Lithium-ion batteries (LIBs) are promising devices for high capacity, rechargeable electrical energy storage; however, LIBs are currently limited by the low specific capacity of the cathode compared to the anode. In previous work, our group demonstrated the viability of a novel cathode material, Li8ZrO6 (LZO), through computational and experimental results. Here we report a general synthesis for transition-metal-doped LZO, and we study the effects of doping on electrochemical delithiation and relithiation. A synthesis using transition-metal (M) doped ZrO2 nanoparticle/carbon black composites as precursors produces doped M-LZO with grain sizes between 35 and 67 nm. The materials were tested as electrode materials. Specific capacities of the doped materials depend on the transition metal and on the Li:Zr ratio used in the synthesis, but they are generally higher than in similarly prepared undoped LZO. In this set of cathode materials, Fe3+-doped LZO/C composites showed the highest specific capacities, with an initial discharge capacity higher than two Li ions per formula unit, a specific capacity of 175 mAh/g maintained after 140 cycles, and a specific capacity greater than 80 mAh/g at a rate of 5C. The effects of doping were also investigated by density functional calculations of dopant locations, band gaps, and delithiation energies. We found that all of the dopants that we studied are more favorably located in Li ion sites than in Zr ion sites. The calculated doping effects on structural parameters agree well with experiments. We also found that doping with any of these ions leads to smaller band gaps. Electronic structure calculations with the HSE06 exchange-correlation functional show that deintercalation after doping with Ce3+, Cu2+, or Co2+ at a Li site decreases the attainable cell voltage, whereas Fe3+ doping at a Li site increases it. Because of the large polarization and high carbon content of the M-LZO/C composite electrodes, further materials optimization will be needed before they become practical for LIBs.
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The authors thank Yuan Fang and Nam Tran for helpful discussions and collaboration on other aspects of the project, Professor Lee Penn for use of her X-ray diffractometer, and Rick Knurr for assistance with the ICP-MS analysis. This material is based upon work that was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Award Number DE-SC0008662. Parts of this work were carried out in the University of Minnesota Characterization Facility, which receives partial support from the NSF through the MRSEC, ERC, MRI, and NNIN programs. Computations were performed using resources of (1) the Molecular Science Computing Facility in the William R. Wiley Environmental Molecular Sciences Laboratory of Pacific Northwest National Laboratory sponsored by the U.S. Department of Energy, (2) the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, and (3) Minnesota Supercomputing Institute.
© 2016 American Chemical Society.