Rechargeable lithium-ion batteries (LIBs) have become the dominant power sources not only for the consumer electronic devices but also for the pure electric vehicles (EVs) and plug-in electric vehicles (PHEVs) because of their high energy density, light weight and long cycle life. The increasing need on minimizing global warming and environmental pollution have accelerated the development of EVs. With the state of the art LIBs used in automobile industry, the driving range of EVs is still far behind those of conventional vehicles using internal combustion engines. Therefore, there is an urgent need to further improve both energy density and power capability of LIBs, to improve the driving range and decrease the recharging time. To meet these requirements, a lot of effort has been made in the search of high energy density electrode materials, especially the cathode materials.

 Recently, Ni-rich layered cathode materials LiNi_xMn_yCo_zO₂ (x ≥ 0.6) have received great attention as a potential cathode alternative due to their high achievable discharge capacity (200~220 mAh g^-1),¹ representing a significant improvement in energy density (~780 Wh kg^-1) as compared to LiCoO₂ (~570 Wh kg^-1) and spinel LiMn₂O₄ (~440 Wh kg^-1). Ni-rich cathode materials also show much higher lithium ion diffusion coefficients, signifying a superior power density as compared to other LiNi_xMn_yCo_zO₂ cathodes with lower Ni contents.² The other advantage of Ni-rich cathode materials is the reduction of production cost owing to the reduced Co content. However, there are still some technical challenges hindering the mass applications of Ni-rich cathode materials, including (i) Li/Ni cation mixing due to the difficulty in maintaining all the Ni in 3+ valence state, (ii) micro strain and micro crack formation due to the significant volume variation during charge/discharge processes, (iii) safety issue because of the aggressive thermal reactions between the delithiated Ni-rich materials and the electrolyte.

 Much effort has been devoted to improving the electrochemical performance of the Ni-rich cathode materials.³ Representative approaches include lattice doping, surface treatment/modification, tuning the material compositions, and a smart design of core-shell or concentration gradient structures. Other than these useful treatment and design, the synthetic conditions, such as co-precipitation pH value, particle size, lithium content, calcination temperature, calcination atmosphere (O₂, air etc.), heating/cooling rates, also play an important role in manipulating the material structural properties and thus influencing the electrochemical performances of prepared Ni-rich cathode materials.

 Herein, we systematically investigated the effects of calcination temperature on the primary particle size, cation mixing and the electrochemical performances of Ni-rich cathode materials. It is found that higher calcination temperature leads to considerable increase of primary particle size, which is supposed to largely reduce the side reactions between cathode material and the electrolytes. However, the materials prepared at high temperatures exhibited much poorer long-term cycling stability both at room temperature and high temperature of 60^oC, in addition to their inferior rate performance. Therefore, the fundamental reasons behind this phenomenon are investigated in detail, which will be presented and discussed during the meeting.

 Acknowledgements

 This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, the Advanced Battery Materials Research Programs of the U.S. Department of Energy (DOE). The microscopy and spectroscopy measurements were performed at the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.

 References

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