Lithium Difluorophosphate as an Effective Additive for Improving the Initial Coulombic Efficiency of a Silicon Anode

Abstract

Silicon anodes usually endure low initial coulombic efficiency (ICE) in applications for lithium-ion batteries (LIBs), although the volume expansion and structural stability has been greatly improved under many efforts in the recent years. During the first charge and discharge, a large number of lithium ions can participate in the formation of a solid electrolyte interface (SEI) on anode surface, such as LiF, Li 2 CO 3 and many Li-based compounds, resulting in low efficiency, particularly as the nanosized silicon is widely applied to solve the volume effect. Herein, we find that the lithium difluorophosphate (LiPO 2 F 2 , LiDFP) additive shows some special properties, which greatly improve the reversible capacity in the first discharge. The silicon nanoparticles can deliver an ICE of 70.6 % with 2 wt% LiDFP in the electrolyte, which is increased by 17.7 % compared with the cell without LiDFP (ca. 52.9 %). By comparing the XPS and NMR results, it appears the LiDFP may reduce the number of lithium ions involved in the SEI reaction in the electrolyte during the first discharge, and thus the ICE can be improved.

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“…Additionally, the initial CE can be further improved by tailoring the anode structural design, engineering the electrolyte interface, utilizing prelithiation techniques, constructing an artificial SEI, and tuning the composition of the electrolyte. [ 101 ]…”

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“…The former is related to the complexity of design solutions proposed for mitigating the latter, which is mainly caused by the disadvantages mentioned above. The tremendous volume variations during Charge/discharge lead to a direct capacity loss (severe cracking), and broken/renovated SEI films, which consumes Li ions and the electrolyte, hampering ionic conductivity and reversibility (the initial formation of an SEI layer is responsible for very low Initial Coulombic efficiency [ 92 ]); similarly to Ni-rich cathodes, mechanical stresses induced by volumetric expansion/contraction lead to severe crack initiation/propagation, and particle pulverization [ 87 ]. The advantages of using nanoparticles in composite materials are closely related to increasing ionic conductivity and designing porous structures capable of accommodating large volume changes.…”

“…Using NMC333 as the counter electrode, the capacity retention was 89.8% after 600 cycles and below 50% after 500 cycles, respectively. In another recent work, LiPO was added to LiPF (LiFDP) for increasing the ICE and capacity retention of nano Si (∼100 nm) anodes [ 92 ]. Half-cells using LiFDP and LiFP (control samples) were tested.…”

Si/FeSi2 Nanoparticles Prepared by Thermal Plasma with Stress‐releasing Effect for Li‐ion Storage

“…These studies have made use of the alloy components to utilize its buffer effect on volume expansion and achieved stable electrochemical performances, which provide feasibility for practical Si‐alloy anode materials. However, it is difficult to achieve high ICE for practical applications or requires additional processing [44–46] . Especially, it remains a considerable challenge that the mechanism of Fe−Si alloy has not been fully discussed.…”