Modulating the lattice structure via Cr3+ doping in LiFe0.4Mn0.6PO4 cathode for improved rate behavior and promoted cyclic performance

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Although LiFe0.4Mn0.6PO4 cathode shows high potential in battery industry, the cyclic performance and rate behavior are still the concerns to overcome. In this paper, Cr3+ doping in the cathode is confirmed very effective to improve both the performance via increased Li migration kinetics and suppressed lattice expansion during the redox process. It is found from the XRD refinement that Li − O bond length is increased upon Cr3+ intercalation, which in turn leads to the decrease in migration energy barrier (from 48.41 KJ·mol−1 to 19.57 KJ·mol−1) and the increase in Li diffusion coefficient. GITT investigation reveals a big promotion of DLi from 7.38 × 10−11 cm2·s−1 to 4.47 × 10−10 cm2·s−1, which is regarded as the reason for the excellent rate capacity of 126.62 mAh·g−1 (80.8 % of the value at 0.1C) at 5C current density. Further study with ex-situ XRD investigation reveals that the lattice expansion during redox process is suppressed after Cr3+ doping due to the smaller radius and stronger bond energy of the substituted cations. The suppressed volume change decreases both the internal resistance and the polarization of the electrode, which therefore leads to an improved capacity retention. The initial discharge specific capacity of LFMP-2%Cr cathode material reaches 156.8 mAh·g−1 at 0.1C, and the capacity retention rate of the material is maintained as 98.4 % after 100 cycles at 0.5C. The above results reveal the effectiveness of Cr3+ in promoting the electrochemical performance and further confirm the Cr- doped LFMP cathode high potential in future application.