Fe and V Substituted Li2MnSiO4/C As Potential Cathode Material for Li-Ion Batteries

Rechargeable batteries can store energy in the form of chemical energy and facilitate it with a high conversion rate when needed. Moreover, rechargeable batteries are used in almost all kinds of portable consumer electronics, hybrid and pure electric vehicles. Thus, the development of advanced battery technologies is a major field of scientific focus. Li-ion technology has been shown to be superior compared to other battery concepts in performance, cycling stability, self-discharge and expected lifetime.[1] However, commercially used cathode materials (LiMO₂) exhibit low capacities compared to the graphite based anodes (~300 mAh/g) and thus limits the battery performance. In the last decade polyanion based materials gained interest and in 2005 Nyten et al. reported on Li₂FeSiO₄as a new Li-battery cathode material [2]. Lithium transition-metal silicate based materials are promising candidates for next generation’s Li-ion batteries since they allow Li extraction/insertion beyond one Li ion per formula unit. Furthermore, they consist of cheap, non-toxic and abundant elements [3].

This work focuses on Li₂MnSiO₄ (LMS), which can theoretically deliver two Li-ions per formula unit since the transition metal ion possesses two redox couples (Mn³⁺/Mn²⁺ + Mn⁴⁺/Mn³⁺). Synthesis of phase pure LMS material has, however, turned out to be challenging. Here, synthesis of LMS with high phase purity was demonstrated by an acidic, PVA assisted sol-gel method using metal nitrates and TEOS as precursors, which is also suitable for upscaling. The dried precursor was pre-calcined and then mixed with a given amount of corn-starch as carbon source. To obtain the desired phase and coat the material in a single step, the powder/starch mix was then carbothermally reduced at 700 °C for 10 h. The atmosphere and starch content are seemingly crucial in both heat treatments to achieve high phase purity in the final cathode powder. Best results were achieved when both heat treatments were carried out in 95% Ar 5% H₂ and with starch contents ≥ 25 wt-%. Figure 1 shows powder XRD patterns of LMS with optimized parameters and different starch contents and a full pattern refinement.

The synthesized materials were micro- and meso-porous powders with a thin uniform carbon coating (confirmed by TEM), offering high external surface areas of more than 30 m²g^-1 (excluding micropore area which is inaccessible to the electrolyte). In addition to pure Li₂MnSiO_4, this work also focuses on the synthesis and electrochemical performance of Fe and V substituted LMS. Fe substitution should provide increased cycling stability since Li₂FeSiO₄ has shown relatively good long-term stablility [2,3]. The doping of V on either the Mn or the Si site of LMS is currently under investigation. V doped LMS is believed to offer improved electrochemical performance since V offers 3 redox couples in the accessible voltage window of the electrolyte of a Li-ion battery [4].

[1]     Tarascon, J. M., Armand, M, Issues and challenges facing rechargeable lithium batteries, Nature, 2001, 414, 359-367.

[2]     Nyten A., Abouimrane A., Armand M., Gustafsson T., Thomas J. O., Electrochemical performance of Li2FeSiO4 as a new Li-battery cathode material, Electrochemistry Communications, 2005, 7, 156-160.

[3]     Saiful Islam M., Dominko R., Masquelier C., Sirisopanaporn C., Armstrong A. R., Bruce P. G., Silicate cathodes for lithium batteries: alternatives to phosphates?, J. Mater. Chem., 2011, 21, 9811-9818

[4]     Li, Y, Cheng, X, Zhang, Y, Achieving High Capacity by Vanadium Substitution into Li₂FeSiO₄, Journal of The Electrochemical Society, 2012, 159 (2) A69-A74.