The transformation from LiNi1-x-yCoxMnyO2 (NCM) cathodes to Co-free LiNi1-xMnxO2 (NM) cathodes is considered as an effective solution for the electric vehicle (EV) industry to deal with the high cost of cobalt. However, severe Li/Ni disorder, structural instability and poor cycling stability are the main obstacles to their practical application. Al doping has proven to be an effective method to improve the electrochemical performance of Ni-rich NCMs. However, with regard to Ni-rich Co-free NM cathodes, the influence of Al doping on the structural stability and electrochemical performance of NM cathodes is still not clear. In this work, Al doped LiNi0.8Mn0.2-xAlxO2 cathodes are designed and their structural stability and electrochemical performance are investigated by a combination of XRD, SEM, TEM, CV, GITT, cycling testing and EIS techniques. As a result, Al doping can effectively inhibit Li/Ni disorder and improve the structural and thermal stability. In detail, 5% is the optimal doping amount for LiNi0.8Mn0.2O2 cathodes to obtain the best electrochemical performance and the LiNi0.8Mn0.15Al0.05O2 cathode shows an excellent capacity retention of 91.97% after 300 cycles at 3.0-4.3 V. This work provides an effective strategy for the development of Ni-rich Co-free NM cathodes.