Glucose-assisted combustion synthesis of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ cathode materials with superior electrochemical performance for lithium-ion batteries

Abstract

Lithium-rich layered Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ cathode materials have been successfully fabricated by a glucose-assisted combustion method combined with a calcination treatment. The effect of the amount of glucose fuel on the properties of the prepared materials is investigated by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and electrochemical measurements. The results show that the nano-sized cathode material obtained at a fuel ratio of φ = 1 exhibits uniform fine well-crystallized particles with the largest specific surface area, leading to excellent cyclic capability and rate performance. It delivers the highest initial discharge capacity of 280.5 mA h g⁻¹ with a capacity retention of 84% after 50 cycles at 0.1C (25 mA g⁻¹). Besides, after cycling at an increasing rate from 0.2C to 3C, the electrode retained 90.3% (230.2 mA h g⁻¹) of the initial discharge capacity when the rate was recovered back to 0.2C.

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1. Introduction

Lithium ion batteries (LIBs) with high energy density have gained enormous attention as a promising energy provider for electric vehicles (EVs) and hybrid electric vehicles (HEVs). To achieve extensive application, various cathode materials such as layered LiCoO₂, spinel LiMn₂O₄, and olivine LiFePO₄ have been explored to be utilized in Li-ion batteries. However, these conventional cathodes cannot fully satisfy the energy and power density demands of the electric vehicle applications. Therefore, it is urgent to search for the next generation of high performance LIB cathode materials.^1–3

Li-rich layered cathode materials, mostly described as a solid solution between layered Li₂MnO₃ and LiMO₂ (M = Ni, Co, Mn, etc.), have attracted much attention for their high specific capacity, increased energy density and relatively lower cost compared with layered LiCoO₂.^4–6 For example, 0.5 Li₂MnO₃·0.5 LiMn_1/3Ni_1/3Co_1/3O₂, which is equivalent to Li_1.2Ni_0.13Co_0.13Mn_0.54O₂, can deliver a large capacity exceeding 250 mA h g⁻¹ with an average operating voltage of about 3.5 V (vs. Li/Li⁺).^7–9 However, there are still several intrinsic problems such as (1) oxygen release during first charge, (2) high first cycle irreversible capacity loss (IRC), (3) substantial voltage decay and (4) poor rate capability mainly induced by the insulating Li₂MnO₃ component, which have hindered the real application of Li-rich cathode materials.^10–12

It is well understood that the synthetic route is a main factor for the electrochemical performance of the cathode materials.^13–16 Low temperature combustion synthesis (LCS), based on a highly exothermic oxidation–reduction reaction between metal salts and organic fuels,^16,17 has been successfully used to synthesize various cathode materials for LIBs, such as LiMn₂O₄,¹⁸ LiMn_1.5Ni_0.5O₄,^17,19,20 LiNi_1/3Co_1/3Mn_1/3O₂,²¹ LiVO₃,²² etc. The obtained powder shows nanometric particle size and improved electrochemical performance. Recently, several reports have been concerned with this method to prepare Li-rich cathode materials. Different carbonaceous compounds, such as alcohol,²³ sucrose,²⁴ gelatin²⁵ and mannitol,²⁶ have been used as fuels. However, the effect of organic fuel amount on the morphology, crystallinity and electrochemical property of the product has not been investigated comprehensively.

In this paper, the Li-rich cathode materials Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ were successfully prepared via a glucose-assisted combustion synthesis followed by calcination. Herein, nitrate serves as oxidizer, while glucose plays the role of fuel and complexing agent. The structure, morphology, particle size distribution and electrochemical properties of the Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ compounds synthesized with different fuel-to-oxidizer ratios were thoroughly investigated.

2. Experimental

2.1 Synthesis procedure

Fig. 1 illustrates the synthetic process of the Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ cathode materials by the glucose-assisted combustion method. Simultaneously, the proposed reaction equation can be presented as:

1.2LiNO₃ + 0.13Ni(NO₃)₂ + 0.54Mn(NO₃)₂ + 0.13Co(NO₃)₂ + nC₆H₁₂O₆ → Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ + gases evolved

Fig. 1 Schematic illustration of glucose-assisted combustion method for Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ materials.

According to this equation, the combustion process should be a redox type of reaction, in which nitrates act as oxidant and glucose acts as fuel. Furthermore, the atoms H, C, Li, Ni, Co and Mn are considered with valencies +1(H), +4(C), +1(Li), +2(Ni), +2(Co) and +2(Mn), respectively. The oxygen atom is considered as an oxidizing agent with a valency of −2 and nitrogen atom is taken as neutral with a valency of 0. Therefore, the oxidizing valency of LiNO₃ is −5, that of Ni(NO₃)₂, Mn(NO₃)₂ and Co(NO₃)₂ is −10, while the reducing valency of the fuel glucose is +24. And the fuel-to-oxidizer ratio (φ) is defined as the ratio between the total valencies of the fuels and nitrate oxidizers of the components.^17,20,27

During the synthesis, stoichiometric amount of LiNO₃, Ni(NO₃)₂·6H₂O, Mn(NO₃)₂ and Co(NO₃)₂·6H₂O according to the element ratio of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ were sufficiently dissolved in distilled water to form nitrate solution, and 5% excess LiNO₃ was added to compensate for possible loss during calcination. Then different amount of glucose was added with organic fuel to oxidizer ratios (φ) of 0.75, 1.0, 1.25 and 1.5. The mixtures were stirred and evaporated at 90 °C to obtain a swelled gel, and then ignited at 200 °C in air. The combustion reaction lasted for about 2–3 min to obtain brown precursors named as S75-P, S100-P, S125-P, S150-P, respectively. Finally, the brown ash was calcined orderly at 450 °C for 3 h, 900 °C for 12 h in ambient atmosphere and subsequently cooled naturally in the furnace to get the layered Li_1.2Ni_0.13Co_0.13Mn_0.54O₂. The synthesized oxides were named as S75, S100, S125 and S150 for short.

2.2 Characterization and electrochemical measurements

The morphologies and crystal structures of the obtained Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ materials were characterized by scanning electron microscopy (SEM, Hitachi S-4800, Japan) and X-ray diffraction (XRD, Rigaku D/Max-2400, Japan). The specific surface area of the prepared materials was measured by the multipoint Brunauer–Emmet–Teller (BET) method. The oxidation valence states of all the elements were determined through X-ray photoelectron spectroscopy (XPS) by Thermo VGESCALAB250 X-ray photoelectron spectrometer. Thermal gravimetric analysis of the precursors was performed with a thermal analyzer (TG, STA 449F3, NETZSCH, Germany).

The working electrode was prepared by a slurry coating procedure. The slurry was formed by mixing Li_1.2Ni_0.13Co_0.13Mn_0.54O₂, carbon black and polyvinylidene fluoride (PVDF) with a weight ratio of 8 : 1 : 1 and was coated on to an aluminum foil. After drying in an oven at 120 °C for 8 h, the sample was pressed and weighed. The loading of the active material was controlled between 3 and 4 mg cm⁻², with thickness of about 20 μm. The CR2025 coin-type cells were assembled inside a glove box full of high-purity argon, using 1 M LiPF₆ in 1 : 1 ethylene carbonate (EC)/dimethyl carbonate (DMC) as electrolyte. Cells were cycled galvanostatically at various C rates for the charge–discharge tests using LAND CT 2001A system between 2.0 and 4.8 V vs. Li/Li⁺ at room temperature. Here, 1C rate means a discharge of the cell in 1 h, corresponding to a current density of 250 mA g⁻¹. Cyclic voltammograms were measured on an electrochemical workstation (CHI 660D) between 2.0 and 4.8 V (vs. Li/Li⁺) with a scanning rate of 0.1 mV s⁻¹. Electrochemical impedance spectra (EIS) measurements were performed in the frequency range of 100 kHz to 0.01 Hz with ac amplitude of 10 mV.

3. Results and discussion

3.1 Material characterization

According to Jain's thermochemical calculation method, the parameter φ < 1, φ = 1, φ > 1 indicates a fuel-lean condition, stoichiometric redox mixtures and a fuel-rich condition respectively, which shows significant impact on crystallinity and purity of the product obtained from the combustion reaction.^16,27 As shown in Fig. 2a, the precursors obtained after gel-combustion at different fuel ratios contain a uniform mixture of lithium carbonate and amorphous oxides of nickel, manganese and cobalt. Meanwhile, the diffraction peaks in the XRD patterns for Li₂CO₃ become more obviously strong with the increased fuel ratios, indicating a side reaction between the lithium salt and the carbonaceous residues resulting from incomplete combustion in the fuel-rich condition.^26,28 TG analysis is performed under air atmosphere from room temperature to 1000 °C with a heating rate of 10 °C min⁻¹ to characterize the properties of the precursors, as shown in Fig. S1 (ESI†). The TG data shows that the mass loss processes consist of three steps. In the first region below 250 °C, the continuous mass loss may be caused by the evaporation of water. The second one in the range of 250–500 °C can be ascribed to the decomposition of the intermediate products such as carbonaceous residues.²⁷ The third one above 500 °C can be associated with the decomposition of Li₂CO₃ and the evaporation of lithium in the high temperature region.^26,29 Obviously, the weight loss of the precursor tends to increase as the amount of glucose fuel increases due to the incomplete combustion with more carbonaceous residues left.

Fig. 2 XRD patterns of the samples: (a) GCS-prepared precursors and (b) calcined products at different fuel ratios.

Fig. 2b shows the XRD patterns of the calcined products obtained after a heat treatment. All the samples at various fuel ratios display a well-defined hexagonal α-NaFeO₂ layered structure with a space group of R3̄m, while the weak superlattice peaks between 20° and 25° are characteristics of the monoclinic Li₂MnO₃ phase with a LiMn₆ cation ordering in the transition metal layers, corresponding to the space group of C2/m.^30,31 Notably, no obvious peak characteristic of Li₂CO₃ could be found, which means an absolute reaction with transition metal oxides during the calcination. Meanwhile, all diffraction peaks are narrow, indicating a high crystallinity structure. The determined structure parameters of the samples prepared with different fuel ratios are further examined. The lattice parameter ratios of c/a can be taken as an indicator of the hexagonal ordering.^23,32 As given in Table 1, all samples show a high c/a ratio (larger than 4.9), which implies an explicit layered structure. Furthermore, the peak intensity ratio of I₍₀₀₃₎/I₍₁₀₄₎ is widely used to indicate the cation mixing in the layered LiMO₂ (M = Ni, Co, Mn) structure. And it is suggested that the cation mixing is negligible when the value of I₍₀₀₃₎/I₍₁₀₄₎ is larger than 1.2.^33,34 It can be found that the c/a and I₍₀₀₃₎/I₍₁₀₄₎ ratios for samples S100 and S125 are slightly larger than those for sample S75 and S150, indicating a higher crystal orientation and a lower cation mixing.

Table 1 Lattice parameters, I₍₀₀₃₎/I₍₁₀₄₎ intensity ratios, mean particle sizes and BET specific surface areas for all samples

	a (Å)	c (Å)	c/a	I(003)/I(104)	Mean size (nm)	BET (m^2 g^−1)
S75	2.8576	14.2199	4.9762	1.4248	596	1.4
S100	2.8489	14.2179	4.9907	1.7669	414	5.4
S125	2.8519	14.2240	4.9876	1.7690	518	3.2
S150	2.8555	14.2156	4.9783	1.4079	614	1.1

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Fig. 3 displays the morphology and particle size distribution of samples obtained at different fuel ratios. It can be found that all samples are composed of nanoparticles with clear and smooth surface, indicating good crystallization. More detailed information shows evident distinction in particle size for the materials prepared with different fuel ratios. The sample S75 and S150 show relatively wide particle size range of 100–1500 nm and 100–1200 nm respectively. Meanwhile, sample S100 demonstrates the narrowest distribution ranging from 100–600 nm. As shown in Table 1, the mean particle size of the sample at different fuel ratios follows an order of S150 ≈ S75 > S125 > S100. BET analysis is carried out to investigate the further information of the sample surface areas. As shown in Table 1, sample S100 exhibits the highest BET surface area of 5.4 m² g⁻¹. And the specific surface areas are 1.4 m² g⁻¹, 3.2 m² g⁻¹ and 1.1 m² g⁻¹ for sample S75, S125, S150, respectively. Moreover, the BET surface area decreases with the fuel ratio increases from 1.0 to 1.5, which is ascribed to serious aggregation after an incomplete combustion in the fuel-rich condition with more carbonaceous residues left. Typically, nanosized particles with a high specific surface area can reduce the Li⁺ diffusion distance and offer better contacts between electrode and electrolyte.^23,35

Fig. 3 SEM image of the Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ samples at different fuel ratios: (a) S75; (b) S100; (c) S125; (d) S150. Insets are the particle size distributions of the corresponding samples.

Fig. 4 shows Co 2p, Ni 2p and Mn 2p XPS spectra of the Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ samples at different fuel ratios. The oxidation valence states of the transition metal ions are further determined. It can be detected that four signals for Co 2p and Ni 2p spectra, including two main peaks and two corresponding satellite peaks. The binding energies of Co 2p_3/2 for all samples are around 780 eV with satellite peaks around 790 eV. Moreover, binding energies for Co 2p_1/2 level are around 795.5 eV with satellite peaks around 805 eV, conforming that the main chemical valence state is Co³⁺ for Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ samples.^36–38 For Ni 2p spectra, the main line for Ni 2p_3/2 and Ni 2p_1/2 locates around 855 eV and 872.5 eV, and two corresponding satellite peaks are around 861.5 eV and 880 eV, respectively. All of this indicates that Ni ions for all samples are in an oxidation state of +2.^14,39 According to Mn 2p spectra shown in Fig. 4c, Mn⁴⁺ is the majority constituent for all samples at different fuel ratios with respect to the peaks around 642.5 eV (Mn 2p_3/2) and 654 eV (Mn 2p_1/2). However, there are two other peaks in Mn 2p spectra for sample S125 and S150: one peak around 653.3 eV and the other around 641.6 eV. These data suggest that a certain amount of Mn³⁺ exists on the surface of the sample prepared at fuel ratios of 1.25 and 1.5.^15,28,40 The incomplete oxidized manganese ions can be attributed to the decomposition of Li₂CO₃ and carbonaceous residues during the calcination process, in which the released gas CO₂ could reduce the local oxygen partial pressures and thereby resulted in the presence of Mn³⁺.^26,28 However, the trivalent manganese ions are also known to suffer from Jahn–Teller distortion and dissolution in the electrolyte, which may further destroy crystal structure and impact the cycle stability.⁴¹

Fig. 4 XPS spectra of (a) Co 2p, (b) Ni 2p and (c) Mn 2p for Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ samples at different fuel ratios.

3.2 Electrochemical analysis

Fig. 5 exhibits the charge–discharge profiles of all Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ electrodes at a current density of 25 mA g⁻¹ (C/10) between 2.0 and 4.8 V for the initial and second cycles. As shown in Fig. 5a, similar to normal Li-rich layered cathodes, there are two charge plateau regions in the initial cycle due to different lithium de-insertion processes. The initial slope region A (<4.5 V) is attributed to the lithium extraction from the layered component accompanying with the oxidation of Ni²⁺/Ni⁴⁺ and Co³⁺/Co⁴⁺.^42,43 The other plateau region B (around 4.5 V) is assigned to the simultaneous extraction of lithium and oxygen from the Li₂MnO₃ component which results in high irreversible capacity loss.⁴⁴ As shown in Fig. 5b, the plateau region around 4.5 V disappears in the second cycle due to the irreversible activation process. The initial discharge capacities at a current density of 25 mA g⁻¹ are 244.2, 280.5, 270.3 and 266.1 mA h g⁻¹ for samples S75, S100, S125, S150, respectively. Moreover, the initial coulomb efficiencies are calculated to be 74.4% for S75, 77.4% for S100, 75.6% for S125 and 77.5% for S150. The lost capacity is ascribed to the irreversible removal of Li₂O from the Li₂MnO₃ region and side reactions with the electrolyte at high operating voltage.^45,46 It is clear that S100 exhibits the highest charge and discharge capacities indicating a relatively easier activation of Li₂MnO₃ component than the other samples.

Fig. 5 (a) First and (b) second charge–discharge curves at 0.1C in a voltage range of 2.0 to 4.8 V for all samples.

Fig. 6 depicts the cycle performance of the Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ electrodes at different rates. As shown in Fig. 6a, after 50 charge–discharge cycles, samples S75, S100, S125, and S150 deliver high discharge capacities of 202, 235.6, 211.8 and 192.6 mA h g⁻¹, respectively, with the corresponding capacity decay rates of 17.3%, 16%, 21.6%, and 27.6%. However, the discharge curves of all Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ electrodes move to lower voltage plateaus with successive cycling, as shown in Fig. S2 (ESI†). The origin of such voltage decay could be related to the gradual change of layered structure to a spinel-like structure during cycling.^34,47 Due to its high crystallinity and structural integrity, the sample S100 suffers a relatively slower drop in the discharge midpoint voltage which decreases from 3.52 V to 3.23 V after 50 cycles. The cycle performance performed at 1C is shown in Fig. 6b. There is an activation process appearing for about 10 cycles before achieving the maximum of discharge capacity for all electrodes, followed by a gradual capacity fading upon cycling. The maximal discharge capacities for sample S75, S100, S125 and S150 are 186.6, 213.9, 205.8 and 195.7 mA h g⁻¹, respectively. Moreover, a discharge capacity of 182.6 mA h g⁻¹ is retained for sample S100 after 100 cycles. And the retained discharge capacities are 111.6, 152.5 and 125.8 mA h g⁻¹ for sample S75, S125 and S150, respectively. It is obvious that S100 exhibits the highest discharge capacity with excellent cyclic stability both at 0.1C and 1C due to its perfect layered structure. Comparatively, the serious capacity decay suffered by sample S125 and S150 are mainly attributed to the dissolution of the Mn³⁺ on the surface of the sample during charge–discharge process. This mechanism can be explained as: Mn³⁺ → Mn⁴⁺ + Mn²⁺. The dissolution will seriously destroy the oxide surface and further form unsatisfactory SEI films. Besides, the relatively lower chemical valence state of manganese ions aggravates the Jahn–Teller distortion during the cycling process which also affects the cycle stability.^23,32 This result is accordant with the previous discussion of XPS spectra.

Fig. 6 Cycle performance for all samples at (a) 0.1C and (b) 1C in the voltage range of 2.0 to 4.8 V.

Fig. 7 exhibits the rate capability of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ electrodes at current rates of 0.2–3C (1C = 250 mA g⁻¹) between 2.0 and 4.8 V at room temperature. It is clearly observed that the sample S100 synthesized with a fuel ratio of φ = 1 exhibits a superior rate capability than the others. And the cathode delivers average discharge capacities of 254.8, 229.3, 201.2, 172.9 and 150.22 mA h g⁻¹ at 0.2C, 0.5C, 1C, 2C and 3C, respectively. About 90.3% (230.2 mA h g⁻¹) of the initial discharge capacity can be recovered back at 0.2C. The excellent rate capability is probably attributed to the well-formed layered structure. Moreover, small particle size with a high specific surface area can offer shorter Li⁺ migration distance and better contacts between the active electrode and electrolyte, which can effectively enhance the kinetics of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ cathode and contribute to an improved rate capability.³⁵ Furthermore, S125 and S150 exhibit a relatively better rate capability. This result can be ascribed to the increased conductivity and Li⁺ ion diffusion coefficient for the presence of mixed valence state Mn⁴⁺ and Mn³⁺ ions.^26,28,33

Fig. 7 Rate performance for all samples from 0.2C to 3C in the voltage range of 2.0 to 4.8 V.

CV tests were performed to further investigate the redox reaction during the charge–discharge processes. As shown in Fig. 8, all the electrodes exhibit similar shape with two anodic peaks at about 4.0 V and 4.7 V in the initial positive scanning. The peak at low voltage is associated with the oxidation of Ni²⁺ to Ni⁴⁺ and Co³⁺ to Co⁴⁺, while the second peak is assigned to the extraction of Li⁺ and O²⁻ from Li₂MnO₃ component, corresponding to the activation of Li₂MnO₃ and the formation MnO₂.^48,49 The CV features of the second cycle show obvious differences as compared with the first one. The strong activation peak at ∼4.7 V disappears or reduces in the second cycle, which is corresponding to the large irreversible capacity of the initial cycle. And new broad peaks appear at ∼3.9 V, corresponding to the oxidation of Ni²⁺ and Co³⁺ ions.⁵⁰ Though the presence of Mn³⁺ for S125 and S150 has been observed through the XPS analysis, there is no obvious oxidation peak of Mn³⁺ in the first oxidation process, which may indicate that small amounts of Mn³⁺ only exist on the surface with no intrinsic change.¹⁵ Furthermore, it is obvious that the CV curves of the S100 are symmetrical and sharp, implying a superior kinetics behavior for the insertion/extraction of lithium during cycling process.

Fig. 8 CV curves of the Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ samples at different fuel ratios: (a) S75; (b) S100; (c) S125; (d) S150 for the initial two cycles (scan rate: 0.2 mV s⁻¹, voltage range: 2.0–4.8 V).

Electrochemical impedance spectroscopy (EIS) experiments were carried out to investigate the electrochemical properties of the Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ electrodes at the charge state of 4.5 V in the initial cycle. Nyquist plots and equivalent circuits of all samples are shown in Fig. 9. In general, the spectra are composed of two well-defined semicircles in the high and medium frequency ranges and a slope line in the low-frequency range. The small intercept in high frequency corresponds to the solution resistance (R_s). The first semicircle in the higher frequency range represents the impedance (R_f) of Li⁺ ion diffusion through the surface film. The second semicircle at lower frequency is assigned to the charge transfer resistance (R_ct) at the electrode/electrolyte interface. The slop line in low frequency range refers to the Warburg impedance (Z_w), which is related to Li⁺ ion diffusion in the bulk electrode material.^32,51 Furthermore, the calculated values of the R_s, R_f and R_ct for all electrodes are presented in Table 2.

Fig. 9 EIS spectra and equivalent circuits of Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ samples in the initial cycle at the charge state of 4.5 V.

Table 2 The impedance parameters of equipment circuits

Sample	Rs (Ω)	Rf (Ω)	Rct (Ω)
S75	0.53	184.4	627.4
S100	1.00	120.2	141.9
S125	1.32	143.8	204.2
S150	0.91	179.6	355.4

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As listed in Table 2, R_s values for all samples at the initial cycle are so small (∼1 Ω), which indicates that the effect of solution resistance is neglectable compared with that of R_f and R_ct. The value of R_f is ascribed to the solid electrolyte interface film (SEI film) resulted from the reaction between active material and electrolyte on the electrode surface. A larger value of R_f may be destructive to electrode and hinder the migration of Li⁺ during the deintercalation and intercalation process.¹² The parameter R_ct plays a dominant role in the charge transfer process, which is associated with the delithiated process from the Li₂MnO₃-like region in this work. It is obvious that sample S100 shows the lowest values of R_f (120.2 Ω) and R_ct (141.9 Ω), indicating an enhanced kinetics of the Li⁺ through the SEI film and a relatively easier activation of Li₂MnO₃ compound, which is corresponding to its largest first discharge capacity and superior rate capability.

4. Conclusion

In this study, lithium-rich layered Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ cathode materials have been successfully prepared by a glucose-assisted combustion synthesis followed by calcination treatment. The amount of glucose fuel has a considerable effect on morphology, specific surface area and electrochemical properties of the products. It is suggested that the sample obtained at a fuel ratio of φ = 1 exhibits excellent cyclic capability and rate performance with the smallest charge transfer resistance, which can be ascribed to two main aspects. On the one hand, its largest surface area could increase electrode/electrolyte contact area and provide more active sites for Li⁺ ions. On the other hand, the enhanced crystallinity with the absence of trivalent manganese could reduce the loss of manganese ions and improve the structure stability during cycling process. Therefore, the present results indicate that the glucose-assisted combustion synthesis is an effective approach for the synthesis of lithium-rich layered cathode materials.

Acknowledgements

This work was supported by the National Basic Research Program of China (973 Program) (2013CB934001), National Natural Science Foundation of China (51274017), International S&T Cooperation Program of China (2012DFR60530), Shanghai Academy of Space Technology (SAST201467).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra15639h

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