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 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.

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%.

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