Design And Synthesis Of Nano LiFePO₄/C Hybrid Materials And Their Studies In Lithium Storage

Rechargeable lithium-ion batteries(LIBs), as an advanced energy storage technology, have rapidly conquered the consumer market of portable electronics, and now stand as a very promising candidate for the upcoming high power and long-life battery applications such as hybrid or plug-in electric vehicles. As the most crucial component in LIBs systems, the positive electrode almost governs the development of LIBs in all respects involving capacity, operating voltage, cost, and more importantly safety. In recent decades, olivine-type family LiMPO4(where M=Fe, Mn, Co, and Ni) has attracted a tremendous level of attention as alternative Li-storage cathodes for LIBs, of which LiFePO4 is of particular interest and now taken as potential cathode candidate for the large-scale battery applications(e.g. transportation, storage power station, etc.), depending on the appealing features of its own such as low price, non-toxicity, proper operating voltage(3.45 V vs. Li/Li+), strong overcharge tolerance, intrinsic thermal safety, and minor bulk volume expansion(6.8 %) upon cycling. Unfortunately, the intrinsic sluggish mass and charge transport enormously restrain their further applications in high power battery field. Current schemes for ameliorating the issues mainly focus on dimension reduction, conductor coating and ions doping, although the later still remains controversial. Size tailoring in conjunction with conductive surface modification, typically with carbon materials, is considered as the technology of choice. With respect to the methods of such kind currently available, the procedural complexity and/or economic feasibility may conflict with the modern industry expectations.To begin with, we present our detailed work on a novel strategy for the lithiation of amorphous hydrated FePO4, typically a Fe PO4/polyaniline(FePO4/PANI) nano hybrid, by a facile H+/Li+ ion exchange reaction proceeding in nonaqueous medium. The mechanism has been attentively deduced and studied with the help of several chemical/physical analytical techniques. Briefly speaking, the protons belonging to the –OH and/or –OH2 groups of amorphous FePO4·xH2O, under the specified non-aqueous condition, favor the exchange of Li+ ions. The resulting Li-derivative(Li-FePO4/PANI) is proved to be a desirable precursor for fabricating LiFePO4/C nanomaterial with ideal structural features containing highly crystalline LiFePO4 nanoparticles completely coated with N-doped conductive carbon. More importantly, the LiFePO4/C nanomaterial is capable of offering excellent rate capability. For instance, even at super high rates of 60 C and 100 C(1 C=170 mA g-1), the reversible capacities could also reach as high as 95.2 mAh g-1 and 80.3 mAh g-1, respectively. Moreover, Li FePO4/C nanomaterial features outstanding capacity retention, with merely less than 3 % discharging capacity loss over 600 cycles at 10 C. The superior galvanostatic charge-discharge characteristics were well supported by the results of cyclic voltammetry(CV) and electrochemical impedance spectroscopy(EIS) tests.Secondly, we further develop a general and promising methodology to yield LiFePO4/C nanomaterials with appealing structural and lithium-storage properties by ingeniously combining polymer-rich FePO4·x H2 O, typically a proper nano-sized FePO4·x H2O/poly(furfuryl alcohol)(FePO4/PFA) composite, with the rapid H+/Li+ ion exchange reaction introduced above, followed by direct calcinations. The whole synthesis dispenses with any process of additional carbon coating or tedious grinding. The LiFePO4/C created by such syntheses structurally consists of a Fe·Li defect-free LiFePO4 nanocrystal “core” fully coated by a thin semi-graphitized carbon “shell”(1.6-2.0 nm) with overall particle sizes in a range of 20-50 nm, capable of sustaining a super-high charge/discharge rate of up to 100 C(discharge capacity of 109.6 mAh g-1) and 150 C(discharge capacity of 95.4 mAh g-1), and taking on an extremely good long-term cycling stability over 1000 cycles at 10 C rate(less than 2 % capacity loss). The superior charge/discharge properties are strongly supported by the kinetic analyses(e.g. interfacial kinetics, bulk lithium ion diffusion, etc) based upon CV and EIS. On the top of that, the material can afford a high power density(~64.5 kW kg-1) comparable to those in supercapacitors, yet with energy density(~261.3 Wh kg-1) almost one order of magnitude higher.Finally, we design and propose a bottom-up synthetic strategy for the synthesis of graphene-modified LiFePO4/C(LiFePO4@C/rGO) hierarchical microspheres by a one-pot mixed-solvothermal process(precursor of LiFePO4OH@RF/GO) followed by direct calcinations, using a ferric iron salt as raw material. The whole synthesis dispenses with any process of additional carbon coating or tedious grinding. The LiFePO4@C/rGO hierarchical microspheres created by such syntheses show a quite uniform distribution of particle size with the average diameter of 2-3 μm and structurally consist of LiFePO4@C nanocrystals with mean particle size of 65 nm, on which a large number of ultrathin nanosheets associated with rGO nanosheets can be clearly visualized. It is interesting to note that partial rGO nanosheet is embedded into the interor of the microsphere, while the rest wraps around the outer surface. Distributing the rGO in such a manner would provide an integrated and robust conductive network for LiFePO4@C microspheres from the interior to the outside and also interparticles. Benefiting from this unique hierarchical architecture, these microspheres can be densely packed together, giving a relatively high tap density of ~1.3 g cm-3, and simultaneously, the primary LiFePO4@C nanoparticles can provide proper electrochemically available surface for enhancing the rate capability of the lithium ion insertion/deinsertion reaction. Typically, Li FePO4@C/rGO can afford discharge capacities of 86.5 and 60.8 mAh g-1 when testing at high rates of 30 C and 60 C, respectively. Besides, the total capacity loss is merely less than 5 % when cycling for 300 cycles at 2 C rate followed by another 700 cycles at 10 C rate.