Due to the fast reduction of fossil fuels and greater environmental pollution caused by automobiles, more attention has been focused on electric vehicles (EVs) or hybrid electric vehicles (HEVs). Lithium-ion batteries (LIBs) are the fundamental power source for the EVs or HEVs. The reversible capacity of the LIBs greatly appertains the cathode material. Olivine-type LiFePO4 is observed as one of the most important cathode materials because of its great theoretical capacity (170 mAh g-1), relatively inexpensive, nontoxicity, and its environmental friendliness 1. However, LiFePO4 generally represents an insignificant rate capability due to its low inherent electronic conductivity (~10-9 S cm-1) and slow lithium-ion di’usion (~10-18 Cm2 S-1) 2. To surmount these problems, a variety of solutions have been offered containing coating LiFePO4 particles with conductive substances 3, generating materials with tiny particles 4 and metal doping 5. Covering of the LiFePO4 facing with Cu, Ag, carbon, or conducting polymers can significantly improve the electrical conductivity along the surface of LiFePO4 particles 6. Carbon coating 7 or use of nanosized electrode materials 8 is really effective for improving the rate performance of LiFePO4, but the tap density falls dramatically when the carbon content increases and/or particle size drops. Thus, ion doping is commonly used for the modification of LiFePO4 without tap density reduction 9. Commutation of a little quantity of Li+, Fe2+ or O2- by other ions greatly increases the intrinsic conductivity and improves the kinetics of materials in terms of capacity delivery, cycle life and rate performance 10. The doping at the Li-site, namely Li1-xMxFePO4 (M= alien metal) or Fe-site, namely LiFe1-xMxPO4, resulting in two kinds of solid solutions 11. The doping at the Li-site is important to amend the electrochemical property of LiFePO4. It has been found that LiFePO4 has one-dimensional lithium ion diffusion path 12, 13, and the doped high valence transition metal ions at the Li site in LiFePO4 would close the one-dimensional diffusion path, which contributed to the poor electrochemical efficiency 14. The first-principles calculations prove that the electronic conductive features and Li ion diffusion velocity property of LiFePO4 can be amended by the Li-site doping with Na and desirable for high-rate performance 15. Yin et al. 16 prepared the olivine structured Li1-xNaxFePO4/C (x=0, 0.01, 0.03, 0.05) particles by in situ polymerization restriction carbon thermal reduction process. The Li0.97Na0.03FePO4/C powder represented the highest discharge capacity and cycle performance, which might ascribe to its larger lattice parameters than the others. The improved electrochemical activity, demonstrated by CV and EIS experiments, showed that Na ion doping would help to increase the electrochemical performance of LiFePO4/C. Moreover, the doping at the Fe-site, namely LiFe1-xMxPO4 (M=Ni, Co, Ti, Al, etc.), can efficiently stabilize the crystal structure and also increase the transfer of electrons and probably Li ion 17, 18. First-principle calculation demonstrates that doping at a Fe site with alkali metal ions helps the diffusion of Li+ ions along the one-dimensional path, which can enhance both the electronic and ionic conductivity 19. Wang et al. 20 synthesized Na+ and Cl- co-doped LiFePO4/C samples via a solid-state route. The Na+/Cl- co-doped LiFePO4/C composites showed specific capacities of 157 mAh g-1 at 0.2 C, 115 mAh g-1 at 10 C, and 98 mAh g-1 at 20 C, respectively. The betterment can attributed to the improved electronic conductivity and electrode kinetics due to the structural reformation boosted by co-doping. A better electrochemical performance reported for the Li0.97Na0.02K0.01FePO4/C composite among Li1-x-yNaxKyFePO4/C (0’x’0.03, 0’y’0.03, x+y=0.03) systems synthesized by sol gel method 21.
In this paper, Na and K co-doped LiFePO4/C samples with controlled Na and K sites, i.e., the Li1-x-yNaxKyFePO4/C and LiFe1-x-yNaxKyPO4/C (x=0.02, y=0.01) have been firstly synthesized via a common solid-state synthesis. The effects of the metal occupied site on the structure, morphology and electrochemical performance of LiFePO4/C are studied and discussed in particular. The results offer that the structure, polarization, reversible capacity, and cycle performance of the Na and K co-doped LiFePO4/C compounds very dependent to occupied sites by dopant. Besides, because of it is important to minimize the carbon content in LiFePO4/carbon hybrids to increase volumetric energy density and tap density, In this work only 3 wt.% carbon black was added in the cathode preparation procedure.