Contents lists available at ScienceDirect ## Journal of Power Sources j o urnal homepage: www.elsevier.com/locate/jpowsour ## Suppressing capacity fading and voltage decay of Li-rich layered cathode material by a surface nano-protective layer of CoF2 for lithium-ion batteries Shaokun Chong a , Yuanzhen Chen a , Wuwei Yan a , Shengwu Guo a , Qiang Tan a , Yifang Wu b , Tao Jiang a , Yongning Liu a, * - a State Key Laboratory for Mechanical Behavior of Materials, School of Material Science and Engineering, Xi'an Jiaotong University, Xi'an, 710049, PR China b Northwest Institute for Nonferrous Metal Research, Xi'an, 710016, PR China ## h i g h l i g h t s -  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. ## a r t i c l e i n f o Article history: Received 8 July 2016 Received in revised form 2 September 2016 Accepted 6 September 2016 Available online 29 September 2016 Keywords: Lithium-ion battery Cathode material CoF2 coating layer Capacity fading Voltage decay * Corresponding author. E-mail address: ynliu@mail.xjtu.edu.cn (Y. Liu). ## g r a p h i c a l a b s t r a c t ## a b s t r a c t 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. © 2016 Elsevier B.V. All rights reserved. ## 1. Introduction To meet more demanding requirements for portable electronics, electric vehicles and hybrid electric vehicles, rechargeable lithium- ion vacancy in TM layers during initial discharge procedure resulting in a large irreversible capacity loss [31,42,43], corresponding sche- matic view discharge of extraction/insertion process in Fig. 5(b) presents the CV pro fi les of all the samples for the initial cycle. The oxidation peaks of Ni well as the peaks of the release of O2 at about 4.75 V can be observed clearly for all the samples during initial charge process, while the peak intensity of the release of O2 weakens gradually as the increase exhibit 1 and CF-1.0 the discharge 1 capacity and coulombic ef fi - ciency of CF-1.0 slightly decrease compared to CF-0.5, which is layer restraining Fig. 6(a) shows the cycling stability of all the cathode materials 4.8 V. CF-0.1, CF-0.5 of 207.2 mAh g 1 , 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 þ /Mn 3 þ couple at about 3.25 V during 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- | chemical | activity after the activation | of Li 2 MnO 3 , | consistent with the | |------------------|---------------------------------|--------------------|-----------------------| | change tendency | of the release of | 2 . | | | As shown | in Fig. 5(a), all the | samples deliver | similar initial | | charge capacity, | while CF-0.1, | CF-0.5 and CF-1.0 | samples exhibit | | higher initial | discharge capacities | of 254.3mAhg 1 , | 264.4mAhg 1 | and 259.1 mAh g as well as larger fi rst coulombic ef fi ciencies of 82.1%, 1 86.2% and 84.4% than bare LNMO (244.0 mAh $ g 1 and 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]. of 63.0% for bare LNMO is obtained at 0.1C. The reason for this 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 Fig. 5. (a) Initial charge-discharge curves of bare LNMO, CF-0.1, CF-0.5 and CF-1.0 from 2.0 V to 4.8 V at 0.1C and (b) corresponding cyclic voltammetry pro fi les; (c) schematic view of extraction/insertion of Li stability. Fig. 6(c) presents energy density fading curves upon cycling, which re fl ect the integration stability of capacity and voltage. CF- 0.1, CF-0.5 and CF-1.0 reveal much higher initial energy densities of 914.9 Wh kg 1 , 949.1 Wh kg compared with 872.9 Wh kg 1 for LNMO, as a result of improved the one hand, the relatively thicker coating layer not only sup- but also prevents the extrac- in layered structure, resulting in low capacity. On the other hand, the low reduction potential of 2.854 V for CoF2/ LiF can reduce the operating voltage of CF-2.0. Moreover, CF-2.0 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 ## supplementary materials presents the operation voltages versus speci fi c capacities of 2.0 wt% CoF2-coated sample (CF-2.0) and other four samples after 1st cycle and 100 th cycle, respectively. The initial discharge capacity of 236.0 mAh g 1 and operating potential of 3.444 V for CF-2.0, lower than other four samples, are obtained. On 1.0. However, CF-2.0 delivers the much lower energy density of 694.6 Wh kg 1 than CF-0.5 and CF-1.0 owing to its low capacity and voltage. Thus, CF-1.0 reaches the optimal integrated performance between high capacity, high operating voltage and outstanding cycling stability. The excellence in rate capability, performed at various current densities from 0.1 C to 5 C between 2.0 and 4.8 V, further highlights the advantage of the CoF2 coating, as exhibited in Fig. 6(d). Distinctly, the rate performances enhance gradually as the content of CoF2 coating increases. Such that even at the current density of þ Fig. 6. (a) Cycling performance, (b) discharge median voltage, (c) energy density and (d) rate discharge capability curves of all the samples at 0.1C between 2.0 V and 4.8 V upon cycling. 5 C, CF-1.0 exhibits the highest capacity of 167.5 mAh g 1 , yet that of LNMOis only 120.1 mAh g 1 . Moreover, when the current density is Fig. 7. back to 0.1 C, the discharge capacity returns to 98.7% of its initial value for CF-1.0, implying good reversibility and structural stability after high-rate measurements. The primary reason for the signi fi cantly improved rate capability of CF-1.0 is that the introduction of CoF2 nano-coating layer, which accelerates the electrons transport and Li þ diffusion, and inhibits the interfacial reaction between electrolyte and active materials. 3.3. Improved electrochemical kinetics in CoF2 nano-coating layer To get insight into the causes for signi fi cantly improved cyclic stability and rate performance of CoF2-coated samples, EIS mea- fi tted with the equivalent circuits in the inset. Fig. 7(a) presents the EIS curves of the cells at open circuit potential before cycling. The intercept on the x-axis at the highest frequency is related to ohmic resistance of the electrolyte (Rs), the diameter of the semicircle EIS pro fi les of LNMO, CF-0.1, CF-0.5 and CF-1.0 (a) at open circuit potential and (b) at charged state of 4.3 V after 100 cycles. surements of all fi ve samples were performed respectively, whose represents charge-transfer resistance (Rct), and the slope line at the low frequency is assigned to the Warburg impedance (Zw) exhibiting the diffusion of Li þ in the electrode material. It can be obviously seen that a signi fi cant decrease in the Rct with the increase of CoF2 coating amount except for the similar values of Rs and Zw, exhibiting better electrochemical kinetics on the electrode/electrolyte interface for CF-1.0. The EIS spectra at the charged potential of 4.3 V after 100 cycles at 0.1 C, with a high-frequency semicircle ascribed to the resistance (Rf) of the solid electrolyte interface (SEI) and a medium-frequency corresponding to charge-transfer resistance (Rct), are presented in Fig. 7(b). As expected, CF-1.0 appears the lowest values of Rf and Rct, indicating that the formation and Fig. 8. (b3 (a) Schematic view of structural transformation from layered to spinel phase upon cycling for LNMO active materials; (b1) TEM image, (b2) corresponding SAED pattern and and b4) dark fi eld images of LNMO after 50 cycles; TEM images and corresponding SAED patterns of: (c1 and c2) LNMO after 100cycles, (d1 and d2) CF-1.0 after 100 cycles growth of the unacceptable SEI fi lm as well as the interfacial reaction have been mitigated by CoF2 coating layer. Thus, the improvement of electrochemical kinetics, the suppression of the formation of SEI fi lm and side reaction between electrolyte and electrode, are responsible for the outstanding stability of capacity and rate capability of CF-1.0. 3.4. Delayed phase transition in CoF2 nano-coating layer upon diffraction patterns can be observed, consisting of [111] zone axis of hexagonal layered structure and [111] zone axis of cubic spinel structure. Furthermore, the dark fi led images, corresponding to the lattice planes of (101)R and (04 ð Þ 4)C as shown in Fig. 8(b3 and b4), respectively, clearly reveal the structure transformation from layered to spinel phase after 50 cycles and the spinel structure originating from the surface of active material. The phase transition upon cycling is ascribed to the migration of TM ions from TM layers to Li layers induced by the irreversible release of oxygen and extraction of Li þ after the initial cycle [10,46 e 48], which leads to between 2.0 and 4.8 V at 0.1 C. The index marked by subscript R and C is related to the rhombohedral (layered LiNi0.5Mn0.5O2) phase and cubic (spinel LiMn2O4) phase. serious capacity fading and voltage decay, as illustrated in Fig. 8(a). After 100 cycles for LNMO, a thick layer, namely, SEI fi lm formed on the surface of grain, indicative of the severe side reaction between electrolyte and electrode as shown in Fig. 8(c1). And the SAED pattern exhibits [100] zone axis of cubic spinel phase on the whole cycling To disclose the reasons for improving the capacity fading and voltage decay upon cycling of CoF2-coated samples, LNMO and CF1.0 after several cycles were observed using TEM. Fig. 8 (b1 and b2) presents TEM image and corresponding SAED pattern of LNMO after 50 cycles at 0.1 C between 2.0 and 4.8 V. Two types of particle in Fig. 8(c2), indicating complete structure transition after 100 cycles. However, the thin SEI fi lm mixed with CoF2 coating layer can be observed in Fig. 8(d1) for CF-1.0 after 100 cycles owing to the inhibition of meanwhile it the release still which is in good agreement with the best stability of capacity and operating voltage. Therefore, the phase transition from layered to spinel can be delayed effectively by CoF2 nano-coating layer upon cycling. 4. Conclusions In this paper, a nano protective layer of CoF2, in situ synthesized by means of a common wet chemistry technology, was coated on LNMO. The results of SEM, TEM, EDS and XPS demonstrate that CoF2 coating, belong to cubic structure, distributes homogeneously on the surface of LNMO, and the cell tests chemical performances are enhanced obviously for CoF2-coated samples. 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