Guoxian Liang

LiFePO4–graphene as a superior cathode material for rechargeable lithium batteries: impact of stacked graphene and unfolded graphene

In this work, we describe the use of unfolded graphene as a three dimensional (3D) conducting network for LiFePO4 nanoparticle growth. Compared with stacked graphene, which has a wrinkled structure, the use of unfolded graphene enables better dispersion of LiFePO4 and restricts the LiFePO4 particle size at the nanoscale. More importantly, it allows each LiFePO4 particle to be attached to the conducting layer, which could greatly enhance the electronic conductivity, thereby realizing the full potential of the active materials. Based on its superior structure, after post-treatment for 12 hours, the LiFePO4–unfolded graphene nanocomposite achieved a discharge capacity of 166.2 mA h g−1 in the 1st cycle, which is 98% of the theoretical capacity (170 mA h g−1). The composite also displayed stable cycling behavior up to 100 cycles, whereas the LiFePO4–stacked graphene composite with a similar carbon content could deliver a discharge capacity of only 77 mA h g−1 in the 1st cycle. X-ray absorption near-edge spectroscopy (XANES) provided spectroscopic understanding of the crystallinity of LiFePO4 and chemical bonding between LiFePO4 and unfolded graphene.

Hierarchically porous LiFePO4/nitrogen-doped carbon nanotubes composite as a cathode for lithium ion batteries

A porous composite of LiFePO4/nitrogen-doped carbon nanotubes (N-CNTs) with hierarchical structure was prepared by a sol–gel method without templates or surfactants. Highly conductive and uniformly dispersed N-CNTs incorporated into three dimensional interlaced porous LiFePO4 can facilitate the electronic and lithium ion diffusion rate. The LiFePO4/N-CNTs composites deliver a reversible discharge capacity of 138 mA h g−1 at a current density of 17 mA g−1 while the LiFePO4/CNTs composites only deliver 113 mA h g−1, demonstrating N-CNTs modified composites can act as a promising cathode for high-performance lithium-ion batteries.

Surface aging at olivine LiFePO4: a direct visual observation of iron dissolution and the protection role of nano-carbon coating

LiFePO4 has attracted much attention as a potential cathode material for advanced lithium-ion batteries due to its superior thermal stability. In spite of this, LiFePO4 still suffers from fast capacity fading at high temperature and/or moisture-contaminated electrolyte. The influence of moisture and the detailed corrosion mechanism is still not clear. Here, for the first time, we present a direct visual observation of the surface corrosion process at olivine LiFePO4 stored in moisture-contaminated electrolyte, and found the direct relationship between iron dissolution and LiFePO4 corrosion. By using the LiFePO4 ingot sample with a flat surface as model materials, iron dissolution and surface chemistry change can be clearly observed and identified by field-emission scanning electron microscopy (SEM), time-of-flight-secondary ion mass spectroscopy (TOF-SIMS), and X-ray absorption near-edge structure (XANES). These iron dissolutions at some corrosion sites evoked the overall LiFePO4 surface corrosion. A significant improvement of the surface stability of LiFePO4 was obtained by nano-carbon coating, and the carbon surface layer protects LiFePO4 from direct contact with corrosive medium, effectively restraining the surface corrosion and preserving the initial surface chemistry of LiFePO4.

In situ self-catalyzed formation of core–shell LiFePO4@CNT nanowires for high rate performance lithium-ion batteries

In situ self-catalyzed core–shell LiFePO4@CNT nanowires can be fabricated by a two-step synthesis, where one-dimensional LiFePO4 nanowires with a diameter of 20–30 nm were encapsulated into CNTs, and 3D conducting networks of CNTs were obtained from in situ carbonization of a polymer. LiFePO4@CNT nanowires deliver a capacity of 160 mA h g−1 at 17 mA g−1, and 65 mA h g−1 at 8500 mA g−1 (50 C, 1.2 minutes for charging and 1.2 minutes for discharging).

Soft X-ray XANES studies of various phases related to LiFePO4 based cathode materials

LiFePO4 has been a promising cathode material for rechargeable lithium ion batteries. Different secondary or impurity phases, forming during either synthesis or subsequent redox process under normal operating conditions, can have a significant impact on the performance of the electrode. The exploration of the electronic and chemical structures of impurity phases is crucial to understand such influence. We have embarked on a series of synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy studies for the element speciation in various impurity phase materials relevant to LiFePO4 for Li ion batteries. In the present report, soft-X-ray XANES spectra of Li K-edge, P L2,3-edge, O K-edge and Fe L2,3-edge have been obtained for LiFePO4 in crystalline, disordered and amorphous forms and some possible “impurities”, including LiPO3, Li4P2O7, Li3PO4, Fe3(PO4)2, FePO4, and Fe2O3. The results indicate that each element from different pure reference compounds exhibits unique spectral features in terms of energy position, shape and intensity of the resonances in its XANES. In addition, inverse partial fluorescence yield (IPFY) reveals the surface vs. bulk property of the specimens. Therefore, the spectral data provided here can be used as standards in the future for phase composition analysis.

Size-dependent surface phase change of lithium iron phosphate during carbon coating

Carbon coating is a simple, effective and common technique for improving the conductivity of active materials in lithium ion batteries. However, carbon coating provides a strong reducing atmosphere and many factors remain unclear concerning the interface nature and underlying interaction mechanism that occurs between carbon and the active materials. Here, we present a size-dependent surface phase change occurring in lithium iron phosphate during the carbon coating process. Intriguingly, nanoscale particles exhibit an extremely high stability during the carbon coating process, whereas microscale particles display a direct visualization of surface phase changes occurring at the interface at elevated temperatures. Our findings provide a comprehensive understanding of the effect of particle size during carbon coating and the interface interaction that occurs on carbon-coated battery material--allowing for further improvement in materials synthesis and manufacturing processes for advanced battery materials.

Atomistic modeling of site exchange defects in lithium iron phosphate and iron phosphate

A new set of potentials is presented that allows for modeling of the entire lithium insertion range of the lithium iron phosphate system (LixFePO4, 0 ≤ x ≤ 1). By comparing calculated values to experimental crystallographic, spectroscopic and thermodynamic data, the potentials ability to reproduce experimental results consistently and reliably is demonstrated. Calculations of site exchange defect thermodynamics and diffusion barriers for lithium and iron inside the lithium diffusion path suggest that the site exchange defect related capacity loss may be justified exclusively by thermodynamic considerations. Moreover, a low activation barrier for iron transport in the lithium diffusion channel in FePO4 brings into question the significance of the antisite iron ion as an obstacle to lithium diffusion.

Ultrafast charging of LiFePO4 with gaseous oxidants under ambient conditions

Lithium iron phosphate is a lithium-ion battery positive electrode material with widespread use, as well as, unusually complex redox chemistry. Here we report on the discovery of a direct gas–solid delithiation reaction. Unique to this reaction, in addition to the lack of solvent, is remarkably fast kinetics. In situ X-ray diffraction, corroborated by elemental analysis, shows for the first time that LiFePO4 bulk diffusion supports nearly complete delithiation/charging of carbon coated LiFePO4 micropowder at ambient temperature in less than 60 seconds.

LiFePO4 synthesized via melt synthesis using low-cost iron precursors

LiFePO4/C material has been prepared using fast-melt synthesis method followed by grinding and carbon coating. The low-cost iron ore concentrate (IOC) and purified iron ore concentrate (IOP) were used as iron precursors in the melt process to reduce significantly the cost of LiFePO4/C. The same product was also synthesized using pure Fe2O3 under similar conditions as reference. The physical-chemical and electrochemical properties of samples were investigated. The X-ray Diffraction (XRD) results confirm the formation of an olivine structure of LiFePO4 with a minor amount of Li3PO4 and Li4P2O7 impurities for all the samples but no Fe2P. The power performances of LiFePO4/C using low-cost iron precursors were close to the sample using pure Fe2O3 precursor although capacity in mAh g−1 is somewhat lower. With the inherent presence of silicon and other metals species, multi-substitution may take place when using IOC as source of iron leading to a Li(Fe1-yMy)(P1-xSix)O4 general composition. Multi-substitution, however, allows a better cycling stability. Therefore, these iron precursors present a promising option in this field to reduce the cost of a large-scale synthesis of LiFePO4/C for Li-ion batteries application.

Structural Transformation of LiFePO4 during Ultrafast Delithiation

The prolific lithium battery electrode material lithium iron phosphate (LiFePO4) stores and releases lithium ions by undergoing a crystallographic phase change. Nevertheless, it performs unexpectedly well at high rate and exhibits good cycling stability. We investigate here the ultrafast charging reaction to resolve the underlying mechanism while avoiding the limitations of prevailing electrochemical methods by using a gaseous oxidant to deintercalate lithium from the LiFePO4 structure. Oxidizing LiFePO4 with nitrogen dioxide gas reveals structural changes through in situ synchrotron X-ray diffraction and electronic changes through in situ UV/vis reflectance spectroscopy. This study clearly shows that ultrahigh rates reaching 100% state of charge in 10 s does not lead to a particle-wide union of the olivine and heterosite structures. An extensive solid solution phase is therefore not a prerequisite for ultrafast charge/discharge.