Elsevier

Electrochimica Acta

Volume 178, 1 October 2015, Pages 504-510
Electrochimica Acta

Synthesis of hierarchical conductive C/LiFePO4/carbon nanotubes composite with less antisite defects for high power lithium-ion batteries

https://doi.org/10.1016/j.electacta.2015.08.053Get rights and content

Abstract

The low electronic conductivity and Li ion diffusion ability are two major obstacles to realize its wide application for LiFePO4 materials. The material with hierarchical conductive structure and lower antisite defects concentration can effectively enhance the electronic conductivity and Li ion diffusion ability. We firstly report here a modified solvothemal process for the fabrication of hierarchical conductive C/LiFePO4/CNTs composite with less antisite defects. It is found that the modified solvothemal process is facilitated to decrease FeLi antisite defects and enhance the electronic continuity between LFP and CNTs. In favor of its unique properties, the C/LFP/CNTs composites can deliver superior rate capability and cycling stability. Remarkably, even at a high rate of 20C (3400 mA g−1), a high initial discharge capacity of 91.6 mAh g−1 and good cycle retention of 95% with almost 100% coulombic efficiency are still obtained after 100 cycles.

Graphical abstract

The hierarchical conductive C/LiFePO4/CNTs composite with less antisite defects is synthesized by a modified solvothemal process and delivers superior electrochemical performance with high rate capability and good capacity retention.

  1. Download : Download full-size image

Introduction

The high power Li-ion batteries have attracted increasing attention for its applications in hybrid electric vehicles (HEVs) or electric vehicles (EVs). As a cathode material for Li-ion batteries, olivine-type lithium iron phosphate (LiFePO4, or LFP) proposed by Padhi et al. is a promising candidate due to its low production cost, environmental compatibility, superior capacity retention, high thermal stability and safety [1], [2], [3]. However, its low electronic conductivity and Li ion diffusion ability lead to a high initial capacity loss and a poor rate capability, which retards its wide application in energy storage system [4]. So far, tremendous efforts have been reported to tackle the problems, including incorporating electrically conductive materials [5], [6], [7], doping with metal ion [8], [9], and controlling morphology [10], [11], [12].

Among all endeavors, the strategy to design hierarchical conductive structure is regarded as a more efficient way to improve the poor electronic conductivity of LFP [13], [14], [15], [16]. An electrode consisting of a carbon coating, a high-crystalline LFP layer and tridimensional (3D) conductive networks in the nanoscale is quite attractive for cathode materials to achieve excellent performance at a high charge-discharge rate. For fabricating an effective conductive network, carbon nanotubes (CNTs) are superior electron bridges for hybridization of active materials. It has been proven that incorporation of CNTs into different cathode materials can remarkably improve the electrochemical performance of the composites [17], [18], [19], [20], [21].

For the improvement of Li ion diffusion ability, the FeLi antisite defect in crystals is quite a critical factor because the partial occupation of Li sites by Fe atoms inevitably impedes ion transport by blocking the diffusion pathways [22], [23], [24], [25], [26], [27], [28], [29]. In particular, the computational [30], [31] and experimental [3] studies of LFP have demonstrated that lithium ions can diffuse only through a one dimensional tunnel in the crystal. Therefore, the Li ion diffusion of LFP is susceptibly influenced by the presence of FeLi antisite defects. Hydrothermal/solvothemal synthesis has been demonstrated one of the most effective route to synthesize LFP [15], [18], [32]. However, LFP synthesized by hydrothermal/solvothemal method at low temperatures inherently form FeLi antisite defects [22]. These intrinsic antisite defects will block the Li+ diffusion tunnels, and thus severely slash the electrochemical performance [24], [25], [26], [27], [28], [29], [30], [31]. Recently, Park et al. [28] reported that the types of defects and recombination mechanisms in LFP. The type of cation disorder can be an edge-shared Li/Fe defect or a corner-shared Li/Fe defect. Although researches showed that the FeLi defects in LFP can be recombined above 500 °C with heat treatment [33], the edge-shared defect would not recombine for having no apparent reordering route. Therefore, the hydrothermal/solvothemal synthesis is critically important to obtain final LFP with low antisite defects concentration. To date, the hydrothermal/solvothemal synthesis of LFP with low antisite defects remains challenges in a general platform.

Herein, with the purpose to improve both poor electronic conductivity and Li ion diffusion ability, we firstly propose a modified solvothemal process to fabricate the hierarchical conductive C/LFP/CNTs composites with low antisite defects concentration. The facile synthesis was keeping the precursor matured every 2 hours with the increase of every 10 °C for the heating process from 100 °C to 180 °C. We suggest that the maturing process is facilitated to decrease FeLi antisite defects and enhance the electronic continuity between LFP and CNTs. The esterification reaction followed by a calcination procedure was applied to further improve the conductivity of the LFP/CNTs composites. The synthesized composites are characterized with highly hierarchical conductive structure. In favor of the less antisite defects and hierarchical conductive structure, the C/LFP/CNTs composites can deliver superior rate capability with the discharge capacities of 163.6 mAh g−1, 157.1 mAh g−1, 150.5 mAh g−1, 136.4 mAh g−1 and 119 mAh g−1 at 0.5C, 1C, 2C, 5C and 10C, respectively. Remarkably, it still showed a high initial discharge capacity of 91.6 mAh g−1 and good cycle retention of 95% even at a high rate of 20C (3400 mA g−1) with almost 100% coulombic efficiency after 100 cycles.

Section snippets

Preparation of LFP/CNTs-S.

The standard LFP/CNTs (LFP/CNTs-S) were prepared by a regular solvothemal method as follows. FeSO4·7H2O, H3PO4, and LiOH·H2O were used as starting materials in a molar ratio of 1: 1: 3 and ethylene glycol (EG) was applied as solvent. In a typical procedure, 0.047 g CNTs (2 wt.%) was ultrasonically dispersed in 25 ml EG to obtain a homogeneous suspension. And then 0.045 mol of LiOH was dissolved in the above suspension. After stirring for a while, 0.015 mol of H3PO4 was slowly added to the LiOH/CNTs

Results and discussion

Fig. 1 shows the XRD patterns of the obtained C/LFP/CNTs composites, and their Rietveld refinement plots were shown in Fig. S1 Fig. S1. The crystal phases of samples are in accordance with the ordered olivine structure indexed orthorhombic Pnma (JCPDS Card No. 83-2092), and no extra reflection peak of impurity phase is observed. Besides, there is no evidence for amorphous carbon due to its low content.

To investigate the differences of the microstructure in two samples, Rietveld refinement of

Conclusions

In summary, we report a modified solvothemal process for the fabrication of hierarchical conductive C/LiFePO4/CNTs composite with less antisite defects. The synthesized composite reveals excellent electrochemical performance with high rate capability and good capacity retention. The prominent properties can be attributed to the less antisite defects and highly hierarchical conductive structure which can facilitate the fast Li ion diffusion and electron transfer during charge/discharge process.

References (36)

  • G. Shao et al.

    Study on the initial electrodeposition behavior of Ni–P alloys

    Mater. Chem. Phys.

    (2005)
  • A.K. Padhi et al.

    Phospho-Olivines as Positive-Electrode Materials for Rechargeable Lithium Batteries

    J. Electrochem. Soc.

    (1997)
  • J. Wang et al.

    Understanding and recent development of carbon coating on LiFePO4 cathode materials for lithium-ion batteries

    Energy Environ. Sci.

    (2012)
  • S. Nishimura et al.

    Experimental visualization of lithium diffusion in LixFePO4

    Nat. Mater

    (2008)
  • L.X. Yuan et al.

    Development and challenges of LiFePO4 cathode material for lithium-ion batteries

    Energy Environ. Sci.

    (2011)
  • C.R. Sides et al.

    A High-Rate, Nanocomposite LiFePO4/Carbon Cathode

    Electrochem. Solid-State. Lett.

    (2005)
  • Y.S. Hu et al.

    Improved Electrode Performance of Porous LiFePO4 Using RuO2 as an Oxidic Nanoscale Interconnect

    Adv. Mater.

    (2007)
  • I. Bilecka et al.

    Microwave-assisted solution synthesis of doped LiFePO4 with high specific charge and outstanding cycling performance

    J. Mater. Chem.

    (2011)
  • Cited by (21)

    • High-temperature solid-phase synthesis of lithium iron phosphate using polyethylene glycol grafted carbon nanotubes as the carbon source for rate-type lithium-ion batteries

      2022, Journal of Electroanalytical Chemistry
      Citation Excerpt :

      1580 cm−1 is the G peak, which originates from the E2g vibrational mode of the graphite plane and is a response to the orderliness of CNTs[32]. Meanwhile, the intensity ratio of the d-band to the G-band (ID/IG) is used to estimate the disorder of graphitic materials[33,34]. After grafting with PEG, the ID/IG of CNTs decreased from 0.93 to 0.73.

    • Hierarchically porous MXene decorated carbon coated LiFePO<inf>4</inf> as cathode material for high-performance lithium-ion batteries

      2021, Journal of Alloys and Compounds
      Citation Excerpt :

      The final products were washed with ethanol and deionized water several times to get rid of the remaining ions and then moved to an oven kept at 60 ℃ for 12 h to obtain LFP nanoplates. The LFP@C was synthesized with citric acid and ethylene glycol as carbon source followed by heat treatment [51]. Typically, 0.8 g LFP nanoplates were dispersed in 10 mL deionized water, followed by adding 0.29 g citric acid (C6H8O7·H2O) and 0.35 g ethylene glycol (EG), and then heated at 95 °C in thermostat water bath until the sample presented gelatinous.

    • B-axis oriented alignment of LiFePO<inf>4</inf> monocrystalline platelets by magnetic orientation for a high-performance lithium-ion battery

      2019, Solid State Ionics
      Citation Excerpt :

      One improvement in the high rate and cyclic performance of LiFePO4 would help to consolidate its advantages in power batteries, as well as energy storage [9] and step utilization [10,11]. Doping and coating with other elements [12–16], particle-size reduction [17–19] and morphological control [1,17] are commonly used to enhance the high rate and cyclic performance of LiFePO4 by shortening the diffusion path of ions in the solid phase [20,21]. In olivine-structured LiFePO4, lithium-ion (Li+) diffusion is confined to one-dimensional channels along the b axis [11,24,25].

    • LiFePO<inf>4</inf>/carbon hybrids with fast Li-ion solid transfer capability obtained by adjusting the superheat temperature

      2019, Journal of Alloys and Compounds
      Citation Excerpt :

      It can be usually found in the synthesized LFP materials through low temperature and one-step solvent method. The Fe–Li anti-site defect apparently blocks the Li+ diffusion path, greatly affecting the solid transfer rate of Li+ in LFP [35]. Fourier Transform infrared spectroscopy (FTIR) is an effective qualitative technology to detect the Fe–Li anti-site defect as the infrared absorption peak of P–O symmetric stretching vibration in PO4 tetrahedron normally appears at 957 cm−1 in the LFP material without any Fe–Li defect, while shifting to higher wavenumber area with the increasing of defect [1,5,10].

    View all citing articles on Scopus
    View full text