electrolyte.Ben-e?cial effects have also been attributed to an Al(OH)3layer,which has led to its use as a coating for LiCoO2[308].Mixed aluminum oxides,including aluminum-cobalt oxide[309],Y3Al5O12(YAG) [310]and La3Al5O12(LAG)[311],have also been used to improve retention of capacity during cycling.Other forms of aluminum that have been shown to improve electrode performance and stability include AlF3[312–315],AlPO4[91,316–318]and(NH4)3AlF6[319].
Magnesium doping generally improves the performance of phosphate electrodes,including LiFePO4[138,174,320,321]and LiMnPO4[126,140].The bene?cial effect is generally attributed to magnesium increasing the lattice parameter,which facilitates delithiation and stabilizes the structure.Magnesium doping has been reported to improve oxide electrodes by modifying the microstructure(e.g.LiCoO2[322]),or reducing charge transfer resistance(e.g.LiNi0.8Co0.2O2[323]),but more often has little,or even a detrimental,effect on electrode performance(e.g.LiCoO2 [23,271],LiNi0.8Co0.2O2[324],Li(Ni1/3Mn1/3Co1/3)O2[260]).How-ever,when present in MgO[325]or Mg3(PO4)2[271]coatings,
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magnesium additions improve electrode performance by reducing the reaction of LiCoO2with the electrolyte.On the other hand,dop-ing with a larger alkaline-earth cation(e.g.calcium[326])can lead to an increase in lattice parameter of LiCoO2and improved capacity.
Lanthanum additions have been shown to improve the per-formances of LiFePO4[327,328]and LiCoO2[329]cathodes.One of the bene?ts of the lanthanum additions is that with LiCoO2a Li-conducting phase(La2Li0.5Co0.5O4)is formed,which improves ion transfer across the electrode–electrolyte interface.Such con-tact resistance is particularly important in solid–solid contacts and oxide additions can also improve contact between cathode particles or between the electrode and electrolyte materials.Some examples include CeO2,which has been shown to reduce contact resistance in LiFePO4cathodes[330],and Li2SiO3(and to a lesser extent SiO2), which has been shown to improve the performance of LiCoO2with a solid electrolyte[331–333].
As mentioned above,some additions improve performance by reducing reaction with the electrolyte.Other oxides added to improve capacity retention by reducing reaction and/or forming a bene?cial reaction product with the electrolyte include Y2O3[334], YPO4[305],B2O3[182],SiO2[335],Bi2O3[91,267],Sb2O3[336]and SnO2[286].Some of these same elements when added as a dopant have been reported to improve cycling performance by stabiliz-ing the crystal structure.For example,bismuth has been used as a dopant in LiMnO2[215]and LiMn2O4[337],while tin has been used as a dopant in LiMn2O4[338].In such cases there may be multiple bene?ts as some the oxide may dissolve in the electrode(when an oxide coating is used)or some of the dopant may form a sepa-rate phase on the electrode surface(when the cathode is doped). Other dopants that stabilize the crystal structure include rhodium in LiCo0.3Ni0.7O2[339],copper in LiMnPO4[140]or LiMn2O4[215] and indium in LiMnO2[340].The effectiveness of indium doping is enhanced by co-doping with sulfur,which occupies the oxygen site [340].
Another dopant that occupies the anion site is?uorine,which is a common element in lithium ion batteries.Fluorine is present in the commonly used LiPF6-based electrolytes[7],as well as in?uo-rides,such as carbon?uorides,that are used as anodes in lithium ion batteries[315,341].Although?uoride compounds have been reported as cathode materials(e.g.iron oxy?uoride[342,343]),in cathodes?uorine is more often added to replace oxygen or in a compound as an additive to oxide cathode materials[315].For example,?uorine doping has been shown to improve the capac-ity of spinel cathodes,LiMn1.5Ni0.5O4[344]and LiMn1.8Li0.1Ni0.1O4 [345],by increasing the lattice parameter and decreasing the aver-age manganese valence.Fluoride additions have also been shown to improve the cycling performance of Li(Ni,Mn,Co)O2,either as a dopant[346]or a second phase(e.g.LiF[347],SrF2[348])by reducing reaction with the electrolyte.
The electrode reaction involves lithium ions and electrons,so one approach to improving electrode performance is to add a con-ducting phase to enhance charge transfer.A sputtered gold layer has been used to improve the performance of LiFePO4[349],but this involves an expensive material and process.Silver is lower in cost than gold and has been shown to improve the performance of LiCoO2[207,350],LiMn2O4[207]and LiNi1/3Mn1/3Co1/3O2[351] cathodes.The bene?cial effect of silver is generally attributed to increased conductivity,but increases in lattice parameter have also been reported in LiCoO2[350]and LiNi1/3Mn1/3Co1/3O2[351]. Although these metals can improve performance,the most com-mon addition for improving electrode conductivity is carbon.
Carbon is commonly added to cathodes to more effectively utilize the active cathode material,especially at high discharge rates.For example,carbon additions improve the performance of LiNi1/3Mn1/3Co1/3O2[175,176,352],spinels(LiMn2O4[353] and LiMn1.5Ni0.5O4[354]),and especially LiFePO4[161,355–357],which has a relatively low electrical conductivity.The effectiveness of carbon additions depends on the amount and type of carbon used.
A thin carbon layer can provide a path for electrons without block-ing access for lithium ions[358,359].There is an optimal amount of carbon,which depends on the microstructure and operating condi-tions,but is typically on the order of2–10wt%[197,198,360–364]. Graphitic carbon generally provides higher conductivity and thus higher rate capacities at large discharge rates,so carbons with large sp2/sp3ratios are generally preferred[111,199,202,365].The parti-cle shape is also important,as nano-scale?bers[158,191,366–370] and high surface area forms of carbon,such as acetylene black [192,371],have been shown to be effective in improving cathode performance.In addition to providing paths for electrical conduc-tion,nano-scale?bers can also be used to disperse the cathode particles for increased contact with the electrolyte[372].The type and morphology of the carbon deposits depends on the source of the carbon[373],which include glucose[374–376],sucrose[156] and malonic acid[377,378],as well as oligomers[379]and poly-mers[380,381].In addition to being used as precursors for carbon, polymers can be used in the electrode.In particular,semicon-ducting[170,382]or conducting polymers(e.g.polypyrrole(PPy) [162,201,383,384]and polyaniline[385,386])have been used as additives in cathodes.
6.Effect of microstructure and morphology
As mentioned above,electrode performance depends on the electrode microstructure and morphology.Although the focus of this paper is on the materials rather than morphology,some general aspects of electrode morphology will be discussed.Intercalation and deintercalation occur along speci?c crystallographic planes and directions,so higher crystallinity improves electrode perfor-mance(e.g.LiCoO2[387],LiMn2O4[388],Li1.02Mn1.5Ni0.5O4[389], LiFePO4[155,390–392].
The electrode reaction occurs at the surface and requires transport of ions into the electrode material,so small particles, which provide high surface area[388]and short diffusion dis-tances[393],and are generally desired(e.g.LiCoO2[164,394,395], LiNi1/3Mn1/3Co1/3O2[396],LiMn2O4[181,397,398]and LiFePO4 [155,157,171,399–402].However,particles can become too small, so that processes other than the surface reaction,such as diffu-sion of ions in the electrolyte to the particle surface,become rate limiting.In addition,the high reactivity of nanosize particles can be disadvantageous in terms of safety and stability during long operational lifetimes[12,403].Thus,intermediate particle sizes sometimes exhibit the best performance[404–406].For example, analysis of the discharge of individual LiCoO2particles suggests that commercially available micron-size particles have suf?cient capacity,so nanoparticles may not be necessary[407].Similarly, micron-scale carbon?bers have been used to provide a framework for dispersion of LiFePO4particles to improve access to lithium ions in the electrolyte[408].In addition,control of porosity is important to allow for access of the electrolyte to the electrode surface[194,409–412].For example,a recent report has shown that nanoporous micron-size particles perform better than nanoparti-cles[413].
Particle shape is important for electrode performance.For LiMn2O4spinel electrodes,spherical particles have been reported to provide the best performance[414,415],which may be related to the more isotropic lithium transport in the spinel struc-ture,as compared to layered structures.For layered structures, including LiCoO2[416]and LiNi1/3Co1/3Mn1/3O2[417],however, non-spherical particles have been shown to improve perfor-mance,which has been attributed to high tap density.Rod-or needle-shaped particles,which have large surface-to-volume
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ratios,have also shown good performance,especially during cycling and at high discharge currents[418–420].Rod-shaped par-ticles can also be useful as precursors to produce small nano-scale spherical particles[421].Plate-like particles,which re?ect the lay-ered crystal structure,have also been used[204,422,423],and can better accommodate expansion/contraction during intercala-tion/deintercalation and reduce cracking.In addition to the overall particle shape,the surface morphology is also important.For exam-ple,a“desert rose”surface morphology on LiCoO2produces a high surface area,which results in a high capacity[424].
The production of small power supplies for miniature devices requires thin-?lm based batteries,so thin-?lm deposition of cath-ode materials is required for these applications[425].Thin-?lm deposition techniques,such as pulsed laser deposition and sputter-ing,have been used to produce cathode materials including LiCoO2 [426–430],LiMn2O4[431–435],LiFePO4[436].The small batteries have high energy densities,but the energy density can be fur-ther improved by using three-dimensional architectures[437,438]. Such three-dimensional geometries require templates for control of the size,shape and arrangement of the battery components [439].Approaches for producing such complex geometries can be inspired by biological systems[440],and can even use biologi-cal processes for fabrication[441–443].The processes required to fabricate these complex geometries will likely place additional con-straints on materials selection,so further developments in cathode materials and fabrication processes will be required.
7.Conclusions
The development of improved cathode materials is a challenge for meeting current and future energy storage requirements.Sev-eral transition metal based cathode materials can provide high voltages and good capacities.Full utilization of these materials for numerous recharging cycles and at high discharge currents continues to be a challenge.Speci?cally,stabilizing the desired crystal structure,especially during delithiation,and preventing reaction with the electrolyte are important for long operational life, while improved transport to and in the electrode are important for achieving high discharge current.Progress has been made by engineering the electrode composition,microstructure and mor-phology,but additional improvements are needed.
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