Influence of the temperature-induced ordered-disordered phases for iron difluoride on the electrochemical performance

Iron difluoride (FeF 2 ) is one of the promising cathodes to achieve high energy-density lithium-ion batteries (LIBs). However, the sluggish ion transportation kinetics in the FeF 2 bulk phase may cause large voltage hysteresis and lower cell efficiency, which hinders its commercial adoption. Introducing a disordered phase into the ordered bulk FeF 2 is proposed to promote ion transportation and increase ion diffusion channels. Here, varying ratios of the disordered phase are incorporated into the ordered matrix of FeF 2 by adjusting the annealed temperatures. The ion transportation is improved due to the optimal ratio of the disordered to ordered (RDO) phase. Among four electrodes, the RDO25% electrode demonstrates the least electrochemical polarization of 1.22 V and the lowest voltage hysteresis of about 1.0 V, in addition to delivering a discharge capacity of 128 mAh g −1 after 100 cycles in the carbonate-type electrolyte. The introduction of the disordered phase enables FeF 2 to exhibit a distinct intercalation and conversion dual-mechanism cathode. Additionally, appropriate ratios of the disordered-ordered phase can mitigate nanoparticle agglomeration, establish a conformal cathode electrolyte interface (CEI), reduce interfacial resistance, and thereby enhance ion transport.