Effect of Fixed Mn Content in Full Concentration Gradient Structure on the Electrochemical Properties of Layered Cathode Materials

Introduction

The lithium batteries are being developed as power sources for plug-in hybrid electric and electric vehicles, and for these applications, high energy cathodes are necessary to reduce battery size because the mounting space is significantly limited.[1] Hence, layered lithium nickel-cobalt-manganese oxide, Li[NixCoyMnz]O2 (x + y + z = 1), which delivers a high discharge capacity of over 170 mAh g⁻¹, has been intensively studied for high energy density lithium-ion batteries.[2] However, these cathodes exhibit some serious problems, such as capacity fade and poor thermal stability, which hinder their use in vehicle applications. In particular, oxygen evolution from the delithiated cathode can cause serious safety concerns. These limitations necessitate the development of new materials to resolve the above-mentioned difficulties.

Recently, we introduced unique materials, which have a core−shell (core with gradient shell) or full concentration gradient (FCG) structure in a particle level.[3] The structure is basically composed of a Ni-rich core that delivers high capacity and a Mn-rich shell that provides outstanding thermal stability. Our latest report included a new type of FCG material composed of long rod-shaped primary particles approximately 2.5 μm in length,[4] which demonstrated good electrochemical performance and excellent thermal properties due to lower grain boundary resistance ascribed to lower specific surface area contacting with the electrolyte during cycling. However, the Mn concentration could not be varied because of the difficulty of the rod-shaped particle synthesis.

Here, we report the design of FCG Li[NixCo0.16Mn0.84−x]O2 (x = 0.64, 0.59, and 0.51) cathode materials with fixed Mn contents of 20, 25, and 33% in the transition-metal layer. We also report the effects of Mn concentration in terms of structural, electrochemical, and thermal characteristics of the FCG materials.

Experimental

Spherical FCG [Ni_xCo_0.16Mn_0.84-x](OH)₂ (x = 0.51, 0.59, and 0.64) precursors were synthesized by a coprecipitation method. A Ni-poor aqueous solution (molar ratio of Ni:Co:Mn = 0.47:0.20:0.33, 0.55:0.20:0.25, 0.60:0.20:0.20,), consisting of NiSO₄·6H₂O, CoSO₄·7H₂O, and MnSO₄·5H₂O as starting materials, was slowly pumped from tank 2 into a Ni-rich (molar ratio of Ni:Mn = 0.67:0.33, 0.75:0.25, 0.80:0.20) 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. Simultaneously, a 3.0 mol L^-1 NaOH solution (aq.) and the desired amount of a NH₄OH chelating agent solution (aq.) were pumped separately into the reactor. The concentration of the solution, pH, temperature, and stirring speed of the mixture in the reactor were carefully controlled. During the early stage of the coprecipitation process, Ni-rich hydroxide (center composition) first precipitated. Nickel-cobalt-manganese hydroxide of different compositions then gradually layered onto the initially formed [Ni_xCo_0.16Mn_0.84−x](OH)₂ (x = 0.51, 0.59, and 0.64) particles, resulting in a linear composition change of Ni and Co toward the outer surface of the particles. Precursor powders were obtained through filtering, washing, and drying at 100 °C overnight. The obtained FCG [Ni_xCo_0.16Mn_0.84−x](OH)₂ (x = 0.51, 0.59, and 0.64) was mixed with LiOH·H₂O, and the mixture was calcined at various temperatures for 10 h in air: 780 °C for Li[Ni_0.64Co_0.16Mn_0.20]O₂, 845 °C for Li[Ni_0.59Co_0.16Mn_0.25]O₂, and 920 °C for Li[Ni_0.51Co_0.16Mn_0.33]O₂.

The chemical compositions of the resulting powders were determined by atomic absorption spectroscopy (AAS, Vario 6, Analyticjena). Line scans for the full-gradient materials were obtained with an electron probe X-ray microanalyzer (EPMA, JXA-8100, JEOL).

Results and discussion

To judge the relative merits of the FCG Li[Ni_xCo_0.16Mn_0.84−x]O₂ cathodes, coin-type half-cells were tested at 55 °C and C-rates of 0.1 and 0.5 (20.6 and 102.8 mA g⁻¹) with a cutoff voltage of 4.3 V, and the results are presented in Figure 1. The trend observed at 25 °C was similarly found for cycling at the elevated temperature. The Li[Ni_0.64Co_0.16Mn_0.20]O₂ cell again

delivered the highest initial capacity of 201.7 mAh g⁻¹ and exhibited the worst capacity retention, whereas the Li[Ni_0.51Co_0.16Mn_0.33]O2 cathode with the lowest discharge capacity of 184.8 mAh g⁻¹ best retained its initial capacity after 100 cycles. It is generally known that a high Mn content in Li[Ni_xCo_yMn_1−x−y]O₂ is detrimental to the rate capability of the Li-ion battery.

Figure 1. Initial charge-discharge curves of the FCG Li[Ni_xCo_0.16Mn_0.84-x]O₂ (x = 0.51, 0.59, and 0.64) (a) between 2.7 and 4.3 V at 55 °C (current density of 0.1 C-rate corresponds to 20.6 mA g^-1). Corresponding cycling performance of half-cells (b) between 2.7 and 4.3 V at 55 °C by applying a constant current of 0.5-C rate (102.8 mA g^-1).