# Abstract

Li-rich layered oxides have been regarded as valuable cathode materials for high energy density lithium-ion batteries. However, high initial irreversible capacity, bad rate capability, as well as serious capacity fading and voltage decay hinder their commercial application. In this paper, a nano CoF2 protective layer is coated on the surface of Li1.2Ni0.2Mn0.6O2 via a facial wet chemistry method. A high initial discharge capacity of 264.4 mAh g   1 is obtained for 0.5% CoF2-coated sample and 259.1 mAh g   1 for 1% CoF2-coated sample owing to the suppression of irreversible release of O2 and the contribution of electrochemical conversion of CoF2/LiF. Furthermore, 1% CoF2-coated sample exhibits the excellent rate capability of 167.5 mAh g   1 at 5 C rate, the superior cycling stability with the capacity retention of 241.0 mAh g   1 and the ameliorative voltage drop of 0.312 V at 0.1C after 100 cycles. The enhanced rate performance as well as stability of capacity and voltage can be attributed to the nano coatings which inhibit the electrolyte-electrode side reaction, enhance the electrochemical kinetics and mitigate structure transition from layered to spinel phase.

# Body

 Nano-CoF2 is fi rstly coated on Li1.2Ni0.2Mn0.6O2 via a wet chemistry method.  0.5% CoF2-coated sample delivers the improved initial coulombic ef fi ciency of 86.2%.  Excellent stability of capacity and voltage are observed for 1.0% CoF2coating.  The mechanisms of serious fading of capacity and voltage are discussed in details.  Structure transition is suppressed by CoF2 nano-coating evidenced by TEM analysis.

# 1. Introduction

To meet more demanding requirements for portable electronics, electric vehicles and hybrid electric vehicles, rechargeable lithium- matic view of discharge process cycle. The oxidation peaks of Ni and coulombic ef fi - layer restraining 4.8 V. CF-0.1, CF-0.5 1 207.2 mAh g of CoF2 coating amount in consequence of effective suppression of the loss of O2 by nano-coating layer. And the reduction peaks of Ni 4 þ / Ni 3 þ /Ni 2 þ at approximately 3.75 V as well as the weak reduction 225.3 mAh g   and 241.0 mAh g   with the capacity retention of 81.5%, 85.2% and 93.0% after 100 cycles, respectively, while much lower discharge capacity of 153.7 mAh g   1 and capacity retention peaks corresponding to Mn 4 /Mn 3 initial discharge process are observed. Besides, the intensity of reduction peaks of Mn 4 þ /Mn 3 þ wanes with increasing the amount of CoF2 coating as a result of the reduction of MnO2 with electro- 1 and 259.1 mAh g as well as larger fi rst coulombic ef fi ciencies of 82.1%, 86.2% and 84.4% than bare LNMO (244.0 mAh 75.5%), respectively, which mainly attributed to the suppression of side reaction between electrolyte and active material at high potential by CoF2 protective layer, less electrolyte oxidation resulting from less oxygen removal, proved by CV results (Fig. 5(b)), and additional capacity provided by reduction reaction of CoF2 during initial discharge process, the electrochemical conversion reactions of CoF2 shown as follows: CoF2 þ 2Li þ þ 2e   / Co þ 2LiF [31]. 1 1 of 63.0% for bare LNMO is obtained at 0.1C. The reason for this couple at about 3.25 V during superior cyclic stability, which generally enhances as the content of CoF2 nano-coating increases, involves two aspects. One reason can be the subdued interfacial reaction and the reduction of TM dissolution bene fi ting from the protection of coating layer. The other reason is the reduction of Li þ irreversible dissolution and TM migration owing to less amount of O2 release, which suppresses the phase transformation from layer to spinel structure upon cycling. Voltage decay resulting from the layer-spinel structure conversion, which gives rise to serious decrease in energy density upon cycling, is one of the major issues for Li-rich cathode materials [44,45]. The discharge voltage stability curves of all the samples upon cycling shown in Fig. 6(b), are employed to evaluate the voltage decay. The LNMO cathode material suffers from the serious voltage fading with a large voltage drop of 0.675 V at 0.1C after 100 cycles, while the values of voltage drop for CF-0.1, CF-0.5 and CF-1.0 are 0.542 V, 0.482 V and 0.312 V, respectively. Obviously, the voltage fading rate ef fi ciently slowed down as the increase of CoF2 coating content, which can be attributed to the weak electrode/ electrolyte interfacial reaction and the enhancement of structural 1 g and Fig. 5. extraction/insertion of Li stability. 1 of 914.9 Wh kg 1 compared with 872.9 Wh kg for LNMO, as a result of improved but also prevents the extrac- exhibits the improved stability with the capacity retention of initial discharge capacities and similar operating voltage after CoF2 coating. Accordingly, the excellent stability of capacity and voltage 93.6% and the value of voltage drop of 0.299 V after 100 cycles as shown in Fig. S1(inset a and b), which is close to the results of CF- for CF-1.0 delivers the highest retention value of 84.9% after 100 cycles despite reducing on the fi rst discharge capacity than CF-0.5, while LNMO exhibits the lowest retention value of 51.1%.

# In order to investigate the capacity and voltage stability for the

sample whose CoF2 coating content is higher than 1.0 wt%, Fig. S1 in