Richard Stocker explains how vehicle engineering and test specialist Horiba Mira is working to tackle the problem of lithium ion battery deterioration
Lithium ion cells are essential for how we live in modern times. They can be found in everyone’s mobile phones, laptops and cameras, allowing powerful equipment to be portable. They are also increasingly being combined into large, high energy battery packs that can power passenger vehicles, and eventually even commercial buses and trucks, over long distances.
Despite strong recent improvements, battery cell technology is not perfect, and in particular has one large flaw: ageing. Lithium ion (Li-ion) cells age both with time and usage, reducing energy storage capability and increasing resistance to its access.
Battery cell deterioration is already something very noticeable in mobile phones and laptops. With a new phone, battery life can last for a full day or more without charge. Fast forward two years and 4-5 hours leads to full energy depletion. Apple has even admitted to slowing old phones down to account for performance decrease. This, while an inconvenience for portable devices, is a serious issue for electric vehicles. The main concern for electric vehicles is available range, and cell ageing directly reduces that. The concept of battery pack deterioration, and the uncertainty around it, is a primary reason people are nervous about buying second hand electric vehicles, causing depreciation for electric vehicles to be very high.
So how do we solve the problem? The first step is understanding what causes cells to age. Li-ion cells will always age, due to innate chemical reactions between the negative electrode and electrolyte. This will even happen while the vehicle is not being used, a phenomenon called ‘calendar ageing’. The rate at which ageing occurs is a complex relationship between cell design and cell usage. Certain things significantly increase the rate of ageing, such as storing at full charge for long periods, applying high charge currents, charging at low temperatures (<10°C), storing or using cells at high temperatures (>40°C) and cycling cells in the extremes of their operating voltage range.
The conditions in which ageing is accelerated pose a particular problem for automotive applications. Due to concerns with range anxiety, it is unlikely users will accept starting journeys on a partially charged car. For an electric vehicle to be considered a replacement for its ICE contemporary, fast charging is essential for giving confidence to users that the vehicle can effectively handle long distance journeys.
A modern vehicle is also designed to be sold around the world, with an associated wide temperature range. This causes a dilemma for cell designers, with there being a direct trade-off between designing to minimise ageing at high and low temperatures. At low temperatures, the negative electrode’s reactions with the electrolyte cause a protective layer to form on the negative electrode surface. As this layer never completely protects, it becomes gradually thicker over cell lifetime, consuming lithium that would otherwise contribute to cell capacity. The more surface area the negative electrode has, the more layer required to cover it, increasing capacity fade.
At high temperatures, the chemical reactions accelerate making them the defining contribution to ageing. Low temperatures, however, bring a reduction in the ability of the negative electrode to accept Li-ions during charge and an increased risk of the arriving lithium to plate on the electrode surface. This effect is negated by increasing the surface area of the negative electrode, contrasting directly with high temperature design.
The ageing sensitivity to the useable capacity range of the cell is also difficult, as this contradicts directly against another key requirement, vehicle range. A larger capacity range will allow the vehicle to travel further initially but may impact its ability to meet automotive warranty conditions.
The problem of ageing is sensitive to all aspects of the battery pack system. From a cell manufacturer’s perspective, developments in cathode compounds could improve performance and cycle life, but this could be offset by silicon compound anodes, which bring higher energy density but introduce a much bigger mechanical degradation element. Alternative chemistries are also being developed with significantly longer lifetimes, although currently at the expense of energy density.
Battery pack design is improving, particularly knowledge around optimum cell thermal management, which will have a large effect on lifetime. The most significant challenge however, could lie in the Battery Management System, which needs to allow the required performance while delivering the maximum possible lifetime for a given cell. Progress has been made in this area, with tailored charging profiles to avoid lithium plating, intelligent charge timing to reduce State of Charge (SoC), and temperature compensation in operational strategies.
Despite this, a solution to ensure an acceptable battery lifetime in all conditions is not currently found and will only be achieved through a combination of further improvements in cell design, battery pack construction and usage optimisation, in conjunction with understanding of the fundamental ageing mechanisms of emerging cell chemistries.
Richard Stocker is a battery management system research and development scientist at Horiba Mira