High rate capability lithium iron phosphate wired by carbon nanotubes and galvanostatic transformed to graphitic carbon

Abstract

Lithium iron phosphate (LiFePO₄) electronically wired by multi-walled carbon nanotubes (MWCNTs) and in-situ transformed graphitic carbon for lithium-ion batteries are discussed here. Presence of MWCNTs up to a maximum of 0.5% in porous LiFePO₄ (abbreviated as LFP-CNT) resulted in remarkable reversible cyclability and rate capability compared to LFP coated with highly disordered carbon (abbreviated as LFP-C). In the current range (30-1500) mAg⁻¹, specific capacity of LFP-CNT (≈150-50 mAhg⁻¹) is observed to be always higher compared to LFP-C (≈120-0 mAhg⁻¹). At higher currents of 250-1500 mAg⁻¹ LFP-C performed poorly compared to LFP-CNT. LFP-C showed considerable decay in capacity with increase in cycle number at intermediate high currents (≈250 mAg⁻¹) whereas at very high currents (≈750 mAg⁻¹) it is nearly zero. The LFP-CNT showed no such detrimental behavior in battery performance. The exemplary performance of the LFP-CNT is attributed to combination of both enhanced LFP structural stability, as revealed by Raman spectra and formation of an efficient percolative network of carbon nanotubes which during the course of galvanostatic cycling gets gradually transformed to graphitic carbon.