The electrochemical performance of LiFePO 4 /C composites in lithium cells is closely correlated to pressed pellet conductivities measured by AC impedance methods. These composite conductivities are a strong function not only of the amount of carbon, but of its structure and distribution. Ideally, the amount of carbon in composites should be minimal (less than about 2 wt. %) so as not to decrease the energy density unduly. This is particularly important for plug-in hybrid electric vehicle applications (PHEVs) where both high power and moderate energy density are required. Optimization of the carbon structure, particularly the sp 2 /sp 3 and D/G (disordered/graphene) ratios, improves the electronic conductivity while minimizing the carbon amount. Manipulation of the carbon structure can be achieved via the use of synthetic additives including iron-containing graphitization catalysts. Additionally, combustion synthesis techniques allow co-synthesis of LiFePO 4 and carbon fibers or nanotubes, which can act as "nanowires" for the conduction of current during cell operation.
To increase the power density of battery materials, without significantly affecting their main advantage of a high energy density, novel material architectures need to be developed. Using the example of LiFePO 4 , we demonstrate a simple, sol-gel-based route that leads to large (up to 20 µm) primary LiFePO 4 particles, each of which contains hierarchically organized pores in the meso and macro range. As the pores are formed due to vigorous gas evolution (mainly CO and CO 2 ) during degradation of a citrate precursor, they are perfectly interconnected within each particle. Elementary carbon, the other citrate-degradation product, is deposited on the walls of emerging pores. The superposition of a continuous 1-2 nm thick carbon film (electron conductor) on pores (ion conductor when filled with electrolyte) represents a unique architecture in which the electrons and ions are simultaneously supplied to the site of insertion in the particle interior. The material can operate at current rates up to 50 C while preserving a high tap density of ca. 1.9 g cm -3 .
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