A soft chemistry approach to conducting polymer coating of LiFePO4

Lithium-ion batteries have an important role to play in reducing atmospheric pollution, by enabling the use of clean energies like solar, hydro, and wind for transportation. Numerous chemistries have been or are being developed for lithium batteries. Particularly interesting for the cathode is olivine-LiFePO4[1] due to its environmentally friendly and inexpensive constituents, iron and phosphate. However, the use of LiFePO4 as a cathode requires that its poor electronic conductivity be overcome.[1-2] Several schemes have been put forth to circumvent this drawback such as metal ion doping of the structure with foreign metal ions[3], however the most common scheme remains coating with carbon. Coatings are commonly formed by mixing an organic precursor with preformed LiFePO4 before a heat treatment at high temperature (500-700 oC) in an inert or reducing atmosphere.[4] The decomposition of the organic constituent leads, in addition to formation of carbon, to the formation of volatile organic compounds (VOC’s), CO and CO2, which pose environmental problems.[5] More critical for battery applications is however that irregular coating of LiFePO4 can lead to poor connectivity of the particles and hence performance loss.[6] It would therefore be an important improvement to the current LiFePO4 system if low temperature methods could be found to coat LiFePO4 uniformly without the formation of VOC’s, CO or CO2. Previously, it has been shown that conducting polymers, including redox polymers[7] can have a positive effect on the performance of LiFePO4[8] and other cathode materials such as Li1.03Mn1.97O4[9] and LiCoO2[10]. Several means have been used to make polymer/LiFePO4 composites, including electropolymerization from a suspension of LiFePO4 particles[8d], polymerization using a chemical oxidant in the presence of the particle[8b] or, more recently, formation of a colloidal suspension of the polymer immediately before the introduction of the LiFePO4 particles[11]. Herein, we present a methodology that significantly improves the fabrication and use of conducting polymer/LiFePO4 composites. First, the method relies on the intrinsic oxidation power of Li(1x)FePO4 rather than an external oxidant as the driving force of the polymerization process. This eliminates the risk of residual oxidant or oxidant by-products leaching from the polymer into the battery electrolyte, which would wreak havoc on the anode electrode process. Second, the propagation of polymerization requires the reinsertion of lithium into lithium iron phosphate, as well as the transport Li+ ions and electrons through the excising polymer coating. In turn, these are also the functionality characteristics of an effective conducting coating for LiFePO4. As such, the propagation reaction intrinsically favours the functionality of the final product. Moreover, compared to the classical carbon coating technology, this approach is devoid of high temperature processing and VOC’s, CO and CO2 formation. Third, an environmentally benign process based on H2O2 is used to form Li(1-x)FePO4 from the standard olivineLiFePO4. Finally, the conducting polymer/LiFePO4 composite made by our method can be used directly in a “no-carbon-added” cathode. The first processing step is delithiation of LiFePO4. Several oxidants are known to delithiate LiFePO4 such as nitronium [1,12] and Br2[13]. However, these cannot generally be considered environmentally benign. Instead inexpensive hydrogen peroxide is used here because its degradation product is water. Importantly, it has previously been shown that LiFePO4 is stable in water.[14] The first reaction step is therefore