CHAPTER- output of a battery is dependent on the

CHAPTER-
I

INTRODUCTION

We Will Write a Custom Essay Specifically
For You For Only $13.90/page!


order now

1.1.
Introduction to Li- ion batteries:

Energy
demand and supply has always been one of the crucial factors for the evolution
of civilization. Energy in the form of electricity is produced from solar,
wind, nuclear power, burning fossil fuels, etc; however, production of
electricity from renewable sources like, solar and wind need a storage device
for their effective usage during depletion time. In this context, electrochemical
energy storage devices such as batteries play an important role in the
efficient use of renewable energy. Battery is a collective arrangement of
electrical cells that stores and produces electricity by chemical reaction;
storage and release is realized by electrons and ions.1
The
first battery, Voltaic pile was developed by Volta in the year 1800. It
consists of a series of copper and zinc discs separated by card boards
moistened with salt solution. With more than 200 years of development, battery
technology has achieved an era that batteries are safely used for transport of
electricity without heat loss. They can be made in all varieties of sizes and
shapes and useful for various applications.

Batteries
are mainly of two types, viz.

1.
Primary Batteries.  

2.
Secondary batteries.

Primary
batteries are assembled in charged condition and the electrochemical reaction
is mostly irreversible. Examples: Lechlanche, alkaline MnO2, silver oxide,
and zinc/air batteries.2Electrochemical
reaction associated with secondary batteries is reversible. Hence the battery
can be charged/ discharged for a number of cycles and are named as rechargeable
batteries. Examples: lead-acid, nickel-cadmium, nickel-metal hydride and
lithium ion batteries. According to the chemical reaction involved,
rechargeable batteries can further be classified as lead-acid, nickel-metal-hydride,
zinc-air, sodium-sulphur, nickel-cadmium, lithium ion, Li-air batteries etc.

Among
the various rechargeable batteries aforementioned, Rechargeable Li-ion
batteries have gained considerable interest in recent years in terms of highest
specific energy, cell voltage, good capacity retention and negligibly small
self discharge.3

It
is desirable that the energy delivered by a battery during its discharge should
be as high as possible. The energy output of a battery is dependent on the
equivalent weight of active material present in it. Specific capacity i.e.
capacity per gram of active material= 26.8/Equivalent weight Ah.g-1.

Lithium
metal being the third lightest element, lithium based materials with low
molecular weight can effectively produce batteries with high capacity. Further,
lithium ion batteries employs non aqueous electrolyte that offer high operating
voltage (>4V) in comparison to other batteries with aqueous electrolyte
(1-2V). Thus, low weight, compact lithium ion batteries established a strong
market place for portable electronic devices and could find central application
if lithium ion batteries in electric vehicles become reality.4-5

 

1.2.
Basics of Lithium ion batteries

 

Lithium
ion battery consists of three main components, positive and negative electrode
separated by a separator dipped in electrolyte. Negative electrode is normally
an electron donor group which is electropositive in nature like lithium metal.6
Positive electrode is normally an electron acceptor which is strongly
electronegative (e.g. LiMO2 (M= Co, Ni, Mn, etc) compounds). During discharge
process, the negative electrode electrochemically oxidised and releases
electron. This electron moves through outer circuit to the positive electrode
which accepts electron. In Fig. 1.1, the schematic of lithium ion battery
operation is explained using LiCoO2 as positive electrode and carbon
as negative electrode.

 

Positive
electrode:   LiCo3+O2          Charge/Discharge   xLi++
Li1-xCo4+
xCo3+
1-xO2
+
e-

 

Negative
electrode:  C+ xLi+
+
e-   Charge/Discharge   LixC

 

Overall:                    LiCoO2+
C    Charge/Discharge  Li1-xCoO2+
LixC

 

Figure 1.1,
Schematics of Li-ion battery using LiCoO2 as
cathode and graphite as

anode.

During charge,
Li+
moves
from LiCoO2 to carbon through the electrolyte
which causes oxidation of Co3+ to Co4+
and
the reverse happens during discharge; Li+ moves
from carbon to LiCoO2. Role of electrolyte
is to act as a medium for the transfer of ions between the two electrodes. In
general, lithium salt dissolved in organic solvent is used as electrolyte in
lithium ion batteries. Main requirements for the electrolyte are,7
(i)
it should be a good Li-ion conductor and electronic insulator (ii) stability
over the operating voltage window (iii) chemical compatibility with cell
components and electrodes (iv) thermal stability and (v) there should not be
any charge accumulation and concentration polarization. Material that undergoes
chemical reaction producing current during battery operation is known as active
mass or active material.2
In
batteries the electrode itself takes part in chemical reaction apart from being
a charge transfer media. Consequently, the chemistry associated with electrode-electrolyte
interface and bulk of electrode are the main factors that determine the battery
performance. Thus performance of lithium ion battery crucially depends on the
nature of electrode material used.

 

1.3
Parameters of Electrode Quality

Parameters that
are used to validate the quality of electrode material are,

·        
Cell voltage

·        
Conductivity

·        
Specific capacity

·        
Coulombic efficiency

·        
Capacity retention (stability/ cycle
life)

·        
Gravimetric and volumetric energy
density

·        
Power density

·        
Cost, toxicity and safety issues

 

Cell
voltage: Cell voltage is represented by open
circuit voltage (voltage between the two terminals when no external current
flows) or closed circuit voltage (voltage between the two terminals when it is
connected with external circuit).5
Open circuit voltage Voc is calculated from the
chemical potential of the negative (mNLi)
and positive electrode (mPLi)
as,

                          Voc=

 

Where,
F is Faraday constant 96485 JK-1.

Thus
chemical potential of positive electrode should be high and that of negative electrode
should be low in order to achieve high cell voltage. In addition for the electrode
to be thermodynamically stable, redox energies of the electrode should lie within
the band gap of electrolyte material Eg.

 

Conductivity:
Electrode material should be capable of conducting electrons as well as lithium
ions for better battery performance. But many lithium ion insertion materials
are semi conductors by nature. Some of them are even highly insulating
(conductivity <10-9 S.cm-1). Conductivity of the electrode is usually improved by mixing the electrode material with conducting carbon.   Coulombic efficiency: Coulombic efficiency is the ratio of number of charges that enter the battery during charging to the number of charges that can be extracted during discharge. Secondary reactions associated with electrolyte side reactions, structural instability of the electrode material and impurity in electrolyte etc., cause loss of electron during charging and reduce the coulombic efficiency. Electrode material with more than 95% of coulombic efficiency is considered as better electrode. Specific capacity: Specific capacity i.e. capacity per gram of active material= F ? ?X / Molecular weight Ah.g-1 Where, F is the Faraday constant 26.8Ah. F= 96,485 Coulombs = 96,485 A. Sec / 3600 Sec = 26.8 Ah ?X is the amount of reversible lithium. Materials with low molecular weight and higher lithium reactivity would deliver high capacity. Capacity retention/ stability/ cycle life: Capacity retention is derived from the number of cycles that the cell undergoes charge-discharge processes. Although factors like electrolyte stability, temperature, etc, influence the capacity degradation; phase stability of the electrode is the prime component in determining the cycle life of a cell/ battery. Gravimetric and volumetric energy density: Energy density is the energy per unit weight (Gravimetric Wh.kg-1) or unit volume (volumetric Wh.l-1). Gravimetric energy density= (Specific capacity/ kg) ? Cell voltage Volumetric energy density= (specific capacity/ litre) ? Cell voltage Power density: Power density (W/kg or W/ litre) is the power of the battery per unit weight that represents the speed at which the energy can be delivered to the load. Power density= Current (A/kg or A/ litre) ? Voltage (V) = Energy density/ Time. Power of the battery depends on the cell impedance, lithium diffusion through the electrode, electrolyte and other components. In many cases lithium ion diffusion through the electrode material is the limiting factor and determines the power of the battery.                               1.4. Electrode materials for Li- ion batteries:   Compounds that accommodate Li-ions (Li+) either in vacant sites or having open channels (1D, 2D or 3dimensional) for Li-ion migration are capable of reversible lithium insertion. Thus transition metal compounds with vacant sites to accommodate lithium have been studied as lithium insertion material. Further they have the ability to exist in various oxidation states and hence lithium exchange could get compensated with electron flow in the outer circuit. Other materials that do not have vacant sites for accommodating lithium have also been found to be potential candidates for the lithium ion batteries. They react with lithium by a different mechanism such as conversion,8 alloying9 etc; that is different from that of insertion mechanism. Some of the technologically important positive and negative electrode materials are discussed briefly in the following section   1.4.1. Positive electrodes (cathodes): Electrode materials that exhibit a potential of greater than 2.25V vs. lithium metal are considered as positive electrode materials. The main lithium ion battery cathode materials are layered compounds such as lithium transition metal oxides (LiMO2), spinels like LiMn2O4, olivines such as LiFePO4, compounds crystallizing in NASICON related structure and other transition metal oxides like MnO2, V2O5 etc.10   1.4.1. LiFePO4 as cathode materials for Li-ion batteries: The cathode materials discussed before, Spinels and layered compounds are reported to show improved electrochemical performance. However, they suffer from capacity fading on long cycling for instance, greater than 1000 cycles. The stability of fully charged material is also poor. In this regard, an olivine type compound LiFePO4 with high safety and long cyclability had been identified as the strongest candidate for Li-ion battery application.11 LiFePO4 is a natural mineral known by the name Triphylite. Its crystal structure was first analysed by Yakubovich in the year 1977.12The structure consists of corner shared FeO6 octahedra and edge shared LiO6 octahedra which are linked by PO4 tetrahedra (Fig. 1.5). Lithium reactivity of LiFePO4 was first recognised by Padhi et.al., in the year 1997.13 They showed that lithium can be electrochemically extracted from LiFePO4 thus leaving iron phosphate (FePO4) with same space group of LiFePO4. Cycling behavior of LiFePO4 versus lithium is shown in Fig. 1.6. About 1 lithium can be electrochemically extracted from LiFePO4 which accounts to a capacity value of 170 mAh.g-1 which is close to the theoretical capacity (complete removal of one lithium). Lithium extraction and reinsertion proceeds via a two-phase process and the ordered olivine framework maintains during cycling.13-14 Hence, LiFePO4 is capable of cycling for thousands of cycle without capacity loss.15 Electronic conductivity of LiFePO4 is very low (10-9- 10-10 S.cm-1) due to strong covalent nature of bonds.16 Lithium diffusion through these olivine structures is restricted to tunnels along   Figure 1.5, Structure of LiFePO4 that is built with FeO6 octahedra linked by corners to SO4 tetrahedra. Figure 1.6, Voltage vs. composition curve of LiFePO4 cycled in the voltage window of 2.75- 4 V. the 'b'-axis and are one dimensional ionic conductors. Poor electronic conductivity and slow diffusion of lithium requires charge-discharge process at much slower rate to realize the full capacity. With increased specific current rate, insertion/extraction limited to 0.6 lithium. However, conductivity and hence rate capability of LiFePO4 is shown to be improved by, (i) making small particles in the nano regime (ii) coating with conducting carbon (iii) doping with other metal ions to achieve increased intrinsic electronic conductivity.17-19 Another drawback of LiFePO4 is the low redox potential (3.5 V), thereby lower energy density (specific capacity ? voltage). Thus LiNiPO4 (5.2 V),20 LiMnPO4 (4.1 V),21 LiCoPO4 (4.8 V)22 and other substituted phosphates are studied as alternatives; nevertheless, they have not exceeded the capability of LiFePO4 till date.23   1.4.2. Negative electrodes (Anodes):   Ideally lithium metal should be the right anode material for lithium batteries since its low molecular weight and higher specific capacity. Capacity of 3.861 Ah.g-1 could be attained using lithium metal as anode. However use of metallic lithium as anode in rechargeable batteries cause difficulty in terms of safety and reversibility. The morphology of lithium deposited during charging process is different from that of lithium metal. The needle like deposit thus produced is known as dendrite7 and may become electrically isolated from the lithium metal due to non-uniform dissolution of lithium at different portions during continuous charge-discharge cycles leading to capacity loss.   1.5 Graphene (Introduction and its role) Carbon is a very crucial element present in nature. This non-metal is the most fascinating element in the periodic table. Diamond, Graphite, fullerenes and nanotubes are the allotropic forms of carbon. 1 Graphene is the latest discovered allotropic form of carbon and has excellent mechanical and electrical properties. A two-dimensional form of carbon atom discovered recently is named as graphene. The carbon atoms are arranged in hexagonal form. Each unit cell has two atoms. a The Close resemblance of graphene with the diral spectrum makes it alluring for the research.b The discovery of the two-dimensional allotrope of carbon in 2004 revolutionized the carbon based electronics field. Graphene exhibit excellent properties like high young's modulus (1TPa)c, high thermal conductivity, high charge carrier mobility (200,000 cm2V-1s-1). d The exceptional chemical and physical properties exhibited by Graphene has made it a potentially used material in various fields like polymer composites, energy storage devices, sensors, field effect devices etc. 1.1Structure of Graphene The structure of graphene consists of a single layer of graphite with sp2 hybridized carbon atoms packed in hexagonal form in a single layer of graphite. Various types of graphene are as follow 1.      Monolayer- the sheets are not stacked and available in rippled form. 2.      Bi-layer- it is also known turbo static graphene without any stacking order. 3.      Few-layer Graphene – has various stacking arrangements like stacking, rhombohedra stacking etc. 3 The Zigzag motifs or the armchair edges of the graphene leads to different magnetic and electronic properties. 1.6 Role of carbon coating on LiFePO4 cathode materials.   Coating of the LiFePO4 surface with Cu, Ag, carbon, or conducting polymers is very effective in improving the electronic conductivity of the powders.45 Although metal additives may effectively improve the conductivity of LiFePO4, it is difficult to achieve a uniform metal dispersion on the surface of LiFePO4.45 Metal oxidation to form insulating films or soluble ions that might interfere with the negative electrode cyclability is also to be taken into account while carbon is well known for its stability and compatibility and use in practical composite cathode. Furthermore, use of a high-cost metal additive is not suitable for large-scale applications of the LiFePO4 products. The conductive polymers such as polypyrrole generally have poor mechanical properties and instability.46 Although recently poly-(3,4-ethylenedioxythiophene) (PEDOT) chemically grown on partially delitiated LiFePO4 seems to lead to good cyclability,47by comparison, carbon coating has been particularly attractive with respect to its high conductivity using carbon concentrations as low as 0.5–2 wt%,50–53 its low cost, and simplicity of introduction during or after the LiFePO4 synthesis and most importantly its proven use as a conductivity additive in composite electrodes and its chemical stability in the battery. A combination of carbon coating and fine particle size can improve the rate performance and material utilization for LiFePO4 54–56 and has been commercialized.13,56 To make high capacity, low cost LiFePO4 batteries with a suitable size, it is desirable to reduce the amount of carbon and improve the carbon coating quality. Although numerous reports in the literature have focused on C/ LiFePO4 composite materials and great progress has been achieved in this field in the recent years (Fig. 4),57,58 the state-of-art understanding of the role of carbon in the C/LiFePO4 composite material is still scarce.