Active material particles spatial arrangement in combination with the nature of the electrochemical reduction mechanism were found to be the major cause of excessive swelling in cathodes in Li/(CF(x))n cells. A better understanding of the chemical reaction mechanism, a possible new role for the carbon, and a model for cathode growth are discussed

Baxam, Carl C.

Margalit, Nehemiah

English

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lhGor Technology Resources 
THE SWELLING MECHANISM OF CATHODES 
IN L ~ / ( C F X ) ~  CELLS 
N. MARGALIT AND C.C. BAXAM 
BATTERY TECHNOLOGY CENTER 
TRACOR TECHNOLOGY RESOURCES 
ROCKVILLE, MD 20850 
PRESENTED AT THE 
NASA AEROSPACE BATTERY WORKSHOP 
OCTOBER 29-31, 1991 
MARSHALL SPACE FLIGHT CENTER 
HUNTSVILLE, AL 
1991 NASA Aerospace Battery Workshop 
-135- Primary Technologies Session 
https://ntrs.nasa.gov/search.jsp?R=19920013504 2020-03-17T11:40:03+00:00Z
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ABSTRACT 
Active material particles spatial arrangement in combination with the 
nature of the electrochemical reduction mechanism were found to be the 
major cause of excessive swelling in cathodes in L~/(CF'X)~ cells. A 
better understanding of the chemical reaction mechanism, a possible new 
role for the carbon and a model for cathode growth are discussed. 
1991 NASA Aerospace Battery Worhhop 
- 136- Primary Technologies Session 
Thwr Technology Resources 
IlOTRODUCTIOB 
Early developers of lithium-organic electrolyte cells were attracted 
to (CFx), as a potential cathode material. This attraction was based 
on calculated theoretical energy values of which the most outstanding was 
a theoretical specific energy approaching 2000 wh/kg*. Calculations of 
solid volume changes in the L~/(CFX)~ cells, based on 
"crystallographic" densities, indicated a volume reduction caused by 
active materials transforming into products of about 35%. However, in 
practice, loading of active materials was limited by an unexplained 
severe cathode swelling which resulted in cell choking and/or bulging of 
flat cell walls. 
This investigation was aimed at trying to understand the cathode 
swelling phenomenon and, if possible, find a way to increase the active 
material loading in L~/(CFX)~ cells. 
The results of the first part of this project were reported in the 
1990 NASA Aerospace Battery Workshop. This presentation covers a review 
of the earlv results, new insight into the role of carbon in the cathode 
reaction mechanism, a swellina model based on microscopic observation and 
cells swelling behavior and conclusion. 
* This number is based on the assumption that the EMF of the 
electrochemical reaction is between 2.8 and 2.9 volts. See - J. P. 
Gabano, "An Overview" in "Lithium Batteries" (J. P. Gabano ed) P. 2, 
Academic Press, New York, NY, 1983. 
1991 NASA Aerospace Battery Workshop Primary Technologies Session 
brcw Technloo~y Resources 
VOLUME CHANGES OF CATHODES IN BR-2325 CELLS DUE TO DISCHARGE 
WET CATHODES VOLUME BEFORE DISCHARGE 
PC/DME 
GBL 
3 Volume *(Cm ) 
0.261 
0.251 
CATHODES F'ROM DISCHARGED CELLS 
32OF and 120°F Discharge Across a 37,500 ohm Load 
AVolume* ( X )  
PC/DME 
GBL 
75OF Discharge Across a 15,000 ohm Load 
AVolume* ( X )  
PC/DME 
GBL 
1991 NASA Aerospace Battery Workshop Primary Technologies Session 
Tncor Technology Resources 
APPROACH 
The problem is illustrated by the data obtained from measurements 
performed on cathodes from fresh and discharged BR-2325 cells. 
Approximately doubling of electrodes volume was reported for 
experiments conducted with essentially free "standing" electrodes.* That 
report indicated that no temperature effect was observed with the highly 
porous "free standing" electrodes. 
Initially, the source of the problem was thought to be related to the 
reaction mechanism, hence the prevailing mechanism for the electrode 
reduction required a careful review. 
As no mechanistic answer on the atomic level could explain the 
observed magnitude of electrode swelling, microscopic observations of 
active cathode material particles behavior during discharge were 
investigated in search for an explanation for the excessive growth. 
Finally, an atomic-microscopic model was evolved which can explain 
the observed swelling phenomenon. 
* D.M Pasquariello, E.B. Willstaedt and K.M. Abraham, The 
Electrochemical Society, Fall Meeting, Paper No. 17, Seattle, 
Washington, October 1990. 
1991 NASA Aerospace Battery Workshop Ptimary Technologies Session 
Tmcor 7'echndooy Resources 
ELECTROCHEMICAL REDUCTION OF 
PTFE (CFx), BY LITHIUM 
PROPOSED HECMISMS 
n Li + (CF),-)[CFL~],*~~L~F + nC 
* Prevailing literature intermediate [CF-Li(S)n] 
PTFE (Dousek et al) 
Fast 
Li + PTFE -->
I I 
F F F  
I l l  
iF-C -C -C- 
I l l  
F F F  
I I 
Li Li 
Slow 
4 LiF + 
[-crc-],-)to more ordered form 
L i F j  to larger crystals 
PROBLEM : 
In both cases x-ray diffraction patterns do not show 
intermediate; hence 
HOW DOES THE LITHIUM REACH REACTION SITES? 
1991 NASA Aerospace Battery Workshop Primary Technologies Session 
T m r  kchnology Resources 
ROLE OF CABBOtl 
(Reaction Mechanism) 
Reaction mechanisms proposed, in the literature, for the (CFx), 
electrode reduction by lithium assume lithium ion penetration to reaction 
sites between layers of active material via an intermediate. A similar 
mechanism is proposed for the reduction of PTF% by lithium metal. 
X-ray diffraction patterns show only the presence of LiF on both 
reduction products and disordered carbon on the surface of discharged 
(CFx), particles. Hence, lithium has to penetrate the discharging 
active material particles in both cases through the discharge products as 
complete disintegration of particles down to the "molecular" level was 
not observed. 
Based on carbon intercalation chemistry literature, which is 
abundant, and the expected slow lithium ion diffusion through LiF 
crystals, we proposed last year that lithium could be first 
"intercalated" into the discharged carbon and then proceed to (CFx), 
sites. Experiments designed to verify the proposed lithium migration 
path gave interesting results. 
I991 NASA Aerospace Battety Workshop Primary Technologies Session 
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U 
"Lithium passage through carbon" cell. 
Schematic Drawing of the "lithium passage through carbonw 
experimental setup. 
A1 - Resistor used to measure total current flow. 
A2 - Resistor used to measure current flow to (CFX)~. 
A3 - Resistor used to measure current flow to carbon. 
L - Resistive load controlling current flow. 
1991 NASA Aerospace Battery Workshop 
- 142- Primary Technologies Session 
Tncw Technology Resources 
ROLE OF CARBON 
(Experimental) 
The experimental setup designed to understand the role of carbon in 
the cathode reaction consisted of a Li/Li salt in an organic 
s~lvent/(CF~)~ cell separated into two compartments by a piece of 
Pyrolytic Graphite - Coated Graphite (Pyrotech, PT101). 
This cell was discharged through a large resis~al~~z, L and the 
currents flowing from the lithium, to the carbon and to the (CF ) x n 
electrodes were measured through smaller calibrated resistors Al, A3 and 
A2, respectively, using DVMs. The voltage between the two electrodes, Li 
and (CF ) was also monitored during the experiments. 
x n 
Initial results with a 1MLiAsF6 in DMSI solution indicated no 
current flow through A2 unless the carbon was loaded with some lithium. 
Loading the carbon, by disconnecting A2, and unloading the carbon, by 
disconnecting Al, resulted in increasing and decreasing the ability of 
the cell to deliver current at several predesigned discharge voltages 
indicating limits on cell performance dependent on lithium presence in 
the carbon. 
However, gas formation was detected in the positive electrode 
compartment as the experimental work progressed. Couscquently, the 
experiments were repeated with a lMLiAsF in PC. 6 
1991 NASA Aerospace Battely Workshop Pninaiy Technologies Session 
, ! f I I 
5 , I , f 
0 10 B 3 0  40 50 60 70 8 0 9 0 1 0 0  
Time (boun) 
Current distribution for cell B at the start of loading. 
(Load @ 102K ohm) 
" ~ l l l ! l ~  24 0 10 20 30 40 SO 60 70 80 
TIME (HOURS) 
Cell A - Total cell voltage, vs. time, for 
beginning of loading (first run load @ 200K). 
1991 NASA Aerospace Battery Workshop 
-144- Primary Technologies Session 
Tracor Technology Resources 
ROLE OF CARBON 
(Results) 
Results obtained with the PC solution varied some with the piece of 
carbon used. In the case of one cell similar results to those obtained 
with DMSI were obtained. 
However, more interesting were results which were obtained with, what 
could be assumed to be, not completely sealed surfaces of the carbon. 
dith sucn sIightly open faced pieces current started flowing L L L ~ ~ L  .:i\i 4 
to both the carbon and the (CFx), with the sum of the currents equal, 
within the measuring accuracy (two significant figures), to that flowing 
from the lithium electrode. Furthermore, current was flowing to the 
carbon even as the cell voltage was significantly above 2 volts. 
Such behavior indicates carbon loading with lithium while the cell is 
discharging. Hence, if one assumes that (CFx) is fully covered with 
n 
discharge product, even if slightly cracked, then the observed discharge 
voltages could be attributed to mixed potentials involving lithium 
loading on carbon and its ultimate reaction with the positive active 
material. 
Attributing an ionic current conducting role to carbon in the cathode 
reaction could account for some positive electrode svelling due to 
lithium presence in carbons in the (CFx), electroae, . . , LC) c:l the 
discharge product and the conductive additive. 
Still the observed magnitude of swelling cannot be explained in terms 
of the carbon-lithium reaction above. 
I991 NASA Aerospace Battely Workshop Pri~naty Teclznologies Session 
Tracor Technology Resources ORIGINAC PAW 
84_A@H AND i?.iHiTE PHOTOGRAPH 
a. Cathode p e l l e t  a f t e r  some scraping of adherent ma te r i a l s .  
b .  Cathode p e l l e t  a f t e r  i n t ens ive  scrap ing .  
Current c o l l e c t i n g  su r f aces  of p e l l e t s  from PC t e s t  tube 
c e l l s  a f t e r  discharge.  
1991 NASA Aerospace Battery Workshop Priinaiy Technologies Session 
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MICROSCOPIC OBSERVATION 
Visual and microscopic examination of specially made "electrodes" 
gave better insite into the swelling mechanism. 
These electrodes consisted of a pellet made from shiny high purity 
graphite powde~ on which gray (CFx), powder was sprinkled and allowe? 
to discharge slowly vs lithium (CCV 1.8 volts) under flooded condition;. 
in a 1MLiAsF6 in PC solution. 
Surprisingly, partially discharged material on the surface of these 
electrodes hardened into a porous matrix and had to be scraped for 
further observation. Scraping of such partially discharged electrodes 
revealed a gray upper layer of partially discharged material on the top 
of the electrode and a dull black layer on top of the shiny graphite 
surface of the pellet. 
Further examination of the layers revealed that even the partially 
discharged particles were fused together allowing for sizable pores 
between particles. Some shrinkage of particles, but not the disappearing 
of cavities, was observed with the dark material. 
Furthermore, in all cases the fused active material particle?- were 
fused also to the graphite. 
1991 NASA Aerospace Battery Workslzop Primary Technologies Session 
ltaror Technology Bssouaces 
ORIGINAL PAW 
'~I.ACI( AND VJMfPE PlaOTOGRAPh 
a .  Gray s u r f a c e .  
b .  Black s u r f a c e .  
SEM m i c r o g r a p h s  o f  t h e  g r a y  and b l a c k  s u r f a c e s  o n  a  s c r a p e d  
c u r r e n t  c o l l e c t i n g  s u r f a c e  o f  t h e  g r a p h i t e  p e l l e t  Prom c e l l  LT 2. 
1991 NASA Aerospace Batte y Workslzop -148- Primary Technologies Session 
Ihc# Technology Resources 
SWELLING MODEL 
Based on the above, a reasonable model for (CFx), cathode swelling 
will consist of the following steps: 
1. Electrons reach (CFx), particles' surfaces through the 
conductive additive - carbon, and lithium ions reach reaction 
sites via solution. 
2. Reaction taking place at sites of least resistance, e-g., the 
conductive carbon surface, resulting in fusion of discharging 
particles to the carbon. 
3. The reaction proceeds by advancing through partially discharged 
particles to undischarged particles which touch them resulting 
in particles fusion at touching surfaces before full reduction 
of the fused particles. 
4. Any further reduction of the fused particles matrix which 
results in particle swelling, i.e., incorporation of lithium in 
carbon, LiF crystal formation and further layer separation will 
result in amplified three dimensional swelling around the fused 
matrix cavities. 
5 .  Such swelling will continue unimpeded in free standing 
electrodes but will be limited if the e1ec+.roa is swelling 
against a solid metal wall. In the latter case the wal; 
pressure could result in particle matrix breakage and 
densification resulting in less bulging and cell chocking. 
One could also expect the matrix-wall interaction to be more 
sensitive to temperature than the swelling of a free standing electrode. 
I991 NASA Aerospace Battery Workshop 
-149- Primary Technologies Session 
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CONCLUSIONS 
The major contributor to cathode swelling in Li/(CFx) cells is the n 
formation and three dimensional expansion of the fused partii.le ratrix. 
Lithium reaction with carbon in the (CFx), e1ec;;ocir 1;rovides for 
an acceptable explanation for lithium ion migration during discharge and 
might explain observed voltages during discharge. 
AKr?OWLEDGEMERT 
We would like to thank Ms. L. M. King, from our laboratory, for her 
assistance in carrying out the experimental work. 
1991 NASA Aerospace Battery Workshop 
-150- Primary Technologies Session 


The swelling mechanism of cathodes in Li/(CFx)(sub n) cells

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