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Design and Synthesis of AdvancedHigh-Energy Cathode Materials

Guoying ChenLawrence Berkeley National Laboratory

June 9, 2016

Project ID: ES225

Overview

Timeline• Start date: October, 2012

• End date: September, 2016

• Percent complete: 90%

Budget• Total project funding

- FY2014 $500K- FY2015 $500K

Partners• Collaborations: LBNL, UCB,

Cambridge, ORNL, PNNL, NCEM, ALS, SSRL

• Project lead: Venkat Srinivasan

Barriers Addressed• Energy density

• Cycle life

• Safety

• Obtain fundamental understandings on phase transition mechanisms, kinetic barriers, and instabilities in high-energy cathode materials.

• Control cathode-electrolyte interfacial chemistry at high operating voltages and minimize solid-state transport limitations through particle engineering.

• Develop next-generation electrode materials based on rational design as opposed to the conventional empirical approaches.

Relevance/Objectives

Milestones

Date Milestones and Go/No-Go Decision Status

December 2015MilestoneEstablish synthesis-structure-electrochemical property relationship in high-voltage Li-TM-oxides.

Completed

March 2016

Go/No-Go DecisionDownselect alternative high-energy cathode materials for further investigation, if the material delivers > 200mAh/g capacity in the voltage window of 2-4.5 V.

Completed

June 2016

MilestoneDetermine Li concentration and cycling dependent transition-metal movement in and out of oxide particles. Examine the mechanism.

On schedule

September 2016MilestoneIdentify key surface properties and features hindering stable high-voltage cycling of Li-TM-oxides.

On schedule

Approach/Strategy

3) Design and synthesize optimized electrode materials based on the structural and mechanistic understandings.

2) Perform advanced ex situ and in situ diagnostic studies at both particle and electrode levels for insights. Establish direct correlations between physical properties, performance, and stability.

Li OMnNi

3D compositional mapping (APT/PNNL)

10 nm

Phase distribution (TXM/SSRL) and atomic imaging (HRTEM/NCEM)

1 µm

10 µm

1 µm 1 µm

1 µm

1) Remove the complexity in active particles – synthesize crystal model systems with defined properties for well-controlled studies of solid state chemistry, kinetic barriers and instabilities in high-energy cathode materials.

Previous achievementsa b

d

50 nm50 nm

e f

1 µm 1 µm

1 µm 1 µm 1 µm

Plate Needle c

1 µm

L-Poly

S-Poly (∼ 10x more SA) Box Octahedron

• Li- and Mn-rich layered oxide crystals (Li1.2Ni0.13Mn0.54Co0.13O2, LMR-NMC141) with varied surface terminations and surface areas synthesized.

Needle crystal

10 nm

• Comprehensive studies on entire particles made possible by using our well-formed crystal samples.

Shukla et al, Nature Communications, 6, 8711 (2015)

Bulk structure

• Bulk – a single phase with domains of three monoclinic variants in random distribution. Critical new insights on the design and synthesis of advanced LMR-NMC cathode materials.

• Pristine surface properties differ from that of the bulk. This year’s focus is therefore to define these surface/bulk heterogeneities and determine their role in cathode performance and stability.

Bulk

Surface

Previous achievements

Recent technical accomplishments: overviewI. Revealed facet-dependent surface properties on pristine LMR-NMC particles.

• Elemental – selective Co and/or Ni enrichment on certain facets• Chemical – selective TM reduction on certain facets• Structural – selective defective spinel formation on certain facets

Implication – in order to interpret cycling-induced changes using conventional, randomly orientated secondary particles, pristine samples need to be carefully characterized.

II. Established the relationship between pristine surface chemical composition and facet reactivity; correlated the reactivity to cathode structural, cycling and thermal stabilities. Implication – particle engineering important! Improved stabilities on oxide particles with maximum expression of surface facets resistant to TM reduction.

III. Investigated coating approach for surface engineering of particles with given facets. • Coating modifies surface properties without changing particle surface

termination or surface area. • Coating leads to selective chemical modification on surface. • Coating effect is facet-dependent.

Implication – coating investigation needs to take account of particle morphology and other surface attributes.

IV. Visualized phase distribution and revealed the phase transformation mechanism in a first-order intercalation cathode (LMNO).

Accomplishment I – pristine surface elemental composition depends on termination

HAADF STEM imaging andXEDS mapping (incollaboration with ChongminWang, PNNL)

Plate (LMR-NMC141)

• Selective Ni enrichment on (200) and Co enrichment on (20-2) planes.• Roles of surface Ni and Co enrichment in structural stability and cathode performance

currently under investigation.

25 nm

P. Yan et al, Advanced Energy Materials, 1502455 (2016)

Accomplishment I – pristine surface crystal structure depends on termination

HAADF STEM imaging, experimental and simulated SAED patterns (NCEM)

S-Poly

20 nm

Simulated SAED

• Layered • Spinel

• Selective surface facets are covered with a spinel layer.

Accomplishment I – pristine surface layer has a defective structure

HAADF STEM imaging and atomic simulation (Alpesh, LBNL)

• Surface spinel has antisite defects, with columns of mixed Li and TM atoms (TM in the Li sites).

• EELS analysis shows reduced Mn and Co in the surface layer – changes in chemical composition.

Soft XAS L edge spectra collected on composite electrodes with carbon and binder (SSRL beamline 10-1)

850 860 870 880

Energy (eV)

Plate

Needle

L-Poly

S-Poly

Octahedron

640 650 660

Norm

aliz

ed In

tens

ity

Energy (eV)

Plate

Needle

L-Poly

S-Poly

Box

Octahedron

770 780 790 800 810

Energy (eV)

AEY TEY FY

NiMn Co

L2L3L2L3

• Depth-resolved XAS shows reduced Mn and Co on the top surface (a few nm) while Ni remains at 2+ in entire particle.

• Effects of surface termination and surface area – TM reduction least on Plate and L-Poly samples while most on S-Poly and Box samples.

Mn3+/Mn2+Co2+

Accomplishment I – surface chemical composition depends on termination

Fluorescence Yield (FY, 50 nm depth)

Auger Electron Yield (AEY, 1–2 nm depth) Total Electron Yield (TEY, 2–5 nm depth)

Accomplishment II – pristine surface chemical composition indicator of facet reactivity

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Plate Needle L-Poly S-Poly Box Octahedron

Pristine

L3 p

eak

ratio

Co3+

Co2+

0.4

0.5

0.6

0.7

0.8

0.9

1.0 AEY TEY FY

After 45 cycles

• Higher reduced TM content (normalized to area) on pristine suggests more reactive facets with stronger tendency in further TM reduction during cycling – a surface structural stability indicator.

more reactive facetsmore surface area

• Facet effect more dominant than surface area.

• Pristine surface with reduced TM does not passivate but further promotes TM reduction into the bulk upon cycling.

772 776 780 784 788

Plate Needle L-Poly S-Poly Box Octahedron

Nor

mal

ized

inte

nsity

Pristine

772 776 780 784 788

Nor

mal

ized

inte

nsity

Pristine

772 776 780 784 788

Nor

mal

ized

inte

nsity

Energy (eV)

Pristine

772 776 780 784 788

After 45 cycles

Energy (eV)

772 776 780 784 788

After 45 cycles

772 776 780 784 788

After 45 cycles

AEY

TEY

FY

Accomplishment II – facet reactivity and side reactions

• Effect of both surface termination and surface area – parasitic reactions most on S-Poly and box samples, shown by the highest marching rates.

0 50 100 150 200 250 3002.0

2.5

3.0

3.5

4.0

4.5

5.0

Cel

l Vol

tage

(V)

Capacity (mAh/g)

S-Poly

0 50 100 150 200 250 3002.0

2.5

3.0

3.5

4.0

4.5

5.0Plate

Cel

l Vol

tage

(V)

Cyc 5 Cyc 15 Cyc 25 Cyc 35 Cyc 55 Cyc 75

0 50 100 150 200 250 300

Box

Capacity (mAh/g)

0 50 100 150 200 250 300

Needle

0 50 100 150 200 250 300

L-Poly

0 50 100 150 200 250 300

Octahedron

Capacity (mAh/g)

• The extent of parasitic reactions correlates with the trend of reduced TM content on pristine – a side reactivity indicator.

0 10 20 30 40 50 60 700

25

50

75

100

125

Dis

char

ge c

apac

ity e

ndpo

ints

(m

Ah/g

)

Cycle number

Plate Needle L-Poly S-Poly Box Octahedron

Co3+

Co2+

0.1

0.2

0.3

0.4

0.5

0.1

0.2

0.3

0.4

0.5

0.1

0.2

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0.5

Plate Needle L-Poly S-Poly Box Octahedron

0.1

0.2

0.3

0.4

0.5

FY

TEY

AEY

Pristine

Co

L3 p

eak

ratio

Accomplishment II – facet reactivity and side reactions

Accomplishment II – facet reactivity and TM dissolution

• Soft XAS (L edge, TEY mode) detects M2+ (M=Mn, Co and Ni) on recovered separators. M2+ content increases with cycling but morphology dependent.

635 640 645 650 655 660

S-Poly_45 cycleN

orm

aliz

ed in

tens

ity (a

.u)

Box_30 cycle

Needle_30 cycle

Plate_30 cycle

Pristine electrode

Plate_45 cycle

L-Poly_45 cycle

Energy (eV)775 780 785

Energy (eV)

S-Poly_45 cycle

Needle_30 cycle

Plate_30 cycle

Pristine electrode

Plate_45 cycle

L-Poly_45 cycle

845 850 855 860Energy (eV)

Needle_30 cycle

Plate_30 cycle

Pristine electrode

Plate_45 cycle

L-Poly_45 cycle

Mn4+ Co3+ Ni2+Recovered separators

Recovered electrolytes (45 cycles)

• Extent of TM dissolution in electrolyte correlates with facet reactivities.

Plate Needle L-Poly S-Poly Box Octahedron0

5

10

15

20

25

30

35

40

TM C

once

ntra

tion

(ppm

)

Mn Co Ni

Plate Needle L-Poly S-Poly Box Octahedron0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.4

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1.0

0.4

0.5

0.6

0.7

0.8

0.9

1.0

FY

TEY

AEY

Co

L3 p

eak

ratio

After 45 cycles

Accomplishment II – facet reactivity and cycling stability

• Capacity retention correlates with the extent of cycling-induced TM reduction.

Co3+

Co2+

0 20 40 60 80 1000

25

50

75

100

20 mA/g2.5-4.6 VD

isch

arge

cap

acity

rent

entio

n (%

)

Cycle numberPlate Needle L-Poly S-Poly Box Octahedron

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.4

0.5

0.6

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0.9

1.0

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1.0

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1.0

0.4

0.5

0.6

0.7

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0.9

1.0

0.4

0.5

0.6

0.7

0.8

0.9

1.0

FY

TEY

AEY

Co

L3 p

eak

ratio

After 45 cycles

High-Pressure DSC Capsules

• Samples were fully delithiated by chemical method. Evolved heat measured in the presence of a liquid electrolyte.

• Surface with more reduced TMs improves safety – least heat from Box and most from Plate and L-Poly samples.

160 180 200 220 240 260 280

Hea

t Flo

w (m

W)

Temperature (°C)

Plate Needle L-Poly S-Poly Box Octahedron

Accomplishment II – facet reactivity and thermal stability

Plate Needle L-Poly S-Poly Box Octahedron0

50

100

150

200

250

Ons

et te

mpe

ratu

re (ο

C)

0

150

300

450

600

Hea

t evo

lved

(J/g

)

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Accomplishment III – surface coating with fluorinated agents (an example)

• NH4F (5 wt.%) mixed with L-Poly LMR-NMC141 crystals and heated in air at 300°for 6 h.

• Decomposition of NH4F to NH3 and HF allows modification of the crystal native surface without introducing an add-on coating layer.

Accomplishment III – coating leads to selective surface chemical modification

• Coating has no effect on Mn and Ni but enhances Co reduction in the pristine –decoupled chemical modification of TMs.

• STEM/EELS studies on structural and elemental modifications in progress (collaborative effort with PNNL).

NiMn Co

635 640 645 650

L-Polycoated

L-Poly

Nor

mal

ized

Inte

nsity

Energy (eV)

S-Poly

775 780 785Energy (eV)

850 855 860

AEY TEY FY

Energy (eV)

Co2+

• Particles with less accessible surface can be modified by coating to achieve better activation, utilization and rate capability.

Accomplishment III – coating improves utilization and kinetics

0 50 100 150 200 250 300 3502.0

2.5

3.0

3.5

4.0

4.5

5.0

L-Poly L-Poly_coated S-Poly

Cel

l Vol

tage

(V)

Capacity (mAh/g)

20 mA/g

5 10 15 20 25 300

50

100

150

200

250

2.5-4.6 V L-Poly L-Poly_coated S-Poly

Cap

acity

(mA

h/g)

Cycle number

10 20

40 80

150 200mA/g

1 µm

L-Poly(LMR-NMC141)

Accomplishment III – coating lowers capacity retention

0 50 100 150 200 250 300 3502

3

4

5L-Poly_coated

Cel

l Vol

tage

(V)

0 50 100 150 200 250 300 3502

3

4

5S-Poly

Cel

l Vol

tage

(V)

Capacity (mAh/g)

1100

0 50 100 150 200 250 300 3502

3

4

5

L-Poly

Cel

l Vol

tage

(V)

0 25 50 75 1000

50

100

150

200

250

Cap

acity

(mAh

/g)

Cycle number

L-Poly L-Poly_coated S-Poly

20 mA/g2.5-4.6 V

0 20 40 60 80 1000

25

50

75

100

Cap

acity

rent

entio

n (%

)

Cycle number

• Trade-offs between utilization (capacity) and capacity retention.

0 20 40 60 80 10050

100

150

200

250

Cap

acity

(mA

h/g)

Octahedron Octahedron_coated

0 20 40 60 80 1000

20

40

60

80

100

Cap

acity

Ret

entio

n (%

)

Cycle number0 50 100 150 200 250

2.0

2.5

3.0

3.5

4.0

4.5

5.0Octahedron_coated

20 mA/g2.5 - 4.6 V

Cel

l Vol

tage

(V)

Capacity (mAh/g)

Accomplishment III – coating effect morphology dependent

• Coating effect not obvious on octahedron sample – facet dependent behavior of coating.

1 µm

Octahedron(LMR-NMC141)

0 50 100 150 200 2502.0

2.5

3.0

3.5

4.0

4.5

5.0Octahedron

Cel

l Vol

tage

(V)

1100

Accomplishment IV – phase transition mechanism and kinetic barriers in first-order intercalation cathodes

• Why study phase transformation?

o Two-phase reactions with large lattice misfit between the phases are commonamong electrode materials. Well-known examples are LiFePO4 (LFP) andLiMn1.5Ni0.5O4 (LMNO).

o Both LFP and LMNO show good rate capability despite two-phase reactionsassociated with large lattice volume changes (ca. 7% and 6%). High rate in LFPonly possible at small size where the existence of metastable phases and solid-solutions mitigates the lattice strain and enables fast rate. LMNO has fast rateeven at micron size regime.

o Conventional wisdom says first-order phase transition reduces rate capability andcyclability but the relationships between kinetics, strain and phase boundarymovement are currently unknown.

o High spatial- and chemical-resolution visualization of phase boundary propagationat the particle level provides us with fundamental understanding of the phasetransformation mechanism and associated mechanical damage/fracture. This willguide the design of particle morphology and other properties to achieveimproved electrode performance.

Accomplishment IV – chemically delithiated LixMNO

• Two 2-phase transitions among threecubic phases of LMNO, L0.5MNO and MNO.

• As-prepared LixMn1.5Ni0.5O4 (LixMNO)samples are varying mixtures of the three phases.

• Volume changes were 2.5% (LMNO andL0.5MNO ), 3% (L0.5MNO and MNO ) and 5.5% (LMNO and MNO). 35 40 45 50 55 60 65 70 75 80

Li 0.40

2 Theta

Li 0

Li 0.06

Li 0.11

Li 0.25

Li 0.51

Li 0.71

Li 0.82

Li 0.90

Li 1

36 37 38 39

LMNO MNOL0.5MNO

1.0 0.8 0.6 0.4 0.2 0.00

20

40

60

80

100

Pha

se w

t %

Lithium content

LMNO L0.5MNO MNOLMNO

Accomplishment IV – standard XANES spectra

1

2

3

45

6

7

8

9

10

11 12

13

14 15

1625 µm

25 µ

m

Li0.82

1

23

45

6

7

8Li0.71

1

23 4

56

7

8

9

11

10

12

1314

Li0.51

8330 8335 8340 8345 8350 8355 8360

Nor

mal

ized

inte

nsity

(a.u

)

Energy (eV)

LMNO L0.5MNO MNO

• Standard XANES spectrafor LMNO and MNO end members were collected.

• Pure L0.5MNO XANESspectrum was extrapolated for the first time, using linear combination fitting of L0.40MNO data with LMNO and MNO spectra.

Field of view of Full-field Transmission X-ray Microscopy combined with X-ray Absorption Near Edge Structure imaging (FF-TXM-XANES)Collaboration with Dr. Yijin Liu (SSRL, BL 6-2c)

1 2 3 4 56

8330 8340 8350 8360

Nor

mal

ized

inte

nsity

(a.u

)

Energy (eV)

Point 1 Point 2 Point 3 Point 4 Point 5 Point 6

1

6

1 2 3 4 5 60.0

0.2

0.4

0.6

0.8

1.0

LMNO L0.5MNO

Pha

se fr

actio

nPoint

Accomplishment IV – Li0.82MNO 2D FF-TXM-XANESChemical phase map LMNO L0.5MNO

LMNO

MNOL0.5MNO

1

2

34

5

6

71 2 3 4 5 6 7

0.0

0.2

0.4

0.6

0.8

1.0 LMNO L0.5MNO MNO

Pha

se fr

actio

nPoint

8330 8340 8350 8360

Nor

mal

ized

inte

nsity

(a.u

)

Energy (eV)

Point 1 Point 2 Point 3 Point 4 Point 5 Point 6 Point 7

7 1

Accomplishment IV – Li0.51MNO 2D FF-TXM-XANESChemical phase map LMNO MNOL0.5MNO

12

34

65

7

1 2 3 4 5 6 70.0

0.2

0.4

0.6

0.8

1.0

L0.5MNO MNO

Pha

se fr

actio

n

Point

7 1

Chemical phase map MNOL0.5MNO

Accomplishment IV – Li0.25MNO 2D FF-TXM-XANES

8330 8340 8350 8360

Nor

mal

ized

inte

nsity

(a.u

)

Energy (eV)

Point 1 Point 2 Point 3 Point 4 Point 5 Point 6 Point 7

Accomplishment IV – Li0.51MNO 3D FF-TXM-XANES

LMNO

MNOL0.5MNO

Yellow Magenta

Cyan

XZ

Delithiation mechanism of LMNO

• The observation of all three phases on a single particle supports the 3-phase mechanism.

Responses to Previous Year Reviewers’ Comments A total of three reviewers evaluated the project. In general, the reviewers’ comments were very positive and they noted that “the approach was solid and strong”, “the work has been superbly carried out and the results are really insightful” and “there is outstanding collaboration”. Only reviewer 3 submitted specific comments and suggestions which are addressed below:Recommendation/Comment: The project team needed to provide statistics on the performance data in order to rank S-poly, L-poly and plate results and to identify one morphology with overall good performance.Response/Action: The performance data of each sample were statistically evaluated based on at least five coin cell testing. We thank the reviewer for pointing out the omission of this information. Recommendation/Comment: Some explanations are needed on how the surface spinel group affected the voltage fade which was thought to be induced by bulk structural change. The impact of electrode fabrication, for example, grinding, mixing, etc., on the morphology of the crystals, should be quantified since the morphology might not be maintained after the electrode fabrication and after the first activation charge when O2 gas was evolved at high cut-off voltage. Response/Action: We thank the reviewer for the suggestions. Extensive studies are needed in order to assess the contribution of surface processes in performance issues such as voltage fade and this part of research is ongoing. Our electrodes were made by simple mixing of crystals, carbon and binder without the use of mechanical grinding. SEM studies confirmed that morphology remained intact during the fabrication process. First-cycle activation induced morphology damage is likely but not unique to our crystals. Our crystal samples, however, are ideal for investigating cycling-related morphology changes and we plan to look into this in our future studies. Recommendation/Comment: The project team needed to propose more specific surface modification techniques to improve the cathode stability by leveraging insights gained on the surface defect spinel. Response/Action: We have initiated the surface modification effort this year. Studies on coating and its effect on surface chemistry were performed and the preliminary results are included in this year’s review.

DOE Merit Review, June 2015

Collaborations • Drs. Marca Doeff (LBNL), Apurva Mehta and Yijin Liu

(SSRL) – synchrotron in situ and ex situ XRD, XAS and FF-TXM-XANES studies

• Drs. Ethan Crumlin, Jinghua Guo and Tolek Tyliszczak (ALS) – synchrotron XPS, XAS and STXM studies

• Dr. Phil Ross (LBNL) and Prof. Simon Mon (GwangjuInstitute of Science and Technology, Korea and SPring-8, Japan) – Hard X-ray Photoelectron Spectroscopy

• Prof. Clare Grey (Cambridge) – NMR studies

• Dr. Ashfia Huq (ORNL) – neutron diffraction

• Dr. Chongmin Wang (PNNL) – STEM/EELS

• Dr. Arun Devaraj (PNNL) – atom probe tomography

• Dr. Jagit Nanda (ORNL) – new cathode material synthesis and characterization

Remaining Challenges and Barriers • Surface properties of the pristine layered oxides were shown to be different from

that of the bulk. The next phase of study need to address the following:o What is the effect of synthesis conditions on the surface properties of the

oxides?o What is the effect of cathode chemistry on pristine surface properties?o What is the contribution of surface processes in LMR-NMC performance issues

such as voltage fade and impedance rise at low SOC?o Pristine surface properties are facet dependent – comparative studies of

pristine and cycled samples always need to take consideration of crystalline orientation which may be challenging.

• Cathode/electrolyte interfacial chemistry is very complex and needs comprehensive understanding. It is important to know what reactivity is due to cathode instability and what reactivity is due to electrolyte instability at high voltages.

• Cathode particles needs to be optimized/engineered to bring out the best of the material itself can offer.

• Coating effect likely depend on coating materials and coating techniques.

Proposed Future Work• Further investigate the impact of surface properties and bulk structure on the

performance and stability of LMR-NMCs. Identify key properties and features hindering stable cycling of LMR-NMC cathodes.

• Perform studies to understand synthesis-bulk/surface properties-performance relationships in oxide cathode materials.

• Obtain comprehensive understanding on the cathode/electrolyte interfacial reactions and processes, particularly the roles of cathode chemistry (LCO, NMCs, LMR-NMCs etc.), surface properties and electrolyte stability.

• Determine the contribution of surface-related processes in the overall failure mechanisms of LIB cathodes.

• Understand coating effect on facet-specific surfaces and cathode/electrolyte interfacial reactivity.

• Explore other aspects of particle engineering and surface modification to improve cathode performance and stability at high operating voltages.

Summary• Surface properties of LMR-NMCs differ from that of the ideal bulk. Crystal structure,

elemental and chemical compositions of the surface are facet dependent.

• Materials by design important – oxide structural and cycling stabilities improve with maximum expression of surface facets stable against TM reduction.o Surface termination determines material’s tendency in TM reduction, both

during synthesis and cycling. o Extent of surface TM reduction on pristine material is an indicator for surface

reactivities. o TM reduction increases with cycling and progresses from the surface to bulk.

• Decomposable NH4F coating improves first-cycle activation and overall kinetics but decreases capacity retention.

• Coating effect facet dependent. It is important that coating investigation takes account of particle morphology and other surface properties.

• Chemical phase mapping shows the coexistence of Li-rich and Li-poor regions on a single LixMNO particle, suggesting concurrent phase transformation instead of the particle-by-particle process.

Technical Back-Up Slides

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!"##$%"&$'"()*+,-).&/)0+1+()2+3)455627)82)9:;

Standard soft XAS spectra for transition metals

Journal of Electron Spectroscopy and Related Phenomena 190, 64–74 (2013) Physical Review B 84, 014436 (2011)

Journal of Electron Spectroscopy and Related Phenomena 114–116,

855–863 (2001)