Due to the structural stability of LiMPO4 resulting from covalent bonds therein, many attempts are made to develop LiMPO4 as advanced cathode active material for a lithium secondary battery. However, because LiMPO4 has very low conductivity, conductivity of LiMPO4 should be improved in order to commonly use it as electrode active material. Therefore, research and development are made intensively to improve the conductivity of LiMPO4.
Typically, two types of methods are used to improve the conductivity of LiMPO4. One method that is used generally includes a step of adding carbon during a mixing step preceding heat treatment in preparing LiMPO4. By doing so, carbon is coated on surfaces of active material particles formed of LiMPO4 to provide LiMPO4 having improved conductivity. Another method that is used recently includes substituting Li or M sites of LiMPO4 with a metal having a different oxidation number. JP 2002-117903 discloses an electrode active material comprising a compound represented by the formula of LixFe1-yMyPO4, wherein M is Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B or Nb; x is a number of between 0.05 and 1.2; and y is a number of between 0 and 0.8.