Spray pyrolysis synthesis of mesoporous lithium-nickel-manganese-oxides for high energy Li-ion batteries

Uniform, spherical powders consisting of nanostructured Li-excess Li( _1+x)Ni _0.25Mn _0.75O _(2.25+x/2) (0.375 ≤ x ≤ 0.5) were synthesized via a spray pyrolysis approach using mixed nitrate precursors. Among the materials produced, the layered Li _1.5Ni _0.25Mn _0.75O _2.5 (R3m) attained a capacity of over 250 mAhg ^-1, which is comparable to that of the Li-excess cathode materials produced via coprecipitation. Li content was varied and found to affect the crystal structure, morphology and electrochemical properties of the materials, and the sintering temperature increased with Li content. Li ₂MnO ₃-type structures were observed on XRD diffraction patterns at high annealing temperatures for x ≤ 0.5 due to a phase transformation. The Li ₂MnO ₃-type structures degraded the electrochemical performance of these materials due to the high kinetic barrier for Li transportation. In particular, Li _1.5Ni _0.25Mn _0.75O _2.5 annealed at 800°C and Li _1.375Ni _0.25Mn _0.75O _2.4375 annealed at 700°C predominately adopted a layered composite structure. Nano-domains (5∼10 nm) of spinel structure were observed by HR-TEM to be structurally integrated with the layered structure for Li _1.375Ni _0.25Mn _0.75O _2.4375 annealed at 700°C. Nonetheless, the layered Li _1.5Ni _0.25Mn _0.75O _2.5 materials showed the best performance in terms of capacity, cycleability and rate-capability among these materials. The superior rate capability of these materials is aided by the nano-structured morphology, which consists of nano-sized primary particles and highly porous secondary particles.