Ionic conduction and vibrational characteristics of Al
3þ
modified
monoclinic LiZr
2
(PO
4
)
3
Tanvi Pareek
a
, Birender Singh
b
, Sushmita Dwivedi
a
, Arun Kumar Yadav
a
, Anita
a
,
Somaditya Sen
a
, Pradeep Kumar
b
, Sunil Kumar
a, *
a
Discipline of Metallurgy Engineering and Materials Science, Indian Institute of Technology Indore, Simrol, Indore, 453552, India
b
School of Basic Sciences, Indian Institute of Technology Mandi, Mandi, 175005, India
article info
Article history:
Received 2 November 2017
Received in revised form
30 December 2017
Accepted 13 January 2018
Keywords:
LiZr
2
(PO
4
)
3
Sol-gel synthesis
Solid electrolytes
Raman spectroscopy
Impedance spectroscopy
abstract
Effects of Al
3þ
substitution for Zr
4þ
in LiZr
2
(PO
4
)
3
on its structure and lithium ion conduction are
investigated. Li
1þx
Zr
2-x
Al
x
(PO
4
)
3
samples prepared via a sol-gel route and calcined at 900
C crystallize in
monoclinic structure with P2
1
/n space group and show a reduction in cell volume with an increase in x.
Raman spectra showed an increase in broadening of higher frequency v
1
& v
3
vibrational modes and a
spectral weight transfer between v
2
& v
4
bending modes of PO
4
tetrahedra with the increase in Al
3þ
doping. Analysis of Raman spectra further suggested that the renormalization of the mode frequencies in
doped samples is controlled by Li-ion motion via strongly interacting with internal bending modes of PO
4
tetrahedra. A significant improvement in ionic conductivity was observed in Al-doped samples, and the
highest conductivity of 1.83 10
4
U
1
m
1
and lithium diffusion coefficient of about 5.7 10
19
m
2
s
1
was observed for Li
1.25
Zr
1.75
Al
0.25
(PO
4
)
3
at room temperature. Li transference number suggested that the
conductivity in Li
1.25
Zr
1.75
Al
0.25
(PO
4
)
3
is predominantly ionic. Activation energy was found to decrease
from 0.58 eV for LiZr
2
(PO
4
)
3
to 0.47 eV for Li
1.25
Zr
1.75
Al
0.25
(PO
4
)
3
.
© 2018 Elsevier Ltd. All rights reserved.
1. Introduction
Fast growing markets of mobile electronic devices, electric ve-
hicles, and smart grids raised the demand for high energy density
lithium batteries. Such high energy density batteries can be real-
ized by utilizing the metallic lithium as an anode. However, the use
of high capacity lithium anode is hampered by the serious safety
issues related to the formation of dendrites during lithium depo-
sition/dissolution cycles [1e5]. Replacing flammable and volatile
liquid electrolytes with inorganic solid electrolytes in rechargeable
lithium batteries could circumvent the safety issues to a large
extent [1 ,2,6e10]. This has led to a renewed focus on fast lithium
ion conducting materials in recent years. Li
x
M
2
(PO
4
)
3
phases is one
such group which is being widely investigated as the potential solid
electrolytes for next-generation rechargeable lithium battery ap-
plications [11e24]. Structural framework of these materials consists
of corner sharing MO
6
octahedra and PO
4
tetrahedra forming a 3D
network of interstitial tunnels through which lithium ion
movement is facilitated. Several compounds with M ¼ Ti, Zr, Sn, Hf,
Sc, etc. have been studied extensively for their structure and elec-
trochemical behavior [21 ,24e29]. Rhombohedral LiTi
2
(PO
4
)
3
based
compounds show highest Li
þ
conductivity among various
Li
x
M
2
(PO
4
)
3
. Electrochemical stability against lithium metal, high
grain boundary resistance, high interfacial resistance, and difficulty
in sintering remain serious issues with the effective use of these
materials as a solid-electrolyte in lithium batteries
[1 ,2,9,22,26,30e34].
LiZr
2
(PO4)
3
(LZP) is another important compound in view of the
high reduction potential against lithium metal which makes it an
attractive candidate for use as an electrolyte in high energy density
batteries. LZP crystallizes in monoclinic (b
0
phase) or triclinic (a
0
phase) structures at room temperature depending on the synthesis
conditions [12e16,18,19,35e39. While there have been some re-
ports on the structure and lithium ion conduction in rhombohedral
LiZr
2
(PO
4
)
3
(a phase), reports on monoclinic LZP are scarce
[13, 16, 18, 19,35,38]. An orthorhombic (b) to monoclinic (b
0
) phase
transition around 300
C on cooling has been reported in low
temperature synthesized b-phase [35,40]. In the b
0
structure
(Fig. 1), fully ordered Li is located in a regular tetrahedral sur-
rounding with no vacant neighboring site available for facile
* Corresponding author.
E-mail address: sunil@iiti.ac.in (S. Kumar).
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
https://doi.org/10.1016/j.electacta.2018.01.087
0013-4686/© 2018 Elsevier Ltd. All rights reserved.
Electrochimica Acta 263 (2018) 533e543