English

• 1. INTRODUCTION

  Lithium ion batteries (LIBs) are now extensively used in portable electronics and electric vehicles as the energy storage device due to high energy density and long cycle life^[1]. With function upgrades and getting better experience in smart devices, the more energy needs to be stored in the limited space, meaning that the gravimetric energy density and volume energy density of LIBs must be promoted, both of which are mainly constrained by cathodes. As a promising candidate of high energy density cathode (> 1000 W·h·kg^-1) in LIBs, Li-rich layered oxide has been focused on by researchers since it was created by Dahn and Thackeray^{[2, 3]}. It is regarded as a combination of rhombohedral transition metal (TM) layered oxide LiTMO₂ (TM = Ni/Co/Mn, space group R-3m) and monoclinic Li₂MnO₃ (space group C2/m)^[4-6]. To deliver the high discharge capacity (> 250 mA·h·g^-1), the latter Li₂MnO₃ needs to be activated in the first charging process above 4.5 V involving oxygen redox^[7-9]. However, the extensive Li⁺ extraction during activation will lead to lattice oxygen loss, in which oxygen vacancies are formed, resulting in TM migration to Li layers and structural instability^[10-12]. As a consequence, the initial layered phase usually transforms to spinel-like phase and finally evolves to disordered rock-salt phase during subsequent cycles, resulting in capacity degradation and voltage decay, which hinders the commercial application of Li-rich layered cathode^[13-15].

  To improve the cycling stability, lattice doping and surface modification are widely adopted. Lattice doping with Mg, Al, Ti, Nb or B in Li-rich cathode materials has been reported^[16-20]. Surface coating with AlF₃, Li₄Mn₅O₁₂ or TiO₂ effectively improves the cycling stability^[21-23]. In addition, reconstructing the surface to form a core-shell structure is also an effective method. To act as a protective shell, there are two requirements: (1) facilitating Li⁺ diffusion to penetrate the shell; (2) electrochemically robust to protect the bulk in the core. Therefore, LiMn₂O₄-like spinel phase was usually adopted as a shell to construct layered-spinel core-shell heterostructure in Li-rich cathodes by post-synthesis treatments with some oxides like Na₂S₂O₈, (NH₄)₂SO₄, etc^[24-26]. In contrast, NiO-type rock-salt phase is usually considered electrochemically inert and Li⁺ blocking, and so barely used as a shell in Li-rich layered oxide cathodes. Nevertheless, very recently, Piao etc. reported a rock-salt shell in the high voltage cathode LiNi_0.5Mn_1.5O₂, which exhibited improved cycling stability^[27]. It inspires us to apply such a protective rock-salt shell to solve the concern of cycling stability in Li-rich layered oxides.

  Herein, we develop a facile soft chemical method to construct core-shell structure with a unique (Li_xTM_1-x)O (TM = Ni, Co, Mn) rock-salt shell in situ. Systemic structural and chemical analysis demonstrate that such a rock-salt shell can not only efficiently transfer Li⁺ due to Li/TM mixing, but also greatly hinder Mn dissolution and O evolution due to the intrinsic structural robustness of rock-salt phase. Such a bifunctional shell leads to the enhanced cycling stability with an excellent capacity retention of 92.7% after 200 cycles. This work provides a new way to modify particle surface with an electrochemically-inert shell for enhanced cycling stability in Li-rich layered oxides.

  2. EXPERIMENTAL

  2.1 Sample preparation

  Li-rich layered oxide Li_1.2Ni_0.18Co_0.08Mn_0.54O₂ was prepared by high-temperature calcination. Firstly, the nickel-cobalt-manganese carbonate precursor (Hai'an zhichuan battery material technology co. LTD) and stoichiometric (Li: TM = 1.23:0.8) lithium carbonate (Aldrich, ACS ≥ 99%) were thoroughly mixed by an ethanol-involved wet-milling process. 3 at% excessive lithium carbonate was used to compensate for lithium loss during the calcination. Secondly, the mixture was rent in two. Both were sintered at 480 ℃ for 3 h and subsequently heated up to 900 ℃ for 10 h in air, and then quenched in air and 1 wt% glucose solution, respectively. The sample quenched in air was denoted as normal. The powder quenched in glucose solution (denoted as Quench) was washed by deionized water, filtered and dried at 105 ℃ for 12 h in a vacuum oven.

  2.2 Materials characterization

  X-ray diffraction (XRD) patterns were collected from a Bruker D8-Advance diffractometer with CuKα radiation (λ = 1.5406 Å) at 45 kV and 100 mA. The morphology of the samples was detected by the scanning electron microscope (SEM, ZEISS Supra 55). High resolution transmission electron microscope (HR-TEM) was performed by a FEI TecnaiG2 F30 TEM microscope. The concentrations of Li, Ni, Co and Mn in samples were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES, JY2000-2). The distribution of elemental valences was analyzed on an X-ray photoelectron spectroscopy (XPS, ESCALAB 250XL), and the spectra were corrected by C 1s peak at 284.8 eV.

  2.3 Electrochemical tests

  The cathode electrodes were the mixture of 80 wt% sample powders, 10 wt% acetylene black and 10 wt% poly(vinylidene fluoride) (PVDF) coated on an aluminum foil current collector. CR2032 coin-type cells were assembled in an argon-filled glove box and shelved for 12 hours before electrochemical measurement. Lithium foils and polymer membranes were used as the anodes and separators. 1 M LiPF₆ solution with the mixture solvent of EC, EMC and DMC (1:1:1 by ratio) was used as the electrolyte. The cells were charged and discharged on a NEWARE battery test system using galvanostatic mode at different rates (1 C = 250 mA·h·g⁻¹) in the voltage range of 2.0~4.8 V (vs Li⁺/Li) at 25 ℃. The electrochemical impedance spectra (EIS) were collected by the electrochemistry workstation (1400 cell test system, Solartron) in the frequency range of 100 kHz~0.1 Hz.

  3. RESULTS AND DISCUSSION

  3.1 Design of core-shell structure by the soft chemical quench method

  As shown in Scheme 1, a new soft chemical quench method is developed to construct a core-shell heterostructure in situ during the synthesis of Li-rich layered oxides. In a typical process, the powder is fast transferred to a dilute glucose solution for quenching just after completing the calcination at the highest temperature. The glucose molecules in the solution possess typical aldehyde groups (-CHO) with strong reducibility, which can initiate a series of reduction reactions at the particle surface (as depicted in Eq. 1 by taking Li₂MnO₃ as an example) by utilizing the residual heat carried by individual particles. By tuning the concentration of glucose solution and the transfer time, the extent of such reduction reactions could be finely controlled, and leads to the changes of TM chemical valences, and furthermore the structural transformation at the particle surface. Eventually, a perfect core-shell heterostructure can be perfectly constructed within each secondary particles of Li-rich layered oxides.

  $ 2 \mathrm{Li}_2 \mathrm{Mn}(\mathrm{IV}) \mathrm{O}_3+2 \mathrm{CHO}+4 \mathrm{OH} \rightarrow \mathrm{LiMn}(\mathrm{III} / \mathrm{IV})_2 \mathrm{O}_4+2 \mathrm{COOLi}+\mathrm{OLi}+3 \mathrm{H}_2 \mathrm{O} \rightarrow\left(\mathrm{Li}_x \mathrm{Mn}_{3-x}\right) \mathrm{O}_4 \rightarrow\left(\mathrm{Li}_x \mathrm{Mn}_{1-x}\right) \mathrm{O} $

  (1)

  Scheme 1

  

  Scheme 1.  Scheme illustration for achieving core-shell heterostructure in Li-rich layered oxide cathode by a new soft chemical quench method. Spherical secondary particles of Li-rich layered oxides were poured into a dilute glucose solution for fast quenching, wherein a reduction reaction occurred at the particle surface due to the reductive aldehyde groups in glucose molecules, which may induce structural transformation at particle surface and eventually a core-shell structure

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  3.2 Identification of core-shell heterostructure

  Two Li_1.2Ni_0.18Co_0.08Mn_0.54O₂ samples prepared by the normal calcination (denoted as Normal) and the soft chemical quench method (denoted as Quench) were prepared (see details in Experimental section). To identify the influences on the crystal structure by the soft chemical quench method, X-ray diffraction was carried out on Normal and Quench samples. XRD patterns and corresponding Rietveld refinement are shown in Fig. 1a~1b. As expected, both patterns exhibit similar diffraction peaks, indicating negligible effect on the layered phase in the bulk. The strong peaks in the patterns are characteristic of a classic α-NaFeO₂ type layered structure (space group R-3m), and the magnified weak peaks in the 2θ range of 20° to 25° (insets in Fig. 1a~b) are the typical superlattice peaks of Li₂MnO₃-type monoclinic structure (space group C2/m), corresponding to Li/Mn ordering in the pattern of Li@Mn₆ benzene-like ring (six MnO₆ octahedra linked like a ring (Mn₆) with a central LiO₆ octahedron) in TM layers^{[28, 29]}. The refined parameters are listed in Table S1, which shows no discernible difference between Normal and Quench. This means that the lattice does not change after the soft chemical quench.

  Figure 1

  

  Figure 1.  Identification of core-shell heterostructure constructed by the soft chemical quench method. X-ray diffraction (XRD) patterns and Rietveld refinements of normal sample (a) and quench sample (b). The superlattice peaks are marked by the green dashed rectangles, and magnified in the insets. HRTEM images of normal sample (a) and quench sample (b) to analyze the local structure. The corresponding FFT maps were deposited in the insets. The spots in FFT maps were indexed to the corresponding layered phase and rock-salt phase

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  To further evaluate the effect on the local structure, especially at the particle surface, high resolution transmission electron microscopy (HRTEM) was performed on both samples. As shown in Fig. 1c, HRTEM image of normal sample exhibits the continuous and well-defined layered lattice fringes from the surface to the bulk, demonstrating a well-ordered layered structure, further confirmed by the corresponding fast Fourie Transform (FFT) map in the inset^{[30, 31]}. In comparison, HRTEM image of Quench in Fig. 1d is obviously different from that of normal, and exhibits an apparent core-shell heterostructure. It comprises highly ordered layered phase in the core and coherently disordered rock-salt phase with a thickness of approximately 10 nm in the shell, which is confirmed by FFT map in the inset. The spots in the FFT map are indexed by NiO-type rock-salt phase, and corresponding interplanar distances of (1-11), (310) and (220) spots are measured as 2.6, 1.4 and 1.6 Å, which are larger than the corresponding values (2.41, 1.32 and 1.47 Å) in typical NiO, hinting that partial Li⁺ may residue in the rock-salt shell.

  The influence of the soft chemical quench on the elemental compositions is determined by ICP. As deposited in Table S2, the normalized content of Li for quench decreases significantly than that of normal, while the Ni/Co/Mn ratio remain unchanged. It indicates that Li⁺ is largely extracted during the soft chemical quench. Combining with the unchanged lattice parameters by XRD above, we can deduce Li⁺ loss mainly occurs at the particle surface, which leads to the formation of rock-salt phase observed above. The morphology was detected by SEM. As shown in Fig. S1, both are micro-size secondary particles, which consist of primary nanoparticles with the size of 200~500 nm.

  3.3 Chemical analysis in the rock-salt shell

  To further understand the composition and property of the rock-salt shell, chemical analysis was tested at the particle surface. The element distribution was detected by TEM energy dispersive X-ray (EDX) technique. As shown in Fig. S2, the contrast difference in the image demonstrates an apparent core-shell heterostructure in Quench sample, and Ni, Co and Mn are uniformly dispersed both at the surface and in the bulk. The valence states of TM cations at the surface were analyzed by XPS. As shown in Fig. 2a-b, Mn 2p peaks overall shift to low binding energy after the soft chemical quench. To quantify the composition of valence states, two separated peaks associated with Mn³⁺ (at 641.7 eV) and Mn⁴⁺ (at 642.6 eV) are used to fit Mn 2p_3/2 peak, as depicted by blue and green colors, respectively. The ratios of the integrated areas of the two peaks (denoted as Mn³⁺/Mn⁴⁺) are distinctly different, and plotted in Fig. 2c. The Mn³⁺/Mn⁴⁺ values of Normal and Quench are 0.22 and 1.51, respectively, which indicates more Mn³⁺ in the rock-salt shell. It can be interpreted by Mn reduction at the surface due to the reducibility of aldehyde group in glucose molecules during the soft chemical quench. Ni and Co 2p XPS spectra were also recorded. As shown in Fig. S3, the same position of Ni 2p_3/2 (at 854.6 eV) indicates Ni²⁺ at the particle surface in both samples^[32]. Similar with Ni, there is only Co³⁺ at the particle surface, demonstrated by the same position of Co 2p_3/2 peak (at 780.1 eV) in both samples^[33]. In brief, only Mn reduction without Ni and Co reduction occurs during the soft chemical quench. The existence of Mn³⁺ and Co³⁺ provides the opportunity of Li⁺ co-occupancy in NiO-type rock-salt phase for charge balance. Meanwhile, C 1s spectra shown in Fig. S4 were used to check the influences of soft chemical quench on the surficial C species. The spectra were fitted by multiple peaks from different C species. The C−C peak (at 284.8 eV) is associated with adventitious carbon contamination^[34]. The increased peak area of -COO and C−O peaks in Quench sample, compared with those in Normal sample, may come from the abundant -OH radicals in the glucose molecules. Moreover, the peak of Li₂CO₃ (at 289.8 eV) disappears in Quench sample in comparison with that of the Normal one, indicating the elimination of the residual Li₂CO₃ at the particle surface by water solution. It is consistent with the previous reports about liquid (water or ethanol) washing, and may be beneficial to electro-chemical performance^[35-40].

  Figure 2

  

  Figure 2.  Chemical analysis into the rock-salt shell. Mn 2p XPS spectra and the peak fitting for Normal sample (a) and Quench sample (b). (c) The ratio between Mn³⁺ content and Mn⁴⁺ content (denoted as Mn³⁺/Mn⁴⁺), represented by the ratios of peak areas between Mn³⁺ and Mn⁴⁺ 2p peaks in (a) and (b). Thereinto, Mn³⁺/Mn⁴⁺ value for Normal sample could be used to represent the composition in the bulk of core-shell structure for Quench sample

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  3.4 Enhanced cycling stability

  To evaluate the impact of this unique rock-salt shell on the electrochemical performance, two samples were fabricated into the cathodes and assembled into coin cells for systemic electrochemical tests. As shown in Fig. 3a, the charge/discharge curves in the 1^st cycle at 0.05 C (at 25 ℃) are provided for two samples. The discharge capacity is increased to 287.3 mA·h·g^-1 with a higher coulombic efficiency of 81.1% (74.6% for Normal), which should be related to the less surface side reactions due to the elimination of Li₂CO₃, demonstrated by XPS above. The normalized dQ/dV curves are compared in Fig. S5. The peak at 4.5 V, associated with the activation of Li₂MnO₃, rises up for Quench sample compared with that for the Normal one, suggesting more Li⁺ extraction from Li₂MnO₃ component during activation. Correspondingly, the reduction peaks for Quench sample maintain the same positions, and increased the areas in comparison with that for Normal sample. The cycling performances of both samples were tested at 0.5 C (Fig. 3b). After 200 cycles, the discharge capacity and the retention for Quench sample are 203.2 mA·h·g^-1 and 92.7%, respectively, which is much better than the Normal sample (166.5 mA·h·g^-1 and 82.6%). The corresponding charge/discharge curves as well as the deduced dQ/dV ones at the selected cycles are shown in Fig. 3c~3f. R1 peak above 4.1 V represents the anionic reduction of O^n-/O^2-, R2 peak at 3.8V associates with the reduction of Ni⁴⁺/Ni³⁺/Ni²⁺and Co⁴⁺/Co³⁺, R3 and R4 peaks correspond to the reduction of Mn⁴⁺/Mn³⁺ in layer structure and in spinel-like phase, respectively. It is obvious that R1 of Quench has less attenuation compared with that of Normal after 200 cycles, meaning that Quench owns better reversibility of anionic redox. R3 peak of Quench just presents a little change, while R3 peak of Normal disappears after 200 cycles. Meanwhile, R4 peak of Normal becomes much more pronounced after long-term cycling than that of Quench, indicating the much serious structure degradation from the layered phase to spinel-like phase^[41]. The average voltage decay is also compared in Fig. S6. Normal electrode exhibits a lower discharge voltage in the initial cycle at 3.52 V and decrease to 3.24 V after 200 cycles, while Quench electrode shows a higher discharge voltage during cycling (from 3.59 V to 3.31 V). Combining the capacity and the average voltage together, Quench sample displays a better energy density (> 670 W·h·kg^-1 at 0.5 C after 200 cycles), shown in Fig. S7, the retention of which is 85.3% after 200 cycles, higher than that of the Normal sample (76.4%). Above all, Quench sample not only possesses a higher discharge voltage with an excellent capacity, but also displays the enhanced cycling stability.

  Figure 3

  

  Figure 3.  Enhanced cycling stability by the rock-salt shell. (a) The charge/discharge profiles during the 1st cycle at 0.05 C (1 C = 250 mA·h·g^-1) for Normal and Quench samples. (b) The cycling performance of Normal and Quench samples after 200 cycles at 0.5 C. The charge/discharge curves of selected cycles for Normal (c) and Quench (e) samples. The corresponding dQ/dV curves for Normal (d) and Quench (f) samples

  DownLoad: Full-Size Img PowerPoint

  The representative rock-salt phase NiO is usually considered to be electrochemically inert, and a barrier for Li⁺ diffusion. In order to study the impact of Li-contained (Li_xTM_1-x)O rock-salt shell, the impedance of two samples was measured by EIS. As shown in Fig. S8, Quench sample exactly presents a little higher impendence compared with the Normal one, which may be due to the poor electronic conductivity in the rock-salt shell and also Li/TM mixing in this unique (Li_xTM_1-x)O rock-salt phase. And their rate capabilities are rendered in Fig. S9. The corresponding discharge capacities of Quench sample are 257.2, 249.9, 225.1, 206.1, 184.1 and 157.5 mA·h·g^-1 at 0.1, 0.2, 0.5, 1, 2 and 5 C, respectively, superior to that of Normal sample (246.7, 237.1, 216.9, 198.1, 180.1 and 153.2 mA·h·g^-1 at 0.1, 0.2, 0.5, 1, 2 and 5 C, respectively). Both samples present excellent rate performance. Notably, the discharge capacity of Quench is still better even at a high rate of 5 C, in spite of the lager impedance of the rock-salt shell.

  3.5 Mechanism for the protective role of rock-salt shell

  To further demonstrate the structural robustness and the protective role of such a rock-salt phase, HRTEM images were collected on Quench sample after 5 cycles to analyze the local structure. As shown in Fig. 4a, the rock-salt shell is still preserved after 5 cycles, and the thickness is still thin (< 20 nm), demonstrating the good mechanical robustness. The layered structure is still well preserved in the bulk, and the seamless connection between the layered core and the rock-salt shell due to the lattice matching ensures the mechanical stability of such a core-shell heterostructure. In addition, the XPS spectra of Ni/Co/Mn/O after 5 cycles were also detected, as shown in Fig. S10. The peak areas of Ni 2p and Mn 2p are integrated to observe the content variation. It is obvious that the Mn 2p peak area of Normal is lower than that of Quench, while they both have similar Ni 2p peak area, meaning more Mn dissolution in Normal compared with Quench. It further highlights the role of (Li_xTM_1-x)O rock-salt shell in suppressing Mn dissolution. Meanwhile, O 1s spectra were compared in Fig. S10d. O^- related peak was considered to be related with the oxygen redox reaction and the oxygen evolution at high voltages^{[42, 43]}. Much lower peak of O^- related peak in Quench sample than that in Normal sample means that the oxygen evolution is greatly suppressed by the unique rock-salt shell.

  Figure 4

  

  Figure 4.  Mechanism illustration for the protective role of the unique rock-salt shell. (a) HRTEM images as well as the corresponding selected-region FFT maps (insets) for Quench sample after 5 cycles. (b) Schematic illustration for the protective role of bifunctional rock-salt shell in Li-rich layered oxide cathodes

  DownLoad: Full-Size Img PowerPoint

  In summary, a scheme is drawn in Fig. 4b to clearly illustrate the protective role of such a bifunctional rock-salt shell. On one hand, it inherits the structural stability of cubic lattice in rock-salt phase, which can effectively inhibit the TM dissolution and oxygen evolution. On the other hand, Li-containing rock-salt shell is Li/TM mixed, which creates available routes for Li⁺ diffusion, thereby solving the electrochemical inertness of pure NiO rock-salt phase. Both functions finally lead to the actually improved the cycling performance.

  4. CONCLUSION

  In this work, a soft chemical quench method, utilizing the strong reductivity of aldehyde groups in glucose molecules and the residual heat after calcination, is successfully developed to construct a new core-shell heterostructure in Li-rich layered oxide with a unique (Li_xTM_1-x)O rock-salt shell. It not only inherits the chemical stability of traditional NiO-type rock-salt phase, but also facilitates Li⁺ diffusion due to the co-occupancy of Li⁺ and TM cations. Compared to Normal sample, it shows a higher capacity retention of 92.7% after 200 cycles at 0.5 C with higher average voltage, thereby a higher energy density (> 670 W·h·kg^-1 at 0.5 C after 200 cycles). The finding will provide new guidance to design and synthesize new Li-rich layered oxides with excellent cycling stability.

  
  
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  Scheme 1  Scheme illustration for achieving core-shell heterostructure in Li-rich layered oxide cathode by a new soft chemical quench method. Spherical secondary particles of Li-rich layered oxides were poured into a dilute glucose solution for fast quenching, wherein a reduction reaction occurred at the particle surface due to the reductive aldehyde groups in glucose molecules, which may induce structural transformation at particle surface and eventually a core-shell structure

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  Figure 1  Identification of core-shell heterostructure constructed by the soft chemical quench method. X-ray diffraction (XRD) patterns and Rietveld refinements of normal sample (a) and quench sample (b). The superlattice peaks are marked by the green dashed rectangles, and magnified in the insets. HRTEM images of normal sample (a) and quench sample (b) to analyze the local structure. The corresponding FFT maps were deposited in the insets. The spots in FFT maps were indexed to the corresponding layered phase and rock-salt phase

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  Figure 2  Chemical analysis into the rock-salt shell. Mn 2p XPS spectra and the peak fitting for Normal sample (a) and Quench sample (b). (c) The ratio between Mn³⁺ content and Mn⁴⁺ content (denoted as Mn³⁺/Mn⁴⁺), represented by the ratios of peak areas between Mn³⁺ and Mn⁴⁺ 2p peaks in (a) and (b). Thereinto, Mn³⁺/Mn⁴⁺ value for Normal sample could be used to represent the composition in the bulk of core-shell structure for Quench sample

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  Figure 3  Enhanced cycling stability by the rock-salt shell. (a) The charge/discharge profiles during the 1st cycle at 0.05 C (1 C = 250 mA·h·g^-1) for Normal and Quench samples. (b) The cycling performance of Normal and Quench samples after 200 cycles at 0.5 C. The charge/discharge curves of selected cycles for Normal (c) and Quench (e) samples. The corresponding dQ/dV curves for Normal (d) and Quench (f) samples

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  Figure 4  Mechanism illustration for the protective role of the unique rock-salt shell. (a) HRTEM images as well as the corresponding selected-region FFT maps (insets) for Quench sample after 5 cycles. (b) Schematic illustration for the protective role of bifunctional rock-salt shell in Li-rich layered oxide cathodes

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