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Imaging Battery Fading Energy Science

The mechanism of capacity fading of a composite positive electrode was investigated with
operando neutron powder diffraction combined with transmission X-ray microscopic methods.

ACTIVITY REPORT 2016
he development of electric vehicles might allevi- composite positive electrode within a full cell contain-
T ate our reliance on fossil fuels; among the ener- ing a Li 4Ti 5O 12 (LTO) negative electrode. They devel-
gy-storage devices developed for such applications, oped cooperatively the measurement of an operando
lithium-ion batteries (LIB) have a larger energy densi- transmission X-ray microscope (TXM) at TLS 01B1,
ty than other battery types and have become a lead- and combined the operando neutron powder-dif-
ing candidate. The capacity of an electrode is related fraction (NPD) technique to allow the correlation
to both the number of lithium ions that can be revers- of atomic-scale crystallographic and morphological
ibly inserted into it and its molecular mass. Positive detail to understand the electrode function. This
electrodes have much less theoretical capacity than combination revealed in detail the underlying phase
typical negative electrodes, and thus become a ma- transformations and mechanisms that are respon-
jor performance bottleneck. Rapid developments in sible for the initiation and intensification of particle
advanced techniques such as electric transport have cracking that likely leads to pulverization and capacity
motivated the search for novel positive-electrode fading of this electrode.
materials with improved performance characteristics
such as a large rate capability and energy density. Operando TXM images of a Li 2MnO 3·LiMO 2 particle
The xLi 2MnO 3·(1-x)LiMO 2 (Li 1+ zMO 2 in which M = Ni, surrounded by super-P (carbon black for improved
Co, Mn) system, consisting of Li 2MnO 3 and LiMO 2, is
a promising positive electrode with a capacity ~250- (a)
300 mA h g . Despite this feature, this material has
-1
several drawbacks, including a poor rate and cycling
performance, and a large hysteresis in the charging
and discharging curves, particularly in the first few
cycles. Decreasing the capacity loss of Li 2MnO 3·LiMO 2
(M = Li, Ni, Co, Mn) is thus a major focus of LIB re-
search.

Understanding the atomistic and molecular-scale or-
igin of a battery performance is key to improving the
capacity and cycling performance of electrode mate-
rials. Most layered-oxide positive electrodes undergo
a predominantly solid-solution reaction within their
normal operational voltage window, although a two- (b) (c) (d)
phase reaction during overdelithiation might occur at
higher voltages. Charging beyond the normal operat-
ing range to increase the energy density and capacity
results in a two-phase reaction that is similar to that
occurring in spinel-type materials such as LiMn 2O 4 .
The coexistence of multiple phases over a wide range (e) (f) (g)
of lithium content results in phase bordering and
interface movement through material grains, and
maintaining the structure type during lithiation might
avoid such phase-border shifts and result in better
cycle life.

Fig. 1: Charge and discharge curves of a battery containing
In this work, Ru-Shi Liu (National Taiwan University), Li2MnO3·LiMO2 (a) during operando TXM of a Li2M-
Yen-Fang Song (NSRRC) and Chun-Chieh Wang (NSR- nO3·LiMO2 particle at (b) OCV, after charging to (c) 4.5 V,
RC) investigated the structural and morphological charging to (d) 4.6 V and (e) 4.7 V, and discharging to (f)
+
evolution of the Li 2MnO 3·LiMO 2 (M = Li, Ni, Co, Mn) 3.6 V and (g) 2.0 V vs. Li /Li. [Reproduced from Ref. 1]