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AUTOMOTIVEENGINEER

SEPTEMBER 2020

Electric vehicle li-ion battery technologies

The Institute of the Motor Industry www.theimi.org.uk I 02

AUTOMOTIVEENGINEER SEPTEMBER 2020

This paper is about electric vehicle (EV) batteries and some of the management and control methodologies that can be used to get the most from them and prolong their life. As a way to set the scene and as a reminder of the diversity of approaches to vehicle electrification, Table 1 shows the differences between some of the common forms of EVs.

Electric vehicle li-ion battery technologies01

Battery management is important even on 12V ICE cars, but it becomes even more so on higher-voltage lithium-ion (li-ion) batteries. The key aspects to consider include ways to:

■ Improve the chemical reaction;

■ Control temperature; and

■ Accurately control charge levels and charge rates.

1.1 Introduction

1.2 Lithium-ion (li-ion) batteries

Lithium-ion technology has become many manufacturers’ battery technology of choice, but it still has potential for further development. Today’s batteries have an energy density of around 140Wh/kg – or more in some cases – but the technology has the potential to go as high as 280Wh/kg. Much research into cell optimisation is now taking place to create a battery with a higher energy density and therefore an increased range. Lithium-ion technology is currently considered the safest way to do this.

ICE’s relative importance

Battery system

Mains charging

Electric traction

CO2 reduction versus ICE

****

12V

No

n/a

****

12V / 48V

No

10-20kW

20%

***

LV / HV

No

15-60kW

30%

***

LV / HV

Yes

40-80kW

50%-75%

**

LV / HV

Yes

40-80kW

50%-75%

LV / HV

Yes

>80kW

100%

Internal

combustion

engine (ICE)

Mild

hybrid

(M-HEV)

Full

hybrid

(F-HEV)

Plug-in

hybrid

(P-HEV)

Range-extended

EV

(RE-BEV)

Pure electric

vehicle

(BEV)

TABLE 1: Electrification methods compared to ICE



FIGURE 1: Basic representation of a lithium-ion cell

A li-ion battery works as follows. A negative pole (anode) and a positive pole (cathode) make up the individual cells of a lithium-ion battery, together with the electrolyte and a separator. The anode is a graphite structure, and the cathode is made up of layered lithium oxide. Lithium ions are deposited between these layers. When the battery is charging, the lithium ions move from the anode to the cathode and take on electrons. The number of ions therefore determines the energy density. When the battery is discharging, the lithium ions release the electrons to the anode and move back to the cathode.

The electrode of a battery that releases electrons during discharge is called the anode; the electrode that absorbs the electrons is the cathode. The battery anode is always negative, and the cathode is always positive.

Useful work is performed when the discharged electrons flow through a closed external circuit. The following equations show one example of the chemistry, in units of moles, making it possible to use coefficient x. Note: it is not necessary to understand the details of these equations. They are simply a guide to the chemical process.

The cathode (marked +) half-reaction is:

The anode (marked -) half-reaction is:

One issue with this type of battery is that in cold conditions, the lithium ions move more slowly during the charging process. This tends to make them reach only the electrons on the surface of the anode, rather than those inside it. Also, using a charging current that is too high creates elemental lithium, which can be deposited on top of the anode, covering the surface and sealing the passage. This is known as lithium plating. Research into this is ongoing, and one possible solution could be to warm up the battery before charging.



FIGURE 2: Basic operation of a lithium-ion battery (Source: Bosch Media)

High-voltage electrolytes can further boost battery performance, raising the voltage within the cell from 4.5V to 5V. The technical challenge lies in guaranteeing safety and longevity while improving performance. Sophisticated battery management can increase the range of a car by up to 10%, without altering the cell chemistry (Denton, 2020).

1.3 Solid-state batteries

A new battery technology for EVs is the solid-state cell. These could be a breakthrough technology, as the batteries charge faster and are smaller than existing technologies. They could also be production-ready by 2024.

FIGURE 3: Solid-state batteries could

offer double the energy content of

today’s cells, while also being less

combustible and 75% smaller

(Source: Bosch Media)

Bosch is now working on post-lithium-ion batteries, such as those made using lithium-sulfur technology, which promises greater energy density and capacity. However, there are several other ways to improve battery performance. For example, the materials used for the anode and cathode play a major role in the cell chemistry. Most of today’s cathodes consist of nickel manganese cobalt (NMC) or nickel-carboxyanhydrides (NCA), while anodes are typically made of graphite, soft or hard carbon, or silicon carbon.



FIGURE 4: Solid-state cells (Source: Bosch Media)

The performance of an energy storage device can be improved in various ways. For example, in cell chemistry, the materials that the cathode and anode are made of play a major role. One of the reasons why energy capacity is limited in current lithium-ion batteries is that the anode largely consists of graphite. Solid-state technology means that the anode can be made out of pure lithium instead, which considerably increases storage capacity. In addition, the new cells function without ionic liquid, which means they are not flammable.

1.4 Battery life and recycling

Up to now, the industry’s declared target has been to double battery energy density and halve costs by the end of this decade. The new solid-state cells have the potential to achieve both of these targets and could even exceed them. A comparable EV with a driving range of 150km today would be able to travel more than 300km without recharging thanks to a solid-state system, at a lower cost. Engineers are working on further refining the technology, and in doing so, making electromobility a more practical proposition.

At the time of writing, battery cells make up about 30% of the price of a pure electric vehicle. One problem to solve going forward is the growth in demand. By 2030, for example, demand for raw materials in Europe is expected to be 25 times higher than it was in 2015. Reuters reports that prices for key elements such as cobalt, lithium, nickel and copper could increase exponentially (May 2019). One way to help with this demand problem is the 5R solution shown in Figure 5.

FIGURE 5: The 5R solution (Source: Autocraft Solutions)



(Autocraft Solutions, 2019)

Cells age differently, so only a proportion will be unusable at the end of a battery pack’s life. The usable cells (or modules) can then be sorted and repurposed. By replacing between 5% and 30% of a battery’s cells, EV battery packs can theoretically be remanufactured to a state of health (SOH) of almost 100%, multiple times.

1.5 State of charge

State of charge (SOC) is a measure of the amount of energy left in a battery compared with the energy it had when it was full. It gives the driver an indication of how much longer a battery will continue to work before it needs recharging. It is considered a measure of the short-term capability of a battery.

However, it is more difficult to define SOC than you might expect. It is typically defined as the available energy capacity, expressed as a percentage of a given reference figure. This could be its:

■ Rated capacity (as if it were new); or

■ The latest charge-discharge capacity.

This can be a problem, particularly on an EV. A vehicle that has a range of, say, 100km on a fully charged, brand-new battery could reasonably expect a range of 50km when it’s 50% charged. However, after several years, the capacity of the battery when fully charged may only be 80% of what it used to be. An indication of 50% charge would now only give a 40km range.

Because EVs use SOC to determine range, it should ideally be an absolute value based on the capacity of the battery when new. Several methods of estimating a battery’s SOC have been used. Some are specific to particular cell chemistries. Most depend on measuring a parameter which varies with the state of charge.

The easiest way to monitor SOC is a voltage measurement, but this does depend on several factors. An open circuit voltage will be higher than when current is flowing, due to the cell’s internal resistance. Temperature also has a big effect. What’s more, lithium-ion batteries have a cell voltage that doesn’t change that much between fully charged and fully discharged. Most are actually operated between 80% and 20% charged, as this reduces degradation over time. The voltage changes are therefore even smaller. Nonetheless, taking all factors into account, a voltage measurement under a known load gives a reasonable estimate of SOC.

The Autocraft EV battery system identifies different grades of recovered packs:

■ Grade A packs (Repaired) for use in a vehicle at OE specifications

■ Grade B packs (Remanufactured) for use in a vehicle at a lower specification

■ Grade C packs (Reused) for use in alternative markets

■ Grade D packs (Recycled) made safe for material recycling partners



It is also possible to calculate SOC by measuring current and time (in or out). Current multiplied by time gives a suitable value for SOC. Unfortunately, there are a few problems with this:

■ The discharge current changes non-linearly as the battery discharges;

■ To know how much charge it contained, the battery must be discharged; and

■ There are losses during the charge/discharge cycle.

A battery will always deliver less during discharge than was put into it during charging. This is sometimes described as the Coulombic efficiency (CE) of the battery. Temperature is once again an issue. However, if all factors are considered, a reasonable figure for SOC can be calculated. Most battery manufacturers use Coulombs in and Coulombs out as a benchmark for their warranties.

Li-ion has one of the highest CE ratings in rechargeable batteries. It offers an efficiency that exceeds 99%. However, this is only possible when charged at a moderate current and at cool temperatures. Ultra-fast charging lowers the CE because of losses through charge acceptance and heat. The same problem arises with a very slow charge, where self-discharge comes into play. For comparison, the CE of a lead acid battery is about 90%.

1.6 State of health

The state of health (SOH) of a battery is a measurement that indicates the general condition of a battery and its ability to perform compared with a new battery. It considers charge acceptance, internal resistance, voltage and self-discharge. It is considered a measure of the long-term capability of a battery.

SOH is an indication, not an absolute measurement. Over its lifetime, a battery’s performance deteriorates due to physical and chemical changes.

Unfortunately, there is no agreed definition of SOH. Cell impedance (or cell conductivity) is often used as a reasonable estimate of SOH. More complex systems monitor other parameters and involve a range of calculations. Because SOH is a figure relative to the condition of a new battery, the measurement system must collect and save data over time and monitor the change.

Counting the charge/discharge cycles of the battery is one way to measure battery usage and can be used to indicate SOH, if compared with the expected values over time. This is because the capacity of a lithium-ion cell deteriorates quite linearly with age or cycle life. The remaining cycle life can therefore be used as a measure of the SOH.



1.7 Fast charging batteries

The flow of electrical current in battery cells and the associated connections causes heat, so cooling is vital. This heating effect is proportional to the square of the current flowing, multiplied by the internal resistance of the cells and connections (I2R). A cell’s internal resistance rises when it’s cold.

FIGURE 6: A lithium-ion battery pack Many lithium battery cells should not be fast charged when their temperature is below 0oC. Battery management and control methodologies can handle this low temperature issue, but lithium cells also begin to degrade quickly if their temperature is too high (typically over 45oC), and there are safety concerns associated with operation at high temperatures. The temperature of cells also needs to be kept consistent across the pack, as uneven temperatures can lead to performance degradation and potential thermal events.

FIGURE 7: Representation of heat generated inside a battery pack – the penguin effect There are two options for

efficient liquid cooling:

■ Indirect cooling using a liquid that is pumped around the battery and passed through a cooling radiator or similar; or

■ Direct immersion cooling, where the battery cell components are covered in a cooling agent.

Cooling the battery with a dielectric oil (the cooling agent) which is then pumped out to a heat exchanger system is an effective solution and offers good control. Figure 8 shows an example of this.

FIGURE 8: Battery pack with coolant flow connections (Source: Porsche Media)



Currently, the most widely used cooling method is indirect cooling using a traditional cooling agent. As the demands of volumetric density, safety and power density increase, there is an urgent need for a safer, more efficient cooling system design, and new cooling agents.

Direct liquid cooling is more effective than indirect cooling and takes up less space, provided that the heat transfer fluid is a safe and stable dielectric oil. Direct immersion cooling offers a safe, efficient, simplified design that enables more compact packaging.

Selection of the liquid for direct immersion cooling mustn’t be made on the basis of heat transfer characteristics alone. Chemical compatibility of the coolant with the cells, electronics (control units) and other packaging materials must be a prime consideration to help avoid:

■ Short-circuiting;

■ Corrosion;

■ Cross-contamination; and

■ Flammability risk.

There are several material factors to consider, because fast charging has four main limiting factors:

■ Lithium plating;

■ Particle cracking;

■ Atomic rearrangement; and

■ Temperature increases.

These four factors must be taken into consideration when determining the optimal fast charge profile, else a significant reduction in cell capacity could result. Increasing the charging time by changing cell chemistry or design will have a cost in terms of either battery life or energy density.

One way to improve the thermal conditions in a battery pack is to use a liquid gap filler in place of a thermal pad. This method is shown as Figure 9 and is now used by many manufacturers.

Claimed charging times on some new battery designs vary from a few minutes to a few hours. The more ambitious fast charge times would have to overcome significant barriers to become a reality. As a bare minimum, a highly efficient thermal management system and detailed attention to component packaging will be required.

FIGURE 9: Gap fillers can have a huge impact on thermal conditions in a

lithium-ion battery (Source: Solvay)



1.8 Thermal runaway

If overheated, lithium-ion batteries can go into a thermal runaway process. This can be separated into three stages:

FIGURE 01: Thermal runaway stages. The SEI layer is a component of lithium-ion batteries, formed from the decomposition of materials associated with the electrolyte of the battery

An exothermic reaction (one that produces heat) increases the battery temperature and therefore the internal pressure of the li-ion battery. Gas evolution also increases the pressure of the cell. If the cell is equipped with a pressure relief valve (not found on pouch cells), this valve will open and release flammable organic compounds. A pouch cell (mostly used in laptops) may burst if internal pressure is too high.

White smoke may indicate the emission of organic carbonates. During further heating of the cell, the colour of the smoke turns into grey as active electrode material is emitted (mainly graphite particles). This thermal runaway process heats the cell up to 700oC – 1,000oC. This high temperature may affect adjacent cells and cause a chain reaction.

If a thermal runaway occurs, a large number of different chemicals are generated. Combustion reactions mainly create:

■ CO and CO2 from organic materials;

■ NOx;

■ HF; and

■ Low-molecular-weight organic acids, aldehydes and ketones.

Although a li-ion battery fire should not ideally be extinguished with pure water, using plenty of water may be reasonable because it cools the surrounding cells to avoid a chain reaction. Additionally, many of the emitted particles and toxic gaseous compound will bind and be diluted by the water.

Anodic reactions start at about 90oC

Decomposition of the solid electrolyte interphase (SEI) >120oC

Reduction of the lithiated negative anode

Exothermic reactions at the positive electrode >140oC

Rapid oxygen evolution

Positive oxidation decomposition

Electrolyte oxidation >180oC, high rate exothermic process

Temperature rise: 100oC/min

Dangers: Requirements: Challenges:

Safety Performance Multi-cell control

TABLE 2: BMS safety, performance and cell control challenges (Source: NXP)



1.9 Battery management systems (BMS)

A BMS is used for managing remaining battery capacity and controlling battery charging and discharging. The voltages of the individual cells are monitored, and the balance controlled by lithium-ion battery monitoring integrated circuits (BMICs). The system can manage remaining battery capacity and battery charge control. It is like the brain of an EV’s battery pack, and its performance greatly affects the driving range and battery life of the vehicle.

The main functions of a BMS are summarised in Table 2:

■ Excess voltage

■ Extra heat

■ Unstable chemistry

■ Thermal runaway

■ Low temperature charging

■ Safe and fast charging

■ Discharge optimisation

■ State of charge (SOC)

■ State of health (SOH)

■ Hundreds of cells

■ Manufacturer mismatch

■ Capacity degradation

■ Lifetime degradation

■ Voltage, current and temperature measurement

■ Ensuring safety

■ Voltage, current and temperature measurement Coulomb counting

■ Internal resistance calculation

■ Monitoring available energy

■ Monitoring ageing

■ Cell balancing

■ Increasing battery life

■ Enhancing stored energy

Key BMS

Functions

The design shown in Figure 11 is an example of a complete automotive BMS. It can be used for up to 70 series-connected li-ion cells. The microcontroller communicates with the multi-cell BMICs to monitor cell voltage, pack temperature and current, record significant fault detection, and control cell balance (Renesas, 2020).

A daisy chain hardware layout provides robust, isolated communication between batteries. The BMS can interface with multiple CAN, LIN and other buses and control multiple pulse-width modulation (PWM) outputs. The intelligent power devices (IPDs) are for external load control via a fan or pump. This high degree of control will extend driving range and battery life performance (NXP, 2020).



Two key system features are:

■ Cell voltage accuracy of 0.8mV

■ Integrated 300 mA cell balancing

FIGURE 11: Battery management system

1.10 Cell balancing

When new, high-quality lithium-ion cells have a very uniform capacity and low self-discharge rate. However, balancing is advantageous because the performance of individual cells can decrease at slightly different rates. If just one cell in a series connection loses capacity or develops increased self-discharge, the whole string is affected.

Passive balancing uses a resistor that can be connected across a cell to divert some of the charge current. This is usually done when the cells are between 70% and 80% charged. Active balancing moves any extra charge from higher-voltage cells during discharge to those with a lower voltage.

Active balancing is the method used by manufacturers for many EV batteries. It is more complex than passive balancing, as it requires DC-DC converters, but it is more beneficial overall. The corrections to charging current are in the mA range. Because of the heavy loads during acceleration, which are often followed by fast charging from regenerative braking, it is essential to keep the cells balanced to extend battery life. FIGURE 12: The principle of cell balancing



There are several parts of the Electricity at Work Regulations 1989 (UK)1 that apply to working on high-voltage vehicles2. However, the following are key:

1.11 TechSafe™

TechSafeTM registration will fulfil the requirement that anyone working on high-voltage vehicles must be competent in line with the Electricity at Work Regulations 1989. ADAS, hydrogen fuel cells, cybersecurity and other areas will be covered in a similar way. TechSafeTM means the technician’s safe, which in turn means the customer’s safe.

The IMI’s TechSafe™ professional registration scheme is designed to ensure that complex automotive technologies are repaired effectively and that technicians work safely – particularly in the UK, but also internationally. To be added to the register, a technician must successfully complete a specified qualification (eg. Electric/Hybrid Vehicles Level 2, 3 or 4) or IMI Accredited programme, join the IMI Professional Register, and complete specified annual CPD to remain registered.

Regulation 3(1)(a) states that: “It shall be the duty of every – (a) employer and self-employed

person to comply with the provisions of these Regulations in so far as they relate to matters

which are within his[/her] control. 3(2)(b) reiterates the duty for employees.

Regulation 16 states that: “No person shall be engaged in any work activity where

technical knowledge or experience is necessary to prevent danger or, where appropriate,

injury, unless he possesses such knowledge or experience, or is under such degree of

supervision as may be appropriate having regard to the nature of the work.”

Regulation 29 states that: “In any proceedings for an offence consisting of a contravention

of Regulations 4(4), 5, 8, 9, 10, 11, 12, 13, 14, 15, 16 or 25, it shall be a defence for any person

to prove that he took all reasonable steps and exercised all due diligence to avoid the

commission of that offence.”

FIGURE 13: The IMI’s TechSafeTM logo

1 HSE Guidance: www.hse.gov.uk/pubns/books/hsr25.htm

2 The definition of high voltage can be confusing, for example when comparing vehicles to National Grid powerlines. For vehicle use, any figure in excess of 60V DC is described as high-voltage: www.hse.gov.uk/mvr/topics/electric-hybrid.htm

???



1.12 Summary

Incremental developments in battery technology and battery management technology will continue in the coming years, but the principles outlined in this paper will remain broadly the same. There may be an innovative breakthrough, but the key aspects will continue to be:

■ Advanced materials to improve the chemical reaction;

■ Thermal management; and

■ Accurate control of charge levels and charge rates.

1.13 Questions

1 What is meant by each of these words or phrases?

(a) Anode and cathode

(b) Chemical reaction

(c) State of charge

(d) State of health

(e) Thermal runaway

2 Why is careful control of charge levels important?

3 How is cell balancing achieved?

4 Why is a suitable 5R process (or similar) important?

5 What are the advantages and disadvantages of fast charging?

6 Why is TechSafeTM important?

1.14 References/Bibliography

Autocraft Solutions (2019). "5R Solution", 2019, from www.autocraftsg.com

Denton, T. (2020). Electric and Hybrid Vehicles 2nd Ed., Routledge.

NXP (2020). "Battery Management", from www.nxp.com

Renesas (2020). www.renesas.com

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