1180 Chem. Mater. 2010, 22, 1180–1185 DOI:10.1021/cm902627w
Influence of Manganese Content on the Performance of LiNi0.9-yMnyCo0.1O2 (0.45 e y e 0.60) as a Cathode Material for Li-Ion Batteries† Jie Xiao,‡ Natasha A. Chernova, and M. Stanley Whittingham* Department of Chemistry and Materials Science and Engineering Program, State University of New York at Binghamton, Binghamton, New York, 13902-6000. ‡Current address: Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA 99354 Received August 25, 2009. Revised Manuscript Received October 21, 2009
The layered oxide cathode material LiMO2, where M = Ni0.9-yMnyCo0.1 and 0.45 e y e 0.60, was synthesized by a coprecipitation method. X-ray diffraction analysis shows that the maximum manganese content in the stoichiometric material, i.e. with Li:M = 1, cannot exceed 50%; otherwise, a second phase is formed. Rietveld refinement reveals that increasing manganese content suppresses the disorder between the lithium and nickel ions. Magnetic measurements show that part of the Mn4þ ions in the manganese rich compounds is reduced to Mn3þ; this results in a larger hysteresis loop due to the increased magnetic moment of the resulting ferrimagnetically ordered clusters. LiNi0.4Mn0.5Co0.1O2 and LiNi0.45Mn0.45Co0.1O2 show similar electrochemical capacities of around 180 mAh/g (between 2.5 and 4.6 V at 0.5 mA/cm2) for the first discharge. However, subsequent cycling of LiNi0.4Mn0.5Co0.1O2 results in faster capacity loss and poorer rate capability indicating that manganese rich compounds, with Li:M = 1:1, are probably not suitable candidates for lithium batteries. 1. Introduction Layered materials have attracted extensive attention since the original work on TiS21,2 and LiCoO2.3-5 The majority of commercialized cathode materials for lithium ion batteries are based on LiCoO2. However, the scarcity and high price of cobalt limit its use for large scale systems such as those required for electric vehicles or for utility load-leveling. Although there has been much effort on the related LiMnO2 because of its lower cost and the environmentallybenign properties of manganese, LiMnO2 with the R-NaFeO2 structure is metastable. Thus, it cannot be formed using traditional high temperature methods but must be formed at lower temperatures, for example, by ion exchange from R-NaMnO26,7 or hydrothermally.8 Even then, it transforms to the spinel structure upon Li cycling.6-10 This † Accepted as part of the 2010 “Materials Chemistry of Energy Conversion Special Issue”. *Corresponding author. Tel.: þ1-607-777-4623. Fax: þ1-607-777-4623. Email address:
[email protected].
(1) Whittingham, M. S. Science 1976, 192, 1126–1127. (2) Whittingham, M. S. Chalcogenide battery. U.S. Patent 4,009,052 and U.K. Patent 1468416, 1973. (3) Mitzushima, K.; Jones, P. C.; Wiseman, P. J.; Goodenough, J. B. Mater. Res. Bull. 1980, 15, 783–789. (4) Goodenough, J. B.; Mizuchima, K. Electrochemical cell with new fast ion conductors. U.S. Patent 4,302,518, 1980. (5) Whittingham, M. S. Chem. Rev. 2004, 104, 4271–4301. (6) Armstrong, A. R.; Bruce, P. G. Nature 1996, 381, 499–500. (7) Capitaine, F.; Gravereau, P.; Delmas, C. Solid State Ionics 1996, 89, 197–202. (8) Chen, R.; Whittingham, M. S. J. Electrochem. Soc. 1997, 144, L64–L67. (9) Xiao, J.; Zhan, H.; Zhou, Y. Mater. Lett. 2004, 58, 1620. (10) Shao-Horn, Y.; Hackney, S. A.; Armstrong, A. R.; Bruce, P. G.; Gitzendanner, R.; Johnson, C. S.; Thackeray, M. M. J. Electrochem. Soc. 1999, 146, 2404–2412.
pubs.acs.org/cm
conversion does not require oxygen ion rearrangement as both structures are cubic close-packed (ccp). Two approaches to stabilize LiMnO2 have been proposed. In the geometrical approach, a non-ccp, for example tunnel, structure is considered or “pillars” as in (VO)yMnO2 are placed between the layers, which prevent conversion to the spinel phase.11 In the electronic stabilization approach, Mn is partially substituted by other transition metals such as the more electronegative Ni. When there are equal amounts of Ni and Mn, the Ni is found in the 2þ state and the Mn in the 4þ state eliminating any Jahn-Teller Mn3þ which can cause structural instability. Thus, the most successful electronically stabilized layered oxides contain equal amounts of Ni and Mn. LiMn0.5Ni0.5O2 has the highest theoretical capacity among these layered oxides provided by the oxidation of Ni2þ to Ni3þ to Ni4þ; however, it suffers from an 8-10% Li/Ni exchange which is thought to be detrimental to high-rate performance.12,13 For the purpose of layered structure stabilization, solid solutions between LiMn0.5Ni0.5O2 and LiCoO2, LiNiyMnyCo1-2yO2, have been successfully developed,14-16 with LiMn0.33Ni0.33Co0.33O2 (11) Zhang, F.; Ngala, K.; Whittingham, M. S. Electrochem. Commun. 2000, 2, 445–447. (12) Ohzuku, T.; Makimura, Y. Chem. Lett. 2001, 744–745. (13) Yabuuchi, N.; Ohzuku, T. J. Power Sources 2003, 119-121, 171– 174. (14) MacNeil, D. D.; Lu, Z.; Dahn, J. R. J. Electrochem. Soc. 2002, 149, A1332–A1336. (15) Hwang, B. J.; Tsai, Y. W.; Carlier, D.; Ceder, G. Chem. Mater. 2003, 15, 3676–3682. (16) Kim, M. G.; Shin, H. J.; Kim, J. H.; Park, S. H.; Sun, Y. K. J. Electrochem. Soc. 2005, 152, A1320.
Published on Web 11/12/2009
r 2009 American Chemical Society
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being commercialized. We have recently shown that the Co content in LiNiyMnyCo1-2yO2 can be reduced to 0.2 in LiNi0.4Mn0.4Co0.2O2 without loss of electrochemical performance17-19 and that further reduction of the Co content to 0.1 does not result in significant loss of electrochemical capacity.20 In this study, we set out to determine the impact of increasing the manganese content in LiNi0.45-δMn0.45þδCo0.1O2 on the structural, physical, and electrochemical properties 2. Experimental Section LiNi0.9-yMnyCo0.1O2 (0.45 e y e 0.60) was synthesized from Ni0.9-yMnyCo0.1(OH)2 coprecipitate. Stoichiometric amounts of the soluble salts, Mn(OAc)2 3 4H2O, Co(OAc)2 3 4H2O, and Ni(OAc)2 3 4H2O (all from Sigma-Aldrich, 99%) were dissolved in deionized water. An excess amount of LiOH solution (LiOH 3 H2O, ACS, 98%, Alfa Aesar) was added to precipitate the mixed hydroxide; the solid was then thoroughly washed twice with deionized water to remove residual Li salts. The coprecipitate of nickel, manganese and cobalt hydroxides was dried at 65 °C overnight and then mixed with the LiOH 3 H2O in an amount providing 5% excess of lithium to compensate the evaporation of lithium at high temperatures. The well-ground powders were pressed into pellets and heated first at 450 °C for 12 h, then reground, repressed, and fired at a higher temperature for 8 h. For the y = 0.4520 material, 800 °C was found to be optimal; for the y = 0.5, samples were annealed at 700, 800, 900, and 1000 °C; and for samples with y = 0.55 and 0.65 only 900 °C was chosen as the y = 0.5 formed at that temperature showed the best electrochemical performance. The final products were cooled slowly to room temperature. X-ray diffraction (XRD) data was collected from 10 to 90 2θ° with a step size of 0.02° and an exposure time of 10 s at room temperature on a Scintag XDS2000 θ-θ diffractometer equipped with a Ge(Li) solid state detector and Cu KR sealed tube (λ = 1.54178 A˚). Chemical analysis of the samples was done by direct current plasma spectroscopy (DCP, ARL Fisons SS-7). The morphology of the Li(Ni0.45Mn0.45Co0.1)O2 was examined by using a scanning electron microscope (SEM, Hitachi S-570, Japan). A SQUID magnetometer (Quantum Design MPMS XL-5) was used to measure the dc magnetic susceptibility (χ = M/H, M is magnetization, H is applied magnetic field) of the samples from 400 to 2 K in a magnetic field of 1000 Oe. Magnetization curves were measured at 5 K in magnetic fields up to 5 T. The sample was zero-field cooled to 5 K before the magnetization data was taken. The electrochemical properties were tested in 2325-type coin cells on a VMP2 multichannel potentiostat (Biologic). The cathode was constructed by mixing the oxide with acetylene black and poly(vinylidene fluoride) powder in the weight ratio of 80:10:10. Pure lithium foil (Aldrich, thickness 23 μm) was used as the counter and reference electrode. The electrolyte used was 1 M LiPF6 (lithium hexafluorophosphate) in a mixture of DMC (dimethyl carbonate) and EC (ethylene carbonate) with 1:1 volume ratio (LP30 from EM Industries). The assembly of the coin cells was processed in a glovebox filled with helium gas. (17) Yang, S.; Song, Y.; Ngala, K.; Zavalij, P. Y.; Whittingham, M. S. J. Power Sources 2003, 119, 239. (18) Ngala, J. K.; Chernova, N. A.; Ma, M.; Mamak, M.; Zavalij, P. Y.; Whittingham, M. S. J. Mater. Chem. 2004, 14, 214–220. (19) Ma, M.; Chernova, N. A.; Toby, B. H.; Zavalij, P. Y.; Whittingham, M. S. J. Power Sources 2007, 165, 517–534. (20) Xiao, J.; Chernova, N. A.; Whittingham, M. S. Chem. Mater. 2008, 20, 7454–7464.
Figure 1. XRD patterns for LiNi0.9-yMnyCo0.1O2, for 0.45 e y e 0.6, and synthesized at different temperatures. Asterisks indicate spinel peaks. Table 1. Lattice Parameters of Layered LiNi0.9-yMnyCo0.1O2 y
synthesis T (°C)
a (A˚)
c (A˚)
c/3a
Rp (%)
0.45 0.50 0.50 0.50 0.50
800 700 800 900 1000
2.872(2) 2.872(1) 2.871(4) 2.871(3) 2.875(3)
14.256(2) 14.274(5) 14.273(2) 14.271(1) 14.277(1)
1.655 1.657 1.657 1.657 1.655
5.9 6.3 6.1 6.4 9.7
The coin cells were cycled within different voltage ranges at a current density of 0.5 or 0.1 mA/cm2 at room temperature.
3. Discussion We have previously described the structural properties of LiNi0.45Mn0.45Co0.1O2.20 Here, we report on the composition limits of the layered structure as the manganese content is increased. We first increased the manganese content in the LiNi0.9-yMnyCo0.1O2 system to y = 0.5 giving LiNi0.4Mn0.5Co0.1O2. The XRD patterns for LiNi0.4Mn0.5Co0.1O2 synthesized at different temperatures (Figure 1) can be indexed to the R-NaFeO2 layered structure without any indication of a second phase. The Rietveld refinement results (Table 1) indicate that the c/3a ratios are the same regardless of the synthesis temperature, which is consistent with that observed for LiNi0.45Mn0.45Co0.1O2.20 Both the a and c lattice parameters show a slight increase when the synthesis temperature is 1000 °C. This is consistent with some loss of lithium with additional Ni going to the Li site. The best electrochemical performance was obtained for the 900 °C LiNi0.4Mn0.5Co0.1O2 as opposed to 800 °C found for LiNi0.45Mn0.45Co0.1O2; thus, the samples with higher Mn content were synthesized at 900 °C. Rietveld refinement of the X-ray powder data shows that the c parameter increased, while the a parameter stayed almost the same as the manganese content increased from 0.45 to 0.50: for LiNi0.4Mn0.5Co0.1O2, c = 14.271(1) A˚ and a = 2.871(3) A˚, whereas for LiNi0.45Mn0.45Co0.1O2, a = 2.872(2) A˚ and c = 14.256(2) A˚. The Rietveld refinement also showed slightly less Ni2þ/Liþ cation disorder as the manganese content increased,
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Figure 3. Temperature dependences of the magnetic susceptibilities of LiNi0.9-yMnyCo0.1O2 (0.45 e y e 0.60). The inset shows the reciprocal susceptibilities and their fit to the Curie-Weiss law.
Figure 2. SEM images of (a) LiNi0.3Mn0.6Co0.1O2; (b) LiNi0.35Mn0.55Co0.1O2; (c) LiNi0.4Mn0.5Co0.1O2; and (d) LiNi0.45Mn0.45Co0.1O2.
decreasing from 6.2% (y = 0.45) to 5.7% (y = 0.50). This decrease in disorder is also reflected in the c/3a ratio, which increases from 1.655 for LiNi0.45Mn0.45Co0.1O2 to 1.657 for LiNi0.4Mn0.5Co0.1O2. The a lattice parameters, which reflect the size of the oxygen anions, are essentially the same because the average metal oxidation states remain the same, and thus, the electron density on the oxygen is unchanged. The increase of the c parameter is a reflection of the reduction of Ni/Li disorder as the Ni content decreases. Increasing the Mn content above 0.5 in LiNi0.9-yMnyCo0.1O2 results in the formation of a spinel impurity phase (Figure 1). The cubic a lattice parameter of the spinel phase present in the LiNi0.35Mn0.55Co0.1O2 and LiNi0.3Mn0.6Co0.1O2 compounds is 8.617 and 8.577 A˚, respectively. Both are much larger than that, 8.235 A˚,5 of the pure manganese spinel LiMn2O4 which are possibly formed with the layered oxide when y > 0.5. Thus, this discrepancy may be attributed to the lithium deficiency in the mixtures, as the a lattice parameter of the spinel LiMn2O4 increases with decreasing lithium content.21 SEM images for the LiNi0.9-yMnyCo0.1O2 (0.45 e y e 0.65) compounds are shown in Figure 2. Initially when the manganese content is 60% (y = 0.6), the particle size is between 300 and 600 nm with no clear hexagonal shapes. As the manganese content decreases and the nickel content increases, the particle size is reduced to less than 300 nm. The aggregation of the particles is also (21) Amatucci, G. G.; Pereira, N.; Zheng, T.; Tarascon, J.-M. J. Electrochem. Soc. 2001, 148, A171–A182.
Figure 4. Magnetization curves of LiNi0.9-yMnyCo0.1O2 (0.45 e y e 0.60) at 5 K.
Table 2. Magnetic Parameters of LiNi0.9-yMnyCo0.1O2 y
C, emu K/mol TM
θ, K
μ, μB
μtheor, μB
0.45 0.50 0.55 0.60
1.233(1) 1.304(5) 1.341(7) 1.360(5)
-89.9(1) -78.9(8) -79(1) -68.9(8)
3.14(1) 3.23(1) 3.28(1) 3.30(1)
3.22 3.41 3.58 3.76
more significant and the porous character of LiNi0.45Mn0.45Co0.1O2 is similar to the one observed for LiNi0.4Mn0.4Co0.2O2. In other words, the relative content of the transition metals in the final compound does influence its morphology and thus the electrochemical properties. The presence of Mn3þ in the compounds was determined through magnetic measurements (Figures 3 and 4). The temperature dependences of the magnetic susceptibility follow the Curie-Weiss law above 200 K for all the compositions, which allows the determination of the average effective magnetic moment μ of the transition metal ions (Table 2) from the linear fit of the reciprocal susceptibility (inset of Figure 3). An increase of the effective magnetic moment is observed as Mn3þ (S = 2, μ = 4.90 μB) replaces Ni2þ (S = 1, μ = 2.82 μB) and Mn4þ (S = 3/2, μ = 3.87 μB). However, the experimental values of the effective magnetic moment are somewhat lower than expected. At lower temperatures, an increase of the magnetic susceptibility is observed caused by the local ordering of magnetic clusters
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Figure 5. Schematic of the magnetic ordering in the layered LiMO2 oxides assuming flower-type transition metal ordering (a) in the areas where no Ni2þ in the transition metal layer is present; (b) around Ni2þ ions in the transition metal layer; and (c) increase of the cluster magnetization when Mn3þ ions are present. Ni ions are in green, Mn ions are red, and Li ions are yellow.
Figure 7. Cyclic voltammetry of (a) Li/LiNi0.3Mn0.6Co0.1O2, (b) Li/ LiNi0.35Mn0.55Co0.1O2, (c) Li/LiNi0.4Mn0.5Co0.1O2, and (d) Li/LiNi0.45Mn0.45Co0.1O2 coin cells at a sweep rate of 0.1 mV/s between 2.5 and 4.6 V.
Figure 6. Electrochemical charge-discharge profiles of LiNi0.9-yMnyCo0.1O2 (0.45 e y e 0.60) at 0.5 mA/cm2 between 2.5 and 4.6 V at room temperature.
around the interslab Ni2þ ions.22 With increasing Mn3þ content, these clusters start to order at higher temperatures and develop larger magnetization, which is also reflected in the increase of the magnetic hysteresis loops (Figure 4). The large hysteresis loops observed for the y = 0.55 and 0.6 compounds are consistent with the coformation of a spinel phase. In this case, not only Ni2þ, but also Mn ions may occupy interslab positions resulting in local ferrimagnetic ordering. However, the layered y=0.45 and 0.5 compounds have similar amount of Ni2þ in the lithium layer, thus a different reason for the strong magnetic clustering should be found. We showed earlier that the magnetic clusters nucleate at Ni2þ ions in the lithium layer (NiLi), whose spins are strongly antiferromagnetically coupled to the Ni2þ ions in the transition metal layer (NiTM) through a 180° NiLi-O-NiTM bond and, then, propagate within the transition metal layers through the 90° magnetic exchange (see Figure 5).22 However, if Ni2þ, Co3þ, or Mn4þ ions appear at the end of the 180° bond, such as in NiLi-O-CoTM, the magnetic cluster propagation is (22) Chernova, N. A.; Ma, M.; Xiao, J.; Whittingham, M. S.; Breger, J.; Grey, C. P. Chem. Mater. 2007, 19, 4682–4693.
terminated due to a much weaker magnetic exchange. In contrast to Co3þ and Mn4þ ions, the Mn3þ ion with one t2g electron and Jahn-Teller distortion is capable of strong 180° antiferromagnetic exchange with Ni2þ ions in the lithium layer, enabling larger magnetic clusters. Also, the higher effective magnetic moment of the Mn3þ ion leads to higher magnetization values. Therefore, strong magnetic clustering in LiNi0.4Mn0.5Co0.1O2 directly reflects the presence of Mn3þ ions in this compound. The effect of Mn3þ as well as of the spinel impurity phase on the electrochemical performance of the materials is shown in Figure 6. Clearly, the layered LiNi0.45Mn0.45Co0.1O2 with all Mn4þ shows the best electrochemical behavior. The first discharge capacity (lithium intercalation) reaches 180 mAh/g while the irreversible capacity is around 50 mAh/g. The polarization for this compound is about 0.3 V. LiNi0.4Mn0.5Co0.1O2 delivers a similar discharge capacity in the first cycle, but the polarization is much larger. As the manganese content continues to increase, the capacity drops substantially with a huge polarization perhaps due to the coexistence of two phases. The voltage of the charge plateau increases with increasing manganese content. This is also observed in their cyclic voltammogram (CV) redox peaks: the higher the manganese content, the higher the potential of the oxidation peaks (Figure 7). The oxidation peaks shift from 4.05 to 4.07, to 4.25, and finally to 4.46, for y = 0.45, 0.50, 0.55, and 0.60, respectively. For the two lower manganese content samples, more detail is observed in the cyclic voltammograms, Figure 7c and d. For LiNi0.45Mn0.45Co0.1O2, the major redox peaks at ca. 4.0 and 3.7 V in the first cycle correspond to the transition between Ni2þ and Ni4þ ions. These peaks remain essentially unchanged in position on subsequent sweeps, showing that the structure of this composition is quite stable. This is also consistent with the results in Figure 6. In contrast, the materials with higher manganese content show a shift to higher voltage in the second and
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Figure 9. Discharge and charge capacities of LiNi0.4Mn0.5Co0.1O2 when cycled to cutoff potentials of 4.4 and 4.8 V.
Figure 8. Voltage profiles of LiNi0.4Mn0.5Co0.1O2 with an upper cutoff voltage of 4.4 and 4.8 V cycled at 0.5 mA/cm2.
subsequent sweeps. In the case of the LiNi0.4Mn0.5Co0.1O2 material, this suggests the possible formation of some spinel phase, which is already present in the two higher manganese content phases. The first charge of LiNi0.4Mn0.5Co0.1O2 shows a broad oxidation peak beginning above 3.0 V meaning that some Mn3þ/Mn4þ oxidation occurs; at higher voltages, the CV looks similar to the Ni2þ/Ni4þ oxidation peak of LiNi0.45Mn0.45Co0.1O2. However, as noted above, the 4.07 V peak shifts to higher potentials on subsequent cycles. Upon discharge, the reduction peak of Mn4þ/Mn3þ is more pronounced and becomes even more so on the second and subsequent discharges with continuously increasing capacity below 3 V. This is also reflected in the cycling curves in Figure 6, where the y = 0.5 composition shows much more capacity than the y = 0.45 material below 3 V. Thus, in the 0.5 composition material manganese is electrochemically active, in contrast to the 0.45 material where there is no Mn3þ in the material. The impact of the charging potential on the electrochemical behavior of LiNi0.4Mn0.5Co0.1O2 is shown in Figure 8 for cutoff voltages of 4.4 and 4.8 V. At 4.8 V, the first charge capacity is 248 mAh/g, close to the theoretical value of 275.5 mAh/g indicating that most of the lithium can be removed at this potential. A 200 mAh/g portion of lithium can be reinserted. Charging only to 4.4 V limits the lithium removal to around 200 mAh/g, and the discharge capacity is only 123 mAh/g. However, our earlier work on the composition LiNi0.45Mn0.45Co0.1O2 showed much more stable cycling when the charging voltage was limited to 4.4 V.20 We thus evaluated the capacity of LiNi0.4Mn0.5Co0.1O2 when cycled extensively, and the results are shown in Figure 9. The initial high capacity of 200 mAh/g dropped to about
Figure 10. Rate capability of LiNi0.4Mn0.5Co0.1O2 cycled between 2.5 and 4.6 V.
160 mAh/g at the 10th cycle when the material was cycled between 2.5 and 4.8 V with an average decay rate of 1.7% per cycle. When the cutoff voltage is decreased to 4.4 V, the material is quite stable during the first 20 cycles and its capacity decayed to 106 mAh/g at the 28th cycle. The average capacity fade rate between 2.5 and 4.4 V is only 0.5% per cycle. After 20 cycles, both of the capacities decreased to around 125 mAh/g. Thus, more stable cycling is observed for the lower charge potential of 4.4 V. The rate capability of LiNi0.4Mn0.5Co0.1O2 when cycled between 2.5 and 4.6 V at different C rates is shown in Figure 10. All the coin cells were charged and discharged once at a C/20 rate between 2.5 and 4.6 V. Then the rate was varied up to the 1C rate and was equal on both charge and discharge. The discharge capacity decreased significantly from 180 to 150 mAh/g when the C rate was doubled from 0.05 to 0.1C. However, from 0.2 to 1C, the capacity did not drop significantly. The difference between the capacities at 0.2 and 1C is about 12 mAh/g. Further studies are needed to determine whether charge or discharge is the slower reaction and controls the overall capacity. 4. Conclusions LiNi0.9-yMnyCo0.1O2 (0.45 e y e 0.60) forms a solid solution for y up to 0.5; for higher y values, a spinel
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impurity phase is formed. LiNi0.4Mn0.5Co0.1O2 has a slightly lower but comparable Ni2þ-Liþ disorder to LiNi0.45Mn0.45Co0.1O2. Magnetic measurements show that part of the manganese is reduced to 3þ in LiNi0.4Mn0.5Co0.1O2, but the concentration is not high enough to induce a static Jahn-Teller distortion. As the manganese contents continue to increase, the formation of spinel LiMn2O4 leads to an increase of the oxidation potentials. LiNi0.4Mn0.5Co0.1O2 has capacities of 218 and 181 mA h/g for the first charge and discharge, respectively, close to those of LiNi0.45Mn0.45Co0.1O2. However,
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the capacity retention and rate capability of the former are inferior to those for LiNi0.45Mn0.45Co0.1O2. When the manganese content is above 50%, the capacities drop substantially and the polarization increases significantly. Acknowledgment. We thank the U.S. Department of Energy, Office of FreedomCAR and Vehicle Technologies, for financial support through the BATT program at Lawrence Berkeley National Laboratory. Financial support from the National Science Foundation, DMR 0705657, is also greatly appreciated.
