Challenges in the synthesis of high voltage electrode materials for lithium-ion batteries: a review on LiNiPO4

The current array of commercially produced cathode materials for advanced lithium-ion batteries is poorly suited to today's energy supply demands. A current example of this problem is the unsatisfactory range of commercially available electric vehicles or even of mobile electronic devices. The bottleneck of capacity is the availability of cathode materials like LiCoO2 that also has associated safety risks. Lithium conducting phosphates with the olivine structure LiMPO4 (M = Fe, Mn, Co, Ni) with a theoretical capacity of approximately 170 mAh g(-1) might provide recourse. LiNiPO4 has received increasing interest in recent years, mainly because of its similar structure to LiFePO4, which was shown to have beneficial electrochemical properties after a few years of intensive research. An optimized nickel phosphate would in theory possess an even higher energy density, because of its redox potential of about 5.1 V vs. Li/Li+. Nevertheless advances in ionic and electronic conductivity as well as electrochemical reversibility of LiNiPO4 are rare and the reasons are not fully understood. This review presents an overview of recent progress in the fabrication of LiNiPO4 powders and the general synthesis approaches to circumvent the drawbacks of LiNiPO4. The impacts of these fabrication methods on the purity, structure, and electrochemical performance of LiNiPO4 powders are discussed. Particular attention is paid to electrochemical activation of nickel in the olivine structure and recent trials to apply LiNiPO4 in aqueous and nonaqueous lithium-ion batteries are reviewed. On the basis of this rigorous study a complete picture of the state of the art of LiNiPO4 as a possible cathode material and its perspectives is given.