Surface and Interface Issues in Spinel LiNi0.5Mn1.5O4: Insights into a

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Surface and Interface Issues in Spinel LiNi0.5Mn1.5O4: Insights into a Potential Cathode Material for High Energy Density Lithium Ion Batteries Jun Ma,† Pu Hu,† Guanglei Cui,*,† and Liquan Chen†,‡ †

Qingdao Industrial Energy Storage Research Institute, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China ‡ Key Laboratory for Renewable Energy, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: Spinel LiNi0.5Mn1.5O4 with high operating voltage (∼4.7 V vs Li/Li+), high theoretical capacity of 148 mAh g−1, fast lithium ion diffusion kinetics, and potentially low cost is the most potential candidate material for high energy density LIBs used in plug-in hybrid electric vehicles and pure electric vehicles. However, the high operating voltage of LiNi0.5Mn1.5O4 challenges the electrochemical stability of other components in the batteries and induces diverse interfacial side reactions, leading to irreversible capacity loss, poor cycling performance, and safety issues, especially at the elevated temperatures. Thus, a basic understanding of the intrinsic surface properties of LiNi0.5Mn1.5O4 and the mechanism of interfacial interactions between each component in the electrochemical system is a critical requirement for developing substantial enhancements of LiNi0.5Mn1.5O4-based batteries. In this review, we summarize the surface/interface reactions and challenges in the whole cell system of LiNi0.5Mn1.5O4-based LIBs. Perspectives and strategies for LiNi0.5Mn1.5O4-based high energy density batteries used in PHEV/EVs are also proposed at last.

1. INTRODUCTION With the scientific and technological progress as well as the policy support, the plug-in hybrid electric vehicle (PHEV) and pure electric vehicle (EV) are expected to play a significant role in relieving the energy crisis and environmental pollution. In the development of PHEV/EVs, energy storage technology is the key factor. Among recent energy storage technologies used in automobiles, the lithium ion battery (LIB) has become more and more popular because of its high energy/power density, long lifespan, less memory effect, and low self-discharging rate.1−6 Nowadays, the LIB has been utilized in golf carts, electric bicycles, utility vehicles, and cars, such as Tesla. Nevertheless, today’s state-of-the-art LIB is not enough to match the performance of internal combustion vehicles.2,7−14 There are still some scientific challenges in extending driving distances, increasing rate capabilities, lowering costs, and eliminating safety hazards for LIBs in PHEV/EVs. It is therefore necessary to sustain the research and development efforts to improve the energy/power density and safety of LIBs while reducing their cost. Because the commercial high conductive carbonaceous anode potential is ∼0 V, the energy/power density of LIBs is mainly determined by the cathode materials. Table 1 summarizes the characteristics of various cathode materials for LIBs in the present market.15 According to recent research, the potential cathode materials for power LIBs are layer© 2016 American Chemical Society

structured transition metal oxides LiMO2 (M = Ni, Co, Mn, Al), olivine structure LiFePO4, and spinel lithium manganese oxides.8,9,14,16−18 The energy densities of recent commercial cathodes for power LIBs are shown in Figure 1.15,19,20 Though Li(NiCoAl)O2 with high energy density has been used in Tesla vehicles successfully, the expensive and toxic Co element will raise cost and hinder mass production of EVs. For LiFePO4, though it displays high safety and cycling performance, its low volume specific energy and complicated production process bring difficulties to the reduction of vehicle size and battery cost, respectively. Spinel LiMn2O4 is the most widely used cathode material in both EVs and PHEVs. However, its energy density is low and facing the problem of poor high temperature stability. Ni doping in LiMn2O4 (LiNi0.5Mn1.5O4) is reported to be an effective method to solve the above problems.21 Thus, spinel LiNi0.5Mn1.5O4 with high operating voltage (∼4.7 V vs Li/Li+), high rate performance, and potentially low material cost is the most potential cathode material for power LIBs used in vehicles. Recently, great efforts have been made to understand the fundamental chemistry and material issues of LiNi0.5Mn1.5O4 and propose modification strategies to overcome its intrinsic challenges for power LIBs, as introduced in Received: March 8, 2016 Revised: May 12, 2016 Published: May 13, 2016 3578

DOI: 10.1021/acs.chemmater.6b00948 Chem. Mater. 2016, 28, 3578−3606

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Chemistry of Materials Table 1. Characteristics of Various Cathode Materials for LIBs Based on Cell Level15

characteristics LiCoO2/graphite NMCa/graphite NCAa/graphite NMC/graphite LMOa/graphite LiFePO4/graphite a

working potential (V vs Li/Li+) 2.5−4.2 general; 2.5−4.35

average voltage (V) 3.7

weight specific energy (Wh kg−1) 170−240, cylinder; 130−200, polymer

volume specific energy (Wh L−1)

weight power energy (W kg−1)

volume power energy (W L−1)

continuous rate capability (C)

400−640,

∼1000

∼2000

2−3

5

>500

pulse rate capability (C)

cycle life under 100% DOD (until 80% capacity)

250−450

undetermined 2.5−4.2

3.7

100−150

350

∼4000

∼10000

above 30

above 100

>500

2.5−3.6

3.3

60−100

125−250

∼4000

∼10000

10−125

up to 250

>1000

NMC, NCA, and LMO represents LiNi1−x−yMnxCoyO2, Li(NiCoAl)O2, and LiMn2O4, respectively.

detail in several reviews17,22−26 and papers.27−29 Particularly, Kim et al.17 have pointed out that the most critical barrier for the successful commercialization of LiNi0.5Mn1.5O4 in power LIBs is the electrolyte decomposition and concurrent degradation reactions at the cathode/electrolyte interfaces at high voltages. It is worthy to note that in addition to electrolytes, the compatible anodes, separators, and the other inactive components, such as binders, current collectors, conductive additives, and current collectors, also face challenges and suffer from detrimental interface reactions in high operational voltage LIBs.30−37 Therefore, the successful commercialization of LiNi0.5Mn1.5O4 in PHEV/EVs is not only determined by its own electrochemical performance but also depends on the understanding and simultaneous optimization of numerous interfaces between all components in LIBs at high voltages. However, most reviews focus on the interface reaction between LiNi0.5Mn1.5O4 and electrolyte, not enough emphasis is laid on the other interface reactions. Here, the status and scientific issues of LiNi0.5Mn1.5O4-based power LIBs are reviewed from the perspective of surface/ interface chemistry. The main content focuses on the surface/ interface reactions and challenges in the whole cell system before and during cycling, as well as the current strategies to overcome these surface/interface challenges in LiNi0.5Mn1.5O4-

Figure 1. Energy densities of recent commercial LIBs with different cathode materials based on cell level. LFP, NCM, LMO, LNO, and NCA represents LiFePO4, Li(NiCoMn)O2, LiMn2O4, LiNiO2, and Li(NiCoAl)O2, respectively. The estimated practical energy density of LiNi0.5Mn1.5 (LNMO)-based LIBs is also present in the last column.15,19,20

Figure 2. Illustration of the interface reactions in LiNi0.5Mn1.5O4/graphite battery. 3579


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Figure 3. Incremental capacities and diffusion coefficients during the charge (black) and discharge (red) of (a) plates and (b) octahedrons. Rate cycling profiles of (c) plate and (d) octahedron sample. Reproduced with permission.74 Copyright 2012, Royal Society of Chemistry.

Figure 4. Morphology, surface structure, and electrochemical performance of LiNi0.5Mn1.5O4 with different crystal orientations. Reproduced with permission.75 Copyright 2013, Royal Society of Chemistry.

based batteries. Finally, perspectives and approaches will be introduced for high voltage battery designs and new chemistry

combinations to increase the cycling and safety of full cells used in PHEV/EVs. 3580


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Figure 5. Rate capacity, cycling performance, and SEM images after 200 cycles at 55 °C for chamfered polyhedral (a, c, e) and octahedral (b, d, f) LiNi0.5Mn1.5O4, respectively. Reproduced with permission.79 Copyright 2014, Royal Society of Chemistry.

2.1. Surface Chemistry of LiNi0.5Mn1.5O4. The surface chemistry of cathode materials, including crystal structure and plane, termination atomic layer, bond-breaking energy, and element distribution, plays an important role in the lithium ion transfer dynamics, CEI formation, and chemistry compatibility between electrode and electrolyte.38−45 As for LiNi0.5Mn1.5O4, the thermodynamics and dynamics properties, such as redox reaction, phase transition, Li ion diffusion, and electron transfer, are closely related with the crystal structure, cation ordering, oxygen vacancy, and impurity phases, which are varied with synthesis method and thermal treatment conditions.46−73 Accordingly, the diverse surface structure and composition of LiNi0.5Mn1.5O4 will display an effect on the electrochemical performance. Furthermore, the three-dimensional Li ion diffusion network of LiNi0.5Mn1.5O4 makes the kinetics of Li ion intercalation/deintercalation quite sensitive to the crystal orientations. Besides the influence of original surface characteristic on the electrochemical performance, the dynamic surface structure transformation of LiNi0.5Mn1.5O4 during Li ion intercalation/deintercalation is also worthy of attention. Therefore, a comprehensive knowledge of the surface chemistry

2. SURFACE AND INTERFACE CHEMISTRY In a typical configuration of the LIB, electron migration and Li ion diffusion take place inevitably on the surface of active material particles and various interfaces between each component. In addition, side reactions, including electrolyte decomposition, electrode corrosion, formation of unstable solid electrolyte interface (SEI) films on the anode surface and/or cathode−electrolyte interface (CEI) layer between cathode and electrolyte, and even corrosion of inactive components by electrolyte, seriously deteriorate the stability of LiNi0.5Mn1.5O4 and electrolyte and exacerbate the loss of active material and cell impedance, and thus lead to low initial Coulombic efficiency and poor cycling stability of LiNi0.5Mn1.5O4 electrode at high work potential. Thus, the electrochemical performance of LiNi0.5Mn1.5O4 is determined not only by its intrinsic properties but also by a series of surface and interface chemical properties in LIBs, as illustrated in Figure 2. To improve the electrochemical performance of LiNi0.5Mn1.5O4, it is essential to understand and modify its surface and interface chemistry. 3581


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Figure 6. SEM images of (a) a typical cathode electrode before cycling; (b) Li[Ni0.5Mn1.5]O4, (c) Li[Ni0.475Cr0.05Mn1.475]O4, and (d) Li[Ni0.45Cr0.1Mn1.45]O4 electrodes after 50 cycles at 55 °C; (e) the atomic ratio of Ni:Mn in LiNi0.5−xCr2xMn1.5−xO4 (0 ≤ 2x ≤ 0.8) before/after cycling at 55 °C; (f) cycling performances of the LiNi0.5−xCr2xMn1.5−xO4 (0 ≤ 2x ≤ 0.8) at 55 °C. Reproduced with permission.88 Copyright 2010, The Electrochemical Society.

(111) surface planes in Poly 1 were considered to weaken the electrolyte decomposition at high operating voltages and resulted in excellent cycle life, capacity retention, and rate capability (Figure 4e,f).75 However, there are inconsistent conclusions among different research groups.77,79,80 For example, the work of Chen et al.77,79 demonstrated that the chamfered polyhedral LiNi0.5Mn1.5O4 with faces of {111}, {001}, {110}, and so on provided superior capacity retention and rate capability to that of the octahedral structure with only {111} surface (Figure 5). They attribute this result to the existence of {110} and the reduction of {111} crystal faces, which can improve the Li ion diffusion, suppress the Mn ion dissolution and lattice strain caused by the Jahn−Teller distortion during Li ion intercalation/deintercalation. We have two opinions about the above results and controversy. On the one hand, as the electrochemical performance of LiNi0.5Mn1.5O4 is not only determined by crystal orientation, it is inappropriate to compare simply the results from different groups and ignore the bulk crystal structure (cation ordering, oxygen vacancy, and impurity phases) and particle size. Several works have reported that the effect of particle size on the electrochemical performance of LiNi0.5Mn1.5O4.53,81−84 Compared with microsized particles, nanosized LiNi0.5Mn1.5O4 improves the high rate performance but worsens the cycling stability. This is because the small particle size is beneficial to the shortened lithium ion diffusion path, whereas high surface area aggravates the dissolution of transition metal ions and then exacerbates the side interface reactions, impedance increase, and active lithium loss. Thus, there is still much work to do to understand further the influence of crystal orientation on the rate capability and capacity retention by eliminating other structure and particle size factors. Up to now, an enlightening work has been carried out by Manthiram et al.76 They carefully compared the influence of crystal orientation, degree of cation ordering, and phase segregation on the rate capability and capacity retention and found that the crystal orientation has a dominant effect in spite of the cation ordering. On the other hand, in terms of Mn ion dissolution and CEI formation on the LiNi0.5Mn1.5O4/

of LiNi0.5Mn1.5O4 is critical for understanding the reactions on the electrode surfaces and their influences on the electrochemical performance. 2.1.1. Crystal Orientation. Crystal orientation, showing up as different crystal morphologies, has been demonstrated to influence the electrochemical performance of LiNi0.5Mn1.5O4.74−79 Table S1 shows the different electrochemical performance of LiNi0.5Mn1.5O4 with various crystal orientations. In general, when the measuring parameter is determined, LiNi0.5Mn1.5O4 with (111)-faceted octahedral crystals had better electrochemical performance than that with other faces. One possible reason for this result is the anisotropic kinetic of Li ion diffusion in LiNi0.5Mn1.5O4. Hai et al.74 have reported the different chemical diffusion coefficients in plate-shaped and octahedron-shaped LiNi0.5Mn1.5O4. The plate-shaped LiNi0.5Mn1.5O4 displayed the large surface facets corresponding to (112) crystal planes, which accounted for more than 90% of the surface area. On the other hand, the octahedron-shaped sample showed all the surface facets corresponded to (111) crystal planes. Both samples had a similar average size of 2 μm. During charging and discharging, diffusion minima were observed in both samples, which correlated well with the capacity maxima in both processes (Figure 3). In addition, the value of chemical diffusion coefficient in octahedrons is much higher than that in plateshaped sample, indicating higher Li ion diffusion through (111) crystal planes, which is consistent with the higher capacities and better rate properties of octahedron-shaped LiNi0.5Mn1.5O4. The other reason is thought to be the superior interface stability between the (111) crystal plane and electrolyte. Manthiram et al.75 prepared the octahedral particles consisted entirely of the {111} family of planes (name as Poly 1) and the truncated octahedral particles composed of {111} and {100} planes (name as Poly 2) (Figure 4a,b). The comparison of Li ion arrangement in both samples (shown in Figure 4c,d) indicated that the (100) plane had a less dense arrangement of Li ions, higher surface energy, and seemed to be more vulnerable for Mn dissolution. Furthermore, stable CEI has been reported on the (111) plane in spinel cathode.39 Thus, the 3582


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Figure 7. TOF-SIMS depth profiles of the LiMn1.5Ni0.5−xMxO4 (M = Cr, Fe, and Ga; x = 0, 0.08) samples (a) before and (b) after postannealing at 700 °C. Reproduced with permission.90 Copyright 2012, American Chemical Society.

disproportionation reaction (2Mn3+ → Mn2+ + Mn4+),86 the amount of Mn2+ dissolved in the electrolyte increased with the concentration of Mn3+ in the spinel LiNi0.5Mn1.5O4. Thus, to reduce the dissolution of Mn2+, it is required to decrease the concentration of Mn3+ on the surface of LiNi0.5Mn1.5O4. However, the role of Mn3+ in the electrochemical performance of LiNi0.5Mn1.5O4 is still under debate. The atomistic simulations conducted by Xiao et al.59 suggested that the formation of Mn3+ promoted Ni−Mn site disorder, which can facilitate the Li+ transport, especially at high rates. The roomtemperature conductivities of LiNi0.5Mn1.5O4 also showed that the electronic conductivity of cation disordered Fd3̅m spinel LiNi0.5Mn1.5O4 was 2.5 orders of magnitude higher than that of ordered P4332 sample because of the electron hopping from the increased content of Mn 3+ (absence in ordered Li-

electrolyte interface, most work was based on the results of LiMn2O4.38,39 It is feasible to take knowledge from LiMn2O4 when studying the surface chemistry of LiNi0.5Mn1.5O4 because both cathode materials are spinel structures. Whereas, the different atomic arrangements caused by Ni2+ and Mn4+ ions make the surface energy, atomic arrangement, and reaction with electrolyte between LiNi0.5Mn1.5O4 and LiMn2O4 are really distinctive. It is therefore quite necessary to conduct theoretical calculation and experimental work on the relation between crystal orientation and Mn ion dissolution as well as CEI formation mechanism of LiNi0.5Mn1.5O4. 2.1.2. Surface Element Distribution. It has been demonstrated that the surface element distribution has some impact on the electrochemical performance of LiNi0.5Mn1.5O4. The research by Chen et al. 85 showed that due to the 3583


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Figure 8. STEM images showing the surface and subsurface regions of the pristine (left) and first-charged (right) LiNi0.5Mn1.5O4. Reproduced with permission.95 Copyright 2015, American Chemical Society.

Figure 9. (a) Charge−discharge curves of a typical LiNi0.5Mn1.5O4/Li half cell in the first cycle; (b) charge−discharge capacities and Coulombic efficiency for the first 100 cycles. Reproduced with permission.95 Copyright 2015, American Chemical Society.

Figure 10. Schematic of the energy versus density of states plot, showing the relative positions of the redox couples in Li[Ni0.5Mn1.5]O4 spinel and the highest occupied molecular orbital (HOMO) of carbonate electrolyte. Reproduced with permission.65 Copyright 2012, American Chemical Society.

Ni0.5Mn1.5O4), which was beneficial to the capacity retention at high temperature.70,71,87 As a result, the balanced influence of Mn3+ on the electrochemical performance of LiNi0.5Mn1.5O4 has to be optimized in the future.

Besides the impact from Mn ion distribution, the Ni4+/Ni2+ couple can oxidize the electrolyte to form CEI film on the 3584


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substitution for Ni2+ and Mn4+ segregated the Ni ion from the surface to the bulk and improved the cycling performance of LiNi0.5Mn1.5O4 at 55 °C (Figure 6f). It means that the Nideficient surface of LiNi0.5Mn1.5O4 is beneficial for the stabilization of the electrode/electrolyte interface and the decrease of surface resistance, which has been further proved by time-of-flight-secondary ion mass spectroscopy (TOF-SIMS) depth profiles and electrochemical impedance spectroscopy (EIS) results in the work of Goodenough et al.65,89 To produce a surface layer deficient in Ni, cation substitution for Ni and/or Mn has been adopted.90−93 These works indicated that besides the segregation of Ni, the doping cations also segregated preferentially to the surface and affected the electrochemical performance of LiNi0.5Mn1.5O4. It has been reported that the Cr-, Fe-, and Ga-doped LiNi0.5Mn1.5O4 displayed a more stable cathode/electrolyte interface and remarkably improved cyclability and Coulombic efficiency, which was benefited from the surface element segregation of the dopant ions, as demonstrated by the TOF-SIMS depth profiles (displayed in Figure 7). A recent work of Qiao et al.94 reported that there are electrochemically inactive Ni2+ and Mn2+ phases on the surface of LiNi0.5Mn1.5O4, which are detected by the surface sensitive soft X-ray absorption spectroscopy. This work offers new knowledge about the surface chemistry of LiNi0.5Mn1.5O4 and makes the relation between surface element distribution and electrochemical performance much more complicated. It is better to explore this question by combining the advanced

Figure 11. Reflectivity profiles collected in situ for the LMNO film (left) charged to 4.75 V. Corresponding scattering length density (SLD) plots representing the film thickness (right). The schematics in the middle represent the layers formed from the silicon substrate out. Reproduced with permission.108 Copyright 2014, American Chemical Society.

surface of cathode particles and thus impede the Li ion diffusion across the LiNi0.5Mn1.5O4/electrolyte interface, as proposed by Goodenough et al.88 By charactering the surface morphological changes and the near-surface Ni segregation of LiNi0.5−xCr2xMn1.5−xO4 (0 ≤ 2x ≤ 0.8) after cycling at 55 °C (Figure 6a−e), they observed pulverization of the LiNi0.5Mn1.5O4 electrode and attributed this phenomenon partially to the CEI layer formation, which was correlated with the reaction between the electrolyte and the oxidized Ni ions at the surface. Their work also demonstrated that Cr3+

Figure 12. Possible formation pathway of the metal complexes upon DEC oxidation at the LixNi0.5Mn1.5O4 surface by two proton-coupled electron transfer (PCET) processes associated with ligand adsorption. Reproduced with permission.112 Copyright 2015, American Chemical Society. 3585


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Figure 13. Electrodes cycled with (a) C/4, (b) C/2, and (c) 5 C for approximately 25 cycles. Green, Mn; red, Ni; blue, transmission. The angled green area in b was caused by a piece of copper tape used to fix the electrode in its pouch. Reproduced with permission.149 Copyright 2015, American Chemical Society.

Figure 14. (a) Lattice constant evolution during charging and discharging. Yellow highlighted points correspond to the states shown in panel c. (b) Open circuit voltage at which the measurements were taken. (c−f) Strain evolution in both the core and the shell of the particle. Reproduced with permission.153 Copyright 2015, Royal Society of Chemistry.

show rocksalt-like structure, caused by partial occupation of the empty octahedral sites by transition metal ions. It is inferred that the occupation of transition metal ions in Li sites in charged LiNi0.5Mn1.5O4 will block the Li ion diffusion, resulting in the increased charge transfer impedance and poor first cycle Coulombic efficiency (Figure 9). Furthermore, the distorted structures in the subsurface and bulk structure may become nucleation centers for the growth of Mn3O4 and rocksalt phases during cycling for prolonged cycles or at high temperatures, leading to structural and electrochemical performance failure of LiNi0.5Mn1.5O4. The authors suggested that preoccupation of the tetrahedral Li sites in the surface of spinel LiNi0.5Mn1.5O4 by a small amount of insoluble ions may possibly be the key to stabilizing structure and improving electrochemical performance of LiNi0.5Mn1.5O4. This work has provided fundamental understanding and opened new research field in exploring the relation between surface chemistry and electrochemical performance of LiNi0.5Mn1.5O4 in atomic scale. To understand further the influence of surface structure changes on the electrochemical performance of LiNi0.5Mn1.5O4 in the charge

surface sensitive characterization techniques and electrochemical methods. 2.1.3. Surface Structure Transformation. Because of the difficulty in characterizing surface structure during charging and discharging, there is little knowledge about the relation between surface structure transformation and electrochemical performance of LiNi0.5Mn1.5O4. Recently, the effect of surface structure transformation of LiNi0.5Mn1.5O4 during charging and discharging on the capacity degradation and poor first-cycle Coulombic efficiency has been put forward. Lin and co-workers95 first studied the local atomic structure of LiNi0.5Mn1.5O4 through aberration-corrected scanning transmission electron microscopy (STEM). The local atomic structures of LiNi0.5Mn1.5O4 electrodes before charging, and after charged to 4.9 V are displayed in Figure 8. Compared with the pristine LiNi0.5Mn1.5O4, the charged LiNi0.5Mn1.5O4 exhibits a thin layer of the Mn3O4-like structure (∼2 nm) in the surface regions, associated with the tetrahedral Li sites partially occupied by transition metal ions. Meanwhile, the subsurface regions and some isolated bulk regions (close to the surface) 3586


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Figure 15. (a) Modulus of the applied current during charge and discharge as a function of measurement time shown on logarithmic scale. (b) Average along the Debye−Scherrer ring for all diffraction patterns shown on a logarithmic scale. Lithium concentration 1−δ as in Li1−δNi1/2Mn3/2O4 and three different structural phases are indicated. (c−e) Hysteretic structure transition upon cycling. Reproduced with permission.154 Copyright 2014, American Chemical Society.

crucial to the cycling life of LiNi0.5Mn1.5O4-based LIBs. Several reviews have highlighted the effect of LiNi0.5Mn1.5O4/electrolyte interface reaction on the cycling performance of high voltage LiNi0.5Mn1.5O4-based batteries.17,24,100 Here, we will focus on the reaction mechanism of LiNi0.5Mn1.5O4/electrolyte interface at high voltages. 2.2.1. Electrolyte Decomposition. Electrolyte decomposition is the most studied interface issue in high voltage LIBs. Xu25,101 has discussed the oxidation and degradation of conventional carbonate-based electrolytes in detail. With the existence of LiPF6 or LiBF4 in carbonate-based electrolyte, it has been reported that the CEI is composed of inorganic species such as LiF, ROM, ROCO2M (R is organic group, M = Li, Ni, Mn), LixPFyOz or LixBFyOz as well as organic species such as polyethers and polycarbonates.102−104 Though the polymeric CEI film resulting from the oxidation of ethylene carbonate (EC) or propylene carbonate (PC) may stabilize the interface during subsequent cycling, its protective function is still doubted due to the incomplete coverage on cathode surfaces.105,106 To understand better the degradation mechanisms of electrolyte, Dedryvere et al.107 studied the interface reaction process between LiNi0.4Mn1.6O4 and EC/PC/dimethyl carbonate (DMC) electrolyte by chemical sensitive X-ray photoelectron spectroscopy (XPS) and electrochemical sensitive EIS techniques. They found that the electroadsorption mechanism was predominant at the first cycle whereas the filmformation mechanism caused by electrolyte decomposition

and discharge process, we suggest to analysis the corresponding variation of interface impedance, Mn ion dissolution, and stability of electrode/electrolyte interface simultaneously. 2.2. LiNi0.5Mn1.5O4/Electrolyte Interface. To obtain thermodynamic stability in LIBs, the electrochemical potential of the electrode is required within the electrochemical window of the electrolyte.11 For cathode materials, the electrochemical potential should above the highest occupied molecular orbital (HOMO) of the electrolyte. However, in LiNi0.5Mn1.5O4-based LIBs, the Fermi energy of two nickel redox couples, Ni3+/Ni2+ and Ni4+/Ni3+, is beyond the thermodynamic stability window of the commercially available liquid carbonate electrolyte, as demonstrated in Figure 10. Thus, the surface chemical reactivity of LiNi0.5Mn1.5O4 with electrolyte leads to the electrolyte oxidation and generate diverse organic and inorganic products, which may cover the surface of LiNi0.5Mn1.5O4 as a cathode−electrolyte interface (CEI) layer and result in capacity fading and reduced cycle life. Furthermore, Ni and Mn ion dissolution is usually accompanied by the electrolyte oxidation on the LiNi0.5Mn1.5O4/electrolyte interface, which results in transition metal ions deposition and electrolyte decomposition on the surface of Li and graphite anodes.31,32,35,96,97 Especially, when graphite serves as an anode, the electrolyte decomposition and Mn ion deposition on the surface of graphite will accelerate the loss of active lithium and thus cause fast capacity fading of LiNi0.5Mn1.5O4/graphite full cell during cycling.98,99 Hence, the LiNi0.5Mn1.5O4/electrolyte interface chemistry is 3587


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Figure 16. (a−d) Deposition of nanoparticle (∼30 nm) (highlighted with a yellow box) on the cycled graphite particles. (e) Cycle lives of LiNi0.5Mn1.5O4/Li and graphite/Li cells by applying C/10 rate at 30 °C. (f) Cycle lives of LiNi0.5Mn1.5O4/graphite cells with different lower cutoff voltages at 30 °C. Reproduced with permission.35 Copyright 2013, American Chemical Society.

became predominant during subsequent cycling. Recently, Browning et al.108 proposed an in situ characterization of the LiNi0.5Mn1.5O4/electrolyte interface by neutron reflectometry (NR) to overcome the drawbacks of ex situ analyses and avoid the destruction and pollution of CEI structure and chemistry during manipulations. Combined in situ NR and ex situ XPS techniques, for the first time, they detected a dense 3.1 nm thick fluorine- and phosphorus-rich CEI layer on the surface of the LiNi0.5Mn1.5O4 electrode during the delithiation at 4.75 V, as displayed in Figure 11. This work provides experimental validation of interface chemistry and the resultant layer thickness during the initial delithiation and paves new way to understand the various interface chemistry in LIBs. 2.2.2. Transition Metal Dissolution. The mechanism for Ni and Mn ion dissolution is commonly accepted as the disproportionation reaction (2Mn3+ → Mn2+ + Mn4+)86 and side reaction between LiNi0.5Mn1.5O4 and electrolyte. LiPF6 is the usually used salt in carbonate-based electrolyte. However, LiPF6 is very sensitive to traces of water (form HF) and unstable at high temperatures. 103,109−111 As a result, LiNi0.5Mn1.5O4 is easily attacked by HF with the following reaction: 2LiNi0.5Mn1.5O4 + 4H+ + 4F− → 3Ni0.25Mn0.75O2 + 0.25NiF2 + 0.75MnF2 + 2LiF + 2H2O, which is first proposed by Pieczonka et al.35 Recently, Jarry et al.112 postulated the heterogeneous catalysis mechanism to describe interfacial reaction processes, such as electrochemical oxidation of carbonate esters and Ni/Mn dissolution at the LiNi0.5Mn1.5O4/organic carbonate electrolytes (diethyl carbonate (DEC) and EC) interface. Through X-ray absorption and optical fluorescence spectroscopy and imaging experiments, they demonstrated that the carbonate esters electrochemically oxidized at potentials >4.2 V and results in fluorescent Ni2+ and

Figure 17. (a) SEM image of the carbon- and binder-free LiNi0.5Mn1.5O4 electrode; (b) current (top) and integrated fluorescence intensity (bottom) variations during three CV sweeps between 3.5 and 5.0 V at 0.05 mV/s of the carbon- and binder-free LiNi0.5Mn1.5O4 electrode. Reproduced with permission.161 Copyright 2013, Elsevier. 3588


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Figure 18. (a) Capacity retention and (b) Coulombic efficiency of LNMO and LNMTO paired with Li (half cells, 3.5−4.9 V) or graphite (full cells, 3.4−4.8 V) with C/5-rate at 30 °C; (c) variation of surface film resistance (Rfilm) in LNMO and LNMTO full cells with number of cycles; (d) illustration of the formation mechanism of the CEI on LNMTO cathode. Reproduced with permission.126 Copyright 2015, John Wiley and Sons.

Mn2+/3+ complexes with β-diketonate ligands and Ni2+ and Mn2+ oxalates and carbonates. To explain the interface reactions between LiNi0.5Mn1.5O4 and carbonate esters electrolyte, stepwise and all-concerted proton-coupled electron transfer (PCET) reaction mechanisms were proposed, as illustrated for DEC in Figure 12. The continuous creation of oxygen vacancies was postulated to enhance the reaction rate. In addition, the Ni/Mn dissolution is mainly caused by the adsorption of β-diketonate chelate ligands at Ni4+/Mn4+ surface sites. However, the possible influence of fluorescent Ni2+ and Mn2+/3+ complexes with β-diketonate ligands on the Li+ transport in the CEI layer and the degradation of electrochemical performance are still unclear. Strategies to reduce the interfacial reactions between LiNi0.5Mn1.5O4 and liquid organic electrolyte have been carried out widely. In terms of LiNi0.5Mn1.5O4, changing particle sizes and shapes,53,113−115 coating,111,116−124 and element doping90,93,125−128 are usually adopted. In terms of electrolyte, the reported solutions include search novel high voltage electrolytes,129−136 optimization the conventional electrolyte by other additives,137−145 and application of ionic liquids.146−148 Recently, another intricate aging mechanism different from the general dissolution of transition metals in the electrolyte has been proposed by Boesenberg et al.149 Using fast micro X-ray fluorescence spectroscopy (XRF) scanning technique with medium spatial resolution (500 nm), they visualized changes in elemental distribution in LiNi0.5Mn1.5O4 composite electrodes cycled at different rates, as shown in Figure 13. It demonstrates that higher sweep rates increase morphological and chemical inhomogeneity, as evidenced by the formation of craters or holes at high cycling rate. Furthermore, the Ni-enriched regions in the direct vicinity of the holes, and the Ni depleted and Mn enriched area around the holes, indicating a possible aging mechanism that Ni atoms dissolute first and then redeposit

around the eroded spots. This aging mechanism is more complicated than the simple dissolution of transition metal ions in the electrolyte. However, the origin of Ni depleted and Mn enriched areas has not been evidenced. Further operando measurements are expected to answer this question. 2.2.3. Interfacial Li+ Ion Diffusion. In addition to the above interfacial reactions, the mechanism of Li+ ions diffusion at the LiNi0.5Mn1.5O4/electrolyte interface is also worthy of attention because the interfacial Li+ ions diffusion is an essential step during the electrochemical Li+ ion insertion and extraction process in LIBs. A two-step desolvation reaction during the Li+ ions insertion in LiMn2O4 in nonaqueous electrolyte has been evidenced in view of the different solvation energy of the electrolyte.150 Similarly, because of different solvation energies in various electrolytes, the Li+ ions diffusion at the LiNi0.5Mn1.5O4/electrolyte interface may depend on the kind of electrolyte. Xu et al.151 studied the rate performance of LiNi0.5Mn1.5O4 in various electrolytes and revealed that ECDMC and EC-ethyl methyl carbonate (EMC) have similar rate capabilities and better rate performance than EC-DEC. Besides the different Li+ ion conductivities in each electrolyte, the interfacial Li+ ions diffusion rate may be another reason for their distinct rate performance. In addition, on the basis of the calculation of spin-polarized total energy and semiempirical model, Seyyedhosseinzadeh et al.152 determined the diffusion coefficient for Li+ at the LiNi0.5Mn1.5O4/electrolyte (1 M LiClO4 in EC/DMC (1:1)) interface is 10−8 cm2 s−1, which is 3 orders of magnitude higher than that in bulk LiNi0.5Mn1.5O4 (10−11 cm2 s−1). Though this work evidenced the higher diffusion coefficient for Li+ at the LiNi0.5Mn1.5O4/electrolyte interface, the Li+ diffusion across the CEI layer as well as the resistance from the side reaction products is still unclear. Recently, by observing the 3D strain evolution throughout a single LiNi0.5Mn1.5O4 nanoparticle during the charging and 3589


Review


by the formation of a cathode/electrolyte interface layer with low ionic conduction, which is consistent with the pulverization phenomenon detected by Goodenough et al.88 Figure 14a,b shows the average lattice constant of the single LiNi0.5Mn1.5O4 particle and corresponding open circuit voltage during charging and discharging, respectively. The remarkable deviation of measured lattice constant from the theoretical value (connected particle behavior) after 8 h indicated the changed conduction pathways between particles after charging and discharging. After 8 h, the lattice constant changed slightly in the theoretical direction. This suggested that the particle still respond but ion transport was much slower than before, which was consistent with the formation of a LiNi0.5Mn1.5O4/electrolyte interface layer with poor ion conductivity. The 3D strain evolution throughout a single LiNi0.5Mn1.5O4 nanoparticle during charging and discharging process further demonstrated the reason for disconnection (Figure 14c−f). For the discharged state (t = 6 h, disconnection happened at this state, Figure 14d), both the core and the shell had significant strain inhomogeneity. In addition, the strain increased from the top to the bottom of the particle. According to the knowledge of phase transition evolution in LiNi0.5Mn1.5O4 during cycling, this discharged state was in the solid solution regime. Thus, the compressive/tensile strain can be attributed to Li poor and Li rich regions, respectively. It means that the strain evolution is accompanying with the inhomogeneous lithiation. After the disconnection (t = 7 and 15 h, Figure 14e,f), a large tensile strain region still existed in the particle boundary, which may be caused by the lithium trapped in LiNi0.5Mn1.5O4/electrolyte interface layer. Furthermore, an ionic blockade layer different from CEI has also been proposed on the LiNi0.5Mn1.5O4/electrolyte interface. By comparing the coherent X-ray diffractive data with electrochemical data measured simultaneously on the same operando battery, Singer et al.154 detected a hysteretic structure transition upon cycling (displayed in Figure 15) and attributed this hysteretic behavior to an ionic blockade layer formed on the LiNi0.5Mn1.5O4/electrolyte interface. This blockade layer was formed during charging to about 4.7 V, whereas it cannot be found during discharging. This blockade layer seems not to be CEI, because CEI equally exists during charging and discharging and builds up with cycling.155 However, this conclusion was obtained based on the comparison between electrochemical and coherent X-ray diffractive data, instead of direct experimental characterization at this interface. Thus, the LiNi0.5Mn1.5O4/electrolyte interface becomes much more intricate and need further research to understand these interfacial characteristics and reactions. 2.3. LiNi0.5Mn1.5O4/Anode Interaction. Though the cathode and anode are spatially separated by the separator within a battery, they are actually not isolated from each other. Growing research has evidenced the interactions between each other and emphasized a holistic system approach to address these strongly interrelated processes.25 Material exchange between electrodes, like interfacial species migrating from one electrode to another, is the direct evidence of cathode/anode interactions. The material exchange between cathode and anode was once detected in secondary amorphous V2O5/Li cell,156 as the cathode species dissolved in the electrolyte and migrated through the separator and then deposited on the Li anode. As for LiNi0.5Mn1.5O4 in LIBs, two forms of compounds exchange between two electrodes have also been reported.31,32,35,157,158 One form is the deposition of transition

Figure 19. (a) Conceptual illustration demonstrating a selective Li+ transport through the solid electrolytes while blocking decomposed products (DP). TEM images of cycled graphite electrode using (b) normal coin cell (inset is an EDS spectrum of nanoparticle, Cu signal is from a sample grid) and (c) MEC. The charge−discharge voltage curves of LiNi0.5Mn1.5O4 full cells with C/20-rate at 25 °C measured by (d) coin cell and (e) MEC. Reproduced with permission.171 Copyright 2013, Elsevier.

Table 2. General Requirements for Separators Used in Lithium-Ion Batteriesa parameter chemical and electrochemical stabilities wettability mechanical property thickness pore size porosity permeability (Gurley) dimensional stability thermal stability shutdown

requirement stable for a long period of time wet out quickly and completely >1000 kg cm−1 (98.06 MPa) 20−25 μm

Surface and Interface Issues in Spinel LiNi0.5Mn1.5O4: Insights into a

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