Study Of Electric Vehicle Battery Reliability And Improvement (Term Paper Sample)

STUDY OF ELECTRIC VEHICLE BATTERY RELIABILITY AND IMPROVEMENT
NAME:
REG No:
UNIT CODE: SCH 506
UNIT NAME: Advanced Electrochemistry
DATE:
LECTURER
ABSTRACT
Due to the collective efforts of governments and NGO’s to control environmental pollution by setting emission-restrictions to industrial and automotive sectors, car manufacturers are seeking alternatives. Electric vehicles are increasingly replacing internal combustion engines taking advantage of the gasoline environmental pollution and high demand and prices of fossil. Car battery sits at the heart of electric vehicles and its performance and reliability entirely determines its success now and into the future. A lot of research continues to be done in order to improve battery technology. This term paper is particularly focused on Lithium-ion rechargeable car battery as a source of power for electric cars and ways of improving its performance. It also looks into ways in which Li-ion batteries often fail as well as methods of performance and reliability measurements. Li-ion battery is the center of focus in this term paper over other energy sources for electric cars such as fuel cells because they are the most common and a battery of choice for many Electric car manufacturers.
INTRODUCTION
The automotive industry is estimated to produce between 15 and 25 % of the total pollutants such as carbon dioxide, carbon monoxide, particulate matter, nitrogen oxide that adversely affect the environment. This leads to serious issues such as climate change and global warming. Recently Australia, Amazon have faced the wrath of forest fires as a result of climate change. Governments and NGO’s are taking up the responsibility of environmental protection by setting up emission restriction fuels (Liu, Y. and Helfand, 2009). Apart from setting regulation on the amount fuel consumption, other governments are taking up stringent measures of setting a deadline for automotive manufacturers to completely phase out internal combustion engine for electric cars. France for instance hopes that its roads will be all electric vehicles by 2040, Germany, Israel, India, Ireland, and Netherland by 2030 (Jochem et al, 2015). The demand and rising prices of fossil fuels have also become another motivating factor in favor of electric cars. Electric vehicles uses the power of secondary rechargeable battery technology as its source of power for propulsion in place of the internal combustion engines. Power batteries for propulsion in electric vehicles are predominantly lithium ion batteries. This is because they possess high energy density as well as excellent specific energy of about 140 Wh/kg. Li-ion batteries also retain power for long periods of time with low magnitude of self-discharging rate, long life cycles and are also environmentally friendly (Nayak et al, 2018).
Today, all electric car companies such as tesla, Think, Coda automotive, Wheego electric cars, Fisker automotive, Tango, BYD, Venturi etc have rolled out efficient car models that have challenged the traditional combustion engine car manufacturers. Automotive giants such as Toyota, Nissan, Honda, Ford, Lexus, and Mercedes etc have also manufactured hybrid and all electric. Electric vehicles being cost effective, 100 percent ecofriendly and fast accelerating makes them the cars of the future. The biggest issue that electric vehicle manufacturers face is finding a way of maintaining the performance and dependability of the battery with minimal or no failures over long periods of time. Performance and reliability of electric car batteries is pegged on its ability to provide enough power for sustained long range drive, stable acceleration and high safety mechanism. This are the basic features that customers consider which dictate sales and development of electric car industry (Lu, L Han 2013). Therefore reliability and performance of the lithium-ion batteries in electric vehicles is in the heart of EV development and its future growth.
The very first step of understanding and evaluating the performance and reliability of electric vehicle batteries is by identifying the types of failures they experience. After the identification and evaluation of the failures, appropriate test methods are applied to ensure the EV batteries performance is up to expected standards. This enables the identification of possible design efficiencies, safety levels of the batteries for development and improvement
Basics of Li-ion Batteries
The electrochemical principle
Lithium ion batteries comprise of cathode (positive electrode), anode (negative electrode) and the electrolyte. The electrolyte in the Li-on batteries is usually a salt of lithium (LiPF6 ) dissolved in organic solvent. The electrolyte separates the two electrodes and allows electron transfer between them. During the application of load/ discharging process electrons will move from the anode through the electrolyte to the cathode due to the external applied load. During the charging process, electrons reverse direction and move from the cathode to the anode. At the cathode, reactions use up Lithium molybdenate LiMO2 where the M is the transition metal reactions take place as follows;
LiMO2 xLi+ + xe- + Li1-xMO2
At the anode, the reaction that take place;
xLiC6 xLi + xe- + xC6
Overall reaction;
LiMO2 + C6 Li1−xMO2 + LixC6
Cathode material
The material used at the cathode must be able to produce Li ions since the Carbon anode does not supply ions. The commonly used transition metal-lithium materials are LiCoO2, LiMoO, and LiFePO4. Lithium Cobalt Oxide was first successfully used as cathode but the price and availability of cobalt inhibited continued use. Since then, LiMnO4 has been widely used as the cathode material for Li-ion batteries. It is abundant hence easily accessible. LiMnO4 is also lowly priced and has satisfactory electrochemical properties over LiCoO2 (Chen, x et al, 2012). At high temperatures however, significant dissolution of manganese lowers the capacity.
Anode material
Popular anodic materials are based on carbon, lithium alloys and transition metal chalcogenides (TMC). The electrochemical performance characteristics such as cycle-ability, energy density and rate of charging will depend on anodic material. Carbon was the first anode materials and to date, it still remains as the anodic material of choice in form of graphite. Graphite has excellent cyclability is due to the fact that it has a layered structure that increases mobility of lithium ions.
TMCs such as Cobalt sulfide (CoS), tin disulfide (SnS2), Molybdenum disulfide (MoS2), Tungsten disulfide (WS2 ) are also being reviewed for use as anodes in Lithium-Ion batteries due to their high capacity and safety properties. They however face problems of low electronic conductivity. Lithium alloys such as tin and silicon can replace carbon as the anode.
The separator
Each battery cell consists of a separator that electrically isolates anode from the cathode. Usually made of a polymer membrane made up of porous propylene or polyethylene. Separators are very important as they affect the performance, lifecycle, safety and reliability of the battery.
TYPES OF ELECTRIC VEHICLE BATTERY FAILURES
Lithium-ion battery performance highly depends on the temperature and operation voltage. The cell operating voltage and its temperature must be maintained within required limits to avoid permanent damage of the battery cell. The cell failure can be due to;
Voltage
Over-voltage
In the instance that the voltage is increased past the optimum recommended that is normally 4.2 V, the excess current will lead to two problems;
* Lithium plating - The high currents slows the accommodation of lithium ions between the intercalation layers of carbon anode and as a result, lithium ions accumulate onto the anode surface. The accumulated lithium ions are deposited as metallic lithium in a phenomenon known as lithium plating. The negative effect is a reduction in number of Li free ions that lead to irreversible capacity loss. Since the plating after lithium plating is no longer homogenous (dendritic form), the battery can short-circuit between the electrode (Zinth et al, 2014)
* Overheating – Excessive charging current leads to increased joule heating of the cell hence increase in temperature. This causes adverse effects seen below.
Under-voltage or over-discharge
Allowing the cell to fall way below the stipulated voltage normally 2 Volts leads to progressive damage of the cell electrodes.
* Anodes- The copper plate current collector at the anode will dissolve into the electrolyte at low discharge voltage. This occurrence further increases the rate of self-discharge of the cell. Now, when the battery cell is recharged back up past 2 Volts, the copper ions that had dissolved into the electrolyte are precipitated as metallic copper and can potentially short-circuit the electrodes (Richardson et al, 2010).
* Cathodes- Prolonged over-discharge periods gradually breaks-down the cathode through release of oxygen by the LiMnO4 and LiCoO2. This occurs over many cycles and ends in permanent capacity loss.
Temperature effects
Heat is a serious battery killer. Excess heat or low heat presents the same dangers of battery destruction.
* Low temperature operation- From Arrhenius Law, chemical reactions will decrease with decrease in temperature. At lower temperatures, the rates of transformations of active chemicals in the cell are reduced. The current carrying capacity during both charging and...