1    
INTRODUCTION

 

Lithium batteries power an increasing range of devices
in industrial and scientific applications. Lithium-ion batteries, widely known via
their use in laptops and consumer electronics, dominate the most recent group
of EVs in development.
They are the power source for the modern electric vehicles. They are a type of
rechargeable battery. They are one of the most popular types for portable
consumer electronics such as cameras, laptops and iPads. They have one of the
highest energy densities of any energy storage technology. They are
increasingly being considered for large scale applications such as hybrid
electric vehicles. Li-ion batteries will replace all batteries in the defence
and space sector. Batteries are a key component in all variants of submarines.
Migrating from the familiar lead-acid variant of batteries to the lithium-ion
type for their high energy and high power density capability is the need of the
hour. This variant of batteries has the potential to sustain for a longer
period.

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The classic sealed lead acid battery technology of
1900 is too heavy, too big, and does not last long enough to make these
solutions as practical or efficient as desired. Battery solutions delivering
more electrical capacity at smaller size and less weight is a prodigy in the
subsea Oil and Gas deep water field equipment industry, (Adams and White,
2013).

 

Recent work on new
materials shows that there is a good likelihood that the lithium ion battery
will continue to improve in cost, energy, safety and power capability and will
be a formidable competitor for some years to come. While improvements in specific energy and power
requirements are clearly desirable, improvements in the safety of lithium cells
are also necessary. Throughout this review, developments are noted in the
components of lithium cells that may lead to safer batteries.

 

                 2     Lithium ion CELLS

In today’s world of portable electronics, one can’t go
a day without being in contact with a lithium-ion rechargeable battery. These
batteries power up cellphones, smart watches, tablets, laptops, cars, bikes,
homes, and planes. Lithium-ion batteries are common
in home electronics. They are one of
the most popular types of rechargeable batteries for portable
electronics, with a high energy density, tiny memory effect and
low self-discharge. Li-ion batteries
are also growing in popularity for military, battery electric vehicle
and aerospace applications. For example, lithium-ion batteries are becoming a
common replacement for the lead–acid batteries that have been used
historically for golf carts and utility vehicles. Instead of heavy lead plates
and acid electrolyte, the trend is to use lightweight
lithium-ion battery packs that can provide the same voltage as
lead-acid batteries, so no modification to the vehicle’s drive system is
required.

 

The term lithium-ion battery refers to an entire family of battery chemistries. It is beyond
the scope of this report to describe all of the chemistries used in
commercial lithium-ion batteries. The improvement over
previous aqueous systems was due to the high cell potential and low atomic
weight of lithium compared to all other negative electrode materials that had
been investigated. Lithium-ion has been a
transformative electrochemical cell technology for consumer and industrial
applications, where size or weight or both are driving factors. The following
is a summary providing a basic context for understanding the developments
reviewed in this paper.

 

 2.1. Construction of a Li-ion cell

A typical Lithium-ion cell consists of a
positive electrode (cathode) which is a lithium containing compound, and the
negative electrode (anode) is most commonly a lithiated graphite. Fig 1 shows
overall reaction of a Li-ion cell.

Fig 1: Schematic of a typical lithium-ion cell showing overall reaction of
Li-ion cell.

(Source:
http://www.nexeon.co.uk/technology-2/about-li-ion-batteries/)

 

At
the anode:

LiCoO2
à Li(1-x)CoO2
+ x Li+ + xe-

At the cathode:

xLi+ + xe- + graphite à xLi – graphite

Overall cell reaction:

LiCoO2 + graphite  à xLi – graphite + Li(1-x)CoO2

 

 

Fig 2
shows a schematic where the cathode is a generic lithium metal oxide, shown as
a layered structure in which lithium-ions can be trapped between layers in a
process known as intercalation. At the anode lithium-ions are intercalated
between plates of the graphite. Intercalation is the key process in lithium ion
cells; the anodes and cathodes must have an atomic structure that allows
intercalation and the dimensional changes it imposes. Other cell components
are:

•              The current collectors for each
electrode;

•              An electrolyte that is compatible
with the materials within

    the cell, is stable at the cell voltage and
forms the crucial     

    solid electrolyte interface layer on first
charge; and

•              A separator that allows passage of
lithium-ions, but not

    electrons
(which would internally short-circuit the cell), and

    blocks
the ionic current if the cell overheats.

Fig 2: Schematic of a typical lithium-ion cell showing how lithium-ions are
stored within intercalation materials at the positive electrode (here a lithium
metal oxide) and the negative electrode (here a lithiated graphite). Electrons
flow from anode to cathode in the external circuit, while positively charged
lithium-ions flow from anode to cathode within the cell.

 

Under normal operating conditions, there is no
lithium present in the cell as a free metal. Erikson and Ghanty (2013) provide
a wide-ranging review of future developments in lithium-ion battery technology,
written from the perspective of electrochemists. This simplified review takes each
cell component in turn, and looks at developments taking place against the
background of current cell performance.

 

2.2. Working

During
charge, an external potential is applied between the two electrodes. Lithium
from the cathode moves into the anode. During discharge, Lithium from the anode
moves through the electrolyte as Li+ ions while an electron moves
through the external circuit and perform electrical work.

 

2.3. Properties of electrode

One
of the most prime properties of a Lithium ion battery electrode is voltage,
which is a measure of the difference in free energy of Li between the anode and
cathode. It is given by the following expression:

 

 

Where
VOC is the open circuit voltage and e is the magnitude of the
electronic charge. While generally higher voltage means higher energy
densities, current electrolytes can only support voltages of up to 5V.

 

Another
important property of an electrode is its capacity. The capacity is a measure
of how many Li can be packed in an electrode per unit weight or volume of
material. Energy density and specific energy are other important properties.
They measure how much energy is in the material per unit volume or weight. It
is given by the voltage multiplied by the capacity of the electrode. An
additional property to be discussed is the volume change upon de-lithiation.
Large volume changes during de-lithiation may lead to structural instability.
The stability of a material is another important property of an electrode.
Unstable materials may be difficult or impossible to synthesize. Finally, the
critical oxygen chemical potential is the measure of safety of the material.
Electrodes that evolve oxygen at high oxygen chemical potentials are
potentially unsafe.

 

2.4. Types of electrode

Intercalation
electrodes are the most common type of electrode used. They are essentially a
host structure that can contain lithium. Examples of intercalation electrodes
include LiCoO2, LiFePO4, and graphite. Another type of
electrode material are conversion electrodes. They are less common and they
function by undergoing chemical reaction where the electrode structure is
created and destroyed during operation. Examples include FeF3 and Li2O2.

 

2.5. Positive electrodes (cathodes)

The
cathode material is often the most distinctive feature between different types
of lithium-ion cells. Xu
et al. (2013) gives a critical review of the ‘rather limited number of
cathode materials of significant promise’, summarized as in Fig 3. Most of the
cathode materials under investigation have a theoretical capacity of less than
twice that of lithium cobalt oxide (LiCoO2) prevalent today. LiCoO2,
used in the first commercial cells, has a specific capacity of ~155 mAh/g and a
cell voltage of 3.9 V.

Fig
3: Electrode potentials for anodes and cathodes of
various materials for lithium cells with the theoretical capacity for each
electrode material. Not shown are lithium metal at 0 V and 3860 mAh/g and
silicon at ~0.2 V and 4200 mAh/g as anodes.

 

It is
relatively expensive and more susceptible to thermal runaway when abused,
compared with more recent materials. Other cathodes in use and under
incremental development are described in the following paragraphs.

 

The high cost, toxicity,
chemical instability in the deep charges state, safety concern, and limited
capacity of LiCoO2 have prevented its large scale applications in
transportation and stationary storage. LiMn1.5Ni0.5O4
is a plausible substitute to LiCoO2 for high energy applications due
to its relatively high working voltage of ~4.7V (~4.1V for LiCoO2). Due
to its low cost, low toxicity, high safety, and excellent cycling performance,
LiFePO4 is yet another suitable candidate, promising a specific
capacity of ~170 mAh/g.

 

Lithium
manganese oxide (LiMnO2) is inexpensive and more tolerant of abuse,
but due to structural instability, it has a lower specific capacity than LiCoO2
at ~100 mAh/g – 125 mAh/g. Partial substitution of the manganese leads to
greater cycle life, particularly at over 40°C (Tao et al., 2011). Lithium iron
phosphate (LiFePO4) presents a specific capacity to LiCoO2,
at ~160 mAh/g – 170 mAh/g at 3.45 V, but is much safer. Possessing a very
stable atomic structure, it has great resistance to degradation over charge
discharge cycles and is capable of operating at over 60°C. The main drawback
was low electrical conductivity, leading to low specific power. However,
carbon-coating and/or substituting copper for part of the iron, integrated with
measures to increase electron transport within the cathode (such as the use of
nanoparticles), has led to LiFePO4 cells being available with
specific power in excess of 1000 W/kg. The consequence is that these measures
add to the manufacturing complexity.

 

Although lithium
nickel oxide (LiNiO2) has a high specific capacity at ~275 mAh/g
when pure, is less toxic and is cheaper than the LiCoO2, it is not a
viable option due to its less stability structurally. After a certain number of
charge-discharge cycles, high vulnerability is observed in the layered
structure, generating less capacity for hosting lithium-ions. Experimental
additives which include aluminum, gallium, magnesium and titanium has been
substituted as a part of the nickel, after extensive research in order to
achieve stability among the layers. (Tao et al., 2011). Two types of nickel
compounds – lithium nickel manganese cobalt oxide and lithium nickel cobalt
aluminum oxide – are now found in commercial cells. LiNiMnO2 is more
tolerant of abuse, has a longer cycle life and is cheaper than LiCoO2.
Depending on composition, its specific capacity ranges from 140 mAh/g to 180
mAh/g at 3.8 V. Lithium nickel cobalt aluminium oxide has a higher specific
capacity at ~200 mAh/g at 3.73 V, but has a thermal runaway risk similar to
LiCoO2. Lithium nickel manganese oxide has the highest specific
capacity of cells currently available at ~275 mAh/g at 3.8 V, but has a lower
specific power than the chemistries listed earlier. Chikkannanavar et al.
(2014) evaluates the performance of cells with these blended cathode materials.
A sound number of cathode materials are available that guarantee a potential
performance gain in at least one aspect that determines its importance (i.e.
capacity, cycle life, safety or cost). But, one or more difficulties need to be
overcome if any of these materials are to bloom as a potential replacement of
the current cathode materials. These materials include high-voltage cathodes of
lithium transition metal oxides and lithium-excess transition metal oxides;
High-voltage cathodes of lithium transition metal oxides promise a considerable
increase in capacity over existing cells (Fig 3) achieved primarily through
high cell voltages (4.5 V–5.1 V). However, present day electrolytes are
discordant with these high cell voltages. Although lithium-excess transition
metal oxides having a cell voltage of 4 V are compatible with existing
electrolytes, they undergo a large capacity loss on first charge (Xu et al.,
2013). Lithium transition metal phosphates is another part of the vast cathode
family (Zaghib et al., 2013), but the materials show a low conductivity and
currently cycle life is poor. Having twice the capacity of LiCoO2, conductivity
of lithium orthosilicates is below the already low transition metal phosphates
exponentially. Laboratory experiments have shown that incorporating conductive
polymers and carbon nanotubes can result in cells with usable discharge rates
(Chen, 2013).

 

2.6. Negative electrodes (anodes)

The
anode in a lithium-ion cell plays an integral role in determining the
efficiency of the cell. While being able to support the intercalation process, its
structure must be resistant to damage over the required number of charge-discharge
cycles. Other desirable properties include good ionic and electron
conductivity, tolerance to the electrolyte, and thermal stability. The large
majority of cells in use today use carbon in one form or other. However, there
are several issues with carbon, most notably a theoretical specific capacity at
~372 mAh/g that is lower than alternative materials, Fig 3. A general tendency
for lithium-ions to become lithium metal and to plate the surface of the carbon
electrode during high charge or discharge, especially at low temperature
hinders the intercalation process and risks formation of sharp dendrites of
lithium metal, which may pierce through the separator and lead to an internal
short circuit and thermal runaway. The following paragraphs will elaborate
about the other anode materials under investigation. Having twice the
theoretical capacity of other forms of carbon, Graphene is an ideal candidate
for anode. Yoo et al.
(2008) demonstrated a 50% increase in anode capacity, and showed that a
doubling was possible with the incorporation of carbon nanotubes.(show Chen along with Yoo for this) Lithium metal
was the material used in the first lithium secondary cells by researchers in
the 1970s, with a theoretical capacity of ~3860 mAh/g. Problems such as
reaction with the electrolyte resulting in a high resistance layer, or gas
evolution, combined with the risk of forming lithium dendrites, however, goes
hand in hand with its uses. Addition of additives in the electrolyte can help
reduce formation of dendrites that improve ionic conduction. However, safety of
lithium metal cells are at stake, according to the discoverer of the concept
that intercalation could be applied to lithium secondary cells, conceding that
dendrites ‘always’ form (Whittingham, 2013). Lithium, in the form of lithium
metal nitrides (where the metal can be cobalt, nickel or copper) has roughly
double the capacity of carbon, but these nitrides are unstable if moisture is
present. Another high profile candidate for an anode, projecting a potential
specific capacity of 4200 mAh/g (11 times that of carbon) is Silicon, but disintegration
after a few cycles due to the ~400% volume change during intercalation is a
drawback.

 

Li et
al. (2014) discusses mesoporous silicon (explicitly a sponge with holes of ~20
microns), recording primacy through a high specific capacity of 750 mAh/g and
capacity retention of over 80% to 1000 cycles as well as its ability to resist
pulverization. With all these properties in reserve, particle expansion on full
lithiation was still 30%, and the current density was low at 1.5 mA/cm2.
This suggests that this technology could provide cells with high specific energy
but modest specific power. Just when tin-based alloys seemed to be promising,
the anodes suffered rapid degradation with cycling. A rugged anode was later
designed by incorporating a transition metal and carbon with the tin. Launching
in 2005, Sony was first to use this material in its Nexelion cells. Recent 18650
size cells claim a capacity of 3.5 Ah, above the range of 2.5 Ah – 3.3 Ah for present-day
cells using carbon anodes. The 18650 (18mm
by 65mm) battery is a size classification of lithium-ion batteries. Showing
better long-term capacity retention than the plain material and lower
capacity loss on first charge/discharge as well as better long-term capacity
retention than the plain material and with a stable specific capacity of ~600
mAh/g (over 60% increase on carbon), tin-based oxides vows to be a possible
substitute Wang et al. (2011). A similar specific capacity has been reported by
Chen (2013) for an anode comprising tin oxide particles enclosed within carbon
nanospheres.