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Effects of fast lithium-ion conductive coating layer on the nickel rich layered oxide cathode material
Ceramics International 45 (2019) 3177–3185
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Effects of fast lithium-ion conductive coating layer on the nickel rich layered oxide cathode material ⁎
T
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Meng Wanga, , Yongqiang Gonga, Yijie Gub, , Yunbo Chena, Lin Chena, Hua Shia a b
Beijing National Innovation Institute of Lightweight Ltd., Beijing 100083, China College of Materials Science and Engineering, Shandong University of Science and Technology, Qingdao 266510, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Lithium-ion battery Cathode material Fast ionic conductor Coating layer Li+ diffusion coefficient
In this study, a series of ultra-thin fast ionic conductor (Li3PO4, Li2ZrO3, Li4Ti5O12) layers were successfully deposited on the surface of LiNi0.8Mn0.1Co0.1O2. The effects of three typical ionic conductors were systemically compared for the first time. The influences of coating layers on the microstructures and electrochemical properties of the cathode material were investigated by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), high-resolution transmission electron spectroscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and electrochemical tests. Analysis indicated that the coating layers existed on the surface of the cathode material and did not cause any noticeable change in the crystal structure. Electrochemical tests proved that all the surface-modified samples exhibited excellent cycling performance and rate capability compared to the bared sample. The inferior electrochemical performances of the bared sample were related to the formation of thick solid-electrolyte inter-facial layer during cycling, while the coating layer could minimize the side-reactions between the cathode and electrolyte during cycling. The electrons transfer and Li+ diffusion coefficient of the Li3PO4 coated sample were superior to that of Li2ZrO3 and Li4Ti5O12 coated samples, which were beneficial to the rate capability of LiNi0.8Mn0.1Co0.1O2.
1. Introduction The nickel-rich layered transition metal oxides LiNixMnyCo1-x-yO2 (x ≥ 0.8) are highly potential cathode materials for lithium-ion batteries used in electric and hybrid electric vehicles, due to their high specific capacity, relatively low toxicity and good rate capacity. However, the high nickel content in these cathode materials induces inherent structural instability, which invariably leads to degradation in battery performance during cycling [1–3]. The highly delithiated nickel-rich cathode materials become unstable due to the appearance of abundant Ni4+ ions, which react with organic electrolyte and then increasing the impedance and lowering the cyclability of batteries [4,5]. Besides that, other side reactions such as dissolution of the metal ions from the cathode into the electrolyte and the formation of an inactive solid electrolyte interface (SEI) on the cathode surface also have negative effects on the cycling and rate properties [6–8]. For improving the electrochemical performance of nickel-rich cathode materials, one widely used method is to coat a protective layer on the surface of them. Various surface modification compounds have been reported, such as TiO2 [9], Al2O3 [10], SiO2 [11], Mn3(PO4)2 [12],
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LiPON [13] and AlF3 [14]. These coating layers can obviously improve the cycling performance compared to the pristine materials. These improvements are mainly attributed to the coating layers reducing reactions between electrode and electrolyte, minimizes the impact of side reactions during extensive cycling [15–17]. However, these coating materials are usually lithium ion insulator, which is detrimental to the rate capability and discharge capacity of cathode materials. An ideal coating material is act as a protective layer as well as a lithium-ion migration promotion layer [18]. Therefore, some lithium-ion conductors are employed as surface coating materials to maximize the performance of the cathode materials, such as Li3PO4 [19], Li2SnO3 [20], LiAlO2 [21], Li2ZrO3 [22,23] and LiPON [13]. These coating materials have higher ionic conductivity than oxides, because they provide conduction channels which facilitate rapid transportation of lithium ions from the cathode material to the electrolytes [22,24,25]. Although there are some reports about adopting lithium ions conductive materials as an effective coating materials for protecting the surface of cathode materials, there is few report that systematically compare the effects of different lithium ions conductive coating layers on the electrochemical performance of the cathode materials. In this
Corresponding authors. E-mail addresses: [email protected] (M. Wang), [email protected] (Y. Gu).
https://doi.org/10.1016/j.ceramint.2018.10.219 Received 31 August 2018; Received in revised form 17 October 2018; Accepted 26 October 2018 Available online 28 October 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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study, the Li3PO4, Li2ZrO3, Li4Ti5O12 were successfully deposited on the surface of the LiNi0.8Mn0.1Co0.1O2 by hydrothermal treatment. And aim to explore the relatively excellent lithium ions conductive coating materials to achieve the outstanding electrochemical performance for nickel-rich layered cathode materials. 2. Experimental The Ni0.8Mn0.1Co0.1(OH)2 precursor was a commercial material obtained from Ningbo Jinhe New materials Co.,Ltd. Other raw materials, such as (NH4)2HPO4, Zr(NO3)3, C16H36O4Ti, LiOH and polyving akohol (PVA) were produced by Sinopharm Chemical Reagent Co., Ltd. To prepare the Li3PO4 or Li2ZrO3 or Li4Ti5O12 coated LiNi0.8Mn0.1Co0.1O2, Ni0.8Mn0.1Co0.1(OH)2 powder was dispersed in 70 ml deionized water by strong stirring. Then the stoichiometric ratio of (NH4)2HPO4 or Zr(NO3)3 or C16H36O4Ti with LiOH and 5 wt% PVA were added to the precursor suspension water and dispersed by strong stirring for 2 h. The well-dispersed suspension was transferred into a 100 ml hydrothermal reaction vessel and maintained at 180 °C for 24 h. The prepared mixtures were first preheated at 450 °C for 6 h and then annealed at 800 °C for 12 h in a continuous flow of O2 to obtain the 1.0 wt% Li3PO4 or Li2ZrO3 or Li4Ti5O12 coated LiNi0.8Mn0.1Co0.1O2 (were abbreviated as LPO, LZO and LTO), respectively. For comparison, the bared LiNi0.8Mn0.1Co0.1O2 (was abbreviated as B) was prepared by the same hydrothermal reaction and heat treatment process. The preparation process is shown in Scheme 1. The X-ray diffraction (XRD) was performed with Rigaku Ultima IV185 (Cu Kα radiation) between 10° and 80° 2θ at a scan rate of 8° min−1. The morphology of materials was observed by a FEI Quanta 250 field emission scanning electron microscope (FESEM). The high resolution transmission electron microscopy (HRTEM) was applied to observe the micro-structure of the materials on a JEM2100. The X-ray photoelectron spectroscopy (XPS, PerkineElmer, PHI 5600) measurements were performed to get information on the surface of the materials. The cathode electrodes were prepared by mixing cathode material, acetylene black and polyvinylidene fluoride (PVdF) in a mass ration of 85:10:5. The obtained slurry was coated on the surface of aluminum foil and dried at 55 °C for 8 h. The CR2025 coin type cells were assembled with metal lithium foil as the counter electrode and 1 M LiPF6 in ethylene carbonate (EC)-dimethyl carbonate (DMC) (1:1, v/v) as the electrolyte. The galvanostatic charge-discharge tests were performed on CT2001A Land instruments between 2.8 and 4.5 V at room temperature, and the current density of 0.1 C is defined as 15 mA g−1. The cyclic voltammetry (CV) was investigated on a CHI604D electrochemical workatation within scanning speed at 0.1 mV s−1. The electrochemical impedance spectroscopy (EIS) measurements were carried out using a CHI604D impedance analyzer, with an amplitude voltage of 5 mV and the frequency range of 10−2–105 Hz.
Fig. 1. Powder XRD patterns of the sample B, LPO, LZO and LTO.
NaFeO2-type structure with a space group of R3¯m . The clear split peaks of (006)/(102) and (108)/(110) suggests a typical layered structure for all the samples. No peak shifts are observed in the XRD patterns of the coated materials, indicating the coating layers do not cause any noticeable change in the crystal structure. The lattice parameters are calculated from the diffraction data and displayed in Table 1. The lattice parameter c represents the average metal–metal interslab distance, and the large c is generally associated with fast Li-ion insertion/extraction. The c and c/a values are increased after coating, indicates that the lattice has the priority to grow along the c axis and thus to promote electrochemical reactions. The I003/I104 of LPO, LZO and LTO are higher than that of sample B, which is partially due to the proper coating materials can reduce the mixing degree of Ni2+ with Li+ in 3a site. The higher I003/I104 value implies coated materials will exhibit superior electrochemical performances [26]. The SEM images of the Ni0.8Mn0.1Co0.1(OH)2 precursor are shown in the Fig. 2(a),(b). It can be seen that the precursor particle is spherical with a diameter of about 13 µm which are composed of many fine particles. The surface morphology of the sample B, LPO, LZO and LTO are compared in Fig. 2(c)-(f). It can be seen that all the particles are secondary agglomeration spherical particles with approximately 10 µm in diameter. Contrary to the smooth and clean surface of B, the surface morphology of the coated sample LPO, LZO and LTO exhibit rough surface morphology. HRTEM images were further used to observe and analyze the microregion structure of the samples. As shown in the Fig. 3, the sample B presents a smooth edge line without any layer on the surface. By contrast, an evenly distributed amorphous coating layer of about 2–4 nm is clearly seen on the surface of all the coated materials. It demonstrates that the hydrothermal coating method used in this study is an effective way to obtain a uniform and ultra-thin coating layer on the surface of LiNi0.8Mn0.1Co0.1O2. The Fig. 3 shows that all the samples presents the well-defined layered structure, and the spacing of two adjacent lattice fringes is equal to 0.47 nm corresponding to the d − value of (003) plane. The typical R3m structure can be identified by the diffraction spots in the fast Fourier transformation (FFT), and the FFT of the sample B and LPO are inserted in the corresponding HRTEM images. The FFT of the sample B indicates the (003) and (101) planes of LiNi0.8Mn0.1Co0.1O2, while that of the sample LPO shows an array of hexagonal symmetry dots and demonstrates an in-plane [ 3 × 3 ] R30° ordering in the transition-metal layers [27]. The FFT and lattice
3. Results and discussion 3.1. X-ray diffraction and morphology Fig. 1 shows the XRD patterns of the sample B, LPO, LZO and LTO. All the reflections could be indexed to a well-defined hexagonal α-
Table 1 Lattice parameters of the NMC, LPO&NMC and H-LPO&NMC materials.
Scheme 1. Schematic illustration of the synthesis of coated cathode material. 3178
Samples
a(Å)
c(Å)
c/a
I(003)/I(104)
B LPO LZO LTO
2.872 2.873 2.875 2.872
14.190 14.224 14.241 14.196
4.941 4.951 4.953 4.943
1.114 1.389 1.368 1.397
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Fig. 2. FESEM images of the (a,b) Ni0.8Co0.1Mn0.1(OH)2 precursor magnified to 5000 and 10000 times; (c) sample B, (d) sample LPO, (e) sample LZO and (f) sample LTO magnified to 20000 and 30000 times.
2p3/2 are located at 795.1 eV and 780.4 eV, while that of Mn 2p1/2 and Mn 2p3/2 are at 642.6 eV and 653.8 eV, respectively. The values suggest that the valance states of Co and Mn are + 3 and + 4, respectively [30,31]. For the sample LPO, there is a strong P 2p peak at at 133.5 eV which is consistent with PO43 − that reported in other literature [32]. For the sample LZO, there are two strong peaks at 181.9 eV and 184.3 eV, which are corresponding to the typical Zr 3d5/2 and Zr 3d3/2 peaks [33,34]. For the sample LTO, the two peaks of Ti 2p around 458.0 eV and 464.1 eV can be attributed to Ti4+ [35,36]. According to XPS analysis, the Li3PO4, Li2ZrO3 and Li4Ti5O12 layer has been successfully deposited on the LiNi0.8Mn0.1Co0.1O2 surface, respectively.
fringes indicating that the coating layers have no effect on the cathode crystal structure. The oxidation states of surface elements in B, LPO, LZO and LTO are analyzed by XPS measurement, and the corresponding spectra are illustrated in the Fig. 4. Both materials have Ni2p, Co2p and Mn2p peaks without remarkable binding energy shift, which indicate that the valence states of transition metal ions of the materials are not changed after coating. According to the researches, the Ni 2p3/2 peaks of Ni2+ and Ni3+ are located at 854.4 eV and 855.8 eV, respectively [28,29]. Accordingly, a strong Ni 2p3/2 peak appears at 855.4 eV can be attributed to lots of Ni3+ and a small amount of Ni2+ in the nickel-rich materials. Furthermore, the binding energy peaks of Co 2p1/2 and Co 3179
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Fig. 3. HRTEM images of the sample B, LPO, LZO and LTO.
delivered a discharge capacity of 211.7 mAh/g. This is a little higher than those of the coated samples, which are 210, 200.5 and 208.4 mAh/ g for the LPO, LZO and LTO, respectively. This is mainly due to the decreased amount of electrochemical active material that is replaced by the coating materials. Although there is a slight decrease in discharge capacities for the coated samples, the initial coloumbic efficiencies are
3.2. Electrochemical performance Fig. 5 shows the initial charge/discharge voltage profiles of the sample B, LPO, LZO and LTO tested between 2.8 and 4.5 V at 0.1 C at room temperature. All the samples exhibit similar smooth charge/discharge profiles with a typical potential plateau at 3.75 V. The sample B
Fig. 4. XPS spectra of Ni 2p, Co 2p, Mn 2p, P 2p, Zr 3d and Ti 2p of the sample B, LPO, LZO and LTO. 3180
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Fig. 5. Initial charge/discharge voltage profiles of the sample B, LPO, LZO and LTO cathode electrodes at 0.1 C.
there is no obvious difference in the capacity retention rate for the coated samples, the sample LPO delivered the highest discharge capacity of 192.4 mAh g−1 at the 45th cycle, while sample B only has 172.4 mAh g−1 after 45 cycles. The coloumbic efficiency of each sample is maintained at about 97–100%. The improved capacity retention for the coated samples may be attributed to the surface modification of Li3PO4, Li2ZrO3 and Li4Ti5O12, which can hinder the direct contact with electrolyte and inhibit the side reactions between the electrode and electrolyte caused by HF etching [40–42]. To further evaluate the coating effects, the rate capability property was investigated for the samples and the results are shown in the Fig. 7a,b. Each cell was charged at 0.1 C and then discharged at 1.0–5.0 C. All the samples exhibit gradual drops of discharge capacity with the increase of the current density. Noticeably, the coated samples exhibits superior enhanced rate performance compared with the bare sample, especially at high C-rates. The capacity retentions of LPO, LZO and LTO at 5.0 C are 91.0%, 84.4% and 82.6%, respectively, while that of B is only 77.0%. The results implies the modification by ions conductive materials provide a fast channel for rapid migration of lithium ions between electrolytes and cathode materials which improves the rate property of sample. The discharge curves of the bare and coated samples at the various current density are shown in the Fig. 7c1–4. The discharge plateau of the bare sample decreases dramatically with the increasing of current density from 0.1 C to 5.0 C. But for the coated samples, especially the LPO samples, the discharge plateau decreases tardily. Moreover, the B sample presents discharge capacities of 211.7, 187.9, 178.0, 170.7, 167.1 and 163 mAh g−1 at 0.1, 1.0, 2.0, 3.0, 4.0 and 5.0 C, respectively. Contrast to the bared sample, the LPO/LZO/ LTO samples delivered discharge capacities of 210.0/200.7/208.4, 207.4/190.7/193.8, 201.4/184.2/186.6, 197.4/179.4/181.7, 193.9/ 174.6/176.7 and 191.1/169.3/172.2 mAh g−1 at 0.1, 1.0, 2.0, 3.0, 4.0 and 5.0 C, respectively. Apparently, the LPO exhibits the most outstanding discharge capacity retention at 5.0 C (91%). This can be due to the ionic conductivity of Li3PO4 is 10−8 S cm−1 [43,44] which is higher than that of Li2ZrO3 (10–12 S cm−1) [45] and Li4Ti5O12 (10–13 S cm−1) [46,47]. Therefore, the high ionic conductivity of Li3PO4 can further improve the diffusion of Li ion at the interface of the electrode. Cyclic voltammetry (CV) is a useful technique to study the electrochemical performance and electrode kinetics of oxide materials. Fig. 8 shows the CV curves of the cells with the sample B, LPO, LTO and LZO as the cathode electrodes. The cells are tested at a scan rate of 0.1 mV s−1 in the range of 2.5–4.8 V. The CV curves of the four asprepared materials exhibited almost the same features, three couple oxidation and reduction peaks existed in the voltage range of 3.6–4.3 V.
increased to 88.0%, 83.8% and 86.0% for the LPO, LZO and LTO, respectively, when compared to that of the sample B (80.8%). This owing to a well developed lithium-ion conductors forming on the surface of the LiNi0.8Mn0.1Co0.1O2, which facilitate Li ions diffusion and thus lithium utilization. Besides that, the coating layer also act as a physical protection barrier to reduce the side reactions between the cathode materials and electrolyte, and then suppress the formation of SEI film and the depletion of lithium ions. Among the coated samples, the initial coloumbic efficiencies of LPO is the highest, while that of the LTO is relatively the lowest. This presents the Li3PO4 is much more effective than Li4Ti5O12 as the protective coating layer. After activation by one preconditioning cycles at a rate of 0.1 C, the cycle performance of all the samples are tested at 1.0 C, and the results are shown in the Fig. 6. The initial discharge capacity of the sample B is 203.3 mAh g−1, compared with 206.4, 196.8 and 204.2 mAh g−1 for the sample LPO, LZO and LTO. After 45 cycles, the sample B exhibited a rapid capacity loss to 172.4 mAh g−1 and the capacity maintains only 84.8% of its initial capacity. This undesirable performance is ascribed to the instability of the Ni-rich layered host structure caused by multiple phase transitions and transition metal dissolution of cathode surface resulting from HF attack which causes severe serrations of the cathode particle surface [37–39]. After coated, the capacity retention after cycling is remarkably increased to about 93.3% of the coated samples, which illustrate a more stable cycle life after coating. Although
Fig. 6. Cycling performance and of the sample B, LPO, LZO and LTO cathode electrodes at 1.0 C. 3181
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Fig. 7. Rate capability of the sample B, LPO, LZO and LTO cathode electrodes at various rate (from 0.1 to 5.0 C): (a) discharge capacity at various rate; (b) capacity retention rate at various rate; (c1–4) discharge curves of the samples at the various current density.
To further investigate the possible reasons for enhanced performance of the coated samples. The electrochemical impedance spectroscopy (EIS) can be used to investigate the kinetic process of lithium intercalation/deintercalation into electrodes. Fig. 9(a) shows Nyquist plots of cells assembled by using bared and coated samples as cathode active materials. The data points are experimental while red solid lines are fitted data by the Zview2 software. As shown in Fig. 9(a), each of the plots consists of two well-defined semicircles and an inclined line. The intercept of the semicircle at the highest frequency with the real axis (Z′) is related to uncompensated ohmic resistance (Rs). The semicircle at high frequency region refers to the resistance of the surface film (Rsf) due to the formation of an insulating solid electrolyte
One peak is sharp while the other two peaks are weak, which illustrating phase transitions taken place during charge and discharge process. The three oxidation peaks between 2.8 and 4.3 V are caused by the phase transition from hexagonal to monoclinic (H1 to M), monoclinic to hexagonal (M to H2) and hexagonal to hexagonal (H2 to H3) during the Li-ions extraction process [48]. The phase transitions are reversed during the discharge process, and the reduction peaks are a little lower than the oxidation peak. The corresponding potential difference between anodic peak and cathodic peak (ΔV) for the sample B is the largest, while that of the sample LPO is the smallest of all. This indicates a worse reversibility of Li+ extraction/insertion process in the coated samples, and a larger reaction polarization occurred in the sample B. 3182
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Fig. 8. CV curves at the scan rate of 0.1 mV s−1 for the sample B, LPO, LZO and LTO cathode electrodes.
Fig. 9. (a) Nyquist plots of the sample B, LPO, LZO and LTO cathode electrodes at a charged state of 4.2 V after 45 cycles; (b) the corresponding Rs, Rsf, Rct.
interface (SEI) film or the deposition of organic compounds on the electrode surface by electrolytic decomposition [49,50]. The semicircle at medium frequency are related to the charge transfer resistance through the electrolyte/electrode interface (Rct), respectively. The sloping line at low frequency is attributed to the solid-state diffusion of lithium ions in the cathode materials (Wo). The fitted resistance values are presented in Fig. 9(b). it is noted that Rs (which reflects solution resistance of the cell) for all the samples is very small and can be ignored. As can be seen, the Rsf of the coated samples are obviously suppressed compared with the bare one, which from 179.4 Ω (B) to 75.1 Ω (LPO), 65.14 Ω (LZO), 111.0 Ω (LTO), respectively. We can conclude that the surface coating layer of Li3PO4, Li2ZrO3 and Li4Ti5O12
can depress migration resistance of lithium ions through the SEI film. This implies that coating layer can effectively protect the surface of cathode materials from fast impedance growth during the (de)intercalation processes. This indicates that coating layer can obviously reduce the continuous decomposition of electrolyte on the cathode surface and then effectively suppress the growth of SEI film. Besides that, the Rct value of B sample after cycling is 350.5 Ω which is much larger than that of the coated samples LPO (112.2 Ω), LZO (142.9 Ω) and LTO (118.9 Ω). this analysis implies that coating layer could prevent the host LiNi0.8Mn0.1Co0.1O2 from directly exposed to the electrolyte, which could be easily decomposed under high voltage. The highest Rct value of B can be ascribed to both the HF erosion and the oxygen release form 3183
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the highly lithium deintercalation [51]. Furthermore, this layer also play an important role in protection of the cathode particle from electrolyte decomposition, thus facilitating electron transport through the electrolyte/electrode interface [21]. Therefore, the reason why LPO has the best electrochemical performance can be explained by its the lowest solid electrolyte interface resistance and charge transfer resistance. As mentioned above, the straight sloping line at low frequency region is attributed to the lithium ions diffusion in the bulk of the materials. Therefore, the diffusion coefficient of lithium ions (DLi+) can be calculated from the slop in the low frequency region according to the following equation [52,53]:
DLi =
R2T 2 2n4F 4A2 CLi2 + σ 2
Fig. 11. Schematic diagram of the effects of different fast ionic conductor coating layers on the electrons and lithium ions transfer.
(1)
CLi+ is the concentration of lithium ions in the materials, R is the gas constant, T is the absolute temperature, F is the Faraday constant, A refers to the electrochemically active surface area (here is surface area of the electrode), n stands for the number of electrons per molecule during redox process, σ is the Warburg factor which can be obtained by the following equation: Z ′ = Rs + R ct + σω1/2
coefficient which are the reasons for the improved rate capability and enhanced cycling stability of the LPO. LPO cathode with high-rate capability and long cycling life would be a promising high power density material, which might apply in electric vehicles and hybrid electric vehicles.
(2)
4. Conclusion
Z ′ is the real part of impedance, Rs is the electrolyte resistance, Rct is the charge transfer resistance and ω is the angular frequency. Fig. 10(a) shows the linear relationship of Z ′ and ω−1/2 and the slope of the fitted straight line indicates the σ value. According to the Eqs. (1) and (2), DLi+ values are calculated and compared in Fig. 10(b). It is clearly that the DLi+ values increased after coating, which is higher than that of the sample B (8.65 × 10−10 cm s−1). Among the coated samples, the LPO reaches to 9.56 × 10−9 cm s−1, which is one order higher than that of the pristine sample. This proves that the Li3PO4 coating can promote the migration rate of lithium ions which may account for better electrochemical performance of LPO sample. As analyzed above, the EIS results show that the Rsf+ct of the coated samples after cycling are much lower than that of the sample B, and the order of them for the samples are as follows: B (529.9 Ω) > LTO (229.9 Ω) > LZO (208.4 Ω) > LPO (187.3 Ω). This indicates that electrons transfer across the SEI film and electrode/electrolyte interface of LPO is the easiest of all. For the coated materials are the fast ionic conductors, the diffusion coefficient of lithium ions can diffuse fast in the layer structure of the coated samples, and the order of DLi+ for the samples are as follows: LPO (9.56 × 10−9 cm s−1) > LZO (3.08 × 10−9 cm s−1) > LTO (1.10 × 10−9 cm s−1) > B (8.65 × 10−10 cm s−1). Schematic diagram of the effects of different fast ionic conductor coating layers on the electrons and lithium ions transfer is shown in Fig. 11. Among the samples, the LPO possesses the lowest charge transfer resistance and highest lithium ions diffusion
In summary, a series of fast lithium-ion conductive layer are successfully coated on LiNi0.8Mn0.1Co0.1O2 cathode material by a hydrothermal treatment method. By this method, an ultrathin layer with thickness about ~ 4 nm is uniformly coated on the surface of LiNi0.8Mn0.1Co0.1O2, while its morphology and crystalline structure are well maintained. The XRD, HRTEM and XPS results indicate that the coating layer exists on the cathode particles, rather than interacting with the core material. After 45 cycles at 1.0 C, LPO maintains 93% of the initial discharge capacity. The surface and charge-transfer resistances of the coated samples are obviously suppressed after 45 cycles. Among them, the LPO shows the lowest Rsf (75.1 Ω) and Rct (112.2 Ω) compared with those of the pristine sample (179.4 and 350.5 Ω). Besides that, the lithium diffusion rate of the modified electrode are also significantly enhanced. This improvement is attributed to minimization of the side-reaction between the cathode and electrolyte by the protecting layer, the formation of an undesirable SEI layer is suppressed. Therefore, surface modification by lithium conductive material is an effective way to improve the performance of cathode materials for lithium-ion batteries. Moreover, although the coating materials used are lithium ion conductors, their ionic conductivity is different, which result in different performance of the coated cathode samples. Compared to Li2ZrO3 and Li4Ti5O12, the utilization of Li3PO4 significantly improves the performance of LiNi0.8Mn0.1Co0.1O2. This discovery is an important step towards understanding the fast lithium-ion
Fig. 10. (a) the profiles of Z ′ vs. ω−1/2; (b) DLi+ obtained from the EIS data. 3184
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conductive coating effects.
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Effects of fast lithium-ion conductive coating layer on the nickel rich layered oxide cathode material ⁎
T
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Meng Wanga, , Yongqiang Gonga, Yijie Gub, , Yunbo Chena, Lin Chena, Hua Shia a b
Beijing National Innovation Institute of Lightweight Ltd., Beijing 100083, China College of Materials Science and Engineering, Shandong University of Science and Technology, Qingdao 266510, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Lithium-ion battery Cathode material Fast ionic conductor Coating layer Li+ diffusion coefficient
In this study, a series of ultra-thin fast ionic conductor (Li3PO4, Li2ZrO3, Li4Ti5O12) layers were successfully deposited on the surface of LiNi0.8Mn0.1Co0.1O2. The effects of three typical ionic conductors were systemically compared for the first time. The influences of coating layers on the microstructures and electrochemical properties of the cathode material were investigated by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), high-resolution transmission electron spectroscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and electrochemical tests. Analysis indicated that the coating layers existed on the surface of the cathode material and did not cause any noticeable change in the crystal structure. Electrochemical tests proved that all the surface-modified samples exhibited excellent cycling performance and rate capability compared to the bared sample. The inferior electrochemical performances of the bared sample were related to the formation of thick solid-electrolyte inter-facial layer during cycling, while the coating layer could minimize the side-reactions between the cathode and electrolyte during cycling. The electrons transfer and Li+ diffusion coefficient of the Li3PO4 coated sample were superior to that of Li2ZrO3 and Li4Ti5O12 coated samples, which were beneficial to the rate capability of LiNi0.8Mn0.1Co0.1O2.
1. Introduction The nickel-rich layered transition metal oxides LiNixMnyCo1-x-yO2 (x ≥ 0.8) are highly potential cathode materials for lithium-ion batteries used in electric and hybrid electric vehicles, due to their high specific capacity, relatively low toxicity and good rate capacity. However, the high nickel content in these cathode materials induces inherent structural instability, which invariably leads to degradation in battery performance during cycling [1–3]. The highly delithiated nickel-rich cathode materials become unstable due to the appearance of abundant Ni4+ ions, which react with organic electrolyte and then increasing the impedance and lowering the cyclability of batteries [4,5]. Besides that, other side reactions such as dissolution of the metal ions from the cathode into the electrolyte and the formation of an inactive solid electrolyte interface (SEI) on the cathode surface also have negative effects on the cycling and rate properties [6–8]. For improving the electrochemical performance of nickel-rich cathode materials, one widely used method is to coat a protective layer on the surface of them. Various surface modification compounds have been reported, such as TiO2 [9], Al2O3 [10], SiO2 [11], Mn3(PO4)2 [12],
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LiPON [13] and AlF3 [14]. These coating layers can obviously improve the cycling performance compared to the pristine materials. These improvements are mainly attributed to the coating layers reducing reactions between electrode and electrolyte, minimizes the impact of side reactions during extensive cycling [15–17]. However, these coating materials are usually lithium ion insulator, which is detrimental to the rate capability and discharge capacity of cathode materials. An ideal coating material is act as a protective layer as well as a lithium-ion migration promotion layer [18]. Therefore, some lithium-ion conductors are employed as surface coating materials to maximize the performance of the cathode materials, such as Li3PO4 [19], Li2SnO3 [20], LiAlO2 [21], Li2ZrO3 [22,23] and LiPON [13]. These coating materials have higher ionic conductivity than oxides, because they provide conduction channels which facilitate rapid transportation of lithium ions from the cathode material to the electrolytes [22,24,25]. Although there are some reports about adopting lithium ions conductive materials as an effective coating materials for protecting the surface of cathode materials, there is few report that systematically compare the effects of different lithium ions conductive coating layers on the electrochemical performance of the cathode materials. In this
Corresponding authors. E-mail addresses: [email protected] (M. Wang), [email protected] (Y. Gu).
https://doi.org/10.1016/j.ceramint.2018.10.219 Received 31 August 2018; Received in revised form 17 October 2018; Accepted 26 October 2018 Available online 28 October 2018 0272-8842/ © 2018 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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study, the Li3PO4, Li2ZrO3, Li4Ti5O12 were successfully deposited on the surface of the LiNi0.8Mn0.1Co0.1O2 by hydrothermal treatment. And aim to explore the relatively excellent lithium ions conductive coating materials to achieve the outstanding electrochemical performance for nickel-rich layered cathode materials. 2. Experimental The Ni0.8Mn0.1Co0.1(OH)2 precursor was a commercial material obtained from Ningbo Jinhe New materials Co.,Ltd. Other raw materials, such as (NH4)2HPO4, Zr(NO3)3, C16H36O4Ti, LiOH and polyving akohol (PVA) were produced by Sinopharm Chemical Reagent Co., Ltd. To prepare the Li3PO4 or Li2ZrO3 or Li4Ti5O12 coated LiNi0.8Mn0.1Co0.1O2, Ni0.8Mn0.1Co0.1(OH)2 powder was dispersed in 70 ml deionized water by strong stirring. Then the stoichiometric ratio of (NH4)2HPO4 or Zr(NO3)3 or C16H36O4Ti with LiOH and 5 wt% PVA were added to the precursor suspension water and dispersed by strong stirring for 2 h. The well-dispersed suspension was transferred into a 100 ml hydrothermal reaction vessel and maintained at 180 °C for 24 h. The prepared mixtures were first preheated at 450 °C for 6 h and then annealed at 800 °C for 12 h in a continuous flow of O2 to obtain the 1.0 wt% Li3PO4 or Li2ZrO3 or Li4Ti5O12 coated LiNi0.8Mn0.1Co0.1O2 (were abbreviated as LPO, LZO and LTO), respectively. For comparison, the bared LiNi0.8Mn0.1Co0.1O2 (was abbreviated as B) was prepared by the same hydrothermal reaction and heat treatment process. The preparation process is shown in Scheme 1. The X-ray diffraction (XRD) was performed with Rigaku Ultima IV185 (Cu Kα radiation) between 10° and 80° 2θ at a scan rate of 8° min−1. The morphology of materials was observed by a FEI Quanta 250 field emission scanning electron microscope (FESEM). The high resolution transmission electron microscopy (HRTEM) was applied to observe the micro-structure of the materials on a JEM2100. The X-ray photoelectron spectroscopy (XPS, PerkineElmer, PHI 5600) measurements were performed to get information on the surface of the materials. The cathode electrodes were prepared by mixing cathode material, acetylene black and polyvinylidene fluoride (PVdF) in a mass ration of 85:10:5. The obtained slurry was coated on the surface of aluminum foil and dried at 55 °C for 8 h. The CR2025 coin type cells were assembled with metal lithium foil as the counter electrode and 1 M LiPF6 in ethylene carbonate (EC)-dimethyl carbonate (DMC) (1:1, v/v) as the electrolyte. The galvanostatic charge-discharge tests were performed on CT2001A Land instruments between 2.8 and 4.5 V at room temperature, and the current density of 0.1 C is defined as 15 mA g−1. The cyclic voltammetry (CV) was investigated on a CHI604D electrochemical workatation within scanning speed at 0.1 mV s−1. The electrochemical impedance spectroscopy (EIS) measurements were carried out using a CHI604D impedance analyzer, with an amplitude voltage of 5 mV and the frequency range of 10−2–105 Hz.
Fig. 1. Powder XRD patterns of the sample B, LPO, LZO and LTO.
NaFeO2-type structure with a space group of R3¯m . The clear split peaks of (006)/(102) and (108)/(110) suggests a typical layered structure for all the samples. No peak shifts are observed in the XRD patterns of the coated materials, indicating the coating layers do not cause any noticeable change in the crystal structure. The lattice parameters are calculated from the diffraction data and displayed in Table 1. The lattice parameter c represents the average metal–metal interslab distance, and the large c is generally associated with fast Li-ion insertion/extraction. The c and c/a values are increased after coating, indicates that the lattice has the priority to grow along the c axis and thus to promote electrochemical reactions. The I003/I104 of LPO, LZO and LTO are higher than that of sample B, which is partially due to the proper coating materials can reduce the mixing degree of Ni2+ with Li+ in 3a site. The higher I003/I104 value implies coated materials will exhibit superior electrochemical performances [26]. The SEM images of the Ni0.8Mn0.1Co0.1(OH)2 precursor are shown in the Fig. 2(a),(b). It can be seen that the precursor particle is spherical with a diameter of about 13 µm which are composed of many fine particles. The surface morphology of the sample B, LPO, LZO and LTO are compared in Fig. 2(c)-(f). It can be seen that all the particles are secondary agglomeration spherical particles with approximately 10 µm in diameter. Contrary to the smooth and clean surface of B, the surface morphology of the coated sample LPO, LZO and LTO exhibit rough surface morphology. HRTEM images were further used to observe and analyze the microregion structure of the samples. As shown in the Fig. 3, the sample B presents a smooth edge line without any layer on the surface. By contrast, an evenly distributed amorphous coating layer of about 2–4 nm is clearly seen on the surface of all the coated materials. It demonstrates that the hydrothermal coating method used in this study is an effective way to obtain a uniform and ultra-thin coating layer on the surface of LiNi0.8Mn0.1Co0.1O2. The Fig. 3 shows that all the samples presents the well-defined layered structure, and the spacing of two adjacent lattice fringes is equal to 0.47 nm corresponding to the d − value of (003) plane. The typical R3m structure can be identified by the diffraction spots in the fast Fourier transformation (FFT), and the FFT of the sample B and LPO are inserted in the corresponding HRTEM images. The FFT of the sample B indicates the (003) and (101) planes of LiNi0.8Mn0.1Co0.1O2, while that of the sample LPO shows an array of hexagonal symmetry dots and demonstrates an in-plane [ 3 × 3 ] R30° ordering in the transition-metal layers [27]. The FFT and lattice
3. Results and discussion 3.1. X-ray diffraction and morphology Fig. 1 shows the XRD patterns of the sample B, LPO, LZO and LTO. All the reflections could be indexed to a well-defined hexagonal α-
Table 1 Lattice parameters of the NMC, LPO&NMC and H-LPO&NMC materials.
Scheme 1. Schematic illustration of the synthesis of coated cathode material. 3178
Samples
a(Å)
c(Å)
c/a
I(003)/I(104)
B LPO LZO LTO
2.872 2.873 2.875 2.872
14.190 14.224 14.241 14.196
4.941 4.951 4.953 4.943
1.114 1.389 1.368 1.397
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Fig. 2. FESEM images of the (a,b) Ni0.8Co0.1Mn0.1(OH)2 precursor magnified to 5000 and 10000 times; (c) sample B, (d) sample LPO, (e) sample LZO and (f) sample LTO magnified to 20000 and 30000 times.
2p3/2 are located at 795.1 eV and 780.4 eV, while that of Mn 2p1/2 and Mn 2p3/2 are at 642.6 eV and 653.8 eV, respectively. The values suggest that the valance states of Co and Mn are + 3 and + 4, respectively [30,31]. For the sample LPO, there is a strong P 2p peak at at 133.5 eV which is consistent with PO43 − that reported in other literature [32]. For the sample LZO, there are two strong peaks at 181.9 eV and 184.3 eV, which are corresponding to the typical Zr 3d5/2 and Zr 3d3/2 peaks [33,34]. For the sample LTO, the two peaks of Ti 2p around 458.0 eV and 464.1 eV can be attributed to Ti4+ [35,36]. According to XPS analysis, the Li3PO4, Li2ZrO3 and Li4Ti5O12 layer has been successfully deposited on the LiNi0.8Mn0.1Co0.1O2 surface, respectively.
fringes indicating that the coating layers have no effect on the cathode crystal structure. The oxidation states of surface elements in B, LPO, LZO and LTO are analyzed by XPS measurement, and the corresponding spectra are illustrated in the Fig. 4. Both materials have Ni2p, Co2p and Mn2p peaks without remarkable binding energy shift, which indicate that the valence states of transition metal ions of the materials are not changed after coating. According to the researches, the Ni 2p3/2 peaks of Ni2+ and Ni3+ are located at 854.4 eV and 855.8 eV, respectively [28,29]. Accordingly, a strong Ni 2p3/2 peak appears at 855.4 eV can be attributed to lots of Ni3+ and a small amount of Ni2+ in the nickel-rich materials. Furthermore, the binding energy peaks of Co 2p1/2 and Co 3179
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Fig. 3. HRTEM images of the sample B, LPO, LZO and LTO.
delivered a discharge capacity of 211.7 mAh/g. This is a little higher than those of the coated samples, which are 210, 200.5 and 208.4 mAh/ g for the LPO, LZO and LTO, respectively. This is mainly due to the decreased amount of electrochemical active material that is replaced by the coating materials. Although there is a slight decrease in discharge capacities for the coated samples, the initial coloumbic efficiencies are
3.2. Electrochemical performance Fig. 5 shows the initial charge/discharge voltage profiles of the sample B, LPO, LZO and LTO tested between 2.8 and 4.5 V at 0.1 C at room temperature. All the samples exhibit similar smooth charge/discharge profiles with a typical potential plateau at 3.75 V. The sample B
Fig. 4. XPS spectra of Ni 2p, Co 2p, Mn 2p, P 2p, Zr 3d and Ti 2p of the sample B, LPO, LZO and LTO. 3180
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Fig. 5. Initial charge/discharge voltage profiles of the sample B, LPO, LZO and LTO cathode electrodes at 0.1 C.
there is no obvious difference in the capacity retention rate for the coated samples, the sample LPO delivered the highest discharge capacity of 192.4 mAh g−1 at the 45th cycle, while sample B only has 172.4 mAh g−1 after 45 cycles. The coloumbic efficiency of each sample is maintained at about 97–100%. The improved capacity retention for the coated samples may be attributed to the surface modification of Li3PO4, Li2ZrO3 and Li4Ti5O12, which can hinder the direct contact with electrolyte and inhibit the side reactions between the electrode and electrolyte caused by HF etching [40–42]. To further evaluate the coating effects, the rate capability property was investigated for the samples and the results are shown in the Fig. 7a,b. Each cell was charged at 0.1 C and then discharged at 1.0–5.0 C. All the samples exhibit gradual drops of discharge capacity with the increase of the current density. Noticeably, the coated samples exhibits superior enhanced rate performance compared with the bare sample, especially at high C-rates. The capacity retentions of LPO, LZO and LTO at 5.0 C are 91.0%, 84.4% and 82.6%, respectively, while that of B is only 77.0%. The results implies the modification by ions conductive materials provide a fast channel for rapid migration of lithium ions between electrolytes and cathode materials which improves the rate property of sample. The discharge curves of the bare and coated samples at the various current density are shown in the Fig. 7c1–4. The discharge plateau of the bare sample decreases dramatically with the increasing of current density from 0.1 C to 5.0 C. But for the coated samples, especially the LPO samples, the discharge plateau decreases tardily. Moreover, the B sample presents discharge capacities of 211.7, 187.9, 178.0, 170.7, 167.1 and 163 mAh g−1 at 0.1, 1.0, 2.0, 3.0, 4.0 and 5.0 C, respectively. Contrast to the bared sample, the LPO/LZO/ LTO samples delivered discharge capacities of 210.0/200.7/208.4, 207.4/190.7/193.8, 201.4/184.2/186.6, 197.4/179.4/181.7, 193.9/ 174.6/176.7 and 191.1/169.3/172.2 mAh g−1 at 0.1, 1.0, 2.0, 3.0, 4.0 and 5.0 C, respectively. Apparently, the LPO exhibits the most outstanding discharge capacity retention at 5.0 C (91%). This can be due to the ionic conductivity of Li3PO4 is 10−8 S cm−1 [43,44] which is higher than that of Li2ZrO3 (10–12 S cm−1) [45] and Li4Ti5O12 (10–13 S cm−1) [46,47]. Therefore, the high ionic conductivity of Li3PO4 can further improve the diffusion of Li ion at the interface of the electrode. Cyclic voltammetry (CV) is a useful technique to study the electrochemical performance and electrode kinetics of oxide materials. Fig. 8 shows the CV curves of the cells with the sample B, LPO, LTO and LZO as the cathode electrodes. The cells are tested at a scan rate of 0.1 mV s−1 in the range of 2.5–4.8 V. The CV curves of the four asprepared materials exhibited almost the same features, three couple oxidation and reduction peaks existed in the voltage range of 3.6–4.3 V.
increased to 88.0%, 83.8% and 86.0% for the LPO, LZO and LTO, respectively, when compared to that of the sample B (80.8%). This owing to a well developed lithium-ion conductors forming on the surface of the LiNi0.8Mn0.1Co0.1O2, which facilitate Li ions diffusion and thus lithium utilization. Besides that, the coating layer also act as a physical protection barrier to reduce the side reactions between the cathode materials and electrolyte, and then suppress the formation of SEI film and the depletion of lithium ions. Among the coated samples, the initial coloumbic efficiencies of LPO is the highest, while that of the LTO is relatively the lowest. This presents the Li3PO4 is much more effective than Li4Ti5O12 as the protective coating layer. After activation by one preconditioning cycles at a rate of 0.1 C, the cycle performance of all the samples are tested at 1.0 C, and the results are shown in the Fig. 6. The initial discharge capacity of the sample B is 203.3 mAh g−1, compared with 206.4, 196.8 and 204.2 mAh g−1 for the sample LPO, LZO and LTO. After 45 cycles, the sample B exhibited a rapid capacity loss to 172.4 mAh g−1 and the capacity maintains only 84.8% of its initial capacity. This undesirable performance is ascribed to the instability of the Ni-rich layered host structure caused by multiple phase transitions and transition metal dissolution of cathode surface resulting from HF attack which causes severe serrations of the cathode particle surface [37–39]. After coated, the capacity retention after cycling is remarkably increased to about 93.3% of the coated samples, which illustrate a more stable cycle life after coating. Although
Fig. 6. Cycling performance and of the sample B, LPO, LZO and LTO cathode electrodes at 1.0 C. 3181
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Fig. 7. Rate capability of the sample B, LPO, LZO and LTO cathode electrodes at various rate (from 0.1 to 5.0 C): (a) discharge capacity at various rate; (b) capacity retention rate at various rate; (c1–4) discharge curves of the samples at the various current density.
To further investigate the possible reasons for enhanced performance of the coated samples. The electrochemical impedance spectroscopy (EIS) can be used to investigate the kinetic process of lithium intercalation/deintercalation into electrodes. Fig. 9(a) shows Nyquist plots of cells assembled by using bared and coated samples as cathode active materials. The data points are experimental while red solid lines are fitted data by the Zview2 software. As shown in Fig. 9(a), each of the plots consists of two well-defined semicircles and an inclined line. The intercept of the semicircle at the highest frequency with the real axis (Z′) is related to uncompensated ohmic resistance (Rs). The semicircle at high frequency region refers to the resistance of the surface film (Rsf) due to the formation of an insulating solid electrolyte
One peak is sharp while the other two peaks are weak, which illustrating phase transitions taken place during charge and discharge process. The three oxidation peaks between 2.8 and 4.3 V are caused by the phase transition from hexagonal to monoclinic (H1 to M), monoclinic to hexagonal (M to H2) and hexagonal to hexagonal (H2 to H3) during the Li-ions extraction process [48]. The phase transitions are reversed during the discharge process, and the reduction peaks are a little lower than the oxidation peak. The corresponding potential difference between anodic peak and cathodic peak (ΔV) for the sample B is the largest, while that of the sample LPO is the smallest of all. This indicates a worse reversibility of Li+ extraction/insertion process in the coated samples, and a larger reaction polarization occurred in the sample B. 3182
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Fig. 8. CV curves at the scan rate of 0.1 mV s−1 for the sample B, LPO, LZO and LTO cathode electrodes.
Fig. 9. (a) Nyquist plots of the sample B, LPO, LZO and LTO cathode electrodes at a charged state of 4.2 V after 45 cycles; (b) the corresponding Rs, Rsf, Rct.
interface (SEI) film or the deposition of organic compounds on the electrode surface by electrolytic decomposition [49,50]. The semicircle at medium frequency are related to the charge transfer resistance through the electrolyte/electrode interface (Rct), respectively. The sloping line at low frequency is attributed to the solid-state diffusion of lithium ions in the cathode materials (Wo). The fitted resistance values are presented in Fig. 9(b). it is noted that Rs (which reflects solution resistance of the cell) for all the samples is very small and can be ignored. As can be seen, the Rsf of the coated samples are obviously suppressed compared with the bare one, which from 179.4 Ω (B) to 75.1 Ω (LPO), 65.14 Ω (LZO), 111.0 Ω (LTO), respectively. We can conclude that the surface coating layer of Li3PO4, Li2ZrO3 and Li4Ti5O12
can depress migration resistance of lithium ions through the SEI film. This implies that coating layer can effectively protect the surface of cathode materials from fast impedance growth during the (de)intercalation processes. This indicates that coating layer can obviously reduce the continuous decomposition of electrolyte on the cathode surface and then effectively suppress the growth of SEI film. Besides that, the Rct value of B sample after cycling is 350.5 Ω which is much larger than that of the coated samples LPO (112.2 Ω), LZO (142.9 Ω) and LTO (118.9 Ω). this analysis implies that coating layer could prevent the host LiNi0.8Mn0.1Co0.1O2 from directly exposed to the electrolyte, which could be easily decomposed under high voltage. The highest Rct value of B can be ascribed to both the HF erosion and the oxygen release form 3183
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the highly lithium deintercalation [51]. Furthermore, this layer also play an important role in protection of the cathode particle from electrolyte decomposition, thus facilitating electron transport through the electrolyte/electrode interface [21]. Therefore, the reason why LPO has the best electrochemical performance can be explained by its the lowest solid electrolyte interface resistance and charge transfer resistance. As mentioned above, the straight sloping line at low frequency region is attributed to the lithium ions diffusion in the bulk of the materials. Therefore, the diffusion coefficient of lithium ions (DLi+) can be calculated from the slop in the low frequency region according to the following equation [52,53]:
DLi =
R2T 2 2n4F 4A2 CLi2 + σ 2
Fig. 11. Schematic diagram of the effects of different fast ionic conductor coating layers on the electrons and lithium ions transfer.
(1)
CLi+ is the concentration of lithium ions in the materials, R is the gas constant, T is the absolute temperature, F is the Faraday constant, A refers to the electrochemically active surface area (here is surface area of the electrode), n stands for the number of electrons per molecule during redox process, σ is the Warburg factor which can be obtained by the following equation: Z ′ = Rs + R ct + σω1/2
coefficient which are the reasons for the improved rate capability and enhanced cycling stability of the LPO. LPO cathode with high-rate capability and long cycling life would be a promising high power density material, which might apply in electric vehicles and hybrid electric vehicles.
(2)
4. Conclusion
Z ′ is the real part of impedance, Rs is the electrolyte resistance, Rct is the charge transfer resistance and ω is the angular frequency. Fig. 10(a) shows the linear relationship of Z ′ and ω−1/2 and the slope of the fitted straight line indicates the σ value. According to the Eqs. (1) and (2), DLi+ values are calculated and compared in Fig. 10(b). It is clearly that the DLi+ values increased after coating, which is higher than that of the sample B (8.65 × 10−10 cm s−1). Among the coated samples, the LPO reaches to 9.56 × 10−9 cm s−1, which is one order higher than that of the pristine sample. This proves that the Li3PO4 coating can promote the migration rate of lithium ions which may account for better electrochemical performance of LPO sample. As analyzed above, the EIS results show that the Rsf+ct of the coated samples after cycling are much lower than that of the sample B, and the order of them for the samples are as follows: B (529.9 Ω) > LTO (229.9 Ω) > LZO (208.4 Ω) > LPO (187.3 Ω). This indicates that electrons transfer across the SEI film and electrode/electrolyte interface of LPO is the easiest of all. For the coated materials are the fast ionic conductors, the diffusion coefficient of lithium ions can diffuse fast in the layer structure of the coated samples, and the order of DLi+ for the samples are as follows: LPO (9.56 × 10−9 cm s−1) > LZO (3.08 × 10−9 cm s−1) > LTO (1.10 × 10−9 cm s−1) > B (8.65 × 10−10 cm s−1). Schematic diagram of the effects of different fast ionic conductor coating layers on the electrons and lithium ions transfer is shown in Fig. 11. Among the samples, the LPO possesses the lowest charge transfer resistance and highest lithium ions diffusion
In summary, a series of fast lithium-ion conductive layer are successfully coated on LiNi0.8Mn0.1Co0.1O2 cathode material by a hydrothermal treatment method. By this method, an ultrathin layer with thickness about ~ 4 nm is uniformly coated on the surface of LiNi0.8Mn0.1Co0.1O2, while its morphology and crystalline structure are well maintained. The XRD, HRTEM and XPS results indicate that the coating layer exists on the cathode particles, rather than interacting with the core material. After 45 cycles at 1.0 C, LPO maintains 93% of the initial discharge capacity. The surface and charge-transfer resistances of the coated samples are obviously suppressed after 45 cycles. Among them, the LPO shows the lowest Rsf (75.1 Ω) and Rct (112.2 Ω) compared with those of the pristine sample (179.4 and 350.5 Ω). Besides that, the lithium diffusion rate of the modified electrode are also significantly enhanced. This improvement is attributed to minimization of the side-reaction between the cathode and electrolyte by the protecting layer, the formation of an undesirable SEI layer is suppressed. Therefore, surface modification by lithium conductive material is an effective way to improve the performance of cathode materials for lithium-ion batteries. Moreover, although the coating materials used are lithium ion conductors, their ionic conductivity is different, which result in different performance of the coated cathode samples. Compared to Li2ZrO3 and Li4Ti5O12, the utilization of Li3PO4 significantly improves the performance of LiNi0.8Mn0.1Co0.1O2. This discovery is an important step towards understanding the fast lithium-ion
Fig. 10. (a) the profiles of Z ′ vs. ω−1/2; (b) DLi+ obtained from the EIS data. 3184
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conductive coating effects.
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