Suppressing Voltage Decay of Lithium-Rich Cathode Material by

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Suppressing Voltage Decay of Lithium-Rich Cathode Material by Surface Enrichment with Atomic Ruthenium Huaifang Shang, Fanghua Ning, Biao Li, Yuxuan Zuo, Shigang Lu, and Dingguo Xia ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06271 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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ACS Applied Materials & Interfaces

Suppressing Voltage Decay of Lithium-Rich Cathode Material by Surface Enrichment with Atomic Ruthenium Huaifang Shanga† , Fanghua Ningb† , Biao Lib , Yuxuan Zuob, Shigang Lu*a, Dingguo Xia*b a

China Automotive Battery Research Institute Co., Ltd., Beijing 101407, P. R. China

b

Key Lab of Theory and Technology for Advanced Batteries Materials, College of Engineering, Peking University, Beijing 100871, P. R. China

†

These authors contributed equally to this work.

KEYWORDS: High energy density lithium-ion batteries, Lithium-rich layered oxides, Ruthenium segregation, Suppressing voltage decay, DFT calculations, ABSTRACT: Lithium-rich layered oxides are promising cathode materials for high energy density lithium-ion batteries. However, the development of cathode materials based on these layered oxides has been limited by voltage fading, poor rate performance, and the low tap density of these materials. In this work, we prepared a material consisting of micrometer-scale spherical lithium-rich layered oxides particles with a three-dimensional conductivity network design and modified the surface of the primary particles with ruthenium. The as-obtained product with a maximum tap density of 2.1 g⋅cm-3 shows superior high reversible capacity with 280 mAh⋅g−1 at 0.1 C, capacity retention of 98.1 % after 100 cycles, and outstanding rate capability. More importantly, enrichment of the primary particle surface with ruthenium can effectively suppress voltage decay. This cathode is feasible to construct high-energy and high-power lithium ion batteries. This novel design may furthermore open the door to new material engineering applications for high-performance cathode materials. 1. INTRODUCTION Lithium-rich Mn-based layered oxides (LLOs) have garnered considerable interest as promising cathode materials for high-energy density lithium-ion batteries owing to their high specific-capacity exceeding 250 mAh⋅g−1, low cost, and low toxicity.1-4 The common feature accounting for the abnormal high capacities of LLOs is supposed to be related with the activation of the LiMn6 ordering component, which involves the two subsequent redox processes of the transition metal cation and oxide anion.5-10 During the activation process, the loss of Li2O from the surface could result in the formation of lithium- and oxygen- vacancies. The existence of these vacancies could induce the migration of transition metal manganese ions and result in the transition of 1 / 15

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the layered phase to the spinel phase, resulting in voltage fade, which is one of the major shortcomings hindering the practical application of LLOs. 11-17 Previously, LLO materials were widely prepared by the co-precipitation method,18-22 which can easily be scaled up for industrial production. However, the size of the as-obtained particles is usually about 150 nanometers in most of the LLO materials and the tap density of products is very low, resulting in a low energy density of actual full batteries. The large specific area caused by the small particles also aggravates the side reactions, causing electrolyte exhaustion and capacity decline. Increasing the size of particles by the assembly of primary particles can increase the tap density but leads to unsatisfactory rate performance and voltage retention. Therefore, the investigation of high tap density LLOs with high rate performance and stable operating voltage is highly desirable for the preparation of batteries suitable for practical usage.23-24 Here, we report the series of materials Li1.2Ni0.13Co0.13Mn0.54-xRuxO2 (Rux-LLOs, x = 0, 0.01, 0.02, and 0.03), surface-enriched by Ru, and prepared by a facile co-precipitation and subsequent heat-treatment method. The enrichment of ruthenium in the surface of primary particles leads to the construction of a uniform three-dimensional conductivity network, which serves to protect the LLO cathodes from corrosion induced by organic electrolytes and effectively stabilizes the crystal structure of the LLOs. DFT calculations were employed to further explain the origin of the Ru surface enrichment and the suppression of voltage fading. The resulting materials not only show significantly stable operating voltage but also exhibit an enhanced rate capacity. 2. EXPERIMENTAL SECTION 2.1. Synthesis of Rux-LLOs. Stoichiometric amounts of C4H6NiO4·4H2O, C4H6CoO4·4H2O, C4H6MnO4·4H2O, and RuCl3 (in the molar ratio of Mn : Ru : Co : Ni = 0.54-x : x : 0.13 : 0.13, x = 0, 0.01, 0.02,0.03) were dissolved in distilled water in a continuously stirred reactor. Aqueous Na2CO3 and a specific amount of NH3·H2O were simultaneously pumped into the reactor. The pH of the reaction solution was carefully maintained at 8.0. The precipitate was collected by filtration, washed with deionized water, and then dried at 80 °C for 5 hours under vacuum. Next, stoichiometric amounts of the obtained carbonate precursor and Li2CO3 and boric acid were mixed thoroughly in an agate mortar. The boron, which is known to increase the cycling stability of the bulk phase, was added to enable the surface investigation to be carried out in this work, as reported in our previous work.25 The mixed powders were calcinated at 900 °C for 12 h under air to finally obtain the Li1.2Ni0.13Co0.13Mn0.54-xRuxO2 (Rux-LLOs, x=0, 0.01, 0.02, and 0.03) cathode materials. 2.2. General characterization. The X-ray diffraction patterns were collected using a Bruker D8-Advance diffractometer (Bruker, Germany) equipped with a Cu Kα radiation source (λ = 1.5406 Å) and operated at 40 kV and 40 mA. A scanning electron microscope (HITACHI, S-4800), transmission electron microscope 2 / 15


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(TECNAI, F20), high-resolution transmission electron microscope (JEM-2100F), and scanning transmission electron microscope (FEI Titan G2 60-300) were employed to observe the morphology and identify elemental distribution. Powder X-ray photoelectron spectroscopy (AXIS, ULTRA) was performed to analyze the chemical valence states of elemental Ru and data was calibrated using the adventitious C1s peak with a fixed value of 284.8 eV. 2.3. Electrochemical test. Electrochemical tests were conducted with 2032-type coin cells. The counter electrode was lithium metal. The cathode was fabricated by casting a slurry of 80 wt% active oxide, 10wt % PVDF binder in N-methyl-pyrrolidinone solvent and 10 wt% acetylene black onto an Al foil substrate. Cells were cycled galvanostatically at room temperature using a battery station (NEWARE CT-4800) between 4.8 V and 2.0 V (1 C = 200 mAh⋅g-1). 2.4. DFT+U calculations. All the DFT calculations in the present work were performed by using the Vienna Ab-initio Simulation Package (VASP).26 A plane-wave basis set with the projector augmented wave (PAW) method27 and the spin-polarized generalized gradient approximation (GGA) with the 28 Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional were employed. We used the GGA+U method29-30 to account for the strong onsite Coulomb repulsion of the Mn-3d and Ru-4d electrons with Hubbard U values of 4.5 and 2.9 eV respectively, according to previous reports.31-32 The Monkhorst–Pack scheme33 with 2 × 2 × 1 k-point mesh was used for integration in the irreducible Brillouin zone. A cutoff energy of 550 eV was employed. The total ground state energy converged within 10-5 eV. The final force on each atom is less than 0.05 eV/Å. 3. RESULTS AND DISCUSSION 3.1. Morphology and Surface Structure. The morphology and surface structure of the as-obtained material was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) equipped with EDX. Figure S1 shows the XRD patterns of precursors with different amounts of Ru (Mn : Ru : Co : Ni = 0.54-x : x : 0.13 : 0.13, x = 0, 0.01, 0.02, and 0.03). All the peaks can be assigned to MnCO3 of the R-3c space group without other impurity phases. Figure 1a shows the XRD patterns of the products, which were obtained using a prolonged scan of 10 hours. All the XRD peaks can be assigned to the α-NaFeO2 structure with a C2/m space group. The weak intensity peaks in the 2θ range of 21–35° can be indexed well to the super lattice reﬂection resulting from the LiMn6 cation ordering in the transition metal layers. No impurity peaks related with the ruthenium compounds are observed in the XRD patterns. Figure S2a shows the SEM images of the precursors with different amounts of Ru. An increase in the amount of Ru causes the spherical particles to change from compact to loose and the size of the particles decreases. After heat treatment, the Li1.2Ni0.13Co0.13Mn0.54-xRuxO2 (Rux-LLOs, x = 0, 0.01, 0.02, and 0.03) products become more compact as the amount of Ru increases, and demonstrate morphologies 3 / 15



of well-crystallized spherical particles of approximately 8 µm in size, as shown in Figure 1b and Figure S2b. The spherical particles show a high tap density of 2.1 g cm-3 and moderate surface area of 5.6 m2 g-1.

Figure 1. a) XRD patterns of Rux-LLOs, x = 0, 0.01, 0.02, and 0.03; b) SEM image of Li1.2Ni0.13Co0.13Mn0.52Ru0.02O2.

The state in which elemental Ru exists was determined by obtaining the cross-sectional TEM image of spherical particles of Li1.2Ni0.13Co0.13Mn0.52Ru0.02O2, as shown in Figure 2a. It can be seen that secondary spherical particles are formed by aggregation of many primary particles with diameters in the range of 50 - 100 nm and many mesopores are distributed among the primary particles. These pores could store the electrolyte and are conducive to promoting the diffusion of Li ions. Figure 2b demonstrates the energy dispersive spectroscopy (EDS) line scan results along two primary particles (see Figure S3). It can be seen that the elements Ni, Co, and Mn are uniformly distributed among the bulk particles. This result is consistent with the STEM mapping images of the elements in Figure S4. However, it can be seen that the cps of Ru is enhanced at regular intervals in Figure 2b, corresponding to the edges of primary particles, indicating an obvious enrichment of the surface with elemental Ru. Considering the XRD results, this enrichment means that the surface lattice of the Li-rich material is doped with additional ruthenium atoms. Previous work confirmed that Li2RuO3 exhibits stable operating voltage due to the large atomic size of Ru. Moreover, the uniform distribution of transition metal elements can decrease the lattice stress.34 Therefore, the uniform distribution of the elements Ni, Co, and Mn and the enrichment of the surface by elemental Ru can be expected to improve the capacity stability of a cathodic material during cycling.

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Figure 2. a) TEM cross-sectional image of Li1.2Ni0.13Co0.13Mn0.52Ru0.02O2 material; b) Distribution of the elements Mn, Co, Ni, and Ru along two particles.

ICP is an advanced technique to probe the specific atomic concentration semi-quantitatively. Here we provide the results of the ICP atomic concentration of the Ni0.13Co0.13Mn0.54-xRux(CO3)0.8+x (x = 0.02) and Li1.2Ni0.13Co0.13Mn0.54-XRuxO2 (x = 0.02) samples, of which the approximate composition is equal to the theoretical value (see Table S1). XPS was used to analyze the surface compositions of the as-prepared materials, as shown in Figure S5 and Table S2. For the Ru0.02-LLOs sample, the oxidation state of Ni, Co, Mn, and Ru can be indexed as Ni2+, Co3+, Mn4+ and Ru4+, respectively, and the surface composition of the sample is determined to be Li1.2Ni0.13Co0.13Mn0.50Ru0.04O2 whereas the precursor is determined to be Ni0.13Co0.13Mn0.52Ru0.02(CO3)0.82 combined with the XRD results. The Ni: Co: Mn ratio is consistent with the amount of precursors that was experimentally added, but the surface Ru concentration is higher than the average atomic ratio of Ru in the bulk after heat treatment. This further confirms that Ru enriches on the surface. Moreover, the formation of Li2CO3 on the surface when Li-rich material is exposed to air has been reported.35 The presence of Li2CO3 on the surface causes the electrochemical performance of Li-rich cathode materials to deteriorate. A quantitative comparison of the O 1s peaks demonstrates that the amount of Li2CO3 is reduced by about 60 % on the surface of the as-prepared Ru0.02 - LLOs material compared to the Li1.2Ni0.13Co0.13Mn0.54O2 sample. This means that the surface of the sample enriched by Ru has higher stability in air than the sample that was not doped with ruthenium. Therefore, based on the above discussion, the Ru enrichment can be expected to effectively protect cathode materials from electrolytic corrosion and greatly promote the electrochemical performance. 3.2. Density Functional Theory Calculations. Density functional theory (DFT) calculations with respect to the Ru-enriched surface of the sample were conducted in an attempt to clarify the mechanism underlying the enhanced performance. For succinctness, Li2MnO3 was selected as the calculation model and the (010) planes were selected as the predominant surfaces of the particles based on the diffusion 5 / 15



channel of the Li ions. The Li2MnO3 (010) surface was simulated by a symmetric periodic slab model, with consecutive slabs separated by a 10 Å vacuum layer. We chose a supercell slab model of Li2MnO3 with a slab thickness of ~ 17 Å, containing 72 Li, 104 O, and 32 Mn atoms. The Ru-doped slab systems were modeled by substituting one out of 32 Mn ions by a Ru ion. The relaxed geometrical structure of the (010) slab of Li2MnO3 is shown in Figure S6. The total energies of Ru-doped Li2MnO3 with different doping sites are listed in Table S3. One can see that the energy of the Ru-surface-doped case is ~ 1 eV lower than that of the other three cases, which demonstrates that the Ru ion prefers to remain on the surface of the Li2MnO3 compared with its location in the bulk material, that is, surface doping or surface enrichment is achieved. This result is consistent with that of the above-mentioned XPS analysis. Moreover, compared with the gap of 1.9 eV for Li2MnO3, that of Li2MnO3 doped with Ru decreases to 0.6 eV, as shown from the total density of states in Figure S7. Liu′s work also revealed the lowering of the activation barrier resulting from the increase in the Li interslab distance upon Ru doping.36-37 Both of these results suggest that the surface electrical conductivity would be enhanced owing to enrichment of the surfaces of primary particles by Ru. In other words, a three-dimensional conductivity network forms at a depth interior to the secondary particles, as shown in Figure S8. The optimized atomic structure of a fully delithiated Li2MnO3 (010) slab is shown in Figure 3a. The surface oxygen ion continues approaching its nearest oxygen ion and simultaneously moves away from the surface during optimization progress, as is shown in the enlarged view of the surface structure variation in Figure 3b. Finally, the surface oxygen ions (2.48 Å) form oxygen molecules (1.23 Å) and are released into vacuum, changing the sixfold coordinated Mn ions into fourfold coordinated Mn ions (i.e., transforming the MnO6 octahedron into an MnO4 tetrahedron). Thus, we confirm that the oxygen ions on the (010) surface are extremely unstable and tend to release oxygen molecules upon deep delithiation. Oxygen loss on the surface facilitates cation migration, 9, 38 which is related to voltage fade.

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Figure 3. a) Optimized atomic structure of (010) slab of fully delithiated Li2MnO3; b) surface structure variation during optimization progress. The marked values indicate the bond length/distance in angstrom.

The Ru-doped fully delithiated Li2MnO3 (010) slab is shown in Figure 4a, where Ru remains on the surface. Interestingly, something entirely different happens near the Ru-doping sites, where the surface oxygen ions continue to bond with the Ru ion, and the O–O distance on the surface increases from 2.48 Å in the original MnO6 octahedron to 2.78 Å after relaxation (Figure 4b), meaning that oxygen molecules are not formed. In addition, the comparison of the charge density of atomic oxygen further confirms the ability of Ru to suppress the oxygen participation in charge compensation, as shown in Figure S9. Hence, the DFT calculation demonstrates that Ru-doping can efficiently stabilize the oxygen ions and prevent the release of O2 from the Li2MnO3 (010) surface in the deep delithiated state.

Figure 4. a) Optimized atomic structure of Ru-surface-doped (010) slab of fully delithiated Li2MnO3; b) surface structure variation during the optimization process. The marked value indicates the bond length/distance in angstrom.

Then, we considered the extreme case, in which all of the Mn ions on the surface are substituted by Ru ions. As shown in Figure 5, one can clearly see that all the surface oxygen ions are stably bonded with Ru ions without O2 release. This further confirms that surface enrichment with Ru substantially stabilizes the oxygen ions on the (010) surface of Li2MnO3 during the delithiation processes.

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Figure 5. a) Optimized atomic structure of highly doped Ru surface of the (010) slab of fully delithiated Li2MnO3; b) surface structure variation during the optimization process. The marked value indicates the bond length/distance in angstrom. 3.3. Electrochemical Performance. The electrochemical performance of the as-obtained samples with different amounts of Ru is shown in Figure 6. These results show that the ruthenium-doped samples show an obvious electrochemical performance enhancement under a current density of 1 C in the voltage range of 2-4.8 V (Figure 6a). Even at the low current density of 0.1 C, which means a greater possibility of side reactions at high voltage, the Li1.2Ni0.13Co0.13Mn0.52Ru0.02O2 sample continues to show excellent cycling stability with a capacity retention of 95 %, i.e., 268 mAh⋅g−1 at the first cycle and approximately 255 mAh⋅g−1 at the 100th cycle, whereas the sample without Ru doping shows fast capacity degradation with only 41 % capacity retention, delivering 268 mAh⋅g−1 at the first cycle and only 109 mAh⋅g−1 at the 100th cycle, as shown in Figure S10. This result could be attributed to an increase in the stability of surface lattice oxygen owing to the enrichment of the Ru surface. The decreased capacity of the Li1.2Ni0.13Co0.13Mn0.51Ru0.03O2 sample may be related to the low capacity of Ru oxides.25 Although the materials with different amounts of Ru dopants show little difference in their discharge capacities at a current rate of 0.1 C, the discharge capacities at high current rates differ. The ruthenium doped sample (x = 0.02) shows the highest reversible capacity of 150 mAh⋅g-1 at the rate of 5 C, which is ∼50 % higher than that delivered by the sample without Ru. After the high-rate measurement, the specific capacity of the ruthenium-doped sample cycled under a current density of 0.1 C could be recovered to its initial value, as shown in Figure 6b, implying its good reversibility. This enhancement of the rate performance could be attributed to the three-dimensional conductivity network to the interior of secondary particles. More importantly, voltage fading was significantly suppressed after Ru doping, demonstrating a voltage decay of 0.25 V for x = 0.01, 0.15 V for x = 0.02, and even the complete absence of voltage decay for x = 0.03, whereas the sample without Ru exhibits rapid voltage fading from 3.20 V (10th) to 2.72 V (100th) after 100 cycles at 1 C, as shown in Figure 6c and Figure S11. The differential capacity curve also shows the Li1.2Ni0.13Co0.13Mn0.51Ru0.03O2 sample can stabilize the discharge potential (see Figure 6d). As mentioned in the introduction, the loss of Li2O from the surface would result in the formation of lithium and oxygen vacancies. The existence of vacancies would induce the migration of transition metal manganese ions and result in the transition of the layered to the spinel phase, resulting in voltage fading. Therefore, the resultant electrochemical behavior, stable operating voltage, excellent cycle life, and rate capability, validate that surface enrichment by Ru can stabilize the electrode structure during cycling.

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Figure 6. a) Cycling performance of Rux-LLOs samples under 1C; b) Rate capability of Rux-LLOs samples; c) Normalized capacity discharge profiles of the Li1.2Ni0.13Co0.13Mn0.51Ru0.03O2 sample from 10th to 100th cycles. d) Differential capacity dQ / dV versus V plots for the Li1.2Ni0.13Co0.13Mn0.51Ru0.03O2 sample from the 10th to the 100th cycle.

The influence of ruthenium enrichment of the surface on the kinetics of Li+ insertion/extraction into Li1.2Ni0.13Co0.13Mn0.54-xRuxO2 (Rux-LLOs, x = 0 and 0.03) was investigated by recording the electrochemical impedance spectra of the two samples at a charge state of 4.5V and 4.8V, respectively, at the 50th cycle with a voltage amplitude of 5 mV, in the frequency range 100 kHz–0.01 Hz. Figure 7 shows the Nyquist plots of the as-prepared electrodes, and all the Nyquist plots present the same characteristics, including a small semicircle in the high-frequency range, a large semicircle in the high- to medium-frequency range, and a quasi-straight line in the low-frequency range.39 The small semicircle in the high-frequency range corresponds to the impedance of Li+ migration across the SEI film. The large semicircle in the high- to medium-frequency range is related to the impedance of charge transfer. The quasi-straight line in the low-frequency range is connected with the impedance of Li-ion migration in the cathode. Figure 7a and b show that the electrode doped with Ru (x=0.03) exhibits a lower value than the undoped material (x=0); therefore, the samples subjected to surface modification by Ru exhibit superior performance in terms of their discharge voltage retention. The same phenomenon appears at 4.8 V (Figure 7c, d), indicating that surface enrichment by Ru stabilizes the oxygen ions on the surface.

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During the charge and discharge processes, the side reaction between the cathode and electrolyte deposits by-product to form an SEI film, thereby decreasing the values of the sample of which the surface is modified by Ru. This implies that side reactions associated with the electrode subjected to Ru doping are weak.

Figure 7. a) and b) Electrochemical impedance spectroscopy (EIS) of Li1.2Ni0.13Co0.13Mn0.54-xRuxO2 (Rux-LLOs, x = 0 and 0.03) performed at a charge state of 4.5 V at the 50th cycle.; c) and d) EIS of Li1.2Ni0.13Co0.13Mn0.54-xRuxO2 (Rux-LLOs, x = 0 and 0.03) performed at a charge state of 4.8 V at the 50th cycle.

4. CONCLUSIONS In summary, a material consisting of micrometer-scale spherical lithium-rich layered oxides with a three-dimensional conductivity network was synthesized by simple co-precipitation combined with a solid-state reaction. Because the surface of the electronic structure was subjected to Ru enrichment, the materials showed an enhanced rate performance and cycling stability, maintaining 95 % capacity retention between 3.0 and 4.8 V after 100 cycles and a reversible capacity of 150 mAh⋅g-1 at the 5 C rate. More importantly, 99 % average discharge voltage retention can be obtained for the Li1.2Ni0.13Co0.13Mn0.51Ru0.03O2 sample after 100 cycles. We attribute this enhanced electrochemical performance to the stable surface structure, which may alleviate the formation of oxygen vacancies during the charge-discharge process. This novel strategy to suppress voltage fading can be expected to lead to the development of other approaches to cathode preparation.

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ASSOCIATED CONTENT Supporting Information. This Supporting Information is available free of charge on the ACS Publication website.

Supporting Information available: Figure S1 X-ray diffraction (XRD) patterns of precursors with different Ru amount (Mn : Ru : Co : Ni = 0.54-x : x : 0.13 : 0.13, x = 0, 0.01, 0.02, and 0.03). Figure S2 a) SEM images of the precursors with different Ru doping amount (Mn : Ru : Co : Ni = 0.54-x : x : 0.13 : 0.13, x = 0, 0.01, 0.02, and 0.03); b) SEM images of Rux-LLOs (x = 0, 0.01,0.02, and 0.03) products. Figure S3 TEM image of two Li1.2Ni0.13Co0.13Mn0.52Ru0.02O2 particles. Figure S4 a) TEM cross sectional image of Li1.2Ni0.13Co0.13Mn0.52Ru0.02O2 material, which shows Ni, Co, and Mn elements are uniformly distributed in the bulk particles. Figure S5 XPS patterns of the surface compositions of the as-prepared Li1.2Ni0.13Co0.13Mn0.52Ru0.02O2 materials. Figure S6 The optimized geometry structure of the (010) slab of Li2MnO3. Figure S7 Total density of states in Li2MnO3 and Li2MnO3 doped Ru. Figure S8 Formation process of three-dimensional conductivity network in the interior of secondary particle. Figure S9 The average charge of the oxygen atoms in the corresponding oxygen layer from Bader charge analysis in pure, B-surface-doped, and B Ru-surface-doped (010) slab of fully delithiated Li2MnO3, respectively. Figure S10 a) The cycling performance of Rux-LLOs samples under 0.1 C; b) The discharge capacity retention of Rux-LLOs samples. Figure S11 a) Voltage decay of 0.4 V for Li1.2Ni0.13Co0.13Mn0.54O2 sample;. b) Voltage decay of 0.25 V for Li1.2Ni0.13Co0.13Mn0.53Ru0.01O2 sample; c) Voltage decay of 0.15 V for Li1.2Ni0.13Co0.13Mn0.52Ru0.02O2 sample. Table S1 XPS analysis the surface compositions of the as-prepared materials. Table S2. The total energies of the Ru-doped (010) slab of Li2MnO3 with corresponding doping sites. AUTHOR INFORMATION Corresponding Author

* E-mail: [email protected] * E-mail: [email protected] ORCID Huaifang Shang: 0000-0001-6543-8079 Fanghua Ning: 0000-0001-6590-958X Biao Li: 0000-0002-3156-4381 Dingguo Xia: 0000-0003-2191-236X 11 / 15



Author Contributions

H. S. and F. N. contributed equally to this work. Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENT

This work was financially supported by the new energy project for electric vehicle of national key research and development program (2016YFB0100200), National Natural Science Foundation of China (51671004, U1764255), and National Postdoctoral Program for innovative Talents (8201400801). The computations were supported by High-performance Computing Platform of Peking University. REFERENCES (1) Park, S. H.; Kang, S. H.; Johnson, C. S.; Amine, K.; Thackeray, M. M. Lithium-Manganese-Nickel-Oxide Electrodes With Integrated Layered-Spinel Structures for Lithium Batteries. Electrochem. Commun. 2007, 9, 262-268. (2) Kim, S.; Cho, W.; Zhang, X.; Oshima, Y.; Choi, J. A Stable Lithium-Rich Surface Structure for Lithium-Rich Layered Cathode Materials. Nat. commun. 2016, 7, 1-8. (3) Luo, K. M.; Roberts, R.; Hao, R.; Guerrini, N.; Pickup, D. M.; Liu, Y. S.; Edstrom, K.; Guo, J. A.; Chadwick, V.; Duda, L. C.; Bruce, P. G. Charge-Compensation in 3d-Transition-Metal-Oxide Intercalation Cathodes Through the Generation of Localized Electron holes on Oxygen. Nat. Chem. 2016, 8, 684-691. (4) Nayak, P. K.; Erickson, E. M.; Schipper, F.; Aurbach, D. Review on Challenges and Recent Advances in the Electrochemical Performance of High Capacity Li- and Mn-Rich Cathode Materials for Li-Ion Batteries. Adv. Energy Mater. 2017, 1702397. (5) Armstrong, A. R.; Holzapfel, M. H.; Nova´k, P.; Johnson, C. S.; Kang, S. H.; Thackeray, M. M.; Bruce, P. G. Demonstrating Oxygen Loss and Associated Structural Reorganization in the Lithium Battery Cathode Li[Ni0.2Li0.2Mn0.6]O2. J. Am. Chem. Soc. 2006, 128, 8694-8699. (6) Gu, M.; Belharouak, L.; Amine, K.; Thevuthasan, S.; Wang, C. M. Formation of the Spinel Phase in the Layered Composite Cathode Used in Li-Ion Batteries. ACS Nano, 2013, 7, 760-767. (7) Koga, H.; Croguennec, L.; Suard, E.; Weill, F.; Delmas, C. Reversible Oxygen Participation to the Redox Processes Revealed for Li1.20Mn0.54Co0.13Ni0.13O2. J. Electrochem. Soc. 2013, 160, A786 - A792. (8) Li, B.; Xia, D. G. Anionic Redox in Rechargeable Lithium Battteries. Adv. Mater. 2017, 1701054. (9) Yan, H. J.; Li, B.; Jiang, N.; Xia, D. G. First-Principles Study: the Structural Stability and Sulfur Anion Redox of Li1-xNiO2-ySy. Acta Phys. -Chim. Sin. 2017, 33, 1781-1788.

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Suppressing Voltage Decay of Lithium-Rich Cathode Material by

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