Structural, Transport and Electrochemical Properties of LiFePO4 Substituted in Lithium and Iron Sublattices (Al, Zr, W, Mn, Co and Ni)

Abstract

LiFePO4 is considered to be one of the most promising cathode materials for lithium ion batteries for electric vehicle (EV) application. However, there are still a number of unsolved issues regarding the influence of Li and Fe-site substitution on the physicochemical properties of LiFePO4. This is a review-type article, presenting results of our group, related to the possibility of the chemical modification of phosphoolivine by introduction of cation dopants in Li and Fe sublattices. Along with a synthetic review of previous papers, a large number of new results are included. The possibility of substitution of Li+ by Al3+, Zr4+, W6+ and its influence on the physicochemical properties of LiFePO4 was investigated by means of XRD, SEM/EDS, electrical conductivity and Seebeck coefficient measurements. The range of solid solution formation in Li1−3xAlxFePO4, Li1−4xZrxFePO4 and Li1−6xWxFePO4 materials was found to be very narrow. Transport properties of the synthesized materials were found to be rather weakly dependent on the chemical composition. The battery performance of selected olivines was tested by cyclic voltammetry (CV). In the case of LiFe1−yMyPO4 (M = Mn, Co and Ni), solid solution formation was observed over a large range of y (0 < y ≤ 1). An increase of electrical conductivity for the substitution level y = 0.25 was observed. Electrons of 3d metals other than iron do not contribute to the electrical properties of LiFe1−yMyPO4, and substitution level y > 0.25 leads to considerably lower values of σ. The activated character of electrical conductivity with a rather weak temperature dependence of the Seebeck coefficient suggests a small polaron-type conduction mechanism. The electrochemical properties of LiFe1−yMyPO4 strongly depend on the Fe substitution level.

Effect of particle size on dc conductivity, activation energy and diffusion coefficient of lithium iron phosphate in Li-ion cells

“…The ionic component of electrical conductivity is related to the mobility of Li + ions. It is preferential along [010] direction which is characterized by the Ea of 0.55-0.65 eV when compared to [001] direction with Ea = 2.89 eV and [101] direction with Ea = 3.36 eV [9,17]. It is observed that there exists a relationship between the electronic and the ionic conductivity in LiFePO4 [9,16,18,19].…”

Evidence for Al doping in lithium sublattice of LiFePO4

“…It is worth noting that data proving metallic-like properties of pure and (assuming) Li site-substituted LiFePO 4 were published [24,25]; however, these results were questioned, and the observed metallic-like conductivity of the samples was rationalized as originating from the presence of metallic phosphides and/or carbophosphides on the surface of LiFePO 4 grains [33,34]. As described in our earlier work [35], there is still an ongoing discussion about possibility and, assuming so, the range of substitution of lithium by other cations and its expected influence on the properties of the material. This includes also the so-called anti-site defect, where a nominal Li + site is occupied by Fe 2+ ion [12].…”

Electrochemical tuning of olivine-type lithium transition-metal phosphates as efficient water oxidation catalysts

“…Table 1 summarizes the parameters describing the OER performance of the pristine and the delithiated LCoP. 39 Noticeably, there are two distinct improvement steps for LCoNiFeP/rGO ( Figure S11), which are above 4.80 V and 5.00V, corresponding to the redox potential of Co 2+/3+ and Ni 2+/3+ respectively. Such a significant improvement confirmed the efficacy of electrochemical tuning on olivine-type LiMPO4 materials.…”