ArticlePDF Available

New Contact Probe and Method to Measure Electrical Resistances in Battery Electrodes

Authors:

Abstract and Figures

Electrical resistivity is an important measure to qualify electrodes for lithium-ion batteries. A reliable determination of conductivity is of high practical importance with regard to, for example, electrode production improvements and quality control. To complement state-of-the-art measuring techniques, a new method has been developed based on a new “micron-powder probe”. Following a simple measuring procedure, the system allows nondestructive, highly reproducible, and rapid data acquisition. In this paper, we describe the new concept thoroughly and present experimental results. These results demonstrate that an initial determination of resistance values in battery electrodes is beneficial especially if it is combined with an electrical postmortem analysis of cycled cathode disks. The outcome of our investigation is validated with regard to the electrochemical performance of cathodes in half-cells.
A–C) Experimental setup to measure electrical conductivities in electrode sheets. A) The powder probe. Its particles (red spheres) are attached to a magnetic mount and they contact the surface of a composite electrode that consists predominately of dielectric LMO particles (black spheres). The particle size distribution of the powder probe is similar to that of the LMO particles. The red dots on top of the probe particles represent electrically conductive silver deposits (Figures 2 and 3). Different from this, the dashed red lines in (A) and (B) indicate electron pathways that penetrate the matrix of the composite electrode. B) An indented contact stamp and the resulting plastic deformation of the electrode are demonstrated. The schematic of the proposed technique in (A) versus the conventional stamp (B) shows the reduction of parasitical contact resistances in the case of a powder probe application. One explanation for this is that spherical particles shift easily. Therefore, the powder collective conforms to uneven surfaces and will inherently fill in surface gaps or macroscopic roughness. Beyond this, the particular structure of the Ag coating () is believed to promote the conductivity by bridging microscopic voids and efficiently coupling the carbon black conductive network of the electrode, respectively. In contrast, any inflexible stamp will be interstitial even if it is indented. C) Different resistance contributions. Electric flux lines are also plotted.
… 
Content may be subject to copyright.
DOI:10.1002/ente.201600127
NewContact Probe and Method to Measure Electrical
Resistances in Battery Electrodes
Nils Mainusch,*[a, b] Torge Christ,[a] Thammo Siedenburg,[b] TomOQDonnell,[c]
Meylia Lutansieto,[c] Peter-Jochen Brand,[c] Gerhard Papenburg,[d] Nina Harms,[d]
BilalTemel,[e] Georg Garnweitner,[e] and Wolfgang Viçl[a, b]
Introduction
Theamount of energy that can be extracted from lithium-ion
batteries(LIB) is related directly to the voltage levels
that arespanned in the course of cell operation. Phenomena
that cause undesirable voltage losses are electrochemical
polarization and internal resistances.[1] Internal resistances
are partiallyattributed to imperfect electron transfer. LIB
electrodes usually consist of metal foils (current collectors)
that are coated with aparticle-based composite (active
layer).Active layers are designed typically with athickness
of 50–150 mm, which depends on the targeted application
(e.g.,high-power vs.high-energy cells).The electronic path-
ways within the composite are aggravated by insulators (elec-
trode active material) and incoherent components (organic
binder, carbon black, and graphite). As the particles are of
micron or submicronscale,Ohmic drops occur at multiple in-
terfaces.Moreover, it is proventhat electricbarriersoccur
on the surfaceofcurrentcollectors and might hamper con-
ductivity. This can be alimiting factor for the overall cell per-
formance and impactthe usable battery capacity,respective-
ly.[2,3]
State-of-the-art options to maximizeelectron transport
comprise the dopingand coating of cathode active materi-
als,[4–6] incorporation of conductive additives,[7] and current
collectormodifications.[8–10] Apartfrom the useoftailored
preproducts,the entire electrodefabrication affects the quali-
ty of the conductivity network of the electrode. Optimized
manufacturing implies ideal slurry formulation and the per-
fect dispersionofthe constituents as well as an optimal coat-
ing and drying routine in addition to adequate compaction
(so-called “calendering”). Electrical conductivity is an impor-
tant criterion to qualify elaborated electrodes,and quality as-
sessment requires electrochemical tests.These are both time-
and material-consuming so it has to be ensuredthat, in terms
of conductivity,only the best electrodes are submitted to ex-
tensivetesting. In conclusion,itbecomes clear that the deter-
mination of conductivity from electrode sheets is highly ben-
Electrical resistivity is an important measuretoqualify elec-
trodes for lithium-ion batteries.Areliable determination of
conductivityisofhigh practical importance with regard to,
for example,electrodeproduction improvements and quality
control. To complement state-of-the-art measuring tech-
niques,anew methodhas been developed based on anew
“micron-powderprobe”. Following asimple measuring pro-
cedure,the system allowsnondestructive,highly reproduci-
ble,and rapid data acquisition. In this paper, we describe the
new conceptthoroughly and presentexperimental results.
These results demonstrate that an initial determinationofre-
sistancevalues in battery electrodes is beneficial especially if
it is combined with an electrical postmortemanalysis of
cycled cathode disks.The outcome of our investigationisva-
lidated with regard to the electrochemical performanceof
cathodes in half-cells.
[a] N. Mainusch,T.Christ, Prof. Dr.W.Viçl
Faculty of Natural Sciences and Te chnology
UniversityofApplied Sciences and Arts
Hildesheim/Holzminden/Goettingen
Von-Ossietzky-Str.99, 37085 Gçttingen (Germany)
E-mail: nils.mainusch@ist.fraunhofer.de
[b] N. Mainusch,T.Siedenburg, Prof. Dr.W.Viçl
Application Centerfor Plasma and Photonics APP
FraunhoferInstitute for SurfaceEngineeringand Thin Films IST
Von-Ossietzky-Str.100, 37085 Gçttingen (Germany)
[c] T. O’Donnell, M. Lutansieto, Dr.P.-J. Brand
Center for Tribological Coatings
FraunhoferInstitute for SurfaceEngineeringand Thin Films IST
Bienroder Weg54E,38108 Braunschweig (Germany)
[d] G. Papenburg, N. Harms
Technische Universit-tBraunschweig
Institute of Environmentaland Sustainable Chemistry
Hagenring 30, 38106 Braunschweig(Germany)
[e] B. Temel, Prof. Dr.G.Garnweitner
Technische Universit-tBraunschweig
Institute for Particle Technology
Volkmaroder Str.5,38104 Braunschweig (Germany)
The ORCID identification number(s) for the author(s) of thisarticle can
be foundunder http ://dx.doi.org/10.1002/ente.201600127.
T2016 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA.
This is an open accessarticle under the terms of the Creative Commons
Attribution Non-Commercial License, which permits use, distribution and
reproduction in any medium,provided the originalwork is properlycited,
and is not usedfor commercial purposes.
Part of aSpecialIssue on “Li-Ion Batteries”. To view the complete issue,
visit: http ://dx.doi.org/10.1002/ente.v4.12
Energy Technol. 2016,4, 1550–1557 T2016 The Authors. Published by Wiley-VCH Verlag GmbH&Co. KGaA,Weinheim 1550
eficial to refine electrodeproductionand enable qualitycon-
trol.1
However, there is little work publishedonviable methods
to conduct such measurements.Both simpletwo-point and
extendedfour-point probe techniques are well known.[11] The
first generally suffer from “parasitical” resistances,which are
Ohmic losses that arise because of an imperfect couplingof
acontactpin to an electrodesurface (contact resistances,
Rcontact;F
igure 1C). Four-point probe techniques permit the
eliminationofparasitical resistances.Moreover, they can
allow the separate determinationofelectrode composite con-
ductivities (or composite resistance, Rcomposite)a
nd interfacial
resistances, Rinterface,that occur between the current collector
and the composite.
Wang and co-workers[12] presented amethodthat allows
alayerwiseextraction of the electronic resistivity in electrode
sheets. To do so,they impose four contacts in alinear ar-
rangement on top of the electrode. As the electrodes might
be horizontallystratified, such adepth-profileanalysis can
provide informationonelectrical in-planecharacteristics.It
can also give information about functional film properties in
the case of aprevious functionalization of acurrent collector
surface,for example.Inaddition to the determinationofan
electrodecomposite resistivity,they specify current collector
sheet resistances, Rsq. [Wm2]. Here,akey findingisthat
Rcomposite is in generalmuch lower than Rinterface.2Aproblemat-
ic aspect of this method is that all results feature high stan-
dard deviations (case-dependent up to &80 %), and it thus
possesses little reproducibility.Furthermore,the arrangement
requires the use of elongated electrodesamples ( &10 cm in
length), and conductivity analysis is done in ahorizontal ori-
entation, which does not coincide with the predominant di-
rection of electron transfer in cells in operation, which is ba-
sically perpendiculartothe composite layer.
Ender et al.[13] presented an alternativemethod. Their
setup makes use of adisk-shaped electrodesample, and
aconstant current is induced at its centerbymeans of atip.
Surface potentials are evaluated as afunctionofradial dis-
tances, and the determination of composite resistivity as well
as collector sheet resistances is done by numerical simula-
tions that are fitted to experimental surface potential data
for specific radial resistances.The input for the calculation is
the electrodegeometry.Again, an important outcome of the
proof-of-concept study is the collectorsheet resistances that
dominate composite resistivity.The comparison of different
types of cathodes shows that Rinterface is much higher than
Rcomposite.The multiplier of Rinterface to equal Rcomposite ranged
between 105 and 733, which depends on the investigated
cathode sheet. Adrawback of this method is the scattering
of the measured values.Standard deviationsofrepeated
measurements on identical samples are predominantly 20–
30%. This is explained by possiblemisalignment of the tips
of the measurementunit. Anotherproblem occurs on closer
inspection of the given resultsthat specify LiMn2O4(LMO)
cathodes:Afirst samplewith athickness of 144 mmisas-
signedtohave Rcomposite &90 mWand Rinterface &14.9 W,thus
atotal electroderesistance of &15 W.However, athicker
specimen of 170 mmshows Rcomposite &60 mW,Rinterface &6.2 W,
and in total &6.2 W.This appears implausible.The methodol-
ogy makes use of surface potential simulations that are based
on the model of ahypothetical, ideal currentfield distribu-
tion. Here,the presupposition is ahomogeneouselectrode
matrix.This excludes the probable existence of zones with
discrete electric conduction that are localized in-plane within
the matrix. Theinconsistency could be explained because
such zones cannot be considered in the underlying model.
With the intention to address deficiencies in electrical two-
point probe measurements (parasitical contact resistances) as
Figure 1. A–C) Experimental setup to measure electrical conductivities in elec-
trode sheets. A) The powder probe.Its particles (red spheres) are attached to
amagnetic mount and theycontact the surfaceofacomposite electrode that
consists predominately of dielectric LMO particles (black spheres). The parti-
cle size distributionofthe powder probe is similar to that of the LMO parti-
cles. The red dots on top of the probe particles representelectrically conduc-
tive silverdeposits (Figures 2and 3). Different from this, the dashedred
lines in (A)and (B) indicateelectronpathways that penetrate the matrix of
the compositeelectrode. B) An indented contact stamp and the resulting
plasticdeformation of the electrode are demonstrated. The schematic of the
proposed technique in (A) versusthe conventional stamp (B) shows the re-
duction of parasitical contact resistances in the case of apowder probe appli-
cation.One explanation forthis is that spherical particles shift easily.There-
fore, the powder collective conforms to unevensurfaces and will inherently
fill in surfacegaps or macroscopic roughness. Beyond this, the particular
structure of the Ag coating (Figure 2) is believed to promote the conductivity
by bridging microscopic voids and efficientlycoupling the carbon black con-
ductive network of the electrode, respectively.Incontrast,any inflexible
stamp will be interstitial even if it is indented.C)Different resistance contri-
butions. Electric flux lines arealso plotted.
1Notably,the electronic properties will change as dry electrode sheets are saturat-
ed with liquid electrolyte because of matrix swelling.
2Resistances werecalculatedfrom the given layer dimensions in accordance with
the published composite resistivity or sheet resistance,respectively.
Energy Technol. 2016,4, 1550–1557 T2016 The Authors.Published by Wiley-VCH Verlag GmbH &Co. KGaA,Weinheim 1551
well as how to overcomethe discussed disadvantages in four-
point methods (high standard deviations,inconsistentresults)
anew approachhas been developed.The concept involves
essentially two elements:First of all, aspecific “powder
probe” was implemented in an extended two-point measur-
ing setup.With the powder probe,wetarget an initial reduc-
tion of unwanted contactresistances.Second, aparticular
measuring procedure wasestablished.This aims at the iden-
tificationofthose contactforces that are needed to compen-
sate for contact resistances.
Results and Discussion
Given below is adescription of the powderprobe,its setup,
and its methodological characteristics.Afterwards contactre-
sistances for conventionalstamp versus probe measurements
were evaluated. Avalidation of conductivitymeasurements
on LMO cathodes that include the electrochemicalper-
formanceisgiven, and finallysome examples for advanced
probe applicationsare presented.
Method
It is well known that parasitical electrical losses virtually
always occur if arigid contactmedium is applied to a(micro-
scopically) rough surface. One reason for this is an imperfect
adaption. Couplingcan be optimized, for example,byindent-
ing astamp into the surface of an electrode.Bydoing so,the
matrix will be deformed plastically,and the electrical connec-
tion between stamp and the conductive network will be
somewhat improved,however, this is associated with possible
changes in the electrical properties of the composite matrix
as well. This situation is depicted in Figure 1B.Different
from asolid stamp,the system with the new powderprobe
basically consists of acollective of particles that serve as con-
tact media. This collective features high plasticity,thus it
conformsinherently to the microtopography of the surface
as it is driven towards an object.[14]
In contrast to inflexible devices,itisthe contact media
that is deformed and it couples in anondestructive manner
to the conductivity network of an electrode.This setting is il-
lustrated in Figure 1A.The schematic shown in Figure 1C
enlists the various resistance contributions that can be
summed up to an overall resistance.Inthe middle of the
schematic Relectrode indicates all domains that in total repre-
sent the electrode resistance (with the exclusionofRcontact).
Furthermore,wiring andcontacting that is used to induce the
measuring current (typically 1–100 mA) and the separated
circuit for the potential detection is displayed.
Thepowder probe is composed of functional micron-sized
particleswith amass-weighted median diameter d50 of ap-
proximately 15 mm. Theparticles are spherical and are made
from magnetic stainless steel. By means of physical vapor
deposition(PVD), the particle surfaces were covered with
ahighly conductive silver coating. Thereference material,
functionalized particles,the particle collective,and acom-
plete powder probe are shown in Figures 2and 3.
Theactuallaboratory setup is displayed in Figure 4. The
middle of the photograph shows the force sensor and the ap-
pended powder probe.Amagnet is inserted into the copper
mount, which fixes the particle collective.The probe micro-
Figure 2. Documentation of sphericalstainless-steelparticles used for the
design of the powder probe.Scanning electron microscopyand transmission
electronmicroscopy(insets)images of pristine particles (top) and Ag-coated
particles (bottom). In both cases, particles with adiameterofapproximately
10 mmwere chosen for the analysis. As can be seen for the Ag-coated parti-
cles (bottom), the plasmasputter process permits an all-around covering of
the starting material. The silver coating exhibits agranularstructure with
aparticular submicron roughness. The TEM image (bottom inset) indicates
aroundish morphology of an isolated Ag deposit on the powder probe mate-
rial. However,untreated particles possess acomparativelysmooth surface
with characteristic smallfissures and unspecific surface debris (top inset).
Figure 3. Left image:Light microscopy image of stainless-steel particles at-
tached to amagnet. The particles are spread becauseofthe magnetic field.
They adopt an antenna-like and pointed structure that constitutes a3D
shape. The inset documents how the particles are strung together. Right im-
age:The entirepowder probe with acopper mount,aninserted magnet, and
the attached particle collective.
Energy Technol. 2016,4, 1550–1557 T2016 The Authors.Published by Wiley-VCH Verlag GmbH &Co. KGaA,Weinheim 1552
particlesare highly magnetizable,which prevents powder
losses.Nonetheless,after powder probe application residual
powder material lodgedinthe electrode could sometimes be
observed, and these tracescan be dustedoff and removed
thoroughly.Therefore, we do not expect to contaminateelec-
trode sheetswith unwanted material (such as iron) and pre-
serve examined electrodesheets for electrochemical follow-
up investigations.Aforce sensor and apowder probe were
adjusted by using amicrometergauge.Modification from
apowder probe to astamp configurationisfeasible.In
powder probe applications,amask bordersthe contact area.
Themeasuringobjectshown in Figure 4isaLiFePO4(LFP)
stand-alone sheet(i.e., an electrode withoutasubjacent cur-
rent collector, hereafter referred to as “laminate”). Thelami-
nate is placed in between amask with 8mmapertures ar-
ranged linearly and adielectric backplate.
Atypical result of resistance measurements is shown in
Figure 5. In this case the objectisaLMO cathode. Foreach
measurement the resistance (left ordinate) and the applied
force (right ordinate, red dashed curve) were recorded over
aperiod of 100 s. Thecomparison of contact stamp (8 mm di-
ameter, gold coated surface) and powder probe (8 mm mask)
measurements involves an initial baseline acquisition (gray
calibrationcurves)tocontrol the setup performance. This is
done by contacting the grounded copper backplate directly,
and aslightly lower level is obtained in the case of the con-
tact stamp.Inthe examination of the cathode sheet, the
probe renders significantly lower values(&4.7 W)than the
stamp (>65 W). Furthermore,asecond measurementreveals
excellent data conformityfor the probe,whereas the stamp
produces an offset. Adecrease of parasitical resistances and
superiorreproducibility for probe measurements can be
stated.
Resistancesincases of adiscrete force increasefrom 2–
10 Ninthe aforementioned LFP laminate,which again com-
pare stamp and powderprobe measurements,are shown in
Figure 6. Theeffective measuring track is 50 mm, and the
measurements were repeated five times.The dashedlines
represent fit curves.Analgorithm based on apower function
was chosen because this corresponds to the physics of con-
tact resistances under external loads.[15]
It is clear that in the case of astamp,the resistancegradi-
ent is pronounced and, correspondingly,the calculated expo-
nent (&@0.04) is comparatively high (probe&@0.005). For
the probe,only amarginal influence of the contactforce on
the resistance occurs,and saturation at low forces can be
seen. Again, reproducibility for the powder probe is excel-
lent (standard deviation <0.15 W,not plotted here), and the
resistance disparitybetween the two devicesranges from 519
to 362 Wat 2and 10 N, respectively. Asimpleextrapolation
of the fitting curve of the stamp leads to aforce of >45 N
that would have to be imposed to decreasethe resistance of
the contact stamp to the level of the powder probe.This,in
Figure 4. Laboratory setup. The photographdocuments the force sensor (1),
the copper mount with attached powder probe (2), apolymer foil with 8mm
diameterapertures (3), and aLFP laminate (4).
Figure 5. Survey of resistance measurements on aLMO cathode sheet. Comparison between contact stamp and powder probe over 100 smeasuring time.
Energy Technol. 2016,4, 1550–1557 T2016 The Authors.Published by Wiley-VCH Verlag GmbH &Co. KGaA,Weinheim 1553
turn, would provoke sample deformation. Consequently,an
undesired structural change of the measuring objectitself
would take place.
Analysis of contact resistances
To determine the quantity of unwantedcontactresistances
that occur in two-point measurements,investigations on LFP
laminates were performed. Theuse of alaminate allowsan
in-planeinvestigation of Ohmic drops as afunction of the
distancebetween, for example,one fixed contact andits vari-
able counterpartthat is applied to the test structure.Any
length of this test structure defines aspecific “measuring dis-
tance”,and the resultingvaluescorrespond to the sum of dis-
tance-related contactresistances plus aspecific Rcontact.Thus,
the approximation d!0mmgives solely Rcontact.Based on
alinear regression of the data (least-square fit) the determi-
nation of yaxis intercepts and the assignment of Rcontact be-
comes possible,and one outcomeisshown in Figure 7.
Theextrapolation of the probe data (gray dotted line) fea-
tures a yaxis intercept that is close to zero.This indicates
that probe measurements on the LFP laminatehardly impli-
cate contact resistances or that those must be extremely
small compared with the overall resistance.Incontrast, the y
axis intercept of the stamp is >300 W.
Apart from the evaluation of yaxis intercepts,the contact
resistance can be deduced by “multiple contacting” and cu-
mulatingsegment-related resistances. This idea means that
first the resistance over afixed distanceisdetermined in
asingle measurement. Theobtainedvalue is afterwards sub-
tracted fromacumulative resistance value.This value is ach-
ieved from multiple measurements performed at two (or
more)segments that in total equal the originally chosen dis-
tance (e.g.,two measurements over 2cmtracks minus one
measurement over adistanceof4cm). Athird possibility to
assess the contactresistanceofthe stamp is given by simply
compensatingresults with values from correspondingprobe
measurements according to Rstamp@Rprobe.
Contact resistances that accountfor the three methods are
showninTable 1. Good overall data consistency exists and,
in particular, the calculated y-axis intercepts deliver figures
that are in good agreement with those of the multiple con-
tacting method.
Theoverall resistancescales with the elongation of an
electrodelaminate and, in principle,invariant contact areas
in each measurementexist and Rcontact remains constant. At
the same time it decreases proportionally with the increasing
measuring distances and vice versa.The corresponding R/d
ratio for astamp and aprobe are plotted in Figure 8.
Thecurve of the stamp increases steeply at short distances.
Conversely,the curves stay fairly constant as the probe is ap-
plied. This depiction illustrates how parasitical resistances
might dominate the measurement. According to this,for
short measuring distances (e.g.,across a100 mmthin elec-
trode layer), Relectrode would not allow analysis by using
astamp because the obtained value basically represented
parasitic contact resistances.
Validation of LMO cathodes
As stated initially,interfacial resistances can hamper electron
transferbetween the electrodelayer and acurrent collector.
ForLMO cathodes,Ender et al.found that Rinterface exceeds
Figure 7. Electrical resistances in aLFP stand-aloneelectrode sheet as afunc-
tion of measuring distances from 20–100 mm. Comparison betweenstamp
and powder probe with appliedmeasuring forces of 2and 10 N.
Table 1. Contact resistance values in case of the applicationofastamp in
contact withaLFP laminate.
Force [N] Rcontact (y-axis
intercept) [W]
Rcontact (multiple
contact)[W]
Rcontact (probe
compensation) [W]
2477 484 503
4399 400 429
6350 349 425
8337 336 414
10 321 323 367
Figure 6. Electrical resistances in aLFP stand-aloneelectrode sheet as afunc-
tion of contactf
orces of 2–10 N. Comparison betweenstamp and powder
probe measurements with ameasuring distance of 50 mm.
Energy Technol. 2016,4, 1550–1557 T2016 The Authors.Published by Wiley-VCH Verlag GmbH &Co. KGaA,Weinheim 1554
Rcomposite by at least 100 times.[13] Thus,itcan be assumed that
the electrochemical performanceofacathode is improved
by enhancing interfacial conductivity.Toconfirmthis and
validate the practicability of the powder probe,carbon-
coated Al foil was implemented as acurrentcollectorfor
LMO cathodes.The carbon coating aims to substitute the di-
electric Al2O3thin film that always occurs on the surface of
Al foil. Functionalized current collectors were compared to
pristinematerial by using electrical measurements and cur-
rent rate (C rate) tests in ahalf-cell configuration.
Contactresistance compensation
As apreparatory investigation before the determinationof
electrical conductivity,data were acquired in atest series as
the contact force was increased successively.Bydoing so,the
resistance approximates to aconstant limitingvalue (red
curves,see Figures 9and 10). This number can be regarded
as the genuine electrode resistance because parasitical con-
tact resistances are minimized or even completely eliminat-
ed. Moreover, this value can be subtracted from every single
overall resistance at each applied force. As aresult of this
compensation, it is possible to quantify contact resistances.
This in turn enablesustoidentify an appropriate contact
force to be used and warrants the suitability of the method.
Themeasuring results for both types of LMO cathodes are
depicted in Figures 9and 10.
Acomparison of the cathodes provesthat acoating of
carbon on the Al foil enhances electron transfer. Thecon-
ductivity increases by approximately afactor of 7. Contact
resistances diminish with increasing force,and in the case of
the functionalized current collector, they become inferior to
electroderesistances above approximately6N.
Apparently the two electrodes possessdifferent parasitical
contact resistances (2 W,Figure 9vs. 0.45 W,Figure 10, both
at 2N). This seemssurprising as an identical LMOslurry
was cast in ahomogeneous manner on both the pristine and
carbon-coated Al foil. One possible explanationmight be
adistinctelectrodesurface structure or adifferentsurface
roughness caused by the underlyingcarbon coating. As such,
the carbon functional film influences the wetting and drying
behavior of the LMO slurry explicitly.Further investigation
will have to be performed to reveal the influence of the
carbon coating on the electrode morphology.
Half-cell tests
Thebenefit of improved electron transfer in terms of avail-
able battery capacity is evident from the results of the half-
cell tests (Figure 11). LMO cathode sheets with carbon-
coatedcollectors (Al/C,red symbols)clearlyoutperform
cells with apristinecollector foil at high discharge rates (Al-
Ref., black).
In additiontothe initial conductivity enhancement ach-
ieved by carbon coating, postmortem analysis of the cycled
cathodes showedaneven higher resistancedisparity (Al-
Ref.: 10–16 Wvs.Al/C : <1W,see legend insets). Thereason
for this is Al surface corrosion and deterioration. Explicit
corrosion phenomena such as staining and efflorescence
were observed on the surface of uncovered pristinecurrent
collectors.Incontrast, carbon-coated collectors hardly exhib-
ited any defects.Analysiswas performed by using light mi-
croscopy.
Figure 9. Electrical resistances of aLMO cathode sheet. Blade-coated on bare
Al foil. Powder probe with appliedmeasuring forcesof2–10 N.
Figure 10. Electrical resistances of aLMO cathode sheet. Blade-coated on Al
foil with carbon thinfilm functionalization. Powder probe withappliedmeas-
uring forces of 2–10 N.
Figure 8. Ratio resistance/distance plotted for all screened electrode distan-
ces. Comparison between stamp and powder probe measurements.
Energy Technol. 2016,4, 1550–1557 T2016 The Authors.Published by Wiley-VCH Verlag GmbH &Co. KGaA,Weinheim 1555
Advanced powder probe applications
It is expected that the conductivitymeasurements by using
the powderprobe will be superior to that by using acontact
stamp to optimize electrodemanufacture,refine the design
process,and ascertain electrodequality.Two examples under-
line this assumption.
Calendered LMO cathodes
Theresults from acomparative study that includes com-
pressedLMO cathodesare shown in Figure 12. Repeated re-
sistance measurements were performed, and the stamp is
compared to the powderprobe.The standard deviations in
the context of stamp measurements are such that differentia-
tion between the two specimens becomesdifficult. Converse-
ly,explicitdiscriminationispossible if the powder probe is
used.
Carbon-coated current collector
Powder probe measurements are nondestructive.Toempha-
size this statement we performed aninvestigation on an Al
foil that had been coatedpreviously with carbon films of var-
ious thicknesses,and the outcome can be seen in Figure 13.
Both left bars in each series represent results from the direct
contactwith the copper backplate.Inthe case of the stamp
and for Al/C20 nm and Al/C40 nm (red), the values are almostas
low as the lowest basevalue.Only the 60 nm thick carbon
coating is detectable and leads to an increased resistance.
In contrast, powder probe measurements provide succes-
sively decreasing resistancesfor all three foil modifications.
This is aconclusive outcomebecause the carbon coating is
applied on thoroughly cleaned and sputter-etched foil and
the thicker the carbon the better its barrier effect against die-
lectric oxide films (reoxidation). Thus,conductivityisim-
proved. This experiment confirms the nondestructive charac-
ter of the powder probe,whereas effects that arise from the
partialdestruction of the dielectric oxide film on the surface
are visible for the stamp measurement.
Conclusions
Conventional two-point probe techniques applied in conduc-
tivity measurements on thin battery electrodes suffer natural-
ly from parasitical contact resistances.This complicates com-
parativestudies and makes quantitative analysis (resistivity
determination) virtually impossible.Advanced four-point
methods deliver explicit information on sheet resistances and
composite resistivity but high variances restricttheir practic-
ability.
Figure 12. Electrical resistances of aLMO cathode on an Al current collector
with a20nmcarbon film. Comparison between stamp and powder probe
measurements for apristine cathode and acompacted cathode. The applied
measuring force is 2N.
Figure 13. Electrical resistances of Al foil sputter-coated with acarbonfilm.
The carbon film thickness is 20, 40, and 60 nm. Comparison between stamp
and powder probe measurements with an applied measuring force of 2N.
Figure 11. Electrochemical results from atotal of twelvehalf-cell tests using
LMO-electrode disks (specific capacity:0.4 mAh cm@2)that were cycledin
three-electrode cells with Li foil as acounter and reference electrode.The
electrolyte is 1mLiPF6in ethylene carbonate/dimethyl ethylene carbonate
with aratio of 3:7.
Energy Technol. 2016,4, 1550–1557 T2016 The Authors.Published by Wiley-VCH Verlag GmbH &Co. KGaA,Weinheim 1556
Thenew powderprobe techniqueisbased on an initial
minimizationofcontactresistances and requires aspecific,
but simplemeasuringprocedure.This method provides
viable resistance data for lithium-ion battery cathodes.This
enables us to specify and to qualify electrodes.Both aspects
are proven to be significantwith regard to their electrochem-
ical performance.
At present,extendedtestseries are underway to deter-
mine how long one probe can continue to be used. So far,
adecrease in conductivitycaused by the loss or degradation
of the powder probe material has not been observed. Further
experimental work will be conducted to ascertain the applic-
ability of this test in the case of graphite battery anodes,ca-
pacitor electrodes,and thin primer coatings.Investigations
will also be performed to analyze electrical anisotropy in
electrodes.Afully automated measuring system is also under
development.
Experimental Section
Powder probe particle preparation
Adcsputter device was used to produce Ag-coated stainless-
steel particles.Before sputtering, the system was evacuated and
flushed with Ar and finally brought to apressure of 5.0 X10@4Pa.
Deposition was performed under an Ar atmosphere with awork-
ing pressure of 4.9X10@1Pa. Ahighly magnetizable stainless-
steel powder (15 g; obtained from Eckart GmbH) with a d50 of
15 mmwas stirred mechanically during the coating process aimed
at ahomogenous deposition on all particles.Power density on
the target was 0.75 Wcm@2,the target-to-substrate distance was
130 mm, and the coating time was 2h.
Al current collector
Various carbon-coated Al foils (alloy AA 1085, 20 mmthickness)
were produced. Foils were initially sputter-etched to remove
native oxide structures and potential contaminations.All carbon
films were deposited by using adcsputtering system (base pres-
sure 5.0X10@4Pa) with atarget power density of 3.7 Wcm@2and
atarget-to-substrate distance of 60 mm. Thedeposition rate of
carbon was 3.5 nm min@1at aworking pressure of 4.9 X10@1Pa.
Film thicknesses were 20, 40, and 60 nm, respectively.
LMO cathode production
LMO cathodes for electrical and electrochemical measurements
consist of 90 wt %active material LiMn2O4(TODAKogyo
Corp.) with anominal capacity of 108 mAh g@1(first charge ca-
pacity at 0.1 C). Further slurry components are 5wt% carbon
black (C 65) and 5wt% binder (PVDF) dissolved in 17 g
DMSO.Doctor-bladed electrode layers yield aspecific loading
of 0.4 mAh cm@2.
Electrochemical tests on atotal number of 12 LMO-electrode
disks (diameter 18 mm) were performed by using three-electrode
test cells (from EL-cell GmbH) with Li foil as counter and refer-
ence electrode.The electrolyte was 1mLiPF6in ethylene carbon-
ate/dimethyl ethylene carbonate with aratio of 3:7. Formation
was done with atotal number of three cycles (charge rate 0.05 C
up to 4.3 V, 15 min pause,discharge 0.05 Cdown to 3V,15min
pause). Charging in current rate tests was performed with 0.1 C
(until 4.3 V) discharge stepwise (0.1, 0.2, 0.5, 1, 2, 5C)until 3V.
Acknowledgements
The authors would like to thank the Nds.Ministerium fgr
Wissenschaft und Kultur of the State of Lower Saxony for the
financial support of this work with Graduiertenkolleg Ener-
giespeicher und Elektromobilit-tNiedersachsen (GEENI).
Material analysis by TEM was carried out at the Gottfried
Wilhelm Leibniz University Hannover,Laboratory of Nano
and Quantum Engineering.
Keywords: battery characterization ·contactresistance ·
energy storage ·lithium-ion batteries ·scanning probe
[1] M. Park, X. Zhang, M. Chung, G. B. Less,A.M.Sastry, J. Power
Sources 2010,195,7904–7929.
[2] H.-L. Pan, Y. -S.Hu, H. Li, L.-Q.Chen, Chin. Phys.B2011,20,
118202.
[3] M. Gaberscek,J.Moskon, B. Erjavec, R. Dominko,J.Jamnik, Elec-
trochem.Solid-State Lett. 2008,11,A170 –A174.
[4] H. Tukamoto, A. R. West, J. Electrochem. Soc. 1997,144,3164 –3168.
[5] S. Y. Chung, J. T. Bloking, Y. M. Chiang, Nat. Mater. 2002,1,123
128.
[6] H. Li, H. Zhou, Chem.Commun. 2012,48,1201 –1217.
[7] Y. H. Chen, C. W. Wang, G. Liu, X. Y. Song, V. S. Battaglia, A. M.
Sastry, J. Electrochem. Soc. 2007,154,A978 –A986.
[8] H.-C.Wu, H.-C.Wu, E. Lee,N.-L. Wu, Electrochem. Commun. 2010,
12,488–491.
[9] H.-C.Wu, E. Lee,N.-L. Wu,T.R.Jow, J. Power Sources 2012,197,
301–304.
[10] S.-K. Chen, K.-F.Chiu, S.-H. Su, S.-H. Liu, K. H. Hou, H.-J.Leu, C.-
C. Hsiao, Thin Solid Films 2014,572,56–60.
[11] S. L. Bewlay, K. Konstantinov, G. X. Wang, H. K. Liu, Mater.Lett.
2004,58,1788.
[12] C.-W.Wang, A. M. Sastry, K. A. Striebel, K. Zaghib, J. Electrochem.
Soc. 2005,152,A1001–A1010.
[13] M. Ender, A. We ber, E. Ivers-Tiff8e, Electrochem. Commun. 2013,
34,130–133.
[14] N. Mainusch, T. Siedenburg, T. Christ, E. Flade,J.Paulus,W.Viçl,
pending patent:DE102015 212 565.3, 2015.
[15] R. Holm, Electric Contacts:Theoryand Application,Springer,
Berlin,Heidelberg, New York, 2000.
Received:March 4, 2016
Revised:April 19, 2016
Published online on August 25, 2016
Energy Technol. 2016,4, 1550–1557 T2016 The Authors.Published by Wiley-VCH Verlag GmbH &Co. KGaA,Weinheim 1557
... 'Contact probe' is used as expression for all possible contact mechanisms (e.g. powder [3], metal probe or stamp, liquid), each differing in value and properties. As copper (5.96 x 10 7 S/m), aluminum (3.5 x 10 7 S/m) and all probe materials are highly conductive, the bulk resistivity of the current collector and the probes play a minor part. ...
... In order to confirm that a process-depended change in electrode resistivity is detectable by ATPM, even though the contact resistance between coating and probe is included, a second method which reduces this contact resistance is used to verify the results [3]. Furthermore, Indrikova et al. show that two point probe methods are capable of measuring the relative electrode resistivity [29]. ...
... As an alternative measuring method, developed by Mainusch et al. [3], a two point powder probe (TPPP) is used in chapter 3.2 to exemplary validate the trend of the ATPM results. The probe consists of a collective of Ag-coated particles (mean primary particle size x50 = 10 µm) that serve as contact media (see Figure 3). ...
Article
In order to achieve a profound understanding of the production process of electrodes for lithium-ion batteries, methods to determine the (intermediate) product quality are a necessity. Therefore, a new, fast and easy to use two point method to determine the relative resistivity of dry electrodes has been established. The method is used to determine process-induced changes in the electrode’s structure. A materials testing machine is used to ensure a homogeneous and constant mechanical stress during the analysis. By applying a direct current and measuring the voltage drop the electron transport characteristic along the whole electrode cross-section, taking all battery relevant resistances into account, can be determined. The result is an easy to compare relative resistivity value including coating resistance, contact resistance between coating and adhering current collector as well as the contact resistances between sample and probe. Process-induced changes are clearly visible in the results. The influence of the main testing parameters – contact stress and applied current – is determined. To cross-check the results, an established ‘powder probe’ method is used to confirm the relative resistivity changes caused by calendering. Slight calendering of LiNiMnCoO2 cathodes leads to an increase in electrode resistivity as conductive pathways are broken by the applied shear forces. However, increasing the cathode density to 2.95 g/cm³ decreases resistivity by one third compared to uncalendered electrodes by re-establishing and shortening electrical pathways. Furthermore, a relative resistivity of anodes produced with a high energy powder mixing step is measured and shows that applying too much stress to the carbon black leads to a loss in long range conductivity, resulting in electrodes with an increased resistivity of up to 50%.
... To investigate the reason for the reduction in the charge transfer resistance of TiNb 2 O 7 , the sheet resistance of prepared electrodes with reduced samples was further measured using the four-probe method. 60,61 The measured sheet resistance of prepared samples followed the same trend as the results of charge transfer resistance, as shown in Figure 7B. The results indicated that the decrease in the charge transfer resistance of reduction samples was due to the decline of sheet resistance. ...
Article
Full-text available
The discharge capacities and rate capability of TiNb2O7 powders were enhanced through the additional postreduction treatment. X‐ray photoelectron spectroscopy and electron paramagnetic resonance results confirmed the formation of oxygen vacancies in TiNb2O7 powders after a reduction treatment. The appearance of oxygen vacancies in TiNb2O7 powders formed the impurity level in the forbidden gap and decreased the bandgap values of TiNb2O7. Compared with the pristine TiNb2O7 powders, when TiNb2O7 powders were reduced at 400°C for 40 min, the charge transfer resistance of prepared samples was reduced from 43.67 to 19.35 Ω, and the pseudocapacitive contribution of TiNb2O7 was increased from 44% to 59%. In addition, the discharge capacities at 0.1 and 20 C of prepared batteries were increased by 10.84% and 105.85%, respectively. On the other hand, increasing the temperature in the reduction treatment caused the formation of Ti⁴⁺/Ti³⁺ and Nb⁵⁺/Nb⁴⁺ pairs and decreased the amounts of available redox couples, thereby deteriorating the electrochemical performance of prepared batteries. The results in the present study revealed that the discharge capacities and rate capability of TiNb2O7 powders were enhanced through a postreduction treatment.
... The 'Powder Probe' depicted in Fig. 13 offers a method of reducing the contact resistance between the electrode surface and contacting stamp. This probe has inherent plasticity and micron size particles can couple with the topography of the electrode surface [18,149]. ...
Article
Full-text available
In a drive to increase Li-ion battery energy density, as well as support faster charge discharge speeds, electronic conductivity networks require increasingly efficient transport pathways whilst using ever decreasing proportions of conductive additive. Comprehensive understanding of the complexities of electronic conduction in lithium-ion battery electrodes is lacking in the literature. In this work we show higher electronic conductivities do not necessarily lead to higher capacities at high C-rates due to the complex interrelation between the electronically conducting carbon binder domain (CBD) and the ionic diffusion within electrodes. A wide body of literature is reviewed, encompassing the current maxims of percolation theory and conductive additives as well as the relationships between processing steps at each stage of electrode manufacturing and formation of electronic conduction pathways. The state-of-the-art in electrode characterisation techniques are reviewed in the context of providing a holistic and accurate understanding of electronic conductivity. Literature regarding the simulation of electrode structures and their electronic properties is also reviewed. This review presents the first comprehensive survey of the formation of electronic conductivity networks throughout the CBD in battery electrodes, and demonstrates a lack of understanding regarding the most optimum arrangement of the CBD in the literature. This is further explored in relation to the long-range and short-range electrical contacts within a battery electrode which represent the micron level percolation network and the submicron connection of CBD to active material respectively. A guide to future investigations into CBD including specific characterisation experiments and simulation approaches is suggested. We conclude with suggestions on reporting important metrics such as robust electrical characterisation and the provision of metrics to allow comparison between studies such as aerial current density. Future advances in characterisation, simulation and experimentation will be able to provide a more complete understanding if research can be quantitatively compared.
Article
Full-text available
The lithium-ion cell has been successively improved with adoption of new cathode electrochemistries, from to higher-capacity to lower cost . The addition of conductive additives to cathode materials has been demonstrated to improve each type. Four systems have emerged as important cathodes in recent studies: (i) the spinel , (ii) , (iii) the “Gen 2” material, , and (iv) the system. The architectures of model composite cathodes were generated using our prior approach in simulating packing of polydisperse arrangements; conductivity was then simulated for several realizations of each case. A key finding was that the conductive coatings significantly improve overall conductivity. Percolation was achieved for the volume fraction of active material in studied cases, which was larger than the percolation threshold for a 3D spherical particulate system. Neither surface nor bulk modifications of active-material particle conductivities seem desirable targets for improvement of laminate conductivity at present. As part of future work, trade-offs between conductivity and capacity will be considered.
Article
Carbon films have been synthesized by chemical vapor deposition (CVD) on AISI 304 stainless steel (304SS) sheets with various C2H2/H-2 flow ratios at 810 degrees C. The films exhibit three different morphologies and structures: filament, sphere and transition types at different C2H2/H-2 flow ratios, as characterized by scanning electron microscopy, X-ray diffraction and Raman spectroscopy. It was found that the degree of graphitization increased with decreasing C2H2/H-2 flow ratios. The carbon film modified 304SS sheets were used as cathode current collectors and coated with an active layer containing LiMn2O4 active materials, conducting additives and binders for lithium ion batteries. The electrochemical properties of these LiMn2O4 cells with bare and carbon film modified current collectors were investigated. Under high current operation, such as 3000 mA/g, the capacity of the LiMn2O4 cell with transition type carbon film modified current collector is 55% higher than the cell with bare current collector. The enhanced performances of high current density charge-discharge cycles can be attributed to the reduced contact resistance and improved charge transfer efficiency provided by the transition type carbon film modified current collectors.
Article
The rate and cycling performances of the electrode materials are affected by many factors in a practical complicated electrode process. Learning about the limiting step in a practical electrochemical reaction is very important to effectively improve the electrochemical performances of the electrode materials. Li4Ti5O12, as a zero-strain material, has been considered as a promising anode material for long life Li-ion batteries. In this study, our results show that the Li4Ti5O12 pasted on Cu or graphite felt current collector exhibits unexpectedly higher rate performance than on Al current collector. For Li4Ti5O12, the electron transfer between current collector and active material is the critical factor that affects its rate and cycling performances.
Article
The performance of a porous electrode is strongly related to its electrical properties, such as the effective conductivity of the coating and the contact resistance between the coating and the current collector. This work presents a new method to measure both the effective conductivity and the contact resistance with a single measurement. No preparation is necessary for this, other than cutting a disk shaped electrode and measuring the thickness of the coating. The method is applied to three different cathodes and an anode as a proof of concept.
Article
The effects of current collector on the charge/discharge capacity and cycle stability of Li4Ti5O12 (LTO) electrode under high C-rates (up to 20 C) have been investigated by applying five types of current collectors, including a Al foil, an anodization-etched Al (E-Al), the same etched Al with a conformal C coating (C-E-Al), a Cu foil (Cu) and the same Cu foil with a C coating (C-Cu). The C coatings on both metal current collectors are deposited by a chemical vapor deposition process using CH4 at 600 °C. The capacities of the LTO electrodes above 1 C rate are in the order of Al < E-Al < Cu ∼ C-E-Al < C-Cu, exhibiting remarkable enhancement in rate performance by the C-coating for both metals. Surface analyses indicate that the enhancement can be attributed to the combination of two factors, including removal of the native oxide layer and modification of surface hydrophobicity, which improves adhesion of active layer, on the current collector surface. Both contribute to the reduction of the resistance at the current-collector/active layer interface. All electrodes show good cycle stability.
Article
We present order-of-magnitude conductivity data for “carbon-included” lithium iron phosphate (LFP) powders lightly pelletised as used as cathodes in Li-ion batteries. The powders were synthesised by a spray pyrolysis method, with a short ameliorating sinter to optimise phase purity. Carbon was introduced into the materials by adding stoichiometric amounts of sucrose into the starting ingredients. We obtained X-ray diffraction patterns and electrical conductivity estimates for carbon contents of between 0 and 31 wt.%. The resultant conductivities spanned almost seven orders of magnitude.
Article
LiCoOâ the active cathode material in commercial rechargeable lithium batteries, is shown to be a p-type semiconductor, associated with the presence of a small concentration of CO{sup 4+} ions. Its conductivity at room temperature can be increased by over two orders of magnitude, to â¼0.5 S/cm, by partial substitution of CO{sup 3+} by Mg{sup 2+} and compensating hole creation. The electrochemical performance of LiMg{sub 0.05} Co{sub 0.95}Oâ is comparable to that of LiCoOâ; a small reduction in capacity, associated with a reduction in Co{sup 3+} content, occurs but good reversibility is retained and, in contrast to LiCoOâ, the Mg-doped material is single phase throughout the charge/discharge cycle.
Article
The charge/discharge capacity and cycle stability at high C-rate of LiFePo4 (LFPO) electrodes using three types of Al current collectors, including smooth un-etched Al foil, anodization-etched Al foil, and the etched Al foil covered with a conformal C coating grown at 600°C in CH4, were investigated. The results unequivocally demonstrate the strong effects exerted by the surface structure and composition of the Al current collectors on the power performance. In particular, the use of the C-coated current collector not only remarkably increases the power-delivering capability, by 3–7-fold based on different comparison criteria, of the LFPO electrode, but also greatly enhances its cycle stability under high C-rate (5C). The rate enhancement exceeds that of a low-temperature organic-bound C-coating reported in the literature. The enhancements are consistent with observed reduction in overall charge-transfer resistance, which can be attributed to the removal of the native insulating oxide surface layer of the current collector and to the improved adhesion at the active layer/current collector interface. This current collector is also applicable to other cathode and anode (e.g., Li4Ti5O12) materials of Li-ion batteries for the same beneficial effects.
Article
We performed experimental studies to determine electronic properties of multilayered LiFePO4 cathodes in order to quantify reductions in LiFePO4 matrix resistivity and/or contact resistances between matrices and current collectors by addition of carbon black and graphite. In order to extract these layerwise and interlayer properties, we extended the Schumann-Gardner approach to analysis of a four-point probe experiment and solved the resulting coupled nonlinear equations numerically. We studied five cathodes with varying amounts (3-12 wt %) and types (carbon black, graphite) of conductive additives. LiFePO4 particles within the electrodes were precoated with carbon before mixing with additives and binder. Experimental results showed reductions of similar to 62% in electrical resistivities of LiFePO4 matrix with addition of carbon black from 3 to 10 wt %; addition of graphite additives produced only small reductions. For concentrations above 6 wt % of conductive additives, homogeneous electronic resistivities were observed. Contact resistances at interfaces between LiFePO4 matrix and carbon coating of current collector and between carbon coating and current collector were similar in all cases, indicating consistency in manufacturing. Future work will focus on combining models for capacitive loss with models for conductive properties, along with experimental verifications. (c) 2005 The Electrochemical Society.