Surface and Interface Issues in Spinel LiNi0.5Mn1 ... - ACS Publications

Subscriber access provided by ORTA DOGU TEKNIK UNIVERSITESI KUTUPHANESI

Review

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 Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00948 • Publication Date (Web): 13 May 2016 Downloaded from http://pubs.acs.org on May 17, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 93

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Chemistry of Materials

Surface and Interface Issues in Spinel LiNi0.5Mn1.5O4: Insights into a Potential Cathode Material for High Energy Density Lithium-Ion Batteries Jun Maa, Pu Hua, Guanglei Cui*a, Liquan Chena, b a

Qingdao Industrial Energy Storage Research Institute, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China b

Key Laboratory for Renewable Energy, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

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 PHEV/EVs. 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

1

ACS Paragon Plus Environment

and

strategies

for


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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, plug-in hybrid electric vehicle (PHEV) and pure electric vehicle (EV) are expected to play a significant role in relieving 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, lithium-ion battery (LIB) becomes more and more popular because of its high energy/power density, long lifespan, less memory effect, and low self-discharging rate.1-6 Nowadays, LIB has been utilized in golf carts, electric bicycles, utility vehicles, and cars, such as Tesla. Nevertheless, today’s state of art in LIBs 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 reduce their cost. Since 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 present market.15 According to recent research, the potential cathode materials for power LIBs are layer-structured transition metal oxides LiMO2 (M = Ni, Co, Mn, Al), olivine structure 2


Page 2 of 93

Page 3 of 93

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Characteristics of various cathode materials for LIBs based on cell level.15 Cycle Weight

Volume

Weight

Volume

Average specific

Working potential voltage

Characteristics / V vs. Li/Li+

specific

power

energy / Wh

energy / W

kg

L

-1

kg

-1

Pulse

rate

under 100%

rate capability

capability /

DOD (until

/C

C

80%

energy / W

V -1

Continuous power

/ energy / Wh

L

-1

capacity)

170-240, LiCoO2/graphite

2.5-4.2

general; cylinder;

a

NMC /graphite

2.5-4.35

NCAa/graphite

undetermined

400-640,

3.7 130-200,

~1000

~2000

2-3

5

>500

250-450

polymer

NMC/graphite 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

a

LMO /graphite

LiFePO4/graphite

Note: a NMC, NCA, and LMO represents LiNi1-x-yMnxCoyO2, Li(NiCoAl)O2, and LiMn2O4, respectively.

LiFePO4, and spinel lithium manganese oxides.8,9,14,16-18 The energy density of recent commercial cathodes for power LIBs are shown in Figure 1. 15,19,20 Though Li(NiCoAl)O2 with high energy density have been used in Tesla vehicles successfully, the expensive and toxic Co element will raise cost and hinder mass production of EVs. For LiFePO4, though displays high safety and cycling performance, its low volume specific energy and complicated production process bring difficulties to the reduction of vehicle 3


life


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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

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 problem of poor high temperature stability. Ni doping in LiMn2O4 (LiNi0.5Mn1.5O4) is reported to be an effective method to solve its 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 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 4


Page 4 of 93

Page 5 of 93

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 field is not only determined by its own electrochemical performance, but also depends on the understanding and simultaneously 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-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.

2. Surface and Interface Chemistry In a typical configuration of LIB, electron migration and Li ion diffusion take place 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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

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. 6


Page 6 of 93

Page 7 of 93

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 attention. Therefore, a comprehensive knowledge of the surface chemistry 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 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

S1 shows the different electrochemical performance of LiNi0.5Mn1.5O4 with various crystal orientations. In general, when the measuring parameter is determined, LiNi 0.5 Mn1.5 O4 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 coefficient 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.

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. 8


Page 8 of 93

Page 9 of 93

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


corresponded to (111) crystal planes. Both samples had similar average size of 2 mm. 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 plate-shaped 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.

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.

9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. The 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.

The other reason is thought to be the superior interface stability between (111) crystal plane and electrolyte. Manthiram et al.75 prepared the octahedral particles 10


Page 10 of 93

Page 11 of 93

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


consisted entirely of the {111} family of planes (name as Poly 1) and the truncated octahedral particles comprised 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 less dense arrangement of Li ions, higher surface energy and seemed to be more vulnerability for Mn dissolution. Furthermore, stable CEI has been reported on the (111) plane in spinel cathode.39 Thus, the (111) surface planes in Poly 1 was 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 between 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 simply compare 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 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 shorten lithium ion diffusion path, while 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 further understand 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/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 12


Page 12 of 93

Page 13 of 93

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 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 room-temperature conductivities of LiNi0.5Mn1.5O4 also showed that the electronic conductivity of cation disordered Fd-3m 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 Mn3+ (absence in ordered LiNi0.5Mn1.5O4), 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 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), 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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.

they observed pulverization of the LiNi 0.5 Mn 1.5 O4 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+ 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 Ni-deficient on the 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 14


Page 14 of 93

Page 15 of 93

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 post annealing at 700 °C. Reproduced with permission.90 Copyright 2012, American Chemical Society.

(EIS) results in the following 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 work 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 surface sensitive characterization techniques and electrochemical methods. 2.1.3 Surface structure transformation Due to 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 coworkers95 firstly studied the local atomic structure of LiNi0.5Mn1.5O4 through 16


Page 16 of 93

Page 17 of 93

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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

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) 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 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 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

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. 18


Page 18 of 93

Page 19 of 93

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 further understand the influence of surface structure changes on the electrochemical performance of LiNi0.5Mn1.5O4 in the charge 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 electrode is required within the electrochemical window of 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, Ni 3+ /Ni 2+ and Ni 4+ /Ni 3+ , 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 on the surface of LiNi0.5Mn1.5O4 as cathode-electrolyte interface (CEI) layer and result in capacity fading and reduced cycle life. Furthermore, Ni and Mn ion dissolution is usually accompanied with the electrolyte 19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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.

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 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 crucial to the cycling life of LiNi0.5Mn1.5O4-based LIBs. Several reviews have highlighted the effect 20


Page 20 of 93

Page 21 of 93

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 stablize the interface during subsequent cycling, its protective function is still doubted due to the incomplete coverage on cathode surfaces.105,106 To better understand 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 film-formation mechanism caused by electrolyte decomposition became predominant during subsequent cycling. Recently, Browning et al.108 proposed an in situ characterization of the LiNi 0.5 Mn 1.5 O 4 /electrolyte interface by neutron reflectometry (NR) to overcome the drawbacks of ex situ analyses and avoid the 21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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.

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 22


Page 22 of 93

Page 23 of 93

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 firstly 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

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.

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 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 believed 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 coating111,116-124 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. 24


Page 24 of 93

Page 25 of 93

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 13. Electrodes cycled with (a) C/4, (b) C/2, and (c) 5 C for approximately 25 cycles. Green, Mn; red, Ni; and 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.

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 firstly 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 25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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, due to different solvation energy in various electrolyte, 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 EC-DMC and EC-ethyl methyl carbonate (EMC) have similar rate capability and better rate performance than EC-DEC. Besides the different Li+ ion conductivity in each electrolyte, the interfacial Li+ ions diffusion rate may be another reason for their distinct rate performance. In addition, based on the calculation of spin-polarized total energy and semi-empirical model, Seyyedhosseinzadeh et al.152 obtained the diffusion coefficient for Li+ at the LiNi0.5Mn1.5O4/electrolyte (1M LiClO4 in EC/DMC (1:1)) interface is 10-8 cm2 s-1, which is three 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 charging and discharging process based on the coherent X-ray diffractive imaging (CXDI) technique, Ulvestad et al.153 detected particle disconnection 26


Page 26 of 93

Page 27 of 93

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


in LiNi0.5Mn1.5O4 and considered it was caused 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 hours indicated the changed conduction pathways between particles after charging and

Figure 14. (a) Lattice constant evolution during charging and discharging. Yellow highlighted points correspond to the states shown in (c). (b) The open circuit voltage at which the measurements were taken. (c-f) The strain evolution in both the core and the shell of the particle. Reproduced with permission.153 Copyright 2015, Royal Society of Chemistry. 27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 93

discharging. After 8 hours, 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 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 of disconnection (Figure 14c-f). For discharged state (t = 6 h, disconnection has happen 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 h and 15h, 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 28


Page 29 of 93

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


about 4.7 V, while 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

Figure 15. (a) Modulus of the applied current during charge and discharge as a function of measurement time shown on logarithmic scale. (b) The 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) The hysteretic structure transition upon cycling. Reproduced with permission.154 Copyright 2014, American Chemical Society.

29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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 metal ions on the surface of anode, and the other one is the oxidized or reduced electrolyte species migrated across the separator and reach the opposite electrode. 2.3.1 Deposition of transition metals Recently, opinions on the oxidation state of transition metals deposited on the anode are still controversial. Komaba et al.159 once considered the reduction of Mn2+ (Mn2+ + 2LiC6 → Mn + 2Li+ + graphite) on the anode surface of the graphite/Li half-cell resulting 30


Page 30 of 93

Page 31 of 93

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


from the much higher standard redox potential of Mn/Mn2+ (1.87 V vs. Li/Li+) than that of the Li+ intercalation into graphite (< 0.3 V vs. Li/Li+). When LiNi0.5Mn1.5O4 works at high voltage, dissolution of Mn2+ and Ni2+ occurs due to the surface disproportion reaction (2Mn3+ → Mn2+ + Mn4+)86 and side reaction96,97,102,103 between LiNi0.5Mn1.5O4 and electrolyte. Since the standard redox potential of Ni/Ni2+ (2.80 V vs. Li/Li+) is much higher than that of Mn/Mn2+, both Mn and Ni deposition and reduction could happen on the anode surface in LiNi0.5Mn1.5 O4/Li half cells or LiNi0.5Mn1.5O4/graphite full cells.31,32,35 Kim et al.35 analyzed the TEM and EDX measurements of graphite anode

Figure 16. (a-d)The 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. 31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

electrode from a 100-cycle LiNi0.5Mn1.5O4/graphite full cell, as displayed in Figure 16. They regarded the randomly distributed particles containing Ni and Mn elements on graphite surfaces as Mn metals and MnNi alloy and attributed their deposition and reduction to the promoted loss of active Li+ ions through the formation of thick SEI and rapid capacity fading of full cell. However, no further and effective evidence of Ni0 and Mn0 on the anode surface was supplied. In contrast, Park et al.124 observed Mn2+ and Ni2+ signals on graphite electrodes cycled against LiNi0.5Mn1.5O4 through XAS measurements. Furthermore, Xiao et al.122 and Delacourt et al.157 also detected Mn2+ signals on graphite by means of XPS and soft X-ray spectroscopy, respectively. They hypothesized the oxidation of Mn0 caused by the air exposure during sample manipulation prior to measurement or the reaction between Mn0 and electrolyte solvent molecules. Another opinion about the reactions during Mn2+ deposition on the anode surface was a metathesis reaction between Mn2+ and some species on the SEI, instead of a reduction reaction resulting in the formation of Mn metal.160 Thus, it is still need further research to explore the exact composition, nature, formation mechanism of Ni2+ and Mn2+ ions on the anode surface and understand their interactions with SEI. 2.3.2 Deposition of electrolyte decomposition products The migration of oxidized or reduced electrolyte species between two electrodes was concluded by Dedryvere et al. 1 0 7 based on the investigation on the electrode/electrolyte interfaces in LiNi0.5Mn1.5 O4/Li 4 Ti5 O12 full cells. In general, Li4Ti5O12 is considered as a SEI-free electrode material, but in this case, organic species 32


Page 32 of 93

Page 33 of 93

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


were measured on its surface. Reason for this phenomenon was inferred as side reactions firstly took place at the positive electrode and then the products adsorbed at the Li4Ti5O12 electrode surface, either by diffusion or migration of the organic species. Recently, the diffusion of electrolyte oxidation products into electrolyte has been visually demonstrated by Norberg et al.161 They observed the in situ fluorescence signal from a single LiNi0.5Mn1.5O4 particle during cyclic voltammetry measurements of a binder- and carbon-free LiNi0.5Mn1.5O4 electrode (Figure 17). During the cathodic scan, the fluorescence intensity degraded gradually, indicating that most of the fluorescent

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. 33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

electrolyte decomposition products originally accumulated at the surface dissolution of LiNi0.5Mn1.5O4. With cyclic voltammograms (CV) scan number increasing, the residual fluorescence signal at the end of each cathodic scan increased, suggesting some fluorescent electrolyte decomposition products remain at the particle surface of LiNi0.5Mn1.5O4 and accumulate gradually during cycling. However, the effect of soluble fluorescent electrolyte decomposition products on the surface chemistry and stability of anode remains unknown. 2.3.3 Strategies To solve the problem of material exchange between LiNi0.5Mn1.5O4 and anodes, the most important work is to prevent the dissolution of Ni and Mn ions as well as the decomposition of electrolyte on the surface of cathode. In fact, great efforts have been made to suppress these side reactions by cathode surface coating111,119,120,122-124,162-164 and doping,90,125,126 morphology design with oriented growth crystal planes,78 and modification of electrolyte components96,129,130,151,165-167 and additives.137,140,141,145,168-170 Table S2 displays the electrochemical performance of LiNi0.5Mn1.5O4 under these traditional modification strategies to suppress the interface side reactions of LiNi0.5Mn1.5O4/electrolyte and LiNi0.5Mn1.5O4/anode. Kim et al.126 firstly reported the in-situ formation of Ti-O enriched CEI layer on the surface of Ti-substituted LiNi0.5Mn1.2Ti0.3O4 (LNMTO) and demonstrated its function of mitigating electrolyte oxidation and transition metal dissolution. Compared with LiNi0.5Mn1.5O4 (LNMO), LNMTO with Ti-O enriched CEI layer has much higher 34


Page 34 of 93

Page 35 of 93

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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) The 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.

capacity retention and Coulombic efficiency when paired with graphite anode (Figure 18a, b). Furthermore, the LNMTO/graphite full cell shows much slower growth rate of Rfilm than that of LNMO/graphite full cell during cycling (Figure 18c), which indicates the lower amounts of electrolyte oxidation products at the surface of LNMTO/graphite cell. The outstanding electrochemical performances of LNMTO/graphite supports that detrimental side reactions at the cathode/electrolyte interface are mitigated due to the presence of Ti-O enriched CEI. The improvement mechanism of LNMTO is explained as following (as illustrated in Figure 18d): (1) the selective Mn and Ni dissolution and 35



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

concurrent formation of Ti-O enriched CEI layer on the surface of LNMO during the first cycle, (2) in the subsequent cycling, the Ti-O enriched CEI layer suppresses the electrolyte oxidation and HF generation as well as the transition metal ions dissolution, (3) less transition metal ions dissolution results in reduced transition metal deposition on the graphite surface and active Li+ consumption on the SEI layers. It is interesting and enlightening that MahootcheianAsl et al.171 designed a multilayer electrolyte cell (MEC) to eliminate the Mn2+ dissolution problem on graphite negative electrode. Different from the conventional cell, the MEC consists of two liquid electrolytes separated by a solid electrolyte (Li1+x+yTi2-xAlxP3-ySiyO12) which prevents any electrolyte and any potential oxidation or reduction products to pass over while selectively transports Li+ ions, as illustrated in Figure 19a. This design successfully offers individual positive and negative electrode/electrolyte interfacial reactions in a single cell. As a result, the MEC blocked Mn2+ ions migration from LiNi0.5Mn1.5O4 to graphite and therefore prevented the Mn2+ ions deposition and reduction on the surface of graphite, which were proved by comparing the TEM images of graphite cycled in normal coin cell and MEC (shown in Figure 19b, c). Therefore, the MEC had higher capacity and better capacity retention than the normal coin cell, as demonstrated by the electrochemical performance in Figure 19d, e. The Mn2+-capturing separator and functional binder has also been demonstrated to be effective in reducing the Mn2+ deposition on the anode surface in LiMn2O4-based LIBs, which have the similar Mn ion dissolution issue. 172-175 However, these strategies have not been applied in 36


Page 36 of 93

Page 37 of 93

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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.

37



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

LiNi0.5Mn1.5O4-based LIBs. It is suggested to extend the Mn2+-capturing separator and binder to LiNi0.5Mn1.5O4-based LIBs. 2.4 LiNi0.5Mn1.5O4/separator/electrolyte interfaces

Table 2. General requirements for separators used in lithium-ion batteries. Reproduced with permission.176 Copyright 2014, Royal Society of Chemistry.

Parameter

Requirement

Chemical and electrochemical stabilities

Stable for a long period of time

Wettability

Wet out quickly and completely

Mechanical property

>1000 kg cm-1 (98.06 Mpa)

Thickness

20-25 µm

Pore size

Surface and Interface Issues in Spinel LiNi0.5Mn1 ... - ACS Publications

Recommend Documents