C

Co (d) 7

Zn 6 Mn 7

Mn Zn

Zn

Mn

 Co8

Zn 3 Mn 9

5

9

13

17

21

25

6 5 4 3 2 1 0

0

1

2

3

4

20

15

10

5

0

0

ρ nSLD solvent 2, n

2

 (10-6 ) -2 Å

nSLD solvent 1, 

ρ

n

1 (10

Information gain, 

Δ

SOFT MATTER

42 NIST CENTER FOR NEUTRON RESEARCH

I

n recent years it has been shown that concentrated 

electrolytes can form a much more stable, conducting solid 

electrolyte interphase (SEI) as compared to traditional, 

more dilute electrolyte concentrations. The solid-electrolyte 

interface in electrochemical energy storage systems is critical 

for controlling both performance and safety. With this in 

mind, research related to the development of new electrolytes 

remains a key area of focus for battery researchers. It 

is a challenge to understand not only the differences in 

chemistry of the SEI with electrolyte formulation, but also 

the mechanism of SEI formation during cycling, and how 

this connects with the processes of cation intercalation and 

corresponding electrode microstructural changes. To address 

this challenge, the dynamic evolution of microstructure 

and surface chemistry for an ordered mesoporous carbon 

(OMC) electrode was probed during cycling using operando

measurements on the NGB 30 small-angle neutron scattering 

(SANS) beamline [1]. Many different electrolytes have been 

developed, all with tradeoffs in viscosity, electrochemical 

stability and conductivity. The choice of salt and solvent for the 

electrolyte is similarly important, and the standard LiPF6 salt and 

ethylene carbonate (EC) based solvent used in most commercial 

batteries is a compromise amongst other commercially available 

Li salts each with too many potential disadvantages, such 

as toxicity, an unstable or uneven passivation layer (SEI), or 

high corrosion of the current collector. Replacing LiPF6 with 

LiN(SO2CF3)

2

 (LiTFSI) can improve thermal and chemical stability 

of the electrolyte, but unfortunately result in corrosion of 

the cathode current collector (made of Al) at high voltages. 

In addition, replacing the standard EC-based solvent with 

propylene carbonate (PC) results in an electrolyte with better 

ionic conduction at room temperature, but the PC suffers from 

an inability to form a stable SEI and thus produces exfoliation 

of graphite in battery anodes. However, it has been shown 

that by increasing the salt concentration in electrolytes, 

Al corrosion and exfoliation of graphite is suppressed. It 

is reported that the salt in concentrated electrolytes are 

sacrificially reduced to form more stable passivation layers 

high in LiF content. These LiF based SEIs are superior in 

adhesion to the electrode surface, and thinner and denser with 

superior mechanical properties. Given the importance of the 

inorganic salt layer, a key question is therefore to understand 

how the concentration of LiTFSI, along with the increase in 

electrolyte viscosity, influences the formation of inorganic salt 

products (i.e., LiF, LiOH) versus carbonaceous products in the SEI 

during the critical initial discharge cycle.

Given the sensitivity of neutron scattering to light elements 

(e.g., H, Li, C, F), SANS is a powerful technique to probe 

the formation of the SEI layer during battery discharging. In 

order to study the SEI evolution operando, a suitable neutron 

scattering battery cell has been designed, as shown in Figure 

1, that provides minimal contribution to the scattering data, 

along with minimal absorption. Separate cells for dilute and 

concentrated LiTFSI/PC electrolytes on mesoporous hard carbon 

were simultaneously cycled on the beamline. The resulting data 

(Figure 2), demonstrate that operando SANS, in combination 

with ex-situ X-ray photoelectron spectroscopy (XPS), provides 

unprecedented detail on the concentration dependent filling 

of mesopores and micropores with Li-rich and carbonaceous 

products, co-intercalation of solvated Li+, and corresponding 

electrode microstructure expansion.

There are several key findings from this work derived from the 

SANS and supporting XPS data. SANS intensity changes indicate 

a displacement in the pore surface electrochemical processes to 

lower voltages, which is a physical effect related to pore filling 

– the filling of pores is shifted to lower voltages with the higher 

viscosity concentrated electrolyte, which can be influenced by the 

viscosity of the electrolyte and wetting of the surface. Moreover, 

the pore filling influences the expansion of the carbon framework, 

Electrolyte concentration dependence on electrode 

SEI formation revealed by operando SANS

C. J. Jafta,1 X.-G. Sun,1 G. M. Veith,1 G. V. Jensen,2 S. M. Mahurin,1 M. P. Paranthaman,1

S. Dai,1 and C. A. Bridges1

1 Oak Ridge National Laboratory, Oak Ridge, TN 37831

2 NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899

FIGURE 1: Schematic view of the operando SANS cell.
A

12d

A

12d

A

12d

right-h

A

C

6

Å-2

)

solv

ρn

 

0.05

Momen

solv

ρn

 

SOFT MATTER

2019 ACCOMPLISHMENTS AND OPPORTUNITIES 43

with electrolyte concentration dependent results. The expansion 

occurs just below the open circuit voltage in the 1 mol/L cell and 

below 1.1 V in the 4 mol/L cell. The size of the solvation shell 

around Li+ is found to influence the framework expansion during 

the first discharge, with the larger PC coordinated Li+ in 1 mol/L 

electrolyte producing an overall 3.5 Å (≈ 4 %) increase in the 

pore–pore separation as compared to the 1.4 Å (≈ 1.5 %) increase 

for the Li(PC)-TFSI aggregates. The overall change in framework 

volume during the discharge can be explained by the combination 

of pore filling and co-intercalation into graphitic layers, which 

are reflected in SANS intensity changes during operando cycling. 

Furthermore, the intensity changes can be correlated with the 

electrochemical discharge profile to provide more information 

on the mechanism of SEI formation (Figure 2(c, d)). Both the 

low and high concentration electrolytes show electrochemical 

double layer (EDL) formation by adsorption of solvated PC to 

the electrode surface. In the 1 mol/L electrolyte system it is 

known that the inner Helmholtz layer is dominated by free PC 

molecules and that of the 4 mol/L by salt anions. Despite the 

prevalence of PC in the inner Helmholtz layer of the 1 mol/L 

electrolyte system, the TFSI- anions present at the anode surface 

begin to reduce at a higher potential and at a faster rate as 

compared to the PC, due to the higher reduction potential of 

the TFSI- anions. We find that Li-rich salts (e.g., LiF, LiOH) form 

at higher voltages for the 1 mol/L electrolyte, but that in the 

sloping discharge region of 1.1 V to 0.9 V the salts begin to 

dominate the mesopore scattering for the 4 mol/L electrolyte, 

while carbonaceous products such as C–O and C–F containing 

compounds begin to dominate for the 1 mol/L electrolyte. 

By the end of the first discharge the 4 mol/L electrolyte has a 

passivation layer richer in low SLD Li-rich reduction products than 

the 1 mol/L electrolyte. Finally, with the subsequent charges and 

discharges PC or solvated PC is desorbed (charge) and adsorbed 

(discharge), which is shown to have significant reversibility in 

the mesopores but not in the micropores. As we have shown, 

operando SANS studies can provide knowledge about the 

dynamic cell chemistry that is microstructure specific in new 

candidate electrolyte systems for high-energy density batteries, 

which cannot be seen from other probes such as electrochemical 

cycling or XPS studies. This approach has the capability to impact 

a wide range of electrochemical systems, taking advantage of 

the unique contrast provided by neutron scattering.

References

[1] C. J. Jafta, X.-G. Sun, G. M. Veith, G. V. Jensen, S. M. 

Mahurin, M. P. Paranthaman, S. Dai and C. A. Bridges, 

Energy Environ. Sci., 12, 1866 (2019).

FIGURE 2: Scattering intensity is shown as a function of the scattering vector 

q, first discharge time and voltage for (a) the 1 mol/L and (b) 4 mol/L LiTFSI/PC 

(propylene carbonate) electrolyte systems. The development of carbonaceous 

and fluoride-rich reduction products in the SEI differs based on concentration as 

can be clearly seen from the extracted data for (c) the 1 mol/L and (d) 4 mol/L 

electrolyte systems.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.7

0.8

0.9

1.0

0 10 20 30 40 50 60

90

91

92

93

94

0 131 524 1213 0

Voltage (V)

535 645

0.9 V0.3 V

Peak intensity

1.1 V

c)

0.00

0.05

0.10

0.15

0.20

MPSI(cm-1)

Capacity (mAh/g)

Time (h)

P-P spacing

Micropore scatt. intensity

Pore-pore spacing

Norm. peak intensity

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.7

0.8

0.9

1.0

0 10 20 30 40 50 60

90

91

92

93

94

434

Voltage (V)

1.1 V0.9 V0.3 V

0 147 471 1010 0 425 552 504

Peak intensity

0.00

0.05

0.10

0.15

0.20

MPSI(cm-1)

d) Capacity (mAh/g) P-P spacing

Time (h)

Norm. peak intensity

Micropore scatt. intensity

Pore-pore spacing