Lithium Iron Phosphate/Carbon (LFP/C) Composite Using Nanocellulose as a Reducing Agent and Carbon Source

Abstract

Lithium iron phosphate (LiFePO₄, LFP) is the most promising cathode material for use in safe electric vehicles (EVs), due to its long cycle stability, low cost, and low toxicity, but it suffers from low conductivity and ion diffusion. In this work, we present a simple method to obtain LFP/carbon (LFP/C) composites with different types of NC: cellulose nanocrystal (CNC) and cellulose nanofiber (CNF). Microwave-assisted hydrothermal synthesis was used to obtain LFP with nanocellulose inside the vessel, and the final LFP/C composite was achieved by heating the mixture under a N₂ atmosphere. The resulting LFP/C indicated that the NC in the reaction medium not only acts as the reducing agent that aqueous iron solutions need (avoiding the use of other chemicals), but also as a stabiliser of the nanoparticles produced in the hydrothermal synthesis, obtaining fewer agglomerated particles compared to synthesis without NC. The sample with the best coating-and, therefore, the best electrochemical response-was the sample with 12.6% carbon derived from CNF in the composite instead of CNC, due to its homogeneous coating. The utilisation of CNF in the reaction medium could be a promising method to obtain LFP/C in a simple, rapid, and low-cost way, avoiding the waste of unnecessary chemicals.

Keywords: lithium iron phosphate (LFP); lithium-ion batteries; nanotechnology.

Figure 1

XRD diffractograms of LFP obtained with ethanol before and after the heat treatment (E samples) and with NC (N samples) to form LFP/C composites under different synthesis conditions: (A) 150 °C and 15 min; (B) 150 °C and 30 min; (C) 205 °C and 15 min; (D) 205 °C and 30 min.

Figure 2

FE–SEM images of LFP/C composites with NC (N samples): (A) N1; (B) N2; (C) N3; (D) N4; (E) N5; (F) N6; (G) N7; (H) N8; (I) N9; (J) N10; (K) N11; (L) N12; (M) N13; (N) N14; (O) N15; (P) N16.

Figure 3

(A) Raman spectra of LFP/C composites produced with 1% wt. CNC (red line) and 1% wt. CNF (blue line); LFP synthesised without NC is added for comparison (black line). (B) TGA–DSC analysis of CNC and CNF.

Figure 4

(A) XDR of both composites. (B) Cyclic voltammetry of LFP/C composites obtained with CNC and CNF at the same weight percentage of carbon (4 % wt.) in LFP/C, measured at 0.1 mV s⁻¹ between 2.8 and 4.1 V versus Li/Li⁺ in 1 mol L⁻¹ LiPF₆ with EC/DMC 1:1 v/v, using the coin cell CR–2032. (C) TEM images of LFP/C composites created with CNF (above) and CNC (below).

Figure 5

(A) Cyclic voltammetry of an LFP/C composite obtained with 12.6% wt. carbon derived from CNF and LFP synthesised without NC, measured at 0.1 mV s⁻¹ between 2.8 and 4.1 V versus Li/Li⁺ in 1 mol L⁻¹ LiPF₆ with EC/DMC 1:1 v/v; second cycle. (B) Charge–discharge curve at 0.1 C–rate between 2.8 and 4.1 V versus Li/Li⁺ using the coin cell CR–2032; second cycle.

Figure 6

TEM images of composites formed with CNF, at different size scales: (A) 500 nm; (B) 200 nm and (C) 100 nm.

Figure 7

(A) High-resolution XPS Fe 2p spectra of synthesised LFP and LFP/C. (B) XRD diffractograms of the samples, along with their respective pictures.

Figure 8

High–resolution XPS spectra of the LFP/C composite before cycling and after cycling for 10 cycles at 0.1 C–rate: (A) C 1s spectrum; (B) O 1s spectrum; (C) F 1s spectrum; (D) Fe 2p spectrum.