Since Padhi et al. demonstrated the reversible electrochemical lithium insertion–extraction for LiFePO4 in 1997 [1], this compound has been attracting much research and development interest. LiFePO4, with a theoretical capacity of ~170 mAhg−1 and a flat charge/discharge potential at 3.45V vs. Li+/Li [1-3], is considered as one of the most promising cathode materials for lithium ion batteries owing to its various advantages over other cathode materials such as LiCoO2. These advantages include high energy density, good thermal stability and having abundant raw materials, cheap, nontoxic, non-hygroscopic and environmentally benign [1, 4-7]. Li ion batteries with LiFePO4 as cathode materials also have great potential as power sources for electric vehicles (EV), hybrid electric vehicles (HEV), etc.

However, pristine LiFePO4 has the disadvantage of poor rate performances due to its low electrical conductivity  [8] and low lithium ion mobility. In order to meet the requirement of energy storage devices that can perform both high-power and high-energy operation which are needed for systems by which the power generation is distributed and for electric vehicles, considerable efforts have been made to increase LiFePO4’s electrical conductivity. 

Efforts through which LiFePO4’s electrical conductivity was increased to as high as 4.8×10−2 Scm−1 [9, 10] include carbon coating [11], metal-rich phosphide nanonetworking[12], or high-valence ion doping [9,13]. Chung et al. proposed that LiFePO4 doped with high-valence metal ions could improve its electronic conductivity as high as eight orders of magnitude [9, 10]. Shi et al. not only proved the feasibility of Cr3+ ion doping LiFePO4 in theory, but also synthesized the sample of Li1-3xCrxFePO4 and confirmed an enhancement of the electronic conductivity up to eight orders of magnitude comparing with pure LiFePO4 with x=0.01 and 0.03 [14, 15]. Wen et al. also investigated the structure and properties of LiFe0.9V0.1PO4 and indicated that its cathode properties, including reversible capacity, cycle number and charge–discharge characteristics, were better than those of pure LiFePO4 [16]. Sun et al. prepared single-phase vanadium-doped LiFePO4 and studied its structural characteristics and electrochemical Li+ intercalation performances [8]. It is a comparatively effective method to improve electrochemical property of LiFePO4 by high-valent ion doping, because the addition of carbon will decrease the practical density of cathode material and volumetric capacity of cell is difficult to increase. 

Means explored to enhance LiFePO4’s Li+ ion diffusion include reducing its particle size [17] and cation (Ni, Co or Mg) doping at Fe-site [18]. Rho et al. proposed that sub-µm or nanoscale LiFePO4 particles minimized the path length for Li+ transport [17]. Wang et al. [18] reported that the rate capability and cyclic stability of LiFePO4 were greatly enhanced by bivalent cation (Ni, Co or Mg) doping at Fe-site. Under a high rate of 10 C at room temperature, the specific capacity of LiFe0.9Co0.1PO4 maintained at 90.4 mAhg−1 [13]. Fe-site doping increased the ionic mobility and diffusion coefficient probably by weakening Li-O interaction [8]. 

Even though a lot of efforts have been taken, the increased electrical conductivity and lithium ion motion ability did not result in the improvement of the rate performances of LiFePO4 as expected, and there are still drawbacks for the use of LiFePO4 as a commercial cathode material. This mainly results from four reasons. First, the particle size of LiFePO4 powders is still not small enough. Second, it is not easy to dope ion uniformly in the solid phase, which may limit the amount of doping [20]. Third, it is difficult to synthesize LiFePO4 because of iron oxidation state [21]. Fourth, recent evidence indicates that Li+ ions can only move into the bulk of the crystal in the [010] direction [23]. Recently, Lim et al. prepared nanowire and hollow LiFePO4 as cathodes for high-performance lithium batteries using hard-templates [19]. One-dimensional nanostructured electrode showed unique rate capabilities, because the distance that Li+ must diffuse is restricted to the radius direction, which may be as small as several to tens of nanometers. It is significantly smaller than that in the usual powder electrode [22]. Yang et al. put forward an improved co-precipitation route to prepare Cu2+ doped LiFePO4 [20]. In this method, olivine LiFePO4 was gained by the precursor which was prepared by co-precipitation sintered in the N2, in which inexpensive and stable Fe3+ compound was used as raw material and inert gas was not used during the processes of co-precipitation. This is an economical way for production and is in favor of industrialization. Moreover, by using this method it is easy to dope ion uniformly in LiFePO4 in the liquid state. Kang et al. obtained LiFePO4 particles with an amorphous structure on the surface by a solid state method, and increased lithium ion diffusion across the surface towards the (010) facet [23]. As a result, the LiFePO4’s specific power is two orders of magnitude higher than typical power rates for lithium ion battery materials. The amorphous nature of the coating removes the anisotropy of the surface properties and enhances delivery of Li+ to the (010) facet of LiFePO4 where it can be inserted.

All the advantages of these three methods which can obtain LiFePO4 with high energy density and high power density could be obtained by our electrical spinning method, and our method is expected do even better. We prepared very long, thin LiFePO4 nanowires and doped Co ions uniformly within the LiFePO4 nanowires because our wire precursors come from uniform solutions containing various ions. We also prepared LiFePO4 and carbon complex nanowires. These nanowires are expected to retain high capacities when charging and discharging under high currents and have excellent cycle lives.

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