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INTRODUCTION

LITHIUM-ION batteries (LIBs) have attracted broad attention due to their high energy and power densities, long lifespans, wide operating temperature, and environmental benignity.[1–3] Layered-LiCoO2 and spinel-LiMn2O4 are the first commercial cathode materials for LIBs but have some limitations for large-scale applications due to their high cost, low safety, and toxicity.[4–6] Transition-metal phosphate Li3V2(PO4)3 (LVP) has attracted considerable interest as the cathode materials for rechargeable LIBs due to its intrinsic structural and chemical stability brought by the three-dimensional polyanion (PO4)3 framework, a wide insertion/extraction voltage of 4.0 V (~ 0.6 V higher than LiFePO4), and a high theoretical capacity (133 mAh g1 in the potential range of 3.0 to 4.3 V).[7] Nevertheless, LVP still faces challenges such as a negligible electronic conductivity (2.4 9 107 S cm1) HE WANG, LONGFANG LI, SHULAN WANG, and XUAN LIU are with the Department of Chemistry, School of Science, Northeastern University, Shenyang 110819, P.R. China. Contact emails: [email protected], [email protected] LI LI is with the School of Metallurgy, Northeastern University, Shenyang 110819, P.R. China. Contact emails: [email protected], [email protected] Manuscript submitted August 15, 2018.

METALLURGICAL AND MATERIALS TRANSACTIONS A

and a low ionic conductivity due to the intrinsic separated VO6 octahedral arrangement.[8,9] To address this issue, many methods have been explored, including doping with metal ions,[5,10,11] surface coating with conductive materials,[12] introducing other oxides,[13] and reducing particle sizes of the active materials.[14] Metal ion doping is viewed as an effective approach to improve the microstructure and charge transfer as well as electrochemical performances of LVP.[15] To date, many metal ion dopants, including K+, Fe3+, Ni2+, Al3+, Zn2+, Mg2+, Ca2+, and B3+, have been investigated for replacing V3+ or Li+ in LVP.[8,10,16–20] K-doped LVP with N-doping carbon coating Li2.99K0.01V2(PO4)3/C+N shows an enhanced conductivity and electrochemical performance.[8] Al-substituted C-Li3V1.98Al0.02(PO4)3 cathode displays a high discharge capacity of 182 mAh g1 in the potential range of 3 to 4.8 V.[10] The discharge capacity of B-doped Li3V2(P0.97B0.03O4)3/C is as high as 130 mAh g1, with a high capacity retention of 98 pct after 100 cycles.[20] Surface carbon coating can reduce contact resistances between the LVP active material and the electrolyte to achieve enhanced electrochemical performances and, therefore, is considered as a feasible and economic method.[21–23] However, most conventional carbon loading methods are limited to nanoscales, while hierarchical pore structures, including micro-, meso-, and macropores, are highly desirable structures that can provide fast transport channels of charges and

result in enhancement of the electrochemical performances of the materials.[24] Herein, Na- and Cr-doped hierarchical porous LVP/C composite materials with full trimodal pores wer