Cycle Life Analysis of LiMn2O4-DCld Made Via in Situ Combustion Synthesis and Reduced-Time Solid State Processing

Abstract

Lithium manganese oxide spinels exhibit promising electrochemical performance and good thermodynamic and kinetic stability when used as a cathode in lithium and lithium-ion electrochemical systems. This can be attributed to its inherent three-dimensional structure that allows lithium ions to easily flow in and out of the lattice. A common disadvantage of this chemistry is its limited cyclability as a result of the energy barriers for removing lithium from the octahedral sites. Additionally, the dissolution of manganese ions into the electrolyte causes both capacity fading and cycle life degradation in an electrochemical system. CERDEC has previously demonstrated enhanced electrochemical performance at both low and high voltage ranges, with the reversible region occurring between 2.0 V to 4.75 V. This material incorporates chlorine into the lithium manganese spinel structure using a reduced-time solid state processing method, where the material is calcined in air at 600 °C for 2 hours. This method drastically reduces the processing time and calcining temperature in comparison to the traditional hydrothermal fabrication method or the standard solid state method for lithiated manganese spinels. The general formula for the halogenated lithium manganese spinel material is LixMn2O4-dXd, where x ≈ 1 and d ranges from 0.005 to 0.3. This work focuses on further evaluating the cycle life performance of electrochemical cells utilizing this LiMn2O4-dCldcathode material in the high voltage operating range. Additionally, an alternative processing method is analyzed for halogenated lithium manganese spinel, where the material is made via in situ combustion synthesis in order to further promote lithium deinsertion from octahedral sites. Incorporating chlorine into the lithium manganese spinel structure further stabilizes the lattice to prevent manganese dissolution into the electrolyte when utilized in an electrochemical cell. The alternative processing method involves using a glycine nitrate combustion process, where the ash is collected and calcined in air at 600 °C for 2 hours. Material analysis was performed using X-ray diffraction, X-ray fluorescence, thermogravimetric analysis, BET surface area analysis, and scanning electron microscopy to evaluate and verify the composition and structure of the chlorinated spinel material, as well as its particle morphology. This alternative processing method allows for a more uniform structure in the desired spinel phase with higher surface area. Electrochemical coin cells were fabricated utilizing LiMn2O4-dCld against a lithium anode, where the cathode material was synthesized using both processing methods. Test cells were cycled between 3.5 V and 4.75 V at 0.5-1 mA/cm2 to determine cycle life capability and capacity degradation processes. A comparative performance evaluation was also performed by incorporating undoped cathode material into electrochemical coin cells and conducting galvanostatic testing under the same test regime. A cycle life graph is shown displaying the capacity retention of electrochemical coin cells incorporating an undoped lithium manganese spinel, LiMn2O4-dCld synthesized using a glycine nitrate combustion process (GNP), and LiMn2O4-dCld synthesized using the reduced time solid state process (RTP). These cells were cycled between 3.5 V and 4.75 V at 0.5 mA/cm2. The data shows increased capacity retention, above 95% at 130 cycles, using the alternative processing method, GNP. Additionally, a clear degradation and recovery of capacity of the undoped lithium manganese spinel cell is shown, which is attributed to the cell being cycled at a voltage range that is higher than optimal. Figure 1