Mössbauer analysis of silicate Li2FeSiO4 and delithiated Li2− x FeSiO4 ( x = 0.66) compounds

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.

Section: Resultssupporting

confidence: 72%

Rate-dependent phase transitions in Li2FeSiO4 cathode nanocrystals

Wei

Chiu

et al. 2015

“…Then, a clear correlation cannot be found between polymorphism and magnetic features of Li 2 FeSiO 4 . Rather, the T N value of the lithium iron silicates can be influenced by the coexistence between Fe ions with different oxidation states32. Also the results shown above for the Li-Mn silicates are in agreement with these hypotheses: the small T N shift from 12 K ( i.e.…”

Section: Discussionsupporting

confidence: 75%

“…A higher μ eff value has instead been obtained for Fe-900 (μ eff ≅ 5.15), which could imply, for example, the contribution, in addition to the one of divalent iron ions, from Fe ions with higher oxidation state. Nevertheless, this should not be the case because a T N value of 20 K in this compound is related to magnetic interactions between Fe 2+ spins only, while the coexistence of Fe 2+ and Fe 3+ ions should give rise to AF ordering with a T N value higher than 20 K32. On this basis, an extrinsic contribution can be instead invoked to explain the higher estimated μ eff value for Fe-900, as, for example, the one due to the small amount in the sample of the ferrimagnetic Li 3 Fe 5 O 8 phase not fully saturated in the whole temperature range considered to estimate Curie and Weiss constants.…”

Section: Resultsmentioning

confidence: 95%

Polymorphism and magnetic properties of Li2MSiO4 (M = Fe, Mn) cathode materials

Bini

Ferrari

Ferrara

et al. 2013

Transition metal-based lithium orthosilicates (Li 2 MSiO 4 , M 5 Fe, Ni, Co, Mn) are gaining a wide interest as cathode materials for lithium-ion batteries. These materials present a very complex polymorphism that could affect their physical properties. In this work, we synthesized the Li 2 FeSiO 4 and Li 2 MnSiO 4 compounds by a sol-gel method at different temperatures. The samples were investigated by XRPD, TEM, 7 Li MAS NMR, and magnetization measurements, in order to characterize the relationships between crystal structure and magnetic properties. High-quality 7 Li MAS NMR spectra were used to determine the silicate structure, which can otherwise be hard to study due to possible mixtures of different polymorphs. The magnetization study revealed that the Néel temperature does not depend on the polymorph structure for both iron and manganese lithium orthosilicates. P olyanion framework compounds based on PO 4 or SiO 4 structural units are now under intense study for the application as cathode materials in lithium-ion batteries. All of these materials are characterized by low costs and toxicity, are environmentally friendly, and highly safe 1 . Along with the well-known phospho-olivine, lithium orthosilicates appear especially promising because they can afford more than one electron reversible exchange per transition metal atom, so increasing the overall cathode capacity. In fact, Li 2 MnSiO 4 can reach the capacity of 333 mAhg 21 , while Li 2 FeSiO 4 could deliver 166 mAhg 21 for the extraction of one Li ion 2 . However, the low electronic conductivity of silicates has to be overcome in order to reach the theoretical capacity and different approaches have been tried to improve their electrochemical performances, e.g. by mixing Fe and Mn

“…This value is indeed compatible with the presence of Fe 3+ ions on the Fe 2+ sites. In fact, a T N higher than 20 K for Li 2 FeSiO 4 was already related to a consistent amount of Fe 3+ in the orthosilicate phase by Mössbauer measurements performed at low temperature onto de-lithiated samples41. The authors reported a T N = 20K for Li 2 FeSiO 4 , containing only Fe 2+ , and a T N = 28 K for a de-lithiated sample with Li 1.34 Fe 2+ 0.33 Fe 3+ 0.66 SiO 4 composition, suggesting that stronger AF interactions take place when Fe 2+ ions are partially substituted by Fe 3+ ions.…”

Section: Discussionmentioning

confidence: 87%

New materials for Li-ion batteries: synthesis and spectroscopic characterization of Li2(FeMnCo)SiO4 cathode materials

Ferrari

Mozzati

Lantieri

et al. 2016

Improving cathode materials is mandatory for next-generation Li-ion batteries. Exploring polyanion compounds with high theoretical capacity such as the lithium metal orthosilicates, Li2MSiO4 is of great importance. In particular, mixed silicates represent an advancement with practical applications. Here we present results on a rapid solid state synthesis of mixed Li2(FeMnCo)SiO4 samples in a wide compositional range. The solid solution in the P21/n space group was found to be stable for high iron concentration or for a cobalt content up to about 0.3 atom per formula unit. Other compositions led to a mixture of polymorphs, namely Pmn21 and Pbn21. All the samples contained a variable amount of Fe3+ ions that was quantified by Mössbauer spectroscopy and confirmed by the TN values of the paramagnetic to antiferromagnetic transition. Preliminary characterization by cyclic voltammetry revealed the effect of Fe3+ on the electrochemical response. Further work is required to determine the impact of these electrode materials on lithium batteries.