INSIGHTS | PERSPECTIVESsciencemag.org SCIENCEGRAPHIC: A. KITTERMAN/SCIENCELITHIUM BATTERIESReining in dissolved transition-metal ions
Approaches are needed for stabilizing transition metals in lithium-ion battery cathodes
By Hooman Yaghoobnejad Asl and
Arumugam ManthiramW
ith an increasing deployment of
large-size lithium-ion batteries
(LIBs) for applications beyond
consumer electronics, critical
questions surround their life span
and safety. The LIB technology is
based on oxide cathodes and graphite an-
odes developed in the 1980s ( 1 ). The current
~350 cycle-life warranty (based on 100,000
miles and 300-mile range per full charge)
( 2 ) provided by major electric vehicle man-
ufacturers falls short of the 1000 cycle-life
target sought by the U.S. Department of
Energy ( 3 ). As a result, a major focus has
been to understand the fundamental fac-
tors that cause the degradation of LIBs.
Among them, dissolution of transition-
metal (TM) ions from the cathode into the
liquid electrolyte has been recognized as a
leading cause. We discuss the causes of the
dissolution of certain metal ions from the
cathode into the electrolyte, including the
possible role of electronic structure.
A typical commercial lithium-ion cell has
a lithiated TM oxide (TMO) cathode (posi-
tive electrode), such as Li(Ni1/3Co1/3Mn1/3)O 2.
These cathodes are variants of the very sta-
ble but expensive LiCoO 2 cathode in which
some of the Co is replaced with less costly
TMs. The challenge is that these replacement
TM cations tend to dissolve into the liquid
electrolyte and migrate and deposit on the
graphite anode. Once there, they increase
the solid-electrolyte interphase (SEI) layer
thickness on graphite, trap cyclable lithium
in the SEI, and raise the cell impedance,
thereby limiting the cell cycle life ( 4 , 5 ). The
thicker SEI on the graphite anode also pro-
motes lithium dendrites, which can result in
fire and safety hazards.
The problem of TM-ion dissolution is ag-
gravated by the cathode chemistries that
involve Jahn-Teller (J-T) distorted TM ions
and can be attributed to trace amounts of
hydrofluoric acid (HF) generated in the
electrolyte. For example, metal dissolution
is substantially higher for cathode composi-
tions containing the J-T–active Mn3+ ions ( 6 ,
7 ). This finding raises the issue of delineat-ing the complex chemistry involving the H+
acidic species, J-T distortion of TMOs, and
the enhanced TM-ion dissolution observed.
The electronic structure of TM cations is
modified by J-T distortion. Most of the TM
cations in TMOs adopt an aggregation of six
oxide ligands arranged symmetrically along
the Cartesian axes, giving rise to an octahe-
dral (Oh) coordination (see the first figure,
left). The Oh ligand electric field separates
the d orbitals of the TM cation into triply
degenerate t2g (dxy, dxz, and dyz) and doublydegenerate eg (dz 2 and dx (^2) - y 2 ) sets. This ar-
rangement is preferred for TM cations that
have a cubically symmetric electron density
that is achieved when the eg orbital set is
completely filled, half-filled, or empty (for
example, t2g^3 eg^0 , t2g^3 eg^2 , and t2g^6 eg^0 ).
However, for certain TM cations, includ-
ing high-spin Mn3+, high-spin Fe4+, and low-
spin Ni3+, the electron density follows a non-
cubic distribution as a result of the single
occupancy of the doubly degenerate eg set
(for example, eg^1 ). This unfavorable state is
relaxed according to the J-T theorem by an
elongation of two axial TM–O bonds and a
shrinkage of the other four equatorial ones,
which reduces the TMO 6 symmetry from cu-
bic to tetragonal (see the first figure, right).
Note that the Co3+/4+ (high-spin or low-spin)
ions in the commonly used LiCoO 2 cathode
(Co3+: t2g^6 eg^0 ; Co4+: t2g^5 eg^0 ) in LIBs avoid the
problematic eg^1 electronic state and result-
ing metal-ion dissolution and help account
for their successful implementation in first-
generation LIBs.
The J-T effect can vary in strength with a
global, macroscopic distortion of the crystal
structure, as is the case often with Mn3+, or
a local, microscopic distortion, as is usually
observed with TMOs with Ni3+ and Fe4+.
In the local distortion case, the individual
TM–O bond lengths may vary without af-
fecting the average macroscopic structure,
which leads to a dynamic lattice instability.
The strength of J-T distortion also corre-
lates inversely with the covalency of TM–O
bonds, which increases in the order Mn3+ <
Ni3+ < Fe4+ because of an increase in the ef-
fective nuclear charge (Mn < Fe < Ni) com-
bined with a higher formal charge on Fe4+.
Thus, the local J-T distortion in TMOs with
dynamic lattice instability also makes them
reactive enough toward acidic species and
susceptible to TM-ion dissolution.
Understanding how J-T distortion,
whether global or local, can cause TM-ion
dissolution in acidic electrolytes can be ad-
dressed best through molecular orbital (MO)
considerations. The axial-equatorial splitting
of TM–O bonds alters the degree of overlap
of the atomic orbitals of TM cations and O
ligands. The eg^1 electronic configuration
(e.g., dz 21 dx (^2) - y 20 ) allows the empty dx (^2) - y 2 or-
Materials Science and Engineering Program and Texas
Materials Institute, University of Texas at Austin, Austin, TX
78712, USA. Email: [email protected]
xy
Octahedral feld
Mn4+ (3d^3 )
Tetragonal feld
Mn3+ (3d^4 )
Jahn-Teller (J-T) distortion + electron
xz yz
z^2 x^2 - y^2 z^2
eg
t2g
x^2 - y^2
xy
Energy-level splitting of d orbitals
xz yz
Mn4+
O
O
Mn3+
140 10 JULY 2020 • VOL 369 ISSUE 6500
Jahn-Teller (J-T) distortions
The ligand field in manganese oxides used in lithium-ion battery cathodes distorts the six equivalent
metal- oxygen bonds (left) into two longer axial bonds and four shorter equatorial bonds (right). Charge is
transferred from the metal cation to the axial oxygens, which become more basic.