Carbon nanotube reinforced batteries: towards larger capacities and longer lifetimes

Carbon nanotube reinforced batteries: towards larger capacities and longer lifetimes

Maxwell C. Schulze and Amy L. Prieto

Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523

How can rechargeable batteries store more energy?

Electrodeposition of Sb/CNT composite films

2

Electrochemical performance of Sb vs. Sb/CNT composite anodes

2

Outlook: Irreversible capacity losses still limit cell lifetimes

Questions:

How can we keep Sb anodes from cracking during cycling?

How can Sb anodes continue to reversibly store Li

+

_{even if they do crack?}

working electrode

(Sb/CNT composite film) separators and electrolyte (salts in carbonate solvents) counter/reference electrode (Li metal foil)

We would like to thank Dr. Pat McCurdy (Central Instrumentation Facility at CSU) for assistance with the SEM–EDS. Funding for this research is provided through NSF SSMC grant #1710672.

Hypothesis:

Incorporating carbon nanotubes (CNT’s)

into Sb anodes can help extend their cycle

lifetime by preventing cracking or keeping

cracked portions electrically connected.

•Above: Large portions of Sb/CNT composite film remain intact and able to

reversibly store Li+ _{after cycling despite substantial film delamination.}

•The Sb/CNT composite anode exhibits a larger reversible capacity for more cycles.

•The Sb anode without CNT’s exhibits

a smaller reversible capacity for fewer cycles.

•Below: The Sb anode is substantially cracked and pulverized after cycling,

leading to poor electrical connectivity and less reversible Li+ _{storage in the anode.}

•Above: Li-ion half cell used to test

electrochemical performance of anodes.

•Below: The resulting potential vs. time

t r a c e s m e a s u r e d d u r i n g r e p e a t discharge-charge cycles of the cell at constant current.

•Right: The plot of capacity vs. cycle

number calculated using the following:

time * current ÷ mass_Sb+CNT = capacity

(hours) (mA) (g) (mAh/g)

5 µm Ni Sb O ~3.5µm 20 µm 200 µm 20 µm

Sb film

after

cycling

Sb film

before

cycling

±Li

Ni Sb O 5 µm ~5µm 20 µm 20 µm ~30µm 200 µm Ni substrate Sb/CNT

Sb/CNT composite film before cycling

Sb/CNT composite film after cycling

±Li

discharge

charge current = 100 mA/g

IC₁ = (capacity_discharge – capacity_charge)

IC₂

IC₃

IC₄

IC₅ Irreversible capacity (IC)

A discharge-charge cycle is not perfectly efficient; some capacity is irreversible when charge is lost to side reactions.

Graphite (C) (intercalation anode) 372 mAh/g 800 mAh/cm3 10% volume change

C

₆

+

Li

+

_{+ e}

-

_Li

_C

6 Antimony (Sb)

(high capacity alloying anode)

660 mAh/g 1892 mAh/cm3

129% volume change

Sb

+

3Li

+

_{+ 3e}

-

_Li

3

Sb

negatively biased nickel (Ni) foil substrate

dissolved antimony cations (Sb3+₎

carbon nanotubes (CNT’s) suspended by

cationic surfactant

e- _e- _e- _e- _e- _e- _e- _e- _e-

3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+ 3+

Electrodeposition mixture in water

Positively charged CNT’s are electrostatically attracted to negatively charged Ni while Sb deposits onto Ni and CNT’s surfaces via reductive electrodeposition:

Sb3+ _{+ 3e}- _Sb

(metal)

1)  M. N. Obrovac and V. L. Chevrier, Alloy Negative Electrodes for Li-Ion Batteries, Chem. Rev. 2014, 114 (23), 11444–11502.

2)  M. C. Schulze, R. M. Belson, L. A. Kraynak, and A. L. Prieto, Electrodeposition of Sb/CNT composite films as anodes for Li- and Na-ion batteries, Energy Storage Materials, 2018, manuscript in progress. 3)  M. Winter, The Solid Electrolyte Interphase–the Most Important and the Least Understood Solid Electrolyte in Rechargeable Li Batteries, Zeitschrift für Physikalische Chemie, 2009, 223 (10-11), 1395–1406.

cycling in Li-ion battery

CNT’s maintain mechanical and electrical connectivity between cracked anode portions

e-

✔

±Li

Sb/CNT composite film

•Irreversible capacity is due to side reactions between Li+ _{and electrolyte components:}

•These reactions form a layer on anode surfaces called the solid-electrolyte-interface (SEI) that is extensively studied in literature.3

•The SEI irreversibly traps Li+_{, making a cell}

useful only for as long as capacity losses are less than the initial reversible capacity (!).

•This mode of cell failure is especially bad for alloy anodes and needs to be addressed for these anode types to have useful lifetimes.

Li+ Li₂CO₃ LiF R-CO₂Li electrolyte components Li+ trapped in SEI A cell’s energy can be

increased by substituting graphite (anode used in today’s Li-ion batteries) with a higher capacity anode like antimony (Sb).1 cell energy = (V_cathode – V_anode) * capacity

(mWh/g) (volts) (mAh/g)

Capacity is the amount of working ions (i.e. Li+) _{that can be}

stored in each electrode and is measured in units of (mAh/g) or (mAh/cm3_).

However, higher capacity anodes exhibit limited useful lifetimes due to large volume changes during battery cycling (charging and discharging) that crack and isolate parts of the anode. Rechargeable Li-ion battery cell discharging charging positive electrode (cathode) negative electrode (anode) current collector liquid electrolyte current collector Li+ e- Li+ e- e-

anode pieces become isolated and unable to

reversibly store Li+

volume changes during cycling cause cracking

e-

✔

±Li

film of anode material

Over many cycles the irreversible capacity accumulates as capacity losses that can far exceed the initial reversible capacity.

Cumulative capacity loss

IC

_n n=1 n=cycle #

∑

initial reversible capacity !