Formation of size-dependent and conductive phase on lithium iron phosphate during carbon coating

Carbon coating is a commonly employed technique for improving the conductivity of active materials in lithium ion batteries. The carbon coating process involves pyrolysis of organic substance on lithium iron phosphate particles at elevated temperature to create a highly reducing atmosphere. This may trigger the formation of secondary phases in the active materials. Here, we observe a conductive phase during the carbon coating process of lithium iron phosphate and the phase content is size, temperature, and annealing atmosphere dependent. The formation of this phase is related to the reducing capability of the carbon coating process. This finding can guide us to control the phase composition of carbon-coated lithium iron phosphate and to tune its quality during the manufacturing process.