Solid solution material Li1.2Ni0.16Co0.08Mn0.56O2 (0.5Li2MnO3∙0.5LiNi0.4Co0.2Mn0.4O2) is obtained through rheological phase method and further treated in ammonium persulfate solution. The post-treatment significantly decreases the charging capacity above 4.5 V and enhances the columbic efficiency in the initial cycle. Along with the higher efficiency, the cycling stability and the rate capability both get improved. The improvement mechanism is investigated in terms of XRD, XPS, Raman spectrometry, and ICP-AES. The results confirm that (NH4)2S2O8 treatment leads to Li+ removal from Li2MnO3 component while the layered structure of the solid solution phase is well maintained. After being treated in 30% (NH4)2S2O8 solution, 95% columbic efficiency is observed on Li1.2Ni0.16Co0.08Mn0.56O2 in the first cycle and it also shows a near 200 mAh g−1 capacity at 4C current rate.