Background: Lithium manganese oxide spinel is a strong candidate for lithium-ion battery cathodes due to its high operational voltage, three-dimensional network of lithium pathways within the structure, low toxicity, reasonable capacity, and low cost of raw materials. However, spinel suffers from diminished cycle life due to loss of crystallinity or fracturing of the structure either through the alternative phase formation or Jahn-Teller distortion. Historically, cycle life enhancements are achieved through stabilization of the spinel structure via transition metal doping on the B-site of the lattice; creating disorder necessary to stabilize the lattice structure during cycling. Previously, we have demonstrated a stabilization of this structure can also be achieved with a chlorine dopant on the oxygen site that results in enhanced performance at voltages below 3.0V when homogenously dispersed throughout the crystal, giving a desirable added capacity in a robust chemistry protected by this low voltage plateau. Single B-site dopants have been incorporated into these chlorinated spinels but rarely have several dopants been studied for optimizing and customizing cathode performance as well as anion dopant feasibility. Raw materials cost must be taken into to consideration when designing a novel cathode. An ideal cathode would limit scarce or unethically sourced materials like cobalt, and instead focus on considering inexpensive and available materials like iron. This effort focuses on the manganese substitution of up to three metals, iron, cobalt, and nickel, in order to stabilize the structure for further cycling while minimizing the need for cobalt incorporation. Several variations for LiaMn1-x-y-ZFexCoyNizO2-dCld will be presented that exhibit opportunities for 5.0+ V operation. Experimental: A wet chemical gel combustion method was used to synthesize metal-doped chlorinated lithium manganese oxide spinel. Stoichiometric amounts of Li(NO3), Mn(NO3)2∙4H2O, a metal-chloride salt appropriate for each metal, (Fe, Co, and/or Ni)(NO3)n, and a chelating agent (glycine or citric acid with ethylene glycol, Alfa Aesar/Sigma Aldrich) were dissolved into an aqueous or ethanol solution. The solution was heated to evaporate the water and form a gel, which was heated further to 250 °C when auto ignition occurs. The resultant ash was fired between 600 – 700°C for 2 to 12 hours to achieve the desired phase. Alternatively, an acetate-based synthesis is performed with metal acetate salts less a chlorine or lithium additive dissolved in distilled water. NH4OH is added to form a gel and is heated to evaporate the water. The gel is ignited above 200 °C and calcined at 400 °C in air. The resultant oxide is mixed with LiClO4 and LiOH and ground in solid state prior to a final calcination at 600 °C for 4 hours. Material characterization was performed using X-ray diffraction, X-ray fluorescence, thermogravimetric analysis, and scanning electron microscopy. Prepared active cathode materials were mixed with carbon black and polytetrafluoroethylene in a weight ratio of 85:10:5 of active to carbon to Teflon binder and calendared to 0.04 cm, punched into disks and pressed onto aluminum mesh opposite a Li metal anode. CR2025 button cells were filled with 1 M LiPF6 in a proportional mixture of diethyl carbonate, dimethyl carbonate, and ethylene carbonate. Electrochemical characterization was performed using electrochemical impedance spectroscopy (Solartron 1260) and cycling on an ARBIN MSTAT4 battery cycler system. Conclusion: Electrochemical data is presented for B-site dopants with two or more of the metals iron, cobalt and/or nickel. The results demonstrate an enhanced high voltage operation above 5.0 V coupled with a deep discharge capability below 3.0 V. In the case of LiaMn0.77Fe0.06Ni0.17O2-dCld, a cobalt-free cathode material exhibits a capacity of 210 mAh/g from 5.0V to 2.25V.An example electrochemical performance result is shown in the attached figure where LixMn0.945Fe0.05Co0.055O2-dCld is the active cathode material and lithium is the active anode material. For this example, the cell was cycled between 4.5 V and 3.5 V for the initial ten cycles. For the next three cycles, the cell was cycled between 5.25 V and 3.5 V. The next three cycles included a deep discharge between 5.25 V and 2.25 V before returning to cycling between 5.25 V and 3.5 V. The cycle plots indicate no loss of capacity when cycling between the aforementioned voltages. When discharging deep, there is a 50% increase in capacity and the return to high voltage charging without the deep discharge indicates the material is reversible and does not undergo permanent structural deformation. Importantly, a two-fold increase in capacity is shown when cycling to 4.5V (cycles 1-10) vs. 5.25V (cycles 11-13). Electrolytes with 5.0+ V stability will need to be studied further to enable the full potential of these presented cathodes. Figure 1
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Dec 9, 2020
Sanjib Chowdhury + 6
