In this paper we have studied the magnetism of a new cathode material with a high energy density, Li(AlxCo1-x)O2(x=01—05), which is used in rechargeable lithium batteries. We found that doping Al can change the spin state of d electron in Co3+， leading to the emergence of high spin d electrons. Accompanied with this, there are also some changes of the structure development which is indicated by the obviously decreasing rate of the increasing value of c/a. This explains the positive deviation of the structure from the Vegard law. This is very important to the design of the microstructure and properties of materials.