Fundamental Insight into Zr Modification of Li- and Mn-Rich Cathodes

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Fundamental Insight into Zr Modification of Li- and Mn-Rich Cathodes: Combined Transmission Electron Microscopy and Electrochemical Impedance Spectroscopy Study Xing Li,*,† Kangjia Zhang,† David Mitlin,‡ Zhenzhong Yang,*,§ Mingshan Wang,† Yao Tang,† Fei Jiang,† Yingge Du,*,§ and Jianming Zheng*,∥,⊥ †

The Center of New Energy Materials and Technology, Southwest Petroleum University, Chengdu, Sichuan 610500, China Chemical & Biomolecular Engineering, Clarkson University, Potsdam, New York 13699, United States § Physical and Computational Sciences Directorate and ∥Energy and Environment Directorate, Pacific Northwest National Laboratory, 902 Battelle Boulevard, Richland, Washington 99354, United States ⊥ Research Institute (RI), NingDe Amperex Technology Limited, Ningde, Fujian 352100, China

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‡

S Supporting Information *

ABSTRACT: While zirconium-based coatings are known to improve the cycling stability of a number of lithium ion battery cathodes, the microstructural origin of this enhancement remains uncertain. Here we combine advanced transmission electron microscopy (high-resolution transmission electron microscopy, high-angle annular dark field, electron energy loss spectroscopy, and energy-dispersive X-ray spectroscopy) with electrochemical impedance analysis to provide new insight into the dramatic role of Zr surface modification on the electrochemical performance of Liand Mn-rich (LMR) cathodes (Li[Li0.2Ni0.13Co0.13Mn0.54]O2). It is demonstrated that a Zr-based rock-salt structure layer with a thickness of 1−2 nm is formed on the surface of the LMR. This layer is effective in suppressing the deleterious phase transformation of LMR from initial layered composite combining Li2MO3 and LiMO2 to the disordered rock-salt phase, leading to an enhanced long-term cycling performance and rate capability. Electrochemical impedance spectroscopy analysis demonstrates that the Zr coating does not affect the cathode electrolyte interface (CEI), with the surface film impedance (Rsf) being virtually identical in both cases after 100 cycles, at 45.1 versus 45.6 Ω. Conversely, the Zr coating tremendously stabilizes the cathode interfacial structure. The charge-transfer impedance (Rct) in the baseline unmodified LMR increases from 34.2 Ω at cycle 3 to 729.2 Ω at cycle 100. For the Zr-modified specimen, Rct increases dramatically less, from 19.7 to 76.9 Ω. The key finding of this study is then that Zr is actively incorporated into the structure of the cathode but does not affect CEI stability. This fundamental result should guide future surface modification strategies for a range of cathode materials.

1. INTRODUCTION

Two key issues prevent broader utilization of LMR cathode materials.9 First is the irreversible phase transformation from layered to spinel-like and then disordered rock-salt structure during the electrochemical cycling, which leads to both a voltage fade and a capacity fade.10,11 The second is the formation of a thick cathode electrolyte interface (CEI) at high operating voltages due to oxidization of carbonate-based electrolyte and the detrimental attack of acidic species (HF, etc.).12 The formation of thick CEI likewise results in cycling induced polarization and capacity decay.13 There have been a number of experimental approaches employed to improve the overall performance of LMR materials, often not clearly

A major global effort is being devoted to developing higher energy and longer lasting lithium ion batteries (LIBs) to boost the driving range of electric vehicles (EVs).1,2 The search for cheaper, higher capacity, and safer layered-structure cathode materials to substitute for LiCoO2 remains a critical challenge.3 Among many candidates, Li- and Mn-rich (LMR) layered structure cathode material (e.g., Li[Li0.2Ni0.13Co0.13Mn0.54]O2) is of particular interest because of its high discharge capacity (exceeding 250 mA h g−1), low cost, good thermal stability, and high operating voltage.2,4−8 LMR cathode material can be considered as a composite combining Li2MO3 (C2/m) and LiMO2 (R3̅m) (M = Ni, Co, Mn, or combinations), for example, Li[Li0.2Ni0.13Co0.13Mn0.54]O2 could also be rewritten as 0.5Li2MnO3·0.5LiNi1/3Co1/3Mn1/3O2. © 2018 American Chemical Society

Received: November 21, 2017 Revised: March 23, 2018 Published: March 25, 2018 2566

DOI: 10.1021/acs.chemmater.7b04861 Chem. Mater. 2018, 30, 2566−2573

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edges of Ni, Co, Mn, O, and Zr.11 The Zr-modified and pristine Li[Li0.2Ni0.13Co0.13Mn0.54]O2 samples after the electrochemical cycling were scraped from the Al current collector. Then they were thoroughly washed with pure dimethyl carbonate (DMC) solvent to remove the residual electrolyte and dried for several hours to evaporate the solvent. To avoid the effect of the air atmosphere, the disassembly of the coin cells was conducted in a high-purity argon glovebox. The asprepared and cycled specimens were also kept in a glovebox prior to transmission electron microscopy (TEM) characterization. The STEM images, EDX, and EELS mapping data were obtained from several regions of the specimens. As the obtained data were consistent from different regions, therefore, only the representative data are presented. 2.3. Electrochemical Measurements. The cathode electrodes were prepared by casting a slurry containing the pristine or Zrmodified Li[Li0.2Ni0.13Co0.13Mn0.54]O2, acetylene black, and poly(vinylidene fluoride) binder with a weight ratio of 80:10:10 onto Al current collector foil. Then it was punched into disks with the diameter of 1.27 cm and dried under vacuum overnight before use. The active material loading amount was about 5 mg cm−2 on the asprepared electrodes. The R2032 coin-type cells consisting of the asprepared cathodes, the separator (Celgard 2500), and the electrolyte (1.0 M LiPF6 in ethyl carbonate:ethyl methyl carbonate (EC:EMC)= 4:6) were assembled in an argon filled glovebox with both the water and oxygen content being less than 0.1 ppm. The rate performance evaluation of the assembled cells was performed at different rates of C/ 10, C/5, C/3, C/2, 1C, 2C, and 3C (1C = 250 mA g−1) on the BTS-5 V20 mA cell testing instrument (NEWARE Electronic Co., Ltd.) in the voltage range of 2.0−4.8 V. The cycling performances of the cells were tested at both 30 and 60 °C, while the rate performance of the cells was tested at 30 °C. EIS of the cells was measured at the charged state of 4.3 V at the frequency from 105 to 10−2 Hz with potential perturbation amplitude of 10 mV using the electrochemical workstation of CHI660D.

distinguishing whether the methods stabilize the structure, stabilize the CEI, or both. Representative strategies include the surface coating/modification,14−21 partial substitution of transition-metal (TM) ions,22−24 and/or oxygen anion,25,26 and application of electrolyte additives. 6,13 Employing zirconium and zirconium compounds to modify the surfaces of the high-voltage cathode materials has been shown to be particularly effective.27−32 However, a detailed mechanistic understanding of the role of zirconium in improving performance remains unclear, especially for the case of LMR cathode. Little is known regarding the crystallography and chemistry of a modified surface, specifically how the Zr affects either the actual LMR structure or the cathode electrolyte interface. The goal of this study is to clearly elucidate the role of a Zr coating in stabilizing the electrochemical performance of LMR, considering both the structure effect and the CEI effect. Aberration-corrected scanning transmission electron microscopy (STEM) was employed to characterize the crystal microstructure of the pristine versus Zr-modified Li[Li0.2Ni0.13Co0.13Mn0.54]O2 before and after high-voltage electrochemical cycling. Our unique analytical approach combined high-angle annular dark field (HAADF)-STEM analysis with electrochemical impedance spectroscopy (EIS) to clearly differentiate the two contributions. Per the EIS analysis, we observed no change to the CEI growth behavior with the Zr modification. Employing high-angle imaging with energy dispersive X-ray spectroscopy (EDX) and electron energy loss spectroscopy (EELS) mapping, it is proven that the zirconium treatment prevents the irreversible cycling-induced phase transformation from layered- to spinel-like structure in LMR.

3. RESULTS AND DISCUSSION 3.1. Electrochemical Performance. Figure 1a presents the cycling performance of the pristine and of the Zr-modified LMR. After the initial three formation cycles at C/10, the specimens were tested at the rate of C/3. It may be observed that the Zr-modified LMR exhibits substantially improved cycling stability as compared to the baseline pristine specimen.

2. EXPERIMENTAL SECTION 2 . 1 . P r e p a r a t i o n of Z r - M o d i fi e d M a t e r i a l . T h e Ni0.13Co0.13Mn0.54(OH)2 was prepared by a coprecipitation method.33 It can be briefly described as follows: the stoichiometric amounts of nickel acetate, cobalt acetate, and manganese acetate were dissolved into deionized water, and then the excess lithium hydroxide solution was added drop by drop. The final Ni0.13Co0.13Mn0.54(OH)2 was obtained by thoroughly washing the as-prepared mixed hydroxide. Then the Ni0.13Co0.13Mn0.54(OH)2 precursor was blended with a stoichiometric amount of LiOH·H2O in deionized water to form a uniform mixture via vigorous magnetic stirring. Finally, the mixture was dried at 80 °C, followed by a precalcination at 500 °C for 5 h and further calcination at 900 °C for 15 h in air to obtain the final product of Li [Li 0 . 2 N i 0 . 1 3 Co 0 . 1 3 Mn 0 . 5 4 ] O 2 . T he Z r - m od ifi e d L i [Li0.2Ni0.13Co0.13Mn0.54]O2 was prepared with a wet chemical method. The ZrO(NO3)2·H2O was first dissolved into the deionized water and then the as-prepared Li[Li0.2Ni0.13Co0.13Mn0.54]O2 was added. The above mixture was dried at 80 °C with vigorous stirring. Finally, the dried mixture was calcinated at 400 °C for 6 h in air to obtain the Zrmodified Li[Li0.2Ni0.13Co0.13Mn0.54]O2. The amount of the Zr element was set as 1 wt % in the Li[Li0.2Ni0.13Co0.13Mn0.54]O2. This was based on our previous studies, where we optimized the Zr content for performance in LiNi1/3Co1/3Mn1/3O2 and LiNi0.8Co0.1Mn0.1O2 (with similar particle sizes).27,32 2.2. Characterization Methods. HAADF-STEM observations of the Zr-modified and pristine Li[Li0.2Ni0.13Co0.13Mn0.54]O2 samples before and after electrochemical cycling were performed on a probeaberration-corrected FEI Titan STEM at 300 kV. The image intensity of each atomic column reflects the related average atomic number (∼Z1.7),34 which is chemically sensitive and is termed as Z-contrast imaging, enabling us to visually identify the atomic structural changes. EDX mapping was performed using a Cs-corrected JEM ARM200F STEM. EELS mapping was acquired with a collection semiangle of ∼100 mrad, and a larger dispersion was chosen to cover the range of

Figure 1. (a) Cycling performances of the pristine and Zr-modified LMR at the rate of C/3 after the three formation cycles at C/10 over the voltage range of 2.0−4.8 V at 30 °C; and (b, c) the corresponding charge−discharge voltage curves of the (b) pristine and (c) Zrmodified LMR at different stages of cycling. 2567

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Chemistry of Materials After 100 cycles, the Zr-modified LMR still delivers a discharge capacity of 182 mA h g−1, which corresponds to 86% of its initial discharge capacity (211 mA h g−1). Conversely, the pristine LMR begins to degrade by around cycle 50. After 100 cycles its specific discharge capacity is 87 mA h g−1, corresponding to a capacity retention of only 40%. The charge and discharge voltage profiles of pristine and Zr-modified LMR materials during cycling are further compared in Figures 1b,c. As indicated by the black arrows, pristine LMR shows a significantly worse voltage fade than the Zr-modified counterpart. The role of the Zr modification in promoting the voltage and capacity stability in LMR will be covered later in this article, parallel to the discussion of the TEM findings on the postcycled specimens. For Li2MO3 systems the continuous voltage fade is mainly attributed to the intrinsic phase transformation from layered to spinel-like and rock-salt-like phase, which occurs when charging to high voltages during cycling.34,35 Specifically, the voltage fade is correlated insertion of lithium into octahedral sites instead of the tetrahedral sites.36 The voltage at which lithium ion intercalation occurs into the defect spinel and disordered rocksalt phase is ∼2.8 V, which is lower than that for lithium intercalation into the original layered phase (>3.0 V). In addition to the thermodynamic detriment, there should be a kinetic origin to the hysteresis associated with structure disordering: The channels in the original lattice available for facile Li ion diffusion become blocked by the migrating transition metals. During cycling the surface and subsurface of the pristine LMR will turn into the undesirable defect spinel and then disordered rock-salt phase. As will be demonstrated through TEM analysis in the next set of figures, the Zr treatment suppresses this deleterious reaction. Cycling-induced crack and pore formation would play a role in the cycling observed capacity fade as well as the hysteresis. It is conceivable that the Zr treatment stabilizes the LMR structure against such physical degradation as well. It should be pointed out that the direct relationship between voltage fade and capacity fade in LMR cathodes is likely correlated, but requires further analysis. A related deleterious feature associated with cycling the pristine LMR is the increase in the overpotential during deintercalation (charging). This effect should originate from the progressively more degraded LMR lattice that kinetically impedes the charging process and adds to the net polarization required to extract the lithium ions. Since the host environment of the lithium ions is different upon charge versus upon discharge, both the hysteresis and the voltage fade are not expected to be symmetrical. Indeed, this is what is observed for both the pristine and the Zr-modified LMR materials. Moreover, per Figure 1c, the Zr modification is more effective in reducing the hysteresis upon charging than upon discharging, which could be back-correlated to the kinetics of lithium transport though an “empty” versus a “full” lattice. In many cathode systems the concurrent formation of CEI further drives the kinetic polarization, requiring greater overpotentials at both charge and discharge, while reducing the accessible capacity. As will be elucidated through EIS analysis, for the LMR cathodes examined in this study, cycling-induced CEI formation is a secondary effect. Figure 2 shows the rate capability of pristine and Zr-modified LMR materials. The result illustrates that the Zr-modified LMR offers substantially improved kinetics, especially at higher charge/discharge rates. The discharge capacities of Zr-modified LMR cathode at the ascending rates of C/3, C/2, 1C, 2C, and

Figure 2. Comparison of rate performance of the pristine and Zrmodified LMR evaluated at ascending current rates of C/10, C/5, C/3, C/2, 1C, 2C, and 3C, respectively.

3C are 218, 203, 178, 156, and 143 mA h g−1 versus 210, 192, 164, 118, and 87 mA h g−1 for the pristine LMR. The superior rate capability of the Zr-modified LMR could be ascribed to the enhanced interfacial reaction kinetics and improved Li+ diffusivity, as will be demonstrated by EIS. The enhanced structural stability of the Zr-modified LMR would aid in maintaining facile kinetics at higher currents since the material concurrently is cycled during C rate testing. Conversely, per EIS, there is minimal difference between CEI resistances for pristine versus Zr-modified LMR. This indicates that the cathode electrolyte interface is not in fact responsible for the observed rate behavior disparity. To further confirm the positive effect of Zr modification, the cycling performance of pristine and Zr-modified LMR cathode was also evaluated at an elevated temperature of 60 °C. These results are shown in Figure S1. It is found that both pristine and Zr-modified LMR materials exhibit higher discharge capacity at 60 °C than at 30 °C. This is mainly because that higher temperature is beneficial for reducing electrode polarization and enhancing the electrochemical reaction kinetics to enable more Li+ utilization. The Zr-modified specimen also displays improved cycling performance at 60 °C, similar to what was observed at 30 °C. For pristine LMR, the discharge capacity is only 15 mA h g−1 after 100 cycles at C/3 at 60 °C. This is much lower than the 87 mA h g−1 discharge capacity after 100 cycles at C/3 at 30 °C. The result confirms that the higher temperature readily exacerbates the detrimental phase transformations, resulting in accelerated capacity fading.37 By contrast, the Zr-modified LMR is still capable of delivering a high discharge capacity of 212 mA h g−1 after 100 cycles at C/3 at 60 °C. This is even higher than the 182 mA h g−1 discharge capacity after 100 cycles at C/3 at 30 °C, indicating that the Zr modification remains quite effective at elevated temperature. 3.2. Interfacial Evolution of LMR during Electrochemical Cycling Process. Electrochemical impedance spectroscopy measurements were carried out to track the interfacial evolution of pristine and Zr-modified LMR cathode electrodes. Figure 3 presents the impedance spectra of the pristine and Zr-modified LMR electrodes at 3rd, 10th, 30th, 50th, and 100th cycles during cycling at 1C. The impedance 2568

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is indicative of irreversible microstructure changes occurring at the surface of the pristine LMR during the electrochemical cycling. On the other hand, the Zr modification is effective in preventing these detrimental microstructure changes during the cycling. The lithium ion diffusion coefficient of LMR at the third and 100th cycles could be calculated from the Warburg impedance coefficient (σw) using eqs 1 and 2,40

spectra show a semicircle at the high-frequency region representing the surface film impedance (Rsf, so-called CEI layer). The semicircle located in the high- to medium-frequency region represents the charge-transfer impedance (Rct), and an oblique line located at the low-frequency region represents the Warburg impedance (W), respectively.38 The intercept at high frequency with a real axis is corresponding to the electrolyte resistance. The fitted results of the impedance spectra using the equivalent circuit as inset in Figure 3a are listed in Table 1. As Table 1. Fitted Results of the Impedance Spectra (Figure 3) Using the Equivalent Circuit Inset in Figure 3a sample Zr-modified

cycles

Re (Ω)

Rsf (Ω)

Rct (Ω)

Re (Ω)

Rsf (Ω)

Rct (Ω)

3rd 10th 30th 50th 100th

3.8 4.5 5.4 6.4 10.2

18.5 22.1 30.9 38.5 45.6

34.2 87.9 179.9 309.4 729.2

3.3 3.5 4.4 4.5 7.2

16.8 20.0 30.5 37.2 45.1

19.7 25.1 44.7 68.4 76.9

(1)

DLi = R2T 2/(2A2 n 4F 4Cσw 2)

(2)

where DLi represents the lithium ion diffusion coefficient, R is the gas constant, T is the absolute temperature, A is the effective area of the electrode, n is the number of electrons transferred, F is Faraday constant, and C is the concentration of lithium ions. The Warburg impedance coefficient σw is determined from the slope of Zre as a function of ω−1/2 as shown in Figure S2 of the Supporting Information. The calculated DLi values for the pristine LMR at the 3rd and 100th cycles are 1.49 × 10−8 and 1.02 × 10−10 cm2 s−1, respectively. Hence, it may be seen that cycling decreases the baseline material’s lithium diffusivity by 2 orders of magnitude. By contrast, the DLi values for the Zr-modified LMR after the 3rd and 100th cycles are almost the same, being 1.52 × 10−8 and 0.93 × 10−8 cm2 s−1, respectively. The minimal change of the Li+ ion diffusion coefficient at 100th cycle strongly supports the effectiveness of the Zr-doped rock-salt phase in suppressing the detrimental phase transformations. 3.3. Structural Evolution of Pristine LMR during Electrochemical Cycling Process. To gain fundamental insight into the cycling-induced fade mechanism of pristine LMR, we characterized the structural evolution at atomic scale using aberration-corrected HAADF-STEM. The HAADF image contrast exhibits a relationship of ∼Z1.7, with respect to atomic number Z. In Figures 4a,b the as-synthesized pristine LMR is oriented in the [100] zone axis. In Figure 4c−e the pristine LMR is in the [100] orientation, but shown after 100 cycles at C/3 between 2.0 and 4.8 V. The atomic models are R3m ̅

Figure 3. Nyquist plots of (a) pristine and (b) Zr-modified LMR electrodes at the 3rd, 10th, 30th, 50th, and 100th cycles at 1C between 2.0 and 4.8 V. The impedance spectra were collected at a charged state of 4.3 V. (c) and (d) are enlarged from the red rectangle indicated in (a) and (b). The inset in (a) is the equivalent circuit used for fitting the impedance spectra.

pristine

Zre = (R sf + R ct + σwω−1/2)

expected, the total resistance (Re + Rsf + Rct) for the two electrodes increases with the cycle number.39 Although both materials exhibit similar Rsf and Rct values at the early stage of cycling, the pristine LMR electrode experiences a major increase in Rct upon cycling, whereas the Zr-modified LMR does not. Rct in pristine LMR increases from 34.2 Ω at cycle 3 to 729.2 Ω at cycle 100. For the Zr-modified specimen, Rct increases dramatically less, from 19.7 to 76.9 Ω. Conversely, neither the pristine nor the modified electrode experiences a drastic increase in CEI-related resistance Rsf, indicating its relative stability in both cases. The values of Rsf are virtually identical for both materials at 100 cycles, at 45.1 Ω for pristine versus 45.6 Ω for Zr-modified. From the Nyquist data, it may then be concluded that the Zr coating neither improves nor degrades the CEI, but has a major effect on the kinetic chargetransfer resistance of the electrode. The significant jump in Rct

Figure 4. (a) and (b) HAADF-STEM images of pristine LMR before electrochemical cycling, observing from the [100] zone axis; (c), (d), and (e) after 100 cycles at C/3 between 2.0 and 4.8 V. The atomic models in (b), (d), and (e) are R3̅m layered structure viewed down from the [100] zone axis, and defect Fd3̅m spinel phase and Fm3̅m rock-salt-like phase viewed down from the [110] zone axis, respectively. 2569

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Chemistry of Materials layered structure along the [100] zone axis and defect Fd3m ̅ spinel phase and Fm3̅m rock-salt-like phase are along the [110] zone axis, respectively. It is worth noting that there are a few atomic layers of the rock-salt-like structure as a result of Li and TM cation mixing, as identified on the surface of the LMR per Figure 4b. This is believed to be due to the oxygen nonstoichiometry and/or thermal diffusion during the calcination process.41,42 However, the [100] oriented zone axis images shown in Figure 4a,b demonstrate that overall a well-defined layered structure in the pristine LMR is obtained. This is in agreement with prior studies on the structure of properly synthesized LMR.43,44 The fast lithium ion diffusion pathways are not blocked prior to electrochemical cycling, and hence the pristine LMR material delivers high discharge capacity at the early stage of cycling. Significant microstructural changes are detected after 100 cycles, as presented in Figure 4c−e. From the enlarged HAADF-STEM images (Figure 4d), it is clearly observed that the layered structure of the LMR has been transformed to a defect spinel-like structure in the outer surface region. Moreover, the subsurface of the pristine LMR also has been transformed to a defect rock-salt-like phase. The total depth of the transformed phase is about 10 nm, which demonstrates that LMR is unstable upon electrochemical cycling. It may be assumed that with progressively more cycling, for example, up to 500 cycles, even more LMR will transform, further degrading the voltage profiles and the reversible capacity. This cyclinginduced transformation phenomenon agrees with results by authors in previous references.10,11,45 Detailed analysis shows that there is light contrast at the Li (8a) sites of the outer surface spinel-like structure, which means that the Li (8a) sites are partially occupied by TM cations. The phase change occurred at the outer surface, supporting the mechanistic explanation that the surface-layered structure of pristine LMR sample is not stable. LMR is susceptible to oxygen loss and TM cations dissolution during the electrochemical process,11,46,47 which makes it energetically favorable for the TM ions migrates into the neighboring Li layers. Moreover, HRTEM shows that in the cycled material the subsurface has also been transformed to a rock-salt-like phase. In that unfavorable structure the Li sites become largely occupied by TM ions.48 This directly agrees with the EIS diffusion analysis, where fast lithium ion diffusion pathways are interrupted, considerably slowing down the Li+ ion diffusion during repeated intercalation and extraction. This also explains the reduced rate capability in the pristine LMR specimens. 3.4. Structural Evolution of the Zr-Modified LMR during Cycling. Figure 5 shows the HAADF-STEM image of the as-synthesized Zr-modified LMR, and the corresponding EDX maps of Mn, Co, Ni, O, and Zr elements. Figure 5a shows the HAADF-STEM image, while parts (b)−(f) of Figure 5 give the EDX analytical maps of Mn, Co, Ni, O, and Zr. The inset in Figure 5a is the EDX line-scan profiles of these elements. It may be observed that while Mn, Co, Ni, and O are uniformly distributed, the Zr atoms are at the surface of the particle, forming a coating layer. This may be further confirmed from Figure S3, which shows another example of the elemental distribution. Figure 6a−f shows the HAADF-STEM images of the assynthesized and postcycles 100 cycles Zr-modified LMR. Figure 6a−c show the as-synthesized materials while (d)−(f) show the material after 100 cycles at C/3 between 2.0 and 4.8 V. The atomic models in (c) and (f) are R3̅m layered structure viewed

Figure 5. (a) HAADF-STEM image of Zr-modified LMR material before electrochemical cycling, and EDX mapping of (b) Mn, (c) Co, (d) Ni, (e) O, and (f) Zr elements. The inset in (a) is the EDX line scan profiles of O, Mn, Co, Ni, and Zr elements.

Figure 6. HAADF-STEM images of Zr-modified LMR material observed along the [100] zone axis (a−c) before cycling and (d−f) after 100 cycles at C/3 between 2.0 and 4.8 V. (b) and (e) are enlarged images form blue and red rectangular box areas in (a) and (d), respectively. (c) and (f) are enlarged from (b) and (e), respectively, and overlapped with crystal structure model. The atomic models in (c) and (f) are R3̅m layered structure viewed down from the [100] zone axis and Fm3̅m rock-salt-like phase viewed down from the [110] zone axis, respectively.

along the [100] zone axis and Fm3̅m rock-salt-like phase viewed along the [110] zone axis, respectively. From the enlarged images presented in Figure 6b,c, it is observed that the Zr-rich rock-salt surface phase (brighter contrast) has a thickness of 1− 2 nm. Moreover, it may be seen that there is lattice coherence between the Zr phase and the underlying LMR. Importantly, an EELS map of as-synthesized material demonstrates that Zr has entered into the lattice of the LMR, as shown in Figure S5. This is the first demonstration that the Zr modification could participate in the formation of a rock-salt phase on the surface of the LMR material. During the electrochemical cycling, this rock-salt phase formed via the Zr surface modification will function as an LMR lattice-active protective coating that will suppress the irreversible phase transformation from layered to spinel-like structure. Therefore, the intrinsic layered structure and the fast lithium ion diffusion pathways in the bulk of the LMR are much better preserved during the electrochemical cycling. It is most likely that the Zr layer prevents the oxygen loss because of its own intrinsically high bond strength with oxygen. In effect, the Zr-rich oxide acts as an oxygenimpermeable barrier around the LMR lattice. 2570

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Therefore, the formation of disordered rock-salt phase in the Zr-modified material could be prevented. Meanwhile, cyclinginduced CEI growth is unaffected.

Comparing the as-synthesized versus the postcycled Zrmodified LMR, it is observed that the Zr layer is effectively unchanged. This indicates that the formed Zr-based rock-salt phase is electrochemically stable since it appears unaffected by the charging−discharging and does not react with the electrolyte. In fact, per the EIS analysis, the Zr modification seems to have little effect on the cathode electrolyte interface in general. We should point out that this is the first report in scientific literature where Zr-modification’s atomic-scale structure and role in promoting cycling stability in LMR has been clearly and directly explained. Before this work, while it has been known that Zr treatments do work, the fundamental mechanisms were uncertain. To summarize the effect of the Zr layer: In Li2MO3 materials a voltage-fade is thought to be due to the deleterious phase transformation of the active material from layered to spinel-like and rock-salt-like structures. When this transformation occurs, lithium is thought to move into the octahedral sites rather than in and out of its original positions in the tetrahedral sites. The channels in the lattice available fast Li ion diffusion also become blocked, creating a kinetic barrier, that is, overpotential, for lithiation/delithiation. The transformation is believed to be a result of the loss of oxygen from the structure during cycling. If anion redox sites were also active during charging/discharging, loss of oxygen would further drive the reversible capacity decay. It is demonstrated by combined TEM and EIS analysis that the Zr treatment suppresses this deleterious reaction. Considering the EIS results, it may be seen that the initial performance of Zr-modified and unmodified structures are effectively the same in terms of the charge-transfer resistance and Li diffusivity. It is only after extended cycling that there is substantial contrast between the two materials. We argue that the Zr layer prevents the oxygen loss due its own intrinsically high bond strength with oxygen. For instance, the bond strength of ZrO2 is among the highest in all oxides, being at −1097.46 kJ/mol. Since HRTEM shows that direct incorporation of Zr into the lattice (Zr-rich rock-salt), the above enthalpy serves only as a qualitative illustration of the affinity of Zr atoms for oxygen. In parallel, the Zr layer should also prevent the dissolution of other transition metals into the electrolyte, with that mechanism requiring further study via theory/modeling. Through these means, the Zr modification stabilizes the initial structure of the cathode, allowing the initial voltage/capacity profiles to be better maintained throughout cycling. It should also be pointed out that the Zr addition does not positively affect the Li transport or voltage hysteresis in the starting material. EIS shows no improvement at cycle 1. Rather, Zr reduces the fast deterioration of these properties with charge−discharge cycling because of the reasons discussed.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04861. High-temperature electrochemical cycling stability, relationship between the real resistance and the frequency, TEM images, EDX mapping, EDX line profiles, and EELS mapping (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 28 83037409. Fax: +86 28 83037409. E-mail: [email protected] (X.L). *Tel.: +1 509 372 4762. Fax: +1 509 375 2186. E-mail: [email protected] (Z.Y). *Tel.: +1 509 372 4762. Fax: +1 509 375 2186. E-mail: yingge. [email protected] (Y.D.). *Tel.: +1 509 372 4762. Fax: +1 509 375 2186. E-mail: [email protected] (J.Z). ORCID

Xing Li: 0000-0003-2745-5673 Jianming Zheng: 0000-0002-4928-8194 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was carried out with financial support from the National Natural Science Foundation of China (Grant Nos. 51474196 and 51502250), the Science & Technology Department of Sichuan Province (Grant Nos. 2016RZ0071 and 2017JQ0044), and the Southwest Petroleum University (Grant Nos. 2015CXTD04 and X151517KCL50). J. Zheng is thankful the support by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC0205CH11231, Subcontract No. 18769, under the Batteries for Advanced Transportation Technologies program. STEM, EDX, and EELS studies were supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Early Career Research Program under Award No. 68278. A portion of the work was performed at the W.R. Wiley Environmental Molecular Sciences Laboratory, a DOE User Facility sponsored by the Office of Biological and Environmental Research and located at the Pacific Northwest National Laboratory. D. Mitlin (interpretation of results, preparation of manuscript) was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-SC0018074.

4. CONCLUSIONS To summarize the core mechanism of Zr enhancement: Activation of LMR cathode materials is closely related to lithium ion removal from TM layers accompanied by oxygen loss from the structural lattices at charge to voltage >4.5. In particular, at the outer surface region, the migration of TM ions from TM layers to Li layers becomes energetically favorable because of the reduced oxygen coordination. This results in the formation of defect spinel-like or finally disordered rock-salt structures, with the Zr-based coating layer and rock-salt phase preformed on the surface of the LMR cathode. Hence, the oxygen loss during each charge process is largely alleviated because of the significantly enhanced Zr−O bond strength.

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