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Xianhui Zhang1 Zhenlian Chen1 Liyuan Huai1 Deyu Wang1 Jun Li1

1, Ninbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, , China

The development of novel high-capacity and high-energy cathode materials for lithium ion batteries is a focus of the energy storage technology. Different from LiMO2 and LiFePO4, which only one Li+ can be reversibly cycled, Li2CoSiO4 would allow reversible extraction of two Li+ in principle thus delivering a higher theoretical capacity of 325 mAh/g. Moreover, it has been proved to show a high voltage plateau above 4.0 V, which makes it among the few candidates having theoretical energy density above 1200 Wh/kg.
However, poor electronic conductivity seriously hinders its electrochemical performance. Unfortunately, carbon-coating, which has significantly enhanced the electronic transportation of many polyoxyanion materials, is usually fruitless and frustrating in Co-silicates. In the past decade, the reversible capacity of Li2CoSiO4 has been staying 75 mAh/g above 3.0 V or 107 mAh/g above 1.5 V despite extensive efforts.
To break the development bottleneck of Co-silicates, first-principles calculations had been performed by our group to investigate the changes of lattice and electronic structure for understanding electrochemistry of Li2CoSiO4. With the guidance of theoretical calculations, we had evaluated several mechanisms that may control the electronic structures of Co-silicates in doping. At the same time, a series of innovations in material synthesis were also carried out to improve the conductivity of the materials. Under such a joint effort, a P-doped Li2CoSiO4, whose discharge capacity up to 144 mAh/g above 2.5 V, was successfully synthesized for the first time in 2016. Recently, we develop a novel Mn-doped Li2CoSiO4 with more advance performance. This new scheme not only reduces the proportion of Co, thus enhancing the cost competitiveness, but also improves the electrochemical performance effectively, resulting in a more highly leveraged possibility for the application of cobalt based silicates as high energy cathode materials. The synthesized Li2MnxCo1-xSiO4/C could deliver a reversible capacity up to 208 mAh/g in the voltage window of 1.5-4.8 V, which is almost double of the highest record in the last ten years. Moreover, we are surprising to find that the Mn involves in the redox reaction during charging and discharging. This is in contrast to that of the LiNixCoyMn1-x-yO2 and Mn-silicates. The introduction of Mn may bring a synergistic effect with Co redox, leading to the significant increase in reversible capacity.
In summary, with the joint theoretical and experimental studies, we design and synthesis a capacity record-breaking Li2MnxCo1-xSiO4 and for the first time realize a reversible capacity above 200 mAh/g for Co-silicates. This on the one hand opens a new window for the further research of silicate material, on the other paves an avenue to a new prototype of advanced high energy cathode materials from tetrahedral groups.

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