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Kinetics of jo is the exchange current, α and β = 1–α are the anode
Lithium Nickel Cobalt Oxide Electrode Material for and cathode transfer coefficients, respectively, D is the
Lithium-Ion Batteries DC in solid phase, f=zF/RT, and η is the overpotential.
Boris Ravdel, Frank J. Puglia
The measurements have been performed in a
Yardney Technical Products, Inc. three-electrode cell of a special design with a potentiostat
82 Mechanic St., Pawcatuck, CT 06379 Solartron 1480. An example of the results of the
bravdel@lithion.com measurements and best-fit approximations to the Eq. (A)
are presented in Figure 1.
The determination of the mechanism and kinetics
of electrochemical reaction is one of the primary goals of In the talk, we present the results of the PS
the fundamental electrochemistry. On the other hand, the measurements in the wide range of DoC, and the results
kinetic parameters of the electrochemical reaction are of calculations will be discussed.
closely related to the battery rate capability. The Acknowledgement
parameters being the intrinsic physical properties of the
material create the natural limits of the material The work was internally funded by Yardney
performance. Technical Products, Inc.
In the electrodes of lithium-ion battery (LIB),
expecting the simplest mechanism of the total reaction as
0.6
a. transport of lithium ions through the liquid solution, b.
±4
charge transfer reaction, and c. transport of lithium ions ±7
0.4
through the crystal, these parameters are the exchange ± 20
current (EC) and the diffusion coefficient (DC) of lithium 0.2
in the host’s crystalline lattice.
I, mA/cm²
The average rate of a multi-step process is 0
defined by the rate of the slowest step, which in the
-0.2
present case could be either the heterogeneous charge-
transfer reaction, or the transport of Li+ ions in the solid.
-0.4
The simultaneous determination of both DC and EC of is
not a trivial problem. Further, both parameters depend on
-0.6
lithium content (concentration), or degree of charge 0 0.05 0.1 0.15 0.2 0.25
(DoC). In electrode materials, the DC can vary in few time, s
orders of magnitude going sometimes over several
extremes when DoC varies from 0 to 1. 1-2 Most of the
Figure 1. Experimental (black) and best-fit (colored)
electrochemical calculations suppose constant values of
chronoamperograms at DoC=0.936
the parameters. One of the electrochemical methods
allowing us to solve the problem is the potential-step (PS) References
method 3 based on applying a small potential pulse up to
1. E.Yu.Nikolskaya, K.I.Tikhonov, A.L.Rotinyan,
±20 mV to the equilibrated electrode and calculating the
B.A.Ravdel. – Soviet Electrochemistry, 24, 532
kinetic parameters from the response chronoamperogram.
(1988).
A new approach to the calculation of kinetic 2. E.I.Toroshchina, B.A.Ravdel, K.I.Tikhonov. – Soviet
parameters using the thermodynamic data has been Electrochemistry, 26, 686 (1990).
proposed about two decades ago. 2,4 Recently, new 3. K. J .Vetter. Electrochemical Kinetics. Academic
interest in the thermodynamics of the process itself 5-9 and Press, N.Y., London, 1967. p. 363
in the connection between thermodynamic functions and 4. B.A.Ravdel. – 186th Meeting of The Electrochem.
kinetic parameters 10 arose. Soc., Miami Beach, FL, Oct. 9-14, p.1994. Abstracts.
5. J. Newman, K. E. Thomas. – 11 IMLB, Monterey,
In this work, we applied the approach 2 to the
CA, June 22-28, 2002. Abstracts, , p.346.
reaction of the reduction-oxidation of LiNi0.8Co0.2O2. The
6. R. Yazami, Y. Reyner, B. Fultz. 11 IMLB, Monterey,
thermodynamic analysis 11 proved that the compound
CA, June 22-28, 2002. Abstracts, p.225.
LixNi0.8Co0.2O2 is a phase of variable composition over
7. K.E. Thomas, J. Newman . – J.Electrochem. Soc., 150,
the range 0.25 < x < 0.95. Thus the elementary electrode
A176 (2003)
step may be re-written in the form of the scheme
8. B. Fultz, Y. Reyner, T. Swan-Wood, J. Graetz, P. Rez,
Li+ + e + HM Li(HM) Y. Ozawa, K. Lam, R. Yazami.– 204th Meeting of
The Electrochem. Soc., Orlando, FL, Oct. 12-16,
and the reaction product, Li(HM), may be regarded as a
2004, Abstracts. p.327.
solution of lithium in the Host Matrix (HM) the
9. Y.F. Reyner, B. Fultz, R. Yazami. – 204th Meeting of
composition of which can be expressed in terms of DoC.
The Electrochem. Soc., Orlando, FL, Oct. 12-16 ,
The equation describing the response to the PS 2004Abstracts. p.1361.
disturbance is 10.Q.Zhang, Q.Guo, R.E. White. – J. Electrochem. Soc.,
2
153, A301 (2006).
j (t ) = I (0)e λ t erfc(λ t ) (A) 11.B.Ravdel, S.A.Trebukhova, and J.F.DiCarlo.. – 42nd
Power Sources Conference, Proc. Volume, 2006, 9.3
where erfc(Y) is the complementary error function, the
formal parameters I(0) and λ are
e β fη
( )
I (0) = j 0 e αfη − e − βFη , λ = j 0 ,
zFc 0 D
