Batteries exist by the grace of people who want to be mobile at all times. Lithium ion batteries are especially popular because of their high energy density – up to 200 Watts of energy per kilogram of battery. But also because they can be recharged over and over again without deteriorating significantly. And that’s why they are invariably nearby: in portable electronic devices such as smartphones, notebooks and tablets, but also in electric bikes and electric cars.
Batteries are nothing more and nothing less than charge pumps, driven by chemical reactions inside the battery, which pump the mobile electrons further into the electric wires and to the gadgets they have to power. So batteries do not make electrons themselves; flowing charges pass through the battery and do not accumulate there. The capacity of a battery, in milli Ampere hour or mAh, indicates how much charge a battery can pump around. For example, a battery of 2000 mAh can deliver a current of 1 Ampère for 2 hours.
Charge and discharge
In rechargeable lithium ion batteries, lithium ions are the charge carriers. Inside the battery the lithium can be found in three places: as part of LiCoO2‘s crystal lattice in the cathode, as lithium salt in the electrolyte and in the anode where the lithium is a ‘guest’ between the carbon layers in the graphite. When the battery is fully charged, the anode is maximally full of lithium, with one lithium atom attached to six carbon atoms.
During use (or ‘discharge’), positively charged lithium ions move inside the battery from the positively charged electrode or anode via the intermediate electrolyte to the negatively charged electrode or cathode. Lithium cobalt oxide (LiCoO2) is a predominant cathode material for lithium ion batteries in gadgets, but it feels lithium iron phosphate (LiFePO4) – with a lower energy density but chemically more stable – breathing down its neck. In order to close the circuit, negatively charged electrons move outside the battery from the anode to the cathode, thus powering a connected gadget. During charging, forced by an external power source the lithium ions follow the reverse path inside the battery and go back from the cathode to the anode. The (electro)chemical reactions within rechargeable batteries are reversible.
The reacting compounds at the anode and cathode determine the capacity and voltage of a battery. The materials that constitute these electrodes are chosen in such a way that the anode donates electrons and the cathode absorbs electrons. The extent to which this happens determines the voltage of the entire cell. The elektrolyte between the anode and cathode conducts electricity, in a lithium ion battery of course in the form of lithium (Li+) ions. The combination of graphite as anode, LiCoO2 as cathode and lithium salts as LiPF6 or LiBF4 dissolved in an organic solvent such as carbonate esters gives a nominal voltage of 3.6 Volt for a single cell. This value is the average between a full cell of 4.2 Volt and an empty cell of 3.0 Volt. Higher voltages can be obtained by placing a number of cells in series into a real ‘battery’ of cells. So a cordless drill with a rechargeable battery of 14.4 Volt contains four cells. Lithium ion batteries in these devices, but also in gadgets such as notebooks, are equipped with electronics to protect against overcharging or high temperatures, or to prevent the battery from being discharged too much.
A lithium ion polymer battery deviates from a ‘normal’ lithium ion battery in that a polymer such as polyacrylonitrile or polyethylene oxide has replaced the organic solvent in the electrolyte, and LiCF3SO3 is used as a lithium salt. Electric bicycles use this type of battery.
In the course of time, the performance of rechargeable lithium ion batteries declines. We are not talking here about a battery that is empty at the end of the day, but about the capacity that gradually deteriorates over the years.
Part of this ageing already starts as soon as the battery leaves the factory. On a microscopic scale, defects arise or undesired (crystal) structures grow further, which diminish the capacity and the power of the battery. For example, a ‘solid electrolyte interface’ is formed on the interface between the carbon anode and the electrolyte. Over the years, this SEI ‘rust layer’ grows further and further (corrodes) as a result of undesirable side reactions that occur in the electrolyte, such as decomposition of the electrolyte. In addition, this layer also ‘eats’ lithium, which can no longer be used in the ion transport. So the internal resistance for lithium ion conduction increases over time, whether you use the battery or not.
However, the deterioration is also due to the fact that frequent charging and discharging during operation has a detrimental effect on capacity. For example, the LiCoO2 lattice of the cathode very slowly takes on a different structure when lithium ions, which leave the cathode during charging, can occupy a different position when they return during discharge. This continuous insertion (also known as intercalation) and extraction of the lithium changes the lattice somewhat, causing the ‘lithium-providing’ properties of the lattice to deteriorate.
These are two examples of processes in the ‘dynamic battery’ that determine its service life. After about 3 years or 1000 charges and discharges, you can consider the device to be deceased. So in practice, a rechargeable battery cannot be recharged forever. At best, as a user, you can prolong its life span by (dis)charging tactfully. By inserting and extracting the lithium ions to and from the electrodes, these electrodes swell and shrink a little. Even though this volume difference is merely a few percent – hardly noticeable – this means that the electrodes are always subjected to a mechanical cyclic load during charging and discharging. When this proceeds for a sufficiently long period of time and to a serious extent, the electrode particles will break off from each other in some places. This way, locally the ‘electrical pathway’ will become lost, causing the capacity of the entire cell to deteriorate. Full charging and discharging is a heavier burden to the electrodes than partial charging and discharging. So in order to prolong the service life of such a rechargeable battery, it is better not to charge too much (say up to 80% capacity) and not to discharge too much (~30%), than to charge and discharge completely (0 – 100%) all the time. Of course, this is at the expense of the ‘length of use’ of the electronic equipment in between two charges.
At (too) high temperatures, the desired ion transport in the battery will be faster and faster, but the unwanted side reactions will also be increasingly faster, so that the SEI layer becomes thicker and thicker and the internal resistance for ion transport gradually increases. Too low temperatures result in slow ion transport, which reduces the (discharge) capacity.
It is not convenient to have to charge a smartphone or tablet during the day after frequent use. A larger battery is a simple way to deliver more capacity, but that is at odds with the trend to make gadgets increasingly lighter and smaller. A solution? Manufacture the electrode materials from ever smaller particles or make them (micro)porous, so that the total electrode surface area becomes larger, and more exchange of lithium ions on these surfaces can take place. Silicon is also in the picture to replace graphite as anode material, because silicon has room for more lithium ions than graphite. However, the problem still needs to be solved that silicon swells (or shrinks) more strongly during (dis)charging than graphite.