Enhancing the Structural Stability of Ni-Rich Layered Oxide Cathodes

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Enhancing the structural stability of Ni-rich layered oxide cathodes with a preformed Zr-concentrated defective nanolayer Bo Han, Sheng Xu, Shuai Zhao, Guixian Lin, Yuzhang Feng, Libao Chen, Douglas G. Ivey, Peng Wang, and Weifeng Wei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11112 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on October 30, 2018

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ACS Applied Materials & Interfaces

Enhancing the Structural Stability of Ni-Rich Layered Oxide Cathodes with a Preformed Zr-Concentrated Defective Nanolayer

Bo Han a†, Sheng Xu b†, Shuai Zhao a, Guixian Lin a, Yuzhang Feng b, Libao Chen a, Douglas G. Ivey c, Peng Wang b* and Weifeng Wei a* a

State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan

410083, P.R. China b National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences

and Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, P.R. China c

Department of Chemical & Materials Engineering, University of Alberta, Edmonton, Alberta,

Canada T6G 1H9 † These authors contributed equally to this work Corresponding Author

* [email protected]; * [email protected];

ACS Paragon Plus Environment

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ABSTRACT: Nickel-rich layered oxides (NLOs) exhibit great potential to meet the evergrowing demand for further increases in the energy density of Li-ion batteries (LIBs), due to their high specific capacities. However, NLOs usually suffer from severe structural degradation and undesired side reactions when cycled above 4.3 V. These effects are strongly correlated with the surface structure and chemistry of the active NLO materials. Herein, we demonstrate a preformed _

cation mixed (Fm3m) surface nanolayer (~5 nm) that shares a consistent oxygen framework with the layered lattice through Zr modification, in which Ni cations reside in Li slabs and play the role of a “pillar”. This preformed nanolayer alleviates the detrimental phase transformations upon electrochemical cycling, effectively enhancing the structural stability. As a result, the Zr-modified Li(Ni0.8Co0.1Mn0.1)0.985Zr0.015O2 material exhibits a high reversible discharge capacity of ~210 mAh/g at 0.1 C (1 C = 200 mA/g) and outstanding cycling stability with a capacity retention of 93.2% after 100 cycles between 2.8-4.5 V. This strategy may be further extended to design and prepare other high performance layered oxide cathode materials.

KEYWORDS: Nickel-rich layered oxide cathodes, Zr-modification, preformed defective nanolayer, phase degradation, side reactions, structural stability


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1. INTRODUCTION Currently, lithium-ion batteries (LIB) are extensively employed as a key power source in various portable electronic devices, plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs).1, 2

Moreover, considering the growing energy crisis and environmental concerns around the globe,

a LIB cathode with high energy density, long cycle life, superior safety and non-toxicity is in urgent demand.3 Nickel-rich layered oxides (NLOs), with the chemical formula LiNixTM1-xO2 (x > 0.5, TM = Co, Mn, Al), are considered as promising cathode materials because of their higher specific capacities (170 - 220 mAh /g) and cutoff voltages (> 4.5 V vs Li+ /Li), lower cost and less toxic nature,4-6 when compared with conventional LiCoO2. However, when cycled above 4.3 V, _

NLO undergoes a steady phase transformation from a well-ordered layered (R3m) phase to _

_

disordered spinel (Fd3m) and/or rock-salt (Fm3m) phases, which preferentially forms on the cathode surface and results in subsequent capacity fading and voltage decay.2, 5 The origin of this phase evolution upon cycling stems from site exchange between Ni2+ (0.69 Å) and Li+ (0.76 Å) because their similar ionic radii.6,

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In addition, when charged at elevated voltages and/or

temperatures, the highly delithiated NLO electrode surface, where highly oxidizing Ni4+ species and reactive oxygen redicals are formed, promptly reacts with organic electrolytes.5-9 These side reactions give rise to Ni2+ dissolution and electrolyte decomposition,6-9 thereby impairing battery performance and bringing about safety risks.8, 10 Thus, it is evident that the surface structure and


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chemistry of NLOs are the determining factors regulating electrochemical performance, particularly cycling stability. A number of strategies, including surface coating /modification,11-18 chemical doping,19-21 core-shell structures22-26 and electrolyte additives,27-29 have been adopted to mitigate the unwanted phase degradation and side reactions. Among these strategies, surface modification has been one of the principal remedies since structural degradation typically starts from the surface of the NLO particles2, 5 and well-designed surface structures can restrain this degradation process during the initial stage.5,

12, 30

In this respect, a variety of coating materials, including metal oxides31, 32,

fluorides,33 phosphates34,

35

and Li+ /electron conducting matrixes,36,

37

have been reported to

improve the electrochemical performance. However, these coating materials with different crystal structures generally are not lattice matched with the layered host phase, which accounts for different degrees of lattice volume change or even severe internal stresses as well as cracks between the host and coating components during cycling.5,

38

For this reason, it is extremely

difficult for conventional surface coatings to retain uniformly when integrated with the host phase during long-term cycling. Recently, Kim et al.38 reported a novel Li-rich layered surface bearing a consistent framework with the host, in which nickel cations are regularly arranged and act as “pillars” between the transition metal slabs. This surface structure suppresses unfavorable phase transitions and enhances cycling stability. More recently, Ahn et al.39 designed a nanoscale Zr-rich surface phase on a Li-rich layered oxide matrix, realizing a substantial improvement in electrochemical performance of Li-rich layered oxide materials. In both aforementioned studies, the surface layers possess a matched crystal structure at the atomic level with the bulk phase so that the surface structures can be uniformly formed over the entire particle surface during synthesis and can also remain integrated with the bulk phase over prolonged cycles.


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Inspired by these considerations, we have developed a novel NLO material with a preformed

nanoscale

cation-mixed

surface

layer

through

surface

structural

rearrangement induced by Zr modification, as illustrated in Scheme 1. This Zrconcentrated defective nanolayer shares the same oxygen sublattices with the layered host and consequently remains integrated with the bulk phase during long-term cycling. The substitution of Zr4+ cations may increase the amount of Ni2+ in order to maintain charge balance in the system, resulting in more cation mixing between Ni and Li. The portion of Ni2+ residing in Li slabs could provide an electrostatic repulsive force to prevent continuous Ni2+ migration to the Li layer and, as such, stabilize the surface structure of the particle, namely, “pillar effect”.40 In addition, the enrichment of Zr (depletion of Ni) in the nanoscale surface layer, can serve to alleviate the side reactions between the active materials and the organic electrolyte.9, 41 When the nanoscaled defective layer is introduced in the NLO cathode, its cycling stability at a cutoff voltage of 4.5 V is significantly improved, as compared with different recent works in Table S1. To obtain a better understanding of the structural characteristics behind the enhanced battery properties, detailed microscopic and spectroscopic analyses are carried out.


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Scheme 1. Schematic illustration of NLO cathode material with a preformed cation mixed surface layer induced by Zr modification.

2. EXPERIMENTAL 2.1 Materials preparation [Ni0.8Co0.1Mn0.1]CO3 and [(Ni0.8Co0.1Mn0.1)0.985(Zr)0.015]CO3 compounds were synthesized via a carbonate co-precipitation method. Typically, a 2.0 M aqueous solution of NiSO4·6H2O, CoSO4·7H2O, and MnSO4·H2O, with the Ni/ Co/ Mn = 0.8: 0.1: 0.1, as the starting materials for [Ni0.8Co0.1Mn0.1]CO3 was pumped into a continuously stirring tank reactor (CSTR, capacity of 5L) using a peristaltic pump. Meanwhile, a 2.0 M Na2CO3 solution (aq.) as the precipitation agent and a 0.24 M NH4OH solution (aq.) as the chelation agent were also slowly added into the reactor. The feeding rate of the solution (1.5 mL/min), temperature (55 ℃) and pH (7.8) of the mixture in the reactor were carefully regulated. To prepare the Zr-modified [(Ni0.8Co0.1Mn0.1)0.985(Zr)0.015]CO3, the resultant spherical [Ni0.8Co0.1Mn0.1]CO3 was continuously reacted with Zr(SO4)2·4H2O


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(molar ratio of Zr:TM = 0.015:0.985). Collected products were filtered, washed and dried at 110℃ over 12 h and heated at 450℃ for 4 h to obtain the oxide precursors. Finally, the as prepared precursors with stoichiometric LiOH·H2O (Li/TM = 1.03) were calcined at 800℃ for 20 h under pure oxygen atmosphere to optimize the preparation conditions of the layered Ni-rich oxides Li[(Ni0.8Co0.1Mn0.1)1-x(Zr)x]O2 (x = 0, 0.015). The as-calcined products with and without Zr modification were named pristine 811 and Z811, respectively. 2.2 Electrochemical testing To fabricate the working electrodes, a slurry of the active materials, polyvinylidene fluoride (PVDF) and acetylene black in a weight ratio of 8:1:1 in N-methyl-2-pyrrolidone (NMP) was pasted onto Al foil and dried at 120℃ for 12 h in a vacuum oven. Then the coated foil was punched into circular pieces with a diameter of 1.2 cm and the corresponding active material loading was around 1.8 to 2 mg. CR2012 coin-type half-cells consisting of an as-prepared cathode, a Li metal anode, a Celgard 2400 separator and 1 M LiPF6 in EC-EMC-DEC (1:1:1 by weight) electrolyte solution were assembled in an Ar-filled glove box. Electrochemical tests of the as-assembled coincells were conducted using a battery testing system (LANHE CT2001A, Wuhan LAND Electronics Co., P. R. China) at 2.8-4.3 V (or 4.5 V) at different current densities. Cyclic voltammetry (CV) data were collected at a scan rate of 0.1 mV/s on a Princeton PARSTAT 4000 (AMETEK Co. Ltd.) workstation. Electrochemical impedance spectroscopy (EIS) was carried out using a Princeton PARSTAT 4000 in a frequency range from 10 mHz to 100 kHz with an AC amplitude of 5 mV. 2.3 Materials characterization


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The chemical composition of the as-prepared Li[(Ni0.8Co0.1Mn0.1)1-x(Zr)x]O2 (x = 0, 0.015) materials was determined using inductively coupled plasma-atomic emission spectrometry (ICPAES). The morphology and crystallographic structure were measured by scanning electron microscopy (SEM, FEG250, FEI QUANTA), X-ray diffraction (XRD, Advance D8, Bruker), and transmission electron microscopy (TEM, JEM-2100F, JEOL). Rietveld refinement of the XRD data was accomplished by General Structure Analysis System (GSAS) software.42 Annular bright field (ABF) and high angle annular dark field (HAADF) imaging were performed using a FEI Titan3 G2 60–300 TEM equipped with a double aberration-corrector for both the probe-forming and imaging lenses. The samples analyzed by scanning transmission electron microscopy (STEM) were sliced and thinned using a focused ion beam (FIB). An ESCALAB 250Xi X-ray photoelectron spectrometer (XPS) was applied to determine the chemical state of the elements. Depth profiles of Ni, Co, Mn, and Zr were acquired by etching with Ar+ ions and the etching rate was estimated as 3.0 nm/min. All XPS spectra were calibrated using the C 1s peak with a binding energy (B.E.) of 284.8 eV. Background subtraction and curve fitting were fulfilled using XPSPEAK Version 4.0 software.

3. RESULTS AND DISCUSSION The morphology of pristine 811 and Z811 oxide precursors and their corresponding lithiated active materials is shown in Figure 1a-d. It is apparent that both pristine 811 and Z811 oxide precursors (Figure S1g) are comprised of numerous spherical secondary particles with a diameter of about 6 to 8 μm (Figure 1a and 1c). After the high-temperature calcination, the spherical morphology was maintained whereas the primary particles grew to a larger size of around 500 to 700 nm (Figure 1b and 1d). SEM-EDS maps (Figure S1a-f) of the Z811 oxide precursor particles confirm the effective precipitation and uniform distribution of Zr on the surface. After high-temperature


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annealing process, Zr element exhibits the concentrated distribution with an enrichment in the surface region, as clearly shown in Figure S1h-l. The chemical composition and average structural information of the pristine 811 and Z811 materials were determined by inductively coupled plasma-atomic emission spectrometry (ICPAES) (Figure S2) and powder X-ray diffraction (XRD) (Figure 1e), respectively. XRD patterns _

of both materials can be indexed to a layered α-NaFeO2 structure with R3m symmetry. No extra diffraction peaks are observed in the Z811 material, suggesting that the Zr cations have entered the layered oxide lattice. The clear split of the (018)/(110) peaks demonstrates a well-ordered layered structure for both materials.43 The significant shift toward lower angles of the (003) and (104) reflections, as shown in the enlarged patterns (Figure 1f, g), is indicative of an increase in d-spacings for (003) and (104) planes of the Z811 material.44 For the (110) reflections (Figure 1h), a similar trend is observed while the corresponding shift is relatively smaller.


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Figure 1. (a, b) SEM secondary electron (SE) images of the oxide precursor and as-annealed pristine 811. (c, d) SEM SE images of the oxide precursor and as-annealed Z811. (e) XRD patterns of the pristine 811 and Z811 materials synthesized at 800 °C. (f, g, h) Enlarged regions for the (003), (104) and (110) peaks of the XRD patterns showing peak shifts.

Table 1. Refined crystallographic parameters for the 811 and Z811 samples Sample 811

a (Å) 2.870

c (Å)

Unit volume (Å3) Ni2+ in Li layer (%) I(003)/I(104)

Rp

Rwp

14.2026

101.353

4.49

1.2275

2.10

2.74

14.2131

101.552

4.81

1.2034

2.22

3.13

6 Z811

2.872 3

To obtain the quantitative differences in crystallographic parameters, Rietveld refinement of the XRD patterns was carried out and the results are displayed in Figure S2 of the Supporting Information and Table 1. The enlarged parameters a and c and unit cell volume of the Z811 material can be attributed to the larger ionic size of Zr4+ (0.72 Å), when compared with that of Ni2+ (0.69 Å).2,19 It is generally believed that a larger d-spacing favors Li ion migration and capacity/voltage retention for layered oxide cathodes.45 It is worth noting, however, that the Li/Ni disorder degree of the Z811 material is 4.81%, while that of the pristine 811 material is 4.49%, indicating that Zr modification leads to a slight increase in Li+/Ni2+ interlayer mixing (Table 1). This observation, which also coincides with the variation in the (003)/(104) intensity ratios shown


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in Table 1, may be ascribed to the incorporation of Zr4+ (valence state of 4+) which would account for the higher proportion of Ni2+ (valence state of 2+) to maintain charge balance in the system.46 The excess Ni2+ cations (0.69 Å) tend to occupy Li+ (0.76 Å) sites in the materials preparation process due to their similar ionic radii.47 The electrochemical properties of the pristine 811 and Z811 materials were evaluated using galvanostatic charge-discharge testing, and the initial charge-discharge curves of each sample at a current density of 0.1 C (1 C = 200 mA/g) from 2.8 to 4.3 V at 30℃ are depicted in Figure 2a. For the Z811 material, it is evident that the charge plateaus above 3.8 V shift to slightly higher voltages, whereas the discharge plateaus shift to lower voltages, compared with the pristine 811 material (arrows 1 and 2 in Figure 2a). As a result, the Z811 material delivers an initial discharge capacity of ~200 mAh/g, which is a bit lower than that of the pristine 811 material (206 mAh/g). A similar trend is observed in rate capacity tests, as shown in Figure S3. However, the Z811 material exhibits a higher Coulombic efficiency of 89.66% for the first cycle than the 811 material (88.39%). As displayed in Figure 2b, improved cycling performance is observed in the Z811 material, showing a high discharge capacity of 158 mAh/g (91.5% capacity retention) after 200 cycles between 2.8 and 4.3V. The charge and discharge voltage profiles of the pristine 811 and Z811 materials during cycling are further compared in Figure 2c and 2d. As indicated by arrows (1) and (2), the Z811 material shows a significantly slower discharge voltage decay as well as lower capacity fading than the pristine 811 sample. The effect of Zr modification becomes more prominent when the pristine 811 and Z811 materials are evaluated at an elevated charging cut-off voltage of 4.5V. As shown in Figure 2e and 2f, the Z811 material still exhibits a slight reduction in specific capacity and increase in Coulombic efficiency for the initial cycle, compared with the pristine 811 material. More importantly, the Z811 material retains a high discharge specific


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capacity of 182 mAh/g (93.22% capacity retention) after 100 cycles, while the 811 material delivers a discharge capacity of 164 mAh/g with a much lower retention rate of 85.84% under the same testing conditions. In addition, a slighter peak shift was observed in the CV curves of the Z811 material between 2.8-4.5 V when compared with the pristine 811 material, as marked by the orange arrows in Figure S4c and S4d, suggesting the superior cycling stability of Z811 material. The inferior electrochemical kinetics and better cycling stability observed for the Zr-modified material is closely related to structural variations to both the bulk and surface of the electrode. The XRD analysis, presented above, only provides average bulk structure information; as such, direct evidence of local structure and chemical information of the electrode surface is therefore critical.


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Figure 2. (a) Initial charge–discharge profiles of as-prepared 811 and Z811 materials tested at a rate of 0.1 C (1 C = 200 mAh/g) between 2.8 and 4.3 V. (b) Cycling performance of as-prepared 811 and Z811 materials cycled between 2.8-4.3 V. Galvanic discharge profiles (3rd, 50th, 100th, 150th and 200th cycles) of (c) 811 and (d) Z811 materials at a rate of 2 C. (e) Initial chargedischarge profiles of all samples tested at a rate of 0.1 C between 2.8 and 4.5 V. (f) Cycling performance of as-prepared 811 and Z811 materials cycled between 2.8 and 4.5 V.


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To obtain insight into the origin of limited electrochemical kinetics and enhanced cycling performance for the Zr-modified material, the 811 and Z811 materials were image using aberration-corrected STEM. Figure 3 shows HAADF-STEM images and FFT (fast Fourier transformed) patterns of the pristine 811 and Z811 materials. As shown in Figure 3b, the FFT pattern of square (I) in region 1 at the outermost surface of the pristine 811 material contains two _

_

sets of diffraction patterns. One pattern can be indexed as layered R3m along the [010] zone axis. The other pattern, comprised of diffuse streaks and marked by the white arrows, can be indexed as _

NiO-type Fm3m along the [110] zone axis, indicating the presence of the cation mixed phase. The FFT patterns from square (II) and (III) in region 1 and from region 2 have just one pattern which _

is indexed to layered R3m and confirm that a layered phase continues from the sub-surface to the _

bulk region of the pristine 811 particle. For the Z811 material, in addition to a layered R3m structure in the bulk (square (iii) in Figure 3e and Figure 3f), it is worth noting that a pure NiO_

type Fm3m surface layer (~5 nm thick) is present along certain crystallographic facets, as shown _

_

in square (i) of Figure 3e. A mixture of layered R3m and NiO-type Fm3m phases can be detected in square (ii) of Figure 3e. This observation confirms that Zr modification may generate a thicker _

surface layer with disordered NiO-type Fm3m structure, which is consistent with the higher Li/Ni cation mixing degree based on the XRD refinement analysis.


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Figure 3. (a) SEM SE image (inset) of a single particle and low magnification STEM ADF image of a cross-sectioned 811 particle. (b) Magnified STEM ADF image (left) and corresponding FFT patterns (right) taken from region 1 in (a). (c) Magnified STEM ADF image and corresponding FFT pattern (inset) taken from region 2 in (a). (d) SEM SE image (inset) of a single particle and low magnification STEM ADF image of a cross-sectioned Z811 particle. (e) Magnified STEM ADF image (left) and corresponding FFT patterns (right) taken from region (1) in (d). (f) Magnified STEM ADF image and corresponding FFT pattern (inset) taken from region (2) in (d).

To further analyze the surface structure, atomic resolution HAADF-STEM images were obtained, and Figure 4 shows representative STEM images of the surfaces regions of the pristine 811 and Z811 materials. Figure 4b and 4c are atomic resolution images of regions 1 and 2 (811 material) in Figure 4a, respectively, showing alternative stacking of Li and TM slabs along the


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[001] direction. It is clearly shown that the surface regions 1 and 2 are viewed along the same [010] zone axis projection but have different exposed crystallographic planes. Specifically, both regions 1 and 2 exhibit an uneven NiO-type cation mixed layer with a thickness of 1-2 nm, which is generally considered as an intrinsic surface defect for NLOs and naturally formed during the synthesis process.48 In contrast, as displayed in Figure 4e and 4f, surface regions (1) and (2) of the Z811 sample in Figure 4d show much thicker layers of the NiO-type cation mixed phase (5-6 nm) with bright contrast, compared with regions 1 and 2 of the pristine 811 material. This distinction is further verified by the intensity profiles where the peaks with high intensity represent TM occupation, as shown in Figure 4g and 4h. It is reasonable to conclude that the substitution of TM cations with Zr4+ cations leads to enlargement of cation mixed region at the surface, where the original Ni3+ cations are effectively reduced to Ni2+ to balance the charge difference induced by Zr4+ cations. Some newly generated Ni2+ cations tend to migrate from the original TM sites to Li sites to form the NiO-type cation mixed phase because Ni2+ has a similar ionic radius (0.69 Å) to that of Li1+ (0.76 Å).49 In addition to the slight suppression of electrochemical kinetics, the nanoscale NiO-type phase has been reported to serve as a pillar layer that can block continuous migration of TM ions to Li sites and alleviate the side reactions between the electrolyte and active materials.38, 40 Accordingly, it is reasonable to believe that the remarkable cycling stability of the Z811 material in this work is attributed to the thickened surface cation mixed nanolayer induced by Zr modification.


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Figure 4. HAADF-STEM images taken from the surface regions of cross-sectioned particles from (a) pristine 811 and (d) Z811 materials. Atomic resolution STEM images from (b) Region 1 and (c) Region 2 in (a). Atomic resolution STEM images from (e) Region (1) and (f) Region (2) in (d). (g) Li intensity profiles from selected region A in (b) and selected region B in (e). (h) TM intensity profiles (h) from selected region C in (c) and selected region D in (f).


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X-ray photoelectron spectroscopy (XPS) and depth profiling measurements were conducted to further prove the presence of the cation mixed surface nanolayer induced by Zr modification. The survey spectra (Figure S5a) show typical peaks corresponding to Ni 2p, Co 2p, Mn 2p, O 1s and Li 1s in both the pristine and Zr-modified NLOs. An extra Zr 3d peak (Figure S5c) is present for the Z811 material, which confirms the presence of Zr. The binding energies for Zr 3d5/2 and Zr 3d3/2 are about 181.79 and 184.14 eV,

50, 51

respectively, which is consistent with the values

reported for Zr4+ in ZrO2, suggesting a 4+ valence for Zr. The characteristic XPS spectra for Ni 2p, Co 2p, and Mn 2p for the 811 and Z811 materials before etching were compared, as shown in the Figure 5a-c. For the Z811 material, the Ni 2p and Co 2p peaks shift slightly toward lower binding energies, indicating that the incorporated Zr4+ in the TM layers prefers to substitute for the Ni3+/Co3+ sites.19 To achieve charge neutrality in the material system, Ni and Co tend to shift to lower valence states. After Ni 2p peak fitting, as displayed in Figure S5b, the intensity ratio of Ni 2p3/2 (Ni2+) to Ni 2p3/2 (Ni3+) in the Z811 material is visibly larger than the intensity ratio in the pristine 811 material, suggesting that a higher percentage of the Ni2+ ions resides in the surface region of the Z811 material via Zr modification.52 XPS depth profiles (Figure 5d-f) were obtained to confirm the occurrence of gradient doping of Zr in the surface and sub-surface regions. It is clearly shown that the atomic ratios of Ni, Co and Mn barely change in the pristine 811 material, whereas depletion of Ni (enrichment of Zr) is evident in the surface region of the Z811 material (as marked by the black dotted box). This phenomenon is consistent with the calculated results proposed by Aurbach et al.,19 in which Zr substitution has the highest preference for Ni sites, followed by Co sites and the lowest preference for Mn sites. Consequently, with the assistance of Zr4+ cations, a significant amount of Ni2+ cations preferentially occupy the octahedral sites in the Li slabs of the layered structure and trigger the formation of the nanoscale cation mixed surface


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structure. Since this surface layer is associated with Ni depletion (Figure 5f), the side reactions between the electrolyte and highly oxidizing Ni4+ cations would be effectively mitigated during the charge process.5, 52 Additionally, the nanoscale electrochemically inactive cation mixed surface layer, characteristic of high structural stability, suppresses unfavorable surface structural degradation,38, 40 which accounts for the improved cycling performance in the Z811 material.

Figure 5. Chemical information for the as-prepared 811 and Z811 materials. XPS spectra for (a) Ni 2p, (b) Co 2p and (c) Mn 2p for both samples. The red solid arrows are employed to show the energy shift for the TM 2p peaks. (d) XPS spectra for Zr 3d at various depths in the Z811 sample. (e, f) Compositional change for the TMs and Zr as a function of the etched depth in as-prepared 811 and Z811 materials. The enlarged images (inset) correspond to the black dotted boxes showing the enrichment of Zr (depletion of Ni) in the surface region of the Z811 sample. Electrochemical impedance spectroscopy (EIS) tests were performed to track the interfacial evolution of the pristine 811 and Z811 cathode electrodes. The Nyquist spectra for both cathodes


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after charging to 4.5 V in the 3rd, 50th and 100th cycles are displayed in Figure S6a and S6c. As expected, the spectra contain a small semicircle in the high frequency range (Figure S6b and S6d), a larger semicircle in the high to medium frequency range and an oblique line in the low frequency range, which represent the surface film impedance (Rsf and CPE1), the impedance for charge transfer (Rct and CPE2) and the Warburg impedance (W), respectively.53 The intercept at very high frequency (Figure S6b, d) with the real axis corresponds to the electrolyte resistance (Re).53 The equivalent circuit as an inset in Figure S6c was used to simulate the Nyquist spectra and the obtained Re, Rsf and Rct values are listed in Table S2. Note that the two electrodes show similar Rsf and Rct values during the early stages of cycling. However, as clearly shown in Table S2, Rct in the pristine 811 material increases from 259.2 Ω (3rd cycle) to 1050 Ω (100th cycle), while Rct in Z811 increases from 167.9 to 433.1 Ω after the same number of cycles. Similarly, the Z811 material shows smaller Rsf values, when compared with the pristine 811 material. It is generally believed that the dramatic rise in Rct is indicative of irreversible microstructure degradation typically occurring at the surface of the NLO materials upon cycling.54 The SEM images of pristine the 811 and Z811 cathode after different cycles between 2.8-4.5 V are presented in Figure S7a-h. It is clearly shown that more cracks and severe pulverization could be observed in the pristine 811 cathode when compared with the Z811 cathode after the same cycles, indicating the improved structural stability of Z811 material. Consequently, the pristine 811 suffered from severer transition metal (TM) dissolution due to the dramatic cracks and the resultant newly formed surface,55 as verified in Figure S7i. Therefore, from the aforementioned data, it may be concluded that the preformed cation mixed nanolayer is effective in suppressing the detrimental microstructure degradation processes during electrochemical cycling, which contributes to its superior interfacial stability.


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Figure 6. (a) Low magnification HAADF-STEM image of the surface region of a cross-sectioned particle from the 811 material after 100 cycles at 2.8 to 4.5 V. (b) Magnified HAADF-STEM image (left) and corresponding FFT patterns (right) taken from region 1 in (a). (c) Magnified HAADF-STEM image and corresponding FFT pattern (inset) taken from region 2 in (a). (d) Low magnification HAADF-STEM image of a cross-sectioned Z811 particle. (e) Magnified HAADFSTEM image (left) and corresponding FFT patterns (right) taken from region (1) in (d). (f) Magnified HAADF-STEM image and corresponding FFT pattern (inset) taken from region (2) in (d).

To confirm the improvement due to Zr modification, HAADF-STEM analysis was conducted on the pristine 811 and Z811 materials after 100 cycles at 2.8 to 4.5 V. Figure 6 presents HAADFSTEM images and corresponding FFT patterns of the surface regions of the cycled samples. As shown in Figure 6b, region 1 of the cycled 811 material displays only the rock salt phase with a


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_

space group of Fm3m, as confirmed by the FFT pattern (inset of Figure 6b). In contrast, for the _

_

cycled Z811 material, the mixed rock salt (Fm3m) and layered structure (R3m) phases in region (1) (Figure 6e) were retained; however, the rock salt layer increased slightly in thickness (~10 nm) after the same amount of electrochemical cycling, indicating superior structural stability. Furthermore, it is noted that the rock-salt phase also coexists with the remaining layered structure in region (2) of the cycled 811 material (Figure 6c), while region (2) of the cycled Z811 material (Figure 6f) consists of only the layered phase. This distinction demonstrates that the preformed cation mixed nanolayer can indeed prevent the layered bulk phase from transforming to disordered spinel-like and/or rock salt phases; thereby contributing to the excellent structural stability as well as cycling performance of the NLO materials.

4. CONCLUSIONS

In summary, a Li[(Ni0.8Co0.1Mn0.1)0.985(Zr)0.015]O2 cathode material with a preformed cation mixed layer (~5 nm) was prepared using a Zr-rich precursor coating on [Ni0.8Co0.1Mn0.1]CO3 during the _

precipitation process. The nanoscaled surface defective layer, with a space group of Fm3m, suppressed structural degradation of particles during cycling and mitigated side reactions between the active materials and the electrolyte. Additionally, a moderate amount of Zr cations was doped into the bulk phase, effectively stabilizing the oxygen close-packed structure and improving the cycling stability of the NLO cathode materials. We anticipate that the nanoscaled surface defective layer could also be formed by modification with similar elements such as Sn, V or Mo and applied to other layered oxide cathode materials.

AUTHOR INFORMATION Corresponding Author


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* Email: [email protected] ORCID Weifeng Wei: 0000-0002-3088-6549 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors would like to acknowledge financial support from the National Natural Science Foundation of China (51304248 and 11474147), the National Basic Research Program of China (Grant No. 2015CB654901), the Natural Science Foundation of Jiangsu Province (Grant No. BK20151383), the International Science and Technology Cooperation Program of China (2014DFE00200), the Innovation Program of Central South University (2016CXS003), the State Key Laboratory of Powder Metallurgy at Central South University and the Hunan Shenghua Technology Co., Ltd.

SUPPORTING INFORMATION AVAILABLE The Supporting Information is available free of charge on the ACS Publications website at DOI: ● SEM maps and EDS spectrum of Z811 oxide precursor and the cross-sectional Z811 particle. ●XRD patterns of 811and Z811 oxide precursor. ●XRD Rietveld refinement patterns of 811and Z811 materials. ● Rate capability of 811 and Z811 materials at various C rates. ● Cyclic voltammograms of the pristine 811 and Z811 materials. ● XPS survey spectrum for 811 and Z811 materials. ● Nyquist plots and fitted results of 811 and Z811 electrodes after the 3rd, 50th and 100th cycles.


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● SEM images of the pristine 811 and Z811 cathodes after different cycles between 2.8-4.5 V. ● Ni, Co, Mn, and Zr concentrations in electrolyte for different samples after 100 cycles between 2.8-4.5 V.

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TABLE OF CONTENTS:


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Enhancing the Structural Stability of Ni-Rich Layered Oxide Cathodes

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