Abstract: Currently, the low cost and stable performance of cobalt-free nickel manganese cathode materials is a research hotspot. This study prepared a Ni_0.8Mn_0.2(OH)₂ precursor through a coprecipitation method, used ammonia as a complexing agent and explored the effect of NH₃ concentration on the grain growth and morphology of the coprecipitated Ni_0.8Mn_0.2(OH)₂ precursor, as well as its effect on the crystal structure and electrochemical properties of LiNi_0.8Mn_0.2O₂ of the cathode material for lithium ion batteries. The structure, morphology and electrochemical properties of the materials were characterized by X-ray diffraction, scanning electron microscopy, cyclic voltammetry, AC impedance and battery charge discharge tests. The characterization results showed that at 0.1 C rate with 2.5~4.2 V the initial specific discharge capacity was 167 mAh/g with the charge-discharge efficiency 96%. When the amount of ammonia was 45 mL, the sample had the best cycle performance. At a 1 C rate of 2.5~4.2 V, after 100 cycles, the specific discharge capacity was 139mAh/g with the capacity retention 93.9%. The samples had significantly better electrochemical properties than other materials when the charge and discharge rate was low.