Effect of Fe²⁺substitution on the structure and electrochemistry of LiCoPO₄prepared by mechanochemically assisted carbothermal reduction

Abstract

LiCo 1Ày Fe y PO 4 solid solutions (0 # y # 1) were prepared by the mechanochemically assisted carbothermal reduction of Co 3 O 4 and Fe 2 O 3 . Mechanical activation was performed using a high-energy planetary mill AGO-2. The samples were characterized in detail by X-ray powder diffraction (XRD) using a Rietveld refinement, Fourier transform infrared spectroscopy (FTIR), M össbauer spectroscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), galvanostatic cycling, and galvanostatic intermittent titration technique (GITT). According to XRD, all the samples are single-phase solid solutions, crystallized in an orthorhombic structure (S.G. Pnma). The cell volume of LiCo 1Ày Fe y PO 4 linearly increases vs. the Fe content. All the Fe ions are in the 2+ oxidation state and are octahedrally coordinated. The LiCo 1Ày Fe y PO 4 solid solutions show improved electrochemical performance, compared with LiCoPO 4 . Based on the data from XRD and GITT, the improvement is attributed to the enhanced Li + diffusion, due to the enlargement of the 1D diffusion channels in the polyanion structure of LiCoPO 4 and the reduced cell volume change in the material during the Li extraction/insertion process. Moreover, a systematic decrease in the average potential of the Co 2+ /Co 3+ redox pair is observed with the increased Fe content, leading to the reaction termination in the electrochemical window of conventionally available electrolytes. In situ synchrotron diffraction shows that upon charging LiCo 0.5 Fe 0.5 PO 4 , the twophase mechanism of Li (de)intercalation at the Fe 2+ /Fe 3+ and Co 2+ /Co 3+ redox stages changes to a solid solution-like mechanism, contrary to the pristine LiFePO 4 and LiCoPO 4 materials.

Electrochemical Performance and Delithiation/Lithiation Characteristics of Mixed LiFe1-yMyPO4(M= Co, Ni) Electrode Materials

“…According to Kosova et al, 33 the peak related to the formation of the partly lithiated Co 2+ /Co 3+ couple intermediate phase disappears at higher Fe contents (y ≤ 0.5 in LiFe 1-y Co y PO 4 ), and a single-phase diffusional mechanism replaces the two-phase processes observed in LiFePO 4 and LiCoPO 4 . They also claimed that this would apply to all compositions y ≤ 0.…”

“…Kosova et al 33 Nyten et al 66 suggested that Co substitution at y < 0.75 levels does not lead to structure distortion, but a less stable structure would be found for y > 0.75. This could be related to the changed Co 2+ /Co 3+ redox mechanism, observed for y < 0.6 in our study and by Kosova et al 33 On the other hand, in the computational study of Malik et al, 67 the existence of solid-solution domains in LiFe 1-y Mn y PO 4 materials was speculated to originate from the random distribution of Mn 2+ , which leads to a mutual disturbance of the attractive Mn 2+ -Li + and repulsive Fe 3+ -Li + interactions.…”

“…However, in practice discharge capacities of typically only 100-120 mAh g −117, [19][20][21][22][23][24][25] or even less 18,[26][27][28][29] are achieved, and the capacity fade during continuous cycling has moreover been rapid. The reasonably poor electrochemical performance and its fast deterioration have been attributed to the low electronic conductivity (lower than for LiFePO 4 ), 30,31 slow Li + -ion diffusion, 32,33 possible irreversible structural changes while charging, 26 and to the electrolyte oxidation at high potentials 21,23 and possible hydrogen fluoride generation. 34 Furthermore, the fully-delithiated CoPO 4 phase is unstable in air and undergoes amorphization.…”

“…18,[28][29][30] However, the two-step mechanism seems to be less apparent during reduction/discharge. 19,21,23,24,26,33,36,38 Distinguishing the two separate steps can be hindered by polarization and electrolyte decomposition reactions, 21 while minimizing the particle size is found to improve the resolution of the two steps. 24 …”

Supervalent doping of LiFePO4 for enhanced electrochemical performance

“…Note that the position of the redox peaks on the dQ/dV plots well corresponds to those of pure LFP, LVP and LTP, evidencing that low degree of substitution and the composite formation do not noticeably influence voltage of the redox processes upon cycling. On the contrary, in the case of the olivine-type solid solutions, e.g., LiFe 1-y Mn y PO 4 [20,21].…”