Mechanism Study on the Interfacial Stability of a Lithium

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Article

Mechanism Study on the Interfacial Stability of a LithiumGarnet-type Oxide Electrolyte against Cathode Materials

Nian Zhang, Xinghui Long, Zhi Wang, Pengfei Yu, Fudong Han, Jiamin Fu, Guoxi Ren,Yanru Wu, Shun Zheng, Wencheng Huang, Chunsheng Wang, Hong Li, and Xiaosong Liu

ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01035 • Publication Date (Web): 02 Oct 2018

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Mechanism Study on the Interfacial Stability of a

Lithium Garnet-type Oxide Electrolyte against

Cathode Materials

Nian Zhang,† Xinghui Long,

†, ∆ Zhi Wang,

‡ Pengfei Yu,

† Fudong Han,

§ Jiamin Fu,

†, ∆ GuoXi

Ren,†, ∆

Yanru Wu,†, ∆

Shun Zheng,†, ∆

Wencheng Huang,†, ∆

Chunsheng Wang, § Hong Li,

#,@ and

Xiaosong Liu*,†,⊥,@

† Center for Excellence in Superconducting Electronics, Shanghai Institute of Microsystem and

Information Technology, Chinese Academy of Sciences, 200050, China

‡ Renewable and Sustainable Energy Institute, University of Colorado, Boulder, Colorado 80309,

United States

§ Department of Chemical and Biomolecular Engineering, University of Maryland, College Park,

Maryland 20742, United states

# Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese

Academy of Sciences, Beijing 100190, China

∆ University of Chinese Academy of Sciences, Beijing 100049, China

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ACS Applied Energy Materials

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⊥ School of Physical Science and Technology, Shanghai Tech University, Shanghai 201210,

China

@ Tianmu Lake Institute of Advanced Energy Storage Technologies, Liyang City, Jiangsu

213300, China

* E-mail: [email protected]

KEYWORDS: Garnet-type oxide electrolyte, Solid electrolyte, Thermal stability, Co-sintering,

Interface reaction

ABSTRACT: All-solid-state lithium ion battery is considered as one of the most promising next-

generation battery technologies. Understanding the interfacial evolution of a solid electrolyte and

a cathode electrode during mixing and sintering is of great importance, which can provide

guidance to avoid forming unwanted compounds and decrease the interfacial resistance. In this

work, chemical compatibilities are investigated between a Ta-doped Li7La3Zr2O12 (LLZO) solid

electrolyte and major commercial metal-oxide cathodes LiCoO2 (LCO) and Li(NiCoMn)1/3O2

(NCM) through ball-milling and co-sintering processes. As revealed by X-ray absorption

spectroscopy and transmission electron microscopy, LLZO spontaneously covers the majority of

the large LCO and NCM particles with a thickness of ~100 nm after ball milling. The thickness

of LLZO layer on these cathodes decreases to about 10 nm after co-sintering at 873 K, and an

interfacial layer of approximately 3 nm is observed for NCM/LLZO. LCO shows a higher

thermal stability than NCM. Density functional theory (DFT) based simulations and

electrochemical measurements suggest Ni-La and Ni-Li exchange could happen at the

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NCM/LLZO interface and Li can diffuse from the interface into NCM to occupy the Ni vacancy

at high temperature. The Li depletion layer after diffusion at the interface induces the

decomposition of LLZO and the formation of La2Zr2O7 and LaNiO3 interfacial layer.

INTRODUCTION

Rechargeable Li-ion batteries (LIBs) have been successfully used for consumer electronics in the

past three decades, and are also the devices of power source for emerging technologies including

electric vehicles (EV) and grid-scale energy storages.1-4

However, most current commercialized

LIBs utilize flammable liquid electrolytes which are thermodynamically unstable against lithium

metal and cannot work at high voltages.5-6

These defects limit the capacity and cause the severe

safety and durability issues. Therefore, all-solid-state Li-ion batteries (SSBs), which employ

nonflammable solid electrolytes with a better chemical stability,7 are considered as a promising

next-generation rechargeable battery technology.

Among solid electrolytes, inorganic solid oxide electrolytes (SOEs) have very high chemical and

thermal stability in air and never release toxic gases even in the case of breakdown. However, the

low lithium ion conductivity of SOEs and their insufficient contact with electrodes hinder their

applications.8-11

An important breakthrough was achieved in 2007 by the discovery of a garnet-

type oxide electrolyte, Li7La3Zr2O12 (LLZO), with a high lithium conductivity of ~2×10-4

S cm-

1,12-15

and ~8×10-4

S cm-1

with Nb doping16

at room temperature. SSBs using Nb-doped LLZO

pellets and LiCoO2 (LCO) cathode exhibit a good electrochemical cyclability.16-18

Nevertheless,

the rigidity of the garnet leads point contact between the solid electrolyte and electrode materials,

which limits the active sides and causes high interfacial resistance for SSBs. For anode,

depositing a thin Ge or Au layer on garnet was found greatly decreases the interfacial resistance

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because an alloying reaction occurs between the Li metal and Ge or Au.19-20

Goodenough and co-

workers introduced a cross-linked polymer between garnet and Li metal to effectively improve

their contact.21

But on cathode side, few methods have been found to modulate the interface

effectively. Although electrochemical activities can be achieved through preparation of cathode

using the pulsed laser deposition method,16

a diffusion layer during high temperature deposition

increases the interfacial resistance.22

Introducing additives in the cathode, such as a mixture of

the cathode and the solid electrolytes materials23-24

as the cathode electrodes 25-26

is another way

to modulate the interface. An essential high temperature co-sintering during the mixture to

increase interfacial contact which still leads to unavoidable chemical side reactions. Rapid

thermal annealing decreases the high interfacial resistance effectively and maintains the chemical

stability of both materials,27

however, The harsh reaction conditions of this method make it

difficult to be widely applied.

To find effective methods to modulate the cathode/garnet interface, it is prerequisite to clarify

the mechanisms of the interfacial reaction at high temperture.28

The formation and stability of the

LCO/garnet interface during co-sintering is still under debate. Some experimental studies

demonstrated the formation of an interfacial layer containing Co, La, and O with a large

interfacial resistance,29-30

while others did not observe this phenomenon.16

Miara et al. explored

the cathode/electrolyte interphases for lithium garnets using first-principles calculations and

showed that cathode/electrolyte interphases are more sensitive to chemical decomposition at high

sintering temperature than to electrochemical decomposition during cycling.31

Understanding the

mechanism and stability of novel solid electrolytes against mixing and sintering with major

commercial oxide cathodes can provide guidance to avoid forming unwanted compounds, to

decrease the interfacial resistance, and to create an excellent coating layer.

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In this work, we investigated the thermal stability and electronic structure of mixture of Ta-

doped Li6.75La3Zr1.75Ta0.25O12 (LLZO) and two cathode materials, LiCoO2 (LCO) or

Li(NiCoMn)1/3O2 (NCM), respectively. The mixed samples were treated through a ball-milling

procedure and co-sintered in ambient air at 873 K for 1 h. We found that LLZO spontaneously

covers the surface of the cathode materials after ball-milling, and the thickness of the surface

layer is reduced to the nanoscale after co-sintering. Our work provides comprehensive insight

into the evolution of solid electrolyte/cathode interface and the impact of the surface morphology

through simple ball-milling and co-sintering processes, which can guide element selectivity and

the pre-treatment of the solid electrolyte and electrode.

METHODS

Ta-doping was used to stabilize the cubic phase of garnet. Li6.75La3Zr1.75Ta0.25O12 (designated as

LLZO) was prepared using a solid-state reaction. Starting materials of LiOH·H2O (99.995%,

Sigma Aldrich), La(OH)3 (99.9%, Sigma Aldrich), ZrO2 (99.99%, Sigma Aldrich), and Ta2O5

(99.99%, Sigma Aldrich), were weighed and mixed based on the stoichiometric ratio. 10 wt. %

excess of LiOH·H2O (10%) was used to compensate for Li loss during the high-temperature

calcination and sintering. The mixture was ball-milled (PM 100, Retsch) in 2-propanol for 24 h

with zirconia balls in a zirconia vial, and then dried and heated in air at 1223 K for 12 h. The

ball-milling and heating were repeated once to enhance the purity.

We found that LLZO spontaneously covers the surface of the cathode materials after ball

milling, and the thickness of the surface layer was dependent on the rate of ball-milling. To

obtain an electrolyte/electrode composite with a suitable size for our soft X-ray absorption

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spectroscopy (sXAS) measurements and magnify the interface reaction, 50wt% LLZO powder

was dry ball-milled with 50wt% commercial LiCoO2 and Li(NiCoMn)1/3O2 (average particle

size:~5µm) at 100 rpm (PM 100, Retsch) for 1h, separately. The composite was then heated at

873 K for 1 h in air. The heating/cooling rate was 5 K/min. Powder X-ray diffraction (XRD)

patterns were obtained using a D8 Advance diffractometer with LYNXEYE and Sol-XE

detectors (Bruker AXS, WI, USA) using Cu Kα radiation.

The soft X-ray absorption spectroscopy (sXAS) experiments were performed at beam line 20A at

the National Synchrotron Radiation Research Center (NSRRC) in Taiwan. The bending magnet

beamline provide photons with energy range from 60 to 1250 eV and the average energy

resolving power of 5000. The L-edge of Co, Mn, and Ni and the K-edge of O X-ray absorption

spectra were collected using surface-sensitive total electron yield (TEY) and bulk-sensitive total

fluorescence yield (TFY) modes simultaneously at room temperature in an ultra-high vacuum

chamber with a base pressure of approximately 5×10-10

Torr. The photon energy was calibrated

with the spectra of the reference samples (CoO, MnO, NiO, and SrTiO3), which were measured

simultaneously. The X-ray absorption near edge structure (XANES) experiments were carried

out at beam line 14W at the Shanghai Synchrotron Radiation Facility (SSRF). Transmission

electron microscopy (TEM) images were collected using a JEOL-2010F electron microscope

operated at 200 kV.

Electrochemical tests were performed in R2032 coin-type cells with lithium foil as the negative

electrode. The positive electrode was fabricated by blending the cathode material, polyvinylidene

fluoride (PVDF), and acetylene black (84:7:7) in N-methyl-2-pyrrolidone (NMP). The slurry was

coated onto the aluminum foil current collector and dried under vacuum at 393 K for 12h prior to

use. The coated foil was cut into 15-mm-diameter circular discs. The electrolyte solution was

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1.0M LiPF6 in an ethylene carbonate (EC)–dimethyl carbonate (DMC) mixture (1:1 ratio by

volume). All cells were assembled in an Ar-filled dry glove box (O2, H2O below 2 ppm).

Galvanostatic charge and discharge cycle tests were carried out at room temperature between 2.5

and 4.4 V using a Biologic electrochemical workstation. The capacity was reported based on the

mass of LCO and NCM.

The first-principles density functional theory (DFT) calculation was performed using the Vienna

Ab-initio Simulation Package (VASP). The practical NCM primary particles were enclosed by

many different facets. Our NCM/LLZO interface for DFT calculation was constructed as

NCM(101�0)/LLZO(100), where NCM had a space group of R-3m, and LLZO was in a slightly

distorted cubic structure (�

�= 1.037). We chose this interface for the following reasons: NCM

(101�0) is one of the most stable facets32

and is reported to provide apparent Li-ion conduction

paths,33

thus having been used in many previous works.34-35

Moreover, we have done the

geometry check for NCM/LLZO interfaces and found that NCM(101�0)/LLZO(100) has the

smallest lattice mismatch (~8.3%), which not only indicates a good stability but also reduces

complexity of modeling. The detailed explanations of the interface modeling and computational

parameters have been given in Supporting Information. To describe the possibility of atom

exchange at the interface, for every pair of atoms {��, ��} that may exchange with each other, we

calculated its exchange energy as

�� ↔ �� = �� ↔ �� − ��

where �� and �� ↔ �� are the DFT total energies of the pristine system and the

system with atom exchange �� ↔ �� , respectively.

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RESULTS AND DISCUSSION

The XRD pattern of the Ta-doped LLZO powder is shown in Figure 1 and matches well with

standard cubic garnet phase.13, 36

After ball-milling process with LCO and NCM (red lines), the

peaks show only the superimposed signal of two compounds, which indicate that no chemical

reaction occurred during the physical mixing. However, peaks labeled with asterisks (*) which

stand for the pyrochlore phase of La2Zr2O7 (JC-PDF, card No.50-0837) can be observed in both

co-sintered samples (blue lines), indicating that side reactions happen at 873K. To confirm the

instability of LLZO is caused by mixed sintering with cathode materials, we heated LLZO

powder individually in ambient air at 873 K for 1 h (see Figure S2), and it still remained pure

cubic phase. The peak intensities of La2Zr2O7 are much stronger for the co-sintered NCM+LLZO

than co-sintered LCO+LLZO, which indicate that Ni or Mn in NCM may influence the

interfacial stability during co-sintering. A peak at approximately 32° labeled with a diamond (♦)

is only observed in co-sintered NCM+LLZO sample which could be assigned to LaCoO3 (JC-

PDF, card No.48-0123), LaMnO3 (JC-PDF, card No.33-0713), or LaNiO3 (JC-PDF, card No.33-

0711). The positions of characteristic peaks for these three compounds are too close to be

distinguished in the XRD spectra. Thus XAS is used to identify the chemical composition, which

will be discussed in detail below. When the co-sintering temperature increases to 973 K, both

LCO+LLZO and NCM+LLZO samples are substantially decomposed and LaCoO3 is observed in

the LCO+LLZO sample as shown in Figure S3. The results reveal that the Li, La, and transition

metals (TMs; i.e., Ni, Co, and Mn) are active elements for high temperature diffusion and side

reactions at the interface.

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Figure 1. Powder XRD patterns show the impact caused by the ball-milling and co-sintering

processes for LCO+LLZO and NCM+LLZO samples; cubic LLZO, LaNiO3 and La2Zr2O7 are

displayed as reference. New peaks which are marked with asterisks (*) appear in both co-

sintered samples; the peaks at approximately 21.7° and 32° marked with a diamond (♦) only

appear in the co-sintered NCM+LLZO sample.

Soft XAS was employed to estimate the composition of the peak at approximately 32° in the

NCM+LLZO 873K sample. Both surface-sensitive TEY (probing depth ~10 nm) and bulk-

sensitive TFY (investigation depth ~100 nm) modes were collected simultaneously and

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compared directly to obtain depth-dependent chemical information.37-38

For all 3d transition

metal (TM) elements, XAS directly measures the unoccupied 3d states through dipole-allowed

2p-to-3d transitions.39-40

Comparing our XAS results with those for the reference samples (CoO,

MnO, NiO), we conclude that Co remains in the +3 valence state in pristine LCO and NCM; Ni

remains mainly in the +2 and may contain a small amount of +3 state as the shoulder C in Figure

2 is higher than reference sample NiO; and Mn is in the +4 state in NCM.

The sXAS results shown in Figure S4-S6 indicate Co and Mn is stable in both LCO+LLZO and

NCM+LLZO samples after ball-milling and co-sintering processes, since no dramatic peak

changes are observed. The TFY spectra of Mn L-edge is significantly distorted due to the severe

self-absorption effect. Therefore, we instead measured the Mn K-edge with larger probing depth

to detect the bulk variations. No peak position and line shape changes are found in the Mn K-

edge spectra (see Figure S7), which indicate Mn is stable in the bulk, thus the XRD peak at

approximately 32° in NCM+LLZO 873K sample may only represent LaNiO3.

To confirm the formation of LaNiO3 after co-sintering, Ni L-edge TEY and TFY spectra were

measured at different stages and are shown in Figure 2. The overall spectral line-shape consists

of three main features (A, B, and C in the graph). In the spectra, peak A represents the La M-

edge, while B and C represent the Ni L3 edge. The energy separation between B and C is 1.5 eV

resulting from the multiplet effect splitting. The relative intensities of B and C indicate the

oxidation state of Ni is mainly in +2 state in pristine NCM. The unexpected strong intensity of

the La M-edge peak (A) in the TEY spectra reveals that the LLZO is the main composition

within the detected 10-nm-thick surface layer after ball-milling and co-sintering. The intensity of

peak A becomes much lower in the TFY spectra than in the TEY spectra after ball-milling (red

line), and almost disappears after co-sintering. These results illustrate that LLZO covers the

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surface of NCM with a thickness between the probing depth of TEY (10 nm) and TFY (100 nm)

after ball-milling, while the thickness of the surface layer further decreases to about 10nm after

co-sintering. The simplified core-shell model is shown in Figure 2(c), NCM materials are

usually big secondary particles, small LLZO particles after ball-milling cover the majority of

large NCM particles with a non-uniform thickness. The thickness of LLZO layer decreases to

around 10nm and partial NCM may be exposed after co-sintering at 873K. The intensity ratio of

peak B and C does not change after ball-milling, while the intensity of peak C significantly

increases in both the TEY and TFY spectra after co-sintering. This variation reveals that Ni in

both surface and bulk material is partially oxidized from the +2 to +3 state, which supports the

formation of LaNiO3 with Ni 3+ and is consistent with our XRD results. In addition, previous

study showed that Li2CO3 is commonly formed on the surface of LLZO when exposed to

ambient air.41

However, we do not observe the diffraction peak of Li2CO3 in any of our XRD

patterns, indicating that Li2CO3 is probably amorphous at the surface. To confirm whether

Li2CO3 participates in the side reactions at the interface during the co-sintering process, we co-

sintered pure Li2CO3 with NCM at 873 K in ambient air for 1 h. No clear valence change of Ni

(pink line in Figure 2) is found, demonstrating that NCM reacts with LLZO rather than Li2CO3,

which causes the changes in the XRD patterns.

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Figure 2. Ni L-edge XAS spectra of NCM+LLZO samples detected in (a) TEY mode and (b)

TFY mode. The spectrum of co-sintered NCM+Li2CO3 is shown to confirm that Li2CO3 is not

the reason for the valence increase of Ni. The inset shows the comparison of the Ni L3 edge. (c)

A brief schematic illustration for the alteration of surface topography during the ball-milling and

co-sintering process.

Figure 3 (a) and (b) shows the normalized O K-edge TEY and TFY spectra of LCO+LLZO and

NCM+LLZO samples, and the pure Li2CO3 is exhibited as a reference sample. The pure Li2CO3

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spectrum (bottom) has a main peak (C) at 534.5 eV, which is assigned to electron transition from

the O 1s orbital to the π* (C=O) antibonding orbital.38

The O K-edge TEY spectrum of LLZO is

quite similar with pure Li2CO3 indicates that the surface of LLZO is fully covered with Li2CO3

(>10 nm thick) after exposure to air. The shoulder B at 532.5 eV in the TFY spectrum is

consistent with the oxygen of LLZO,41

thus the thickness of the Li2CO3-covered layer is much

less than the typical detection depth of TFY mode ~100 nm. When LLZO is ball-milled with

LCO, peak A representing the oxygen of LCO is almost invisible in the TEY spectrum (red line)

and is strong in the TFY spectrum. This phenomenon confirms our conclusion that LLZO covers

the surface of the electrode materials after ball-milling with a thickness between 10 and 100nm.

The intensity of peak A in the TEY and TFY spectra both increases significantly after co-

sintering at 873 K (blue line), supporting that the thickness of the surface layer decreases to

about 10 nm after co-sintering. In Figure 3 (c)-(d), NCM shows a very similar behavior as that

for LCO at the O K-edge, revealing that the coating phenomenon through ball-milling and co-

sintering is common for TM oxide electrode materials.

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Figure 3. O K-edge XAS spectra of the samples and Li2CO3 is shown as a reference sample. (a)

and (b) Spectra of LCO+LLZO at different stages detected by TEY (surface) and TFY (bulk)

mode. (c) and (d) Spectra of NCM+LLZO at different stages by TEY and TFY mode.

For an intuitionistic observation of the interfacial structure, we performed TEM measurements

for the NCM+LLZO powders after ball-milling and co-sintering, respectively. After the ball-

milling process, a layer of LLZO covers the surface of NCM with a thickness of ~100 nm, as

shown in Figure 4(a). The magnified view in Figure 4(b) confirms that the core material is NCM,

which exhibits a local single (003) facet with a lattice distance of 0.476nm. Since LLZO quickly

reacts with air and forms Li2CO3, it is reasonable to expect that the surface of LLZO may have

an amorphous structure,24

thus no clear lattice fringes of LLZO can be found. After co-sintering

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at 873 K for 1 h, the thickness of the surface layer significantly decreases to less than 10 nm, as

shown in Figure 4(c) and (d). We observe clear lattice fringes for NCM and LLZO at the

boundary area, indicating the thickness of the Li2CO3 layer significantly decreases after co-

sintering. The original Li2CO3 around large LLZO particles could be fallen off or decomposed

through the ball-milling and high-temperature treatment, and the regeneration of Li2CO3 may be

depressed when the particle size of LLZO decreases to nanoscale. We find a clear interfacial

layer of approximately 3 nm, which could be created by LaNiO3 and La2Zr2O7. After co-

sintering, the cathode material exhibits a tiny twist of the lattice fringes nearby the interfacial

layer. We speculate that Li which has a similar atomic radius with Ni, diffuses from interface

into the NCM to exchange or occupy the vacancy of Ni. The Li-Ni exchange could be the main

reason for the observed Ni 3+ state in the bulk NCM using TFY mode [Figure 2(b)], as LaNiO3

can only forms at the interface due to the large atomic radius of La.

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Figure 4. TEM image of the NCM+LLZO samples. (a) Sample after ball-milling. (b) Magnified

view of the area outlined by the dotted line in panel (a). (c) Sample after co-sintering at 873 K.

(d) Magnified view of the area outlined by the dotted line in panel (c).

To ensure the diffusion of Li from interface into the NCM at a high temperature 873K and

explore its effect on electrode materials, we performed electrochemical tests using mixed

materials at different stages as the cathode, and the results are shown in Figure 5. For the LCO

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sample, a clear discharge capacity fading from 153.5 mAh g-1

for the pristine sample (black line)

to 100 mAh g-1

for the ball-milled sample (red line) is observed at 0.2 C. This capacity fading

also appears in the NCM sample, but is milder from 164 to 132 mAh g-1

. The distinction may be

caused by the different thicknesses of the LLZO layers on these two cathode materials after ball

milling. An increase of the discharge capacity is observed in both the LCO+LLZO and

NCM+LLZO samples after co-sintering. The capacity for sintered LCO+LLZO sample only

increases to 134 mAh g-1

since rare interfacial reaction happens at 873K. However, sintered

NCM+LLZO sample unexpectedly reaches a capacity of 180 mAh g-1

, which is even higher than

the pristine sample. EIS results in Figure S8 (c) and (d) shows that the charge transfer resistance

(Rct) through the electrode/electrolyte interface increases after ball-milling and decreases after

co-sintering for both LCO and NCM mixed samples. Sintered NCM+LLZO sample has a larger

resistance than pristine NCM, which supports the extra capacity is contributed by the diffusion of

Li from interface into NCM. The diffusion is probably the reason for the initial capacity increase

observed in LLZO-coated LiMn1.95Ni0.05O3.98F0.02 cathodes via the sol–gel route and

subsequently heat treated at different temperatures.42

However, the LCO+LLZO sample exhibits

a more stable cycling performance at different cycle ratios than NCM+LLZO (see figure S8). It

indicates that the formation of interfacial layer and the cathode degradation caused by the mutual

diffusion is harmful for the electrochemical cycle performance.

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Figure 5. The second round galvanostatic charge–discharge curves at 0.2 C for (a) LCO+LLZO

and (b) NCM+LLZO at different stages.

To explore the stability mechanism of NCM/LLZO interface, we have calculated the interface

Ni-La and Ni-Li exchanges using the first-principles density functional theory (DFT). Figure 6.

(a) shows the 3D model of the NCM/LLZO interface and the positions of the 2 Ni, 3 La and 6 Li

atoms involved in the Ni-La and Ni-Li exchanges, while (b) and (c) show the exchange energies

for Ni-La and Ni-Li exchanges, respectively. We have found that the La and Li atoms close to

the Li slab in NCM show lower exchange energies than those distant from the Li slab.

Meanwhile, the exchange energies for Ni-Li are strongly dependent on the Li local environment.

For instance, the 2 Li atoms which have negative exchange energies (Ni1-Li3, Ni2-Li6) are 6-

fold coordinated, making their local environment (LiO6) similar to the Ni local environment in

NCM (NiO6), thus the replacement Ni atoms form the NiO6 octahedrons directly. Other Li

atoms, e.g., Li4 and Li5, only have 4 neighbor oxygens, which lead to higher exchange energies

than Li3 and Li6. We note that because of the atom relaxations, the Li atoms in LLZO interface

may have very different local environment to the ones in the bulk LLZO, i.e., they cannot be

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described using the symmetry symbols in bulk LLZO such as 24 d or 96 h. The lowest exchange

energies for Ni-La and Ni-Li are -1.20 eV and -0.85 eV, respectively, which suggest that

although NCM/LLZO interface is much more stable than the LPS/LCO interface,34

moderate

atom exchanges may still exist. We also calculated the Eex for the interface Co/Mn-Li/La atom

exchanges and the results are shown in Table S1. Co-Li exchange may happen at temperature

higher than 873K as the lowest Eex is -0.45eV, which is 0.4eV higher than Ni-Li. The exchange

energies for Mn-Li are much higher than that of Co and Ni, indicating Mn is the most stable

element in NCM. Co-La and Mn-La have high exchange energies over 2.28eV which suggest

these exchanges hardly happening at the interface.

The very thin diffusion layer (~3 nm) shown in our NCM+LLZO 873K TEM results suggest that

the Ni-La exchange can only happen nearby the relaxed interfacial layer, which can be explained

by the large difference of ionic radius between Ni and La. Li in LLZO, on the other hand, has

similar ionic radius to Ni thus it can diffuse into NCM to exchange with Ni or occupy the Ni

defects under external forces, which can be the reason for Ni oxidation in bulk NCM observed

by XAS and the capacity increasing. Meanwhile, the exchange of Ni-Li and Ni-La nearby the

interface can be the driving forces for further formation of Ni defects in bulk NCM and Li

vacancy at the interface when co-sintered at 873K. The Li depletion layer after diffusion at the

interface can induce the decomposition of LLZO and the formation of La2Zr2O7 and LaNiO3, and

this phenomenon is aggravated with the rising temperature. The formation of LaNiO3 and the

cathode degradation caused by the structural change can be the reason for poor cycling stability

in NCM+LLZO 873K sample.

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Figure 6. (a) The interface structure of NCM(101�0) /LLZO(100). (b) Exchange energies for 3

Ni-La exchanges. (c) Exchange energies for 6 Ni-Li exchanges.

Figure 7 gives a brief schematic illustration of the process during the ball-milling and co-

sintering treatments. Small LLZO particles spontaneously covers majority of large NCM

particles with a non-uniform thickness 40~100 nm during ball-milling, and no chemical reactions

occur at the interface. The thickness of the surface layer decreases to around 10 nm after co-

sintering at 873 K, and an interfacial layer of approximately 3 nm is observed for NCM/LLZO.

Mn is the most stable element in NCM, while Ni is active which can partially exchange with

both Li and La at the interface at 873K. Ni-La exchanges could only happen nearby the relaxed

interfacial layer, while Li can diffuse from interface into NCM to exchange and fill the vacancy

of Ni. The Ni-Li exchanges increase the valence of Ni in the bulk and lead to a higher capacity of

sintered sample than that of the pristine sample. The Li depletion layer can induce the

decomposition of LLZO that forms La2Zr2O7 and LaNiO3. The structural evolution of the

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cathode material and the formation of LaNiO3 at the interface may be harmful for the cycling

stability. Our results suggest that by simple treatments such as ball-milling and co-sintering, one

can easily create a nanoscale surface layer of LLZO on the TM oxide cathode material.

Decreasing the TM vacancy in the cathode material or increasing Ni-Li/Ni-La exchange barriers

may improve the interfacial stability between cathodes and solid LLZO electrolyte at high

temperature.

Figure 7. A brief schematic illustration for the formation process and the structure of the LLZO

surface layer on large NCM particles during the ball milling and co-sintering processes.

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CONCLUSION

We have investigated the chemical compatibilities of garnet-type solid electrolyte Ta-doped

LLZO with major commercial cathodes, LCO and NCM, upon ball-milling and co-sintering

processes. The ball-milling process does not introduce chemical reactions between the electrolyte

and cathode materials. Mn is the most stable element in NCM, while Li, La, and Ni are the active

elements that participate in diffusion at the interface during the co-sintering at 873K. When

mixed with the TM oxide electrode through physical methods, nano-scale LLZO particles cover

majority of the large electrode particles with a thickness of 40~100 nm. After co-sintering at a

high temperature, the thickness of the surface layer decreases to about 10 nm. Li in LLZO nearby

the interface could exchange with Ni and diffuse into NCM to occupy the vacancy of Ni,

forming a Li depletion layer. This Li depletion layer could induce the decomposition of LLZO

that forms La2Zr2O7 and LaNiO3 which is harmful for the cycle performance. Therefore,

stabilizing the TM elements, decreasing the vacancy in TM-based cathodes, and increasing Ni-

Li/Ni-La exchange barriers are necessary to improve the interfacial stability between cathodes

and LLZO at high temperature. Further optimization of ball-milling and co-sintering conditions

to create better coating layer may decrease the interfacial resistance and improve the

electrochemical performance of all-solid-state Li-ion batteries.

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ASSOCIATED CONTENT

Supporting Information. The following files are available free of charge.

Additional details about first-principle calculation mainly on the construction of LLZO(100)/

NCM(110) interface, XRD patterns, XAS spectra for Co and Mn (PDF).

AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval

to the final version of the manuscript.

Funding Sources

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work was supported by the National Natural Science Foundation of China (Grant Nos.

21473235, 11227902, and U1632269), and the One Hundred Person Project of the Chinese

Academy of Sciences. We thank Zhenmin Li at NSRRC for help with some absorption

experiments of this work.

ABBREVIATIONS

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LCO, LiCoO2; LIB, lithium-ion secondary battery; LLZO, Li6.75La3Zr1.75Ta0.25O12; NCM,

Li(NiCoMn)1/3O2; SOE, solid oxide electrolytes; SSB, all-solid-state lithium-ion battery; TEM,

transmission electron microscopy; TEY, total electron yield; TFY, total fluorescence yield; TM,

transition metal; XAS, X-ray absorption spectroscopy; XRD, X-ray diffraction.

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Table of Contents/Abstract Graphic

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Mechanism Study on the Interfacial Stability of a Lithium