Understanding Voltage Decay in Lithium-Rich Manganese-Based

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Understanding Voltage Decay in Lithium-Rich Manganese-Based Layered Cathode Materials by Limiting Cutoff Voltage Jingsong Yang,† Lifen Xiao,§ Wei He,† Jiangwei Fan,† Zhongxue Chen,*,‡ Xinping Ai,† Hanxi Yang,† and Yuliang Cao*,† †

College of Chemistry and Molecular Science, Hubei Key Laboratory of Electrochemical Power Sources, Wuhan University, Wuhan 430072, China ‡ School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China § College of Chemistry, Central China Normal University, Wuhan 430079, China ABSTRACT: The effect of the cutoff voltages on the working voltage decay and cyclability of the lithium-rich manganese-based layered cathode (LRMO) was investigated by electrochemical measurements, electrochemical impedance spectroscopy, ex situ X-ray diffraction, transmission electron microscopy, and energy dispersive spectroscopy line scan technologies. It was found that both lower (2.0 V) and upper (4.8 V) cutoff voltages cause severe voltage decay with cycling due to formation of the spinel phase and migration of the transition metals inside the particles. Appropriate cutoff voltage between 2.8 and 4.4 V can effectively inhibit structural variation as the electrode demonstrates 92% capacity retention and indiscernible working voltage decay over 430 cycles. The results also show that phase transformation not only on high charge voltage but also on low discharge voltage should be addressed to obtain highly stable LRMO materials. KEYWORDS: lithium-rich manganese-based layered oxide, cathode material, voltage decay, limiting cutoff voltage, lithium-ion battery

1. INTRODUCTION The great success in the portable electronics market has promoted the expansion of lithium-ion batteries (LIBs) into electric vehicles (EVs).1−5 The recent rapid development of EVs, on the other hand, requires LIBs to further enhance their energy density, service life, and safety while reducing material costs. Recently, several anode materials with high specific capacities up to 1000 mAh g−1, such as Si and Sn alloys, have been developed. Cathodes with high specific capacities are thus required to match the newly developed anodes to meet the EV application. As a cathode material for LIBs, Li-rich manganese-based oxides (LRMO, Li1+x/(x+2)(M(2−2x)/(x+2)Mn2x/(x+2))O2, M = Mn, Ni, Co, etc.) are very attractive due to their high capacity (>280 mA h g−1), low cost, and safety.6−11 LRMO is composed of two highly integrated layered phases and can be formulated as xLi2MnO3·(1−x)LiMO2.12 At high charge voltages (>4.5 V),12,13 the Li2MnO3 component can be electrochemically activated to provide extra capacity for LRMO. The activation of Li2MnO3 in the high voltage region is accompanied by the extraction of oxygen ions from the crystal lattice through O2− to O214 or O2− to O−15−17 and rearrangement of cations through migration of the transition-metal ions to the Li layers in the crystal lattice. Upon discharge, Mn4+ ions are partially reduced to Mn3+ ions, which results in the layered to spinel phase transition.18−20 Although large reversible capacity is achieved, a series of problems, such as low initial Coulombic © 2016 American Chemical Society

efficiency, continuous working voltage, and capacity decay during cycling and poor rate capability have also arisen.14,21−26 To stabilize the surface and bulk phase structure of LRMOs, a variety of efforts have been made, including surface modification27−29 and ionic doping or substitution.30−34 The surface coating of inert oxides (Al2O3, TiO2, SnO2),35,36 phosphates (AlPO4, CoPO4),37 and fluorides (AlF3)38−40 can effectively suppress the electrolyte side reaction on the edge of material particles, protect the particle surface from corrosive attack of acidic species formed by electrolyte decomposition, and prevent the superficial phase transition from spreading into the interior of particles, thus greatly improving the cycling stability and reducing the initial irreversible capacity of LRMOs. The ionic doping or substitution of Mg, Al, Ti, Cr, Mo, Fe, F, BOx, etc.,30,33,41−45 can modulate the chemical and physical environment of the lattice so as to stabilize the structure and improve rate capability of LRMO by changing the electronic state and ionic diffusion path. For instance, Jiao et al. reported that Cr-doped Li1.2(Ni0.2−x/2Mn0.6−x/2Crx)O2 showed gradually reduced resistance during cycling so that it displayed capacity and rate capability improved over that of the undoped one as x = 0.04.44 He et al. found that introduction of Na ions into the lattice could expand the Li slab space and improve the ionic Received: April 23, 2016 Accepted: July 6, 2016 Published: July 6, 2016 18867

DOI: 10.1021/acsami.6b04849 ACS Appl. Mater. Interfaces 2016, 8, 18867−18877

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ACS Applied Materials & Interfaces

Figure 1. (a) TEM image and (b) XRD pattern of the as-prepared Li(Li0.2Mn0.54Ni0.13Co0.13)O2 sample.

moderate voltage range of 4.4−2.8 V showed the best structural stability and least capacity and working voltage decay upon repeated cycling.

diffusion kinetics. The Na-stabilized LRMO exhibited a high reversible capacity (307 mA h g−1) and rate capability (139 mA h g−1 at 8 C) and excellent cycling stability (89% capacity retention after 100 cycles).30 Li et al. showed that the incorporation of boracic polyanions into LRMO greatly enhanced the oxygen stability in the lattice and provided excellent cycling stability (89% capacity retention over 300 cycles).45 Most recently, Wang et al. demonstrated that Mg doping also can effectively enhance the structural stability of LRMO, giving excellent cycling performance (93.3% capacity retention over 300 cycles) and improved rate capability (114 mA h g−1 at 8 °C).33 Although great improvements of the initial Coulombic efficiency, rate capability, and cycling stability have been achieved by surface coating and bulk doping, severe working voltage decay is still a problem.15,20,43,46 For example, the above-mentioned boracic polyanion-doped LRMO material showed a large voltage decay from ∼3.5 V in the first cycle to ∼2.7 V after 250 cycles.45 Voltage decay indicates that continuous phase transition of the material still happens, although surface coating or bulk doping can somewhat stabilize the transformed phase structure to sustain reversible capacity. Because LRMOs undergo phase transformation at high charge voltages due to the diffusion of the transition-metal cations, a variety of studies have investigated the impact of charge cutoff voltage on the working voltage decay. Wu et al. showed that, when the charge cutoff voltage was lowered to 4.4 V, working voltage decay ceased to occur.20 However, it should be noted that the excess reversible capacity of LRMOs comes from the reduction of activated Li2MnO3 into Mn3+, which usually occurs below 3.5 V.20,46 A higher discharge capacity delivered by LRMOs results in higher amounts of Mn4+ being reduced into Mn3+, leading to potential John− distortion. Zheng et al. demonstrated that the formation of the spinel phase and subsequent fragmentation occurring by disproportionate reaction of Mn3+ are the dominant reasons for the capacity fading during cycling.18 Obviously, the structural transformation emerging in the low voltage region is also an origin of the capacity decay. Herein, X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), line scanning energy dispersive spectrometry (LS-EDS), and electrochemical impedance spectroscopy (EIS) were adopted to investigate the structural variation of the LRMO electrodes cycled in different cutoff voltage regions. Our results show that the LRMO electrodes undergo phase changes both on high charge voltage and low discharge voltage. The electrode cycled at a

2. EXPERIMENTAL SECTION 2.1. Material Preparation. LRMO was prepared by a simple polymer-pyrolysis method.30,33,47 The polymer precursor was prepared by dissolving in sequence LiOH·H2O, Ni(NO3)2·6H2O, and Mn(NO3)2·4H2O in a stoichiometric ratio into acrylic acid/H2O (70/30 wt %) solution under stirring. With a small amount of (NH4)2S2O8 added as an initiator, the solution was held at 85 °C for about 3−5 h and solidified into polyacrylates. After being dried at 120 °C for 24 h, the polyacrylates were decomposed at 450 °C for 5 h and then calcined at 900 °C for 12 h in air to obtain the final product. 2.2. Morphological and Structural Characterization. The crystalline structure of Li(Li0.2Co0.13Ni0.13Mn0.54)O2 was tested using a Shimadzu XRD-6000 diffractometer with Cu Kα radiation. The XRD spectra were collected from 10 to 80° at a scanning rate of 2°/min. The morphologies were examined using a JSM-6700F scanning electron microscope and a JEM-2010FEF transmission electron microscope. 2.3. Electrochemical Characterization. Electrochemical measurements were carried out using CR2032 coin-type cells. The cathode film was fabricated by mixing the as-synthesized Li(Li0.2Co0.13Ni0.13Mn0.54)O2 powders, carbon black, and polyvinylidene fluoride binder (80/10/10 in weight) in N-methylpyrrolidinon. The mixture was then pasted onto Al foil and dried at 80 °C in vacuum. The electrolyte was 1.2 M LiPF6 dissolved in ethylene methyl carbonate and ethylene carbonate (7/3 v/v). The coin cells were assembled in an argon-filled glovebox. The galvanostatic charge− discharge tests were carried out using a Land 2001T battery measurement system. EIS measurements were conducted at an open-circuit voltage in the frequency range of 100 kHz to 1 mHz with a voltage amplitude of 5 mV using an IM6 electrochemical impedance analyzer.

3. RESULTS AND DISCUSSION Figure 1a shows the TEM image of the as-prepared Li(Li0.2Co0.13Ni0.13Mn0.54)O2 powders. It can be seen that the material is composed of nanocrystals with sizes in the range of 50−100 nm. Figure 1b displays the XRD pattern of the Li(Li0.2Co0.13Ni0.13Mn0.54)O2 powders. The diffraction peaks can be indexed as an O3-type layered structure with a space group of R3̅ in addition to several weak reflections around 2θ = 20−25° due to a short-ranged superstructure derived from Li, Mn, Ni, and Co in plane cation ordering. This set of diffraction peaks can be classified as a slightly distorted O3-type layered structure with C2/m symmetry, resulting from a LiMn6 cation arrangement in the transition-metal layers. Thus, the XRD 18868


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also exhibits less voltage decay. The fast voltage decay should originate from the continuous phase transformation from the layered to spinel structure at high charge voltage, as shown in previous reports.20 Also, the high charge voltage could arouse electrolyte decomposition. The surface of the material could be etched by the acidic species generated from the electrolyte decomposition and cause the bulk structure to be vulnerable.18 Thus, lowering the charge cutoff voltage is a practical way to restrain the capacity and voltage decay of the LRMO materials. It is known that upon discharge the activated manganese(IV) oxide can be reduced at a voltage of 1.2, the cation mixing can

The EIS spectra of the electrodes cycled in different voltage ranges of 2.0−4.8 and 2.8−4.4 V were also carried out, as shown in Figure 7. For the electrode cycled at 2.0−4.8 V, Rf increases slightly from 109 Ω at the 2nd cycle to 120 Ω at the 30th cycle, while Rct increases almost two times from 447 Ω at the 2nd cycle to 741 Ω at the 30th cycle (Figure 7a), indicating that the electrodes have undergone a continuous phase change with cycling, leading to a structural instability on the surface and bulk phase. However, for the electrode cycled at 2.8−4.4 V, both the Rf and Rct values were found to decrease slightly after 30 cycles, revealing that controlling charge and discharge cutoff 18871


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Figure 7. EIS spectra of the Li(Li0.2Mn0.54Ni0.13Co0.13)O2 electrode after the 2nd and 30th cycles between (a) 4.8−2.0 V and (b) 4.4−2.8 V.

Figure 8. XRD patterns of the Li(Li0.2Mn0.54Ni0.13Co0.13)O2 electrode after the 2nd and 50th cycles between (a) 4.8−2.0 and (b) 4.4−2.8 V.

be regarded as negligible.50 Figure 8a shows that R decreases from 1.36 to 1 after 50 cycles between 4.8−2.0 V, indicating that a large amount of transition-metal ions immigrate into the Li layers. Additionally, the (018) peak became broader and weaker, implying a decrease in crystallinity. Upon being cycled between 4.4 and 2.8 V (Figure 8b), no discernible variation can be observed from the XRD patterns between the 2nd and 50th cycles; R shows only a slight decrease from 1.36 and 1.28, indicating quite a good layered structure. Thus, XRD results also demonstrate that appropriate charge and discharge cutoff voltages can efficiently retain the structural stability of the LRMO electrodes. EDS line technology was used to analyze the distribution of the transition-metal ions in the Li(Li0.2Mn0.54Ni0.13Co0.13)O2 material after cycling. Figure 9 shows the TEM images of the electrodes cycled for 50 cycles in different voltage ranges of 4.8−2.0 and 4.4−2.8 V, and the EDS line scan curves of the relative atomic ratio (%) of the transition-metal ions as a function of the distance from the particle surface to the interior. Because the relative atomic ratios of Ni, Co, and Mn in Li(Li0.2Mn0.54Ni0.13Co0.13)O2 can be calculated as 16.25, 16.25, and 67.5%, respectively, their ideal distribution was drawn in dashed lines serving as the baseline. When cycled between 4.8 and 2.0 V for 50 cycles (Figure 9a, left), it shows that the relative atomic ratios of Ni and Co show digressive trends from the particle’s surface to the interior. The surface of the particle has Ni and Co content higher than that in the baseline, and the interior of the particle has Ni and Co content lower than that in the baseline (Figure 9a, upper right). On the contrary, the

relative atomic ratio of Mn in the particle gradually increases from the surface to the interior (Figure 9a, lower right). The outer surface has Mn content much lower than that in the standard value, and the interior has Mn content much higher than that in the standard value. These results indicate that, when cycled in a wide voltage range, Mn ions in the particle probably migrate gradually from the surface to the interior; Ni and Co, on the contrary, migrate from the interior to the surface, resulting in accumulation of Ni and Co near the surface and Mn in the interior of the particles, which has also been observed in the previous reference.51 Further, it can be noticed that the content of Ni deviates from the baseline more seriously than that of Co, probably resulting from the much faster migration rate of Ni ions. Apparently, the severely inhomogeneous transition-metal ion distribution indicates severe phase transition or structural damage of the material after cycling between high charge voltage and low discharge voltage. In comparison, when cycled between 4.4 and 2.8 V for 50 cycles (Figure 9b), the contents of Ni, Co, and Mn throughout the particle stay quite close to the baseline, indicating good structural stability. TEM and HRTEM images of the Li(Li0.2Mn0.54Ni0.13Co0.13)O2 electrodes before and after being cycled between 4.8 and 2.0 V for 50 cycles are shown in Figure 10. The pristine particles show high crystallization with obvious grain boundaries (Figures 10a and b). The distance of the lattice fringes is calculated to be 0.47 nm, matching well with the d-value between the (003) planes shown in the XRD pattern. The selected-area electron diffraction (SAED) pattern (inset of 18872


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Figure 9. TEM images and the corresponding EDS line scans of the Li(Li0.2Mn0.54Ni0.13Co0.13)O2 electrodes after 50 cycles between (a) 4.8−2.0 and (b) 4.4−2.8 V.

Figure 10a) reveals that the individual particle of the material has a single-crystalline structure. After being cycled between 4.8 and 2.0 V for 50 cycles, the boundary of the particle becomes obscure (Figure 10c), probably due to a layer of SEI coating. The corresponding SAED pattern (inset of Figure 10c) displays composite diffraction spots (Figure 10e) which can be divided into a set of layered (R3̅m) and a set of spinel (Fd3̅m) signals. The HRTEM and fast Fourier transform (FFT) images further reveal the presence of the composite phases (Figures 10d, e, and f), which are completely different from the uncycled material (Figure 8b). The results apparently demonstrate severe phase transformation occurs on the electrode cycled between 4.8 and 2.0 V. TEM and HRTEM images of the electrodes after being cycled between 4.4 and 2.8 V for 50 cycles are shown in Figure 11. The boundary of the particle also becomes blurred (Figure 11a), similar to that shown in Figure 10c due to the presence of an SEI coating. The SAED pattern exhibits monophasic diffraction spots (inset) which can be indexed to the rhombohedral layered structure. The distance of the lattice fringes in a wide area of the inner part of the particle (Figure 11b) is 0.47 nm, corresponding to the d003 in the XRD pattern, whereas the surface of the particle displays complex lattice fringes belonging to spinel phase (Figure 11b), which may result from the activation process during the first cycle between 4.8−2.0 V. Thus, TEM and HRTEM also demonstrate that the

narrower charge/discharge voltage range between 4.4 and 2.8 V benefits structural stability of the Li(Li0.2Mn0.54Ni0.13Co0.13)O2 electrodes. In all, when cycled upon high charge and/or low discharge voltages, a severe structural variation occurs on the LRMO electrodes, which is clearly observed from the electrochemical impedance, XRD patterns, HRTEM results, and EDS line scans (Figures 6−10). Upon high charge voltage, transition-metal ions will migrate in the transition-metal layer to compensate for the Li-ion vacancies, resulting in an energetically more favorable spinel structure; upon low discharge voltage, the manganese oxide will be over-reduced and result in a disproportionation reaction of Mn3+ by John−Teller effect. The structural transformation is illustrated in Figure 12. The pristine LRMO shows an ideal layered structure, including alternative lithium layers and transition-metal layers with uniform distribution of LiMn6 and M-M lattices (Figure 12a). During the activation process, excessive Li-ion extraction from the material results in Li ions in the transition-metal layers moving to the Li layers and leaving vacancies behind. To balance the accumulated charge, oxygen ions were extracted from the lattice (Figure 12b). Mn ions tend to migrate to the vacancies in the transition-metal layers, and Ni ions tend to move into the Li layers, resulting in the formation of the spinel phase. Because the migration of Mn ions to the Li vacancies in the transitionmetal layers has an energy barrier lower than that for the 18873


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Figure 10. TEM and HRTEM images of (a and b) the pristine Li(Li0.2Mn0.54Ni0.13Co0.13)O2 electrode and (c and d) the electrode after 50 cycles between 4.8 and 2.0 V. (e) Enlarged ED patterns and (f) FFT images of the cycled electrode.

Figure 11. (a) TEM image, ED pattern (inset), and (b) HRTEM image of the Li(Li0.2Mn0.54Ni0.13Co0.13)O2 electrode after 50 cycles between 4.4 and 2.8 V.

migration of Ni ions to the Li layers, a Ni(Co)/Mn concentration gradient inside the particle is gradually

established with cycling (Figure 12c), which is in good agreement with the observation from the EDS line scan 18874


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Figure 12. (a−d) Schematic illustrations of the structural transformation of the Li(Li0.2Mn0.54Ni0.13Co0.13)O2 electrode during the initial activation process.

University (Grant NCET-12-0419), and the Hubei National Funds for Distinguished Young Scholars (Grant 2014CFA038).

(Figure 9a). Upon discharge, the activated Mn oxides are reduced below 2.8 V. Excessive reduction of the activated Mn oxides results in formation of the spinel phase15 or a disproportionation reaction of Mn3+ (Figure 12d). The phase transition causes a resistance increase and a capacity and working voltage loss upon repeated cycling. Further, the ion migration causes the densification of the material surface.51,52

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4. CONCLUSION In summary, the effect of the charge/discharge cutoff voltage on the structure, capacity, and working voltage of the Li(Li0.2Mn0.54Ni0.13Co0.13)O2 electrode upon cycling were elaborately investigated using galvanostatic charge/discharge profiles, EIS, ex-XRD, TEM, and EDS line scan technologies. It was found that, upon high charge and/or low discharge voltages, severe structural variations such as formation of spinel phase and migration of transition-metal ions inside the particles occur on the LRMO electrodes. The electrode cycled at a moderate voltage range of 4.4−2.8 V shows stable structure during cycling demonstrates 92% capacity retention and indiscernible working voltage decay over 430 cycles. The finding that the accumulation of Mn ions in the interior of the material during deep discharge easily leads to John−Teller effect provides a new insight into the search for an appropriate approach to alleviate phase transformation in the low voltage region to achieve highly stable LRMO materials.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the financial support from the National Key Basic Research Program of China (Grant 2015CB251100), the National Science Foundation of China (Grants 21373155 and 21303262), the Program for New Century Excellent Talents in 18875


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Understanding Voltage Decay in Lithium-Rich Manganese-Based

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