Two-step hydrothermal synthesis of submicron Li1+xNi0.5Mn1.5O4−δ for lithium-ion battery cathodes (x = 0.02, δ = 0.12)
A facile two-step hydrothermal method is developed for the large-scale preparation of lithium nickel manganese oxide spinel as a cathode material for lithium ion batteries. In the reaction, nickel is introduced in a first step at neutral pH, followed by lithium insertion under base to form a product having composition Li1.02Ni0.5Mn1.5O3.88. The X-ray diffraction pattern and Raman spectroscopy of the synthesized material support a cubic Fdm structure in which Ni and Mn are disordered on the 16d Wyckoff site, necessary for good cycling characteristics. XP spectroscopy and elemental analysis confirms that Mn remains reduced in the final product (ZMn = 3.82) and that two different chemical environments for Ni exist on the surface. SEM imaging shows a primary particle size of ∼200 nm, and galvanostatic cycling of the material vs. Li+/0 gives a reversible gravimetric capacity of ∼120 mA h g−1 at 1 C rate (147 mA g−1) with reversible cycling up to 1470 mA g−1, supported by rapid Li+ diffusion. The capacity fade at 1 C is substantial, 17.3% over the first 100 cycles between 3.4 and 5.0 V. However, when the voltage limits are altered, the capacity retention is excellent: nearly 100% when cycled either between 3.4 and 4.4 V (where oxygen vacancies are not electrochemically active) or 89% when cycled between 4.4 and 5.0 V (where the Jahn–Teller active Mn4+/3+ couple is not accessed).