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Abstract
It is reported that olivine-type lithium iron phosphate (LFP) for Li-ion batteries is one of the most widely utilized cathode materials, but its high-power applications are limited due to its intrinsically poor ion transfer rate and conductivity. Therefore, it is highly desired to fabricate LFP Li-ion capacitors (LICs) with high power performance and excellent cyclic reversibility, especially in safe, low cost and environmentally friendly aqueous electrolytes. Here, we fabricate LFP/expanded graphite (EG) nanocomposites by a one-step process, in which polyethylene glycol (PEG) is used as the particle growth inhibitor combined with vacuum infiltration of the LFP precursor into EG as a conductive sub-phase, and further investigate their high-power performance in aqueous LICs. Embedding spherical LFP nanoparticles with well-controlled size and agglomeration into the pores of EG and wrapping LFP nanoparticles by EG films contribute to the rapid electron and ion diffusion in LFP/EG composites, resulting in excellent cyclic reversibility and rate performance of LFP/EG composites. The aqueous LFP/EG//active carbon (AC) LICs were assembled in LiNO3 electrolytes with LFP/EG composites and AC as the positive and negative electrodes, respectively. The optimal LIC shows a power density of 2367.9 W kg−1 at an energy density of 6.5 W h kg−1, dramatically favorable rate characteristics and excellent cycle life with 82.1% capacitance retention of its primary capacitance at 2 A g−1 after 6000 cycles, markedly higher than those of the commercial LFP LIC. The presented aqueous LFP/EG//AC LICs with excellent electrochemical performance are expected to have broad high-power appliances that are cost-sensitive and highly secure.
Highlights
With the high-speed development of the world economy, environmental contamination is increasing and fossil fuel is being consumed in large quantities
From the point of application, it is highly desired that LFP/ expanded graphite (EG) composites with small particle size and high electrical conductivity can be fabricated by a simple one-step method and it will greatly contribute to high power performance of LFP Li-ion capacitors (LICs)
A new diffraction peak at 2q 1⁄4 26.6, attributing to the (002) peak of EG, occurs in the LFP/EG composites (S1 and S3) compared with standard LFP. These demonstrate that LFP/EG composites have been successfully prepared and the introduction of EG has no effect on the structure of LFP phase
Summary
Paper display higher power density than LIBs. At present, the organic electrolyte-based LICs have been investigated extensively, while the aqueous electrolyte-based LICs have received a little attention. From the point of application, it is highly desired that LFP/ EG composites with small particle size and high electrical conductivity can be fabricated by a simple one-step method and it will greatly contribute to high power performance of LFP LICs. The assembly process of LICs is crucial for achieving signi cant power and energy density.[32,33] In the process of the charge or discharge, the amount of electrical charge stored or released in the negative and positive electrodes is equal. For the assembled aqueous LFP/EG//active carbon (AC) LICs with LiNO3 electrolytes, through adjusting the mass ratio of the positive and negative electrodes, the cut-off voltage and the electrolyte concentration, we obtain the optimal LFP/EG//AC LIC with a power density of 2367.9 W kgÀ1 at an energy density of 6.5 W h kgÀ1, dramatically good rate characteristics and high cycle life with 82.1% capacitance retention of its primary capacitance at 2 A gÀ1 a er 6000 cycles, markedly higher than the commercial LFP//AC LIC. Our results illustrate a large prospect of our one-step method and aqueous LFP LICs, which are applicable for high-power situations that require low cost and high security
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