Search

Lithium iron silicate (LFS) attracts a lot of attention due to its 330 mAh g−1 theoretical capacity (2 Li+ per formula unit). However, inherently it exhibits poor Li‐ion intercalation kinetics, interfacial reactivity, and complex phase transitions resulting in lower than one Li+ capacity and poor retention. In this work, a core–shell architecture is devised largely overcoming these obstacles. At first, the nanostructure of Pmn21 LFS is annealed via mechanochemical processing enabling the activation of Li‐ion diffusion. Subsequently, the LFS nanocrystals are coated via in situ poly(3,4‐ethylenedioxythiophene) (PEDOT) polymerization involving partial chemical de‐lithiation/re‐lithiation, the latter catalyzed with FeCl3. As a result of the devised mechanochemical/interphasial engineering of the LFS@PEDOT nanocrystals, their Li‐ion storage capacity is augmented to > 1 Li, namely 200 mAh g−1 after 50 cycles or 1.2 Li+ units—the highest capacity reported for the Pmn21 LFS cathode. A key attribute of the new PEDOT coating technique is the generation of a Fe3+‐rich subsurface layer that contributes to structure stabilization via accelerated phase transition to inverse Pmn21 phase, in addition to rendering the nanocrystals electronically conductive and protected against reaction with electrolyte. Such core–shell engineered nanocrystals provide a powerful paradigm in developing viable high energy density cathodes for next‐generation Li‐ion batteries. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Novel Li 2 FeSiO 4 mesocrystals exhibit improved Li-ion storage properties. • EDTA-regulated formation of impurity-free and defect-free hollow Pmn2 1 crystals. • Li 2 FeSiO 4 mesocrystals form via a 4-step hydrothermal crystallization mechanism. Abstract Lithium transition metal orthosilicates such as Li 2 FeSiO 4 (LFS) have been recognized as promising cathode materials for application in Li-ion batteries because of their high theoretical energy density, safety, and benign/abundant element composition. Their development, however, has been hampered by the challenge of obtaining phase-pure and defect-free Li 2 FeSiO 4 nanoparticles as non-optimized crystal properties have an adverse effect on structural stability and electrochemical performance. Considering the sustainable potential of hydrothermal synthesis in producing electrode materials, here we employ this process to systematically study critical synthesis parameters for optimal control of particle size, morphology, phase purity, and defects of Li 2 FeSiO 4 crystallized in the orthorhombic crystal system with space group Pmn 2 1. It is shown that via a combination of elevated FeSO 4 concentration regime and use of EDTA as a complexing agent, phase-pure Pmn 2 1 particle formation can be controlled. Synchrotron-based XRD Rietveld refinement and 57Fe Mössbauer spectroscopy reveal a significant reduction in Li-Fe antisite defects and presence of Fe3+. Additionally, the use of EDTA promotes the formation of unique peanut-shell looking hollow mesocrystals that are advantageous in maximizing electrode/electrolyte contact resulting in higher Li-ion storage capacity than dense LFS particles. On the basis of detailed XRD, SEM, and TEM characterizations, a four-step crystallization mechanism is proposed to explain the formation of the hollow mesocrystals. These findings bring new insight into our pursuit of optimization of Li 2 FeSiO 4 as a cathode material for Li-ion batteries. [ABSTRACT FROM AUTHOR]

Lithium iron orthosilicate (Li 2 FeSiO 4 ) is an important alternative cathode for next generation Li-ion batteries due to its high theoretical capacity (330 mA h/g). However, its development has faced great challenges arising from significant structural complexity, including the disordered arrangement/orientation of Fe/Si tetrahedra, polytypes and its poorly understood Li storage and transport properties. In this context, ab-initio calculations are employed to investigate the phase stability and Li diffusion profiles of both monoclinic ( P 2 1 ) and orthorhombic ( Pmn 2 1 ) Li 2 FeSiO 4 orthosilicates. The calculations demonstrate that formation of Li Fe antisites can induce a metastability competition between both phases, with neither dominating across nearly the entire discharging profile from Li 2 FeSiO 4 through to LiFeSiO 4 . Furthermore, structural instability is shown to be a serious concern at discharge concentrations below LiFeSiO 4 (1 Li extraction) due to the shared occupation of Li donated electrons with oxygen 2p orbitals – rather than the hypothesized transition to a tetravalent Fe 4+ state. This finding is further supported by diffusion calculations that have determined a high activation energy barrier towards fast charging and rapid phase transitions. In summary, these theoretical results provide critical and timely insight into the structural dynamics of lithium iron orthosilicate, in pursuit of high energy density cathodes. [ABSTRACT FROM AUTHOR]

Nanostructured lithium metal orthosilicate materials hold a lot of promise as next generation cathodes but their full potential realization is hampered by complex crystal and electrochemical behavior. In this work Li2FeSiO4 crystals are synthesized using organic-assisted precipitation method. By varying the annealing temperature different structures are obtained, namely the monoclinic phase at 400°C, the orthorhombic phase at 900°C, and a mixed phase at 700°C. The three Li2FeSiO4 crystal phases exhibit totally different charge/discharge profiles upon delithiation/lithiation. Thus the 400°C monoclinic nanocrystals exhibit initially one Li extraction via typical solid solution reaction, while the 900°C orthorhombic crystals are characterized by unacceptably high cell polarization. In the meantime the mixed phase Li2FeSiO4 crystals reveal a mixed cycling profile. We have found that the monoclinic nanocrystals undergo phase transition to orthorhombic structure resulting in significant progressive deterioration of the material's Li storage capability. By contrast, we discovered when the monoclinic nanocrystals are cycled initially at higher rate (C/20) and subsequently subjected to low rate (C/50) cycling the material's intercalation performance is stabilized. The discovered rate-dependent electrochemically-induced phase transition and stabilization of lithium metal silicate structure provides a novel and potentially rewarding avenue towards the development of high capacity Li-ion cathodes. [ABSTRACT FROM AUTHOR]