A novel co-precipitation method for carbon-free LiFePO4 and investigation into potential LiFEPO4-C cathode materials for lithium-ion batteries

Abstract

Lithium iron phosphate (LiFePO4) is a promising cathode material for lithium-ion batteries. In the past few years many improvements have led to consistent cycling capabilities, even at high rates. LiFePO4 is being commercialized as a cathode material in batteries for power tools, and is a serious candidate for the future batteries of hybrid-electric or electric vehicles. It can also be commercialized for other applications requiring a low-cost and safe battery, but its low intrinsic electrical conductivity and low Li-ion diffusion are two major disadvantages. Many groups have shown that battery performance can be enhanced by addition of carbon, during synthesis or post-synthesis carbon coating through various techniques to improve electrical conductivity. Simplification or even minimization of carbon-coating methods is one area of improvement which could help to reduce cost and increase efficiency. These carbon additives can cause multiple effects on purity, crystallinity and the electrochemical performance of the final cathode material (LiFePO4) and therefore makes it difficult to optimise the quantity and specific type of carbon that needs to be added during the synthesis of LiFePO4. All synthetic procedures reported in the literature, however, show that carbon is always present in some form in the final product. In this thesis study, in order to evaluate the effect of various carbon additives unambiguously, a novel one-step co-precipitation method was developed for synthesis of carbon-free LiFePO4. A series of LiFePO4/Carbon composites were prepared where the composites were synthesised at 550, 650 and 750C containing 5, 10 or 20 wt% carbons. Two forms of carbon additives were tested; single wall carbon nanotubes (SWCNT) and carbon black (CB). These carbons were added at one of two different stages; (1) during pre-synthesis, mixed with the LiFePO4 precursors, or (2) in post-synthesis, during the electrode preparation. This approach helped to investigate the effect that the carbon type, carbon content, mode of mixing (pre synthesis or post synthesis) and temperature have on the electrochemical performance of the active component. The topic of electron conductivity and Li-ion diffusion LiFePO4 is also very relevant, especially since this material is now touted as an important high-rate capability cathode. To investigate these effects, cyclic voltammetry, charge-discharge and electrochemical impedance spectroscopy measurements were performed. It was found that the cell discharge capacity, rate capacity and electronic conductivity of the electrode depended on the type of carbon used. The use of a 5 wt. % loading of SWCNTs as conductive additive to LiFePO4 composites prepared at 750C was found to improve the electrochemical performance of cells compared to cells containing CB additives. The LiFePO4 with 5 wt. % SWCNTs mixed pre-synthesis and then synthesised at 750C demonstrates a smaller resistance to charge-transfer (RCT = 59O) and good kinetic behaviour (2.9 x 10-8 cm2/s), and has the highest specific capacity (93 mAh/g and 48 mAh/g at C/20 and C/5 respectively) than any other sample except for the one with 10 wt% SWCNT. The latter demonstrates slightly improved specific capacity at C/20 (94 mAh/g) and better Li-ion kinetic behaviour (3.3 x 10-7 cm2/s) but a worse specific capacity at C/5 (46 mAh/g), probably because the charge-transfer resistance is significantly higher (RCT = 239 O). Therefore, the optimisation of cell performance involves optimisation of Li-ion and electron transport and the charge transfer at the electrode/electrolyte interface. Therefore, it is important to note that the material synthesised according to the novel, single-step, co-precipitation procedure described in this thesis can be applied to many other LiFePO4-carbon composite cathode materials, to compare and evaluate the effect of various carbon additives on the electrochemical performance of cathode materials.

Date of Award	2015
Original language	English