Lithium Rich Layered Material with Excellnt Rate Performance and High Efficiency

Introduction

Lithium rich layered material has attracted much attention because of its high capacity, however, the low effciency in the 1st cycle and poor rate property is a big issue for its practical use. We adopted a (NH₄)₂S₂O₈ treatment on Li_1.2Ni_0.16Co_0.08Mn_0.56O₂ material, the prodcut showed a greatly increased 1^st cycle efficinecy of above 90% and a capacity as high as 200mAh. g ^-1at 4C rate.

Experiment

Li_1.2Ni_0.16Co_0.08Mn_0.56O₂ was prepared by a rheological method. Stoichiometric amount of Li₂CO₃, Ni(CH₃COO)₂·4H₂O, Co(CH₃COO)₂·4H₂O, Mn(CH₃COO)₂·4H₂O and oxalic acid were mixed. And then a proper amount of water was added. After stirring the mixture at 80-90 ºC for 4 h, it was dried at 120 ºC for 10 h, followed by annealing at 850 ºC for 20 h. (NH₄)₂S₂O₈ solution with different concerntration was used to treat the above Li_1.2Ni_0.16Co_0.08Mn_0.56O₂ sample, the product was named as 20%-RL,30%-RL, 40%-RL, 50%-RL according the different concerntration of (NH₄)₂S₂O₈solution.

Cycling test was conducted to evaluate the cyclcing stability and rate property of the products. The samples were characterized in terms of XRD, SEM, XPS and Raman measurement.

Result and discussion

The 1^st charge/discharge curves are compared in Fig.1. The pristine Li_1.2Ni_0.16Co_0.08Mn_0.56O₂ shows an intial caapcity of 224.0 mAh g^-1 but with a low efficiency of 79%. With the increasing of the (NH₄)₂S₂O₈ concerntration, the product shows an increased effciency and reachs 99% in the “40%-RL” sample. Additionally, the (NH₄)₂S₂O₈ treatment leads to an improvement in the cycling stability. In Fig.2, untreated Li_1.2Ni_0.16Co_0.08Mn_0.56O₂ maintains 88% capacity in the 100^thcycle, while the “40%-RL” sample shows capacity retention of 97% after 100 cyles.

The (NH₄)₂S₂O₈-treated samples also present greatly improved rate performance. In Fig.3, the increase of the current rate from 0.1C to 4C causes a capacity decrease from 255mAh.g^-1 to 67mAh.g^-1 for the pristine Li_1.2Ni_0.16Co_0.08Mn_0.56O₂ sample. For the (NH₄)₂S₂O₈-treated samples, they all exhibit a much better rate property than the untreated phase. Especially for the “30%-RL” and “40%-RL” sample, they both present a capcity above 200mAh.g^-1at the 4C rate.

XRD measurements prove that the (NH₄)₂S₂O₈-treatment weakens the super lattice in the Lithium rich layered material. ICP and Raman analysis furtheres reveal that the Li content in Li₂MnO₃ decreased after the (NH₄)₂S₂O₈-treatment, which can be the main reason of the efficiency increase in the 1^st cycle.