Introduction to Lithium-Ion Batteries
1.6. Drawbacks and Failure Modes of Lithium-Ion Batteries
The energy density of the current LIBs is limited by the capacity and potential of the of cathode. So the key to improve energy density of LIBs lies in the choice of appropriate cathode materials, that have higher redox potentials (~5 V) and can reversibly intercalate more than one lithium ion per atom. But, the low electrochemical stability window (~0.8 V – 4.5 V) of the organic carbonate-based electrolyte avoids the usage of high-voltage cathodes. In addition, these organic electrolytes are highly flammable posing serious threats to safety. On battery getting
PF6 BF4
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overheated, there is uncontrolled rise in temperature generating flammable gases during which separator tends to burn, trigger short circuiting, resulting explosion of cell. So, there is an utmost need of safe cathode materials having high thermal stability with high operational voltages and capacity that can guarantee an improvement in the energy density, as well as a suitable electrolyte with high thermal stability and broad electrochemical voltage window that may be well-matched with the high voltage cathodes [329]. The thermal runaway in LIB is schematically shown in Fig. 1.8.
Fig. 1.8: Thermal runaway in a lithium-ion battery (Adopted from Ref. [329])
Another major factor that is critical to the battery performance is solid electrolyte interphase (SEI) formation (below 1V) during lithiation of graphite and silicon anodes. The electrolyte reduces on the electrode surface during initial charge (lithiation) which form a passive layer that is electronically insulating but is ionic conductive. Typically a good SEI thickness is about 10s of nms. SEI acts as an interphase between the metal and the electrolyte solution and stops further degradation of the electrolyte and anode after the second charge. The SEI is composed of inorganic components such as LiF, Li2CO3 as well as organic components like (CH2OCO2Li)2, ROCO2Li which are partial or complete reduction products of the electrolyte solvents and salts (Fig 1.9). Formation of polymeric compounds on the surface causes formation of bad SEI.
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Fig. 1.9: (a) Diagrammatic representation of SEI formation, and (b) the reaction mechanism of SEI formation (Adopted from Ref. [330])
In graphite and Si anodes, SEI formation causes loss in capacity of about 25- 50%, called as irreversible capacity. An efficient, stable SEI stabilizes the electrode surface and helps in good cycle life of the battery, avoiding additional decomposition of electrolyte in succeeding cycles although allowing migration of Li+ through the electrode/electrolyte interface. The development of a surface layer (SEI) is also observed in cathodes, but in contrast to anodes, the decomposition of the electrolyte takes place not only in first charge-discharge cycle but electrolyte decomposition happens even in successive cycles at high operating voltages >4 V, resulting in a thicker SEI which stops the electronic and ionic conduction, resulting in cell failure.
39 1.7. Scope and Objectives of the Thesis
US Department of Energy (DOE) year 2022 goals to develop next generation lithium ion batteries (LIBs) that have high energy density (300-400 Wh kg-1) which enable a large market penetration of hybrid and electric vehicles (HEVs and EVs) [330].
Besides, the LIBs should have reduced cost, improved safety and cycle life. State-of- art LIBs use transition metal (TM, such as Ni, Mn, Co) oxide (LCO, NMC-3332 or LiNi0.8Co0.15Al0.15O2) or olivines (LFP, LMP), spinels (LiMn2O4, LiNi0.5Mn1.5O4
(NMS) based cathodes and graphitic carbon as anode [326]. The nominal capacity of most of these cathodes are in the range of 120-180 mAh g-1 when cycled up to 4.2 V, is only half the specific capacity of graphite anode (theoretical capacity = 372 mAh g-
1) [321]. Thus, there has been an intense research activity during the last decade to develop high capacity-high voltage or high energy cathodes for LIBs. LMR-NMC cathodes which has almost double the capacity (372 mAh g-1 for 1.2 Li transfer) of currently available cathodes have been investigated as a promising cathode material for LIBs [312-318]. To get high capacity from LMR-NMC, the material need to be electrochemically cycled above 4.4 V. During high voltage cycling (> 4.4 V), oxygen release takes place from Li2MnO3 component in the form of Li2O and MnO2 which causes interfacial instability of LMR-NMC electrode. The major issue of LMR- NMC is voltage decay mostly due to structural transformation of layered to the spinel structure. An improved interfacial stability, capacity retention and rate performance of LMR-NMC can be achieved by surface coatings such as metal oxides like Al2O3, ZrO2, TiO2 etc., metal phosphate coatings (AlPO4, LiFePO4 etc.), and solid electrolyte coating like lithium phosphorous oxynitrides or blending with another cathode material [325]. But voltage fade is not properly addressed by these surface coatings or blendings.
To enhance the performance of anodes which meet the requirement of the automotive industry, researchers have been investigating materials which form alloys with lithium to generate anodes that have specific capacities in an order of magnitude higher than graphite. Silicon is an attractive anode material for Li-ion batteries mainly because of its very high theoretical charge capacity of 4200 mAh g-1 (Li4.4Si) [311, 316]. But silicon has various issues of low electronic and ionic conductivity and most
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important one is high volume change of ~400% during lithiation and delithiation leading to structural degradation (pulverization) followed by capacity fade and reduced cycle life.
Thus, there has been an intense research to mitigate volume change during cycling, such as producing Si-NPs, aligned Si-nanowires/nanotubes, dispersing silicon into an active (such as carbon) /inactive (Eg. SiO2) matrix, silicon based thin films, free standing Si-C electrodes and different morphologies of silicon. Taking benefit of high conductivity of carbon and high capacity silicon, recent reports demonstrated carbon-silicon nanocomposites can circumvent the issues associated with silicon and improve the overall electrochemical performance of Si-anode for Li- ion batteries.
So current research emphasis is given to develop high energy density Li-ion batteries through high capacity-high voltage cathodes, high voltage electrolytes and high capacity anodes.
The studies reported in the present thesis are confined to develop high energy density LMR-NMC based cathodes with minimized voltage decay, coupled with high capacity silicon anodes for increasing energy density in LIBs.
1.7.1. The present study
From the above review of literature on the cathode, anode materials for lithium-ion batteries (Chapter 1), it is clear that next generation LIBs can have double the energy and power density of the currently available LIBs with reduced cost and improved safety. With this background, Chapters 2 of the thesis is an attempt to evaluate energy and power density of currently available LIBs with respect to C rate and temperature.
Chapter 3-5 comprise studies on development of LMR-NMC cathode materials with improved energy density. Chapter 6-7 deals with development of Si anode material for the next generation LIBs.
LIBs operating at high-low temperatures and high C rates pose a technical barrier for operating hybrid and electric vehicles is due to substantial reduction in energy and power. Chapter 2 explains simultaneous effect of temperature and C rate on state-of-health, power and energy density of LIBs. Ragone charts are developed on
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LIBs based on LiCoO2 cathode and graphite anode chemistry at various temperatures between -20 and 55 oC and C rates (C/5-10C). The “Energy-Power Index” (EPI) (%) vs. T profile shows a linear increase from -10 C to 25 C and then it makes a semi- plateau between 25 C and 55 C. Besides, for the optimum performance of LIBs operating at low temperatures, low C-rate application is most preferable.
Chapter 3 describes an effort to improve capacity retention, C rate