Comparison of Full Concentration Gradient Li[Ni0.54Co0.16Mn0.30]O2 with Conventional Cathode Materials for Li-Ion Batteries

Lithium-ion batteries are an important power source owing to their high energy density, high voltage, and excellent cycle life. Common applications include portable electronic devices such as portable smart phones, notebook-style personal computers, plug-in hybrid vehicles (PHEVs), and electric vehicles (EVs). However, the energy density and safety of lithium-ion batteries is still insufficient for most applications. At present, the driving range of EVs is less than 160 km and is significantly reduced by the use of air conditioning. As a result, further increases in the energy density, cycle life, and safety are important for their successful application in the automobile industry. The development of new, high-capacity cathode materials is a prerequisite for the next generation of lithium-ion batteries intended for use in EV applications.[1–2]

The layered Li[Ni_xCo_yMn_z]O₂ materials, only Li[Ni_1/3Co_1/3Mn_1/3]O₂ is used in automobile applications in conjunction with stabilized spinel Li_1+xMn_2-yAl_yO₄ in a composite form, resulting in a lower capacity and outstanding safety characteristics characteristics.[3] The capacity of Li[Ni_1/3Co_1/3Mn_1/3]O₂ (155 mAh g^-1) is so low that it limits the driving range of associated EVs to less than 160 km. Another promising cathode material is Li[Ni_0.5Co_0.2Mn_0.3]O₂, which delivers a capacity of 170 mAh g^-1. However, Li[Ni_0.5Co_0.2Mn_0.3]O₂ fails to satisfy requirements for EV applications owing to its poor capacity retention and safety, creating a need for cathode materials with the capacity of Li[Ni_0.5Co_0.2Mn_0.3]O₂ and the safety of Li[Ni_1/3Co_1/3Mn_1/3]O₂.

We report a promising cathode material specifically engineered for EV applications: Li[Ni_0.54Co_0.16Mn_0.30]O₂ with a full concentration gradient (FCG) of Ni and Co ions at a fixed Mn content of 30% throughout the particle. We compared the electrochemical, structural, and thermal stabilities of the FCG material with those of conventional Li[Ni_0.5Co_0.2Mn_0.3]O₂ and Li[Ni_1/3Co_1/3Mn_1/3]O₂materials.

Experimental

Spherical FCG [Ni_xCo_yMn_z](OH)₂ precursors were synthesized through the co-precipitation method. A Ni-poor aqueous solution (molar ratio of Ni/Co/Mn = 0.50:0.20:0.30) consisting of NiSO₄·6H₂O, CoSO₄·7H₂O, and MnSO₄·5H₂O was slowly pumped from tank 2 into a Ni-rich (molar ratio of Ni/Mn=0.7:0.3) stock solution in tank 1. The homogeneously mixed solution was then fed into a continuously stirred tank reactor (CSTR, 4 L) in a replenished N₂ atmosphere. Concurrently, a NaOH solution (4.0 mol L^-1, aq) and the desired amount of a NH₄OH chelating agent solution (aq) were pumped separately into the reactor. The concentration of the solution, pH value, temperature, and stirring speed of the mixture in the reactor were carefully controlled. During the early stage of the co-precipitation process, [Ni_0.7Mn_0.3](OH)₂ (composition of the center of the particle) was first precipitated. Nickel–cobalt–manganese hydroxide at different compositions was then gradually piled onto the formed [Ni_0.7Mn_0.3](OH)₂ particles to result in a linear composition change of Ni and Co toward the outer surface of the particles. The precursor powders were obtained through filtering, washing, and drying at 100 ^oC overnight. The obtained FCG [Ni_0.54Co_0.16Mn_0.30](OH)₂ was mixed with LiOH·H₂O, and the mixture was calcined at 875 ^oC for 10 h in air.

Electrochemical properties of the synthesized FCG [Ni_0.54Co_0.16Mn_0.30]O₂ were evaluated using a 2032-type coin type cell.

Results and discussion

Fig. 1 shows the change of concentrations of Co and Ni in the single rod-shaped primary particle was determined by means of energy-dispersive X-ray spectroscopy (EDS) along with the corresponding elemental distribution of Ni, Co, and Mn around the center region of the FCG particle; the results are shown superimposed on the TEM image. The EDS data show that the Co/Ni atomic ratio decreased from 0.375 to 0.323 along the 1.2 mm length toward particle core, a composition variation similar to that shown by the electron-probe X-ray microanalysis (EPMA) data in Figure 2. Note that the concentration gradient was observed within a single primary particle, which is a characteristic of the FCG particles synthesized by our unique co-precipitation method. Ni was depleted at the particle surface and became slowly enriched toward the particle center whereas Co levels gradually depleted. As expected, the elemental mapping shows that Mn was distributed uniformly throughout the center region of particle.

References

1. J. B. Goodenough, Y. Kim, Chem. Mater. 22, 587 (2010).

2. Y.-K. Sun, S.-T. Myung, B.-C. Park, J. Prakash, I. Belharouak, K. Amine, Nat. Mater. 8, 320 (2009).

3. I. Belharouak, Y.-K. Sun, J. Liu, K. Amine, J. Power Sources 123, 247 (2003).