Lithium-Ion Battery Based on LiMn0.5Fe0.5PO4 Cathode and Lithium Alloying Anode

Polyanion framework olivine materials have attracted large attention as cathodes in LIBs. Recently, the attention of the scientific community has been focused on the substitution of iron by manganese, cobalt or nickel within the olivine lattice [1]. Indeed, Mn³⁺/Mn²⁺, Co³⁺/Co²⁺ and Ni³⁺/Ni²⁺ couples show increasing redox potentials compared to Fe³⁺/Fe²⁺, thus leading to an increased energy density. However, the latter two exhibit redox potential of about 4.8 and 5.2 V vs. Li⁺/Li, respectively, i.e. a value exceeding the conventional carbonate-based electrolytes stability limit [2]. LiMnPO₄ has instead an operating voltage of about 4.1 V vs. Li⁺/Li, still within the working electrochemical window of conventional electrolytes, thus leading to a theoretical energy density of about 700 Wh kg^-1 [3]. LiMnPO₄ suffers from poor electric conductivity in comparison to LiFePO₄ [4]. Several approaches have been studied to improve LiMnPO₄ electrochemical activity in lithium cell. Incorporation of conductive additives, such as carbon, by post-synthesis ball milling or formation of a carbon layer on particle surface can increase the electronic conductivity [5, 6]. Significant decrease of the lithium diffusion pathway and consequent conductivity increase may be reached by reduction of particle size by proper synthetic procedures leading to nano-morphologies [7, 8]. Soft-chemistry methods allow more controlled tailoring of morphologies and size than the conventional solid-state approaches [9]. In particular, hydro- and solvo-thermal methods are simple techniques allowing the precipitation of olivines at low temperature and controlled crystal growth [10]. Further improvement of the cathode performance may be reached by mixed compositions, LiMn_1-xFe_xPO₄. Iron incorporation in the olivine lattice has proven to enhance the electrochemical activity of the Mn³⁺/Mn²⁺ redox reaction by increasing both electronic and ionic conductivity, and by reducing lattice strain due to Jahn-Teller active ion Mn³⁺ [11, 12].

Herein we report the synthesis and characterization of LiMnPO₄, LiFePO₄ and mixed LiMn_0.5Fe_0.5PO₄ materials through solvothermal approach. Careful analysis of the electrochemical properties by potentiodynamic cycling with galvanostatic acceleration and galvanostatic cycling techniques were carried out in order to investigate the features of the Fe³⁺/Fe²⁺ and Mn³⁺/Mn²⁺ redox couples in the olivine structure. In addition, an advanced lithium-ion battery formed by combining a LiMn_0.5Fe_0.5PO₄ cathode with a Sn-C anode is reported. The figure shows the galvanostatic cycling results of a Sn-C/LiMn_0.5Fe_0.5PO₄ cell.

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